Thermal effect of metal forming - Surface phenomena

The surface thermal evolution of metal and tools during cold and hot forming determines the surface quality of the resulting product, lubricant performance, resulting friction, and tool damage and wear patterns. Heat transfer to tooling dictates the surface temperature of the metal: for example, hot extrusion of hard aluminum alloys of the duralumin type gives rise to surface defects known as "palm fronds", if the extrusion speed is too high, preventing the energy dissipated by plastic deformation from flowing into the container and die. The friction between the steel chip and the turning tool heats up the tool considerably, and its service life decreases as its working temperature rises. The working temperature of rolling rolls, both hot and cold, determines the viscosity of the lubricant and, consequently, the amount of lubricant seeping into the metal-tool interface, the chemical reactivity of the lubricant and the resulting friction.

The analysis of these thermal evolutions by finite element calculation codes presents various difficulties due to the high values of the heat flows involved. Furthermore, direct calculation of the established thermal regime of tools would require numerical simulation of continuous operations over very long times (rolling, drawing, turning...) or for a high number of cycles of discontinuous operations (forging, stamping...), which is not currently feasible from an industrial point of view, given the performance of computers and the associated calculation costs. The aim of this article is to specify the orders of magnitude of these thermal phenomena and to propose methods for such numerical simulation work.

The paper first presents a simple "parabolic model" of the thermal evolution of a body induced by a short-duration surface heat flux, followed by various improvements to this model to extend its validity: linearization and inversion of the model, study of the effect of increasing metal surface area and variation in the body's thermophysical properties. We then use these models to analyze the temperature variations on either side of the metal-tool interface induced by the two main phenomena: the temperature difference between metal and tool (either initial, as in hot forming, or due to plastic deformation of the metal) and the energy dissipated by friction. We discuss in detail, based on experimental data, the problem of the surface heat transfer coefficient (or its inverse, the contact thermal resistance), which strongly conditions interfacial heat transfer in hot forming. By way of example, we apply these analyses to various processes: the start of hot extrusion of bars, cold and hot rolling of sheets, wire drawing, machining, piloted and inertial friction welding, and hot forging. Results are compared with experimental data where...