Investigations On Hot Tearing Susceptibility And Mechanism Of Mg-Zn-(Al) Alloys

Magnesium alloys have attracted great attention for applications in automotive and aerospace industries because of their low density, high specific strength and high specific stiffness. Copmparsion with Al alloys, magnesium alloys have excellent casting propreties. So a lots of magnesium products are made by casting process. However, magnesium alloys show a high susceptibility to hot taring during the solidification, especially if casting in a permanent mold. So investigations on hot tearing behaviors and mechanism have very important theoretical and pratical significance for widening its application.For hot tearing susceptibility of magnesium alloys, most of previous inspections were carried out only for Mg-Al series, but seldom concurn about Mg-Zn series. Althouth the effects of age-harding is more obvious then Mg-Al alloys, Mg-Zn binary alloys have coarse micro structure and more sensitive to the hot tearing. All of these limit its applications. So investigation on the hot tearing susceptibility of Mg-Zn binary system and Mg-Zn-Al ternary system help to exclude alloy compositions which are sensitivity to the hot tearing susceptibility during alloys design. And it can optimize the alloys properties. So investigations on hot tearing susceptibility of Mg-Zn-(Al) alloys are not only theoretical but also has important value of engineering applications.The present work first evaluates the hot tearing susceptibility (HTS) of Mg-Zn binary system and Mg-Zn-Al ternary system using thermodynamic calculations based on Clyne-Davies'and Ramseyer's model. And then the influences of content of Zn and Al, initial mold temperature on crack volumes of Mg-Zn binary system and Mg-Zn-Al ternary system are obtained by using the "T" type hot tearing permanent-mold. The hot tearing behaviors of the Mg-Zn binary system and Mg-Zn-Al ternary system are analysed. In order to reliably predict the formation and evolution of hot tearing in casting by numerical simulations, the hot tearing behaviors of Mg-Zn-(Al) alloys are predicted using Flow3D software based on the calculation of temperature field and incremental thermoelastic stress model. It is also essential that the solidification path and Von Mises stress under different initial mold temperatures of the investigated alloys can be accurately described. The influences of solid fractions with variation temperature, local solidification temperatures, temperature gradients and Von Mises stress on the hot tearing susceptibility are investigated. The microstructure of hot tearing regions are observed by optical microscopy, scanning electron microscopy. The hot tearing mechanism of Mg-Zn binary system and Mg-Zn-Al ternary system is analysed and summarized.The results show that, the hot tearing susceptibility tendencies is as a function of Zn conent follows the "λ" shape curve. The hot tearing susceptibility first increases with increasing Zn content, reaches the maximum at about 1.5wt.% Zn and then decreases with further increasing the Zn content, increases with Zn content increasing. But in the Mg-Zn-Al ternary system, the predicted hot tearing susceptibility in Mg-Zn-Al tenary system is the contour plot with variation of Zn and Al content and as function of Zn and Al content. The maximum in the hot tearing susceptibility is found at a composition ranging in around 1.5wt.%Zn and 0-0.75wt.%Al, and another peak in the HTS can also be observed at about 3wt.%Zn and 0.5wt.%Al.The predicted results correlated very well with the cracking volume measurement results. With increasing the initial mold temperature, the crack volume decreases at the same compositons. Under the same initial mold temperatures, the crack volume increases with increasing Zn content, reaches a maximum and then decreases with further increasing Zn content. The typical "λ" shape curve is well reproduced in the current tendency of crack volume. The solidification contraction force recorded at 300℃measured by the force-temperature-time curves can supply some useful information about the hot tearing susceptibility. It is inversely proportional to the crack volume, and aggrement with crack volume measurement. The crack propagation rate increases with increasing the Zn content, reaches the maximum and then decreases with the same initial mold temperature. At the same content of Zn, the crack propagation rate decreases with the increasing the initial mold tempertarue. For Mg-Zn-Al ternary system, at low content of Zn, the crack volume has two peaks with variation of Zn content; at high content of Zn, the crack volume decreases with increasing of Zn content. The numerical simulation results show that, the hot tearing is easily occurs at the postion of "hot spot", i.e. the position of casting near the junction between the sprue and the horizontal rod. The different cooling rate of investigated alloys lead to different temperature gradient and stress concentration. With increasing the initial mold temperature, the temperature gradient and Von Mises stress at postion of "hot spot" decrease, i.e. hot tearing susceptibility decrease. The numerical simulation results are excellent agreement with the experimental data. The theory of interdendritic bridging plays a important role in hot tearing formation mechamism with the low Zn and Al content. The number of Interdendritic bridging is not enough to resist the solidification concentration stress.The interdendritic bridging is broken under solidification concentration stress and will be left behin on the fracture surface. As a result, hot crack forms, which leads to the hot tearing occurs. The bridges are hardly to observe because of they are broken. The mechamism of hot tearing formation when the alloys have high content of Zn and Al, is the results of the liquid film theory and solidification feeding theory played. Due to the thick liquid films around the grain boundary and the separation of dendrites can be feed by remainding liquid, the separated dendrites will be healed and there is no hot cracking formation.