High power Nd-Yag laser welding of super duplex stainless steels

Abstract

The newly developed super duplex stainless steels (SDSS) are highly alloyed duplex stainless steels (DSS) for use in aggressive environments. SDSS have high pitting resistance PREN >= 40 and excellent mechanical properties. Welding of SDSS by conventional techniques such as TIG welding, MIG welding, etc., has been widely investigated and the process conditions for achieving optimum austenite/ferrite (帠/帢) ratio in the weld have been more or less optimised. However, welding of SDSS by high power lasers is far from satisfactory. Most investigations were carried out by high power CO2 lasers and the resulting weld microstructures were worse than those of the parent metals. This project will look into the process of welding SDSS by the newly developed high power Nd-YAG lasers. Extensive literature review on the welding metallurgy of SDSS, methods for measuring the 帠/帢 ratios and the welding of SDSS by CO2 laser were carried out. As high power Nd-YAG laser welding has much more process variables than those of CO2 laser welding, i.e. pulsed output wave form in Continuous wave, Sine wave and Square wave etc., a statistical approach by Taguchi method was used for parameter design, so as to reduce the experimental effort involved. The parameters studied included laser power, focus position, pulse frequency, forms of power output, welding speed, metal thickness (4 and 2.5 mm), shielding gas type (Nitrogen: N2, Helium: He, Argon: Ar) and shielding gas flow rate. Taguchi method was proved to be very efficient and effective in searching for process variables for optimum 帠/帢 ratio in the welded metal. An almost ideal 50/50 帠/帢 ratio was achieved with Nd-YAG laser welding parameters optimised by Taguchi method. This result can be claimed to be the first ever published. Various methods in determining the 帠/帢 ratio in the laser weld were discussed and tried, and the best method was found to be the X-ray diffraction technique. Mechanical and corrosion properties of the laser welds with different 帠/帢 ratios and those of parent metals were investigated by tensile and polarisation tests respectively. The laser weld with optimum 帠/帢 ratio also showed optimum mechanical and corrosion properties. Microstructures of the laser welds were investigated by optical and scanning electron microscopes. The microstructures of the laser welds were varied by using different process parameters. This was due to the difference in thermal cycles during the welding, solidification and transformation processes.