The Effect Of Rare Earth Pr On The Microstructure And Properties Of AZ91 Extruded Magnesium Alloy

As one of the most widely used magnesium alloys,AZ91 traits of low density,high specific strength,high specific rigidity,strong electromagnetic shielding ability and so on,is a promising structural light material.It is widely used in metallurgy,chemical engineering,electronics,nuclear industry,automotive and aerospace industries.However,the active chemistry and mesh-like?-Mg₁₇Al₁₂2 phase,which resulting the low mechanical properties,corrosion resistance,and wear resistance and limiting the use of AZ91 magnesium alloy.The high-energy ultrasound,hot extrusion,rare earth metamorphism and heat treatment processes are used to improve the performance of the material.The effect of rare earth Pr and heat treatment on AZ91magnesium alloys were studies in this study.The results show that the addition of Pr significantly refines the microstructure of the AZ91.When AZ91 modified by 1.0wt.%Pr,the shape of?-Mg₁₇Al₁₂2 transform coarse dendritic into short rod-like,and the distribution is more uniform.There are stripe-like Al₁₁Pr₃ phase and the globular-like Al₆Mn₆Pr phase formed in the alloys.In hot extrusion process,the grains of alloys were crushed and elongated,the sizes of?-Mg₁₇Al₁₂2 phase were reduced from 10?m to 2?m,and the grains are arranged in a certain direction.Rare earth Pr significantly improves the mechanical properties of as-extrusion AZ91 magnesium alloys.Compared to the matrix alloy,the tensile strength?359MPa?,elongation?15.3%?and hardness?95.0HV?of the AZ91 with1.0wt.%Pr were increased by 20.5%,26.0%,18.5%,respectively.The increasing of mechanical properties were mainly due to grain refinement and the formation of hard Al₁₁Pr₃,Al₆Mn₆Pr intermetallic compounds.The fracture mode of extruded AZ91magnesium alloy is quasi-cleavage fracture.After modified by Pr,tensile fracture morphology is characterized by ductile rupture.The friction and wear experiments of as-extrusion AZ91 modified by Pr were carried out.The results show that the wear rate of AZ91-Xwt.%Pr magnesium alloys increases with the increase of external load,and the friction coefficient decreases with the increase of external load.Rare earth Pr can significantly reduce the wear rate and delays the transition from mild wear to severe wear of magnesium alloys.Under 60N applied load,the wear rate(2.35×10^-5g/m)and the friction coefficient?0.217?of AZ91-1.0wt.%Pr magnesium alloy were reduced by 50.1%,33.0%compared with the matrix,respectively.However,further increasing of Pr content would increase wear mass and friction coefficient of the as-extrusion AZ91.Under the load of 30N,the wear mechanism of as-extrusion AZ91 are abrasive wear and delamination wear,while the wear mechanism of as-extrusion AZ91-1.0wt.%Pr is abrasive wear.When the load was increased to 90N,the wear mechanism of as-extrusion AZ91-1.0wt.%Pr was changed to delamination wear.The solution temperature has a significant effect on AZ91 magnesium alloys.With the increasing of the solution temperature,the?-Mg₁₇Al₁₂2 phase is gradually dissolve back into the?-Mg phase.At 410?,the grains of the alloy are the finest and the micro-hardness reaches the highest.However,further increasing of solution temperature would cause grain coarsening.Therefore,the best solution treatment process of the as-extrusion AZ91-1.0wt.%Pr is 410?+6h.The results of aging treatment show that,with the extension of aging time,the hardness of the material increases first and then decreases.The hardness test results show that the best aging treatment process of the as-extrusion AZ91-1.0wt.%Pr is 200?+20h.The peak micro-hardness of the material after solution and aging treatment is 126.7HV,which was increase by 33.4%compared with the as-extrusion AZ91-1.0wt.%Pr.