STUDIES ON DEFORMATION BEHAVIOR AND MICROSTRUCTURAL EVOLUTION OF AA7075 - TAC COMPOSITES

Abstract

Al- Zn-Mg alloys i.e. AA7xxx series are considered as high-performance engineering materials in various applications such as aerospace and automotive industries due to the combination of low density, high specific strength, good fatigue resistance, wear resistance and excellent corrosion resistance. To achieve the desired mechanical properties, these alloys are usually subjected to various age-hardening treatment for precipitation hardening, severe plastic deformation process and adding hard ceramic reinforcements. Through this above process, microstructures can be modified, and mechanical properties can be obtained accordingly. Therefore, the present study is emphasized to investigate the effects of varying amounts of Tantalum Carbide (TaC) on phase analysis, densification, microstructural evolutions, and mechanical properties of AA7075 matrix-based composites developed through stir-casting technique. The study is divided into two phases. The first phase explores the impact of varying proportions of TaC particles (0.1-0.5 wt.%) reinforced to a AA7075 matrix with respect to the microstructural evolution and mechanical properties. Subsequent experiments focus to investigate the effects of heat treatment and severe plastic deformation (SPD) processes through rolling and forging on the AA7075/0.5 wt.% TaC composite. It has found that reinforcement of TaC particles in small amount of 0.1 – 0.5 wt.% has improved the mechanical properties of AA7075 composite through stir-casting. Among all the developed composites, AA7075/0.5 wt.% TaC composite showed the highest hardness of 96 Hv, YS of 171 ± 15 MPa, UTS of 205 ± 26 and 2.7 ±0.6 % El. The AA7075/0.5 wt.% TaC (S-4) composite further subjected to heat treatment (HT) with single step aging, rolling and forging with 40% and 70% thickness reduction at room temperature, elevated temperature, and cryogenic temperature to evaluate and compare the hardness with S-4 composite. The HT sample showed hardness of 136 Hv. The rolled sample at cryogenic temperature of 70% thickness reduction showed the highest hardness of 165 Hv among all the rolled samples processed at different temperature of 40% and 70% thickness reduction. The forged sample at cryogenic temperature of 70% thickness reduction showed the highest hardness of 179 Hv among all the rolled samples, forged sample and HT samples. The incorporation of TaC particles in AA7075 matrix methods significantly improved the mechanical properties of the composite through dispersion strengthening and grain refinement strengthening. The implemented secondary processing methods further enhance the hardness through particle stimulated nucleation (PSN), grain refinement and dislocation strengthening. In the second phase, the effect of TaC particles (1-7 wt.%) on densification, microstructural evolution, and mechanical properties of AA7075 based composites were investigated. The experiments were further extended to investigate the effect of heat treatment with two step aging, forging at elevated temperature of 250 ºC - 450 ºC with and without T6 temper, cryoforging under furnace cooled (FC), air-cooled (AC), and water quench (WQ) cooling mediums with and without T6 temper on AA7075/5 wt.% TaC composite. The results revealed that AA7075/5 wt.% TaC composite (S-4) exhibited the highest hardness of 142 ± 3.2 Hv, YS of 183 ±16 MPa, UTS of 264± 21 MPa and AA7075/1 wt.% TaC composite showed the highest % El. of 3.2 ± 0.14. The hardness, YS, and UTS increases with the addition of TaC from 1 to 5 wt.% and then decreases with further addition of TaC to 7 wt.% in AA7075 matrix. The % El. increases at 1 wt.% TaC in AA7075 matrix and then started decreasing from 3 to 7 wt.% TaC addition in AA7075 matrix. The increment in the mechanical properties is attributed to grain refinement and dispersion strengthening while decrement in hardness and strength due to the agglomeration and poor wetting. The coefficient of thermal expansion (CTE), grain refinement, dispersion strengthening, and Orowan strengthening are the strengthening mechanisms that act simultaneously to increase the strength of the composite. The AA7075/5 wt.% TaC composite (S-4) obtained the highest mechanical properties among all the developed composite of AA7075/1-7 wt.% TaC. Therefore, the experiment was further extended with the purpose to increase the strength and ductility of the composite through heat treatment, hot forging, and cryo-forging. The heat treatment (HT) was carried out on AA7075/5 wt.% TaC composite (S-4) to enhance the mechanical properties of the composite. It was observed that microhardness, yield strength, ultimate tensile strength, and % elongation was higher than S-4 composite. The precipitates developed through two-step aging was the main strengthening mechanism to improve the strength of the composite. The EBSD analysis revealed that the grain structure was fully recrystallized which causes to increase the ductility of the composite. The hot forging with 50% thickness reduction was carried out at 250 ºC - 450 ºC on AA7075/5 wt.% TaC composite. The results revealed that highest microhardness, yield strength, ultimate tensile strength was obtained at 250 ºC while the highest % elongation of was obtained at 450 ºC. The % El. was slightly better than HT sample. The hardness, strength and % El. of HFed composite were higher than undeformed S-4 composite and HT sample. The enhancement in strength is attributed to dislocation strengthening while improvement in % elongation is ascribed to grain boundary strengthening. The hot forging with 50% thickness reduction was further carried out at 250 ºC - 450 ºC followed by T6 temper on AA7075/5 wt.% TaC composite. It was observed that there is remarkable enhancement in microhardness, yield strength, and ultimate tensile strength obtained at 250 ºC while the highest % elongation of 26.4±2.6 was obtained at 450 ºC. The T6 temper simultaneously provides the strength and ductility of the composite. The enhancement in strength is attributed to precipitation strengthening while improvement in % elongation is ascribed to grain boundary strengthening. The AA7075/5 wt.% TaC composite was subjected to cryo-forging with 30% thickness reduction under FC, AC, and WQ cooling mediums. The results shows that the highest microhardness, yield strength, ultimate tensile strength was obtained by CFed (WQ) while the highest % elongation of was obtained by CFed (FC). The improvement in strength is credited to dislocation, grain boundary and precipitation strengthening. It was observed that the hardness and strength attained by CFed (WQ) with 30 % thickness reduction was higher than HFed composite with 50% thickness reduction. The AA7075/5 wt.% TaC composite was subjected to cryo-forging under FC, AC, and WQ cooling mediums followed by T6 temper with an objective to increase the strength and ductility of the composite. It was revealed that the highest microhardness, yield strength, and ultimate tensile strength was obtained by CFed (WQ + T6) while the highest % elongation of 21.4 ± 1.4 was achieved by CFed (FC + T6). The enhancement in strength of CFed (WQ + T6) is ascribed to grain boundary and precipitation strengthening while ductility increased in CFed (FC + T6) is credited to dislocation annihilations. It was revealed from the results that hardness and strength attained by CFed (WQ + T6) sample with 30 % thickness reduction was higher all HFed samples and CFed samples.