| Seamless ring parts can be used to prepare bearing rings,gear rings,flange rings,train wheels and wheels,gas turbine rings,engine rings,anti-aircraft guns,pressure vessels and nuclear reactor reinforcement ring,which are widely applied in energy equipment,aerospace,rocket,heavy machine and vehicles.In the present paper,a two-stage non-isothermal hot die forging process(multiple pre-forging and finish die forging)was proposed based on the knowledge of magnesium alloy forging,which has a simple forming process,low manufacturing cost and can be used in the preparation of high mechanical properties magnesium alloy rings.Two parts contents were included in this paper.Firstly,the commercial AZ31 magnesium alloy was used to verify the feasibility of the ring forming process,and the microstructure evolution and mechanical properties of the magnesium alloy ring during the forming process are systematically analyzed.Secondly,an RE-free,high aging strengthening effect Mg-7.2Zn-1.5Cu-1.0Mn(wt.%)(ZCM721)alloy was designed,and the effects of solution and two-step aging treatment on the microstructure evolution and mechanical properties of the ZCM721 alloy ring were investigated.AZ31 alloy ring was successfully processed by the two-stage non-isothermal hot die forging process.Finite element(FE)simulation results show that the effective strain and temperature distributions of the ring are inhomogeneous.After the hot ring forming process,the effective strain distribution at the centre region of the ring is relatively uniform;in the meantime,the temperature in or near the flash region is higher than the other regions.Combined with the FE simulation and microstructure observation results,it is revealed that,after the hot ring forming process,a refined but inhomogeneous microstructure is obtained in the ring,and the larger the accumulated strain and the lower the temperature are,the finer and more homogeneous microstructure will be achieved.The effects of multi-directional forging(MDF)passes and temperature on the microstructure and mechanical properties of AZ31 alloy were studied.The results show that:(1)in the range of 300~400?,upon MDF process,the microstructure undergoes pronounced grain refinement but an inhomogeneous grain structure is achieved due to the inhomogeneous strain distribution at the center and edge regions.After 36 passes,the microstructure at the center region is composed of finer and more homogeneous grains than that at the edge region.(2)Tensile tests show a significant improvement of mechanical properties after MDF.With the increase of MDF passes,the yield strength both at the center and edge regions first increases and then reduces due to TDRX.Compared to the edge region,the mechanical properties at the center region are superior and less affected by the forging temperatures after 36 passes.Among the experimental processing temperatures,350 °C is favored to get a more uniform mechanical properties across the specimen.The effects of hot ring forging temperature on grain structure,texture and mechanical properties of AZ31 magnesium alloy ring were studied.The results show that:(1)after hot ring forging,the grain structure of the AZ31 alloy ring is significantly refined and the grains are refined gradually with decreasing the finish die forging temperature.(2)The texture feature in the ring changes with the forging temperature,a single peak basal texture is formed after finish die forging at lower temperature,while there exhibits a double-peak texture at higher temperature and the basal poles tilt further away from the center of(0002)pole figure with increasing the finish die forging temperature.(3)Remarkable anisotropy of mechanical property shows in the AZ31 alloy ring,with the tensile yield strength of TD sample is larger than that of the RD and AD samples and with increasing the finish die forging temperature,the yield strength of the AD sample first lower and then larger than that of the RD sample.It is analyized that the texture in the ring leads to the different deformation mechanisms taking into effects at yielding when tensile deformed along different direction,resulting in mechanical property anisotropy.A RE-free,high aging strengthening effect ZCM721 alloy was designed,and the effects of hot extrusion deformation and subsequent solid solution and two-step aging treatment on the microstructure evolution and mechanical properties of ZCM721 alloy were studied,and the aging mechanism of ZCM721 alloy was also investigated.The results showed that: a significant strong aging strengthening response was achieved in the ZCM721 alloy.The yield strength of the extruded ZCM721 alloy was significantly increased after solution and two-step ageing treatment.Compared to the as-solutioned sample,the increments in the yield strength of the as-aged sample were about 173 MPa,resulting in a high yield strength of 372 MPa,which is much higher than most of the Mg alloys which contain large amounts of rare-earth elements.The enhanced aging strengthening response of the alloy was mainly attributed to the high number density of nanoscale rod-shape MgZn2 precipitates perpendicular to(0001)? plane formed during the two-step aging treatment.ZCM721 alloy ring was produced by hot ring forging and the mechanism of texture evolution during the hot ring forging process was investigated,and the mechanical properties between ZCM721 alloy ring and AZ31 alloy ring were also compared.The results showed that:(1)the texture of the ZCM721 alloy ring was gradually changed with the hot ring forming process.After 36 passes of multiple pre-forging,a typical(0002)basal texture was formed in the deformed billet;while during the finish die forging process,with deformation process,the intensities of the(0002)basal texture was first increased and then a non-basal texture was formed in the ring with the basal pole tilt ~50° away from the center of(0002)pole figure to the RD direction.(2)The tensile yield strength of the aged ZCM721 alloy ring along the TD,RD and AD directions is~360MPa,238 MPa and 200 MPa,respectively,which is significantly higher than that of the AZ31 alloy ring that formed in the present work,with the yield strength alongthe TD(~166-213MPa),RD(~92-150MPa)and AD directions(~107-129MPa),respectively.The results show that the RE-free,high aging strengthening effect ZCM721 alloy that designed in the present work can be used to the preparation of magnesium alloy ring with high strength. |
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