| Magnesium alloy is a structural material with high-performance and lightweight,all kinds of industrial products widely used magnesium alloy as raw material.Nowadays we especially promote environmental protection and sustainable development vigorously,many advantages of magnesium alloy is more prominent.But it also has many characteristics that lead to poor welding performance,the joints are prone to coarse grain structure,hot cracks and other quality defects,it has become the main reason to limit its wider application.Isothermal superplastic welding is a new and simple solid welding technology,which not only can effectively reduce the defects of traditional welding such as cracks and spiracles,but also can greatly reduce the welding time,save energy by making full use of the superplastic state characteristics of the material,furthermore it can synchronize with the molding.At present,the research only on the steel,copper alloy and tungsten alloy and in those studies we see the obvious effect,however this welding technology has not been applied to magnesium alloy.In addition,the superplastic compression plays a keyrole in this welding technology,so the study of magnesium alloy compression superplasticity has an essential guiding significance to isothermal superplastic welding.AZ31 magnesium alloy is the research material of this paper,which was preheated by annealing to refine the grains and evenly distributed structure.Then the material after pretreatment is compressed to study the superplastic properties.Finally,isothermal superplastic welding of AZ31 magnesium alloy was further studied according to the superplastic results.The results of annealing pretreatment show that the grain size is refined from 20.79 um to 12.15 um and the refining rate is about 40.7%after annealing at 300 ? for 30 min.After compressing at 250?-480? and 1×10-4 s-11×10-2 s-1,the outer circumferential elongation of the sample is above 200.42%,and all of them achieved good compression superplasticity.The dynamic recrystallization of microstructural occurred in the superplastic compression process,the true stress-strain curve is in coincidence with the dynamic recrystallization curve,the value of flow stress,the grain state and the degree of dynamic recrystallization of compressed structure are affected by the initial strain rate and the compression temperature.The m value is between 0.18 and 0.24,and the average value of m increases slightly along with the increase of compression temperature.After isothermal superplastic welding under the condition ofpre-stress at 30 MPa,temperature of 450? and strain rate of 1×10-4 s-1,joint shear strength of the AZ31 magnesium alloy after pretreated reached42.08 MPa.The stress-strain curve and the microstructure at the weld area of the isothermal superplastic welding all reflect superplastic characteristics.In the main influencing factors,the influence of temperature on the strength of the isothermal superplastic solid-state welded joint is great,the effect of time is second,and the influence of strain rate is relatively small.Compared with diffusion welding,the isothermal superplastic welding can reach the equivalent shear strength with the advantage of saving energy and saving time. |
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