Grain refinement and mechanism of steel in ultrasound assisted wire and arc additive manufacturing

The small volume molten pool of wire and arc additive manufacturing (WAAM) undergoes complex non -equilibrium solidification process, resulting in the formation of columnar grains that severely limits the me-chanical properties of the fabricated components. For refining the microstructure of deposition layers, the optimized ultrasonic equipment and loading methods are installed on a deposition platform, and the ultrasonic energy is coupled synchronously with the WAAM process using ER70S-6 welding wire as the filling material. Experimental results show that high-intensity ultrasound can transform coarse columnar grains into fine equi-axed grains, the area and degree of grain refinement increase with the ultrasonic amplitude. A computational fluid dynamics model of ultrasound assisted WAAM was developed to study the pressure field distribution in the molten pool. The region of acoustic cavitation in molten pools is highly consistent with the region of grain refining, showing that acoustic cavitation is the key factor in changing the grain structure. The collapse of cavitation bubbles generates cyclic alternating local high pressure, resulting in high velocity melt flow near the grain, which causes fatigue fracture of growing columnar grains exceeding the yield strength. A theoretical model is proposed to calculate the maximum length of grain growth under acoustic cavitation, which is in agreement with the experimental statistics of grain size. This work reveals the effect area and mechanism of ultrasound on the rapidly solidified molten pool, and provides a theoretical basis for grain refinement in ul-trasound assisted WAAM.