Fabrication of Premium Magnesium Castings by Semi-solid Forming Process

点击翻译按钮获取中文摘要

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