Detection of tunnel defects in friction stir welded aluminum alloy joints based on the in-situ force signal

Abstract

The demand for high quality and effective manufacturing has raised the need for defect identification in friction stir welding. This work aims to identify the tunnel defect inside the welded joint based on the feature of in-situ force signals. First, we measured the traverse force (Fx), lateral force (Fy) and plunge force (Fz) during the welding process and analyzed the variation characteristic of the force signals for the joints with and without tunnel defect. Then, fast Fourier transform was utilized to obtain the frequency components of the force signal for analyzing the cause of the waveform distortion of the welding force, which showed a direct relationship with the occurrence of the tunnel defect in the joint. The combination result of the frequency components indicated that the waveform distortion of Fx and Fy for the defective joint was mainly influenced by the first harmonic and third harmonic. Thus, a defect detection indicator was proposed by calculating the normalization ratio of the amplitudes of the third harmonic and first harmonic. Tshe best accuracy of the indicator from Fx for identifying the tunnel defect reached 96.42%. Furthermore, the simulation results of the FSW process provide a physical understanding of material flow and defect formation indicated by the first harmonic and third harmonic of Fx, which helps explain the high accuracy of the proposed indicator from Fx signal in the detection of tunnel defect.