Laser welding of niobium to AISI 304 steel using a copper interlayer

Abstract

Cu interlayers with thicknesses of 1, 1.5, and 2 mm were used to join niobium and AISI 304 steel. Fractures occurred in the weld, the Nb base metal, and the unmelted Cu interlayer when the Cu interlayer thickness was 1, 1.5, and 2 mm, respectively. When the thickness of the Cu interlayer was 1 mm, the weld microstructure consisted of austenite with Cu-rich particles along the austenitic grain boundaries and within the austenitic grains, a composite-like structure (the Fe₂Nb lamellae and particles in a γ matrix) embedded with coarse Cu globules, and a mixture of bulk Fe₇Nb₆, Nb-rich dendrites, and Cu matrix. The bulk brittle Fe₇Nb₆ phase embrittled the joint. However, when the thickness of the Cu interlayer was 1.5 mm, the weld microstructure consisted of austenite with Cu-rich precipitates along the austenitic grain boundaries and a Cu-rich phase embedded with Nb-rich particles and dendrites. Solid-solution strengthening of Cu by Fe was responsible for the improved mechanical properties of the joint. The mixture of Nb-rich particles and dendrites in the Cu matrix was also helpful in enhancing the joint strength. Furthermore, when the thickness of the Cu interlayer was 2 mm, the weld microstructure consisted of austenite with Cu-rich precipitates along the austenitic grain boundaries and within the austenitic grains, an unmelted Cu interlayer, and Nb-rich particles and dendrites embedded in a Cu matrix. The unmelted Cu interlayer reduced the joint strength.

Keywords:

• Laser welding
• niobium
• AISI 304 steel
• microstructure
• joint strength

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The authors would like to thank the National Natural Science Foundation of China [Nos. 51605205, 51675248, and 51905332] and the Natural Science Foundation of the Jiangsu Higher Education Institutions of China [No. 15KJB460009] for their support.