Numerical and experimental investigation of the influence of the machining parameters on residual stress distribution of internal thread cold extrusion.

In order to obtain cold extruded internal thread with better quality, the cold extrusion and cutting of 42CrMo4 ultra-high strength steel internal thread are carried out. The tooth shape, microstructure and residual stresses at different orientations are established and discussed. The elastic-plastic finite element model is then established to simulate the cold extrusion processing of internal thread. It is used to analyze the effects of key processing and design parameters (including workpiece bottom hole diameter, machine tool speed, lubricating medium and margin ratio) on tooth shape and the residual stresses around the thread surface. The results show that each parameter has a certain influence on the residual stress of the thread after extrusion processing. The diameter of the bottom hole directly determines the magnitude of the residual stress, and the change of the lubricating medium has no obvious effect on the residual stress. In general, the residual stress along the depth direction of the thread appears as compressive stress on the surface, and shows a trend of increasing first and then decreasing, after reaching the critical point, it quickly changes into tensile stress, finally tends to be flat. The technological parameters of cold extrusion of internal thread are optimized by orthogonal design. After optimization, the maximum residual compressive stress increases by 17.52% and the maximum residual tensile stress decreases by 19.13%. Although the thread height is reduced by 0.05 mm, the surface quality may be improved and the connection strength of the thread still meets the requirements. [Display omitted] • The cold extrusion and cutting of 42CrMo4 ultra-high strength steel internal thread are carried out. • Cold extrusion is beneficial to generate residual compressive stress field near the hole wall, which may eliminate the influence of stress concentration and reduce the sensitivity of fatigue cracks, and finally restrain the rate of crack initiation and propagation. • The effects of key processing and design parameters on tooth shape and the residual stresses around the thread surface are discussed. • After optimization, the maximum residual compressive stress increases by 17.52% and the maximum residual tensile stress decreases by 19.13%. [ABSTRACT FROM AUTHOR]

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