Research On Microstructure Evolution Of Tugsten And Tugsten Alloys In Selective Laser Melting Process

Because of its good performances such as high melting point, high temperature strength, low vapor pressures, low expansion coefficient and outstanding corrosion resistance, refractory alloys has been one of the irreplaceable materials which can be widely used in industry fabrication, military field, medical device and communication equipment. However, due to refractory alloy's bad forming characteristics, the scope of its application was limited. Until now, refractory alloys were usually fabricated through powder metallurgy technology which needs expensive and dedicated tools. Therefore, the developments on new forming methods for refractory alloys become one of the most important focuses. Selective laser melting (SLM), as a new forming method, can directly fabricate parts with complex shapes by melting loose metal powder. The SLMed final components are excellent in mechanical and chemical property which can meet the requirements for direct usage. Due to its flexibility in materials and shapes, SLM method exhibits a great potential for processing refractory alloys'parts with complex shapes.In this paper, selective melting technique was chosen to fabricate refractory alloys (tungsten and its alloys). The effects of processing parameters (such as laser power, layer thickness, scan velocity, scan interval) on forming characteristics and microstructure were systematically investigated. The microstructure transformation and forming mechanism in laser melting-solidification process were also studied. Enhanced sintering technology was applied to optimize the forming property rare earth oxide was added in order to obtain the optimal performance of final parts. Further more, the forming process of refractory alloys (Mo, Nb) was also addressed. The overall results were as follows.Using W as raw material, SLM method was applied to forming three dimensional specimens. Part of W metal particles melted under large energy density of laser beam with the mechanism of liquid phase sintering. The microstructure of SLMed W specimens appeared the acicular dendrite growing along the preferred growth direction. With the enlargement of laser power import, the dimension of acicular dendrite reduced accordingly. When the layer number increased from one to six, the micro hardness of SLMed specimens was decreased from 826HV to 353HV.The effective principle of adding Ni element during SLM process was studied. The forming mechanisms of W-Ni alloys are coexisted liquid phase sintering and melting/solidification of part W particles by laser energy input. Under the equal laser condition, the microstructure of SLMed specimens with the Ni element content of 10wt.%, 20wt.%,40wt.% showed bar shape, dendrites and alveolar, separately. Adding Ni element can decrease the melt viscosity and increase the homogenize microstructure.On the basis of the variation of W-Ni hardness value, a powder shrinkage model can be established according to the thickness of powder layer shrinkage and forming layer number. The variation of real layer thickness and the relevant mathematical explanations are discussed. The results show that the total shrinkage of metal powder layer sharply increases in the initial five layers, and then reaches to a plateau value with the increased processing layers. This value is defined by the ratio of sliced layer thickness (h) to relative density (k) during selective laser melting process.The processing window was established according to experiments of W-Ni-Cu alloys Based on the processing window, final parts was fabricated showing harmonious sintering surface with no cracking and obvious porosity. The sintering metallurgical mechanism transform from LPS to melting/solidification of W particles with the enhancement of laser energy input.To further optimize the microstructure and forming property, rare earth oxide La2O3 was added to W-Ni-Cu alloy powder. Suitable content of La2O3(0.5wt.%-1.0wt.%) can enlarge the laser absorption of powder system, refine the microstructure, decrease roughness of SLMed specimens, reduce microcrack, leading to a better forming characteristics and a higher micro hardness value. Using 90W-7Ni-3Fe as raw material, finite element analysis method was applied to analysis the three dimension temperature field of powder bed under different forming parameters. Experiments on different processing parameters were carried out, and the change of temperature field and microstructure evolution was investigated. Fixing the laser power as a constant value, with a high laser scan velocity(110mm/s), the microstructure of W-Ni-Fe alloys showed un-melted W particles bonding of melting Ni, Fe powder through LPS. Decreasing the laser scan velocity, an increase of dendrites can be obtained, showing an enlargement of molten pool temperature with melting of W particles. Under the laser scan velocity of 20mm/s combined with scan interval of 0.05mm, the microstructure of specimens showed columnar grains with the melting-solidification forming mechanism.The SLM forming process and microstructure evolution of Mo, Nb was investigated. Mo-Ni solid solutions appeared in SLMed specimens with homogenized particulate dispersion and bonding coherence, which were benefit for the density of sintering specimens. Compared to SLMed Mo metal powder, the SLMed mixture 90Mo-Ni powder can fabricate higher relative density and lower surface porosity under the equal laser condition. The microstructure of SLMed Nb specimens showed layered structure with regular arrangement.