Objective Tungsten (W) has been widely applied in electronic and biological fields owing to its high melting point, excellent strength, and oxidation resistance. It has also been selected as the plasma-facing material for diverters in future fusion reactors due to its excellent radiation-shielding properties against heat and plasma fluxes. Recently, tungsten components with complex and fine structures have been required to face the increasing demand of material customization, which is difficult to manufacture through powder metallurgy because of the high hardness and intrinsic brittleness of tungsten. Laser power bed fusion (LPBF), as an important method of additive manufacturing technique, is a rapid forming technology based on 3D models. However, cracks would initiate and propagate due to steep temperature gradient and excessive thermal stress during rapid cooling of the LPBF process. Possible mitigation methods have achieved less crack density via preheating substrates, introducing secondary phases, and optimizing process parameters, with limited success in these cases. This study introduces tantalum carbide (TaC) into the tungsten matrix to explore a new method of reducing the crack density by forming submicron scale substructure strenthening phases. As a high melting point ceramic phase, TaC is well-matched with the lattice constant of tungsten. We reported optimized formability as well as achieving reduced cracks in LPBF tungsten while considering in-situ alloying. The tungsten caibide (W2C) phase formation through in-situ reaction during LPBF process should be focused.
Methods The 5--25 μm tungsten powders are prepared by the electric explosion method of metal wires. The alloying powders of tungsten-5%TaC are prepared by spherical tungsten powders above and 5% spongy TaC powders with 5 μm median diameter and are mechanically mixed using a low-energy blender mixer. Pure tungsten and tungsten-5%TaC parts are fabricated using an SLM280 2.0 machine with appropriate process parameters. Samples were built on stainless substrates. The scanning mode is rotated by 67° between adjacent layers with a “Zig-Zag” scan strategy. To limit oxidation during the LPBF process, the oxygen content inside the chamber is kept at less than 5×10 -4(volume fraction). In the next step, the as-built samples are mechanically grounded and polished, followed by electropolishing in a 1.5% NaOH solution at a voltage of 9.8 V for seconds to reveal the morphology of true cracks. The microstructure of top and side view cross-sections and phase analysis are characterized using a scanning electron microscope and X-ray diffractometer (XRD) with Cu-Kα radiation. The microhardness of tungsten and tungsten-5%TaC parts is also measured.
Results and Discussions Tungsten-5%TaC parts with good surface morphology and formability are obtained with a laser power of 350--400 W and scanning speed of 300 mm/s [Fig. 2(a)]. Warping and interlayer cracking will occur with improper laser energy input [Fig. 2(b)]. With suitable laser energy input, the interaction of powders and laser can achieve complete melt spreading of tungsten-5%TaC. Compared with pure tungsten, the defect density of pores and mesoscopic cracks in the microstructure has been considerably reduced in tungsten-5%TaC system (Fig. 4). After adding TaC particles, the grains are refined and many small-angle interfaces are formed. Further, transgranular cracks and hot cracks are initiated (Fig. 5). According to XRD analysis, the tungsten carbide (W2C) phase appears in the tungsten-5%TaC parts (Fig.6). Submicron substructures with various morphologies are observed in the tungsten-5%TaC microstructure (Fig. 7). During the LPBF ultra-high-speed solidification process, the substructure is formed when the solidification is near the absolute stability limit. The tantalum element exists in the matrix and cell walls, and the W2C phase exists on most of the cell walls. The morphology of the W2C phase may be closely related to the aggregation of tantalum and solidification conditions. These submicron scale substructure combined with good thermal conductivity and high strength of W2C phase segregation at the interface can effectively improve the strength of the material. The meandering crack path at the cell boundary increases the resistance of cracking propagation to some extent. The solid solution of tantalum in the matrix can strengthen the matrix and improve the material's intragranular and grain boundary strength. After adding TaC, the microhardness of the microstructure is increased from 400 HV of pure tungsten to 666 HV (Fig. 9). The refinement of grains can increase crack propagation resistance and alleviate the stress concentration during the LPBF process.
Conclusions This study proposes a new method of strengthening the tungsten matrix and reducing cracks via in-situ reaction by forming submicron scale substructure strengthening phases. By adjusting the appropriate laser process parameters, tungsten-5%TaC samples fabricated by the LPBF technique have achieved good formability and pores in the microstructure have considerably reduced. Neither lower energy input (pores and spheroidization) nor higher energy input (overheated phenomenon) can produce samples with good formability. In the range of appropriate laser parameters, the tungsten-5%TaC system cracks are considerably reduced compared to pure tungsten cracks, which are mainly achieved via alloy strengthening, substructure strengthening, and grain refinement. The microhardness of tungsten-5%TaC samples is increased by 50% compared with pure tungsten, and the intrinsic strength of the material is also improved. Meanwhile, various morphologies of substructures are observed in the microstructure of tungsten-5%TaC, and there exists W2C segregation in the cell walls. The evolution of substructures is mainly affected by melt flow, temperature gradient, and thermal history during LPBF solidification and cooling.
