A Study On Hot-cracking Sensitivity Of Mg-Zn-Y And Mg-Al-Ca Alloys

Magnesium alloys have been quickly developed in these years for their advanced properties,such as light weight,high specific strength and favorable environmental properties.However,magnesium alloys show a high susceptibility of tearing in the solidification process due to the wide range of solidification temperature,poor heat transfer ability and large amount of solidification shrinkage.So investigations on hot tearing behaviors of magnesium alloys have a vital theoretical significance and engineering application value.The effects of Y content of the Mg-Zn-Zr-Y series magnesium alloys on hot-cracking susceptibility are investigated in this paper and the effects of Ca addition into the Mg-Al-Zn-Ca-Mn series magnesium alloys as well.The experimental facility is the stress and shrinkage test system in two-phase region.The maximum of stress gradient and shrinkage of various alloys are the indicators of hot-cracking susceptibility.Metallographic microscope and scanning electron microscope are used.We take use of the metallographic microscope to observe the microstructure of the last part of solidification,and the scanning electron microscope to observe the surface topography of part alloys’ tensile fracture.This article analyzes the causes of the casting crack from the perspective of alloy forced in the solidification and raises a new theory named Stress-strength Ratio.This theory insists that the ingot will probably develop hot crack when the stress approaches to its corresponding tensile strength in the last stage of solidification.Combing the observation of metallographic microstructure,morphologies of hot tearing fracture surfaces with data curves to analyze the results,we can further understand the mechanism of hot crack.The experimental results indicate that for Mg-6Zn-0.5Zr-xY(x=0.5,1,1.5,2)series magnesium alloys,the hot-cracking susceptibility gradually increases with Y content increasing.And a balanced amount of Y can refine the grains of alloy.However,after Y content reaches 1%,the grains can’t continue to be refined.For Mg-1Al-1Zn-xCa-0.2Mn(x=0.5,1,3,5,7)series alloys,the hot-cracking susceptibility decreases with the addition of Ca content when it’s less than 3%.But the hot-cracking susceptibility basically remains unchanged after the Ca content reaches 3%,And the precipitate in grains increases all the time.As for Mg-3Al-1Zn-xCa-0.2Mn(x=0.5,1,3,5,7),small amounts of Ca can coarse grains.The addition of Ca can decrease hot-cracking susceptibility when Ca content is no more than 5%.However,the hot-cracking susceptibility increases when the content of Ca reaches 7%.