Synergistic effects of cold rolling and age hardening on the hardness and tensile characteristics of AA6061 hybrid composites

The present study involves the fabrication of aluminium alloy 6061 matrix hybrid composites with varying weight fractions of silica sand and copper particles by employing the conventional stir casting method. The combined influence of age hardening (AH) and low temperature thermomechanical treatment (LTMT) on the hardness and tensile properties of AA6061 hybrid composites was investigated. The uniform dispersion of the particles in the matrix was confirmed by microstructure analysis and the improvement of Brinell hardness values. The composites exhibited higher tensile strength and hardness than the base alloy. Both AH and LTMT enhanced the properties of the hybrid composites and a comparison between them revealed the best results for LTMT hybrid composites. The LTMT hybrid composite with 3 wt% silica sand and 3 wt% copper (3S3C) subjected to 12% rolling deformation and aged at 100 °C had the highest hardness and tensile strength of 144.26 HV and 290 MPa respectively. The hardness and tensile strength of AA6061-3S3C hybrid composite subjected to LTMT in peak aged condition showed an improvement of 125 and 97% respectively when compared with those of AA6061 alloy. Fracture surface analysis of the thermomechanical treated composites in peak aged condition showed a mixed mode of failure dominant with the ductile fracture.


Introduction
In the relentless pursuit of technological advancements, materials science plays a pivotal role in shaping modern industries.As engineering demands continue to soar, traditional materials often fall short of meeting the requirements of modern applications.To bridge this gap, researchers and engineers have turned their attention to combining the desirable properties of multiple materials to create advanced composites with enhanced mechanical, thermal, and physical attributes.
One such breakthrough in the quest for superior materials is the development of Metal Matrix Composites (MMCs).Since the inception of MMCs several decades ago, they have gained considerable recognition for their ability to replace conventional materials in high-performance applications [1,2].The tailorable nature of the MMCs is the major benefit that provides the manufacturers with numerous choices of materials as matrix and reinforcing materials to achieve specific performance properties suitable for their envisioned application.This resourcefulness of the MMCs has led to their adoption in numerous industries, including aerospace, automotive, defence, electronics, and sporting equipment [3][4][5].
Aluminium is the most prevalent matrix for MMCs due to its high strength-to-weight ratio, superior ductility, good thermal conductivity, and corrosion resistance.Among the different wrought aluminium alloys 2, 6 and 7xxx series alloys are heat treatable alloys.The 6xxx series aluminium alloys contain magnesium (Mg) and silicon (Si) as the primary alloying elements that contribute to formation of stable precipitates (Mg 2 Si).Aluminium alloy 6061 (AA6061) is one such 6xxx series alloy that can be strengthened by work/strain hardening and age/precipitation hardening [6,7].Researchers have also used various ceramic materials in particulate form such as Al 2 O 3 , SiC, B 4 C, TiC and TiB 2 [8][9][10][11][12][13][14][15][16], as reinforcement materials to fabricate Aluminium Matrix Composites (AMCs).Silica sand is naturally available in riverbeds and can be used as strengthening reinforcement as it has a high hardness and is a ceramic material that can withstand high temperatures [17].Additionally, copper (Cu) has a face-centred cubic (FCC) structure that strains the aluminium matrix by supporting the cold deformation.
However, despite their numerous advantages, conventional MMCs often exhibit certain limitations, such as limited fracture toughness, insufficient thermal conductivity, and a high coefficient of thermal expansion.To address these limitations and further optimize the properties, the concept of Hybrid Metal Matrix Composites (HMMCs) emerged [18,19].
The development of HMMCs introduces many advantages and potential applications [10,20,21].The ability to tailor the material properties allows HMMCs to deliver improved mechanical strength, stiffness, and durability compared to conventional materials.Such characteristics make them highly desirable in industries where lightweight and high-strength materials are critical, such as aerospace and automotive engineering [22,23].
Additionally, the pursuit of improved performance and tailored properties has led researchers to explore innovative methods for enhancing the characteristics of HMMCs.One such technique gaining attention is Low-Temperature Thermomechanical Treatment (LTMT).This research delves into the concept and implications of LTMT applied to HMMCs, highlighting its effects on the microstructure, and mechanical properties.The novelty of the work is the addition of harder, naturally available, cheaper reinforcement silica sand into the aluminium alloy matrix and the amalgamation of strain hardening and phase transformation to enhance the properties of the hybrid composites.

Materials and methods
A widely employed 6xxx series aluminium alloy AA6061 [24,25] with a density of 2.7 g cm −13 was used as the matrix in the study.The spectroscopic analysis of the alloy revealed the presence of 0.73, and 0.95 weight percentages (wt%) of Si, and Mg respectively, which are the major alloying elements and are within the permissible limits according to the standard AA6061.Silica sand [26,27] and copper [28,29] particles were used as reinforcements in particulate form to fabricate the hybrid composites.The important parameters of the reinforcement particles considered during the fabrication of the hybrid composites are tabulated in table 1.

