To further investigate the effects of short-term exposure to O3 on HRV, we conducted the first meta-analysis of relevant studies. The percentage change of HRV indicator(s) is the effect estimate extracted for the quantitative analysis in this study. In our meta-analysis, per 10 ppb increase in O3 was significantly associated with decreases in the time-domain measurements, for standard deviation of the normal-to-normal (NN) interval (SDNN) −1.11%  and for root mean square of successive differences (RMSSD) −3.26% ; in the frequency-domain measurements, for high frequency (HF) −3.01%  and for low frequency (LF) −2.14% . This study showed short-term exposure to O3 was associated with reduced HRV indicators in adults, which suggested that the cardiac autonomic nervous system might be affected after O3 exposure, contributing to the association between O3 exposure and CVD risk.At present, ambient air pollution poses a significant threat to patients with cardiovascular disease (CVD). The heart rate variability (HRV) is a marker of the cardiac autonomic nervous system, and it is related to air pollution and cardiovascular disease. There is, however, considerable disagreement in the literature regarding the association between ozone (O At present, it is one of the most important pollutants associated with traffic in urban and industrialized areas and has been linked to a number of health outcomes, including cardiovascular diseases [The formation of ozone  is one of the leading causes of death, and its incidence is expected to rise steadily in the next decade . Environic risks ,8. Theret method ,10. In lOne method of predicting CVD risk is using surrogate markers, and heart rate variability (HRV) has been shown to be a reliable predictor. Specifically, an increase in HRV indicates that the autonomic nervous system (ANS) is well adapted and functioning efficiently, while a decrease in HRV is often an indication that the ANS has not been sufficiently adapted . All HRV3 exposure on HRV metrics; however, these results were inconsistent and a more comprehensive study is needed to elucidate the potential relationships [3 exposure adversely affects the cardiovascular system.Over the past several years, some studies have assessed the effects of short-term Oionships ,15,16. I3 exposure  and HRV metrics in the general population. This review uses the Population, Exposure, Comparator, Outcome, Study Design (PECOS) statement shown in To address this question of surrogate marker of CVD risk related to ozone exposure, we conducted a systematic review and meta-analysis of studies examining associations between short-term OThe protocol of this study was not registered in PROSPERO.The search question was: “Among the general population, what is the effect of a higher exposure to ozone compared to lower level of ozone exposure on HRV indices?”.3 or air pollution) AND   . PRISMA or SDNN)  were fol3 exposure was used to correspond to personal exposure before the HRV protocol. The effect estimates (percentage change (%) and 95% confidence interval (CI)) in the indicators of HRV for an increase in O3 exposure by 10 ppb for a continuous exposure was considered. The outcome was HRV indicators, including RMSSD, SDNN, LF, and HF.The eligibility criteria for the PECOS are summarized in 3 and indicators of HRV published in English. We excluded conference papers, reviews, meta-analyses, and commentaries from the analysis. In order to be eligible for the analysis, studies had to be conducted in the general population and they had to contain original data providing effect estimates on at least one of these four HRV indicators: RMSSD, SDNN, LF, and HF. Those studies with overlapping populations and information were excluded from the review. In this case, we retained the publication providing the most complete information AND/OR the most representative population. Among the studies included in this review were cohort, case–control, and cross-sectional studies that examined the relationships between OWe selected studies following the screening of titles and abstracts by two investigators (Z.Q.Z. and M.Y.Z.), with any discrepancy being resolved by a third investigator (C.Y.H.); next, the retained potential eligible studies were screened on full-text reading.From each study, two investigators (Z.Z. and M.Z.) extracted the following data: author name, publication date, study country, study population, sample size, exposure assessment, indicator(s) of HRV, and adjusted covariates. For the purpose of gathering unpublished data, authors were contacted directly when it was considered appropriate. Meta-analysis was performed using the most fully adjusted effect estimate that represents the greatest control over potential confounders.Cohort, panel, and case–crossover studies were assessed using Newcastle Ottawa Scale (NOS). There are eight items in the NOS, and the items are categorized into three dimensions, including selection, comparability, and outcomes. Studies were evaluated based on an NOS score from 0 to 9, with a score greater than 7 indicating high quality, a score between 5 and 6 indicating moderate quality, and one less than 5 indicating low quality .3 exposure with four common indicators of HRV including two time-domain parameters (RMSSD and SDNN) and two frequency-domain parameters (LF and HF). As most of the included studies reported the HRV measurements on a logarithmic scale, we thus excluded the studies with linear scale models since they were not comparable. The effect estimates of each included study were presented per standard deviation (SD) or interquartile range (IQR) change of the O3 level, and they were converted into per 10 ppb increase in O3. We calculated the percentage change in accordance with the methodology of a previous meta-analysis that assessed the associations of PM2.5 exposure with HRV [Meta-analyses were performed using fixed-effect or random-effects models for the associations of Owith HRV .p-value of Egger’s test less than 0.05 was found to indicate the presence of publication bias. Additionally, the trim-and-fill method was used to evaluate the impact of publication bias when appropriate. Evaluation for presence of heterogeneity was carried out using (1) Cochran’s Q-test with p-value less than 0.05 signifying heterogeneity and (2) I2 statistics, where I2 greater than 50% indicated substantial heterogeneity [In order to determine publication bias, funnel plots and Egger’s regression tests were utilized, and a ogeneity . The potIn total, 8339 records were retrieved from three electronic bibliographic databases. The flow chart shows the detailed screening process . A total3 and indicators of HRV. Per 10 ppb increase in O3 exposure was associated with a decrease in the indicators of HRV. Specifically, meta-analyses on associations of O3 exposure with RMSSD, HF, and LF showed moderate to high between-study heterogeneity and therefore a random-effects model was used, while for SDNN, a fixed-effects model was used due to the low between-study heterogeneity. As shown in 2 = 0.0%) for SDNN and −3.26%  for RMSSD, respectively. Similarly, the pooled estimates were −3.01%  for HF and −2.14%  for LF, respectively.Thirteen studies have examined the relationships between short-term exposure to O3 exposure (≤24 h or others), exposure assessment , and quality of study (high or moderate) , study location (North America or East Asia), length of ECG recording (≤30 min or others), Ooderate) . Subgrou3 exposure on HRV indicators, such as RMSSD, HF, and LF, did not significantly change before and after systematically excluding each study, indicating the robustness of results .Sensitivity analyses were performed to assess the stability of the results. Generally, the pooled estimates of O results . The ind results . When th3 and HRV indicator combination. Vertical funnel plots showed basic symmetry .We constructed vertical funnel plots and Egger’s tests to assess the publication bias for each Osymmetry . The p v3 can increase the risk of cardiovascular diseases in adults. In our meta-analysis, we evaluated the effects of O3 on HRV based on 13 observational studies conducted among the adults. According to the present meta-analysis, short-term exposure to O3 is associated with a decrease in HRV indices. The positive associations indicated that O3 may alter cardiac autonomic function and thus increase the risk of cardiovascular events. To the best of our knowledge, this is the most comprehensive meta-analysis which specifically evaluates the association between O3 exposure and HRV.The purpose of this meta-analysis was to provide evidence that elevated levels of O3 exposure in the short-term and HRV is still scarce. The low number of studies included in the current meta-analysis, as well as the moderate to high heterogeneity observed in the study, may obscure the true association between O3 exposure and cardiovascular disease risk. Previous meta-analyses have shown that short-term exposure to O3 is associated with a variety of adverse health outcomes, including asthma exacerbations [3 exposure and cardiovascular disease. In the present meta-analysis, we emphasized the importance of integrating the results obtained from studies of people with cardiovascular disease with those obtained from studies of healthy individuals. The reason for this is that most previous studies have focused on older individuals with cardiopulmonary disease; there are only a small number of studies that examine associations among healthy and younger individuals. HRV indices have been shown to vary with cardiovascular status and drug mediation, and HRV responses to O3 stimulation are also thought to be affected by health status and drug mediation [In general, the literature on the association between Orbations , pneumonrbations , pulmonarbations , and atrrbations . Despiteediation ,38,39,403 increases CVD risk have yet to be fully determined. It is a well-established fact that imbalance of ANS, as indicated by a disturbance of HRV, is one of the most important mechanisms by which O3 exposure increases the risk of adverse cardiovascular events [3 exposure may activate the HPA and SAM axis, triggering the release of stress hormones, such as corticotropin-releasing factor (CRF), adrenocorticotropic hormone (ACTH), cortisol, adrenaline, and