Mictostructure, Mechanical Properties And Oxide Characterization Of AZ31B Magnesium Alloy Prepared By Solid State Recycling

At present, magnesium alloy is the lightest metal structural materials. It has many advantages including high specific strength and stiffness, good machinability, excellent damping property and good electromagnetic shielding capabilities. Magnesium alloy is being widely used in aviation, spaceflight, automotive, computer, household electrical appliances and communications. However, magnesium alloy waste will increase sharply during producing with application of magnesium alloy increasing. In this paper, magnesium alloy prepared by solid state recycling is investigated, which lead to save energy, reduce gas emission and achieve continuable development of magnesium industry.In this paper, AZ31B magnesium alloy were prepared from magnesium alloy chips by solid state recycling. Analysis and test means of microstructure and mechanical properties were carried out by optical microscope, scanning eletron microscope, transmission electron microscope, electron universal strength testing machine and fatigue testing machine. Recrystallization microstructure of the alloy, tensile properties, morphology and distribution of oxide, interface structure between oxide and matrix, fatigue properties and fracture behavior were investigated. Strengthening mechanism and fracture mechanism of the recycled alloy prepared by solid state process were discussed.Effect of chip size on micrcostructures and mechanical properties of AZ31B magnesium alloy prepared by solid state recycling were studied. Three different size chips were cold-pressed to form compact billets, the pressure is 350MPa and the holding time is 60s. The density of the billets was 1.598g/cm~3, 1.677 g/cm~3 and 1.629 g/cm~3, respectively. Grain size of the alloy prepared by solid state recycling is smaller than that of the alloy prepared by cast extrusion. With S (total chip surface per unit volume in recycled alloy) increasing, the mean grain size of the alloy became smaller and tensile strength of the alloy improved. The elongation of the alloys that have interim S is highest. The two alloys with small chip sizes have higher tensile strength than cast extruding alloy. The elongation of the alloy prepared by solid state recycling is lower than that of cast extruding alloy.Effect of technologic parameters in recycling process on micrcostructures and mechanical properties of AZ31B magnesium alloy prepared by solid state recycling were systematically studied. With extrusion temperatures increasing, grains of AZ31B magnesium alloy became bigger. At begin, tensile strength and elongation of the alloy improved. Then, tensile strength and elongation of the alloy decreased. Recycled alloy that was prepared with extrusion temperature of 400℃showed the most excellent mechanical property, its ultimate tensile strength and elongation to failure were 309.2MPa and 16.3%, respectively. Ultimate tensile strength of the alloy prepared by solid state recycling was higher than that of the cast extruding alloy when extrusion temperature is higher than 350℃. With extrusion ratios increasing from 4:1 to 44.4:1, grains of AZ31B magnesium alloy became smaller. When extrusion ratio is 100:1, grains of AZ31B magnesium alloy became bigger. Ultimate tensile strength of the alloy prepared by solid state recycling and extruded alloy from ingot improved with extrusion ratios increasing. At begin, elongation of the alloy improved. Then, elongation of the alloy decreased. When extrusion ratio come up to 11.1:1, Fracture characteristics along the joint surface between the chip and chip disappear on fracture surface. When extrusion ratio come up to 25:1, ultimate tensile strength of the alloy prepared by solid state recycling is higher than that of the cast extruding alloy.Distribution of oxide in the alloy prepared by solid state recycling was studied. Oxide layer in the surface of chips was destroyed during extrusion. Destroyed oxide is generally parallel to extruding direction. After the chips were seriously oxidized, the recycled alloy has good tensile properties. With extrusion times increasing, destroyed degree and distributive uniformity of oxide increase. Ultimate tensile strength of the alloy is increasing because combination between chips becomes stronger. After third or forth extrusion, microcrack along chip surface on fracture disappear.Morphology of oxide and interface structure between oxide and the matrix were studied. After extrusion, broken magnesium oxide is small spherical particles which size is 15~25nm. When extrusion ratio is low, oxide film is not broken completely. Interface bonding between refine magnesium oxide or unbroken magnesium oxide piece and the magnesium alloy matrix is compact and reaches to atom combination. Broken magnesium oxide particles in the alloy was dispersed in the local area and play a strengthening role to a certain extent.At repeated fluctuating load, fatigue behavior of AZ31B magnesium alloy prepared by solid state recycling is not evidently lower that of the cast extruding alloy. Fatigue limitation of the alloy prepared by 0.1mm,0.4mm,1.2mm chips and cast extrusion is 90.7MPa,86.7MPa,88.1MPa and 87.9MPa, respectively. Fracture process and fracture mechanism of the alloy prepared by solid state recycling and cast extrusion are same. Fatigue source is in sub-surface of the sample and fracture surface of fatigue source region was similar to the steps of cleavage patterns. Transgranular fatigue crack extension and important microscopic characteristics such as fatigue striation and secondary cracks were found on all the sample. Fracture surface of fast fracture zone was composed of different sizes of dimples, and quasi-cleavage step, tear edge and some porous.