Impact of wire feed rate on mechanical properties of GMA Welded joints: a qualitative evaluation using nonlinear ultrasonic technique

Welded assemblies often fail at the weld nugget or heat-affected zone (HAZ) despite stringent design and inspection protocols. This study evaluates the potential of ultrasonic non-destructive testing (NDT) method, specifically nonlinear ultrasonics (NLU), for assessing robotic gas metal arc (GMA) welded mild steel joints fabricated using Cold Metal Transfer (CMT) and pulsed current modes with wire feed rates (WFR) in the range of 4.5–6.5 m/min. Variation in WFR altered the heat input, resulting in distinct microstructural changes in different zones of the welded assembly. Microhardness measurements revealed a minimum (~210 VHN) in the HAZ and a maximum (~290 VHN) in the weld nugget. Yield strength was maximum for both CMT and pulsed current mode with WFR 5.5 m/min. NLU testing indicated a ~20% increase in the nonlinear parameter at the HAZ compared to the virgin alloy, suggesting the formation of a weakest link close to HAZ in the welded structure. This increase is attributed to microstructural heterogeneity and variation in residual stress, with tensile residual stresses being highest near the HAZ–weld interface. The findings highlight that NLU effectively identifies failure-prone regions and provides a reliable, non-destructive approach to assess weld quality without the need for specific sample preparation.