Bey Vrancken | 18-ERD-057
Microcracking and residual stresses are key limitations of laser powder bed fusion (LPBF), an additive manufacturing technique that uses a high-power laser to consolidate successive layers of metal powder. For tungsten, these microcracks find their origin in the combination of a ductile-to-brittle transition between 200°C and 400°C and high residual stresses. This work utilized in situ high-speed video to capture the cracking mechanisms and combined the experimental results with thermomechanical modeling to unveil correlations between crack network morphology and process parameter-related variables, such as the local temperature and residual stress distributions. Consequently, preheating and alloying with rare earth oxides were adopted as possible crack-mitigation strategies, the efficacy of which was tested against the initial fundamental baseline results. Preheating temperatures above 500°C eliminated all cracking, although this threshold is expected to be higher when tungsten powder (higher oxygen content) is introduced. Alloying can serve as an active oxygen-getter in the system, but the rare earth oxides in this work were too large to have significant contributions to crack-mitigation.
This work contributed to Lawrence Livermore National Laboratory's advanced materials and manufacturing core competency and laid the required groundwork for refractory alloy additive manufacturing at Livermore. The project created a new experimental capability, the Flexible Laser Additive Manufacturing in Extreme environments (FLAME) system located in Livermore's Advanced Manufacturing Laboratory, to study laser–material interactions in a controlled atmosphere and at high temperatures. This capability is of interest to potential collaborators, and its focus on refractory/high-temperature alloys aligns with DOE mission goals.
Publications, Presentations, and Patents
Vrancken, B., et al. 2018a. "In-Situ Characterization of Tungsten Microcracking in Selective Laser Melting." 10th CIRP Conference on Photonic Technologies [LANE 2018], Procedia CIRP 74: 107–110. Fürth, Germany, September 2018. doi: 10.1016/j.procir.2018.08.050. LLNL-CONF-748304, LLNL-PRES-757043
——— 2018b. "High Speed, In Situ Monitoring of W Microcracking During SLM." Solid Freeform Fabrication Symposium. Austin, TX, August 2018. LLNL-PRES-756094
——— 2018c. "Microcrack mitigation during Selective Laser Melting of Tungsten." Lawrence Livermore National Laboratory post-doctoral poster symposium. LLNL-POST-752649
——— 2019a. "In Situ Observation of Crack Mitigation Effects of Alloy Additives in Tungsten." Materials Science and Technology 2019 Technical Meeting and Exhibition, Portland, OR, September/October 2019. LLNL-PRES-790821
——— 2019b. "Tungsten Alloying to Reduce Cracking During Laser Powder Bed Fusion." Euromat 2019 European Congress and Exhibition on Advanced Materials and Processes, Stockholm, Sweden, September 2019. LLNL-PRES-788857
——— 2019c "Microcracking During Selective Laser Melting of Tungsten Alloys." Lawrence Livermore National Laboratory post-doctoral poster symposium. LLNL-POST-772853
——— 2020a. "Analysis of Laser-Induced Microcracking in Tungsten Under Additive Manufacturing Conditions: Experiment and Simulation." Acta Materialia 194: 464–472. doi: 10.1016/j.actamat.2020.04.060. LLNL-JRNL-805870
——— 2020b. "Influence of Preheating on Tungsten Microcracking During Laser Scanning." TMS 2020 Annual Meeting and Exhibition, San Diego, CA, February 2020. LLNL-PRES-805429