Processing of silica sand
The processing of silica sand involves several steps before it can be used as reinforcement in the fabrication of the composites.The silica or natural sand is obtained from the riverbed and thoroughly washed with water to remove impurities such as silt, clay and organic matter.The sand is then further cleaned using acetone and distilled water by agitation in an ultrasonic bath sonicator.The cleaned sand is dewatered and dried in a muffle furnace at 200 °C for 2 h.The dried sand is then pulverised in a ball grinder to produce finer particles.These finer particles are then screened using sieves to sort into various particle size specifications.The SEM-EDX analysis of the particles processed is shown in figure 1.

Processing of hybrid composites
The hybrid composites were fabricated through a liquid metal manufacturing technique called stir casting.The process involves melting the matrix alloy 6061 in an electric resistance furnace heated to 760 °C.To improve the wettability of the SiO 2 and Cu particulates with the matrix 3 wt% Mg is added to the molten metal [30,31].Moreover, the SiO 2 and Cu particles were preheated to 500 and 300 °C, respectively to improve the dispersion of the reinforcement particles in the aluminium alloy matrix.The reinforcement particles are uniformly added and mixed in the metal through a mechanical stirring setup.The composite slurry of the reinforcements in the AA6061 maintained at 780 °C is then poured into preheated gravity moulds for cooling and solidification.Table 2 presents the details of the composites processed with different wt% of SiO 2 and Cu.The as cast composite slabs of 15 mm thickness (shown in figure 2(a)) were initially machined using an abrasive water jet machine to dimensions of 120 mm × 40 mm.Subsequently, the composite slabs were sliced into strips of 110 mm × 4 mm.The strips were then machined to produce specimens with thicknesses of 3.4, 3.26, 3.12, and 3 mm using CNC vertical milling machine.To determine the peak hardness the specimens were prepared by cutting strips subjected to LTMT along the length to produce smaller specimens as illustrated in figure 2(b).
The specimen to test the Brinell hardness of the hybrid composite slabs is divided into three sections of 40 mm in length.This was done to segregate the composite surface into three zones namely the top, mid and bottom regions as shown in figure 3.

Heat treatment process
The heat treatment of the composites was carried out through conventional Age Hardening (AH) and LTMT to enhance the mechanical properties.Figure 4 shows the heat treatment cycles employed on the hybrid composites.The conventional AH process comprised heating the composite specimen at 550 °C for 2 h, quenching in water, artificial aging at 100 °C and quenching in water.Similarly, the LTMT process comprised the steps in AH with the addition of cold rolling of the specimen before the artificial aging.The cold rolling was carried out to produce deformation by reducing the thickness of the specimen.The composites were subjected to deformation of 4, 8 and 12% by reducing the thickness of the specimens to 3 mm from 3.12, 3.26 and 3.4 mm respectively.

Mechanical characterization techniques
The macro hardness of the composites to verify the even distribution of the reinforcement in the matrix was carried out using Brinell hardness tester.The surface hardness test of the composites was based on the ASTM E10-18 standard.To determine the peak hardness of the composites subjected to AH and LTMT conditions Vickers hardness test was carried out by following the ASTM E384 standard.An average of 5 readings at different locations of each specimen was considered for the test to ascertain reliable results.The tensile test was carried out to determine the Ultimate Tensile Strength (UTS) values of the composites subjected to AH and LTMT  conditions.The tensile specimens for the test were machined as per the ASTM B557M-15 standards.The specimens were tested in the peak aged condition and to ensure consistent results an average of 3 test readings were considered.Further, to ascertain the mode of failure of the composites SEM images of the fracture surface were analysed.

Results and discussion
Comprehending the material properties is crucial as it plays a vital role in determining the suitability of the material for diverse engineering applications.The microstructure analysis of the AA6061-SiO 2 -Cu hybrid composites was carried out to get an insight into the distribution of the particles in the AA6061.The Brinell hardness test of the as cast composites was executed to quantify the presence of the reinforcement particles.The discussion on the variation in the mechanical properties of the composites is correlated with the fracture surface analysis.

