| As the lightest metal structural material in current engineering applications,magnesium alloy has a wide range of application prospects.Forming the wrought magnesium alloy through the hot extrusion process can refine the microstructure,eliminate casting defects,and improve the overall performance of the product.However,the traditional hot extrusion billet is produced by semi-continuous casting process,which has coarse grain size and many defects.It often requires intermediate auxiliary processes such as homogenization annealing,peeling,and preheating before hot extrusion can be carried out.At the same time,limited by the original structure and deformation mode of the cast slab,the grain size refinement effect of conventional extrusion is often unsatisfactory,and there are obvious deformation textures,resulting in poor plasticity at room temperature and obvious anisotropy.The large plastic deformation process with more obvious grain refinement has a long process flow,high equipment requirements,and small changes in the macroscopic morphology of the material,resulting in high production costs.Therefore,in view of the above-mentioned problems existing in the production of wrought magnesium alloy by the traditional hot extrusion process,this research proposes a new technology that combines the squeeze casting technology with the hot extrusion forming process and improves it into a continuous squeeze casting process.And on the basis of exploring the mechanism of continuous extrusion casting process on the structure and properties of wrought magnesium alloy,by optimizing the extrusion ratio,the AZ31 wrought magnesium alloy with excellent comprehensive performance and higher production efficiency was further prepared.First,in this paper,the squeeze casting billet is obtained by applying bidirectional extrusion to the molten alloy liquid and maintaining the pressure for a period of time.The influence of squeeze casting and gravity casting on the microstructure of AZ31 magnesium alloy was studied.The results show that the matrix grains of the extrusion casting slab are fine,the content of the second phase particles increases,the particle size decreases,and the distribution is uniform.Subsequently,when the billet is cooled to the plastic deformation temperature range,the newly solidified billet is subjected to hot extrusion forming to prepare a continuous extrusion cast and extruded magnesium alloy bar.Compared with the continuous gravity cast-extrusion specimens that are not solidified under pressure,the continuous extrusion-cast specimens have a higher dynamic recrystallization degree,and the overall grain size is smaller.At the same time,the texture strength is significantly weakened and the base texture is strong.The component content is lower,and the non-basal surface texture content that is beneficial to the basal surface <a>slippage increases,so that the plasticity of the extruded magnesium alloy is improved while maintaining higher strength.The average grain size of continuous extrusion casting AZ31 magnesium alloy is 3.0 ± 1.3 μm,and the proportion of ultrafine grains with a grain size of less than 5 μm is 41.9%.Compared with the continuous gravity extrusion material without squeeze casting treatment,its average particle size is reduced by 49.2%,the proportion of fine grains is increased by 280.9%,and the overall texture strength is reduced by 53.1%.Its ultimate tensile strength is 321.1 MPa,yield strength is 210.9 MPa,and elongation is 14.7%,which are increased by 13.3%,31.1% and 20.5%respectively.Finally,based on the new continuous extrusion casting process,the mechanical properties of AZ31 wrought magnesium alloy are further optimized by changing the extrusion ratio.Successfully prepared extruded bars with elongation as high as 20.92%,and can maintain quite excellent ultimate tensile strength(303.19 MPa)and yield strength(215.45 MPa).Increasing the extrusion ratio can obtain a greater amount of deformation,causing strain accumulation,and at the same time,it can promote the dynamic recrystallization process and consume deformation energy.When the extrusion ratio is 25,a larger proportion of dynamic recrystallized grains can be obtained,and at the same time,a large amount of deformation heat generated by an excessively high extrusion ratio can be avoided,which in turn causes significant secondary growth of recrystallized grains.Finally,a microstructure with an average grain size refined to 1.85 ± 1.40 μm was obtained. |
