Thixomolding 공정으로 제조된 Mg-xAl-yZn계 마그네슘 합금의 미세조직과 기계적 물성에 미치는 Ti 첨가 영향

Effects of Ti Addition on Microstructure and Mechanical Properties of Mg-xAl-yZn Magnesium Alloys by Thixomolding Process

Abstract

The microstructural features and relative room temperature mechanical properties were investigated in various compositions of Mg-xAl-yZn alloys by thxiomolding process. The microstructure was composed of ${\alpha}$-Mg particles and mixture of ${\alpha}$-Mg and ${\beta}-Mg_{17}Al_{12}$ eutectic phase. The amount of ${\beta}-Mg_{17}Al_{12}$ eutectic phase in mixture was increased with increasing Al and Zn contents without grain refinement. After adding Ti content, however, the morphology of ${\beta}-Mg_{17}Al_{12}$ eutectic phase transformed from net-like to discontinuous shape and the average grain size reduced. To determine the relationship between microstructural features and their mechanical properties, a tensile test was performed at room temperature. As a result, it was found that the mechanical properties were improved in all of Ti contained alloys due to increased elongation and the mechanisms are discussed in terms of microstructural evolution.

Fig. 1. Binary equilibrium phase diagram of Al-Mg system [4].

Fig. 2. XRD patterns of AZ72, AZ81 and AZ82 alloys in as-cast state, respectively.

Fig. 3. Optical micrographs of (a) AZ72, (b) AZ81 and (c) AZ82 alloys in as-cast state.

Fig. 4. Results of higher magnification optical microscopy observation obtained from (a) AZ72, (b) AZ81 and (c) AZ82,respectively.

Fig. 5. Average grain size of (a)AZ72 and (b)AZ72+Ti alloys measured from higher magnification.

Fig. 6. SEM observations of β-Mg₁₇Al₁₂ morphology obtained from (a) AZ72, (b) AZ72+Ti, (c) AZ81 and (d) AZ82, respectively

Fig. 7. Variation in tensile properties of Mg-xAl-yZn alloys according to change of Al and Zn amount in ascast state.

Fig. 8. Effect of Ti addition on tensile properties of Mg-xAl-yZn alloys in as-cast state.

Table 1. Chemical compositions of magnesium alloys used in this study

Table 2. Summary of room temperature tensile properties obtained from Mg-xAl-yZn and Mg-xAl-yZn+0.4Ti (wt.%) alloys in as-cast state