noradrenaline, which further contribute to cardiovascular and metabolic dysfunction [3 may affect the cardiovascular system. The initial responses to oxidant injury and inflammation may eventually result in endothelial dysfunction, acute arterial vasoconstriction, procoagulant activity, and atherosclerosis. The stimulation of nociceptive fibers in the airways may result in changes in sympathetic and/or parasympathetic tone, which may result in the onset of cardiac arrhythmias [3.The mechanisms by which exposure to Or events ,41. An ir events ,43,44,45r events ,46. Highfunction ,48,49. Ihythmias . In gene3 exposure with RMSSD and HF. We also observed a positive association between O3-HRV  in Asia, where levels of air pollution are much higher than North America [3 levels have increased rapidly in China, during the warm season [3 exposure were used, namely fixed-site monitoring and personal monitoring. The majority of studies on O3 exposure and cardiological diseases used measurements collected at centrally located monitoring stations or fixed-site; however, this method may introduce bias and distort epidemiological associations since it does not take into account the temporal variability of all possible sources of contamination and concentration. Moreover, most short-term effects studies of O3 used mean daily maximum 8-h average (MDA8) as exposure measurements and we only selected studies using 24-h averaged O3 to contain more comparable studies in the present study. Thus, the potential risk of exposure misclassification cannot be ruled out. In order to produce more accurate effect estimates, more detailed information on the measurements of various pollutants based on a fine spatiotemporal scale will provide more reliable understanding of the exposure–response associations [3 exposure also exists. The record of electrocardiograms ranged from 5 min, 7 min, 15 min, 16 min, 30 min, 35 min, and 36 min to 24 h (ambulatory) in our included studies. However, a 5-min recording of electrocardiograms is recommended as longer recordings may be affected by emotions or physical activity [3 concentration was the most commonly used; however, there were studies with O3 exposure periods of 1 h, 2 h, 4 h, 12 h, 24 h, 72 h and 120 h. Various mean period of O3 concentrations and ECG recording lengths do not produce the same effect estimates on HRV indicators, which may also explain the heterogeneity observed in the associations of O3 exposure with RMMSD, LF, and HF [Moderate to high heterogeneity was detected in the meta-analysis and we further performed subgroup analyses. Several possible categorical variables were identified, such as age of participants, that could explain the heterogeneity among the combinations of O America . From 20m season . In the ciations . Heterogactivity . The 24 , and HF .3 exposure and HRV indices were robust, with the exception of the combination of O3 and SDNN , suggesting that the mean period concentration of O3 of 72 h might have been the source of heterogeneity. Vertical funnel plots  in angiotensin receptor blocker (ARB) nonusers, whereas no obvious changes were observed in ARB users [Furthermore, sensitivity analyses showed that our effect estimations for short-term Oand SDNN . The resel plots  and Egge95%CI: 0.−2.07% inRB users . Zhong eRB users . As a re3 exposure and HRV indices in the general population. We were able to perform multiple subgroup analyses to investigate the source(s) of heterogeneity. Finally, this meta-analysis included older adults with CVD as well as healthy young adults, which contributes to our understanding of the exposure–response relationship between O3 and HRV.This meta-analysis has several limitations that should be taken into account when interpreting the results. Firstly, there are a limited number of published studies available for each HRV indicator, which limits the statistical power of the analysis. Secondly, the studies included in the present study were observational, so we were unable to determine whether or not there was a causal relationship. Thirdly, the meta-analysis was based on studies conducted in North America and Asia, which limited the generalization of the results to the different geographical regions. Despite the aforementioned limitations, our meta-analysis has several strengths. As far as we are aware, this is the first meta-analysis conducted to examine the relationships between O3 is associated with alterations in cardiac autonomic function, as measured by HRV in the general population. Further research is recommended to determine effective interventions for improving air quality and reducing incident CVD, and mechanistic studies are needed to determine the cause of the detrimental effects of ozone on the cardiovascular system.In this systematic review and meta-analysis, there is evidence that short-term exposure to O


Dry eye disease (DED) can be triggered using preserved ophthalmic formulations or prostaglandin analogs. In this prospective, nonrandomized, open-label pilot study, we evaluated the efficacy of a 0.15% hyaluronic acid (HA) nonpreserved ophthalmic formulation in decreasing DED symptoms in patients with open-angle glaucoma treated with prostaglandin analogs.  30 patients with DED receiving chronic treatment with prostaglandin analogs for primary open-angle glaucoma or ocular hypertension were administered ophthalmic formulations 3 times daily for 12 weeks. Foreign body sensation, burning, stinging, dryness, pain, frequency of symptoms, Ocular Surface Disease Index (OSDI), conjunctival hyperaemia, corneal fluorescein staining (CFS), tear film break-up time (TBUT), best-corrected visual acuity, Schirmer test results, and 25-item National Eye Institute Visual Function Questionnaire score between the baseline and 4 and 12 weeks were evaluated. p < 0.001) and dryness and pain after 12 weeks of treatment  were reduced significantly. Secondary outcomes confirmed the positive results, with a statistically significant change in the OSDI score and CFS between the baseline and 4  or 12 weeks (both p < 0.001) and TBUT after 4 weeks (p=0.01). Conjunctival hyperaemia improved in both eyes in >90% of cases at 12 weeks of treatment.  The analysis shows that all primary endpoints improved; in particular, burning sensation and the frequency of symptoms after 4 and 12 weeks of treatment ( The present study shows that the ophthalmic formulation containing 0.15% HA has a promising beneficial effect on reducing the signs and symptoms of DED in patients treated with prostaglandin analogs. The ocular surface consists of a continuous epithelium hydrated by the tear film, which also contains protective antimicrobial factors . Tear film stability is crucial for maintaining homeostasis of the ocular surface and is ensured in particular by the mucin-rich gel produced by epithelial cells . In addiAccording to the 2017 International Dry Eye Workshop II report, dry eye disease (DED) is a multifactorial condition characterized by increased osmolarity of the tear film and inflammation of the ocular surface –12. The Because of the enormous variability of clinical signs, there is no consensus on the diagnosis of DED; nevertheless, self-reported questionnaires, such as the Ocular Surface Disease Index (OSDI), are commonly used as tools for assessing the severity of the disease. In addition, other clinical tests are used by physicians, including the Schirmer test, tear break-up time (TBUT), corneal and conjunctival staining, and tear osmolarity .Many causes underlie the occurrence of DED. The most common are the use of contact lenses, refractive laser cataract surgery, and the use of topical formulations containing preservatives and prostaglandin (PG) analogs for glaucoma and ocular hypertension , 18. RecInstead, PG analogs have become the first-line therapy for treating patients with glaucoma due to their efficacy in lowering intraocular pressure (IOP) . While rTo date, treatment options for DED have been based on avoidance of triggering factors, such as cigarette smoking, adverse environments, and others, in conjunction with the use of topical nonpreserved formulations, such as artificial tears and corticosteroids or cyclosporin A-based eye drops , 25. DueEchinacea extract, and amino acids in improving DED symptoms.Hyaluronic acid (HA) is a linear polymer composed of N-acetyl-glucosamine and glucuronate units. Its use in ophthalmology has been studied since the early 1990s, and HA is known to increase tear film stability by stimulating mucin production . ConsistEchinacea extract  in improving DED symptoms as an adjunctive treatment in patients with primary open-angle glaucoma or ocular hypertension undergoing treatment with PG analogs. This ophthalmic formulation is specifically designed to protect the corneal epithelium and helps increase the biological defence of the tear film by better stabilizing and preserving its properties. The inclusion criteria were as follows: (1) an age of ≥18 years, (2) a diagnosis of primary open-angle glaucoma or ocular hypertension and current treatment with PG analogs as monotherapy or in fixed combination/association with beta-blockers for ≥6 months before enrolment, (3) DED symptoms defined by an Ocular Surface Disease Index (OSDI) score of ≥13 points, (4) a Schirmer test I result of ≥5 mm to avoid the inclusion of dry eye patients due to decreased tear production, and (5) conjunctival hyperaemia of ≥2. Patients were excluded if they (1) used artificial tear substitutes in 2 weeks before the start of the study, (2) had a history of ocular trauma, (3) had an active ocular surface infection of any type, (4) had an ocular allergy, (5) had undergone ocular surgery within 30 days prior to enrolment, (6) had another concurrent eye disease associated with ocular surface inflammation , or (7) were pregnant or breastfeeding. We also excluded patients with DED linked to a systemic disease or therapeutic used to treat a systemic disease and patients with known hypersensitivity to any of the components of the study eye drops. The study protocol was assessed and approved by the Internal Commission of the Clinic; the research was conducted in accordance with the Helsinki Declaration, and patients provided informed consent.The aim of this prospective, nonrandomized, open-label pilot study