Research On Selective Electron Beam Melting Of Tungsten Heavy Alloys

Tungsten alloys have high strength,high density,low coefficient of thermal expansion,high thermal conductivity,which are widely used in military,nuclear industry,medical treatment,aerospace and other fields.At present,tungsten alloys are mainly prepared by powder metallurgy,High melting points and high strength have caused many obstacles in the process of processing solid parts containing internal curved surfaces or porous structures.In recent years,the rapid development of metal additive manufacturing technology has significant advantages in the formation of complex components.Selective electron beam melting(SEBM),as a typical metal additive manufacturing technology,has the advantages of short processing cycle,high energy utilization rate,low residual stress of molded parts,and molding under vacuum.It provides an effective way for the formation of complex components such as tungstenbased alloys in refractory metals.In this paper,mainly researched the SEBM melting technology of 90W-7Ni-3Fe alloy,researched the influence of processing parameters(electron beam current,scanning speed and preheating temperature,etc.).The main research contents and results are as follows:The finite element technology was used to carry out the simulation research of SEBM forming pure tungsten.The characteristics of temperature field distribution in the forming process of tungsten under different scanning strategies and process conditions were analyzed.It was found that compared with the outer spiral and inner spiral scanning strategies,the serpentine scanning method can better control the melting and solidification processes of adjacent melt channels,and effectively guarantee the molding quality of the sample.Established the finite element method 90W-7Ni-3Fe electron beam melting temperature selection field model.According to the temperature field variation range projected molding process interval is the scanning speed is 1.4-3 m/s,the scanning current is less than 14 mA,the scanning strategy is serpentine scanning,and the preheating temperature is 700-900?;Based on the results of simulations,SEBM forming 90W-7Ni-3Fe alloys were studied using mixed element powders and pre-alloyed powders as raw materials.The experimental results show that:due to the effect of powder agglomeration and gravity during the powdering process of the mixed element powder,the powder raw material appears segregation,resulting in a low density and non-uniform structure of the formed sample.Most samples are below 90%),so for SEBM forming of tungsten-based alloys,mixed element powder is not the best choice.The use of pre-alloyed powder as the raw material can effectively improve the uniformity and density of the alloy.The densification process is mainly due to the uniform distribution of the bonding phase and the matrix phase of the pre-alloyed powder during the forming process,which can promote the tungsten(dissolutionprecipitation)process and particle rearrangement.Based on the pre-alloy powder raw material,the density of 90W-7Ni-3Fe alloy samples can reach more than 95%.Forming 90w-7Ni-3Fe alloy sample by optimized process parameters,the tensile strength of the alloy can reach 1098 MPa,and the elongation is only 1.3%.Through fracture analysis,the fracture mode of 90W-7Ni-3Fe alloy formed by SEBM is mainly transgranular cleavage fracture of tungsten particles.