Effects Of Pb, Sn Additions And Ageing Treatment On Microstructure And Mechanical Properties Of Die Casting AZ81 Magnesium Alloy

Based on die casting AZ81 alloy, this article focuses on the effects of adding alloying elements and ageing treatment on its microstructure and mechanical properties at room temperature and medium temperature, exploring new way of improving mid-temperature properties of die casting AZ81 alloy. The important characterization methods and techniques included X-ray fluorescence(XRF), X-Ray Diffraction(XRD), optical microscopy(OM), Scanning Electron Microscope(SEM)/Energy Dispersive Spectrometer(EDS), Universal Material Testing Machine, etc.The main research results and conclusions are as follows:1) Effects of ageing treatment on microstructure and mechanical properties of die casting AZ81 alloy: After ageing, more granular and amellar precipitation of Mg17Al12 were observed at grain boundary; tensile strength and yeild strength at ambient temperature increased(increased by 18% and 6.9%, respectively); yeild strength at 180? increased(increased by 6.3%).2) Effects of Pb and ageing treatment on microstructure of die casting AZ81 alloy: Pb in die casting AZ81 alloy exhibited grain refining, the grain size decreased as Pb increased; there was no new compounds contain Pb observed and Pb was dissolved in ?-Mg and ?-Mg17Al12 phase of AZ81 alloy; Mg17Al12 precipitation have been observed not only in ?-Mg but also at grain boundary, and Pb could suppress the harmful precipitation of Mg17Al12 at grain boundary and could promote the favorable precipitation in grain which increased with Pb content.3) Effects of Pb and ageing treatment on mechanical properties of die casting AZ81 alloy: Ageing promoted the ambient temperature strength of die casting AZ81-(0.5, 1.0, 1.5)Pb alloys, and the strength first increased and then decreased with the increase of Pb. The combined effect of both ageing treatment and Pb could promote the strength of AZ81 at 180?, and the value increased with the increase of Pb; AZ81-1.5Pb had highest average tensile strength value of 181 MPa, have been improved by 10.2%. Pb and ageing treatment leaded to brittle fracture.4) Effects of Pb+Sn and ageing treatment on microstructure of die casting AZ81 alloy: Before ageing, there was no Mg2 Sn phase in the alloy of 0.5%Sn, but some polygon Mg2 Sn phase have been found in alloy of 1%Sn of die casting AZ81-0.5Pb-(0.5,1.0)Sn alloys; after ageing, all alloys generated polygon Mg2 Sn particles, which have the advantage of heat resistant performance; in the mean time, Pb suppressed the precipitation of Mg17Al12 at grain boundary, which contributed to promot the medium temperature resistance.5) Effects of Pb+Sn and ageing treatment on mechanical properties of die casting AZ81:Before ageing, the room temperature tensile strength and yield strength of die casting AZ81-0.5Pb-(0.5,1.0)Sn alloys increased with the increase of Sn, the tensile strength and yeild strength of die casting AZ81-0.5Pb-1.0Sn were 211 MPa and 150.5 MPa, which have been increased by 4.4 % and 34.4 %; before ageing, the tensile and yeild strength at 180? have been increased with the increase of Sn, the tensile and yeild strength of die casting AZ81-0.5Pb-1.0Sn alloy were 200.5 MPa and 145.2 MPa, which have been increased by 22.3% and 29.6 % respectively. After ageing, the room temperature tensile and yeild strength of die casting AZ81-0.5Pb-(0.5,1.0)Sn alloys have been increased, moreover, the values increased with the increase of Sn, the room temperature tensile and yeild strength of die casting AZ81-0.5Pb-1.0Sn were 212.5 MPa and 151.5 MPa, which have been increased by 5.1 % and 35.3 % respectively; after ageing, the tensile strength and yeild strength at 180? increased with the increase of Sn, the tensile strength and yeild strength of die casting AZ81-0.5Pb-1.0Sn were 180.5 MPa and 142.8 MPa, which have been increased by 10.1% and 26.8%; the Pb+Sn and ageing have made the fracture mechanism transferred from plasticity to brittleness.