Microstructure Formation Mechanism Of High Content Tungsten Alloy Fabricated By Selective Laser Melting

Tungsten alloys are typical refractory alloys with high density,high strength,good corrosion resistance and excellent ray absorption ability,which make them widely used in many fields such as aerospace,defense industry,marine industry and medical industry.Traditional powder metallurgy is the most common method for preparing tungsten alloys,but it has the problem of cumbersome process when processing complex structural parts,while laser additive manufacturing can complete the direct molding of three-dimensional models to achieve mold-free manufacturing,and the additive manufacturing process of aluminum alloy,titanium alloy and other metal materials has become a research hotspot.However,there are few studies related to laser additive manufacturing of tungsten alloys,and there is no clear explanation for the formation mechanism of the microstructure of the fabricated tungsten alloys.Meanwhile,the problem of poor plastic hinge still exists in additive manufacturing tungsten alloys,which greatly limits the application range of tungsten alloys in the field of additive manufacturing.In this paper,the effect of process parameters,bonded phase doping and heat treatment on tungsten alloys fabricated by selective laser melting is investigated,taking W-Ni-Fe alloy as an example.The process parameters are optimized in order to prepare highly dense and defect-free tungsten alloy specimens.In this study,optical microscopy,scanning electron microscopy and electron backscatter diffraction were used to characterize the microscopic morphology and grain orientation of the selected laser melted tungsten alloys,and the formation mechanism of the typical microstructure of tungsten alloys was analyzed.The mechanical properties and fracture failure mechanism of the selected laser-melted tungsten alloy were investigated by tensile testing of the 80W-14Ni-6Fe alloy prepared under the optimal process parameters.The laser powder bed melting of 90W-7Ni-3Fe was investigated by changing the doping amount of theγ-(Ni-Fe)bonding phase and compared with 80W-14Ni-6Fe alloy to clarify the effect of doping amount on the microstructure of tungsten alloy.The heat treatment process of the tungsten alloy was explored to enhance the toughness of the additively manufactured tungsten alloy.The main findings of this paper are as follows:(1)The densification and microstructure formation mechanism of SLMed tungsten alloy.The optimal process parameters for the SLMed 80W-14Ni-6Fe alloy range from80 J/mm~3-100 J/mm~3.With increased laser energy density,the density of 80W alloy first increases,then decreases,and finally stabilizes.The highest densification of the80W alloy was achieved when the laser energy density was 93.33 J/mm~3.The densification mechanism of the SLMed tungsten alloy includes partial melting of tungsten particles and liquid phase sintering.The typical microstructure of the selected laser melted tungsten alloy includes tungsten grain consolidation,tungsten dendrites,ultrafine crystals,tungsten particles andγ-(Ni-Fe)bonded phase.The formation of typical solidification organization is associated with the partial melting of tungsten grains.(2)The influence of Ni and Fe doping amount on microstructure and mechanical properties of SLMed tungsten alloys.The 90W-7Ni-3Fe alloy specimens were prepared by selected-area laser melting by reducing the amount of Ni-Fe bonding.When the laser energy density was too high,the 90W alloy specimens were subjected to thermal stresses that caused warpage and a large number of cracks,and the main forms of microcracks were plastic loss cracks and solidification cracks.90W alloy had an optimum process parameter range of 119.05 J/mm~3-128.21 J/mm~3 and a maximum density of 95.6%.(3)The exploration of heat treatment process of selected area laser melting tungsten alloy.In this study,two heat treatment processes were designed for SLM 80W alloy,annealing at 1400°C for 2h and annealing at 1400°C for 4h,respectively.The results show that the two-hour annealing treatment can effectively dissolve the tungsten grains to merge and form more spherical tungsten particles.In contrast,the four-hour annealing treatment resulted in tungsten alloy specimens similar to the deposited state,with a large number of tungsten grain mergers.The tensile strength and plasticity of the 80W alloy after both annealing treatments did not change much.