In the FEM simulation model, the combinations of stress and strain can lead to damage. The damage in FEM simulation is analogous to the segmentation development. Experimentally, it is very difficult to capture each moment during the segmentation process; however a step by step analysis with FEM made it possible.
The FEM simulation observations across the ASB show that the precursor of damage is a peak of maximum principal stress combined with an increase in accumulated effective strain. When the accumulated damage reaches the critical damage value, element deletion occurs. The damage stops when there is a drop in the maximum principal stress. This is favored by the increase in temperature, and therefore a decrease in material yield strength.
The FEM simulation along the ASB shows that the increase of strain in the middle of the segment can lead to an increase of damage at the same location, and then later at the free surface. The process will keep alternating with peaking damage in the middle of segment then at the free surface until the completion of the segmentation process. The simulation shows also that, even though the damage value starts building up at the tool edge, it will never reach the critical value necessary to cause element deletion.
An analogy can be made with the results obtained experimentally in the QSD tests, in which crack formation was observed at the free surface with a high strain at the location in the middle of segment. Furthermore, in these experiments no crack occurred at the proximity of the tool tip.
CHAPTER: 4- GENERAL DISCUSSION
Three approaches were used in this research study as presented in Figure 2.1. The first experimental approach with TAGUCHI DOE, was used mainly to identify the optimum cutting parameters for the best tool life and surface roughness. The second and third approaches aimed at identifying the phase transformations in the chips, chip formation with ASB evolution, and FEM. All three approaches were used for the understanding of the tool/particles behavior and for the assessment of the effect of speed and LAM on tool life. The three approaches used throughout this project are summarized below.
In the first experimental approach, this thesis is the first of its kind to seek practical recommendations for defining the cutting parameters and evaluating different cutting tools when machining TiMMC. Following the experimental work and the analysis of results, it was found that cutting TiMMC at higher speeds is more efficient and productive since tool life is increased. This is in opposition with most materials and to the Taylor’s tool wear curve.
SEM microscopy was used to explain the particle behavior in the chips under high stress and strain conditions. The phenomenon of efficient cutting at higher speeds (in terms of enhanced tool life) was explained by the different tool/particles behavior where, at higher speeds, fewer hard TiC particles are broken resulting in reduced tool abrasion wear. In order to further increase the tool life, laser assisted machining was performed. LAM was shown to increase tool life by approximately 180% and to be a good solution for machining TiMMC.
Using SEM microscopy, it was found that the tool/particle interaction during cutting can exist under three forms. The particles can either be cut at the surface, pushed inside the material, or even some of the pieces of the cut particles can be pushed inside the material. No particle de- bonding was observed.
In the second approach that was followed in this research, a thorough understanding of the Adiabatic Shear Band (ASB) formation in the segmented chips was required. To understand the chip formation and the crack initiation in the segmentation process when cutting TiMMC, a new concept of a “Quick Stop Device” was designed and used to freeze the cutting action. The new device has multiple advantages over other models found in literature, in terms of efficiency and safety. Subsequently, using a multi-scale (macro and nano) analysis with Scanning Electron Microscopy (SEM), and Transmission Electron Microscopy (TEM), a new model for the
microstructure and grain evolution inside the ASB was developed. This new model is the first to be developed for TiMMC. The model showed that inside the ASB, new nano-grains and cells are formed and characterized by a low dislocation density. However, no phase transformation was found in the ASB.
In the last approach of this research project, an FEM model was developped in order to analyze the different physical parameters (as temperature, stress, strain, damage) involved when machining TiMMC. This required the development of a new constitutive model for TiMMC. To simulate the segmentation process a Cockcroft-Latham (CL) damage model was defined for the TiMMC material. The damage model is based on the work required in a tensile test until fracture. The model gave good predictions for chip segmentation geometry and cutting forces. In order to have a better understanding of the LAM effect on the cutting mechanism, the LAM heat source was simulated and embedded in the same FEM simulation model. Simulations of segmented chips with LAM have never been performed before for TiMMC or for any other material. The results were unexpected and contrary to our hypothesis, in which a higher tool/chip temperature was expected to reduce tool life when using LAM. The analysis of the FEM simulations showed that although LAM increases the overall chip temperature, the peak tool/chip temperature decreases. This could explain the experimental finding that tool life increases when using LAM. To have a better fundamental understanding of the FEM damage and the segmentation sequence, a comparison was made between the FEM model and the experimental chip formation. In both cases the crack initiation, or damage, was shown to never occur at the tool tip. The precursors of damage were determined using stress and strain profiles along and across the Adiabatic Shear Band. Analysis of incremental steps in FEM simulations showed that the damage occurs in a cyclic manner in the middle of the segment, then at the free surface.