Effects of ultrasonic impact treatment on the tensile properties of DH36 steel welded joints

Argon arc welding was used to prepare DH36 welded joints, and the effects of ultrasonic impact treatment (UIT) on the weld metal, heat-affected zone, and base metal were studied using digital image correlation (DIC) and electron backscatter diffraction (EBSD). Tensile tests of DH36 steel welded joints before and after UIT were conducted using the DIC technique, obtaining the stress-strain distributions in each region of the specimen during the tensile process as well as the true stress-strain curves in different regions. The Swift model was used to fit the mechanical properties such as the strength factor, yield ratio, yield strength, and tensile strength in each region. Additionally, the tensile fracture and microstructure after UIT were characterized by SEM and other techniques. The results show that after UIT, the surface grains of the specimens were significantly refined, and the nanoindentation and microhardness showed significant performance enhancement. Meanwhile, the average ultimate tensile strength increased by up to 10.90%. It is worth noting that the yield ratio of all regions decreased, the largest reduction being from 0.788 to 0.740 for weld metal. This decrease was attributed to the combined strengthening mechanisms of fine grain strengthening, dislocation strengthening, and residual compressive stresses induced by UIT. This further indicates that UIT can achieve a synergistic increase in material strength and plasticity. The plasticity and strength of the material are effectively enhanced. In addition, fracture analysis showed that all the specimens exhibited micropore aggregation fracture as the main mechanism and shear fracture as the auxiliary mechanism. After UIT, the fibrous zone and shear lip dimples showed a larger size, and the plastic deformation zone expanded, indicating that UIT enhances the plasticity of the material. This study provides technical and theoretical support for manufacturing high-quality welded joints.