The Impact Of Trace Elements On The Microstructure And Properties Of Mg-al-zn Magnesium Alloy

Magnesium alloy could offer significant reduction for automotive, especially used for the powertrain components. With a low cost and a proper combination of creep resistance, mechanical properties at room temperature and elevated temperature and low cost, Mg-Al-Zn-Si series alloy had much predominance in automotive components. With a high melting temperature, Mg2Si has benefits for mechanical properties at elevated temperature. However, the addition of Si to magnesium alloys often resulted in poor mechanical properties due to the chinese script type and polygonal shaped type Mg2Si particles. It was necessary to control morphology, volume fraction, distribution and size of Mg2Si to improve the mechanical properties. In this study, effects of Al, Si, Ca, Sr on microstructure and mechanical properties of Mg-Al-Zn-Si series alloy and effects of Sm microstructure and mechanical properties of Mg-6Al-1Zn alloy were investigated using optical microscopy, inductively coupled plasma,X-ray diffraction, scanning electron microscopy,hardness test and tensile test.The evolution of Mg2Si was observed. The results were as follows:(1) The contents of both Si and Al influenced the morphology of Mg2Si.The results showed that chinese script type Mg2Si particles formed, when the addition of Si was small. With increasing the addition of Si or Al, polygonal type Mg2Si appeared and grew up, and at the meantime,Chinese script type Mg2Si decreased.(2) The relationship between contents of Si or Al and microstructure of Mg-Al-Zn-Si alloy was revealed.There was a linear relationship between the Al content and the Si content in the critical transition zone of chinese script type and polygonal type Mg2Si particles. Chinese script type and polygonal type Mg2Si resulted in a reduction in mechanical properties.(3) The suitable additions of Al and Si were 6% and 0.6%, respectively. In Mg-6Al-1Zn-1.0Si alloy, just chinese script type Mg2Si was found,but both chinese script type Mg2Si and polygonal shaped Mg2Si were observed in Mg-6Al-1Zn-1.0Si alloy.(4) The effects of Ca and Sr on microstructure and tensile properties were determined. The proper addition of Ca and Sr were 0.3% and 0.1%,respectively.Both Ca and Sr had a good refinement effect on Mg2Si phase.Chinese script type Mg2Si and polygonal shaped Mg2Si would be modified to～5μm sized Mg2Si particles and 40～50μm sized polygonal Mg2Si,respectively,after microalloyed. Element of Ca and Sr coated on the surface of Mg2Si phase. Excessive of Ca and Sr would lead to the formation of CaMgSi phase and Al4Sr phase,respectively.Stick shaped CaMgSi would become to scissor shape, while Al4Sr would become to needle shape.(5) A combination addition of 0.3%Ca and 0.1%Sr had a better refinement effect on Mg2Si phase.Such improved microstructure of Ca or Sr modified alloys resulted in large improvement in tensile properties at room temperature and 150℃.And the alloy had a better performance after solution treatment.(6) After the addition of Sm to Mg-6Al-1Zn alloy, granular Al2Sm phase was found.Al2Sm particles greatly improved tensile property at 150℃.With increasing the addition of Sm,Al2Sm increased and grew up,and at the same time Mg17Al12 decreased.