Study On Microstructure, Properties And Welding Behavoiur Of The Fine-grain Mg-5Zn-1Mn-xSn Magnesium Alloy Sheets

ZM(Mg-Zn-Mn) series magnesium alloys is a kind of high Zn content and high strength magnesium series. It has the advantage of low price, excellent performance and easy to smelt. However, their developments are severely limited by the coarse grains, low plasticity, poor heat resistance and high thermal cracking tendency.Grain refinement and alloying are good way to improve the mechanical properties, meanwhile, using laser welding can also effectively solve the problem of difficult to weld. In this paper, the effect of minor Sn addition on the microstructure and mechanical properties of ZM51-based alloys were systematically investigated. At the same time, the weldability of Mg-5Zn-1Mn-xSn sheets was also investigated. The research results showed that:(1) Minor Sn addition can effectively refine the grain size of the as-cast ZM51 alloys and change the compositions, distribution and morphology. The ZM51-xSn alloys mainly consists of the a-Mg, Mg7Zn3 and Mg2Sn. The size of Mg7Zn3 phases at the grain boundaries decreases and the number of granular Mg2Sn phases increases with the increasing Sn addition.(2) The Sn addition can refine the original grain size and reduce the stacking fault energy of magnesium alloys, thus it can effectively promote the dynamic recrystallization (DRX) during the HSRR process, refine the recrystal grain size and enhance the mechanical properties of alloy sheets. Rolled plates show the optimal mechanical properties, the tensile strength and elongation of 337 MPa and 21%, respectively under 350? with the Sn addition of 0.6wt%.(3) There are two kinds of liquation phenomena in the PMZ of the ZM51-0.6Sn alloy sheets. One is the liquation network along grain boundaries, and the other is molten pool. The former is associated with the liquation of base metal and the segregation-induced liquation, while the latter is involved with the liquation of the residual second phases. However, the liquation of base metal and the segregation-induced liquation are the main liquation mechanism in the PMZ.(4) The crack sensitivity of welding joints was investigated by the Houldcroft test. The results shows that the welding joints appear low crack sensitivity, there are no obvious cracks in the welding joints. Some microscopic cracks and cavities are found in the PMZ and these cracks and cavities do not develop into macroscopic cracks because of the fine grain size, low heat input, fine and dispersed low-melting-point second phases.(5) The Sn addition has some influence on the micro structure and properties of the welding joints. With the Sn addition increasing, the number of second phases increases and the size of grains decreases. The degree of liquation also decreases with the Sn increases. At the same time, the ultimate tensile strength and elongation also increases with the Sn addition.