Design Of Oil Circulation Temperature Control For Extrusion Die And AZ91D Magnesium Alloy Study On Extrusion Technology

In this paper,AZ91D magnesium alloy is taken as the research object to explore a process of temperature controlled extrusion of magnesium alloy.A kind of oil circulation temperature control extrusion die was designed and manufactured,and compared the extrusion products'surface quality,microstructure and properties of the oil circulation temperature control die and the conventional oil free die under different extrusion speeds,as well as the temperature and extrusion pressure changes of the die during the extrusion process.The influence of oil circulation temperature control extrusion on the cracks,microstructure and properties of magnesium alloy products was studied,which provides a new idea and theoretical guidance for the plastic processing technology of magnesium alloy.The main conclusions are as follows:(1)The influence of oil circulation temperature control on the extrusion crack of AZ91D magnesium alloy at different extrusion speeds was revealed.The critical extrusion speeds of conventional extrusion and oil circulation temperature control extrusion is about 2.6 m/min and 3.0 m/min,respectively.Oil circulation temperature control extrusion can effectively inhibit the extrusion crack of AZ91D magnesium alloy or improve the degree of extrusion crack.However,when the extrusion speed reaches a high level(over 4.0 m/min),the crack degree of oil circulation temperature control extrusion is similar to that of conventional extrusion,and the improvement effect is limited.(2)During the extrusion process of magnesium alloy,the higher the extrusion speed is,the higher the actual deformation temperature of the alloy through the extrusion die is,and the more likely to produce extrusion defects such as extrusion cracks.At the same extrusion speed,oil circulation temperature control extrusion reduces the temperature of metal billet passing through the die hole,so it is beneficial to improve the surface quality of magnesium alloy extruded products.(3)Dynamic recrystallization and grain refinement occurred in AZ91D magnesium alloy after hot extrusion,The increase of extrusion speed promotes the dynamic recrystallization and grain growth of magnesium alloy in the hot extrusion process,while the oil circulation temperature control extrusion can slow down the grain growth speed,which has the effect of refining grain and stabilizing structure.(4)The second phase of AZ91D extruded magnesium alloy is mainly composed of ?-Mg₁₇Al₁₂ phase with fine grains on the grain boundary and Al-Mn phase with large grains in the grain.With the increase of extrusion speed and deformation temperature,the fine?-Mg₁₇Al₁₂ phase with lower melting point decreased,while the granular Al-Mn phase with higher melting point still distributed on the matrix and grain boundary.During the process of oil circulation temperature control extrusion,the deformation temperature of the alloy decreases,and the second phase is difficult to dissolve back to the matrix.There are many?-Mg₁₇Al₁₂ phases on the grain boundary.These dispersed second phase particles can prevent the grain growth and improve the strength of the material,but they have adverse effects on the plasticity of the material.(5)It is profitable to promote the mechanical properties of magnesium alloy products by oil circulation temperature control extrusion.The plasticity and tensile strength of the alloy have little change,but the yield strength has a great improvement,especially at low extrusion speed.Under the experimental conditions of extrusion temperature 420?and extrusion ratio 79,when the extrusion speed is 1 m/min,the comprehensive properties of AZ91D magnesium alloy by oil circulation temperature control extrusion are the best,the yield strength is 258 MPa,the tensile strength is 376MPa,and the elongation is 14.5%.(6)After the oil circulation temperature control,the hardness fluctuation coefficient of AZ91D extruded magnesium alloy bar at the front-to-rear ends and cross sections decreased,showing better properties homogeneity in the longitudinal and cross sections.