was to evaluate the efficacy of a topical HA-based formulation also containing amino acids and 0.2% Echinacea 3 times daily for 12 consecutive weeks. The time points considered were baseline, 4 (±1) weeks, and 12 (±1) weeks. At each visit, as per the TFOS DEWS II Diagnostic Methodology report [Patients, already undergoing treatment with PG analog therapy, were administered 1 drop of ancillary topical therapy containing HA, amino acids, and y report , the pary report : thermosThe primary outcome of this study was the change in ocular surface inflammatory symptoms for each item of the visual analog scale  after 4 and 12 weeks of treatment. The secondary outcomes were the mean change in OSDI score, VFQ-NEI-25, conjunctival hyperaemia, CFS, TBUT, BCVA, and Schirmer test result between baseline, 4 weeks, and 12 weeks after the start of the study.t-test. As a rule of thumb, an anticipated 10% dropout rate has been assumed, and thus, the minimum number of evaluable subjects included in the study was N = 30.A medium clinically relevant effect size equal to −0.50 at 12 weeks for the dryness symptom has been considered primary outcome of this study; all other ocular surface DED symptoms of the visual analog scale, such as foreign body sensation, burning, pain, and frequency of symptoms, were also primary outcomes but were not considered for sample size analysis. A sample size of 27 data pairs achieved a minimum of 80% power to reject the null hypothesis of zero effect size at 12 weeks at a significance level  of 0.10 using the two-sided paired To select the most appropriate statistical analysis, a preliminary between-eye correlation analysis was performed for all eye-specific outcomes; depending on the value of the Spearman correlation coefficient and visual inspection of between-eye scatterplots, the outcome data were analysed using the average of values from the right and left eyes, or on a per-eye basis, with the exception of symptom scores (VAS and OSDI) which were considered on a per-patient basis.β) of univariate and multivariate analyses were tabulated as point estimates along with standard errors and p values for comparisons between the follow-up and baseline. Multiplicity adjustment was performed using the Student maximum modulus method. Individual profiles and boxplots were created for each primary outcome and for each domain of the NEI-VFQ-25 quality of life questionnaire. A normality test was performed for all variables using the Shapiro–Wilk test. All tests were two-tailed and considered significant at the 5% level. All analyses were conducted using SAS version 9.4 .Continuous variables were summarized by count, mean, standard deviation, and/or interquartile range values by time and analysed with a repeated measures-mixed model. Categorical variables were instead analysed with repeated measures logistic regression analysis, using a subject identifier as a random effect. Regression parameters  were female, and the mean age at the baseline was 64.2 years . Participating patients had been treated for an average of 9.1 years with PG analogs or a fixed combination thereof in the form of ophthalmic drugs with preservatives for glaucoma. All patients were diagnosed with primary open-angle glaucoma or ocular hypertension, and all had ocular surface inflammation and DED symptoms, such as foreign body sensation, burning, stinging, dryness, and pain at the baseline . All patients included in the study completed the entire treatment period; demographic and baseline diagnostic and treatment details are reported in p < 0.001), with Spearman's correlation coefficients ranging from 0.12–0.75. A per-eye analysis was carried out only for conjunctival hyperaemia and BCVA score changes with respect to the baseline.The left and right eyes were significantly correlated for all primary and secondary outcomes  of burning decreased by 2.57  and 2.70 , and the mean values calculated for the frequency of symptoms decreased by 1.80  and 2.77  after 4 and 12 weeks of treatment compared to the baseline, respectively. After 12 weeks of treatment, dryness and pain significantly decreased by 2.23  and 1.27 , respectively. We also observed a reduction in foreign body sensation after 12 weeks of treatment; however, the t-test yielded a borderline value, and the difference was not statistically significant . The differences in stinging values at 4 weeks  and 12 weeks  compared to the baseline were also not statistically significant .At both 4 and 12 weeks, all primary outcomes showed a monotonic trend . Treatmep=0.01) as well as the visit time . No other factors were significantly associated with the observed changes in the primary outcome.Age at the baseline was significantly and inversely correlated with burning in both univariate and multivariate analyses  and −8.1 points (p=0.04) at 4 and 12 weeks, respectively (p=0.01) by 1.5 seconds at 4 weeks of treatment compared to the baseline (p < 0.001) and 4.5 at 12 weeks (p < 0.001), respectively, was observed compared to the baseline (p=0.05) (p=0.02) (p < 0.001), which was included in the clinical parameters as a reference value to detect any clinical worsening or any influence of the HA-based formulation on the effect of PG analog therapy.The OSDI score showed a monotonic and significant mean change between the baseline and 4 and 12 weeks. The mean change scores were −7.8 points (ectively . Interesbaseline . A monotbaseline . The res(p=0.05) . Due to (p=0.02) . AnotherEchinacea in the treatment of ocular symptoms associated with DED in patients undergoing treatment with PG analogs. The present study examined several outcomes, all of which may be representative of ocular surface changes. Indeed, daily use of the study ophthalmic formulation resulted in rapid improvement in inflammation-related symptoms and their frequency, with a significant decrease in burning sensation after only 4 weeks of treatment; foreign body sensation, dryness, and pain scores were also statistically significantly lower after 12 weeks of treatment than at the baseline. The improvement in these symptoms is crucial, since lower levels of inflammation help break or at least mitigate the typical vicious cycle of DED, in which inflammation is not only caused by the ocular surface but also becomes a key factor in damage to the eye [The results of the present study support the efficacy of an ophthalmic formulation containing HA, amino acids, and  the eye . These p the eye –34. Inde the eye . Similar the eye . Finally the eye . An inte the eye . Thus, s the eye . In the  the eye . However the eye . In futu the eye ; howeverEchinacea extract are rapid and persist throughout the treatment period (12 weeks). Future randomized controlled trials would be necessary to better define the application regimen and extend the use of the ophthalmic formulation to other ocular conditions.Despite the limitations mentioned above and its open-label nature, the results of this study show that the beneficial effects of eye drops containing HA, amino acids, and


This review article focuses on the aluminum-based metal matrix composites (Al-based MMCs). Studies or investigations of their mechanical and tribological properties performed by researchers worldwide in the past are presented in detail. The processing techniques and applications for Al-based MMCs are also documented here. A brief background on the composite materials, their constituents, and their classification, as well as the different matrix materials and particulates used in Al-based MMCs, can be found in this review. Then, an overview of dual-particle-size reinforced composites, heat treatment of Al alloys, and temper designations used in heat treatment are also included. In addition, the factors influencing the mechanical and wear properties of Al-based MMCs are discussed. The primary objective is that both present and future researchers and investigators will be assisted by the comprehensive knowledge compiled in this article to further explore and work towards the betterment of society in general. Composite materials are engineering materials made by a mixture of different constituents that are distinct from each other and offer several properties that are unattainable by their individual constituents ,2. In coComposite materials can be classified based on their matrix and reinforcement materials. Based on the morphology of the reinforcements, the composites can be classified as fiber, particulate, whisker, or flake. Based on the type of matrix, composite materials are classified into metal-matrix composites (MMCs), ceramic-matrix composites (CMCs) and polymer-matrix composites (PMCs) ,5,6,7. ABased on the matrix material used, composite materials are classified as follows:Metal-Matrix Composites (MMCs): Metal matrix composites consist of two main components. The first is metal/alloy matrix, which is the continuous phase, and the second is the reinforcement, which is the discontinuous phase and can have different morphologies. Commonly used metals and alloys are aluminum, nickel, copper, steel, titanium, and magnesium, which are used for numerous applications spanning from electrical wire to automobile parts. With respect to polymer-matrix composites (PMCs), MMCs offer high specific strength, high wear resistance, ability to deform plastically, better joining characteristics, and enhancement of strength by various methods.Ceramic-Matrix Composites (CMCs): Non-metallic solids with strong ionic bonding are generally known as ceramics and are crystalline in nature. Ceramics possess high temperature operating capability, high chemical inertness, high compressive strength, and high hardness. However, due to their poor fracture toughness, they are not suitable for structural applications. To improve their fracture toughness, ceramics are reinforced with fibers , whiskers , and particulates . Ceramic matrix composites are used for making cutting tool inserts, aircraft engines, brake disks, turbine blades, exhaust ducts, heat exchanger tubes, and bearings.Polymer-Matrix Composites (PMC’s): Here, an organic polymer matrix is reinforced by different types of continuous or short fibers or sometimes even nanoparticles. Two different categories of polymer-matrix composites, namely reinforced plastics and advanced polymer composites, have been