An Investigation on Cutting Tool Temperatures in Composite Machining Assisted With Heat-Pipe Cooling

Abstract

Metal matrix composites (MMC) are difficult to cut materials, and yet only diamond tools have been successfully utilized for such machining applications. Wear of diamond-coated tools is characterized by catastrophic coating failure (peeling off) due to the adhered work materials at the flank wear-land surface and the high stress developed at the coating-substrate interface, associated with high temperatures, because of very different thermal expansion coefficients. Temperature reductions, therefore, may delay the onset of the coating failure and offer tool life extension. A passive heat-dissipation device, heat-pipe, has been tested for cutting temperature reductions in MMC machining. Though it is intuitive that heat pipes may enhance heat transfer and plausibly reduce the tool temperatures, heat pipes may also increase heat partitioning into the tool, and complicate its effects on the heat removal and temperature reduction efficiency. This paper reports aluminum composite machining by diamond-coated tools and investigates the heat-pipe effects on tool temperature reductions. Numerical simulation of heat conduction in the cutting tool system was performed to evaluate cutting tool temperatures without and with a heat-pipe. A 3-D thermal model of the cutting tool system including coating, insert substrate, and tool holder was established. The heat source was characterized as a heat flux, a portion of the frictional heat flux at the rake face, over the chip-tool contact area. To determine the heat-partition coefficient, a separate 2-D chip model was established with a heat flux, balanced the total rake-face heat flux, over the contact and moving with the chip speed. With the tool and chip thermal models and by matching the average temperature at the tool-chip contact of the two models, the heat partition coefficient can be numerically determined. The model has been used to evaluate how the heat-pipe modifies the cutting tool temperatures. Applying heat-pipe cooling inevitably increases the heat partition into the tool despite the enhanced heat dissipation. However, the heat pipe still effectively reduces the tool-chip contact temperatures, depending upon machining conditions. Cutting tool temperatures have also been measured in machining using thermocouples. The simulation results reasonably agree with the experimental measurements.