Distortion suppression mechanism of laser directed energy deposition on thin sheets based on ultrasonic rolling prestressing

Laser directed energy deposition (LDED) on thin sheet enables the fabrication of both overall geometric shape and local functional features. However, the warping and distortion of the sheet triggered by the high-energy input is the bottleneck that restricts its large-scale industrial promotion. Traditional stress regulation methods (like scan path optimization and heat treatment) have limitations in terms of high fabrication costs and low efficiency, and lack rapid distortion suppress methods based on stress neutralization. This study proposes an active distortion suppression method for laser directed energy deposition on thin sheets based on ultrasonic rolling (UR) pretreatment and establishes a multi-physics finite element model for LDED of UR prestressed sheets. Three processing strategies were designed depending on the combination mode of UR and LDED: CG (conventional group without UR), OSP (opposite side processing of UR sheets with LDED), and SSP (same side processing of UR sheets with LDED). Experimental results show that, compared with CG, the OSP strategy greatly suppressed the sheet distortion by 61.8 %, while the SSP strategy had the reverse effect. The suppression of sheet distortion essentially stems from two aspects: stress neutralization and improved material properties. Specifically, the stress coupling simulation results of OSP strategy show that due to the pre-compressive stress on the surface layer of the rolled sheet, the tensile stress of the sheet parallel to the laser scanning direction is significantly reduced, which predominates in suppressing distortion of sheet. Furthermore, microstructural observation demonstrated that the geometrically necessary dislocations density was increased for the surface layer of ultrasonic rolled sheets, and the proportion of low angle grain boundaries also increased from 5.2 % to 31.6 %. The bending strength, yield strength, and elastic modulus were improved, enhancing the resistance of the sheet to distortion, which is another significant factor in suppressing distortion. The proposed processing strategy is reliable for suppressing distortion in LDED, and provides novel insights for precision control of large parts built by additive manufacture.