developed. These two polymer composites are distinguished on the basis of their achieved strength and stiffness. Polymers themselves are classified as thermoplastics  and thermosets . On the other hand, reinforcements such as continuous fibers  and nanoparticles  are used to reinforce polymers. Common applications include non-structural parts such as exterior panels, structural parts such as leaf spring for vehicles, molded propeller assemblies in the naval industry, lampposts, helicopter rotor blades, golf club shafts, and highway culverts for construction purposes.2- and SiC-based multifilament fibers possess very high specific strength when compared to that of a specific modulus. On the other hand, whiskers are monocrystalline, which means they have no grain boundaries, due to which they possess very high strength and high stability and are not degradable. However, their main drawback is the lack of uniform dimensions. Another major drawback of whiskers is poor bonding with the matrix materials; for example, Al2O3 whiskers have difficulties in bonding with metal matrix. Among all available whiskers, silicon carbide and silicon nitride whiskers are quite popular for reinforcing metal matrices. However, one should note that whiskers are known to be hazardous to health, causing serious problems. Since MMCs with particulate reinforcements cost much less than MMCs with continuous fiber reinforcements, the particulate composites are more popular. From the processing point of view, the particulates can be blended more efficiently with the metal matrices when compared with that of whiskers or fibers. Ceramic-based particulate materials are quite popular owing to their low density, high strength, thermal stability, and good mechanical compatibility [Reinforcements are the materials used to strengthen the matrix phase by various strengthening mechanisms. These are used to reinforce the matrix because of their advantages, such as low density, thermal stability, high stiffness, high hardness, and high strength . Generaltibility ,11.Continuous-Fiber MMCs: The reinforcing phase, in this case, is continuous fiber whose aspect ratio is more than 1000 and whose diameter can vary from 3 to 150 µm. Examples of continuous fiber MMCs are Al-SiC, Al-Al2O3, Al-B, Ti-6Al-4V-SiC, Al-Li-Al2O3, Mg-Al2O3, and Mg-C composites [mposites .Short-Fiber or Whisker MMCs: The reinforcing phase is a short fiber or a whisker whose aspect ratio is usually over 10 and whose diameter can be in the range of 0.01 to 1 µm. Examples of short-fiber/whisker MMCs are AlSi12CuMgNi/Al2O3 short fibers, AlSi12CuMgNi/SiC whiskers, and Al/SiC whisker composites [Particulate MMCs: Here, the reinforcing phase is a particle whose aspect ratio lies in the range of 1 to 4, with its size varying from nanometer to micrometer. Common examples of particulate MMCs are Mg/SiC, Pb/Al2O3, AlMgSiCu/Al2O3, Al-4Cu-2Mg/SiC, and Al-12Si-Mg/SiC composites [mposites ,14,15. TThere are various types of metals or alloys available that can be used as matrix materials for making MMCs depending upon the applications. A brief discussion on different matrix materials used to develop MMCs is provided below ,11,16.Aluminum and its Alloys: Aluminum and its alloys are widely used because of their low density (2.7 g/cm3) and reasonable mechanical properties, such as good strength and corrosion resistance. The alloy series such as Al2xxx, A16xxx, and Al7xxx hold special significance when subjected to heat treatment. After heat treatment, these alloys exhibit good strength and toughness, which make them probable candidate materials for the automotive and aircraft industries.Copper and its Alloys: Copper has a density of 8.9 g/cm3. This high density value rules out its application in automotive or aircraft industries as structural components. However, what are more important are its physical properties, such as electrical and thermal conductivity. Due to its high ductility, Cu can be easily cast and worked using secondary processing techniques. In its pure state, it is used for thermal management and electrical applications such as heat sinks, power cables, bus bars, and other electrical components.Magnesium and its Alloys: This is one of the lightest metals, whose density (1.8 g/cm3) is less than that of aluminum. It possesses high specific strength, good damping properties, and good electrical conductivity. Furthermore, it can be cast or formed easily, due to which it is seen as an attractive material for large-volume applications.Titanium and its Alloys: The density of titanium is 4.5 g/cm3, which is lower than iron. Ti exhibits a higher melting point than steel. Ti and its alloys are preferred for structural applications where the temperature ranges from 425 to 595 °C. This metal is passive to almost all mineral acids and chlorides, thereby providing a high degree of corrosion resistance. Microstructure and chemical composition are the two important factors that determine the properties of titanium alloys. Applications of titanium and its alloys range from compressor blades and armor vehicles to hip implants.Particulates used to Reinforce the Metal Matrices: Hard particulate materials, such as borides, nitrides, oxides, and carbides, are used to reinforce the metal matrices. Day by day, increasing concerns for the environment, the depletion of fossil fuels, and the need for advanced engineering materials have posed a big challenge to researchers across the world. Live examples from nature and continuous efforts have led to the development of composite materials. The basic components of a composite material are the matrix phase, which is continuous, and the reinforcement, which is discontinuous. Both phases are chemically distinct and capable of imparting a new range of properties that are otherwise not achieved by individual constituents of the composite material. This new class of materials can be distinguished based on the type of reinforcement or matrix material. The MMCs offer competitive advantages such as improved specific strength, specific stiffness, high toughness, high surface durability, improved joining characteristics, and thermal stability ,18. Init3. In addition to this, particulates are produced by very inexpensive techniques, such as crushing. Based on the application point of view, both matrix and reinforcement are chosen judiciously to meet the design engineer’s requirements for an application. Apart from the selection of constituents of a composite material, their fabrication also plays a prominent role in dictating the properties, as well as its affordability in the commercial market. There are various techniques available for fabrication, from conventional liquid metallurgy and powder metallurgy to newer techniques such as thermal spray and accumulative roll bonding [It is well known that the matrix material in MMCs is either a metal or alloy, while the reinforcement could be either ceramic or metal. Most commonly preferred metal matrices are based on aluminum, copper, cobalt, magnesium, nickel, silver, titanium, and zinc. Aluminum and magnesium are quite popular in the aerospace and automotive industries due to their low densities ,21. Due  bonding . However2O3, TiC, SiC, etc.) to aluminum matrix helps in deflecting the crack at the interface and thereby ceases it from propagating, resulting in an increase in its toughness. In addition, the reinforcement material may help in bridging the cracks, thereby improving the performance of the aluminum matrix. Out of all reinforcements, SiC particulate is quite popular because of its high-temperature stability, high strength, and stiffness [Al-based MMCs are one of the promising materials for applications in the automotive and aerospace industries as they exhibit low density, high specific strength, and good wear and corrosion resistance ,25. Alumtiffness ,27.Cast aluminum alloys are strengthened by the same alloying elements and by the same mechanisms as wrought aluminum alloys . Cast alThe 6xxx series of aluminum wrought alloys are used extensively, whereas the cast alloys of 6xx.x and 9xx.x series are rarely used. Cast alloys contain a large weight percentage of alloying elements, which results in a heterogeneous structure when compared to wrought alloys . However2 or Cu-Al-Si-Mg, and Al-Si eutectic.In order to make any component or part for an instrument or heavy machinery, a material with good castability or formability is often preferred. In case of cast aluminum alloys, the Al-Si-Mg alloys are well known for their excellent castability, due to which they are quite popular in the automotive industry. It is interesting to note that up to 85% of cast aluminum products are made up of Al-Si alloys. Hence, these alloys are extremely important in casting industries related to aluminum ,31. The The element Be is added both to alter the morphology of insoluble Al-Mg-Fe-Si phase to nodular form and to modify the chemistry of insoluble phase to exclude magnesium, thereby ensuring its availability for precipitation hardening. Since it is a heat-treatable alloy, the strength, fatigue, and corrosion resistance are enhanced significantly after heat treatment using T6 temper. Further, the melt is treated with grain refiners like Al-3Ti-B, Al-5Ti-B, or Al-Ti-C master alloys . Melt mo2, and Al2O3 particles to modify the modulus, as well as electrical and thermal properties [The strength and wear resistance of A357 alloy are increased with the addition of ceramic particulates. To this end, efforts have been made to reinforce A357 with graphite fibers, TiBoperties ,34.4C (3 µm/50 nm), Al3BC (5/300 nm), Al2O3 (100/150 µm), and ZrSiO4 (20/125 µm). The idea of using two different sizes of particles in composites is to get the benefit from their size difference. The addition of differently sized particles has a significant effect on grain size and its orientation. In addition, the intensities of texture components change. For instance, during wear testing, the larger particles not only carry the greater amount of load applied on the composites, but also protect the smaller particles from being removed. On the other hand, small-size particles not only help in reducing the plastic flow by minimizing the plastic deformation, but also aid in preventing the adhesive transfer of material during wear