Experimental analysis of hardness and tensile characteristics of copper reinforced AA6061 stir cast composites subjected to thermal and deformation assisted heat treatments

Abstract Aluminium alloy 6061 based composites reinforced with a varied weight percentage of copper particulates are fabricated utilizing a liquid metal stir casting technique. The collective influence of copper reinforcement, age hardening and low temperature thermomechanical treatment on AA6061 was investigated. The age hardened composites displayed better hardness and ultimate tensile strength than as cast composites. Thermomechanical treatment of the composites further enhanced the mechanical properties and hence showed better results over age hardened composites. The study revealed that an increase in the deformation enhanced the hardness and strength of the composite while the aging time to achieve the peak hardness was reduced. Both age hardened and thermomechanically treated composites with 6 wt.% copper reinforcement indicated the best peak hardness and UTS values at the lower aging temperature of 100 °C. The thermomechanically treated composite with 6 wt.% Cu, 15% deformation aged at 100 °C showed 80 and 69% increase in hardness and UTS, respectively, over as cast composite. Fracture surface analysis of the as cast, age hardened, and thermomechanical treated composites showed a mixed mode of fracture dominant with the brittle failure.


Introduction
Present-day technological developments such as lightweight vehicles with energy efficiency, highspeed jets, agile automatons, and naval vessels have been largely aided through advancements in materials evolution.Numerous avenues are being explored to enhance the material properties to fulfil the requirements of technological applications.Metal matrix composites (MMCs) are an excellent prospect for producing properties tailor-made for the material design process (Arslan & Kalemtas, 2009;Bhoi et al., 2020;Haque et al., 2014).A few typical attributes of the MMCs are superior strength, good toughness, excellent resistance to corrosion, high temperature, and wear and other mechanical characteristics (Alaneme, Okotete, et al., 2019;Garg et al., 2019).
The intentions and purpose of MMCs determine the choice of the materials for fabrication of the composites (Hima Gireesh et al., 2018;Ramanathan et al., 2019).Aluminium and its alloys are the most favoured matrix materials to produce MMCs (Das, Mishra, Singh, & Pattanaik, 2014;Madhusudan et al., 2009;Maurya et al., 2019;Vedrtnam & Kumar, 2017).Owing to the presence of Mg and Si as major alloying elements, AA6061, a precipitation hardenable alloy finds its application in automobile and aircraft industries (Jayakumar et al., 2013).The AA6061-based composites are used to manufacture light weight parts with higher strength such as aeroplane wings and fuselage, sailboat hulls and masts, and bicycle frames.The reinforcements in the form of particulates are generally the preferred choice for aluminium matrix composites (AMCs).The particulate reinforcements are readily available, reasonably viable, and easily distributed in the matrix (Ramanathan et al., 2019).
The heat treatment of AA6061 matrix composites consists of solution treatment wherein the composite is heated well below the recrystallization temperature and quenched in water maintained at room temperature.This results in formation of supersaturated solid solution by the dissolution of secondary phases into the parent phase.Later the composites are aged well below the recrystallization temperature for different durations to achieve strength and hardness.Lately, a great deal of study has been focussed on fabricating AMCs that exhibit enhanced mechanical properties inspiring the discovery of age hardening (Ozturk et al., 2010;Poovazhagan et al., 2016;Sekar & Jayakumar, 2020;Tan & Said, 2009).Moreover, numerous encouraging attempts have transpired to produce fine-grained materials which exhibit a significant rise in properties.Work hardening utilizes the deliberate deformation to improve the material properties (Amirkhanlou et al., 2011;El-Sabbagh et al., 2013;Estrin & Vinogradov, 2013;Manjunatha et al., 2015;Rofman et al., 2019;Tao et al., 2018;Zhang et al., 2019).Nevertheless, the study of the integrated influence of aging and work hardening of AMCs has launched a novel scope in research to fabricate good quality composites.
The study presented includes fabrication of AA6061-Cu composites with varied wt.% of Cu reinforcement incorporated through liquid metal stir casting technique.Microstructure study and analysis of the extracted particles of the composites are performed to confirm uniform distribution of the Cu particles in the composites.The composites fabricated are tested to assess the effect of age hardening and low temperature thermomechanical treatment (LTMT) on the hardness and tensile strength of the composites.Additionally, the fracture surface is analysed to understand the mode of failure in the composites.
The stir casting technique was employed to fabricate composites with 2, 4, and 6 wt.% Cu reinforcement particulates.

