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Ultrasonic impact treatment (UIT) is a kind of mechanical assistive technology that can reduce the residual stress of the as-deposited metal parts. Up to now, the mechanism of ultrasonic characteristics affecting the residual stress has not been revealed distinctly. In this work, the 1Cr12Ni3MoVN deposition layer was treated by UIT with multiple ultrasonic characteristics. The results show that UIT transforms the residual tensile stress of the LMD deposition layer into compressive stress. As the amplitude increases from 20 to 30 μm, the maximum and depth of transverse residual stress observed in the vertical direction increased from 354 to 381 MPa and from 0.364 to 0.523 mm. The maximum and depth of longitudinal residual stress increased from 258 to 318 MPa and from 0.482 to 0.785 mm. As the frequency increases from 15 to 25 kHz, the maximum and depth of transverse residual stress observed in the vertical direction increased from 207 to 406 MPa and 0.301 to 0.395 mm. The maximum and depth of longitudinal residual stress increased from 240 to 326 MPa and from 0.341 to 0.517 mm. The influence of ultrasonic frequency is not as prominent as ultrasonic amplitude. The maximum impact velocity of the pin is the major factor determining the residual compressive stress field. The acceleration of the pin is positively correlated with the downward velocity of the horn. Increasing the ultrasonic amplitude can effectively increase the velocity of the horn. Once the impact energy is large enough, the σmax is basically unchanged, but the increase of impact energy can still increase the d. This work is of guiding significance for the elimination and control of residual stress in parts manufactured by LMD and other additive manufacturing methods. [ABSTRACT FROM AUTHOR]

Laser shock peening (LSP) is a surface-strengthening technique that reduces residual stress in metallic parts. However, there is little literature on interlayer LSP strengthening during selective laser melting (SLM). To systematically investigate the effect of LSP on SLM residual stress, we established the removable LSP impact model based on the SLM multiple deposition layers. The mechanism by which the LSP parameters affect the residual stress of SLM is revealed. The results show that the residual tensile stress of SLM transferred to residual compressive stress due to LSP. The peak pressure of LSP is the most crucial factor for determining the residual stress field. The overlap has the second highest effect on the residual stress field due to the higher overlap causing the LSP to be repeatedly impacted at the same location. The effect of LSP diameter and full width at half maximum (FWHM) on residual stress is not as prominent as the first two. However, the appropriate LSP diameter and FWHM can reduce the residual stress hole phenomenon. This work is instructive for eliminating and controlling residual stress in metallic parts manufactured by SLM. [ABSTRACT FROM AUTHOR]

The unique thermal cycle of selective laser melting (SLM) significantly affects the undesirable formability and mechanical properties of the deposited parts, especially for materials with complex compositions. Laser shock peening (LSP) is a strengthening technology that can refine grain, convert tensile stress to compressive stress, and improve fatigue strength. In situ LSP is a technology that combines LSP and SLM without ablative coating. The combination can strengthen the additive manufacturing microstructure layer by layer. Some literature has verified the feasibility of no absorption layer and pressure confining layer LSP. However, little research reported the effects of the in situ combination on the molten pool. In this work, the finite element method (FEM) has systematically investigated the impact of scanning speed and in situ LSP on fluid flow behavior, heat transfer, and the solidification process of the molten pool. The flow velocity and the size of the molten pool decrease as the scanning speed increases. The solidification rate shows an increasing-decreasing-increasing process at low scanning speed during the solidification process. Moreover, the value of R is consistently stable at high scanning speeds. The temperature gradient increases gradually and decreases sharply with the scanning speed increase. The in situ LSP reduces the temperature and the fluid flow of the molten pool, which decreases the heat convection and the value of Peclet number, but has little effect on the solidification process of the molten pool. [ABSTRACT FROM AUTHOR]