Study On Hot Tearing Susceptibility Of Mg-Zn-Y Alloy With High Zn/Y Ratio

Magnesium alloys have been used widely in automotive and aerospace and other project applications because of their low density,high specific strength,high specific stiffness.However,magnesium alloys casting products involves thin wall and complicated structrue,hot tearing becomes one of the bottleneck for commercial applications.Therefore,it is of great significance to study the hot tearing susceptibility and its formation mechanism for improving the range of magnesium alloys in practical applications.Based on the Clyne-Davies model,the hot tearing susceptibility of Mg-5(Zn+Y)-0.5Zr(Zn/Y=1,1.5,3,6;Zn+Y=5wt.%)and Mg-xZn-yY-0.5Zr(Zn+Y=2,3,5,8 wt.%;Zn/Y=3)alloys was predicted.By using the double thermocouple test method,some important characteristic parameters were collected during solidification process of Mg-5(Zn+Y)-0.5Zr and Mg-xZn-yY-0.5Zr alloys,such as the dendrite coherency temperature,characteristic temperature of primary nucleation and solid fractions at different solidification time,etc.The curves of solidification shrinkage stress with temperature(or time)of Mg-5(Zn+Y)-0.5Zr and Mg-xZn-yY-0.5Zr alloys based on the “T” type hot tearing permanent-mold were collected.The microstructure and morphology of hot tearing regions of the alloys were observed by X-ray diffraction and scanning electron microscopy.The relationship between different Zn/Y ratios or total Zn and Y contents and hot tearing susceptibility of the alloys was revealed.The solidification time and temperature of the hot tearing were determined.The behavior and characteristics of the hot tearing of Mg-5(Zn+Y)-0.5Zr and Mg-x Zn-y Y-0.5Zr alloys wew investigated.The micro-mechanism of hot tearing of Mg-Zn-Y-Zr alloy was explored.The experimental results show that the hot tearing susceptibility of Mg-5(Zn+Y)-0.5Zr alloys increases first and then decreases with the Zn/Y ratio increases from 1 to 6,and the hot tearing susceptibility coefficient reaches the highest when the Zn/Y ratio is 3.This is due to the fact that the Mg-3.75Zn-1.25Y-0.5Zr alloy has the widest solidification temperature range,the highest dendrite coherence temperature,the largest ?Tc in the fragile zone,the larger grain size,and the higher temperature of the hot tearing initiation and the feeding ability is poor.For Mg-xZn-yY-0.5Zr alloys,with the total content of Zn and Y increases,the dendrite coherence temperature of alloys decreases in turn,the feeding ability increases in turn,the initial time when hot tearing occurs is delayed,the temperature of hot tearing initiation decreases in turn and the initial solid fraction increases,the grain size decreases gradually,the thickness of the liquid film increases gradually,the strength of the alloys increases and the ability to resist the solidification contraction stress increases in turn,so the hot tearing susceptibility of Mg-xZn-yY-0.5Zr alloys decreases in turn.When the total content of Zn and Y is low,the amount of low-melting eutectic phases is small,resulting in a high hot tearing susceptibility of Mg-xZn-yY-0.5Zr alloys and the main mechanism for forming hot tearing is intergranular bridge theory.When the total content of Zn and Y is high,the liquid film formed by the low-melting eutectic phase is thicker and the intergranular bridging strength is increased which reduces the hot tearing susceptibility of Mg-xZn-y Y-0.5Zr alloys.The mechanism is liquid film theory and intergranular bridge theory.