Effect of tool parameters on residual stress and temperature generation in high-speed machining of aluminum alloys

by Roman V. Kazban The need throughout the machining industry for cost reduction and increases in productivity have contributed to new interest in high-speed machining. Even though, many model for machining exist, most of them are for low-speed machining, where momentum is negligible and material behavior is well approximated by the quasi-static laws. In machining at high speeds momentum could be large and the strain rate can be exceedingly high. For these reasons a fluid mechanics approach to understanding high-speed, very high-speed and ultra-high-speed machining is attempted here. Namely, a potential flow solution is used to model the behavior of the material around a tool tip during machining at high speeds, i.e. greater than or equal to 100 m/s. It is carefully argued that the potential flow solution is relevant and can be used as a first approximation to model the behavior of a metal during high-speed, very high-speed or ultra-high-speed machining events. At a minimum, the potential flow solution is qualitatively useful in understanding mechanics of high-speed, very high-speed and ultra-high-speed machining. Interestingly, the flow solution predicts that there is a “stagnation” point on the rake face, not at the tool tip as is usually assumed. Because the “stagnation” point is not at the tool tip, the flow solution predicts a significant amount of deformation in the work piece resulting Roman V. Kazban in large residual strains and a possible related temperature rise on the finished surface. To verify the fluid flow model, an experimental apparatus has been designed to examine fluid flow in orthogonal machining. Experiments were conducted at room temperature for different Newtonian fluids, cutting conditions and cutting tools. It was seen that, indeed, the “stagnation” point is not at the tool tip. Next, a modified Hopkinson bar apparatus is employed to simulate dry orthogonal machining at 30 m/s cutting velocity. A focused array of Mercury-Cadmium-Tellurium infrared detectors is used to measure the temperature distribution. A three-component quartz force transducer is utilized in measuring the cutting and feed forces. Measurements of the cutting and feed forces contributed to the ability to prove the steady-state conditions as well as to estimate the coefficient of friction on the tool rake face along with the partition of the thermal energy produced during the high-speed machining process. Force measurements show that at this speed, on the upper boundary of the range of cutting velocities for high-speed machining not high enough to be very-high speed or ultra-high speed cutting, the role of momentum is negligible and the cutting event is dominated by material deformation, making the fluid model less applicable. Much higher cutting speeds, beyond the capability of this apparatus, are needed to make the fluid approach accurate. Not-surprisingly, measurements of temperature distributions showed little heating of the finished surface. Therefore, a study of the temperature fields generated during machining with a cutting tool that has a wear-land was performed. It is seen that the wear-land contributes significantly to the heating of the workpiece and, at this speed, is the most likely mechanism for the generation of residual stress and a temperature rise on the finished surface. For my grandfather, Col. Dmitriy Panaschenko, who inspired a little boy to look up at the sky and imagine a world of science.