test. These dual-size particles have shown that they can bring coordinated improvements in mechanical properties such as strength, ductility, and hardness. Furthermore, an important property, such as creep resistance, of composites will also improve by 3 to 6 times for dual-particle-reinforced composites when compared to single-reinforcement composites and alloys. Most of the dual-size particles were used to reinforce pure aluminum, Al-3Cu, Al2124, Al6061, Al6063, LM13 piston alloy, magnesium, and copper.It is well known that the properties, such as wear resistance, of MMCs largely depend on interfacial bonding, matrix properties, type of reinforcement, particle weight, volume fraction, and particle size. Few researchers have reported that the MMCs containing smaller-size particles have shown higher wear resistance when compared to the MMCs with large-size particles. Taking a cue from this, several research investigations have been carried out to study the effect of dual particle size (DPS) on the mechanical and tribological behavior of MMCs. The size difference between these particles will be quite large, that is, for example, the large particle size will be several hundred micrometers, while the smaller particle size will be a few micrometers, or in some case it will be of submicron size. The most explored particulates for DPS composites are SiC (2/163 µm), TiC (2 µm/100 nm), BThere has been steady growth in utilizing aluminum alloy castings for both aircraft and automotive applications after World War II. During this time, the United States of America made P-51 aircrafts that had the fuselage entirely made up of aluminum for weight reduction. Furthermore, a decrease in the weight of the P-51 engine was achieved by employing Duralumin alloy to make the cylinder block, pistons, and crankcase. In the present day, Al-Si alloys are employed in making complex substructures using the investment-casting process for the aerospace industry. For example, the winglet substructure made of F357 alloy for the Embraer Phenom is made up of a single block. The rear frame avionics bracket and tail rotor gearbox are made up of D357 alloy, using precision sand casting technique. Furthermore, Al-Si alloys are used for making powertrain components in the automotive industry by replacing gray cast iron. In making a V8 engine block, if Al-Si alloy is used, then the overall weight of block is 32 kg, whereas if gray cast iron is used, then it weighs 68 kg. Therefore, there is a considerable amount of weight reduction by opting for Al-Si alloys in place of gray cast iron. In addition, lot of efforts have been dedicated by the North American automotive industry to replace iron engine blocks and cylinder heads with cast Al-Si alloys. For example, Chrysler and Ford use Al-Si alloy for making cylinder heads (C351) using a semi-permanent casting technique. On the other hand, General Motors uses aluminum alloys to produce wheel rims by permanent casting. Chevrolet Corvette uses Al alloys for making engine blocks by sand casting process. Many minivans, cross-over vehicles, cars, and trucks employ aluminum alloys for making door handles or roof brackets .Metal-matrix composites offer promising mechanical and physical properties for various applications, yet their applicability has been limited. Processing MMCs using sophisticated fabrication techniques results in a high cost of the end product and is therefore one of the greatest barriers to expanding the applications of MMCs. By developing good and low-cost fabrication techniques, one can improve the commercial applicability of MMCs. Generally, the processing of MMCs is divided into three main classes: (i) liquid-state, (ii) solid-state, and (iii) gaseous-state processing. A brief discussion on all three processing routes and their advantages is documented below with some examples ,39,40,41In this type of fabrication technique, the reinforcement is added when the metal matrix is in a liquid state. This technique is capable of producing large-scale products at a faster rate and has been proven to be more beneficial for low-melting-temperature metals such as aluminum and magnesium. This technique can be further divided into four important categories, namely stir-casting/dispersion, squeeze-casting/pressure infiltration, in situ, and spray processes. Each of these fabrication techniques have their specific applications. No single technique can be considered to be universally applicable for all alloys, casting sizes, and final components. The present work involves casting of MMCs using the stir-casting technique, which is one of the most widely used techniques for fabricating MMCs. Companies such as Duralcan use this technique to produce aluminum composites reinforced with particulates. Hence, only the stir-casting technique along with different liquid-state fabrication techniques are discussed herein. In the case of stir casting, the casting of MMCs is done in semisolid conditions, which means that the temperature is between liquidus and solidus. A simple schematic diagram depicting the stir-casting method is shown in The reinforcement, which is in an agitated condition, does not agglomerate or flow on top of the semisolid slurry; instead, it is trapped by the solid in this slurry with uniform dispersion. Here, the stirring or agitation of the semisolid slurry is very important as it breaks the agglomerates of particulates. Regardless of the lack of wettability of ceramic particulates with metal matrix, this technique helps in entrapping them in the semisolid slurry so that they are uniformly dispersed. The continuous agitation helps in bonding between both the constituents of the MMCs by bringing them in direct contact. The direct and intimate contact helps in bonding and improving wettability, which improves further with an increase in mixing time. Since the agitation causes a decrease in viscosity, the advantage is that this composite mixture with low viscosity can be directly cast into the required shape. The other advantage of this technique is the absence of shrinkage cavities in the final product, which is mainly because of processing at semisolid conditions where the slurry is nearly in solid state .This is another important fabrication technique, in which a high-volume fraction of reinforcement can be incorporated into the metal matrix. In solid-state fabrication, powder metallurgy is the most widely used processing method for the fabrication of metal matrix composites. The reinforcements are mixed with the metal powders using different mixing techniques and consolidated either at room or elevated temperatures. If the powders are consolidated at room temperature, then an additional processing method known as sintering is adopted. In addition to this, secondary processing techniques such as extrusion, rolling, equal-channel angular extrusion, forging, or multi-axial forging techniques are also used. However, such techniques are very expensive and mass production is not possible, thereby restricting them to specific applications. Further, the tooling costs required for the powder metallurgical processing are very high, which also limits its applications ,41.This fabrication technique includes deposition of vapor on the substrate material, with control over the composition of the composite. Two different types of fabrication techniques are the chemical vapor deposition (CVD) and physical vapor deposition (PVD) processes. Here, the control of various factors such as gas mixing, oxygen contamination, and vapor transport are very important for good coating of the composite. However, in the case of gaseous-state fabrication techniques, the availability of metal and the compound target is quite difficult, and deposition rates are slow. In the case of thermal spray techniques, parts with complex shapes are difficult to coat, and most importantly, the starting materials are quite expensive ,41Out of all the processing techniques, the liquid-state fabrication technique is more popular due to its inherent advantages over the solid- and gaseous-state fabrication techniques. Firstly, liquid metal is less expensive when compared to powders, and secondly, the handling of liquid metal is easier and more flexible when compared to that of powders. In addition, a wide variety of shapes can be produced using the liquid-state processing technique, which is not the case with the solid- and gaseous-state fabrication techniques. However, there are some concerns, such as poor control of fabrication parameters and formation of undesirable interfacial products at the interface between matrix and reinforcement. The chemical reactions taking place at the interface can lead to the formation of carbides and oxides, which are highly brittle in nature. Lastly, one needs to consider the economic viability of the liquid-state fabrication technique with respect to its potential for near-net casting ,41.The mechanical performance of composite materials depends on (i) interfacial bonding, (ii) orientation, (iii) material, and (iv) wettability. A brief discussion on all these factors is provided below.There are different types of bonding between reinforcement and metal matrices, namely chemical, mechanical, and reaction. Generally, a metallic coating such as nickel or copper on the surface of fibers or particulates is applied by electroless technique. The thin layer formed on the surface of the reinforcement tends to form a good adhesion with the metal matrix, thereby increasing the bond strength. On the other hand, reaction bonding occurs when the atoms of the reinforcement diffuse into those of the matrix and vice versa. Mechanical bonding between the reinforcement and matrix is created by etching the surface of the reinforcement in such a way that the increased surface roughness causes interlocking. However, high surface roughness is fraught with the danger of formation of interfacial voids owing to incomplete penetration. When a load is applied parallel to the interface, the mechanical bonding plays an important role, wherein the load is efficiently transferred from the matrix to the reinforcements. At the same time, one should also understand that the mechanical bond alone is insufficient for good interfacial bonding between the reinforcement and matrix. Overall, chemical