Effect Of Compression Deformation On The Microstructures Of AZ61

The magnesium alloy has unsatisfactory thermoformability owning to its lattice features,leading to higher processing cost in the wrought magnesium alloy than the cast magnesiumalloy and furthermore restricting the extensive application of wrought magnesium alloy.Nowadays, it is one of the focuses through the optimization of process to improve thethermoformability of wrought magnesium alloy at home and abroad. So far, however, a betterunderstanding of the mechanisms of the microstructure evolution in the deformation process isneeded to improve the formability of magnesium alloy. In fact, the current research focusesmainly on composition optimization and alloy redesign. Compression deformation is one of themost important deformation ways in the process of the magnesium alloy production. And it isalso an important method to understand the mechanical response and microstructure evolutionof magnesium alloy. It is theoretically important and practically valuable to systematicallyinvestigate the influence of compression deformation on the microstructures of magnesiumalloy.AZ61magnesium alloys are one of the commonly used Mg-Al magnesium alloys withhigh Al content. The strength of the AZ61alloy is higher and the thermoformability is worsethan the AZ31alloy because there are many second phases distributed in the AZ61. Thecharacteristics of second phases （type, size, distribution, quantity, shape, etc.） are one of the keyfactors governing the formability and usability. The recently research indicate that there isclose correlation between the second phases and the recrystallization behaviors. Furthermore,the recrystallization processes determinate the usability in magnesium alloys directly. Therefore,the key points are how to control the fragment and dissolution of the second phases in thedeformation process as well as the characteristics of second phases in the as-cast magnesiumalloy. Unfortunately, few researchers have paid attention to these aspects. Thus, it is difficult tooptimize the existing processing technology and develop a new processing technologyaccording to the existing data and theories. Therefore, the systematic investigation on the secondphase characteristics in the as-cast magnesium alloy, understanding the second phase behaviorsof fragment and dissolution during the compression deformation process and building therelatively theoretical model are essential for optimizing existing processing technology,improving the formability and promoting the commercial application of the AZ61magnesiumalloy. And these data and theoretical model are also suit for other wrought magnesium alloy. There is an important guiding significance in promoting the overall development of wroughtmagnesium alloys.Optical Microscopy （OM）, Scanning Electron Microscopy （SEM） and Energy DispersiveSpectrometry （EDS）, X-ray Diffraction （XRD）, Image analysis processing software （Image Pro）and Thermo-Cacl software etc. are used to characterize the size, the number density, the areafraction, the distribution and the morphology of the non-equilibrium eutectic Mg₁₇Al₁₂and theAl-Mn phase in the semi-continuous casting. Based on the above characters, the compressedmicrostructure at room temperature has been observed and the second phase fracture behaviorand the interaction mechanisms between the second phases and twinning have also beenanalyzed. Then the morphological feature of the as-cast predeformation sample compressed atelevated temperature has been analyzed compared to the as-solution predeformation sample.And the differences between the predeformation sample and the as-cast sample compressed atthe different temperature have been distinguished. The dissolution of the eutectic Mg₁₇Al₁₂atelevated temperature has also been observed. The results of the study show that thepredeformation at room temperature can improve the nuclear uniformity of recrystallization andweaken the instability tendency of the matrix at elevated temperature. It is also found that therecrystallization grain size of the as-cast sample containing a bulky eutectic Mg₁₇Al₁₂phase isfiner than the as-solution sample after compressing at elevated temperature. The abovephenomenon suggested that the bulky Mg₁₇Al₁₂phase can improve the twinning content amongthe internal grains to supply more nucleation sites for recrystallization during hot deformationprocess and avoid the instability caused by non-uniform recrystallization. Compared to theas-solution sample, the as-cast sample can get finer recrystallization grain. So the compressionof as-cast sample can improve the strength and plasticity of alloy. And the predeformation canintroduce the crack into the eutectic Mg₁₇Al₁₂phase. It can make the fracture of eutecticMg₁₇Al₁₂phase become finer and the distribution more diffuse after a few minutesheat-preservation treatment to increase the coordination deformation ability of matrix. At thesame time, the defect produced at room temperature and the force work can accelerate thedissolution speed of Mg₁₇Al₁₂. And it is possible to avoid the disadvantages from the bulkysecond phase after deformation.