Fabrication of the composite
As bought, AA6061 alloy billets were cut to smaller sizes and heated to melt in the electrical resistance type furnace at 750 °C.The impurities were removed as slag after the billets had melted entirely.The preheated Cu particles at 300 °C were added down the vortex of the molten metal produced by stirring.A three-blade mild steel stirrer with a stirring speed of 200-300 rpm was used to maintain a consistent vortex while adding the particles to the molten alloy (Sekar et al., 2019).The Cu particles in three weight percentages of 2, 4, and 6 wt.% were added to the alloy to fabricate the composites.After adding Cu particulates through the vortex, the molten composite was mechanically stirred for 5 additional mins.The molten composite was maintained at 760 °C and poured in the preheated permanent moulds maintained at 500 °C.The composite is then let cool off and solidify.

Machining of the specimens
The as cast composite specimens were initially machined and cut to dimensions of 120 mm x 40 mm x 15 mm.Further, the specimens were machined into strips of 120 mm x 4 mm x 15 mm.These strips were then machined to produce specimens of three distinct thicknesses, as listed in Table 1 as initial thickness.The cutting and machining operations were performed using a wire EDM machine.

Sample preparation for microstructure study
The cubical specimens of 10 mm x 10 mm x 15 mm were cut from the as cast composites.Each specimen was polished along the flat surface using silicon carbide sandpapers of grit sizes 80, 100, 220, 400, 600, 1000, 1500, 2000, and 2500 sequentially.These specimens were further polished on selvyt cloth using 3, 1 and 0.25 µm diamond suspension in sequence over a rotary disc type polishing machine.

Low temperature thermomechanical treatment
The aging treatment (Figure 1a) involved heating the specimens at 550 °C for 2 h and quickly quenched in ambient temperature water.The specimens were precipitation hardened at 100 & 180 °C for various duration of time between 1-20 h.

Description
Original thickness (mm)

Final thickness (mm) Average deformation (%)
Age hardened 3.00 3.00 00 The thermomechanical treatment (Figure 1b) involved heating the specimens at 550 °C for 2 h and instantly quenched in ambient temperature water.The samples were cold rolled to accomplish deformation of 5, 10, and 15%.The specimen details for age hardening and thermomechanical treatment is presented in Table 1.These specimens were then precipitation hardened at 100 and 180 °C for various duration of time between 1-20 h.

Reinforcement particle extraction
To confirm the presence of the Cu reinforcement particles in fabricated composites, the particles were extracted, as shown in Figure 2. The known weights of composite specimens were dissolved in 1:1 nitric acid (HNO 3 ) by heating.The heating was continued until the specimen started to react with the solution.A 10% aqueous solution of sodium nitroprusside was added to digest the residue for half an hour on the lowest heating range of a hot plate.The residue was then filtered through counterpoised Whatman filter paper no.41.The filtered residue is washed with distilled water and acidified with dilute HNO 3 .The filter paper with the residue is then dried in a hot air oven at 110 °C.The dry residue was weighed as copper nitroprusside, and the copper weight was calculated using a conversion factor of 0.2274.The conversion factor is calculated as the ratio of the atomic weight of copper to the molecular weight of copper nitroprusside.

