Microstructure And Mechanical Properties Of High-density Tungsten Alloy Fabricated By Laser-directed Energy Deposition

Tungsten and its alloys are widely used in kinetic-energy penetrators,counter-balance weights,radiation shielding materials and extreme service environments,owing to its advantages of high density,high strength,corrosion resistance,etc.In recent years,the additive manufacturing of tungsten alloys has been attracted much attention to meet the urgent needs of special structural components.The interfacial reaction between phases of tungsten alloy is always difficult to control,thus,cracks are easy to form between W particles and binder phase under the external loading,which reduces the plasticity and toughness of the material.In this paper,medium entropy alloy（MEA）is selected as the binder phase of tungsten alloy with the purpose to improve the interfacial bonding strength through its synergistic effect of multiple components,enhancing its plasticity and toughness.In this paper,W-Fe,W-（Fe Cr Ni）and W-（Co Cr Ni）alloys were fabricated using laser-directed energy deposition（L-DED）technology.The effects of printing parameter,W content and the diameter of tungsten powder on the microstructure and dynamic/static mechanical properties of tungsten alloys were systematically studied.The interaction between elements during the printing process and the strengthening mechanism were analyzed.The main results are as follows:（1）The in-situ precipitation behavior of tungsten alloy with single-component Fe as binder phase under the rapid solidification and thermal cycling co-effect was explored and established.Sub-micron sized Fe₂W particles were precipitated in-situ,and Fe₇W₆/Fe₂W intermetallic layers with the thickness of 2～8μm were formed at the interfaces between W particles andα-Fe.With the increase of W content,the compressive yield strength of the alloy increased at first and then decreased,and the compressive yield strength of the alloy with 45%W is the highest（1710±15 MPa）.The in-situ precipitated Fe₂W particles contributed to the strength of tungsten alloy.Coarse intermetallic compounds（layers）brought about adverse effect to the plasticity of W-Fe alloy,and the plasticity of the alloy decreases with the increase of W content.When W content is 85%,the compressive strain before fracture is less than 2%.（2）The key process strategy was determined to fabricate W-（Fe Cr Ni）alloy,and the interaction between the elements was explained.The result showed that spherical W particles were uniformly dispersed in Fe Cr Ni binder with face-centered cubic structure,and W-W connectivity is low.Cr rich particles were precipitated inside and at the boundary of W particles.There was 15 wt.%W dissolved in the binder,and a large number of nano-sized（Fe,Cr,Ni）₇W₆ particles were precipitated.With the increase of W content,the compressive yield strength of the W-（Fe Cr Ni）alloy increased and its plasticity decreased.The compressive yield strength of the alloy with 90%W is 1336±18 MPa,and the compressive strain before fracture is 48±2%.When the W content is lower than 85%,the strain of the alloy exceeded 70%.The thickness of Fe₇W₆ precipitated layer at W/Fe Cr Ni interface was reduced by 4～10 times compared with W/Fe,indicating that the interfacial reaction of W alloy can be significantly inhibited by using Fe Cr Ni as the binder phase,and the deformation ability of the alloy can be improved.（3）By replacing Fe in Fe Cr Ni with Co,which has better wettability of W,tungsten alloy with Co Cr Ni medium entropy alloy as binder phase was prepared.The microstructure before and after compression test was characterized,and the strengthening mechanism of the alloy was illustrated.A supersaturated solid solution with W content up to 20 wt.%was formed in the binder phase of W-Co Cr Ni)alloy,and dispersed（Co,Cr,Ni）₇W₆nanoparticles were precipitated in situ.Compared with W-（Fe Cr Ni）alloy,W-Co Cr Ni)alloy also has the advantages of low W-W connectivity and inhibiting the interfacial reaction of W alloy,and there is no Cr-rich particles precipitation in the microstructure,the concentration fluctuation and solution strengthening effect of W-Co Cr Ni)alloy were greater.The compressive yield strength and fracture strain of 90W-（Co Cr Ni）alloy are about 54 MPa and 5%higher than that of 90W-（Fe Cr Ni）,respectively.The higher strength of W-（Co Cr Ni）alloy is attributed to particle-bearing strengthening of unmelted W particles,Orowan strengthening of dispersed Co₇W₆ nanoparticles,solution strengthening caused by supersaturated W atoms and the gradient diffusion interface strengthening.The good plastic deformation ability of the alloy is attributed to the extremely low W-W connectivity and the inherent high work hardening rate of the Co Cr Ni binder phase.（4）W-MEA alloy exhibited excellent dynamic compression properties with high strength and high adiabatic shear sensitivity.The spherical W particles dispersed in the alloy constantly blocked the shear deformation.With the increase of W content,the dynamic compressive yield strength of the alloy increased,while the strain before fracture decreased.At the same condition,the dynamic yield strength of 75W-（Co Cr Ni）is about 200～300 MPa higher than that of 75W-（Fe Cr Ni）,attributing to the stronger solution strengthening effect in the binder of75W-（Co Cr Ni）.Both alloys show good adiabatic shear sensitivity,with adiabatic shear deformation bands of about 30μm and 50μm respectively.The dynamic constitutive equation of W-（Fe Cr Ni）alloy was fitted by the classical Johnson-Cook constitutive model.