Relationship between microstructure and cold deformation behavior of aluminum alloys using thermodynamic modeling method

Abstract

The current work emphasizes establishment of relationship between microstructure and cold deformation behavior of aluminum-copper -magnesium alloys. Aluminum-coppermagnesium alloys with varying Cu% and Mg% were casted and undergone cold deformation after homogenization, and their microstructures were examined using optical microscope. Using CALPHAD method, phases developed for different levels of Cu and Mg were modeled in JMatPro software package. It has been found that the prediction performed in equilibrium condition matches closely to phase fraction simulation. It can predict several extra phases in comparison to the equilibrium simulation. Image analysis by ImageJ also confirms this finding experimentally. Finally, the effects of deformation were studied by measuring the hardness of those alloys. From this study it was possible to predict the weight fractions of phases formed during solidification and homogenization using CALPHAD modeling. EDX results confirmed the formation of Al2Cu phase (white phase) and Al7Cu2M and Mg2Si phases (black phases) as modeled in CALPHAD. With increasing amount of copper and magnesium in the binary aluminum-copper alloy system, fraction of Al2Cu, Al7Cu2M, Al2CuMg and Mg2Si phases increased which increased hardness values. However, those additions lowered liquidus and solidus temperatures of alloys as investigated from DTA and modeling. The phase fractions of different alloys obtained from CALPHAD were verified using image analysis techniques. Image analysis data showed a convincing conformation of phases that formed during solidification and through solid state diffusion. Homogenization and solution treatment had a negative effect on the hardness values of investigated aluminum alloys due to dissolution of Al2Cu and Mg2Si phases in to the aluminum matrix. On the other hand, deformation increased the number of dislocations by interactions of dislocation during deformation and other defects, which caused an enhancement of hardness values. Deformation also causes microstructural changes by destroying the necklace like shape of Al-Cu-Mg phases initially obtained in as-cast and homogenized alloys. For this reason, with larger amount of deformation, the increment of hardness may not be very significant. After comparing the effects of all the processing parameters i.e. homogenization, deformation and alloy addition on hardness, it was revealed using ANOVA modeling, that magnesium addition and amount of deformation affects hardness of Al-Cu-Mg alloys to a large extent compared to addition of copper and homogenization.