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Non-Destructive Evaluation of Aluminum Resistance Spot Weld Quality

IP.com Disclosure Number: IPCOM000249369D
Publication Date: 2017-Feb-22
Document File: 3 page(s) / 154K

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Non-Destructive Evaluation of Aluminum Resistance Spot Weld Quality

Resistance spot welding (RSW) is a robust and cost-effective joining technique of great significance to the automotive industry. Meeting the strict demands for reduction of fuel consumption and vehicle emissions has necessitated increasing use of aluminum (Al) alloys in vehicles, leading to increased reliance on Al RSW. One of the chief cost penalties associated with Al RSW is teardown and inspection. A Non-Destructive Evaluation (NDE) method, serving as an alternative to teardown, would enable reduced scrap as well as potential ergonomic benefits. Thus, a robust NDE technique that can achieve acceptable weld classification accuracy, for both welded and weld-bonded assemblies, and attain plant acceptance, all at a satisfactory cost, is highly desired.

Ultrasonic techniques are the most capable and prevalent NDE methods for RSW. However, even these methods might fall short in correlating the ultrasonically-measured button diameter to teardown button diameter. Specially, identification of “cold” spot welds is one of the key difficulties in utilizing ultrasonic NDE of Al RSW. Cold welds are defined as those where the input heat and pressure are not sufficient to produce a satisfactory weld nugget that can withstand the expected stresses during service. However, the heat and pressure are enough to form a relatively weak bond across the faying surface. This bond allows the ultrasound frequencies available in instruments that can be robustly used in plants (10-15 MHz) to traverse across the faying surface, leading to mistaken identification of the cold spot weld as satisfactory. Entire welds can be “cold,” or there may simply be an annular region at the perimeter of a properly-formed weld that is cold.

Any viable NDE technique is required to correctly characterize a significant fraction of cold welds, while avoiding misidentification of satisfactory welds, i.e. having sufficiently low false positive and false negative error rates. Though state-of-the-art, plant-ready ultrasonic instruments (using either single-element or array probes) correctly identify a non-negligible fraction of Al RSW cold welds, no system is capable of providing the desired NDE error rates. Another method through which cold welds can at times be identified is visual inspection. The insufficient input heat and pressure lead to cold welds having a relatively shallow indentation, allowing experienced inspectors to correctly surmise problems. However, the false positive and false negative rates for vis...