Fabrication of Premium Magnesium Castings by Semi-solid Forming Process

我们的研究小组旨在建立半固态成形镁合金高质量的基本制造工艺，包括高性能原材料的制造工艺。结果表明：(1)在“寻找流动性好、拉伸性能好的镁合金”中，高Zn和Al含量导致低温结晶的化合物增多，共晶反应产生的热量很大，在Al+Zn含量分别为20mass%和15mass%的合金中，在465℃和535℃时可以实现半固态成形。这些温度比传统镁合金的压铸温度低100℃以上。少量Ca或Sr的加入显著细化了共晶化合物，这有助于半温度下的流动性，从而获得合适的伸长率和与传统压铸合金相当的强度。(2)采用切屑脉冲电流烧结（PECS）法制备了更多的镁基多孔材料。以AZ91D、AM60B合金锭和挤压AZ31合金锭为原料，改变共晶化合物的数量和半固态温度范围。在AZ91D和AM60B合金多孔试样连接良好的部位，Mg-Al化合物结晶，而AZ31多孔试样连接良好部位的铝含量富集。这意味着在PECS过程中，非平衡凝固的Mg-Al化合物优先被快速加热重熔。因此，连接件的显微组织的差异是由合金中铝含量的不同引起的。采用PECS工艺制备连接良好的多孔材料的条件是使用具有大量低熔点化合物和大半固态温度范围的合金，从而在压缩试验中吸收能量大。在1.3 ~ 37.3MPa范围内，多孔材料的平台应力在很大程度上取决于孔隙比。(3)利用聚焦超声在线监测镁合金熔体，针对液体回收过程中夹杂物的原位检测，发现使用喷金属缓冲棒和频率为5MHZ的聚焦超声，即使在镁合金熔体中，也能清晰地检测到铁块反射的脉冲回波。(4)在“基于初生结晶球化机理生产半固态铸造用镁合金”中，采用了一种新开发的工艺路线，在凝固试样前，在熔融金属中插入作为初生结晶成核点的棒材，旋转使初生结晶细化并球化。初生晶体倾向于漂浮在熔融金属的上表面，因为固体密度低于液体密度。因此，使用靠近熔体底部的倒t形棒搅拌熔融金属，然后在从电阻到淬火的温度区间内减少凝固时间。结果表明，该工艺成功制备出初生晶粒均匀且呈球形的铸锭，平均晶粒尺寸小于73μm

Our research group aims to establish elementary fabrication processes for high qualification of magnesium alloys using semi-solid forming process, including that for high performance raw materials. The results are summarizes as follows(1)In "Search for magnesium alloys having good fluidity and adequate tensile properties", the results show that high Zn and Al contents lead to an increase in the amount of compounds that crystallize at low temperatures and the heat generated from the eutectic reactions becomes so large that the semi-solid forming is possible at 465℃ and 535℃ in the alloys containing Al+Zn contents of 20mass% and 15mass%, respectively. These temperatures are more than 100℃ lower than die-casting temperature for conventional magnesium alloys. The addition of small amount of Ca or Sr remarkably refines the eutectic compounds, which are useful for fluidity at semi-temperatures, resulting in suitable elongation and comparable strength with conventional die-casting alloys.(2)M … More agnesium based porous materials are made by Pulse-Electric-Current-Sintering (PECS) method using cut chips. Commercial ingots of AZ91D and AM60B alloys and extruded AZ31 alloy were selected as raw materials in order to change th amount of eutectic compounds and the range of semi-solid temperatures. In well-joined parts of porous samples of AZ91D and AM60B alloy, Mg-Al compounds crystallize, while the aluminum content at the joined part of AZ31 porous sample is concentrated. This means that the non-equilibrium solidified Mg-Al compound is preferentially remelted by rapid heating during PECS. Thus the difference in the microstructures of the joined parts is caused by the different aluminum contents of the alloys. The condition for making the well-joined porous materials using PECS processi s to use alloys that have large amount of low melting compounds and large semi-solid temperature range, resulting in large absorbing energy in compressive test. The plateau stresses of the porous materials investigated in this study largely depend on pore ration in the range from 1.3 to 37.3MPa.(3)In "in-line monitoring of magnesium alloy melt by using focused ultrasound to aim at an in-situ detection of inclusions during liquid recycling process, it is found that an usage of metal-sprayed buffer rod and Focused Ultrasound with a frequency of 5MHZ enable to clearly detect reflected pulse echo from iron block even in a molten magnesium alloy.(4)In "production of magnesium alloys for semi-solid casting on the basis of spheroidizing mechanism of primary crystals", a newly developed processing route is applied in order to refine and to spheroidize primary crystals by insertinga rod, which acts as a nucleation site for primary crystals, into the molten metal and rotating it before solidified specimen. The primary crystals tend to float on the top surface of the molten metal because solid density is lower than the liquid. THerefore, the molten metal is stirred using an inverted T-shaped rod near the bottom of the melt and followed by decreasing the solidification time at the temperature interval from recalescance to quenching. As a result, this process successfully produces an ingot having homogenous fine and spherical primary crystals with an average grain size of 73μm Less