Study On The Mg-Si Alloys Prepared With The Fine Si Powders As The Raw Material

The Mg-Si alloys have important application value and broad application prospect because of the superior heat resistance, low cost and the simple melting process. However, the Mg₂Si phase in the as-cast Mg-Si alloys exhibits the thick Chinese script type or the dendrite like, which has a detrimental effect on their mechanical properties. In order to refine the Mg₂Si phase in the as-cast Mg-Si alloys, the effect of Si material size, melting process and rolling process on the microstructures and mechanical properties of the Mg-Si alloys. The main conclusions were drawn as follows:(1) The primary Mg₂Si phase was less in the Mg-2Si alloy that was prepared with the Si blocks(about 5mm²0mm) as the raw material and the pouring temprature and the holding time of 700℃and 30min respectively.The sectional dimension of the eutectic structure was more than 400μm and the length of the eutectic Mg₂Si rod was more than 400μm. The primary Mg₂Si particles in the Mg-2Si alloy were characteristics of the fine polyhedral shape(≤10μm) and the length of the eutectic Mg₂Si rod was less than 100μm with the addition of fine silicon powders (≤48μm) in the melt at 700℃and the holding time for 3min. A lot of the fine primaryα-Mg grains and the primary Mg₂Si particles were precipitated during solidification since many Si-rich regions and Si-poor regions occurred in the melt when the fine Si powders were added at the lower temperature with shorter holding time. Both of them formed as a dense skeleton, divided the residual melt and prohibited the growth of the eutectics.(2) The primary Mg₂Si phase in the Mg-5Si alloy that was prepared with the Si blocks(about 5mm) or the fine Si powders (270-1165μm) as the raw material and the pouring temperature of 780℃was characteristic of the thick dendritic(about 300μm), less in volume and a non-uniform distribution. It was characteristic of the fine polyhedral shape(≤50μm), more in volume and a uniform distribution with the fine Si powders(270-1165μm) as the raw material and the melting temperature of 680℃. After adding the fine Si particles into the melt below the liquidus temperature of the Mg-5Si alloy, the positive temperature gradient and the negative Si concentration gradient were formed at the melt around the Si particles during the dissolution of the Si particles and plenty of fine primary Mg₂Si nuclei were generated in the melt around the Si particles. After pouring, the growth time for the primary Mg₂Si phase was shortened due to the smaller temperature range between the melting temperature and the melting point of magnesium and thus the size of the primary Mg₂Si phase was reduced obviously. (3) The rod-like Mg₂Si phase in the Mg-2Si alloy could be refined further by large strain rolling process at elevated temperature, and the mechanical properties of the Mg-2Si alloy were improved. For the as-cast Mg-2Si alloy, its tensile strength and elongation were 142MPa and 5.5% respectively, while they were increased to 252MPa and 8.9% respectively after rolling at 450℃with a deflection of 80%.