Effects Of La, RE, Sc Metamorphic Process On Microstructure And Mechanical Properties Of Aluminum Alloy A356

The modification latency of La, RE and Sc was explored and the effects of La modification, RE modification and Sc modification on the microstructure and mechanical properties of A356alloy in the different process parameters and the mechanisms were mainly studied by using metallographic microscope, EDS energy spectrum analyzer, universal tensile machine, Rockwell hardness tester and conductivity test instrument in this paper. Furthermore, the modification effects of La, RE and Sc were compared and analyzed.Conditional test results show that the smelting cycle can be effectively shortened and the dissolution and diffusion of intermediate can be rapidly accelerated by stirring, resulting in more uniform alloy composition. The strength and elongation of the alloy increase14.73%and253.8%respectively by stirring in La modification process. The strength and elongation of the alloy increase11.41%and58.3%respectively by stirring in RE modification process. The strength and elongation of the alloy increase15.37%and33.3%respectively by stirring in Sc modification process. The microstructure observation indicates that modifier atoms can play metamorphism in a short time and almost all of the eutectic silicon phases have changed from the figures of thick sheet to the figures of small pieces and short rods, resulting in a(Al) phase dendrite refinement. However, the microstructure and properties of the alloy will get variation when stirring time passes the optimal time.Melt needs some holding times after stirring, and it is conductive to promote the diffusion and adsorption of modifier atoms and the modifier to play good modification effects. The strength and elongation of the alloy increase27.34%and2.63%respectively after holding in La modification process. The strength and elongation of the alloy increase13.24%and16.6%respectively after holding in RE modification process. The strength and elongation of the alloy increase15.8%and52.38%respectively after holding in Sc modification process. It can be seen from the microstructure that the eutectic silicon phases and a(Al) phases are refined obviously and after holing. But too long holding time will lead to microstructure segregation, modifier atom agglomeration and bad modification effect. Each modifier has a metamorphic "effective amount". The strength and elongation of the alloy increase44.09%and290%respectively comparing with no modification while the addition of La is optimal. The strength and elongation of the alloy increase24.9%and40%respectively comparing with no modification while the addition of RE is optimal. The strength and elongation of the alloy increase106.9%and450%respectively comparing with no modification while the addition of Sc is optimal, which reaches the best modification effect. According to the metallographic analysis results, almost all of the eutectic silicon phases have changed to the shape of creeping point, dispersed evenly in the grain boundary of a(Al) phases, and most of a(Al) phases are refined into spherical crystal after adding optimal La, RE and Sc. But the amount is not the more the better, it is beginning to appear block and strip compounds when both the addition amounts of La and RE are0.5%, which seriously affect the microstructure and performance. The energy spectrum analysis results show that the compounds are rich lanthanum and rich cerium compounds and their compositions are La, Ce, Si and Al.Compared with the effects of La, RE and Sc on the strength, elongation, hardness and electrical conductivity, it can be known that Sc is the optimal modifier in the experiment conditions. However, the price of Sc is very expensive, and it is mainly used in aerospace and ordnance industry and only used on the stage of test study in metal casting industry. The price of La is relatively low, and it can play a good modification effect. Therefore, La can make an optional modifier for A356alloy in industrial production. With the development of metallurgy and materials science and the improvement of Sc preparation process, it should be known that Sc will be widely used in the modification research and production of Al-Si casting alloy.