Reducing the residual stress in micro electroforming layer by megasonic agitation

•Proposed a novel experiment method of megasonic assisted electroforming.•Investigate the effectiveness of megasonic agitation on reducing the residual stress.•Explore the best megasonic power density to reduce the residual stress.•Reveal the mechanism of reducing the residual stress by meagasonic agitation.•Investigate the effect of acoustic streaming and stable cavitation produced by megasonic agitation. In order to reduce the large residual stress in micro elelctroforming layer, megasonic assisted electroforming is proposed here. Micro electroforming experiments were performed with and without megasonic agitation, respectively. Four different megasonic power densities were applied to investigate the influence of megasonic agitation on reducing the residual stress. The residual stress was measured by X-ray diffraction (XRD) method. Experiment results show that the residual stresses fabricated with megasonic agitation are less than that fabricated without megasonic. When the megasonic power density is 2 W/cm2, the residual stress can be the minimum value of −125.7 MPa, reduced by 60% in comparison with the value of −315.1 MPa electroformed without megasonic agitation. For exploring the mechanism of megasonic agitation on reducing the residual stress, the dislocation density and crystal orientation were calculated by the single-line Voigt profile analysis and Relative Texture Coefficient (RTC) method, respectively. The diameters and distributions of pits on the surface of electroforming layer were observed by the STM-6 tool microscope and counted by the Image-Pro Plus software. It reveals that one hand of the mechanism is the acoustic streaming produced by megasonic can strengthen the motion of dislocation in crystal lattice and makes the crystal lattices grow towards the equilibrium shape, which is benefit to crystallization with low residual stress. When the megasonic power density is 2 W/cm2, the dislocation density increases to be the maximum value of 8.09 × 1015 m−2 and the difference between RTC(1 1 1) and RTC(2 0 0) decreases to be zero, which is consistent with the residual stress results. The other hand is that the stable cavitation produced by megasonic can provide residual stress release points during the electroforming process.