Microstructures and Mechanical Properties of 2024 Aluminum Alloy Welded Joint after Ultrasonic Peening Treatment

Aluminum alloy (2024-T4) was TIG (tungsten inert gas) welded using ER5356 welding wire. The surface of the weld zone, weld toe, HAZ and parent zone were treated by ultrasonic peening treatment (UPT). The microstructure and mechanical properties of the welded joint with and without UPT were studied. The strengthening mechanisms of UPT on 2024 aluminum alloy welded joint were discussed. Microstructure analysis demonstrates that the coarse dendritric grain near the surface of welded joint was refined into ultra tine grains after UPT. Defects such as gas cavity and shrinkage porosity in the weld zone were reduced substantially after UPT. The metallurgical structure of the near surface material of the weld zone was densified. The average tensile strength of the weld joint with UPT is about 17.4% higher than those without UPT. The elongation to failure of the specimens with UPT is about 28% higher than those without UPT. Fractographic analysis reveales that the welded joints without UPT are fractured in the weld toe and fusion and exhibit brittle fracture in tensile test, while those with UPT are cracked in the center of the welding bead, present a 45 turning of the cracking path in the strengthened surface layer and show mixed ductile and brittle fracture with shear lip presents in the fracture. The plastic deformation, grain refinement and reduction of welding defects in the welded joint after UPT are the main mechanism for the improvement of mechanical properties of the 2024 aluminum alloy welded joint.