bond is known to be the strongest bond, and mechanical bond is known to be the weakest ,44.The orientation of reinforcements plays an important role since the properties along the direction of reinforcements  are significantly improved. Similarly, in the materials where the fibers are oriented in different directions, the properties such as strength will be high in the direction of fiber orientation. However, in the case of particulates, the orientation does not play a significant role since the properties are the same in all directions, which is contrary to the fiber-reinforced composites. The strength of unidirectional composites is unmatched when compared to that of woven fabric and particulate composites .The strength of MMCs mainly depends on the material of reinforcement. Though the material plays an important role, the application determines the type of reinforcement material. In certain industries such as aerospace, where cost is not the constraining factor, high-strength and high-elastic-modulus fibers based on carbon are used. For applications where both cost and properties are important factors, glass and aramid fibers are utilized. However, for automotive applications, economical materials such as ceramic particulates are utilized. Diamond and nanomaterials such as graphene, which possess higher thermal properties, are used for heat management applications .This is one of the important criteria when metal matrix composites are produced by casting with pressure infiltration techniques. The degree of wettability is usually determined by edge angle adjustment of the drop of molten metal on the base of a solid. One should study the surface and interface strains for composite systems to understand wettability. Based on the interface energy and edge angle adjustment, the angle limit for a wettable system is <π/2, while for a non-wettable system, the angle limit is >π/2. One should understand that as the angle decreases, the wettability of a molten metal with its solid base increases. The wetting of molten metal/drop depends on kinetics, which means that it depends on temperature and time. Furthermore, if any alloying element is added to the metal, then the wettability is affected because the surface tension of the melt droplet changes. For example, in infiltration, if the wettability is good, then one can observe a capillary effect of melt in-between the fibers. However, if the wettability is not good, then such an effect is not seen. In the case of the particulate-reinforced MMCs, poor wetting of particles can result in segregation of particles .The purpose of heat treatment of Al alloys is to achieve the best possible mechanical properties for the desired industrial applications. A sequence of microstructural modifications enhances the mechanical properties based on thermal treatment factors, namely temperature and time ,47,48.The temper designations used for as-cast alloys are F, O, and T, which stand for as-fabricated, annealed, and thermal treatment. Here, the temper designation T is further subdivided into T4, T5, T6, and T7 (commonly used designations), which indicates artificial and natural aging conditions. A schematic representation of the aforementioned temper designations is shown in In addition to heat treatments such as stress annealing for removing residual stresses, the Al-Si-Mg alloy castings are subjected to different types of heat treatment processes, one of which is solution heat treatment, which is carried out with and without quenching. Heat treatment, which is generally adopted for the enhancement of mechanical properties, starts with homogenization and precipitation hardening. The precipitation-hardening process involves solutionizing followed by quenching and artificial aging ,50.(i)Solutionizing: Solutionizing aims to obtain a homogeneous solid solution of the material. Here, the alloy is heated to a pre-set temperature to dissolve all the soluble alloying elements and is retained at that temperature for a sufficient time [ent time ,50.(ii)Quenching: The aim of quenching is to keep the alloying elements trapped in the solution and to achieve maximum super-saturation of the alloying elements [elements ,50.(iii)Artificial aging: Artificial aging aims to reduce the time for precipitating the dissolved impurity or solute phase [te phase ,50.A schematic diagram of precipitation hardening (temperature versus time diagram) is shown in 2Si, Al8Fe2Si, and eutectic Si are formed with solid solutions.For Al-Si-Mg alloys, T6 temper conditions are preferred, which are documented by ASTM standards such as B917 and B91. According to these standards and as documented by several researchers, the solution treatment suggested varies from 450 °C to 560 °C for 4–12 h, and aging temperature ranges from 150 °C to 250 °C for 6–12 h ,52. DiffFurther addition of reinforcements, such as ceramic and carbon particulates, changes the aging sequence and decreases the time required for aging. The aging sequence after the heat treatment process in Al-Si-Mg alloy is shown in Addition of reinforcement to the metal matrix can bring a number of changes in the microstructure of MMCs. Mechanical properties such as strength, modulus, fracture toughness, and creep resistance are increased significantly when ceramic or organic reinforcements are added to the metal matrix. For example, when reinforcement is added to metal matrix, the difference in coefficient of thermal expansion (CTE) can introduce dislocations, especially near the interface region. Apart from dislocations, other defects such as vacancies are found to appear near the reinforcement/matrix region ,50. SuchThis section presents a critical review of the published literature relevant to the present work. Firstly, the prior work on the mechanical properties of Al-based MMCs is discussed. Next, a review of the existing work on their tribological properties is presented. Finally, a subsection on the dual-particle-size reinforced composites is included.The mechanical behavior of materials plays a significant role in the development of new materials. There are different techniques to improve the mechanical properties, which include the addition of reinforcements, heat treatment, and secondary processing techniques. This section summarizes the research work that has been carried out by several researchers on single-particle-size (SPS) reinforced Al-based MMCs, and also the influence of heat treatment on their mechanical properties.First, we look at the effect of SiC-reinforced A357 composite fabricated by stir casting and hot isostatic processing (HIP) techniques. The SiC particulate chosen had an average particle size of 30 µm, and about 15 vol.% SiC was reinforced to A357 matrix. This work reported the yield (163 MPa) and nominal strength (367 MPa) of A357 alloy in as-cast condition. In contrast, the same strength values determined for the A357/SiC composite were much lower. Furthermore, the values of these two properties of the A357/SiC composite obtained from the HIP process were also lower, at 96 and 214 MPa, compared with the base alloy A357 . The reduction in properties of the reinforced composites were attributed to the formation of brittle phases and high levels of porosity  studied 2Cu-based needle-like precipitates of 100 nm length, formed after heat treatment, were mainly responsible for the improvement in the mechanical properties.Electromagnetic frequency’s effect on the mechanical behavior of A357 composite reinforced with SiC nanoparticles was investigated by Mohammadi Badizi et al. . The nan2Si precipitates. The work showed that the critical aging temperature was highly dependent on both the microstructure of the matrix and the amount of Mg2Si precipitates. Bloyce and Summers [During the investigation on the mechanical properties of the Al-Si-Mg-Ti alloy containing TiC nanoparticles and fabricated by the selective laser melting (SLM) method, it was noticed that this alloy showed an increment in its mechanical properties such as UTS (nearly 562 MPa) and elongation (about 8.8 % until fracture) due to the grain refinement effect, as well as the Orowan/load-bearing mechanisms of strengthening the alloy. Gao et al.  also emp Summers  employedFrom the studies on static and dynamic mechanical properties, the authors concluded that solutionizing at 540 °C and aging at 160 °C resulted in higher mechanical properties for both the A357 alloy and the A357/SiC composites.2 particles, a composite with 5 wt.% percentage of TiB2 was produced using the salt reaction technique. This composite was solutionized at 540 °C for about 8 h followed by hot water quenching. Artificial aging was conducted at 170 °C for a duration of about 7 h, followed by mechanical testing. Upon heat treatment, the hardness of the alloy and the composite increased from 62 to 95 HB and from 72 to 105 HB, respectively [2 particles. Furthermore, related to the precipitation-strengthened composites obtained from the cast eutectic alloy in the Al-Si-Mg system, the nanosized SiC powder was the strengthening phase [2 composites. The composites were heat-treated, wherein solutionizing was carried out at 540 °C for about 12 h, followed by cold water quenching. The final step was artificial aging, which was conducted at 155 °C for about 10 h. The YS and UTS of A356 alloy before heat treatment were 97.5 and 151.5 MPa, respectively, while those of the composite were 109 and 164.5 MPa, respectively. After heat treatment, the YS and UTS of A356 alloy increased to 208.1 and 263.1 MPa, respectively, while those of the composite increased to 255.5 and 312.5 MPa, respectively. The formation of fine-size Mg2Si precipitates in the Al grains caused an increase in strength following heat treatment. Kumar et al. [4 and Al2O3 particulates. The composite fabricated using the casting technique was solutionized at 540 °C for about 3 h, followed by cold water quenching. Then, aging was done at 170 °C for different time durations ranging from 0 to 480 min. Compared to zircon-reinforced composites, the one reinforced with alumina showed a high hardness of 118 HV for an aging duration of 360 min. The hardness increased with an increase in aging time for up to 360 min, following which it started to decrease. Formation of Mg2Si precipitates in the matrix material led to an