Strain enhanced strengthening during thermomechanical ageing of an automotive aluminium alloy

Abstract

Thermomechanical treatments as a means of strengthening Al alloys have been of particular interest in recent years. Fabrication of aluminium alloys that can outperform conventional steels are especially attractive for applications in automotive industries. This study aims to gain a fundamental understanding of the effect of metal forming processes on the mechanical properties in shaped aluminium components for optimal performance in automotive applications. This thesis presents a study on the application of a novel thermomechanical process in a 6xxx series Al-Mg-Si-Cu alloy system comprised of 3 key steps: artificial preaging, cold working and artificial final ageing. A process designed to attain improved mechanical performance from the alloy, particularly strength-ductility optimisation. The current work aims to develop an optimised treatment recipe and characterise pertinent microstructural changes affording these mechanical responses. Vickers Hardness and Tensile Testing were used as the main procedures of obtaining mechanical properties. Optical Microscopy, Electron Dispersive Spectroscopy, Scanning Electron Microscopy, Electron Backscattered Diffraction and Transmission Electron Microscopy were used as analytical tools of microstructural evolution in prepared sample specimens. The understanding garnered will be applied in relevant scenarios to generate desired manufacturing induced properties. Flat strips of 3mm in thickness were prepared through DC casting and hot extrusion followed by immediate water quenching, retaining the super saturated solid solution state (SSSS). Experimental results from hardness and tensile tests illustrate enhancements in material properties subject to the preaging treatment at 150o C and 170o C and the subsequent low strain deformation of 4% and 8% through either rolling or stretching. These strength increments are induced by significant increases in dislocation density resulting from the interactions with the precipitate particles formed in the preaging step, which were characterised by EBSD and TEM analysis. These precipitate-dislocation complexes in turn promoted heterogenous nucleation and growth of secondary phase precipitate particles in the final ageing step of the treatment. Thermomechanical conditions in this study investigated changes in material response due to parameters such as ageing conditions and strain mode. It was found that strain mode exhibited considerable impact on the final properties of the material. Based on EBSD data, stretching, and rolling plastic deformation elicited different changes in crystallographic texture components as compared to the as-extruded material. In particular, the development of cube {001}<100> and goss {110}<001> texture components led to differences in the final mechanical properties. Further, selected preaging condition determined the degree of dislocation storage enhancement. The alloy samples preaged to a further point in the precipitation sequence exhibited a greater increase in the dislocation density than those less preaged conditions due to the development of atomic clusters, Guinier-Preston zones and β’’ phase particles. Evidence also suggests that the dislocation structures formed at these varying preaging stages resulted in notably different final ageing kinetics. Optimisation of the novel thermomechanical treatment in the study showed that preaging temperature differences also played a role in determining final mechanical properties. Preaging at 170o C presented a greater hardening response with the optimum thermomechanical treatment recipe: an artificial preaging at 170o C for 4 hours followed by a plastic deformation of 8% stretching strain at room temperature and a final artificial ageing at 170o C for 4 hours. The Optimal thermomechanical treatment route investigated in this research exhibited mechanical properties that were superior those achieved in other studies using similar alloys. Microstructural changes and interactions between crystallite defects discovered in this study improved the understanding of alloy strengthening through application of a thermomechanical process.