In order to improve machining processes, it is important to first comprehend the phenomena occurring at the cutting edge of the tool. Thermomechanical loads occurring in this zone affect both tool wear and the surface integrity of the final piece. Unfortunately, direct experimental assessment of the conditions in the tool-chip contact area presents unprecedented challenges. Consequently, it becomes quite difficult to access variables like pressure or stress using the available experimental techniques, thereby hampering in-depth analysis of what happens directly at the tool-chip contact area. So far, the chip has been observed to harbor distinct signatures of the conditions that are present at the moment of cutting, especially in the secondary shear zone, which is in direct contact with the tool. However, due to the complexity and limited availability of high-end structure characterization techniques in the metalworking community, the structural and mechanical characterization works have been separated and thus an important link between the structure, material properties and machining conditions has not yet been clearly established.
Recently, a consortium of three research groups, Electron-Microscopy Laboratory at CIC nanoGUNE (San Sebastian, Basque Country, Spain) lead by Ikerbasque Professor Andrey Chuvilin, High Performance Machining Laboratory at the Mondragon University lead by Dr. Pedro Jose Arrazola (Mondragon, Basque Country, Spain) and the Department of Materials Science and Engineering at Saarland University (Saarbrucken, Germany) lead by Prof. Dr. Christian Motz – has carried out a composite study entailing the characterization of the evolution of microstructure in the secondary shear zone during high speed machining of AISI 1045 steel.
In particular, they have focused on the strain mode of the chip section in contact with the rake surface of the tool and its influence on the mechanics of material removal. This work, first authored by Dr. Bentejui Medina-Clavijo, is currently published in the research journal Journal of Materials Processing Tech.
In brief, the research method employed entailed three steps: first, machining, where cylinders of annealed steel were subjected to a cutting process using a vertical milling machine. Next, the researchers set up a model to obtain quantitative information about cutting and feed forces, temperatures and plastic strain. During this stage, the material behavior was simulated by a Johnson-Cook flow stress model. Lastly, samples obtained from the preceding steps were prepared, optically examined and finally characterized using various electron/ion microscopy techniques.
The authors noted that the chip morphology, the chip microstructure, the cutting forces and the feed forces experienced an abrupt step-like transition at a cutting speed in the range of 50–60 m/min. This transition is known to occur due to the conversion from built-up edge (BUE) mode developed at low cutting speed to the mode at which the chip slides directly over the tool surface, but the reason for such an abrupt transition was a complete mystery. The study of the chip’s micro-nano structure has revealed that BUE is dissolved as a consequence of a process of dynamical recrystallization (DRX) activated at a relatively low temperature. Generally, a combination of several information sources, from machining tests to ion scanning microscopy, successfully revealed the succession of events that were responsible for the chip flow regime at mid-high cutting speeds.
“Rotational DRX, responsible for macroscopic changes in cutting parameters, is directly related to grain-boundary atom diffusion – atomistic phenomena, whose characteristics are known for many materials. Our work is thus making a step forward in the prediction of industrially significant machining characteristics from the first principles” – says Dr. Bentejui Medina.
In summary, a consortium of three different research groups, based geographically distant from each other, has demonstrated the potential of a complex approach including a systematic microstructural characterization that could be successfully applied to provide a better understanding of the processes that occur in the confined space and at the extreme conditions during orthogonal cutting. Altogether, novel insights into the machining process have been presented, alluding to fundamental microstructural effects to explain the evolution of the tool-chip contact that could help solving tool-chip friction issues in future simulations and practical experiments.
Bentejui Medina-Clavijo, Mikel Saez-de-Buruaga, Christian Motz, Daniel Soler, Andrey Chuvilin, Pedro J. Arrazola. Microstructural aspects of the transition between two regimes in orthogonal cutting of AISI 1045 steel. Journal of Materials Processing Tech. volume 260 (2018) page 87–96.Go To Journal of Materials Processing Tech