Hybrid Laser-Arc Welding of Ni-base Alloy 690
Open Access
- Author:
- Blecher, Jared Jacob
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- March 01, 2019
- Committee Members:
- Tarasankar Debroy, Dissertation Advisor/Co-Advisor
Todd Palmer, Committee Chair/Co-Chair
Long-Qing Chen, Committee Member
Allison Michelle Beese, Committee Member
Robert Carl Voigt, Outside Member - Keywords:
- laser welding
hybrid welding
nickel alloy
keyhole porosity
root defect
inline coherent imaging
heat transfer and fluid flow modeling - Abstract:
- The welding of Ni-base alloy Inconel™ 690 is commonly required during the construction and refurbishing of nuclear power plants with plate thicknesses varying between 1 mm to greater than 25 mm. However, during conventional multi-pass welding of Alloy 690 in this thickness range, micro-cracking in the form of ductility dip cracking (DDC) and solidification cracking can occur, causing significant delays in an already expensive industry. Hybrid laser-arc welding can significantly reduce the number of passes necessary to weld thick sections and, at the same time, lower the risk of forming DDC and solidification cracking. Significant advantages can be achieved by welding with laser and arc energy sources in close proximity. The high intensity laser forms a vapor cavity, or keyhole, leading to a large increase in weld depth, and welding speed is typically higher during laser welding. On the other hand, an arc creates a wide weld pool, which is useful for bridging gaps between plates, and can add material to the weld with a consumable electrode. However, hybrid laser-arc welding can lead to unique defects not found in conventional arc welding, including keyhole porosity and root defects. Porosity from keyhole instability and collapse can lead to very large bubbles (> 1 mm) becoming trapped as pores in the weld metal during partial penetration welds, while, during full penetration welds, weld metal can fall out of the weld and solidify as nuggets, a form of root defect. The solidification microstructure, root defect formation, keyhole porosity, and keyhole dynamics are studied in-depth using a combination of tools including a three dimensional heat transfer and fluid flow model, X-ray computed tomography, and inline coherent imaging. The results of this work produced a solidification map for Alloy 690, clarified the phenomena affecting root defect formation, developed hybrid welding solutions to keyhole porosity, and documented the keyhole growth rates during laser welding.