Measurement of hardness and tensile strength
The surface hardness was measured using a Brinell hardness tester to confirm the uniform incorporation of the reinforcement particles in the composite.The hardness tests of as cast AA6061-Cu composites were carried out in ASTM E10-18 conditions.The cast composite specimens of 120 mm x 40 mm x 15 mm are separated into three regions as shown in Figure 3.To confirm reliable findings, an average of five readings were recorded for each specimen.
The peak hardness of heat treated composites was determined using a Vicker's hardness tester.The test was conducted based on the ASTM E384 standard.An average of five readings at distinct spots on the surface of each specimen were taken.This technique was employed to ensure the results were reliable.
A horizontal universal testing machine measured the ultimate tensile strength (UTS) of the composite specimen's heat treated to peak aged condition.Figure 4 shows the dimensions of the tensile test sample prepared in accordance with ASTM standard B557M-15.An average of three results were accounted for each composite to ensure reliable results.

Microstructure study and hardness test
The microstructural evaluation gives an insight into the quality of the fabricated composites.The two important considerations during the fabrication of composites with particulates are the homogeneous distribution of the particles during the pouring of liquid composite and its solidification.To achieve superior mechanical properties in a composite, uniform dispersion of the Cu reinforcement in the AA6061 matrix is vital.The micrographs (4× magnification) taken through an upright metallurgical microscope in Figures 5a-c evidently show the dispersal of the Cu particles in the AA6061 matrix with no evidence of porosities or voids.
The Brinell hardness test on AA6061-Cu as cast composites was carried out to verify the existence of the Cu particles.The test revealed an increase in the hardness of the composites because of the presence of hard dispersoids that positively contribute to the composite hardness (Zare et al., 2019).Additionally, the increase in the copper wt.% in the composites shows a gradual increase in the hardness.
The effect of reinforcement particles shows improvement in the hardness of the AA6061-Cu composites.The hardness values of AA6061 and AA6061-Cu composites with 2, 4 and 6 wt.% reinforcement particles are measured at distinct zones: bottom, middle, and top regions.Figure 6 shows the hardness values of as cast alloy and composites at three different regions of the casting measured using the Brinell hardness machine.From the graph it can be inferred that the uniform  Increasing the wt.% of reinforcement particles in the composites leads to increased dislocation density.This occurs during the solidification which is triggered by the thermal mismatch between the reinforcement and the matrix.This leads to significant internal stresses and strain that affect the microstructure and mechanical properties of the composites.The volume enlargement of the reinforcement particles is accommodated by the plastic deformation of the matrix leading to the increased dislocation density.This enrichment of the dislocation density in the composites results in a higher resistance to plastic deformation, which constitutes the increased hardness of the composites (Gowri Shankar et al., 2017).During solidification the addition of the Cu particles to the aluminium alloy melt act as heterogeneous nucleation sites and leads to finer grains, improving the mechanical properties (Khalili et al., 2020;Yoo et al., 2020).

Analysis of extracted particles
The purpose of extracting the particles from the composite was to confirm the presence of the copper reinforcement in relevant wt.%.Table 2 contains the actual and measured wt.% of copper in the composite.The study revealed retention of a minimum of 95% of Cu particles in every single composite.SEM and EDS of the particle extracted are shown in Figure 7.It is evident that the particle extracted from the composite are copper particles.

