AZ31B magnesium alloy chips were recycled by three solid-state recycling processes including cold-pressing,hot-pressing followed by hot extrusion and double extrusion.Microstructure and mechanical properties of the recycled specimens and reference specimens were compared.For the recycled specimen by cold-pressing,the grains are refined to a large extent during hot extrusion due to the presence of twins and high density dislocation.The recycled specimens by hot-pressing and double extrusion do not exhibit finer grain than that the recycled specimen by cold-pressing.Consequently,higher ultimate tensile strength of the recycled specimen by hot-pressing and double extrusion is not achieved.For hot pressing process,more compact billet lowers the porosity in recycled material,so elongation to failure of the recycled specimen increases.The recycled specimen fabricated by double extrusion process shows slightly higher elongation than the reference specimen.The second extrusion makes the oxides further crush and distribute more dispersedly,and minimizes porosity,which is responsible for the improved ductility.
A method for recycling AZ91D magnesium alloy scraps directly by hot extrusion was studied.Various microstructural analyses were performed using the techniques of optical microscopy,scanning electron microscopy(SEM)and energy dispersive spectroscopy(EDS).Microstructural observations revealed that all the recycled specimens consisted of fine grains due to the dynamic recrystallization.The main strengthening mechanism of the recycled specimen was grain refinement strengthening and homogeneous distribution of oxide precipitates.The interfaces of individual scraps of extruded materials were not identified when the scraps were extruded with the extrusion ratio of 40-1.Oxidation layers of the scraps were broken into pieces by high compressive and shear forces under the extrusion ratio of 40-1.The ultimate tensile strength and elongation to failure increased with increasing the extrusion ratio.Recycled specimens with the extrusion ratio of 40:1 showed higher ultimate tensile strength of 342.61 MPa and higher elongation to failure of 11.32%,compared with those of the cast specimen.
