| Mg alloy is one of the most important light weight structure materials.AZ31 magnesium alloy("AZ31”for short hereinafter)is the most commonly used wrought Mg alloy.However,the as-cast grains of AZ31 are coarse under normal cast condition.This results in the relatively poor as-cast mechanical property.Besides,the coarse grains are bad for the deformation process.These factors limited the wide application of AZ31.It is thus important to refine the as-cast grains of AZ31 alloy.Adding master alloy is one of the commonly used methods to refine as-cast grains.According to the previous studies,the Al-Ti-B/C master alloys containing TiAl3,TiB2,Al4C3 or TiC could decrease the grain size of AZ31.However,different chemical composition of Al-Ti-B/C master alloy results in different phase composition,which may further affect their refining ability on AZ31.The study focus on this issue is scarce,but it is very important to the grain refiner optimization and the refining mechanism analysis.Rare earth("RE" for short hereinafter)is an important alloying element of Mg alloys.It has obvious fire retardant and reinforcement effect on Mg alloys.However,there are less research focused on the effect of RE on AZ31 grain size.The combined effect of grain refiner and RE on AZ31 also unclear.In this paper,six Al-Ti-B/C master alloys with different phases were prepared by changing their chemical composition and their effects on AZ31 grain size were studied.The phase composition of the best grain refiner can be obtained based on the experiment results.Two kinds of rare earth(RE)elements,i.e.Y and Gd,were selected to study their grain refining effect and mechanism on AZ31 alloy.Based on this,the most effective master alloy and RE were selected to test their combined effect on AZ31 as-cast grains.Besides,the effect of Al-6.1Ti-4.6C,Gd and their combined effect on the as-rolled microstructure,the as-cast/as-rolled mechanical properties of AZ31 were also studied.Details are as follows:(1)Three different Al-Ti(-B)master alloys,i.e.Al-6Ti,Al-5Ti-1B,Al-4.1Ti-1.9B,were prepared by flux assisted synthesis method.The effect of Ti,B content on the phase composition of Al-Ti-B master alloys were studied.With the Ti,B mass ratio declines to 2.2(the Ti,B mass ratio of TiB2),the phase transforms as:TiA13 → TiA13+TiB2 → TiB2.Their refining ability on AZ31 were also different.Compared with Al-6Ti(contains TiAl3)and Al-4.1Ti-1.9B(contains TiB2),the Al-5Ti-1B(contains TiAl3 and TiB2)is the most effective one,which could decrease the AZ31 grain size from 280μm to 149pm(grain refinement rate:46.8%).This is owing to the combined effect of solute Ti released by TiAl3 and TiB2 particle clusters.Three different Al-Ti-C master alloys,i.e.Al-7.8Ti-0.4C,Al-7.3Ti-1.8C,Al-6.1Ti-4.6C,were prepared by the self-propagating high-temperature synthesis in melt method.The effect of C content in the preform on the phase composition of Al-Ti-C master alloys were studied.With the increasing of C content,the phase transforms as:TiAl3+TiC → TiC → TiC+Al4C3.Among the three Al-Ti-C master alloys,the Al-6.1Ti-4.6C(contains Al4C3 and TiC)could refine the AZ31 grains from 280μm to 109μm(grain refinement rate:61.1%),which is superior to Al-7.8Ti-0.4C(contains TiAl3 and TiC)and Al-7.3Ti-1.8C(contains TiC).The Al4C3 and TiC particle could act as the nucleating substrate of a-Mg,which resulted in the finer grains.According to the grain refinement experiments of these six Al-Ti-B/C master alloys,the conclusion could be drawn that the combination effect of nucleating particle and solute element could improve the grain refining efficiency.With addition of Al-6.1Ti-4.6C,the as-rolled grain size of AZ31 decreased.However,the texture weakening was not obvious.The as-cast/as-rolled mechanical properties were also improved with Al-6.1Ti-4.6C addition.(2)The effect of Y,Gd elements on AZ31 as-cast grains was studied.Al2Y phase formed in AZ31 + 1-3%Y alloys.With the increasing of Y content,the AZ31 grain size increased firstly and then decreased.Al2Gd and Al3Gd formed AZ31 + 1-3%Gd alloys.With the increasing of Gd content,the AZ31 grain size also increased firstly and then decreased.This is due to the following two aspects:①the reactions between Y,Gd and A1 consumed Al element and reduced the refining effect of Al,which resulted in the grain coarsening and ②the Al2RE phase formed in the melt could refined the grains when the amount reached a critical level.However,Gd was more effevtive than Y.With 3%Gd addition,the as-cast grains were refined from 280μm to 116μm(grain refinement rate:58.6%).The as-rolled grains were refined and the texture was weakened.Furthermore,the as-cast/as-rolled tensile mechanical properties were improved.(3)The combined effect of Al-6.1Ti-4.6C and Gd on AZ31 as-cast grains was studied.The average grain size refined from 280μm to 78μm(grain refinement rate:72.1%),which was superior to the separate addition.The experiment results indicated that adding the nucleating particles with different kinds could improve the refining ability on AZ31 as-cast grains.With the combined addition of Al-6.1Ti-4.6C and Gd,when the hold time was within 30min,the grain size almost keeps constant.However,longer holding time resulted in the grain coarsening.According to the Stocks equation and the sample analysis,the settling of Al4C3,TiC and Al2Gd particles was the primary reason of the fading behavior.The combined effect of Al-6.1Ti-4.6C and Gd decreased the as-rolled grains from 30μm to 4μm and the texture was also weakened.The max pole intensity of(0002)basal texture was decreased from 14.484 to 12.609.Besides,the mechanical properties were improved:for the as-cast state,the yield strength(YS)increased from 41.7MPa to 67.9MPa,the ultimate tensile strength(UTS)increased from 105.2MPa to 175MPa and the elongation increased from 8.4%to 10.9%.For the as-rolled state,the YS increased from 192.5MPa to 261.6MPa,the UTS increased from 222.8MPa to 286.4MPa and the elongation increased from 5.4%to 7.7%.The strengthening effect was more obvious than the separate addition. |
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