Comparison of CO2 and Nd:YAG laser welding of grade 250 maraging steel, IIW doc. II-A-173-06

Abstract

Laser welding trials were performed on thin walled cylindrical Grade 250 maraging steel tubes to determine its suitability as a joining process for rocket motor casings. Bead-on-plate (BOP) and butt welded samples were produced utilising a 5 kW Trumpf TLF5000HQ CO2 laser (M2 = 1.82) with a 5-axis TLC1005 Lasercell and 200 mm focal length off axis parabolic mirror welding head. The samples were subjected to tensile testing after an aging post weld heat treatment was performed at 480ºC for 3 hours. Acceptable results were obtained although some concerns emerged regarding the ability of the welding jig to ensure repeatability of joint fit-up and alignment of the laser beam with respect to the seam. Alternative techniques such as dual focus (twin spot), scanner welding, addition of filler material and pulsed Nd:YAG welding were investigated to increase the operating window compared to autogenous single spot laser welding by making the process less sensitive to fit up and alignment. Metallurgical investigations were also performed to compare the microstructures produced by the various welding techniques. Twin spot and pulsed Nd:YAG welding increased the tolerance with regards to laser beam offset to 0.5 mm compared to 0.2 mm for single spot autogenous welding. The higher heat input of twin spot resulted in a higher fraction of reverted austenite after PWHT at 480ºC for 3h and reduced mechanical properties compared to pulsed Nd:YAG welding. The presence of contamination and misalignment has been found to have a profound impact on weld integrity. Pulsed Nd:YAG welding is the preferred process due to the large operating window, small amount of reverted austenite and excellent weld geometry