Effect Of La On Microstructure Properties And Hot-tearing Of Al-Cu-Mg Alloy

Al-Cu-Mg alloy has good weldability and toughness,high tensile strength and fatigue strength at room temperature and high temperature.However,due to the serious tendency of hot-tearing of Al-Cu-Mg alloy,the application and development of the alloy have been limited.In order to reduce the hot-tearing tendency of Al-C u-Mg alloy,many scholars at home and abroad have carried out a lot of experiments from different aspects.The results show that rare earth elements ca n effectively reduce the hot-tearing tendency of Al-C u-Mg al oy.In this study,the effect of rare earth element La on the hot-tearing tendency of Al-4.4Cu-1.5Mg-0.15 Zr alloy was studied,and the mechanism of La element was discussed.The hot-tearing tendency of the alloy is characterized by the critical diameter method and thermal sensitivity coefficient.Research shows that with the addition of La,the grain size of the alloy was first decreased and then increased,without addition of La element,the allo y grain is coarse dendrite,the grain boundary of the grid distribution of the second phase of continuous treatment;when the La element content is 0.4%,the grain morphology of the alloy changed into equiaxed crystalt,the grain boundaries of the coarse grid of the second phase becomes discontinuous,a bright white new phase formation.For the Al-4.4Cu-1.5Mg-0.15Zr-xLa alloy,the heat sensitive coefficient of Hsc with the addition of La showed a decreased and increased trend.When the La element was not added,the hot-tearing tendency of the alloy was the highest,and the thermal sensitivity coefficient was 1.64;When the content of La was 0.4%,the hot-tearing tendency of the alloy was the lowest,and the thermal sensitivity coefficient was 1.05.There are three kinds of hot crack formation mechanism,namely,the theory of intergranular bridging,the theory of liquid film and the theory of solidification compensation for Al-4.4C u-1.5Mg-0.15Zr-xLa alloy.When the content of rare earth element La is less than 0.3%,the thermal cracking mechanism of the alloy is mainly intergranular bridging theory;When the addition amount of rare earth element La is greater than or equal to 0.3%,the thermal cracking mechanism of the alloy is mainly caused by the combination of liquid film theory and solidification shrinkage compensation theory.For the hot-tearing resistance of Al-4.4Cu-1.5Mg-0.15Zr-xLa alloy,the optimal La content is 0.4%.At this time,the tensile strength of the Al-4.4Cu-1.5Mg-0.15 Zr alloy was increased from 173 MPa to 258 MPa,and the elongation increased from 7.3% to compared with that of the La alloy without the addition of the element.In order to improve the comprehensive mechanical properties of Al-4.4Cu-1.5Mg-0.15Zr-0.4La alloy and expand its application in practical production,the heat treatment process of Al-4.4C u-1.5Mg-0.15Zr-0.4La alloy was studied in this study.The effects of bipolar homogenization treatment,solid solution time and aging time on the microstructure and properties of the alloy were studied.The results show that the optimal heat treatment process for Al-4.4Cu-1.5Mg-0.15Zr-0.4La alloy is that two-stage homogenization treatment of 350?×8h+480?×16h solution 490±2?×80min 80 min,then 185?±2?×12h artificial aging treatment.After the heat treatment,the tensile strength and hardness of the alloy increased significantly,which increased from 258 MPa to 498 MPa and 85 HV to 164 HV,but the elongation of the al oy decreased from 8.6% to 6.9%.