A new thermographic procedure for the non-destructive evaluation of RSW joints

Resistance Spot Welding is one of most used method to weld two sheet material thanks to its reliability and rapid production that makes it economical. The quality of the produced joints is related to the process parameters such as current, time, electrode pressure and surface condition and generally, destructive tensile tests are used to investigate the effect of each parameter on the quality of joints. However, these latter are time consuming and do not allow for a complete check (100%) of joints. In this work, a non-destructive thermographic technique and a new procedure was used to assess the quality of joints. In particular, different steel joints were obtained by varying two process parameters: current and cycle time. These joints were tested with the pulsed thermography technique adopting a transmission set-up. Thermographic data were acquired by means of a cooled infrared camera capable to acquire thermographic sequences at 400 Hz with a geometrical resolution equal to 0.062 mm/pixel. A flash lamp has been used to produce a thermal pulse of 3000 J with duration 5 ms. The thermal signal was investigated in the time domain with the aim to obtain different features related to the investigated process parameters. Proposed approach allows for detecting the typical defects that affect the joints such as stick and expulsion of material.