Bey Vrancken | 18-ERD-057
Project Overview
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.
Mission Impact
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
