| Magnesium alloy is the lightest metal engineering structural material and it attracts more and more attention for its many advantages.Casting is the traditional processing method of magnesium alloy,the main problem is that the grains are coarse and the defects are easy to occur.Selective laser melting(SLM)is a new technology,it can not only process complex shape structure which is difficult to be realized by the traditional technology,but also prepare high-density components and reduce casting defects.Powder quality is a key factor for SLM technology.Commercial powders of multicomponent mixed rare earth magnesium alloy cannot be purchased at the present time.The powders of Mg-3Y-3.5Sm-2Zn-0.6Zr alloy has been tried to prepare by using the plasma rotating electrode method,and it was found that the quality of ingots had a significant effect on the properties of powders,the densification of ingots directly determined the success of powder preparation.In order to improve the densification of the ingots,the melting experiments of Mg-3Y-3.5Sm-2Zn-0.6Zr alloy were carried out in this paper.By means of OM,density test,X-ray diffraction,SEM and other experimental methods,the changes in microstructure and properties of the alloy were studied,and the related mechanisms were discussed preliminarily.The main conclusions are as follows:Argon protection melting is beneficial to refine the grains and increase the compactness of ingot.After the melt has been treated at 750℃with an argon flow rate of 3L/min,the average grain size decreases from 35.3μm to 20.3μm,and the porosity reduces from 35.5%to 18.9%.The partial pressure difference between inside and outside of the argon bubbles promotes the degassing of the melt,and the flow of the argon bubbles increases the dispersion of solute elements and improves the element segregation in the alloy.The compactness of the alloy increases first and then decreases with the decrease of smelting temperature but the increase of treatment time in the process of ultrasonic treatment.After ultrasonic treatment at 720℃for 2min,the compactness of the alloy is the highest,and the porosity is 12.6%.The pressure difference between the cavitation bubbles and the melt of dissolved gas promotes the diffusion of gas atoms and the nucleation of gas molecules.The ultrasonic flow can transport the cavitation bubbles carrying gas to the melt surface and escape,thus realizing degassing.With the joint action of ultrasonic treatment and argon protection melting,the compactness of the alloy reaches the highest,the obvious shrinkage and porosity have not appeared in the microstructure of the alloy,and the density and porosity are1.915g/cm~3and 8.4%,respectively.The second phase gradually changes from continuous network to fine intermittent rod or needle after the above three treatment methods.The three treatment processes are all helpful for reducing the segregation of Y and Zr elements,and the effect of the joint action of ultrasonic treatment and argon protection melting is the best.The elements of Y and Zr are distributed in the grain boundary and crystal with small spots.After joint action treatment,the tensile strength of the alloy is 270MPa,the yield strength is 162MPa and the elongation is 11.89%.The mechanism is the interaction between the ultrasonic cavitation and argon degassing. |
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