PROCESSING-STRUCTURE-PROPERTY CORRELATION IN ULTRAFINE GRAINED 6061 ALUMINIUM ALLOY

Abstract

The thesis presents the processing-structure-property correlation in ultrafine grained Al 6061 alloy. The coarse grained alloy with grain size in several microns is processed through various thermo mechanical routes to develop ultrafine grained (UFG) structure in it. To ensure the UFG structure in thermo mechanically processed alloy, electron back scattered diffraction (EBSD), Transmission electron microscopy (TEM), X-ray diffraction (XRD) techniques are employed. The mechanical properties of UFG material are evaluated through hardness and tensile testing at room temperature and compared with its coarse grained counterpart. In the course of grain refinement, the precipitates with nanoscale features imparting strengthening effect to the coarse grained matrix undergo several physical and chemical changes. Ultimately, it influences the mechanical properties of the UFG alloy. The precipitation evolution in UFG material is monitored through differential scanning calorimetry (DSC) and TEM. The main objective of the present study is to understand the effect of microstructure developed through various thermomechanical routes on enhancement of strength and ductility of Al 6061 alloy. Al-6061 alloy was processed through cryorolling (CR) up to ~92% thickness reduction. Effect of low temperature ageing and annealing at high temperatures on microstructure and mechanical properties was investigated. The results evidenced that low temperature ageing has resulted simultaneous increment in the strength and ductility of the UFG Al 6061 alloy. EBSD and TEM investigations on microstructure show the UFG structure in the Al alloy is stable up to annealing temperature, 250 °C. The role of second phase particles on retaining the microstructural stability is emphasized. The TEM studies revealed that second phase Mg2Si particles are effectively pinning the grain boundaries due to the Zener drag effect. Owing to the presence of heterogeneities in the material, a duplex structure was observed upon annealing at temperatures, 150 to 200°C. At high temperatures, annealing leads to coarsening of the second phase particles, which reduces Zener drag effect by precipitate particles. Abnormal grain growth was observed after annealing at high temperatures (300°C). To investigate the effect of dynamic ageing along with cryorolling on the mechanical properties and microstructural evolution of Al 6061 alloy, the alloy was subjected to cryorolling followed by warm rolling at 145 °C (WR- Cryorolling+Warm rolling). It was found that the combination of cryorolling and warm rolling is more effective than cryorolling alone. The WR samples showed a significant improvement in tensile strength (376 MPa) and partial ii iv improvement in ductility (5%) as measured from tensile testing. The DSC, XRD and TEM investigations have shown that the superior properties of the alloy obtained through WR were attributed to formation of fine precipitates during warm rolling. DSC results of WR material have revealed that WR material undergoes low temperature ageing to enhance the strength further. The effect of ageing on WR samples was investigated and the optimum ageing condition was found to be 125 °C for 45 hours, which results in improved tensile strength of (406 MPa) and good tensile ductility (10%). The tensile strength of WR + peak aged (WR + PA) sample (406 MPa) was found to be 11% more than that of cryorolled + peak aged (CR + PA) sample (365 MPa). During peak ageing treatment, the strength has been retained by pinning of dislocations through nanosized precipitates generated during warm rolling and it has been improved further by precipitation of the remnant dissolved second phase in the matrix. The thermal and microstructural stability of the WR alloy is found to be better than CR alloy. It was also observed that, by performing warm rolling at high temperatures (~ 200 °), a good combination of strength and ductility in Al 6061 can be obtained. To obtain maximum strengthening contribution from precipitation strengthening in UFG precipitation hardenable Al 6061 alloy, it is essential to understand the evolution and kinetics of precipitates. Hence, a detailed investigation has been made on the effect of CR and WR on precipitation evolution by using DSC and TEM. Hardness measurements were made to substantiate the TEM and DSC results. A significant change has been observed in precipitation sequence even at very low percentage of deformation (5%). The sequence of nanoclusters and major strengthening precipitate phases (β"/β') were altered with increasing deformation. At low temperatures (<150 °C), two distinct cluster peaks were observed upon cryogenic deformation. At high temperatures (>150 °C), the deformed alloy has shown the absence of β′ formation as compared to undeformed coarse grained material. TEM investigation of the deformed alloy revealed bimodal distribution of precipitates with both very fine and coarse structures. Predeformation of the alloy led to the simultaneous formation of β″ and β′ precipitates in both CR and WR materials. Activation energies of the different precipitates in deformed alloys were calculated by adopting Kissinger analysis. It is observed that the activation energy for the formation of various precipitates the alloy with very low percentage of deformation (5%) is decreased as compared to undeformed bulk alloy. However, with increasing percentage of deformation beyond the 5%, the activation energies are significantly higher as compared to the undeformed bulk alloy. With increasing degree of deformation, the diffusivity of the solute iii v species gets decelerated and therefore reaction rate becomes sluggish as evident from the microstructural changes observed in the alloy. The solute atoms are trapped by the tangled dislocations. To understand the effect of microstructure on precipitation behaviour and mechanical properties of the Al 6061 alloy deformed up to larger strains under cryogenic temperature, multidirectional forging (MDF) at liquid nitrogen temperature has been conducted. The solution treated and water quenched alloy was subjected to cryoforging up to cumulative strains of 1.8, 3.6 and 5.4. The evolution of UFG structure with increasing cumulative strains was investigated through EBSD and TEM. The results indicate that microstructure with an ultrafine grain morphology (average size 250 nm) was achieved through cryogenic forging up to cumulative strain of 5.4. Tensile strength has increased from 180 MPa to 388 MPa with 4.5% percentage of elongation to failure. Tensile test results revealed that MDFed material after ageing led to significant improvement in work hardening and its tensile ductility. Strengthening of the matrix through various mechanisms has been quantified with the existing models to estimate the yield strength of the as forged and peak aged material. The precipitation hardening response in UFG material is found to be 35% lower than that of the coarse grained material as observed in the present work. The reasons could be; i) the presence of high dislocation densities has reduced the coherency of the precipitates with the Al matrix, ii) deformation led to transformation of β" (fine needle shaped phase) to β' (rod shaped phase). With increasing MDF strain at liquid nitrogen temperature up to 6, evolution of new UFGs through dynamic recrystallization was observed through EBSD. However, due to non-uniform distribution of the strain in the MDF samples, structural inhomogeneity from center to the outer edges of the sample was observed. The structural inhomogeneity in the samples was investigated through hardness measurements at various cumulative strains such as 1.8, 3.6 and 6. The results suggest that with increasing strain, the degree of inhomogeneity can be reduced.