((1. 蘭州理工大學 甘肅省有色金屬新材料省部共建國家重點實驗室，蘭州 730050；2. 蘭州理工大學 有色金屬合金省部共建教育部重點實驗室，蘭州 730050))
摘 要: 采用光學顯微鏡和掃描電鏡對AZ91D鎂合金組織進行分析，研究AZ91D鎂合金在半固態(tài)觸變成形過程中的組織特征與二次凝固行為。結(jié)果表明，成形件的充型過程是按先底部后上部的順序填充。觸變成形件中心線部位的組織中初生相含量比邊緣部位的高，產(chǎn)生宏觀上的液相偏析。沿高度方向上，中心線處的初生相顆粒尺寸從上部到底部，先減小后增大。二次凝固過程包括3個階段：第1階段是初生α-Mg相生長；第2階段是液相內(nèi)部重新形核并生長成細小的等軸晶；第3階段是剩余的液相發(fā)生共晶反應(yīng)形成共晶體。共晶反應(yīng)按離異方式生長，共晶組織中的Mg17Al12在初生相顆粒周圍形成“暈圈”。
關(guān)鍵字: AZ91D鎂合金；半固態(tài)；觸變成形；二次凝固；離異共晶
（(1. State Key Laboratory of Gansu Advanced Non-ferrous Metal Materials, Lanzhou University of Technology, Lanzhou 730050, China;2. Key Laboratory of Non-ferrous Metal Alloys of Ministry of Education, Lanzhou University of Technology, Lanzhou 730050, China)）
Abstract:Microstructural characteristic and secondary solidification behavior of the remaining liquid in semi-solid AZ91D alloy slurry in the thixoforming process were investigated by the experiments and microstructure analysis using optical microscope (OM) and scanning electron microscope (SEM). The results show that the mould-filling process of the product is from the bottom to the top orderly. There is macro-segregation of liquid between the center and edge of the products whose primary phase fracture at the center is higher than that at the edge. The size of primary phase particles in the formed product decreases firstly, and then increases along the axial (central) line from header to bottom. The secondary solidification occurs for the remaining liquid inside the die cavity involving: the secondary primary dendrites first directly developed from the surface of the primary particles without re-nucleation; then the residual liquid solidified by re-nucleation/growth, and finally eutectic reaction. The eutectic reaction is carried out with the way of divorced growth model so that a halo of Mg17Al12 forms around the primary phase particle.
Key words: magnesium alloy AZ91D; semi-solid state; thixoforming; secondary solidification behavior; nucleation; grains growth; divorced eutectic
