Mechanical Properties And Hot Deformation Behavior Of Extruded And Die Casting AT72 Magnesium Alloy

Mg-Al-Sn series are promising magnesium alloys for application in automotive industry due to better creep resistance, castability and lower cost. AT72 alloy is a structural alloy and protential deformation alloy with excellent mechanical properties. The initial properties analysis and hot deformation behavior of extruded and die-cast AT72 magnesium alloy were investigated in this thesis.Both extruded and die-cast material exhibit well mechanical properties, where the better yield stress of 160 MPa obtained at extruded direction, and the best performance of 235 MPa extension strength and 9.30% elongation appeared at T4 die-cast condition. The temperature and time for aging hardening of extruded and die-cast material were 200℃/12 h and 200℃/24 h with a hardness increment of 20%.Hot compression test simulated on Gleeble 3500 under the temperature of 250-450℃, and the strain rate of 10-3-10s-1. For the true stress-true strain curves, extrusion condition expressed a higher peak stress than casting condition. The concave character on hot deformation cures result from the twinning-dominated deformation mechanism. The constitutive equation was conducted through hyperbolic sine function and the activation energy of as-cast and extrusion conditions were calculated to be 158 kJ/mol,164 kJ/mol, respectively, slightly higher than pure magnesium 135 kJ/mol. n value was calculated to be 5.18 and 5.36, which indicated a climb-controlled deformation.Hot processing maps were developed at the true strain of-0.1,-0.2,-0.3,-0.5 and -0.8. For extrusion condition, we found four suitable regions for hot deformation. There are low speed regions, 250-320℃/10-2.5-10-3,360-420℃/10-2.5-10-3; medium speed region, 350-400℃/10-2-1 s-1; and high speed region,400℃/10. For as cast material, it appear two safe regions, where are medium and low speed region 350-450℃/10-1-10-3; and high speed region 350℃/10. Complete dynamic recrystallization was found above. Finer grain microstructure was expected by die-cast and deformed under 350℃,10 s-1; while extruded material deformed under 400℃ 10 s-1 could obtain weaker texture microstructure.The AT72 magnesium alloys have a texture of [0001] crystal direction parallel to compressed direction after hot deformation, where higher strain rates have a lower max texture and higher DRX percent. Compared with extruded material, die-cast condition show a smaller and finer grain size, due to a well distributuion of second phase. The twin and slip deformation zones were analysis from hot deformation curves, micro structure and lnZ value.