High-speed video analyzes microcracking during additive manufacturing of tungsten
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the depths of resulting melt pools. As the tungsten cooled, it High-speed video analyzes formed transverse cracks on the surfaces microcracking during additive manufacturing of tungsten of the resulting solids. Higher applied laser power increased spacing between cracks from 250 µm to 600 µm until lonost of our day-to-day encounters gitudinal cracks, which ran deeper into with tungsten metal are limited to the bulk of the material, appeared and reseeing its glowing coils in outdated incanlieved resulting stress. Researchers were descent light bulbs. However, the excepalso able to derive the ductile-to-brittle tional thermal conductivity and excellent transition temperature range of 450–650 K radiation shielding properties of this very for tungsten, during which cracks formed dense metal suggest that it is ideally suitlong after solidification. ed to use as a plasma facing material in The researchers were able to extract containment systems in emerging fusion key data from their thermomechanireactors. In order for that to happen, howcal model that describes how the laser ever, researchers must overcome a critical power and scan speed influence the reroadblock. When tungsten cools down— sidual stress and strain in molten tungfrom either high-temperature processing sten. These affect the ductile-to-brittle or after use in extreme environments—it transition, and, in turn, crack formation. stops being ductile and becomes brittle. In particular, a key derivation was the In turn, the material develops microcracks exponential correlation between the acand loses its strength. This is particularly tivation energy of the transition and the problematic for the additive manufacturstrain rate. The results predict that cracks ing of tungsten components, which uses initiate below surfaces and, following the laser heating to melt this metal and conpaths of least resistance along the grain form it to custom curvatures and shapes. boundaries, emerge on the surface. Moreover, while researchers have exam The heating rate and material compoined failed tungsten articles and analyzed sition, including impurities in the matethe networks of fractures that had formed, rial and in the production environment, postmortem analyses have not provided globally affect the structural integrity of much insight into the mechanisms that resulting additively manufactured tungdrive microcrack formation. sten components. Specific alloy compo Researchers from Lawrence Livermore sitions can lower the ductile-to-brittle National Laboratory decided to use transition and strengthen grain boundadvanced videography to address this aries, while preheating of the tungsten will lower thermal gradi15 High-Speed Video ents and similarly 12 Cooldown time to cracking mitigate embrittle9 Crack 6 ment. The diag∆t 3 nostic tools that T (K) 0 the researchers 3000 0 1 2 3 4 5 6 P/v (J/mm) developed in this 2000 800 work will help 700 monitor the addi200 μm 1000 600 tive manufactur500 ing processing of T, σ, ε 400 tungsten and help 300 Ductile-to-brittle transition 0 1 2 3 4 5
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