熔模精密铸造新型Mg-Zn-Al合金的显微组织及力学性能

采用熔模精密铸造法制备了Mg-9wt%Zn-3wt%Al(简写为ZA93,下同)合金.利用扫描电镜,X-射线衍射(XRD),冷却曲线热分析技术(CA-CCA)研究了ZA93合金的显微组织和凝同路径.同时,研究了熔模铸造ZA93合金在铸态、T4(固溶处理)和T6(固溶+时效处理)状态下的显微组织和力学性能.结果表明,ZA93合金在凝固过程中依次析出α-Mg相、准晶I相和共晶ε-MgZn相,对应温度分别为602、342和332℃;T6处理后,在合金晶内和晶界处析出细小强化相,提高了合金强度.     

砂型铸造Mg-6Al-xZn合金凝固行为及晶粒尺寸

采用双电偶热分析技术和SEM表征了Mg-6Al-xZn合金(简称AZ6x合金,x=0,2,4,6,质量分数,%)在砂型铸造过程中的凝固行为和显微组织;采用背散射电子衍射(EBSD)分析对合金的晶粒尺寸进行定量表征.利用Pandat热力学软件计算了合金的平衡截面相图、非平衡Scheil模型凝固过程,以及枝晶生长抑制因子(growth restriction factor,或称为Q值).结果表明,在AZ6x合金的砂型铸造凝固过程中,AZ60合金中只有非平衡凝固的g-Mg17Al12,而AZ62~AZ66合金的铸态组织中除了g-Mg17Al12相,还出现了F-Mg21(Al,Zn)17相,并且随着Zn含量的增加,g-Mg17Al12相减少而F-Mg21(Al,Zn)17相增多.热力学计算结果表明,AZ60~AZ64合金中g-Mg17Al12相和F-Mg21(Al,Zn)17相在一定温度下能够完全固溶到a-Mg中,而AZ66合金中的F-Mg21(Al,Zn)17相在任何温度下都不可能完全固溶.研究结果还表明,Zn含量高的合金具有高的Q值、小的晶粒尺寸及低的枝晶相干点固相分数fs DCP;并讨论了Q值、晶粒尺寸与fs DCP的关系.     

Correlation of recalescence with grain refinement of magnesium alloys

The grain refinement of Mg-Al based alloys with carbon inoculation was investigated by a computer-aided cooling curve analysis(CA-CCA) system.The results show that carbon inoculation decreases the main parameters of the recalescence regime during the initial stage of solidification.These parameters include the recalescence undercooling(Δθ_(rec)),duration of recalescence (t_(rec)),and liquid peak parameter(LPP) which is firstly introduced into magnesium alloys.The resultant grain size decreases with incre...     

AZ6X镁合金的糊状区特性表征

采用冷却曲线热分析法和连续扭矩测试技术对AZ6X(X=0,2,4,5,6)合金系列的糊状区特性进行了表征及分析.结果表明:随着Zn含量的增加,AZ6X合金初晶形核第一特征温度Tn1、初晶形核第二特征温度Tn2以及枝晶干涉点温度Tch均逐步降低,(Tn1-Tn2)温差逐渐增加,(Tn2-Tch)温差变化不大;AZ6X合金的枝晶干涉点固相分数fs-ch约为0.27-0.35,其中AZ60和AZ66合金的fs-ch较高,AZ62合金的fs-ch最低;随着Zn含量的增加,AZ6X合金的枝晶压紧点温度TPk逐渐降低,枝晶压紧点剪切强度τPk先升后降;AZ6X合金的枝晶压紧点固相分数fs-pk约为0.5-0.6,其中AZ60和AZ66合金的fs-pk较低,AZ62合金的fs-pk最高;随固相分数的增长,AZ62合金的糊状区剪切强度τ增长最为缓慢;冷却曲线热分析法和连续扭矩测试技术的结果具有可比性.     

Microstructure and mechanical properties of melt-conditioned high-pressure die-cast Mg-Al-Ca alloy

A new shape casting process, melt-conditioned high-pressure die-casting （MC-HPDC） was developed. In this process, liquid metal was conditioned under intensively forced convection provided by melt conditioning with advanced shear technology （MCAST） unit before being transferred to a conventional cold chamber high-pressure die-casting （HPDC） machine for shape casting. The effect of melt conditioning was investigated, which was carried out both above and below the liquidus of the alloy, on the microstructure and properties of a Mg-Al-Ca alloy （AZ91D＋2%Ca （mass fraction）, named as AZX912）. The results show that many coarse externally-solidified crystals （ESCs） can be observed in the centre of conventional HPDC samples, and hot tearing occurs at the inter-dendritic region because of the lack of feeding. With the melting conditioning, the MC-HPDC samples not only have considerably refined size of ESCs but also have significantly reduced cast defects, thus provide superior mechanical properties to conventional HPDC castings. The solidification behaviour of the alloy under different processing routes was also discussed.