Research On Solidification Contraction Of Mg-Zn Based Magnesium Alloys

Magnesium alloys have attracted people’s attention because of their low density, high specific strength, strong damping performance and good shielding performance. However, magnesium alloys are lack of grades. Developing on new kinds of alloys is an important aspect of research on magnesium alloys in the world. Zn is a very effective alloying element which can generate ageing strengthening and solution strengthening except Al element. So developing on Mg-Zn based magnesium alloys is one of the most important ways on magnesium development. So research on solidification contraction of Mg-Zn based magnesium alloys and deeply understanding their hot tearing behaviors and mechanism can provide theoretical guidance for developing new alloys, solidification characteristics evaluation and technology process.This paper has researched on the influence of Zn element in Mg-Zn based magnesium alloys and the third group elements Zr, Mn, Cu and Y. By acquiring data of stress in the process of blocked solidification contraction and displacement in the process of free linear contraction, time-temperature-stress curve and time-temperature-displacement curve are auquired during the process of solidification. In this study the experimental data are analyzed combining with the microstructure of the last part of solidification, morphologies of hot tearing fracture surfaces and component on crack surface by EPMA.The result shows that, (1) rapidly accumulating stress coefficient "k" in the process of blocked solidification contraction, its fraction solid interval and maximum linear contraction rate can be the characterization of hot tearing susceptibility. And the hot tearing susceptibility of Mg is higher than that of Al. (2) For Mg-Zn series alloys, the hot tearing susceptibility decreases with Zn content increasing and Mg-10wt.%Zn shows the cold tearing susceptibility. (3) For Mg-Zn-Cu series alloys, the hot tearing susceptibility of Mg-2Zn-lCu is higher than that of Mg-2Zn-3Cu with a hot brittle fracture. The hot tearing susceptibilities of Mg-6Zn-xCu and Mg-lOZn-xCu alloys are all low and with Cu content increasing the hot tearing susceptibility further decreases.(4) For Mg-Zn-Zr series alloys, the grain refines and the hot tearing susceptibility decreases with addition of Zr element. And the hot tearing susceptibility gradually decreases with Zr content increasing. Mg-10Zn-0.4Zr alloy and Mg-10Zn-0.5Zr alloy shows the cold tearing susceptibility. (5) For Mg-Zn-Mn series alloys, the hot tearing susceptibilities of Mg-2Zn-xMn are low and the hot tearing susceptibility of Mg-2Zn-lMn is higher. When Zn content reaches at 6wt.%, the hot tearing susceptibility of Mg-6Zn-0.4Mn is higher. Mg-6Zn-1Mn shows cold tearing susceptibility. When Zn content reaches at 10wt.%, Mg-10Zn-0.4Mn shows cold tearing susceptibility and Mg-10Zn-1Mn alloy shows a good comprehensive performance with low hot tearing susceptibility and low cold tearing susceptibility. (6) For Mg-Y series alloys, hot tearing susceptibility tendencies follows the "λ" shape curve with Y content increasing and Mg-2Y alloy shows the cold tearing susceptibility. For Mg-1Y alloy, its hot tearing susceptibility becomes low with the addition of Zn element and the hot tearing susceptibility gradually decreases with Zn content increasing. (7) In the phase of dendrite separation, the low melting point phase solidifies at the last time consisting of liquid membrance that covers the hot crack fracture surface. And the liquid membrance becomes thicker with the increase of Zn content and Mn content. The extent of the tearing liquid membrance and interdentritic "bridge" on the edge of hot tearing fracture is higher than that in the center. But the extent of solute concentration in the center of the hot tearing fracture is higher than that on the edge of the hot tearing fracture. These phenomenons show that solute redistribution in the process of solidification formed the constitutional supercooling zone destroying the stability of solid-liquid interface, which forms tearing liquid membrance and interdentritic "bridge" and the hot crack extends from edge to center. At the time if the solid skeleton is mature and can be fed by residual liquid phase adequately, the propagation of hot crack can be inhibited or even be healed.