集束探触子による薄鋼板抵抗スポット溶接部の超音波試験

Abstract

It is generally impossible to non-destructively inspect spot welds of thin materials by the conventional normal beam technique of ultrasonic testing, because surface and flaw echoes appear so near together on the cathode-ray tube that these echoes can not be distinguished from each other. In this paper, the nugget formation in resistance spot welding of two-plate lap joints of 0.5 mm thick mild steel sheet is examined micrographically, and an improved technique of ultrasonic testing with an immersion-type focusing probe is tried to evaluate the welds. As a result of this study, the welding variables to produce sound fusion or solid-state welds without expulsion during welding process are determined. In the present ultrasonic technique under such a water distance, that the focus of ultrasonic beam locates beneath the weldment, the echoes from back reflection within the weld region and from faying surface reflection within the nonwelded region converge by multiple reflection, and the echo peak appears with an observable delay behind the front surface echo on the A-scan display. By this process, the back and interface echoes can be detected on the scanning inspection too. The scanning graph of the echo height of back and interface reflections versus probe location for each weld mostly records a composite pattern of protrusion and depression from the level of interface echo due to convergence of the back echo and scattering of wave by the indentation of surface respectively. The C-scan display drawn from these scanning graphs indicates shape of the weld region and data corresponding to the indentation.