Microstructure Evolution Of AZ31 Magnesium Alloy Under Rotating Magnetic Field

Magnesium alloy castings are generally characterized by low strength and toughness,difficult subsequent ingot forming at room temperature,low yield and high production cost,which make it difficult to meet the increasing industrial requirements for the performance of light materials.Therefore,improving the properties of magnesium alloy castings by casting process is one of the important research directions of magnesium alloy development.In the solidification process of magnesium alloy under rotating magnetic field,the flow field,temperature field and casting microstructure evolution rules during the solidification process of magnesium alloy under different magnetic field conditions were studied and analyzed by means of numerical simulation and experiment,and the mechanism of strengthening and toughening electromagnetic stirring casting of magnesium alloy was explored.The main research is as follows:(1)Ansoft Maxwell was used to model and simulate the magnetic field for the existing electromagnetic stirring equipment.Under the condition of applying alternating currents of90 A,120A and 150 A respectively and frequency of 6HZ,the magnetic field intensity of 150 A is the maximum and reaches 0.199 T.In addition,the magnetic field intensity of each part of the mold is approximately constant,but the direction is always changing,the change period is the same as the applied current period,and the stirring force is the resultant force of electromagnetic force and centrifugal force.(2)More field coupling simulation process and result: Will the mixing power functions and viscosity of UDF imported into the Workbench simulation under stirring force of liquid magnesium alloy solidification and temperature field,flow field,the simulation results show that compared with the magnetic field of the magnesium alloy melt,when the magnetic field is added with the increase of current temperature drop rate significantly faster,clotting from near the mold to the middle level,flow velocity in the middle part change rule of the largest and from outside to inside first increase in decreasing,and with the increase of time velocity increases first and then as a whole.(3)Results of electromagnetic stirring experiment: According to the mechanical analysis of 90 A,120A and 150 A at different positions along the radial direction,it can be seen that they increase first and then decrease,reflecting the same law as the flow field,and the cooling performance in water is better than that in air.With the increase of current the number of twinned crystals increases and a lot of dislocations are produced.(4)Heat treatment and cryogenic experiment and results: In order to further improve the properties of magnesium alloy,solid solution cryogenic treatment was carried out for magnesium alloy after electromagnetic stirring.Research shows that first,the AZ31 magnesium alloy in the solidification conditions for electromagnetic stirring150 A-6 Hz water cooling conditions,the temperature of 410 °C for 4 h after solid solution treatment temperature-196°C-12 h of cryogenic can get the best performance,tensile strength of 215 Mpa,the elongation can be up to 20.6%.Secondly,the solid solution treatment retained the dislocation in the tissue,forming a small angle grain boundary,and cryogenic treatment promoted the formation of the dislocation and twins in the organization.