| Mg-6Gd-3Y-0.5Zr(wt.%,GW63K)magnesium alloy has great potential applications in transportation,aerospace and defense industries due to its excellent tensile properties and creep resistance.To date,most studies concerning this alloy are focusing on the microstructure evolution and mechanical properties.In these studies,samples are expected to be ideally perfect without any defect.However,a certain amount of porosity resulting from shrinkage during solidification is inevitable for magnesium cast components.Although shrinkage porosity is a bottleneck for the application of cast magnesium alloys,few studies in this aspect have been carried out.In order to promote the lightweight application of GW63K magnesium alloy,a series of investigations about shrinkage porosity have been performed in this dissertation,including prediction of shrinkage porosity,characterization of shrinkage porosity and its effect on the tensile properties,attempt to reduce shrinkage porosity by hot isostatic pressing and investigation of the influence of heat treatment on microstructure and tensile properties.This study helps to the improvement or modulation of tensile properties in GW63K magnesium alloy.The Niyama criterion and the dimensionless Niyama criterion are used to predict the distribution and amount of shrinkage porosity in GW63K magnesium plates.Instead of the Niyama evaluation condition recommended by commercial ProCAST software and precious studies(fraction solid of 0.9),the most appropriate Niyama evaluation condition is suggested as fraction solid of 0.7 for the investigated alloy.The corresponding threshold Niyama values are qualitatively determined as 0.44 and 0.3 ?0.5s0.5mm-1 for gravity-cast and low-pressure sand-cast plates,respectively.For the dimensionless Niyama criterion,it is capable to quantitatively predict the shrinkage porosity volume fraction in low porosity-containing regions,but not in high porosity-containing regions in which the calculated value is obviously lower than the actual value.The relationship between tensile properties and shrinkage porosity has been analyzed both qualitatively and quantitatively.The qualitative analysis shows that both ultimate tensile strength and elongation-to-failure decrease with porosity level increasing.The relationship between tensile properties and porosity level helps to component quality inspection by radiograph.The quantitative analysis shows that the critical local strain model is testified not applicable for GW63K magnesium alloy to calculate the tensile properties.This is probably because of heat treatment efficiency degradation introduced by porosity and interdendritic ligaments distributed throughout the porosity region which is at variance with the assumption in the critical strain model.Instead,strong quantitative correlations between tensile properties and porosity area fraction are obtained through an empirical approach.Hot isostatic pressing(HIP)is applied to GW63K magnesium alloy to reduce shrinkage porosity,thus,to enhance the integrity and reliability of castings.During HIP process,shrinkage porosity is closed by grain compatible deformation and subsequent diffusion across the bonding interface,forming metallurgical bonding interfaces.The amount of initial shrinkage porosity is the key factor for shrinkage porosity closure(i.e.,HIP is more effective in low porosity-containing regions),and the alloying element may have marginal effect on the utility of HIP in Mg alloysThe influence of heat treatment on tensile properties in gravity casting GW63K magnesium alloy is also studied.The tensile yield strength has little dependence on the solution treatents.The solution time/temperature should be as short/low as possible when Mg24(Gd,Y)5 phase is completely dissolved,and the best solution treatment is 4 h at 475 ?.Precious solution treatment variation has marginal effect on hardness evolution.In contrary,aging temperature will significantly influence the age hardening behaviour and tensile properties.In the age hardening plateau,the tensile properties show little dependence on aging time.The best aging treatment is 100 h at 200 ?. |
PDF Full Text Request