Aging curve and peak hardness of composites
Vicker's hardness test was performed to determine the hardness of AA6061-Cu composites in as cast, age hardened and LTMT conditions.The variation in the hardness was noted to be between ±5 HV.The measured hardness of the composite specimens in as cast condition with 2, 4, and 6 wt.% Cu are 68, 70.68 and 73.98 HV, respectively.The as cast composites showed a rise in hardness with the increase in the Cu particles.Figures 8 and 9 show the graphs with hardness distribution against aging time of composites with deformation of 0, 5, 10 and 15%, for isothermal aging temperature (IAT) of 100 and 180 °C respectively.In comparison with as cast composites with 6 wt.% Cu, the composites aged at 100, and 180 °C exhibited a rise in peak hardness of 48 and 42%, respectively.Likewise, compared to age hardened composites, an increase in peak hardness of 21 and 16% was observed for the LTMT composites with 6 wt.% Cu, 15% deformation and aged at 100 and 180 °C, respectively.
The three distinguished regions (underaged, peak-aged, and overaged) are observed in Figures 8  and 9.The graphs with hardness against aging time visibly show the rise in the hardness with time.The hardness steadily increases to achieve the peak value and subsequently reduces.This    behaviour is expected in the composite during age hardening and is similar to the base alloy case.
As the degree of deformation increases, numerous nucleation sites are created for the development of the new phase and with the reduced growth process, the grains produced are finer in nature.The abundance of such phases with fine grains aid in achieving higher strength and hardness (Revankar et al., 2017;Sanyal et al., 2016).The increase in the lattice strain by deformation is caused by the creation of the vacancies, stacking faults, and dislocations.Thus, aging time is reduced by the formation of nuclei at closer intersite spaces (Ahn & Yu, 2001;Terada et al., 2014).
Compared with the hardness trend at 100 °C IAT, lower peak hardness values are observed at 180 °C IAT with reduced peak aging duration, i.e., aging kinetics accelerates due to the increased diffusion rate at higher temperatures (Avner, 2012).Higher the IAT faster the diffusion process with a reduction in the number of intermediary stages during the precipitation of the hard secondary phase (Rajan et al., 2012).
Figures 10 and 11 show the maximum hardness values achieved by composites and the time required to achieve the maximum hardness at both the lower and higher IATs.The higher the IAT lower are the peak hardness values and aging duration compared to that at the lower IAT.Excellent peak hardness values are observed at the given IAT when a more significant number of intermediate stages are present during the spontaneous separation of solute atoms (Mahadevan et al., 2008;Reza et al., 2009).Hence, specimens aged at lower IAT show higher peak hardness values.With the increase in aging temperature the thermomechanically treated AA6061-Cu composites show about 8 to 9.8% reduction in peak hardness and up to 31% reduction in peak aging duration.

Tensile behaviour of age hardened and low temperature thermomechanical treated composites
The study employed a universal testing machine to understand the tensile behaviour of AA6061-Cu composites in various conditions.Variation in UTS was noted to be between ±10 MPa.The measured UTS of the composite specimens in as cast condition with 2, 4, and 6 wt.% Cu are 150, 157 and 165 MPa, respectively.
From Figure 12, it is evident there is a marginal increase in the UTS with the incorporation of Cu particles and both age hardened and LTMT processed composites show improved tensile strength.The strong interface between the Cu reinforcement and AA6061 matrix could be the reason for the superior properties.Additionally, the combination of the cold rolling and aging treatment provided to the composites has provided better tensile results when compared to the composites in as cast condition.This rise in strength of the composite may be credited to the presence of the Cu particles, formation of coherent Mg 2 Si during aging, that acts as blockades for the progress of dislocations, and the higher degree of deformation produced by rolling (Gao et al., 2019;Martinova et al., 2002).
Composites indicated the best UTS with collective influence of the increased wt.% of Cu, higher degree of deformation and aged at lower IAT.The increase in dislocation density, precipitation of secondary phases rich in solute atoms and disparity in coefficient of thermal expansion (CTE) between the reinforcement and the matrix has led to the increase in strength of the composites.The UTS of age hardened composites with varying reinforcement percentages show a minimum of 34% increase over as cast composites considering both IATs.At the lower aging temperature, the maximum UTS of 279 MPa is achieved by composites with 6 wt.% Cu subjected to highest deformation, which is 14 and 69% increase over age hardened and as cast composites, respectively.

