Assessment of weld quality using control chart and frequency domain analysis

Two novel techniques are proposed in this work to distinguish instantly the weld quality. The first technique is based on the recorded voltage during welding while the other is based on the recorded sound. Both voltage and sound are recorded using data acquisition system having 8 KHz sampling frequency and sound recorder respectively. The recorded data are analyzed immediately after welding process using control chart for voltage-based technique and frequency domain analysis for sound-based technique. The result of the first approach revealed that the sample points in the standard deviation or mean control chart scatter beyond the control limits for an unstable process at the defected region whereas they locate within the control limits for stable process. This prominent technique identifies the arc instability caused by the formation of internal minute porosities. The presence of these porosities is confirmed by the radiographic test. For the second technique, the frequency domain analysis of the welding sound proved to have peak magnitude at 216 Hz consistently for the quality welds. Multiple frequencies with significant absolute magnitude away from 216 Hz were the distinct feature observed for the unstable welds. The two methods are found to be highly adaptable for instant monitoring of the quality of the weld.