Study On Microstructure And Mechanical Property Of AZ91D Magnesium Alloy Prepared By Solid State Recycling

Magnesium alloy is the lightest metal structural material in the engineering application up to the present moment. It is known as the most developing recycling metal material in the 21st century. Magnesium alloy is limitedly used in metal structural material because of its high cost including of the lower rate of recovery of magnesium alloy scraps and the higher expense cost. Plenty of burnout and waste residue are produced using the common remelting method and covering flux and refining flux are added in the remelting process.In this paper, AZ91D magnesium alloy chips and scraps were prepared by solid state recycling. In the solid state recycling process, magnesium alloy chips and scraps were directly extruded into the products by plastic deformation. Cold-press or hot-press were carried out and then hot-extrusion was used. Solid recycling magnesium alloy presents high mechanical property due to grain refinement and homogeneous dispersion of oxide precipitates. Solid state recycling is a new efficient method for the recycling of magnesium alloy chips and scraps.Optimal parameters of AZ91D magnesium alloy chips prepared by solid state recycling were obtained and plastic characteristics in solid state recycling were analyzed. At first, AZ91D magnesium alloy chips were cold-pressed to form a compact billet with the pressure of 300³10MPa. The density of the billet was above 1.49g/cm³ and the surface of the chips exhibited some breaking and bonding. Recycled specimen prepared with chip dimensions of (4⁶)mm×(3.5⁴.5)mm×(1.45¹.55)mm, extrusion temperature of 400℃and extrusion ratio of 25:1 showed higher mechanical property. Its ultimate tensile strength and elongation to failure were almost the same as those of the as-cast and extruded specimen. Ultimate tensile strength and elongation to failure of recycled specimen increased with the extrusion ratio increasing. When the extrusion ratio is above 25:1, the elongation to failure of recycled specimen decreased with increasing the extrusion ratio.Relationship between oxygen concentration and total surface area of AZ91D magnesium alloy was studied for the recycled specimens. The accumulated oxygen concentration increased linearly with the total surface area. A small amount of oxide precipitation contributed to higher tensile strength and elongation to failure and resulted in dispersion strengthening. Excessive oxide may be prone to cause the early development of microvoids and contributed to lower elongation.Optimal parameters of AZ91D magnesium alloy scraps prepared by solid state recycling were obtained and corrosive washing technology of AZ91D magnesium alloy scraps was present. Adopting hot-press and hot-extrusion technology, the grain size was uniform and the bonding between scraps was better with the extruding temperature of 450℃. The ultimate tensile strength and elongation to failure can reach 350.24MPa and 11.82%, respectively. When the extrusion ratio exceeded 25:1, the grain became fine and the bonding layer didn't appear. The ultimate tensile strength increased with the extrusion ratio increasing. When the extrusion ratio was 40:1, the ultimate tensile strength could reach 378.05MPa. Adopting hot-extrusion technology, comparing to hot-press and hot-extrusion technology, the ultimate tensile strength and elongation to failure decreased.Relationship between bonding layer and ultimate tensile strength was studied for the recycled specimens adopting AZ91D magnesium alloy scraps. When the bonding layer in the recycled specimens formed continuous curves, the ultimate tensile strength decreased with the thickness of the bonding layer increasing. Forcastable equation of the ultimate tensile strength for the recycled specimens isσ_w = -40 w/3 + 345 + 40/3 when the thixkness of the bonding layer is 0 < w≤7μm. Forcastable equation of the ultimate tensile strength for the recycled specimens isσ_w = -30 w+ 475 when the thixkness of the bonding layer is 7μm < w <11μm. When the bonding layer for the recycled specimens formed discontinuous curves, the ultimate tensile strength increased with the proportion of the length of the bonding layer accounting for the whole measured curve increasing. Forcastable equation of the ultimate tensile strength for the recycled specimens isσ_l = 150l + 245 when the thixkness of the bonding layer is 0.1≤l≤0.7. Forcastable equation of the ultimate tensile strength for the recycled specimens isσ_l = 75l + 297.5 when the thixkness of the bonding layer is 0.7 < l< 0.9.Effect of solution and artificial aging treatment on microstructure and mechanical property of recycled specimens was studied. The ultimate tensile strength and the elongation to failure obviously increased after T6. With the aging time prolonging, the amount ofβ-Mg₁₇Al₁₂ phase obviously increased andβ-Mg₁₇Al₁₂ phase didn't become coarse. With the aging time prolonging, Vickness hardness of the recycled specimens increased. The Vickness hardnesses of recycled specimens adopting AZ91D magnesium alloy chips or scraps were higher than that adopting as-cast and hot-extrusion.Microstructure and mechanical property of recycled AZ91D magnesium alloy in the industrial experiment were studied. Ultimate tensile strength and elongation to failure prepared by solid state recycling were the highest and could reach 346.2MPa and 9.62%, respectively, with the optimal hot-press parameters of the temperature of 400±10℃, the holding time of 300s, the pressure of 400Mpa and the hot-extrusion parameters of the temperature of 450±10℃, the holding time of 300s, the extruding rate of 0.4mm/s. Mechanical property of the recycled specimens prepared by solid state recycling was related with the thickness of the recycled specimens, extrusion streamline and the distribution of the bonding layer.