Study On The Fluidity And Hot Tearing Behavior Of Mg-Gd-Y-Zr Alloys

Mg-Gd-Y-Zr as one of the high-performance magnesium-heavy rare earth alloys is very attractive for aerospace military and racing automotive industries because of their high specific strength and good thermal stability. When casting products of Mg–Gd–Y–Zr alloys are developed to be complex and thin in structure, hot tearing and bad fluidity are becoming a more serious problem than ever before. HCS (Hot Cracking Susceptibility), the solidification behavior and fluidity of Mg-Gd-Y-Zr Magnesium alloys were tested by constrained rod method, hot crack-ring method, hot cracking testing system and the length of fluidity samples. The effects of purification, pouring temperature and Gd, Y and Zr additions on HCS and fluidity of Mg-Gd-Y-Zr Magnesium alloy were also investigated. Finally, the hot tearing mechanism of Mg-Gd-Y-Zr Magnesium alloy was studied.Shock shrinkage and a relevant shock stress occurred, which was caused by eutectic transformation at the end of solidification. The appearance of shock stress was the inducement for hot tearing of Mg–Gd–Y–Zr alloys.Effects of eight different refiners on HCS of Mg-10Gd-3Y-Zr magnesium alloy have been studied by constrained rod method. The results show that, JDMJ +5%YCl3 and RJ-6+5%GdCl3 refiners can significantly decrease the HCS of Mg-10Gd-3Y-Zr magnesium alloy, the HCS of the sample that without purification was 8, and the HCS of the sample that purified by JDMJ +5%YCl3 and RJ-6+5%GdCl3 refiners was 0, which greatly decreased the HCS of Mg-10Gd-3Y-Zr magnesium alloy.Effects of pouring temperature on HCS of Mg-10Gd-3Y-Zr magnesium alloy have been studied by constrained rod method and found that the HCS of Mg-10Gd-3Y-Zr magnesium alloywas increased with the pouring temperature increased. HCS was increased with Gd and Y additions increased, and the HCS was decreased with Zr additions increased and then was increased. When Zr additions reached 0.2%, the HCS was 0.HCS (hot cracking susceptibility) of Mg-10Gd-3Y-Zr Magnesium alloy was evaluated with three methods including constrained rod method, hot crack-ring method and hot cracking testing system. With these methods, HCS of AZ91D Magnesium alloy was also measured and compared with that of Mg-10Gd-3Y-Zr Magnesium alloy. HCS of Mg-10Gd-3Y-Zr Magnesium alloy is 8 and that of AZ91D Magnesium alloy is 32 using constrained rod methods, hot cracking power of Mg-10Gd-3Y-Zr Magnesium alloy is 340N and that of AZ91D Magnesium alloy is 40N which were tested by hot cracking testing system. Critical fracture diameter of Mg-10Gd-3Y-Zr Magnesium alloy is 53mm and AZ91D Magnesium alloy is 48mm using crack-ring method. The study show that the HCS of AZ91D alloy is higher than that of Mg-10Gd-3Y-Zr Magnesium alloy.Effects of eight different refiners including JDMJ series and RJ-6 series on the fluidity of Mg-10Gd-3Y-Zr magnesium alloy have been studied. The results show that, JDMJ +5%YCl3 and RJ-6+5%GdCl3 refiners can significantly improve the fluidity of Mg-10Gd-3Y-Zr magnesium alloy, the length of the fluidity sample that without purification was 780mm, and the length of the fluidity sample that purified by JDMJ +5%YCl3 and RJ-6+5%GdCl3 refiners were 1270mm and 1113mm, which greatly decreased the fluidity of Mg-10Gd-3Y-Zr magnesium alloy. We found that the content of inclusions was reduced significantly after the purification by the JDMJ +5%YCl3 and RJ-6+5%GdCl3 refiners.Effects of casting techniques on the fluidity of Mg-10Gd-3Y-Zr magnesium alloy have been studied. The results show that the fluidity of Mg-10Gd-3Y-Zr magnesium alloy was improved with the pouring temperature increased, the length of the fluidity sample was increased from 595 mm to 995 mm, when pouring temperature increased 690℃to 770℃. But the increasing extent is becoming less when the pouring temperature reached a certain temperature. The fluidity was increased with Gd and Y additions increased, and the fluidity was increased with Zr additions increased and then was decreased. When Zr additions reached 0.2%, the length of the fluidity sample was 1050mm.