Effects Of Melt Superheating Treatment On The Solidification Structure And Mechanical Property Of Az91Magnesium Alloy

In order to enhance the room-temperature mechanical properties of AZ91magnesiumalloy, the melt superheating treatment is carried through using a home-made resistancefurnace. The effects of melt superheating, the joint role of melt superheating treatment and Srmodification on the solidification structure and mechanical properties of AZ91magnesiumalloy are researched, through the optical microscope, Scanning electron microscopy,Rockwell hardometer, X-ray diffraction instrument, Electronic universal test machine andSetaram synthetically thermal analyzer, and the optimal treatment process is obtained. Theresults are summarized as follows:The shape of α-Mg phase in AZ91magnesium alloy is varied from the fir-tree crystal tothe equiaxed crystal with increasing the melt superheating temperature, and the grain size isgradually decreased. When the melt temperature is850℃, the grain size of AZ91magnesium alloy is90μm, while the grain size of AZ91+0.05%Sr magnesium alloy. Namely,the grain size of AZ91magnesium alloy after adding0.05%Sr modification is decreased by26.7%under the same superheating temperature. Exceeding850℃, the grain sizes of AZ91and AZ91+0.05%Sr magnesium alloy are both slightly reduced. The shapes of Mg17Al12phases are changed into from continuous mesh to intermittent lath-shaped as increasing themelt temperature. When the melt temperature reaches900℃, Mg17Al12phases become coarseand are continuously distributed on the grain boundary.The tensile strength, offset yield strength, elongation, compression strength and hardnessof AZ91and AZ91+0.05%Sr alloys are first increased, and then decreased with the increaseof superheating temperature. The mechanical properties of AZ91+0.05%Sr magnesium alloyare better than that of AZ91magnesium alloy under the same superheating temperature. Thesynthesis mechanical properties of AZ91magnesium alloy are the best at850℃superheating.The tensile strength, offset yield strength, elongation, compressive strength, and hardnessreach280MPa,137MPa,7.4%,370Mpa and27.4HRB, respectively. The synthesismechanical properties of AZ91+0.05%Sr magnesium alloy are the best at800℃overheating.The tensile strength, offset yield strength, elongation, compressive strength, and hardness is292MPa,143MPa,8.6%,371MPa and36.3HRB, respectively. While tensile strength andelongation of foreign die-cast AZ91magnesium alloy are250MPa and7%, respectively.The solidification offset temperature of AZ91and AZ91+0.05%Sr is gradually decreasedwith increasing the melt superheating temperature, which leads to the enhance of nucleationdegree of supercooling and the increase of nucleation rate. That is why the grains are refinedand the mechanical properties are improved by increasing superheating temperature.