Friction stir processing (FSP) has been proposed by applying friction stir welding, FSW, as a method for controlling the microstructures to improve mechanical properties. In this study, microstructure of a 7075 aluminum alloy subjected to multipass FSP, MP-FSP, are assessed to obtain fundamental knowledge on MP-FSP. The MP-FSP has been applied to 7075 aluminum alloy plates with T6 and O tempers, and microstructural characterization has been made by means of optical and scanning electron microscopies together with EDX and EBSD analyses. In addition, micro hardness measurement has been carried out. From microstructural observation, a new zone, PBZ (Pass Boundary Zone), has been discovered between stir zones, SZs. The PBZ is composed of two types of (fine-equiaxed and coarse-elongated) grains, where the coarse elongated grain contains many sub-grains. Hardnesses in PBZ are decreased and increased in T6 and O, respectively, by MP-FSP, and are intermediate between those in BM (base metal) and SZ in both tempers.

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The purpose of this study is to develop the testing method in order to evaluate the critical strain for solidification cracking of the heat-resistant magnesium alloys during die-casting. The critical strain for solidification cracking was obtained by the U-type hot cracking test using in-situ observation method. Ductility curves at elevated temperature for tentative heat-resisting magnesium alloys and commercial ones such as Mg–Al–Ca–RE, Mg–Al–Ca–Mn, Mg–Al–Ca–Sn, AE44 and AZ91D were obtained. The solidification cracking susceptibilities of these magnesium alloys were evaluated by the critical strain rate for temperature drop (CST) using these ductility curves. According to these results, the solidification cracking susceptibilities these alloys decrease in order of Mg–Al–Ca–Sn, AZ91D, Mg–Al–Ca–Mn, Mg–Al–Ca–RE, and AE44.

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This study was performed to develop the predicting method of solidification cracking during die-casting of tentative heat-resistant magnesium alloys and commercial ones such as Mg–Al–Ca–RE, Mg–Al–Ca–Mn, Mg–Al–Ca–Sn, AE44 and AZ91D. Both high temperature ductility curve and strain near cracking area of each alloy at liquidus-solidus temperature range were required in order to predict the solidification cracking. High temperature strain was calculated by a casting simulation code,“ProCAST”. High temperature ductility curve of each alloy was measured by the experiment. Comparing with this high temperature strain and ductility at liquidus and solidus temperature range, it was confirmed whether solidification cracking occurs or not in each magnesium alloy. Finally, this predicting method was verified using simple shape of die-casting. Consequently, occurrence of solidification cracking in the simple shape of die-casting in all magnesium alloys used can be predicted by this predicting method.

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