Microstructure and macro hardness study
The presence of reinforcement and uniform dispersion of reinforcement particles in a composite are critical factors in achieving superior properties in composite materials.Figure 5 depicts the photomicrographs of the as cast hybrid composites with different wt% combinations of the reinforcement particles.The micrographs reveal the uniform distribution of the reinforcement particles in the AA6061 matrix.It can be noted that there is no evidence of voids or porosities or blowholes in the matrix alloy.The dark spots in figures 5(a)-(c) represent the reinforcement particles spread in the aluminium alloy matrix.It is evident from the micrographs that the reinforcement particles distributed in the matrix is in larger quantity in 3S3C compared to the other hybrid composites.This is because of the lower density of the SiO 2 , which is at least 3 times lower than the density of Cu.This leads to a higher presence of SiO 2 particles than Cu for the same wt%.To confirm the presence of both the SiO 2 and Cu particles in the matrix SEM-EDX was done on the 3S3C hybrid composite.From figure 6, it is evident that both reinforcements were present in the 3S3C hybrid composite.
The macro hardness measurement was performed on the hybrid composites using a Brinell hardness tester.The as cast composites when tested revealed an increase in hardness with the incorporation of the reinforcement particles into the alloy.Additionally, the increase in wt% of SiO 2 particles showed an increase in the hardness of the hybrid composite.This can be attributed to the increase in the existence of harder dispersoids that positively influences the hardness of the composites [32,33].Figure 7 shows the Brinell hardness values of the three different regions on as cast AA6061 and AA6061 hybrid composites.The uniform hardness in the hybrid composites evidently confirms the uniform distribution of the SiO 2 and Cu particles in the AA6061 alloy.

Microhardness and aging curves
Figure 8 represents the variation in hardness of the AA6061-SiO 2 -Cu hybrid composites against the aging time when subjected to LTMT at 100 °C.The measured hardness values of each composite subjected to LTMT with various degree of deformation is observed to be between the range of ±5 HV.The hardness values of the as cast 1S5C, 2S4C and 3S3C hybrid composites are 78.07,80.41 and 83.67 HV respectively.The incorporation of the  reinforcement particles into AA6061 alloy (63.9 HV) has led to an improvement in the hardness of the hybrid composites.The age hardened hybrid composites showed an increase of 50 and 87% over as cast hybrid composites and AA6061 alloy respectively.The highest increase in hardness is found to be with 3S3C hybrid composites which showed an increase of 30% over the AA6061 alloy.The 3S3C hybrid composite subjected to LTMT with the highest amount of deformation showed the best result of 144.26 HV with the lowest aging time.This is an increase of 61, and 15% over the as cast and AH 3S3C hybrid composite, and 125% over AA6061 alloy.
With the increase in the SiO 2 particles and decrease in Cu particles with a total wt% of reinforcement at 6, the hardness of the hybrid composites increased.This is credited to an increase in the wt% of the harder reinforcement in the composite.The increase in the ceramic particles increases the thermal mismatch between the alloy and the particles during solidification.This causes an increase in the dislocation density which produces large internal stresses and strain [34].Since the density of the SiO 2 is less than the Cu particles the increase in the wt% of SiO 2 particles would cause deformation of the matrix during solidification to accommodate the volume expansion of the ceramic reinforcement.As the low density SiO 2 particle content increases, its volume fraction in the composite increases, providing more nucleation sites for the heterogeneous phase formation for intermetallic [35,36].Additionally, the plastic deformation of the composites during cold rolling adds to the increase in dislocation density.
The graphs show a rise in hardness values of the hybrid composites with reference to aging time, where the hardness slowly increases to reach its peak value and then decreases.This behaviour is typical of age hardenable matrix composites during aging.Adding reinforcement particles in the alloy and the intentional deformation of the composites by room temperature rolling increases the nucleation sites for creating new solid solution strengthening phases.As the precipitates of these phases nucleate and grow the hardness of the composites increases.When the precipitate grows to a critical size and there exists a coherency between the particle and the matrix, the peak hardness is achieved.If the aging is continued further, the size of the precipitates increases beyond the optimum value and loses lattice coherency [37,38].This condition is called over aging.It may also be noted that the increase in degree deformation reduces the aging time, which is caused by the increased diffusion of the solute atoms from the supersaturated alloy phase to various nucleation sites to form solute-rich harder precipitates [39,40].

