Automated Spot Weld Inspection Using Infrared Thermography

Resistance spot welding (RSW) is an essential joining technology used in automotive body structure assembly. Variations in welding conditions, materials, part dimensions, fit-up and other production conditions inevitably occur in the high-volume and complex body-in-white (BIW) assembling process. These variations can result in out-of-tolerance joints that impair the quality and performance of the vehicles. Accurate and robust welding quality inspection and control is important to ensure the safety and performance requirement of auto body.  A traditional weld quality inspection method currently in use is the destructive evaluation in tear-down booths, which is labor intensive, time consuming and expensive.  In this work, an automated non-contact and non-destructive spot weld system based on infrared (IR) thermography are developed for both real-time (online) and post-weld (offline) applications in automotive manufacturing plants. For the real-time application, the surface temperature evolution during spot welding is observed using an IR camera. For the post-weld application, an auxiliary heating device is used to heat up one side of the weld, while the resulting temperature evolution on the other side is observed. A fully-automated and robust algorithm is specifically designed to correlate the IR thermal signatures, extracted from the recorded IR images, to the spot weld attributes such as nugget size. Fig. 1 shows an example of the quantitative determination of weld nugget size in post-weld application. Furthermore, the system hardware and software are optimized for meet the inspection cycle time requirements dictated by the mass production assembly line environment, i.e., 2-3 sec or less for online inspection, 5 sec or less for offline inspection. Application of the IR-based inspection system to a variety of advanced high strength steels, thickness gauges, coatings, and stack-up configurations will be discussed.

Fig.1 Preliminary results illustrating the quantitative correlation of an IR thermal signature to weld nugget size in post-weld inspection