Effect Of Si On Microstructure And Properties Of Al-4.5Mg Alloy During Homogenizing Heat Treatment Process

The 5xxx series Al-Mg alloy has advantages of low density,good plasticity,high strength,excellent weldability,and good corrosion resistance.In this paper,the effect of homogenization,rolling process,simulated brazing and heat treatment on microstructure and properties of Al-4.5Mg aluminum alloy is studied by adding different content of Si into Al-4.5Mg alloy,so as to develop a new type of aluminum alloy suitable for honeycomb core material.The conclusions of this paper are as follows:(1)Through microscopic observation and hardness test,it is found that the second phases in the as-cast microstructure of the alloy mainly consist of Mg2Si phase,Al6(Mn,Fe)phase and Al12(Mn,Fe)3Si phase.The increase of Si increases the amounts of second phases which evolve from dot shape and strip shape to reticular shape;the hardness of the as-cast alloy increases as the Si content increases,and the hardness value of the alloy containing 1.0wt.%Si reaches 81.74 HV.(2)The results of one-step homogenization heat treatment of alloy show that there are dispersed Al(Mn,Fe)Si phases with different amounts and different distributions precipitated in the homogenized alloy structure,and the amounts of dispersed phases increases with the increase of Si content which can significantly improve the distribution of the dispersion region.the optimal one-step homogenization system is“580?/8h".(3)The results of two-step homogenization heat treatment of alloy show that ?' is most favorable for the precipitation of a-AI(Mn,Fe)Si dispersed phase,the optimal low temperature homogenization system is determined as "2350C/6h",and the best two-step homogenization system is "235?/6h+580?/8h".Under this system,the amount of dispersed ?-Al(Mn,Fe)Si precipitates is the largest and the dispersive zone's distribution is the most uniform.The dispersed precipitation phases of the alloy after two-step homogenization heat treatment has more amounts,greater density and better dispersed distribution than that of the one-step homogenization.When the Si content is 1.0wt.%,the volume fraction of the dispersed region of the alloy which is homogenized by two-step reaches the maximum value 76%,which is a significant improvement over one-step homogenization.(4)After rolling process,simulated brazing and T6 heat treatment,it is concluded that the dispersed Al(Mn,Fe)Si phase plays the role of pinning dislocation and hindering subboundary and grain boundary migration in the primary recrystallization and secondary recrystallization of the alloy.More amounts and dispersed distribution make better inhibiting effect on grain growth;the inhibiting effect of alloy after two-step homogenization heat treatment is better than that of one-step homogenization,and the increase of Si makes the ability of the alloy's hindering primary and secondary recrystallization stronger.When Si content is 1.0wt.%and the homogenization system is "235?/6h+580?/8h",the recrystallized grain size is smallest(4.3?m).When Si content is 1.0wt.%and the homogenization system is "580?/8h",the grain size is 14.6 ?m.(5)Through tensile test and intergranular corrosion test,the results show that the tensile strength ?b of the new alloy after T6 heat treatment is 306.42MPa,the yield strength ?s is 269.23MPa,and the elongation rate after fracture is 10.5%,and has good corrosion resistance.