Tensile strength and fracture surface analysis
Figure 9 represents the UTS values of the peak aged hybrid composites subjected to AH and LTMT wherein the variation in the values is between ±10 MPa.The UTS values of as cast 1S5C, 2S4C and 3S3C hybrid composites measured are 169, 180, and 197 MPa, respectively.From the graph, a significant increase is observed in the UTS of hybrid composites with the increase in the SiO 2 particles and the degree of deformation when subjected to AH and LTMT.The increase in the ceramic particles combined with the effects of cold rolling has enhanced the tensile strength.This is due to the formation of coherent Mg 2 Si precipitates during aging, presence of the reinforcement particles and the increased deformation which act as barriers for the movement of the dislocations in the matrix [41].
The AH hybrid composites in peak aged conditions showed an increase of 48 and 70% UTS as compared to as cast hybrid composites and AA6061 alloy (147 MPa) respectively.The 3S3C hybrid composite when subjected to 12% deformation during LTMT showed the best result of 290 MPa at the peak aged condition.This is an increase of 47, and 11% when compared to 3S3C hybrid composite in as cast and AH peak aged condition.Moreover, there is a 97% increase in the UTS of 3S3C hybrid composite when subjected to 12% deformation during LTMT over AA6061 alloy.High particle concentration, aging heat treatment and cold rolling result in increased dislocation density and particle dislocation interactions.This increase in dislocation density also increases the strength of the composite by expediting the nucleation of secondary solute rich precipitates in the matrix during heat treatment.Additionally, the strength of the composites increases when the load is applied, and the presence of the hard particles in the matrix constrains the plastic flow in the ductile matrix, resulting in a pile-up of the dislocation around the particles, increased stress and work hardening of composites.The synergistic effect of dislocation interaction with reinforcement and grain boundary provides for the solid solution strengthening of the hybrid composites.
The prevalent types of fractures in the case of aluminium alloys and aluminium-based composites are ductile and brittle.The AA6061 hybrid composites showed excellent hardness and tensile strength when subjected to LTMT with 12% deformation.Thus, the fracture surface of all three hybrid composites in peak aged condition subjected to tensile test were analysed as shown in figure 10. Figure 10(a) shows fine equiaxial dimples that are shallow.The equiaxial morphology is the indication of lesser strain hardening in localized regions.Accordingly, reduced strength is reflected in the lower UTS values of 1S5C composites in figure 9. Figure 10(b) shows fine elongated dimples with fractured ridges.These indicate severe strain hardening of the matrix.Also, there is an observation of ultrafine dimples, which support an increase in UTS values of 2S4C composites, as depicted in figure 9.In figure 10(c) lengthy narrow river patterns are observed that indicate severe strain hardening on the matrix.Moreover, the finer elongated dimples are locked by the network of river patterns, representing an increase in the UTS by strain hardening This is due to the increased quantity of the harder SiO 2 dispersoids.The micrographs show clear evidence of increased strain hardening with the increase in silica sand content.
The fractured surfaces show fine equiaxial dimples, that are shallow and indicate lower strength.The plastic deformation of the matrix during tensile fracture is evident as river patterns exist.In a few locations, the fracture is through tear and shear which created uneven elongated dimples with tear ridges of irregular thickness which indicate good tensile strength.The ultrafine dimples of different sizes exhibit attainment of higher strength.The finer elongated dimples and narrow river patterns indicate the achievement of high strength at peak aged conditions.The presence of the narrow river patterns, ultrafine, equiaxial and finer elongated dimples clearly show the mixed mode of failure dominated by ductile fracture.So, it may be concluded that the aging and higher degree of deformation have led to the increased mechanical properties of the composites.

Conclusion
The experimental investigation envisioned the incorporation of naturally available ceramic particles together with metal-based reinforcement particles to fabricate an AA6061 hybrid composite.Additionally, the study aimed at implementing a novel heat treatment process to improve the hardness and UTS of the composites.Based on the investigation, the subsequent conclusions are arrived at: • AA6061-SiO 2 -Cu composites with varying wt% of the reinforcement particles are successfully fabricated by stir casting technique.The microstructure study shows uniform dispersion of the reinforcements in the matrix alloy.This is supported by the macro hardness test which indicated similar hardness values in the as cast hybrid composites.
• The AA6061 -3 wt% SiO 2 -3 wt% Cu hybrid composites subjected to LTMT with 12% deformation presented the best results of hardness and tensile strength.The study clearly shows the enhancement in the properties of the composites with the incorporation of higher wt% of silica sand particles.
• The AA6061-3S3C hybrid composites subjected to LTMT with 12% deformation showed maximum hardness and UTS of 144.26 HV and 290 MPa respectively.
• When compared with the as cast hybrid composites the composites subjected to LTMT (12% deformation aged at 100 °C) showed 61 and 47% increases in peak hardness and tensile strength respectively.Similarly, the increase in hardness and tensile strength of LTMT hybrid composites over AH composites was 15 and 11% respectively.
• Fracture analysis of the LTMT processed hybrid composites indicated mixed failure mode dominated by ductile fracture.The increase in hardness and strength of the hybrid composite is justified by the presence of well-defined dimples spread across the fractured surface.

Figure 2 .
Figure 2. (a) As cast hybrid composite slab, and (b) Hybrid composite strips subjected to LTMT and specimens for the peak hardness test.

Figure 3 .
Figure 3.As cast hybrid composite slab with 3 distinct regions.

Figure 7 .
Figure 7. Variation in Brinell hardness values of as cast AA6061 and AA6061 hybrid composites.

Figure 9 .
Figure 9. UTS values of AA6061-SiO 2 -Cu hybrid composites subjected to LTMT with varying degrees of deformation.

Table 1 .
Details of the reinforcement particles.