The Preparation Of Al-2.5C Master Alloy And Its Refining Effect On AZ31 Alloy

Magnesium alloys are the most potential lightweight structural material in the 21 st century.Magnesium alloys have been widely applied in various industries due to their light weight,high specific strength and specific stiffness,as well as their excellent casting and electromagnetic shielding properties.However,Magnesium alloys are also associated with a number of limitations,such as the low absolute strength and poor room temperature ductility ascribed to their hexagonal close-packed structure.Grain refinement has been considered as one of the most effective approaches to simultaneously increase the strength and ductility.In this paper,the preparation of Al-2.5C master alloy and its refining effect on AZ31 alloy were investigated.The Al–2.5C master alloys were fabricated by the powder metallurgy method.Based on the grain refining experiments of AZ31 alloy,the grain refining effect and mechanism of Al-2.5C master alloy on AZ31 alloy were investigated.The results indicated that Al₄C₃ particle is a potent nucleation substrate for primary ?-Mg grain.With 1.0wt.% master alloy addition,the grain size reduces from 204?m to 70?m,the tensile strength increases from 103.91 MPa to 143.20 MPa,and the elongation increases from 14.95% to 23.13%.The size distribution of Al₄C₃ particles in Al-2.5C master alloy was researched.The grain refining efficiencies of Al₄C₃ particles on AZ31 alloy were calculated.And the relationship between the two was investigated.It was revealed that the size distribution of Al₄C₃ particles is well fitted by a log-normal function.The grain refining efficiencies of Al₄C₃ particles on AZ31 alloy are calculated to be 0.04%-0.75% with different addition amount of refiner.Such low refining efficiency is mainly attributed to the very small proportion of large particles which have higher potency to act as heterogeneous nucleation sites.In the present condition,it was proposed that the optimal Al₄C₃ particles size range for significant grain refinement is greater than 5.0?m.Based on the above results,the effect of milling time,sinter temperature and sinter time on microstructure and reaction mechanism of Al-2.5C master alloy was investigated.The possibility of preparing Al-2.5C master alloys containing large-sized particles was studied and the refining effect of master alloys containing large-sized particles on AZ31 alloy was verified.The results revealed that milling,increasing sinter temperature and prolonging sinter time can promote the coarsening of Al₄C₃ particles.When the milled powder was sintered at 800 oC for 1h,1000 oC for 1h and 1000 oC for 2h,the Al₄C₃ particle sizes in master alloy range from 0.18?m to 5.82?m,0.18?m to 7.08?m and 0.21?m to 7.12?m respectively,and the particles greater than 5.0?m account for 0.18%,0.84% and 2.38% respectively.This is because that milling can refine the Al and C powder,increase the contact area between the Al and C powder and promote the solid solution of C to Al.These can provide excellent kinetic conditions for the Al-C reaction.Besides,increasing sinter temperature and prolonging sinter time can promote the diffusion of Al to C particle surface rapidly,so as to accelerate the Al-C reaction and promote the formation and growth of Al₄C₃ particles.It was proved that using Al-2.5C alloy containing large-sized Al₄C₃ particles to refine AZ31 alloy can reduce the addition amount of refiners and improve the utilization ratio of refiners.