The Surface Integrity Of High-speed Milling Complex Surface Hardened Steel Dies

Due to the complex structure, varied processing paths and different local hardness of hardened steel dies, the geometric accuracy of die processing has poor uniformity. The physical and mechanical properties are anisotropic. Local fatigue cracks appear on the die surface, leading to failure as a whole. The fundamental reason is that hardened steel die processing lays more emphasis on structural integrity, without considering the significant effect of die structural geometrical features on surface integrity. At present, finite element technology can only be applied to constant curvature simulation surface, which also restricts the study on the consistency of die surface integrity.Firstly, considering the influence factors of dieing, this paper puts forward die milling characteristic elements and classifies structural components. Based on Boolean operation, this paper solves cutting layer volume of different characteristic points in the surface structure and builds a die surface characteristic analytical model, a cutting layer parametric model and a surface characteristic microelement analytical model.Secondly, applying finite element analysis, combined with the selection of cutting tool processing path, this paper calculates the milling process of spliced hardened steel, transitional domain and sine variable curvature surface respectively and obtains the influence law of die structural geometric features on physical fields. With sine die as the research object, through single factor experiment and analysis, it verifies the validity of simulation and provides theoretical basis for consistency analysis. Based on central composite design, relying on quenched steel milling orthogonal test, this paper carries out a significance analysis of process parameters, builds a mapping model of process parameters and thermal-mechanical properties and studies the variation law of milling force and milling temperature under interaction.Thirdly, based on the cutting layer parametric model and thermal-mechanical coupling analysis, this paper studies the variation law of die “externalities”(surface appearance and surface roughness) with process parameters and structural geometric features. According to the surface characteristic microelement analytical model, it simulates 3D appearance of sine die and verifies reliability of the prediction model, by comparing external waviness with the tested surface. Using an elastic-plastic finite element model, this paper solves residual stress distribution law. Combined with milling process thermal-mechanical property analysis, based on dislocation theory and atom thermal motion energy theory, it studies the formation mechanism of die “internalities”(residual stress, surface metamorphic layer and cold hardening layer) and verifies the consistent selection of process parameters.From the above analysis, applying basic data sets as an evaluation criterion for quenched steel die surface integrity can provide theoretical evidence for the selection of quenched steel process parameters.