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Welding Journal | January 2014

A B welds. In case A, welding current was too low, and the intermediate interface did not fully disappear (melting did not happen) during welding. In B, welding current was high enough and the intermediate interface disappeared (melting began approximately halfway through welding time). Several parameters are used to make a decision about the weld quality, including the presence of a liquid nugget and its penetration into the sheets to determine the weld height — Fig. 6. Ultrasonic 60 JANUARY 2014 time of flight (TOF) through the welded sheets is directly proportional to the amount of heat developed in the stackup. It has been shown that there is a strong correlation between TOF and nugget diameter (Refs. 8, 9). Special software analyzes the M-scan and extracts these parameters automatically. The decision whether the weld is acceptable or not is made right after the weld is made and before the robot moves to the next weld position. This allows the implementation of feedback to the robot to either stop the line or perform rewelding of the unacceptable weld. Expulsion Detection Expulsion detection in welding is a strong area of interest, particularly for the inline system where the effects of a physical expulsion can make the ultrasound scans of a good weld appear to be undersized. In some cases, expulsion can have a significant effect on the quality of the weld and is undesired. The main problem with expulsion in the ultrasound M-scans in applications where thick sheets and 3T stacks are welded is that the heat loss during expulsion may appear as if insufficient heat was generated to produce a significant nugget, when in fact more than enough heat was produced. For this reason, expulsion events must be detected so that the future decision of the weld’s quality can consider this effect. Currently, there are a number of methods used to detect expulsion, predominately using feedback from servo motors to record the displacement of the electrodes. However, setups not using servo motor feedback (e.g., pneumatic guns) require additional hardware to detect expulsion events. For systems already using the inline ultrasound device, expulsion detection is possible without additional hardware or communication with servo motor controllers. The simplest form of expulsion detection is performed by monitoring sudden changes in TOF from the front wall and back wall reflections (lines 1 and 2 in Fig. 7). The back wall, or lower interface, shows the greatest change when an expulsion occurs, since the position of this reflection in the M-scan depends on both the total heat in the weld zone, as well as the total thickness of the workpiece. During an expulsion, both the total temperature and thickness of the workpiece decrease as heated material is ejected from the weld zone. As a result, there is a sudden decrease in the TOF of the lower interface. The front wall, or upper interface, also exhibits a small perturbation during an expulsion. The TOF of this surface generally increases as a result of localized heating at the contact point between the electrode and plates. This arises when substantial indentation results from a large expulsion. At this moment, reduced contact between the electrode and plate surface increases the current density at this interface, which generates a great deal of heat at the surface. This momentary increase in temperature increases the TOF at the front wall reflection; this is a strong indicator of a substantial expulsion. Figure 7 shows an example of both the front wall and back wall shifts in time of flight that make expulsions detectable in real-time M-scans. When an expulsion is detected, the quality decision can compensate for the reduced time of flight to ensure that good welds are not considered bad. Real-Time Integrated Weld Analyzer A real-time integrated weld analyzer (RIWA) was developed to use this technology in an industrial environment. This device is a small unit installed on each welding robot where the quality control Fig. 5 — Real M-scans of the resistance spot welding process. A — Low welding current; B — high welding current. Fig. 6 — Relating ultrasonic M-scan to real spot weld geometry. Fig. 7 — Interface changes at a moment of expulsion.


Welding Journal | January 2014
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