| With the development of super-hard cutting tool materials and high performance machine tools recently, HSM (high speed machining) technology has already been widely applied in many industries. Hard milling, as a branch of HSM technology, is attracting considerable world-wide interest from die and mold industries. Hard milling means directly milling dies and molds at their hardened state, which has many advantages in comparison with the traditional processes, such as the reduction of machining costs and lead time, the improvement of the surface integrity, the elimination of part distortion caused by heat treatment, and the enhancement of metal removal rate.AISI H13 steel combines a very good hot-hardness with toughness and covers a wide variety of applications, such as dies for pressure casting, extrusion, hot forging and extrusion mandrels. Under the severe service conditions (high temperature, high pressure and thermal shock), the machined surface of the die and mold components must satisfy certain requirements of geometrical surface integrity (surface roughness, shape and dimension precision) and physical surface integrity (microstructure, hardness and residual stress). However, the lack of understanding and corresponding control of potential problems induced by cutting force, cutting temperature and plastic deformation, such as residual stress, microcrack, microhardness and phase transformation, which is responsible for the decrease of the fatigue life of the die and mold components hinders the wide employment of hard milling in die and mold industry. Only if stable surface integrity is obtained can hard milling have industrial application in machining die and mold components.In the present research, single factor experiments were conducted to investigate surface roughness, residual stress, microhardness and phase transformation systematically in hard milling of H13 steel.①The surface topography and the effects of cutting parameters on surface roughness are experimentally investigated, and the orthogonal experiment and the experiments with the same metal removal rate were conducted to optimize the cutting parameters to get the desired surface roughness. The results indicate that the combined effects of feed marks and scallops result in the anisotropic surface profile of the milled surface and the roughness of an area not the roughness of a line on the milled surface is adopted in this research; Under a certain metal removal rate, the combination of high cutting speed, small axial depth of cut and high feed, small radial depth of cut produces the best surface roughness in hard milling of AISI H13;②The effect of cutting parameters on microstructure, microhardness and residual stress are studied, and the residual stress distribution between two adjacent machined traces is investigated originally. Under the cutting conditions in this research, very thin white layers (less than 3μm) are formed during most trials; the microhardness of the milled surface showed that a work-hardening effect has been observed; the residual stress along step-over direction is much more compressive than that along feed direction, and the surface residual stress is mainly compressive; the microhardness and residual stress all have a "hook" shaped profile;③Based on the research mentioned above, the high speed milling mechanism of hardened H13 steel is revealed.In this research, surface integrity in hard milling H13 steel is studied and the high speed milling mechanism of hardened H13 steel is revealed, which has a very important theoretical and practical significance to exploit hard milling potential energy further.The work is sponsored by the Shandong Provincial Natural Sciences Foundation (Y2008F41), the Project Sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars (State Education Ministry 2010-609) and the Independent Innovation Foundation of Shandong University (2009TS028). |
