Influence of Tool Convex Hull Structure on Cold Expansion Strengthening Effect of Non-Round Holes

Two types of direct hull mandrel reciprocating cold expansion tools suitable for non-circular holes of nickel-based superalloys, namely arc-shaped and trapezoidal convex hull mandrels, were designed. The finite element models of different hull mandrel reciprocating cold expansion were established by Abaqus finite element software, and the residual stress distribution on the hole wall of GH4169 superalloy after reciprocating cold expansion strengthening of the two types of convex hull mandrels was simulated. The surface morphology, hardness and residual stress of the non-round hole walls of two types of convex hull mandrels after reciprocating cold expansion strengthening were characterized by experimental methods. The results show that the changes of residual compressive stress vs the distance from the hole wall surface on the hole wall after cold expansion strengthening with the two expansion tools by simulation and test was consistent. The relative error of the residual stress near the extreme value of the residual compressive stress was less than 20%, verifying the accuracy of the finite element simulation method. Compared with those after reciprocating cold expansion strengthening with arc-shaped convex hull mandrels, the surface residual compressive stress of the hole wall after reciprocating cold expansion strengthening with trapezoidal convex hull mandrels was about 200 MPa higher, the depth of the residual compressive stress layer was 150–200 μm greater, and the extreme values of the residual compressive stress were not much different; the depth of the residual compressive stress layer, the extreme value of the residual compressive stress, and the range difference of the surface residual compressive stress on the long side, short side and corner of the hole wall was smaller, and the axial force change during the expansion was more gentle. The surface roughness after cold expansion strengthening with the trapezoidal convex hull mandrels was lower, and the range difference between the axial and circumferential surface roughness was smaller; the depth of the axial hardness enhancement layer at different positions of the hole wall （1 020–1 035 μm） was significantly higher than that after cold expansion strengthening with the arc-shaped convex hull mandrels （720–920 μm）, and the range difference was smaller. Trapezoidal convex hull mandrels were superior to arc-shaped convex hull mandrels in improving the uniformity of residual stress distribution in non-round holes, enhancing the surface hardness of the hole wall, and reducing the surface roughness of the hole wall.