Numerical investigation on the formation mechanism of residual stress in metal cutting

Residual stress can be introduced unintentionally into the workpiece with various magnitudes and distributions through all manufacturing processes. These stresses have a significant effect upon the performance of the final component. Understanding the residual stress imparted by machining is an essential aspect of understanding the machining process and overall part quality. Although many investigations have been conducted over the past few decades in measurement, modelling and mechanisms of residual stresses induced by the different manufacturing processes, the insights of residual stresses induced during the machining are still far from being completely understood. Several crucial issues still have to be investigated. In particular, the residual stress generation under different circumstances (segmented chip, the variation of tool geometries, multiple cuts, and curved surface turning) still has to be fully assessed. In this dissertation, finite element method (FEM) was employed to simulate and analysis the residual stress induced by the metal cutting process aiming to contribute to further understanding of the machining-induced residual stress.
The dissertation covers four aspects of research. First, the cyclic residual stress distribution in machined workpiece with a segmented chip was simulated and investigated. It is shown that the feed force increase firstly and then decrease during one segment genesis. It is the increased feed force that cause an increase in the local normal/tangential stress acting on the machined surface, leading to a less tensile residual stress in the lower stress zone.
Second, the effect of tool geometry on residual stresses induced in an orthogonal cutting process was investigated. The thermal and mechanical contribution to the formation of residual stress was also distinguished. The local normal/tangential stress was used to determine the degree of the tensile plastic deformation induced by the tool, providing a reasonable explanation for the variation of subsurface compressive residual stress with the changing of tool geometry.
Third, the influences of multiple cuts and correspondent cutting parameters and tool geometry on residual stresses evolution were explored. For the first time, material loading cycles were developed in multiple cutting operations based on the quantified stress/strain that is obtained numerically. The results indicate that the existence of the previous cut tends to generate more compressive residual stress in the finished workpiece, and this effect is more evident when the previous cut is implemented at the cutting conditions producing a larger compressive stress/tensile strain in the subsurface.
Finally, the residual stress evolution when turning a fillet surface was simulated and analysed. The variation of the size and shape of uncut chip cross-section in outer/end surfaces turning was analyzed. It is shown that the residual stress becomes more compressive when the tool position changes from outer face to end face, although the difference is not significant.
All four aspects of research present new and novel contributions to the field of metal cutting simulations and numerical analysis. The key physical quantities (i.e., stresses, plastic strains, shear angle, material degradations, forces, and temperatures) generated in the cutting processes are thoroughly evaluated and analyzed to significantly increase the interpretation and understanding of residual stresses formation under different aspects.

Machining processes are the techniques using various kinds of tool to remove excess material to shape the workpiece into the desired geometries, dimensions, and surface conditions. It is common that some stresses will be preserved within the finished parts after the machining processes, which is called residual stress. The machining-induced residual stresses have attracted wide attention for the last decades because they were closely related to the quality and performance of the machined components. Residual stresses can be induced with various magnitudes and distributions by any machining operation. The final residual stress depends on the material of the components, and the employed cutting conditions: cutting speed, cutting feed, depth of cut, tool geometries, tool wear, lubrication, etc., and pre-existing stresses in the parts. Therefore, the investigation of the effect of different cutting parameters on residual stress and the underlying mechanism of residual stress formation is very important to optimize the machining process and improve the work performance of the machined products.
There are three commonly used methods for residual stresses investigation: experimental measurement, analytical modeling, and finite element simulation. Generally, cutting processes operate at severe deformation conditions, involving very high strain, strain rate, stress, and temperature. These extreme conditions increases the difficulty in the measurement of the cutting temperatures, stress, plastic strains, shear angle, etc., which are critical parameters to understand the mechanism of residual stresses. The analytical method has been developed is a good alternative to achieve a better understanding about the phenomena occurring during the cutting processes. The drawback of the analytical models is the lack of accuracy of the results due to the significant simplification of the process. Moreover, some essential aspects, such as the multiple cuts and the pre-stress conditions due to prior manufacturing processes, are difficult to investigate with analytical models. With the continuous development of finite element techniques, the model of machining processes has attracted plenty of attention by many researchers over the last decades. This method not only realize the visualization of the cutting process, but also incorporate the complexity of the actual cutting process.
In this dissertation, orthogonal cutting and fillet surface turning models are established to predict the residual stress induced by metal cutting process under different cutting parameters. With the validated models, this dissertation is further aimed to visualize the cutting process and formation of residual stresses. Therefore, some process variables (i.e., stresses, strains, forces, and temperatures) which are not measurable or difficult to measure experimentally can be obtained to explore the underlying mechanisms of residual stress formation. With a deeper understanding of the influence from each process parameter in detail, an optimization of the machining conditions is possible in the practical machining process.