increase in hardness.In a study of the precipitation kinetics of in situ Al-Si-Mg composite reinforced with TiBectively . Similarng phase . Here, tr et al.  investig2 composites fabricated by thixoforming, solution treatment was conducted at 540 °C for a time duration of 8 h, and quenching was perfomed in cold water. Soon after quenching, the samples were aged at 160 °C for a time duration varying from 0 to 24 h. The samples were air-cooled to room temperature after artificial aging and subjected to microstructure analysis. A356 alloy showed a coarse dendritic structure, while after the addition of TiB2 and heat treatment, the structure was transformed into a fine equi-axed structure. This study reported that the time taken for peak aging in the case of A356 alloy was 12 h, while in the case of composites, it was 8 h. The peak aged samples were subjected to transmission electron microscopy studies, which revealed the formation of GP II zones and Mg2Si precipitates. The sizes of these precipitates were in the range of 10 to 20 nm, with most of them located at the grain boundaries [4C3, Al2O3, and MgAl2O4 spinels at the SiC and Al interface. Formation of Mg2Si precipitates were also seen in the microstructure after T6 heat treatment. Abdulwahab et al. [2Si precipitates along with an Al2Mg3 intermetallic compound. On the other hand, plates corresponding to Al-Si-Mg were also seen in the aluminum matrix. Menargues et al. [2Si phases.In the case of the age-hardening behavior of A356/TiBundaries . Samuel undaries  studied b et al. , in theis et al.  reported7SiMg/rice husk ash (RHA) MMCs, where the composites were fabricated by using the stir-casting method. Electrodeposition of nickel was carried out for the base alloy as well as the MMCs through the electrolytic watts bath with conventional stirring. Higher values of the aforementioned mechanical properties were noticed with an increase in the RHA content [2O3 and B4C particles) in terms of weight percentage. In a more recent work by Sakthi Sadhasivam et al. [In a study on the effect of heat treatment on the impact toughness of aluminum alloy A356, the alloy samples were solution-treated for 540 °C for about 6 h, followed by quenching in water. The artificial aging process was conducted at 155 °C for a time duration of 4 h, and it was then air-cooled. The time gap between the solution treatment and artificial aging was less than 30 s, and this also involved quenching in water. The morphology of eutectic silicon was changed from plate to rod, and then to spherical shape after heat treatment. In addition, the coarse α-Al dendrites were transformed to fine equi-axed grains with fine fibrous silicon particles following heat treatment . Ebeneze content . In an i content  also obsm et al.  in 2022,The interaction of two sliding bodies changes when reinforcement is added to one of them. The influence of these interactions can be seen in the coefficient of friction and the wear rate. This is because when two different pure metals or alloys are subjected to a sliding motion, the coefficient of friction and wear rate are very high and can sometimes lead to seizure of the surfaces. On the other hand, significant changes in the microstructure of metals or alloys due to the addition of reinforcement can result in different friction coefficients and wear behavior. Numerous studies have been devoted to reducing the coefficient of friction and wear rate by adding alloying elements, reinforcements, and microstructure modification in the surface region. This sub-section reviews the research carried out on understanding the tribological behavior of single-particle-size (SPS) reinforced Al composites. The review also includes the effect of heat treatment of Al-based MMCs on their tribological properties.10 as the counterface. At a load of 6 N, the alloy exhibited a high wear rate of 169.6 × 10−6 mm3/Nm, while the composite exhibited a low wear rate of 3.1 × 10−6 mm3/Nm. At a load of 74 N, the composite exhibited a high wear rate of 176.8 × 10−6 mm3/Nm, while the A356 alloy exhibited a lower wear rate of 89 × 10−6 mm3/Nm. At lower loads, the dominant wear mechanism was oxidation, with some contribution from three-body abrasion. At higher loads, adhesive wear was more predominant, with detachment of large metallic sheets due to subsurface cracking, and thus the composite exhibited a higher wear rate. Zulfia et al. [3/m wear rate, while the composite with 15% SiC showed a 0.004 mm3/m wear rate. The microstructure showed the formation of MgO.SiO2 and MgAl2O4 spinels at the SiC and Al interface, which offered wear resistance to the composite. The spinels that formed also ensured minimal SiC particle pull out during the wear test, thereby avoiding severe wear.Leonard et al.  reporteda et al.  reported2O3 composite produced by compo-casting. The cast composite was heat-treated, with solutionizing being carried out at 545 °C for about 4 h, followed by aging at 155 °C for about 6 h. The weight loss of the composite was found to increase with an increase in the applied load from 5 to 30 N. For a load of 5 N, the weight loss was 1.8 mg, while at a load of 30 N, the weight loss was 5.8 mg. Similarly, with an increase in the sliding distance and at a constant load of 10 N, the weight loss was found to increase. The worn surface observed for 10 N load tests was composed of deep continuous grooves and extensive plastic deformation. In the case of a higher specific load, the debris morphology was plate-shaped and had sharp edges, which imply that adhesive wear was predominant. Cam et al. (2016) reported the wear properties of A356 in situ composite reinforced with TiAl3 and fabricated using mechanical alloying and powder metallurgy techniques. Here, the wear test was conducted at room temperature and under dry sliding conditions at a load of 30 N and sliding speed of 1 m/s. The weight loss of the composite increased with an increase in sliding distance from 400 to 2000 m. The wear rate of the composite reinforced with 8 wt.% Ti was less than that of the composite reinforced with 4 wt.% Ti. This decrease in coefficient of friction was attributed to the higher amount of intermetallic phases, which play a big role in increase in hardness. For different Ti contents, the coefficient of friction was evaluated, and it was found that the coefficient of friction decreased with an increase in Ti content from 4 to 8 wt.%. The composite with 4 wt.% Ti exhibited the highest weight loss and showed extensive plastic deformation when examined under electron microscope. Sam et al. [2, WC, and ZrO2 and produced by squeeze-casting. The cast composite was solutionized at 520 °C for about 8 h, followed by aging at 165 °C for 4, 6, and 8 h. The wear tests were conducted as per the RSM model with applied load, sliding speed, and distance ranging from 10–50 N, 1–4 m/s, and 500–2500 m, respectively. Optimal results were obtained by solutionizing at 520 °C for 8 h, followed by quenching and aging at 165 °C for 8 h. The worn surface observed for the 50 N test load had deep grooves and extensive plastic deformation. In another work related to LM25 (Al-Si7-Mg)/WC MMCs [The wear behavior of A356-reinforced SiC composites with sliding against grey cast iron was analyzed. The composite with 25% weight percentage of SiC was produced by stir casting and later heat-treated by following T6 temper conditions. At 40 and 60 N loads, the volume loss of cast iron was higher than that of the A356/SiC composite for all cases of sliding velocities. Similarly, at sliding velocities of 2.5 and 3.7 m/s, the volume loss of cast iron was significantly higher than that of the composite for all cases of load . On the m et al.  reported/WC MMCs , a major2, TiO2, and TiC and produced by modified stir-casting, the cast composite was solutionized at 540 °C for about 10 h, followed by aging at 155 °C for 10 h. A linear reciprocating wear test was carried out with applied load and sliding distance ranging from 15 to 55 N and from 500 to 2500 m, respectively [2 composite exhibited the least weight loss when compared to A359/TiO2 and A359/ TiC composites. The worn surfaces of T6-treated A359 composites displayed mild plastic deformation, compared to the severe plastic deformation of untreated composites. Padmanaban et al. [p composites, where the SiCp content varied from 5 to 20 vol.%. There was an enhancement in the hardness and wear resistance in the RDC samples. Furthermore, the wear resistance in the composite with 20 vol.% SiCp was higher by three times compared to the samples without the presence of SiCp reinforcement. The improved hardness of the reinforced samples was also noticed in their work. Vuddagiri and Ravisankar [2) and alumina (Al2O3) particles. The resulting composites were manufactured through the stir-casting method. The process parameters considered in their work were the weight percentages of the reinforcements, sliding velocity, and load. The authors found that both the friction coefficient and wear rate were reduced upon increasing the sliding speed, whereas both of these properties were increased due to the higher load. In other words, in comparison with the base alloy, an improved resistance to wear and lower coefficient of friction were shown by these hybrid composites. In an identical work by Balasubramaniam et al. [In a study on the tribo-mechanical behavior of A359 composite reinforced with TiBectively . A359/Tin et al. , in 2020visankar  investigm et al.  on fly-am et al. . Furtherm et al. . Usman em et al.  fabricatm et al.  fabricat2O3 and SiC particles of two different sizes, namely 170 and 15 µm, respectively, to reinforce the A356 alloy. The Al2O3 and SiC particles used were of 99.7% and 99.4% purity, with irregular and angular morphology. The Ni-P-coated-particle-reinforced composites were fabricated using the stir-casting technique and hot rolling. This work reported that compared to single reinforcement, the dual-particle-size-reinforced composite exhibited high strength and hardness. Sadeghi et al. (2018) studied the mechanical behavior of spark-plasma-sintered Al/Al2O3 bimodal composites. The Al2O3 particles chosen had an average particle size of 20 nm  and 10 µm (larger particle size), and different weight ratio combinations  were used to reinforce the Al. The hardness