Rheological model for physical and mechanical properties surface layer formation during its cutting

Stupnytskyy V.
To analyze the influence of factors shaping power, intensity of vibration process tool parts that form the parameters of accuracy and quality cutting, causing plastic and elastic-plastic deformation of the workpiece machined layer need to know the size and distribution of these dynamic forces. Studies conducted in the dynamics of cutting can be done by 2 methods: 1) analytical solution of the equation system for calculating cutting forces by analyzing the process is a particular case of the process of plastic deformation (shear); 2) determine the cutting forces based on rheological simulation modeling system that implements the finite element method Obviously, integrated residual stress of all these factors is diverse and opposite signs. Set dominant factor and does not take into account the other - is a complex task that requires further experimental studies. In addition, in many cases, the actions of all three factors are approximately equal and interrelated. Therefore, the possibility of rheological simulation modeling - is the only way to prompt and adequate analysis of the impact of cutting process parameters (structure of technological operation, machining conditions, selecting technological environment, etc.) on the formation of residual stress. In the process of cutting machining in the surface layer of the workpiece occurring simultaneously two opposing processes: hardening and softening thermodynamic. The physical condition of the surface layer of the workpiece is defined by the intensity and speed of these processes. And analytically describe this dynamic process extremely difficult. Reducing the actual yield strength can be explained by the predominant influence of the heat factor, leading to softening of the material. Increased shear stress cutting material layer can be explained by the presence of anticipating deformation zones leads to intensive growth dislocation density zone near the shear and therefore to strengthen the material. This is because the layers of material on the main cutting edges, vertex radius of the blade and cutting blade support superimposed on each other in their motion in a plane shift Physical, mechanical, structural and chemical state of the surface layer of the parts (residual stresses and deformation, structural phase, mechanical condition and chemical composition, etc.) is mainly determined aftereffect elastic-plastic deformation that occurs in the chip formation zone, temperature impact and chemical interaction of the material in the area of cutting material cutting tool technology and external environment. In the tie-cutting tool in processed metal wave of plastic deformation, spreading wedge in front of the cutting tool covers not only chips but also metal, the line is cut. Therefore, the material treated surface near the cutting edge subject to normal force and frictional force acting in the direction of the cut line. Average compression force will cause tension and friction - the tension of the surface layer adjacent to the rear facets of the tool. That is, the surface layer of parts subject to non-uniform plastic deformation, which fades steadily cutting depth Another cause of residual stress formation is the thermodynamic and deformation processes deep in the surface to be processed. The outer layer of metal machined workpiece, heated during the cutting process, tends linear expansion, but this prevents colder inner layer thus first subjected to compression, tension and the second subject. With intensive heating current tension on the surface of prevailing yield strength that causes plastic deformation of compression of the outer layer of metal. During further cooling, the outer layer tends to shrink to a size smaller than the original size in more plastic deformation compression. This will prevent the intense inner layer preform. As a result, the outer layer have a tensile stress, and in the interior - tension compression. Thus, depending on the conditions and modes of cutting machining, can be dominant or mechanical factor, and then on the treated surface prevail macrostress compression; or heat factor, and then the surface will macrostress superior traction. Analysis of the impact of the cutting rate, tool materials for the formation of I type residual stresses, determined based on rheological modeling system Deform 3D, revealed some patterns. The impact of cutting speed manifested primarily in changing the thermal action and change the length of heat and power tools in action. Therefore, increasing cutting speed contributes to the appearance of additional tensile stress, which increases the overall value of the tensile residual stresses. When cutting low-carbon steels (eg – AISI 20), increasing the amount of heat in the cutting zone associated with increased cutting speed can lead to hardening of the surface layer. The increase in specific volume of the metal surface layer during its hardening reduces the residual tensile stress generated at low speeds (V = 40 ... 80 m/min), and converting them into compression stress during machining at high speed cutting (V> 100 m/min).
Friday, February 17, 2017
Friday, February 17, 2017