Analyzing the effect of different shielding atmospheres on fiber laser welding parameters for modified 9Cr-1Mo steel

Abstract

The present research aims to investigate the influence of and the relationship between welding speed (v), focal position (F_P), and laser power (L_P) as input parameters for fiber laser to weld P91 under pure argon and CO₂ shielding atmospheres using response surface methodology and using the desirability approach; optimization of input parameters has been performed to maximize the depth of penetration (D_P) while minimizing the top bead width (T_W) and heat-affected zone width (H_W). The results indicate that laser power (L_P) has the most significant impact on depth of penetration (D_P), whereas welding speed (v) affects top bead width (T_W) and heat-affected zone width (H_W). The optimal combination of parameters under argon shielding results in a significantly low heat input of 48 J/mm (0.048 kJ/mm) and 50 J/mm (0.050 kJ/mm) pure argon and CO₂ shielding atmospheres, respectively. Despite the difference in heat input being only 4.08%, an 11.618% increase in D_P is observed under the CO₂ shielding atmosphere. Due to the high-speed nature of the welding process (argon: 82.65 mm/s and CO₂: 79.283 mm/s) combined with the significant reduction in heat input, the heat-affected zone is limited to a mere width of 0.203 mm under an argon shielding atmosphere and 0.263 mm under the CO₂ shielding atmosphere compared with that reported in the other welding processes without the dissolution of precipitates in the fusion boundary of heat-affected zone. Microstructural characterization of the fusion zone has revealed the presence of an untempered lath martensite structure with precipitate dissolution. The precipitate dissolution has resulted in grain coarsening of 47.76% in argon weld and 39.65% in CO₂ weld compared with the base metal grain size of 6.07 µm.