Acoustic Emission Signal Characteristics Of High Strength Steel Weld-bonding Process

In this paper,1.2 mm thick DP780,DP590 dual phase steel and 6061 aluminum alloy were used the orthogonal test of weld-bonded was designed.Acoustic emission acquisition system acquired the acoustic emission signal,which was generated during the weld-bonded process.Parameter of the acoustic emission signal was analyzed.The results showed that there are three characteristic stages in the acoustic emission signal during the weld-bonded process: electrode loading stage,nugget nucleation stage and electrode unloading stage.Because the overflow of the cauterizing gas has an impact on the molten pool during the weld-bonded process,the ringing count and amplitude of the acoustic emission signal,that was in the first half of the nucleation stage of the weld-bonded process,were larger than those of the spot welded joint.The acoustic emission signals of spatter and non-splash were compared.The ringing count and positive peak of the acoustic emission signal,which was generated in the splash nucleation stage,are greater than those of the non-splash nucleation stage.The ringing count and positive peak were fitted with the nugget diameter and the tensile shear load of the weld-bonded joint,which was used to evaluate the quality of the weld-bonded joint.It was found that the ring count and the positive peak were positively correlated with the nugget diameter and the tensile shear load of the weld-bonded joint.The spectrum analysis of the acoustic emission signals of DP780 dual phase steel spot welded and weld-bonded process showed that the dominant frequency bands corresponding to the nucleation stage of the spot welded joint and the weld-bonded joint concentrated in the range of 250~300 kHz and 220~250 kHz.The main energy concentrates in the range of 50~450 kHz.The dominant frequency bands corresponding to the nucleation stage without splash defect were 200~250 kHz and150~300 kHz respectively,and the main energy concentrates in the range of 50~500kHz.The acoustic emission signal generated during the welding process of the spotwelded joint and the weld-bonded joint was decomposed by the 4-layer scale wavelet decomposition of the db4 wavelet basis function,and it was found that the signal in the frequency range of 50 kHz to 400 kHz occupies the most energy of acoustic emission signal,which was the dominant frequency layer of the acoustic emission signal.The Euler-Lagrange finite element model of the weld-bonding was established to analyze the impact of the gasification of the glue layer on the molten pool.The high-pressure gas was drained from the clearance between the upper and lower sheets.The cavity area was formed around the nugget of the weld-bonded joint.The non-destructive tests of the weld-bonded joints were carried out using the ultrasonic scanning microscope.The results showed that the weld-bonded joints can be classified into adhesive layer area,cavity area and nugget area,based on analyzing the A-scan signal and the C-scan image.The ultrasonic testing analysis and simulation results were coincided with each other.