Monitoring of PHS Joining Quality with Non-Destructive Testing (NDT)

PHS is an ultra-high strength material which is difficult to join either by welding or through mechanical joining techniques. Joining PHS always bears the risk of irregularities affecting the static and dynamic strength and of the joint. Therefore, inspecting the joint quality (post-process) and / or monitoring the joining itself (in-process) will be appropriate. This paper gives an overview of Non-Destructive Testing (NDT) methods for this purpose. In case of Resistance Spot Welding (RSW) the strength of the welds are mainly defined by size and quality of the weld nuggets [1]. Fraunhofer IZFP has developed an UltraSonic (US) welding monitor allowing real-time observation of the RSW process with US transducers integrated in the welding gun [2]. With this NDT system nuggets size can be determined and welds and bonds can be distinguished. RSW generates a local tempering and hence a weakening in PHS [3]. Weld spot size as well as hardness profile in the welded and heat-affected zone can be determined with the micro-magnetic 3MA technique. If PHS should be laser welded, the aluminum-silicon coating could be a problem. Therefore, the coating is partially ablated by laser [4]. Again, the 3MA technique can be used in order to verify the success of the partial ablation by determining the thickness of the residual coating layer. The laser welding process itself can be monitored by modern techniques of Acoustic Emission (AE) testing. New transducers and microphones with sensitivity up to the MHz range allow reliable detection of weld seam irregularities, like insufficient penetration, holes and undercut. Also a mechanical joining process can be monitored by AE - for example high-speed tack-setting. Defects like head overhang and breakthrough can be identified by AE. As in case of RSW also tool-integrated US transducers can be used to monitor the mechanical joining. During screwing the pre-tension force can be observed as well as the residual bottom thickness and with it the size of the undercut during clinching. A relatively new approach is the low-heat joining of PHS components by Friction Stir Welding (FSW) or by the special variant of UltraSound Enhanced Friction Stir Welding (USE-FSW). This approach is especially suited for mixed joints (e.g. PHS + Al). The development of this joining method and adapted NDT methods is topic of ongoing research.