A Study On Al-Li-Cu-Mg Cast Under Near-rapid Cooling

Al-Li alloy has more advantages in density and specific strength than other conventional alloys.There have been many studies on the effects of Cu/Li and Cu/Mg ratio on the microstructure and mechanical properties of Al-Li-Cu-Mg alloys,but few studies have been done on the high Li content.In this paper,the composition design,heat treatment process and properties of Al Li alloy were optimized.In this paper,an alloy with five components is designed and named 1#-5#.[Al-5Li-4.5Cu-xMg(x=1,2,3,4,5)].For 5#alloy,the solidification conditions are changed for smelting and casting.The solidification conditions are graphite mold air cooling,iron mold air cooling,sub-rapid solidification water-cooled copper mold,and water quenching.The microstructure and properties of the alloy were analyzed by melting and casting,SEM,XRD,EDS and tensile test.The 1#?5#alloy cast under the air-cooling condition of the copper mold,with the increase of Mg content,the size of the crystal grains first decreases and then increases,and the eutectic structure in the alloy continues to increase.The second phase always contained in 1#?5#alloy is T2(Al6CuLi3).In the process of adding Mg element,the continuous precipitation of S(Al2MgLi)phase causes the concentration of Li atoms in the alloy to decrease,which inhibits the precipitation of ?'(Al3Li).Since the remelting temperature of T2(Al6CuLi3)is higher than that of S(Al2MgLi),5#alloy has the best effect after homogenization treatment of the same system.The tensile strength and microhardness of alloy 1#-5#are improved after heat treatment,but the effect is not obvious.In the experiments of microhardness and aging time,the increase of Mg element and the increase of temperature shortened the time of alloy aging peak;the microhardness of alloy showed a trend of increasing first and then decreasing.After the tensile test of the 1#?5#alloy,the fracture morphology was observed to be brittle fracture,and the elongation was very low below 1%.The alloy obtained by changing the solidification conditions of 5#alloy is 6#?9#.The other groups of experiments are control experiments of 8#alloy under sub-rapid solidification.The increase in solidification rate did not change the type of the second phase in the alloy,but only changed the size,quantity and shape of the second phase.The increase of the solidification rate increases the degree of undercooling and increases the nucleation rate of the alloy,the grains are continuously refined,the second phase is significantly increased and refined,and the segregation of alloy components is also reduced.The homogenization temperature exploration experiment determined that the best homogenization temperature is 540? for 48h.Compared with 5#alloy,the microhardness and tensile strength of 6#alloy are improved under sub-rapid solidification conditions.Due to the continuous brittle second phase and inclusions on the grain boundary,the intergranular fracture and fracture caused by the continuity of the grain boundary are destroyed A stepped fracture trend.