Study On Thermal-stable Phase Precipitation And Hot Workability Of A Novel Heat-resistant Aluminum Alloy

Heat-resistant aluminum alloy is widely used in aerospace,automobile engine,military industry and other fields.Dispersion strengthening is an important strengthening method of heat-resistant aluminum alloy.Al3M is the main strengthening phase of traditional heat-resistant aluminum alloy.In recent years,it has been found that submicron α-Al(Fe,Mn)Si dispersoids has excellent high temperature strengthening effect.The study on the precipitation behavior ofα-Al(Fe,Mn)Si dispersoids during heat treatment and the hot workability of the alloy has important theoretical significance for the exploration of the preparation process of novel heatresistant aluminum alloy and the optimization of heat treatment system.In this paper,taking Al-0.8Mg-0.75Si-0.75Fe-1Mn(wt.%)alloy as the research object,a new type of heat-resistant aluminum alloy with α-Al(Fe,Mn)Si dispersoids as thermal stable strengthening phase was prepared by twin roll strip casting and sand casting.The effects of cooling rate on the solid solid solubility solute elements,the equivalent length and area fraction of eutectic structure,the precipitation density of thermal-stable phase,the equivalent width and area fraction of precipitate-free zone were investigated,the precipitation behavior of a-Al(Fe,Mn)Si thermal-stable phase was clarified,and the hot workability of the alloy was studied,in order to provide theoretical support and technical reference for the composition optimization and process formulation of the new heat-resistant aluminum alloy.The specific conclusions are as follows:(1)In the as-cast structure of the alloy,the area fraction and equivalent length of eutectic structure of mold casting and twin roll strip casting are 3.1%,18.3 μm and 2.3%,3.4 μm,respectively.On the one hand,with the increase of solidification cooling rate,the higher the solid solubility of alloy elements in aluminum matrix,the lower the content of eutectic structure;on the other hand,the eutectic structure in twin roll strip casting samples is broken by rolling deformation,and the distribution is more fine and uniform.(2)After heat treatment at 430℃ for 6 h,the area density of α-Al(Fe,Mn)Si dispersoids in the twin roll strip casting sample increased significantly compared with the mold casting sample,from 11.6 μm-2 to 14.7 μm-2.The precipitation mechanism of dispersed phase is mainly based on metastable Mg2Si phase,dislocation and vacancy nucleation.(3)After heat treatment at 430℃ for 6 h,a precipitate-free zone was formed near the eutectic structure.The area fraction of the precipitate-free zone in twin roll strip casting and mold casting samples was 4.6%and 10.6%respectively,and the equivalent width was 1.3 μm and 4.1 μm,respectively.The low vacancy concentration near the eutectic structure is the main reason for the formation of the precipitate-free zone.(4)The flow stress of twin roll strip casting sample is higher than that of mold casting sample.When the deformation temperature is 300℃ and 400℃,the yield strength of the twin roll strip casting sample is higher than that of the mold casting sample by 9.2 MPa and 7.4 MPa,respectively.(5)After the thermal simulation experiment,there are different degrees of cracks in the samples,and the crack sources are easy to be produced due to the stress concentration caused by the coarse eutectic structure.The twin roll strip casting samples have better hot workability because the eutectic structure area fraction is lower,the size is smaller,and the occurrence of microcracks is delayed during deformation.