Numerical investigation of thermal-microstructure behavior in hybrid CMT and tandem P-GMAW-narrow-gap-welding of HSLA-steel

A comprehensive three-dimensional heat transfer model was developed to investigate the temperature distribution during narrow gap welding (NGW) under three different lead-to-trail wire feed speed ratios (WFSL/T): 1.4, 1.0, and 0.7. The welding sequence began with a root pass using the cold metal transfer (CMT) technique, followed by filling and capping passes performed using tandem pulsed gas metal arc welding (P-GMAW). The model accurately captured the unique arcing and short-circuiting characteristics of the CMT process, including both spiral and triangular arc oscillation patterns. Model predictions closely matched experimentally measured cooling rates, validating its accuracy. Among all weld passes, the root pass and its associated heat-affected zone (HAZ) exhibited the highest cooling rates. A WFSL/T ratio of 1.0 resulted in the slowest cooling rates, while a ratio of 0.7 led to the fastest. Repeated thermal cycling from successive passes led to coarsening of various ferrite phases, resulting in decreased hardness. Notably, the WFSL/T ratio of 1.4 yielded the highest toughness in both the weld metal (97 J) and HAZ (74 J). The corresponding yield strengths for WFSL/T ratios of 0.7, 1.0, and 1.4 were 450 MPa, 430 MPa, and 425 MPa, respectively.