The effect of workpiece microstructure on the mechanics of microscale cutting of 1045 steel

The demand for miniaturization of metallic components and improved machine tool control and accuracy have made ultra-precision machining a more common process in manufacturing environments. As a result, the scale of cutting has been decreasing to the point that it is on the same order or smaller than the workpiece materials microstructure. To date however, very little is known about how material properties and the mechanics of the cutting process change as cutting moves from the macroscale to the microscale.

Using normalised and micro-structurally refined AISI 1045 steel as the workpiece material, macro, meso, and microscale orthogonal cutting tests along with heterogeneous finite element (FE) cutting models were used to demonstrate and subsequently explain the mechanics of microscale cutting a heterogeneous microstructure such as AISI 1045 steel. The basis for defining the scale of cutting as macro, meso, or microscale was based on the size of the grains of the workpiece microstructure. From the experimental cutting tests, the classic continuous chip formation was shown to transition as the scale of cutting decreased. Ultimately, during microscale machining a new chip type, a Quasi-shear-extrusion (QSE) chip was observed. Examination of the machined surfaces also revealed that surface defects always from when machining a heterogeneous workpiece material such as steel. With a definitive link between material microstructure and the formation of surface dimples, surface defects are shown to always occur in a specific direction relative to the workpiece materials microstructure.

To properly model microscale cutting and the effects of material microstructure during multi-scale machining, a series of heterogeneous FE cutting models were developed. From these FE cutting models, the predicted chip forms more accurately reflected the experimental results across all scales, and the formation of surface defects on the machined surface could be observed and analysed. With a single Johnson-Cook constitutive equation, shear instability, shear and strain localization, and thermal softening events could be modeled by incorporating a microstructure into the FE cutting models.