Promoting densification and strengthening effect of ultrasonic impact treatment on Haynes 230 alloy manufactured by laser powder bed fusion

Laser powder bed fusion (LPBF) has been extensively investigated owing to its high geometry formation accuracy and excellent mechanical properties. However, the LPBFed Haynes 230 parts typically display poor tensile and wear properties due to internal porosity. In this work, the ultrasonic impact treatment (UIT) was applied as a post-treatment to the LPBFed Haynes 230 alloy, and porosity and microstructure modulation were performed to improve the strength properties and wear resistance. The pore closure and microstructure were studied by numerical simulations and experiments, and the mechanisms of increasing densification and strength were discussed. Results show that UIT can effectively close pores and reduce porosity, the internal porosity of the ultrasonic impacted layer for one, two, and three times decreases by 63.64%, 71.25%, and 81.97%, respectively. Pore closure is attributed to the residual compressive stress and shear stress introduced by UIT. Besides, the UIT weakened texture strength and refined grains, especially promoting the formation of fine grains. Meanwhile, it also promotes the formation of a high dislocation density and improves the phase structure distribution. Furthermore, the ultimate tensile and yield strengths of the optimal impact process increased by 9.63% and 34.56%, respectively. The improvement in strength was attributed to dislocation, grain boundary, and promoting densification strengthening. The average friction coefficient reduces by 4.90%–14.59% by refining the surface grains and increasing dislocation density. This work has verified the feasibility of improving the mechanical properties and pore closure of the LPBFed Haynes 230 alloy by UIT.