热处理对真空压铸Mg-7Al-2Sn合金组织性能的影响

Effects of heat treatment on microstructure and mechanical properties of Mg-7Al-2Sn alloy prepared by vacuum die-casting

利用光学显微镜、扫描电子显微镜,能谱分析仪、X射线衍射分析及差热分析研究了T4和T6热处理对真空压铸Mg-7Al-2Sn(AT72)合金组织和力学性能的影响。结果表明,真空压铸使得AT72合金组织更加致密,进而通过热处理可以进一步提高合金的力学性能。特别是T4固溶处理(400℃×20 h)后,晶粒尺寸变化较小,Al、Sn元素固溶到基体中产生固溶强化;同时,由于Mg17Al12相分解,使得高熔点的Mg2Sn颗粒相的析出强化效果更加明显。T4处理后的AT72合金的抗拉强度、屈服强度和伸长率达到276 MPa,202.6 MPa和10.6%,其比压铸态合金分别提高了18.2%,7.0%,24.7%。T6(400℃×20 h+200℃×15 h)处理后由于脆性相Mg17Al12的非连续析出以及Mg2Sn相粗化,使得合金的强度和塑性均有所下降。由于耐热相Mg2Sn的存在,提高了Mg17Al12相的开始熔化温度,使得AT72合金表现出比商用AZ91合金具有更好的高温力学性能。

The influences of solution treatment( T4) and aging treatment( T6) on microstructure and mechanical properties of Mg-7Al-2Sn( AT72) alloy prepared by vacuum die-casting( VDC) were studied by means of optical microscope( OM),scanning electron microscopy( SEM),energy dispersive spectrometer( EDS),X-ray diffraction( XRD) and differential scanning calorimetry( DSC). The results show that the mechanical properties of AT72 alloy are improved by heat treatment because the microstructure of the alloy processed by VDC is dense. The grain size of the alloy has a little variation,and the alloy is strengthened due to Al and Sn dissolved into matrix after T4( 400℃ × 20 h) heat treatment. The precipitation strengthening effect of particle Mg2 Sn phases is more apparent due to the dissolution of Mg17Al12. The ultimate tensile strength,yield strength and elongation of the AT72-T4 alloy reach to 276 MPa,202. 6 MPa and 10. 6%,which are increased by 18. 2%,7. 0% and 24. 7% compared with those of die-cast alloy,respectively. The strength and ductility of the AT72-T6 alloy are reduced because of the discontinuous precipitation of brittle Mg17Al12 phases and the coarsening of Mg2 Sn phases. The start of melting temperature of the Mg17Al12 phase is increased due to the existence of Mg2 Sn heat-resistant phases,which make the elevated temperature mechanical properties of the AT72 alloy better than that of AZ91 alloy.