| As a light-weight and high-performance structural material,magnesium alloys are widely used in many industrial products.If the magnesium alloy is welded by conventional methods,it may lead to poor welding performance.As a new simple state welding technology,isothermal superplastic welding?ISW?can suppress the defects such as air holes,cracks and deformation processed by traditional welding.The superplastic compression plays a key role in this welding technology,so studying the change regulation can provide a guidance for the subsequent welding test.Fine and homogeneous grain structure of magnesium alloy is beneficial to subsequent superplastic welding.Therefore,the influence of annealing temperature and holding time on the internal microstructure of AZ91magnesium alloy was studied at the range of 240400?and 15min60min.The result showed after annealing at 320?for 30 min,the grain size of the AZ91 magnesium alloy is refined from 21.09?m to 13.51?m and the refining rate is about 35.94%,and the refining effect is relatively good.Then,AZ91 magnesium alloy are compressed at 250450?and1×10-4s-11×10-2s-11 after preferable annealing heat treatment.Apart from the strain rate of 1×10-4s-11 at 250?,samples processed by superplastic compression achieved preferable superplasticity and the outer circumferential elongation of the samples are above 149.68%.The long rate is above 149.68%,achieving preferable compression superplasticity.The dynamic recrystallization of microstructural occurred in the superplastic compression process,the grain state and the degree of dynamic recrystallization of compressed structure are affected by the initial strain rate and the compression temperature.The strain rate sensitivity exponent?m?is between 0.16 to 0.19,and it increases slightly with increasing of compression temperature.Based on the results of this test,AZ91 magnesium alloy is recommended to use preferable technological parameters according to different application scenarios.Superplastic forming recommended to use:250?,5×10-3s-1;The large deformation recommended:450?,1×10-2s-1;And the ISW recommended:450?,1×10-4s-1.Finally,according to the constant Compression Superplasticity of AZ91magnesium alloy,the ISW test is carried out according to the test rules.During welding,the pretreatment,prepressure and heat preservation time of the welding parts are all consistent,so the factors that affect the quality of the welding joint are the welding temperature,the welding time and the rate of the strain rate.In order to study the effects of these three factors on the strength of welded joints,the ISW test was divided into multi factor orthogonal test L9?33?and single factor welding test.The results of ISW test showed that joint shear strength of the AZ91 magnesium alloy which pretreated under the condition of pre-stress at 30 MPa,temperature of 450?and strain rate of 1×10-4s-11 in 14min reached up to 51.37 MPa and the strenth is about 41%of the original material.In the process of the ISW,the microstructure of the weld seam will be dynamic recrystallized.So avoiding the negative effect of the coarse grain on the welding process is very important.The temperature is the most important point on the strength of the ISW joint in the influencing factors and time is subdominant.While the influence of strain rate on the strength of the ISW joint is relatively small. |
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