Dynamic Crystallization: An Influence on Degree of Prior Deformation and Mechanical Strength of 6063 Aluminum Alloy
This research is aimed at investigating the influence dynamic solidification of melts on degree of mechanical deformation and mechanical strength of 6063 aluminum alloy. Cylindrical samples of 14mm diameter and 140mm long were die cast following two techniques – vibration and static. Prior deformation via forging was imposed on each solidified sample to achieve 7%, 14%, 21% and 28% thickness reductions respectively for each casting technique. Average deformation load, average hammer velocities and the average energy absorbed were recorded. Tensile properties of each sample were studied via the use of Monsanto tensometer. Mechanical agitation of mould and its content increased the machinability of the alloy even at higer pre deformation. This was justified by the failure of the 28% reduction sample cast on static floor during machining to a tensile piece. The energy absorbed during deformation influences the tensile strength of the material. This increases with increase in percentage deformation except for 28% reduction whose magnitude was lower than that subjected to 21% reduction; vibrated samples possessed superior properties. From results obtained, vibrating a sample and subjecting to 21% pre-deformation possessed the best tensile strength.
Effect of Prior Deformation on Tensile and Vibration Fracture Resistance of Friction Stirred 5052 Alloy
In this study, Al-2.5Mg alloy was prepared with different cold rolling reductions, and then friction stir processing (FSP) was performed to investigate the effect of prior microstructure variations on friction stirred materials. The experimental results indicate that the FSP specimens not only had better tensile properties but also better vibration fracture resistance, which would be expected from the microstructural refinement which resulted from the phenomenon of dynamic recrystallization. The stress-elongation curves of all specimens showed the serrated yielding. The higher grain boundary introduced by FSP could hold the mobile dislocations long enough to let Mg atoms form atmospheres around them. Consequently, the effect serration magnitude was more significant on the specimens which were given FSP. This can be ascribed to the grain refinement and the resulting increase of the boundaries which is the main source of obstacles. In addition, the vibration fracture resistance of the FSP specimens shows that the duration of stage I decreases with increasing the prior deformation rate before FSP. Meanwhile, the prior deformation samples possessed better vibration fracture resistance under the same initial deflection amplitude (6.5 mm). Based on the observed microstructures, this can be attributed to a large number of retained dislocation tangles introduced by prior cold rolling, even after FSP, which improved the crack propagation resistance and reduced the crack propagation rate. The experimental results confirmed that vibration fracture resistance can be quantitatively correlated with the crack tortuosity value which corresponds to the crack propagation behavior. [doi:10.2320/matertrans.47.2504]
Effect of Mg content on vibration fracture resistance of friction stirred Al-Mg alloys
In this study, the effect of Mg content on friction stirred Al–Mg alloy is discussed, and the experimental results indicate that grains of parent plate which are refined through FSP (friction stir processing) are effective in enhancing tensile properties of materials through FSP and improving the vibration fracture resistance of friction stirred specimens. It should be noted that the deterioration of vibration fracture resistance can be recognized as Mg content increases, the grain size actually has been refined by FSP. From crack propagation result, an intergranular crack propagation feature can be recognized in the specimen with higher Mg content, and it is reasonable to suggest that the precipitation of β phase plays an important role in the deterioration of vibration fracture resistance. However, all cross-sectioned microstructures at stir zone show that distinct finer grains which can be acquired by dynamic recrystallization during FSP. That meaningful improvement of uniform elongation on friction stirred specimens can be recognized is not only resulted from controlling the grain size factor, but also microstructural characteristics factor which are resulted from high temperature shear deformation.