of the composites increased with an increase in the ratio of nm- to µm-size particles when compared to that of unreinforced Al. Here, the addition of nanoparticles helped in inducing recrystallization, which assists in grain refinement and increasing the hardness of composites. In addition to this, the particles, especially the nano-size ones, help in the formation of new grains by the particle size nucleation mechanism and also inhibit grain growth. Wang et al. [Some recent studies were carried out on reinforcing metal matrices with dual-size  particulates. The composites reinforced with dual-size particles have shown better strengthening and wear-resistance properties compared to single-reinforcement composites. Out of the two different sized particles, the larger-size particles contribute to improvement in hardness and wear resistance, while the small-size particles contribute to improvement in strength. This section summarizes the research carried out on dual-particle size (DPS)-reinforced Al-based composites, including their mechanical and tribological properties. Kheirifard et al. , in theig et al.  used a p2, respectively. The bimodal composites required higher stresses for plastic deformation, and this was considered the primary reason for their high hardness. This study concluded that work hardening in the case of bimodal composites was quite higher than that of unimodal composites. Dhandapani et al. [4C particles) on the density and microhardness of Al composites. The size of carbon nanotubes and B4C particles were 50–80 nm and 150 µm, respectively, with both having purity close to ~99.5%. The composites were fabricated using the powder metallurgy technique, with sintering being carried out in the temperature range of 420–450 °C. The hardness of the single-reinforcement composite (Al/B4C) was 70 VHN, while that of the dual-particle-size composites was 78 VHN. However, with an increase in the CNTs content from 1% to 2%, the hardness decreased to 74 VHN. However, it was still higher than that of the single-reinforcement composite (Al/B4C). In continuation, Chandrashekar et al. [4C powders, namely, 44 µm and 105 µm sizes, in A356 alloy by using the stir-casting method. It was observed that the higher content of B4C powders in the dual-particle-size composites will increase their tensile strength and hardness by up to 35% and 15%, respectively. Khosroshahi et al. [2O3 (~170 µm) and SiC (~15 µm), were used to reinforce the A356 alloy using semi-solid stir-casting and rolling techniques. Dual-particle-size composites showed a fine grain size of 49 µm compared to that of the single-reinforcement composite (SiC) and A356 alloy, which exhibited a grain size of 59 µm each. The effect of grain size was seen on the ultimate tensile strength and yield strength of all materials. For example, the yield strengths of A356 alloy and single- and dual-particle-size composites were 148, 191, and 237 MPa, respectively. Similarly, the ultimate tensile strengths of A356 alloy and single- and dual-particle-size composites was 215, 266, and 302 MPa, respectively. It can be seen that the dual-size-particle composites had very good mechanical properties when compared to both the A356 alloy and single reinforcement composites due to grain growth restriction caused by dual-size particles.Montoya-Dávila et al.  reportedi et al.  studied r et al.  also inci et al.  studied 2) particles through powder metallurgy. The author showed that the addition of bimodal-size particles improves the wear resistance of the composites significantly by comparison with single-size-particle reinforced composites. Furthermore, the author concluded that the formation of a mechanically mixed layer was the main reason for the reduced wear rate in Fe matrix composites reinforced with SiC and SiO2.Dual-sized particles are not only used to enhance the mechanical properties of many metal matrices, but also to tailor the thermal properties. Arpon et al.  studied c. The composite with a higher content of fine SiC particle size showed the lowest cumulative wear (22 mg), while the one with the highest coarse SiC content showed the highest wear loss (24 mg). However, the coefficient of friction for the higher weight fraction of fine-sized SiC particles was higher (0.57) than that of the higher weight fraction of coarse-sized SiC particles (0.56). The composite with the highest content of fine SiC particle size showed a grooving wear mechanism, while the one with the highest coarse SiC content exhibited a ploughing wear mechanism. Arora et al. [−3 mm3/m) when compared to that of the coarse rutile-particle-reinforced composites (8.4 × 10−3 mm3/m). Fine-size particles help in increasing the hardness and refinement in morphology of the eutectic silicon. The extent of hardness enhancement and silicon refinement by fine-size particles was higher than that of the coarse-size particles. In both cases, oxidative wear was the more predominant mechanism, followed by plastic deformation. Arora et al. [2O3. The wear test was conducted using a pin-on-disc machine at a constant load of 29.41 N, sliding speed of 239 rpm, and sliding diameter of 80 mm. The design of the experiments was conducted using a Taguchi L9 array to study the significant parameters responsible for the wear of the composites. The authors found that particle size played a prominent role in the wear rate of the composites. The other significant parameters were hybrid solute proportion and vacuum pressure. Furthermore, it was noticed that when compared to the triple-particle-reinforced composite, the dual-particle-reinforced composite displayed better wear resistance. Sharma et al. [−3 mm3/m, while the composite with higher coarse-size particle content showed a higher wear rate of 9.6 × 10−3 mm3/m. The wear rate of both dual-size composites was found to be considerably lower than that of the single-particle composite, whose wear rate was 12.7 × 10−3 mm3/m. Both dual-particle-size composites showed crack propagation and wear loss of material by delamination at a 5 kg load.Maleque and Karim  reporteda et al.  studied a et al.  also repa et al.  studied a et al. , in theia et al.  reportedIn another work, Kumar et al.  reportedIn another closely related work, Lakshmikanthan et al.  studied Here, we include a separate section on the additive manufacturing processes of Al-based metal matrix composites (AMMCs). In particular, the promising selective laser melting (SLM) technique is discussed.2O3), the authors discussed the key features, results, and advantages of these AMMCs by comparing them with those obtained from conventional methods. For instance, the AMMCs manufactured via the SLM method possessed relatively low strength. However, unlike the AMMCs produced by conventional methods, a significant increase in the relative densities (greater than 99%) as well as the tribological properties and hardness of the AMMCs developed from the SLM technique were noticed, thereby suggesting that this method is favorable for AMMCs with Al2O3 [4C, and Al2O3. Analysis of the fracture surface revealed that the AMMC with B4C reinforcement showed improved interfacial bonding [In a review by Pelevin et al.  on the fth Al2O3 . Conventth Al2O3  employed bonding .2O3 MMC by adding alumina at 10 wt.%. Different parameters such as laser power, scanning speed and hatch spacing were chosen in their study. The authors implemented the layer’s double exposure with a laser beam [10Mg, wherein the effect of different starting particle sizes of SiC was investigated for the production of the in situ AMMCs with Al4SiC4 + SiC hybrid reinforcement. It was noticed that the SiC particles with finer size aided in the reduction of the coefficient of friction and wear rate when compared with the SLM part developed with coarser SiC particle sizes [Nalivaiko et al.  also conser beam . Furtherser beam  conductele sizes . In anotle sizes .2O3 particles as the reinforcement. The resulting composite was developed by the SLM technique [2/Al-Cu-Mg-Si composite that was subjected to heat treatment [Sercombe et al.  and Gu eechnique . Howeverechnique , in 2018reatment . Prashanreatment  examinedA comprehensive literature survey was conducted on the development and processing of Al-based metal matrix composites (MMCs), including their mechanical and tribological properties. In addition, additive manufacturing processes of Al-based metal matrix composites (AMMCs), in particular the promising selective laser melting (SLM) technique, are discussed here.From this review, it can be seen that the emphasis has been to replace cast iron, which is a traditional material used for aerospace and automobile applications, with MMCs. Furthermore, it is reasonable to draw conclusions that the dual-particle-reinforced composites exhibit better properties than the single-particle-reinforced composites. However, a key question arises with respect to the optimal ratio between the two sizes of particulates that should be used to obtain the optimal combination of mechanical and wear properties. Furthermore, there are other notable findings that emerge from this work. These include issues such as the effect of the addition of dual-particle-size SiC on the mechanical and tribological properties of A357 alloy, as well as the effect of heat treatment on the mechanical and tribological properties of dual-particle-sized SiC-reinforced A357 alloy, which have not yet been addressed to date. Hence, this review indicates the vast scope of this interesting field for present and future researchers.Significant research attention has been paid to fabricating composites with additive manufacturing techniques, which might not induce problems with homogeneous distribution of reinforcements in matrix material. Furthermore, porosity, segregation, and agglomeration problems can be eliminated with additive-manufactured products. Thin sections can be fabricated, which are often difficult with conventional MMCs. Furthermore, authors can automate the production of MMCs with an additive manufacturing process, which is impractical with conventional techniques. Researchers need to optimize the additive manufacturing parameters and make recommendation to industries for ease of fabrication with better qualities, which will require intense research in the near future.