Analysis of the fracture surface
The key purpose of fracture surface analysis is to ascertain the mode of fracture encountered by the composite specimen.Generally, in aluminium alloys and AMCs ductile and brittle failures are the prevalent type of fractures.When compared, the AA6061-Cu composites with 6 wt.% reinforcement showed the excellent results of hardness and tensile strength.Therefore, the fracture analysis was performed on composites with the 6 wt.% reinforcement in as cast, age hardened, and LTMT conditions.Generally, as cast composites show a mixed mode of failure, dominated by brittle fracture, which is evident through the river pattern.Figure 13a shows the presence of uniform finer elongated cups (dimples), which indicate ductile failure and a large number of river patterns suggest the failure is dominated by brittle fracture.Figure 13b shows ultrafine cups, river patterns and dendrite like structures in the age hardened composite.The appearance of a dendrite array is also an indication of attaining higher strength.The reduction in the river pattern indicates a reduction in the brittleness in the composite.Since the dendrite array is confined to few locations excellent strength and toughness combination is not obtained.
Figure 13c shows dendrite patches with uniform size spread evenly throughout the fractured surface of LTMT composite aged at 100 °C.Finer and uniform dendrites indicate superior strength and toughness.Since the concentration of the dimples is reduced, the ductile mode of failure is weakened.The uniform spread of the finer dendrite shows the attainment of peak aged condition through isothermal aging.Figure 13d shows the presence of finer dimples combined with a mixture of uniform and non-uniform dendrite arrays with the least amount of river patterns in LTMT composite aged at 180 °C.This combination indicates the attainment of peak hardness during isothermal aging.The coarse and uneven dendrites in the composites, accompanied by the diminished number of finer dimples, suggest lesser strength in comparison to the composite aged at 100 °C.Fracture analysis is well supported by the UTS results shown in Figure 12.

Conclusion
Based on the results of hardness, UTS and investigation of the fractured surface of the age hardened, and thermomechanical treated composites, the subsequent conclusions were arrived at: • The stir casting technique led to the fabrication of AA6061-Cu composites with uniform distribution of reinforcement particles, which is corroborated by microstructure study, extracted particle analysis and measurement of macro hardness using the Brinell hardness test.
• Extracted particle analysis revealed retention of 95% of Cu particles in the composites which is confirmed by SEM and EDS report.
• Measurement of hardness along the length of the fabricated composites with varied wt.% of reinforcement exhibited consistent readings substantiating uniform distribution of the Cu particle in the composites.
• Composite fabricated with 6 wt.% Cu, 15% deformation and aged at lower IAT (100 °C) presented excellent results.However, the time to attain the peak hardness reduced at higher IAT of 180 °C.
• Maximum hardness and UTS of 133.4 HV and 279 MPa, respectively, were obtained for AA6061-Cu composites when the composite with the highest wt.% reinforcement was thermomechanically treated with deformation of 15% at a lower IAT of 100 °C.
• The peak hardness of AA6061-6 wt.% Cu composite subjected to highest deformation showed an increase of 21 and 80% over age hardened and as cast composites at lower IAT, respectively.
• The UTS of AA6061-6 wt.% Cu composites subjected to highest deformation showed an increase of 14 and 69% over age hardened and as cast composites at lower IAT, respectively.
• Fracture analysis of the as-cast, age hardened, and LTMT processed composite specimens showed the mixed mode of fracture dominated by brittle failure.The increase in hardness and toughness of the LTMT processed composite is justified by the presence of a dendrite like structure in the fractographs.
Figure 1.Heat treatment cycles employed on composites (a) age hardening, and (b) LTMT.

Figure 2 .
Figure 2. Procedure to extract the precipitate from the composite.

Figure
Figure 6.Variation in Brinell hardness values in as cast AA6061 and AA6061-Cu composites.

Figure 7 .
Figure 7. SEM and EDS of the extracted particle.

Figure 12 .
Figure 12.Variation of UTS at peak-aged conditions for AA6061-Cu composites.