Preparation Technology And Microstructure Properties Of Magnesium Alloy Strip Produced By Continuous Extrusion Expansion Forming Process

Magnesium alloys have a series of advantages such as low density,high specific strength,high specific stiffness,which are widely used in aerospace,automotive,3C electronic products and other fields.In 2019,the annual output of raw magnesium in China has exceeded 900,000 tons.As the main application product of magnesium alloy,the annual output of sheet and strip is less than 20,000 tons.A large amount of raw magnesium has been exported to foreign countries in the form of raw materials.Therefore,it is an urgent problem for magnesium alloy industry to improve the production and capacity of low cost and high performance magnesium alloy strip.The continuous extrusion and expansion forming technology to prepare magnesium alloy strip can not only save a lot of energy,but also contribute to the realization of industrial production,which has technical innovation.At present,the study of continuous extrusion and expansion forming of magnesium alloy is still in the feasibility exploration stage,and there is a lack of systematic research on the process.Width-thickness ratio is one of the important process parameters of extrusion products.Therefore,this paper has carried out theoretical and practical exploration for strip products with different width-thickness ratio.Firstly,we constructed the material constitutive model and hot working diagram of ZK60 magnesium alloy through hot compression test,and determined the optimal processing areas of ZK60 magnesium alloy as follows: The deformation temperature is 450～500℃,and the strain rate can be selected in a wide range.Then,the finite element simulation and continuous extrusion experiments of ZK60 and AZ31 magnesium alloys with different width-thickness ratios(1.89,4.57,5.43 and 8.06)were carried out based on the above theoretical results.The continuous extrusion process and its effects on microstructure and properties were systematically analyzed in order to provide reference for the production of high-quality magnesium alloy strips by continuous extrusion.The numerical simulation of continuous extrusion expansion forming of ZK60 and AZ31 magnesium alloys was carried out by Deform.Due to the equipment limitation of continuous extrusion machine,the strip with 8.06 width-thickness ratio needs to use a larger continuous extrusion machine.Therefore,two finite element models were established to explore the variation rules of material flow,stress-strain,temperature field and velocity field when two magnesium alloys were prepared with different width-thickness ratio.It is found that the physical field variation trend of the two magnesium alloys is basically the same.The deformation temperature of AZ31 magnesium alloy is slightly higher than that of ZK60 magnesium alloy due to the large activation energy of hot deformation.Due to different continuous extrusion machine,the 14.5mm×1.8mm strip with 8.06 width-thickness ratio at the same rotation speed has faster flow rate and more severe deformation,resulting in higher temperature of the extruded strip.For the strip with a width of 12.5mm or less,the larger the width-thickness ratio is,the greater the stress and strain will be.ZK60 and AZ31 magnesium alloy strips with different width-thickness ratios were prepared by continuous extrusion experiments.The microstructure of the extrusion cavity and the samples were analyzed and the mechanical properties were tested.The results show that the grain size at the inlet of the cavity is the smallest,which is about 1.79μm,and the grain size increases first and then decreases from the inlet to the outlet.This is mainly because the material is subjected to large shear deformation in the right-angle bending region,and the grain breakage is more serious.As the metal flows forward,dynamic recrystallization occurs in the magnesium alloy,resulting in the increase of grain size to 3.81μm in the expanded cavity region.When the metal flows to the position of the die,it begins to undergo severe deformation at the sizing zone,which leads to secondary dynamic recrystallization of the material and grain refinement.The average grain size decreases to about 2.46μm.The strip with a width-to-thickness ratio of 8.06 has a higher extrusion temperature,the dynamic recrystallization process is more complete,the average grain size is larger than that of other sizes,and the temperature difference between the center position and the edge position is large,cause side appear more above 10 microns thick grain.So the bending strength and tensile strength of the strip are also lower,the hardness is smaller,but the plasticity is improved.Due to the difference of deformation temperature and grain size,the optimum size of strip is different.For ZK60 magnesium alloy,the 12.5mm×2.3mm strip with5.43 width-thickness ratio has the best properties,with the average grain size of 2.41μm,tensile strength of 246.1 MPa and bending strength of 276.7 MPa.For AZ31 magnesium alloy,the 10.5mm×2.3mm strip with 4.57 width-thickness ratio has the best performance,with an average grain size of 3.72μm,tensile strength of 238.8 MPa and bending strength of 260.9MPa.