Comparison of real-time ultrasonic M-scans and dynamic resistance curves obtained from resistance spot welding process.

Abstract

The goal of this paper is to present a correlation study between the dynamic resistance curve obtained from a Resistance Spot Welding (RSW) process versus inspections made through real-time ultrasonic measurements. In steel plate samples, with varying thickness, the ultrasonic images were collected during the welding process using a Tessonics ultrasonic system with single-element, 12 Mhz transducer in pulse-echo mode, installed inside a C-type welding gun, with cooling water as the coupling medium. A-scans were collected with an interval of 1 millisecond, in order to construct an M-scan image which is a signature of the welding process. Dynamic resistance curves of each joint were acquired using the Obara Hawkeye adaptive welding controller mounted on a 90 KVA, medium frequency and direct current (MFDC) machine. Welding schedules were prepared under adaptive mode, with both welding current and time set as variables. The M-scan image was synchronized with the respective dynamic resistance curve for each of the collected samples, aiming to identify and correlate the characteristics between both. The experiments were able to find strong correlation between the M-scans and dynamic resistance curves. In the M-scans, we could observe the amount of heat developed directly between the electrodes, liquid metal thickness, and its penetration into the plates, while in the dynamic resistance curve we could observe corresponding features, such as the resistance maximum peak, as well as sudden resistance drops. The results collected and presented will be used for identification of the main correspondences between both approaches, and further development of specialized instrumentation for improvement of the existing resistance curve-based adaptive welding systems.