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The objective of the present study is to evaluate the hot tearing tendency based on the Clyne and Davies model by evaluating the critical times which can be obtained using a newly developed method. A method to determine the critical times required to calculate the crack susceptibility was presented based on the measurement results with Al–Si alloys, and the method to calculate the crack susceptibility coefficient was presented. In the newly developed method named “Signal intensity method,” signals were generated by tapping the edge of a waveguide which is immersed in molten and solidifying sample and the critical solid fractions were obtained from the signal intensity change. The conventional thermal analysis was also performed simultaneously and the corresponding critical points were identified. The method shown in the present study will enable the determination of the crack susceptibility coefficient with higher accuracy.

Magnesium is the lightest of all structural metals. As such, and combined with good mechanical and physical properties, it forms the basis for commercial alloys that have found successful use in a wide variety of applications. The work presented in this thesis has aimed to investigate the characteristics of microstructures and defects in cast magnesium alloys, as well as how they influence the mechanical properties. The microstructural features of binary Mg-Al alloys with various Al concentrations were investigated under a range of cooling conditions. The experiments were done by using a gradient solidification technique to achieve homogeneous and nearly defectfree magnesium alloy specimens, through which the isolated effect of microstructural parameters on the mechanical properties could be evaluated. Some models were developed to describe the relationships between the microstructure and mechanical properties in the Mg-Al alloys. In this work, it was also found that the mechanical properties of cast Mg components were affected by some non-structural factors. It is proposed that mould constraints in the die during cooling can have a significant effect on the yield strength of die-cast magnesium components. Investigations were also done concerning the characteristics of the microstructure, especially the so called pre-solidified crystals, as well as two common defects, segregation bands and hot-tears, in die-cast Mg components. The effects of process parameters on the formation of the microstructure and these defects were thoroughly investigated in some high pressure die cast commercial components. Applying a high intensification pressure was found to have a strong effect in minimizing the segregation band formation, however at the same time promoting the formation of hot tears. A new theory describing the mechanisms for segregation band formation is proposed. The theory was confirmed by simulations of solidification process during die casting, and by experimental v