| In order to develop new magnesium alloys for wheel materials, Mg-x Gd-3Y-2Zn-0.6Zr alloys were prepared by low pressure casting, gravity casting and extrusion and studied heat treatment. First, by controling parameters of mold temperature, filling time and melt temperature to study the formability and mechanical properties of the as-casted alloys.And provided slab for the follow-up study. Gravity casting slabs were prepared to compared with low pressure casting Mg-x Gd-3Y-2Zn-0.6Zr alloys with microstructure and properties and cleared the advantages of low pressure casting method.Then Mg-(4,8)Gd-3Y-2Zn-0.6Zr were subjected to T4 and T6 heat treatment with different temperature, optimized the solid solution and aging process by studying the change of microstructure and phase in the heat treatment process. At the last, gravity casted Mg-x Gd-3Y-2Zn-0.6Zr(x=0,4,8) were made into extrusion bars,and were subjected to T5 treatment at 200?,to study the transformation of microstructures, mechanical properties and texture. The microstructure observation, X-ray diffraction(XRD), mechanics performance test, etc were used to analysis the change of microstructure and mechanical properties. Finally we got the conclusions as followed:The best low pressure casting parameters of Mg-x Gd-3Y-2Zn-0.6Zr(x=4,6,8) alloys were shell mode temperature was 200 ?,filling time was 3~4s. Mg-4Gd-3Y-2Zn-0.6Zr and Mg-(6,8)Gd-3Y-2Zn-0.6Zr alloys' best casting melt temperature range were 710-730 ?and 730-750 ?, respectively.The low-pressure casting Mg-8Gd-3Y-2Zn-0.6Zr obtained good comprehensive mechanical properties:ultimate tensile strength 240 MPa, yield strength 155 MPa,the elongation 10.8%.Gd Mg-4Gd-3Y-2Zn-0.6Zr obtained mechanical properties:ultimate tensile strength 218 MPa, yield strength 127 MPa, the elongation 15.6%.The corresponding low-pressure casting alloys had better performance than the thin-walled metal mould gravity casting alloys.Different casting methods got the same phase species.The casted alloys were mainly consist of ?-Mg, block-shaped LPSO(long-period stacking orderedstructure) Mg10(Gd,Y)Zn at grain boundary, striped LPSO(Mg12Gd Zn), a small amount of Mg3(Gd,Y,Zn)and some Mg RE phase. But the microstructure had a gap. Gravity casting alloys got coarser block-shaped LPSO and fewer striped LPSO and Mg RE.With the increasement of solid solution temperature, it took shorter time to reach the peak hardness. A small quantity of block-shaped LPSO dissolved, intracrystalline striped LPSO dissolved completely and large amounts of Mg RE phase generated at the grain boundary at the 500?-T4 state. More block-shaped LPSO dissolved and hardly generated Mg RE phase at 520?- T4 state. While the LPSO rapidly decomposed, and changed into W-phase when the temperature beyond 540?. Mg RE had benefit on performance, so the Mg-8Gd-3Y-2Zn-0.6Zr obtained the best properties:?b=302MPa,?0.2=162MPa and ?=20.4%.Mg-(4,8)-Gd-3Y-2Zn-0.6Zr had little ?' phase at T6 state and it's unstable, strengthening effect was weak, and solid solution strengthening effect is reduced,so the tensile properties of the alloys had a small decline after aging treatment when it compared with T4 state alloys, accordingly. Due to the developed stripe-shaped LPSO, Cast-T5 alloys' elongation increased greatly.The as-extruded Mg-3Y-2Zn-0.6Zr obtained properties:?b=313MPa,?0.2=245MPa and ?=13.5%. The as-extruded Mg-8Gd-3Y-2Zn-0.6Zr obtained properties:?b=350MPa, ?0.2=282MPa and ?=12.7%. 8% Gd content can improve strengthand weaken the base texture and cylindrical texture.After aging treatment at 200?, Mg-3Y-2Zn-0.6Zr obtained properties: ?b=313MPa, ?0.2=248MPa and ?=14.5%. Mg-4Gd-3Y-2Zn-0.6Zr had a great improvement of mechanical properties: ?b from 300 MPa to 331 MPa, ?0.2 from 251 MPa to 290 MPa, and ? changed from 8.1% to 10.8%.. |
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