Effect Of Rolling-cryogenic Treatment On Microstructure And Properties Of AZ31 Magnesium Alloy

Roll forming is one of the most economical and effective processes for producing magnesium alloy sheets in industrial production.Magnesium alloy rolled sheets are very easy to form strong basal texture,easy to produce edge cracks,poor formability,affecting the subsequent deep processing.Therefore,in order to improve the comprehensive mechanical properties of magnesium alloy sheets and to expand their application,it is of great engineering importance to improve the rolling process and to provide a suitable post-rolling heat treatment for the rolled magnesium alloy sheets.Different from the traditional heat treatment processes,cryogenic treatment is a kind of treatment methods to place the work-piece in liquid nitrogen at-196℃ for a period of time to optimize the material microstructure and properties,because of its easy operation,significant effect and environmentally friendly,as an ideal metal material toughness treatment process,gradually become a research hot spot.In this paper,we mainly focus on rolling-cryogenic treatment AZ31 magnesium alloy sheets as the research object to explore the evolution of microstructure and mechanical properties of the sheets under different rolling-cryogenic treatment processes treatments,to provide theoretical reference and process guidance for cryogenic treatment of AZ31 magnesium alloy sheets after rolling and forming,and the specific conclusions obtained are as follows:(1)The microstructure evolution of hot-rolled magnesium alloy sheets after different times of cryogenic treatment was revealed.The number of second phase in AZ31 magnesium alloy after cryogenic treatment increases and becomes diffuse distribution;twins are produced after cryogenic treatment,and the number of twins further increases with the extension of cryogenic treatment time;after cryogenic treatment of hot rolled sheets,(10-11),(10-13)crystalline peak is increased,and grain orientation towards the(10-11),(10-13)grain plane deflection.(2)The reasons for the improved tensile properties of the hot-rolled magnesium alloy sheets after different times of slow cooling in air followed by cryogenic treatment were analysed.The tensile strength and elongation at break of the rolled-immediately cryogenic treatment samples and the hot-rolled-slow-cooled 15 s followed by cryogenic treatment samples were 278 MPa,12.6% and 262 MPa,15.9%,respectively,compared with the rolled samples(243 MPa,9.6%).The improvement in the mechanical properties of the rolled slow-cooled samples was mainly attributed to the synergistic effects of grain refinement,twins,dislocations,precipitates and non-basal slip.(3)The effects of two-pass with variable paths rolling-cryogenic treatment on the microstructure and tensile properties of magnesium alloy sheets at a rolling temperature of240℃ were investigated.The grain size and distribution of the two-pass variable path rolling-cryogenic treatment treated samples were improved in different ways,producing different types of twins and dislocation slip.The tensile properties of the two-pass cross-rolled with cryogenic treatment samples performed best,with tensile strength reaching 283 MPa and elongation at fracture increasing to 25.8%,an increase of 18.4% and 87.0%,respectively,compared to the normal rolled samples(239 MPa,13.8 %).The increase in strength was mainly the result of the combined effect of twins strengthening,fine-grain strengthening,grain boundary strengthening and dislocation strengthening,and the increase in elongation at fracture was associated with grain refinement and pyramid <c+a> slip activation.(4)The effects of two-pass with variable paths rolling-cryogenic treatment on the microstructure and tensile properties of magnesium alloy sheets at a rolling temperature of360℃ were investigated.After the cryogenic treatment of AZ31 magnesium alloy rolled in two-pass of variable paths,the grain size tends to homogenise,the grain size is significantly refined;the second phase particles along the grain boundary precipitation increases,the alloy tissue produces twins;internal grain rotation,the grain orientation changes.Grain orientation changes,causing the formation of rolling(0002)strong basal texture significantly weakened.The plasticity of the two-pass rolled-cryogenic treatment samples in the same direction was significantly improved,with an elongation at fracture of 23.2%,an increase of 142%compared to the normal rolled samples(9.6%),the increase in elongation at fracture was mainly due to the high degree of dynamic recrystallisation of the samples(87%),after the cryogenic treatment,the average Schmidt Factor of the non-basal texture were higher than the basal texture,and the opening of the non-basal texture slip significantly increased plasticity of the sheets.