Study On Hot Tearing Susceptibility Of Mg-2Zn-3Y Alloy With A1 Addition

Mg-Zn-Y alloys are now widely used in engineering production.In order to further optimize its properties,the conventional method is to add external refiners to the alloy.In this paper,the Al element is added to Mg-Zn-Y alloy to form a compound with the effect of refining grain,so as to improve the alloy and reduce the tendency of hot tearing.The hot tearing susceptibility（HTS）of Mg-2Zn-3Y-xAl（x=0wt.%,0.5wt.%,1wt.%,2wt.%,3wt.%）and Mg-2Zn-（3+0.5x）Y-xAl（x=0wt.%,2wt.%,3wt.%）alloys were evaluated using Clyne-Davies’model.By using the double thermocouple test method,some important parameters were collected during solidification process of Mg-2Zn-3Y-xAl and Mg-2Zn-（3+0.5x）Y-xAl alloys.Such as the solidification path,dendrite interference solid fraction,dendrite coherency temperature,temperature range of fragile zone and characteristic temperature during process of solidification,etc.The microstructure of Mg-2Zn-3Y-xAl and Mg-2Zn-（3+0.5x）Y-xAl alloys were observed by ZEISS optical microscope,scanning electron microscopy,transmission electron microscope and X-ray diffraction,especially the influence of the microstructure and morphology of hot tearing regions and the type and quantity of the second phase on the hot tearing tendency of the alloy.The curves of solidification shrinkage stress with temperature（or time）of Mg-2Zn-3Y-xAl and Mg-2Zn-（3+0.5x）Y-xAl alloys based on the“T”type hot tearing permanent-mold were collected.The experimental results show that the hot tearing susceptibility of Mg-2Zn-3Y-xAl alloys decreases first and then increases with the Al content increases from 0wt.%to 3wt.%.This is due to the fact that dendrite coherency temperature,temperature range of fragile zone and the temperature of the hot tearing initiation decreases first and then increases,the feeding ability and dendrite interference solid fraction increases first and then decreases.During this process,the LPSO phase gradually decreases,and the Al₂Y phase increases.The effect of grain refinement is better than that of the strengthening phase to reduce the alloy.The grain size is the finest and the hot tearing susceptibility coefficient reaches the lowest when the Al content is2wt.%.It could be seen from the microscopic observation that the hot tearing behavior of Mg-2Zn-3Y-xAl alloys have a great correlation with the amount of low melting eutectic phases.When the Al content gradually increases,the liquid film formed by the amount of low-melting eutectic structure are thicker and feeding channels continuously overlap which reduces the hot tearing susceptibility of Mg-2Zn-3Y-xAl alloys.The role of low melting point phase is mainly an intergranular feeding effect on formed separated dendrites and the main mechanism is the feeding of separated dendrites theory and liquid film theory.Meanwhile,the mechanical properties test results show that the Mg-2Zn-3Y-2Al alloy has the highest ultimate tensile strength,yield strength and elongation rate and the Mg-2Zn-3Y-3Al alloy has the highest microhardness in the five alloys.The hot tearing susceptibility of Mg-2Zn-（3+0.5x）Y-xAl alloys decreases first and then increases with the Al₂Y content increases from 0wt.%to 1.5wt.%.The hot tearing susceptibility coefficient reaches the lowest when the Al₂Y content is 1wt.%.This is due to the fact that the alloy has the best feeding ability,the lowest dendrite coherency temperature,the highest dendrite interference solid fraction,the lowest temperature range of fragile zone,the lowest temperature of the hot tearing initiation and the best effect of grain refinement.Meanwhile,the mechanical properties test results show that the Al₂Y content is 1wt.%has the highest ultimate tensile strength,yield strength and elongation rate and the Al₂Y content is 1.5wt.%has the highest microhardness in the three alloys.