Chip Formation Process Research Articles

Quantification of the chip segmentation in metal machining: Application to machining the aeronautical aluminium alloy AA2024-T351 with cemented carbide tools WC-Co

The chip segmentation process has a significant effect on the cutting force fluctuation during machining which could affect tool vibration and tool wear. This paper deals with a quantitative analysis of the chip segmentation phenomenon in metal machining. The notion of intensity of the phenomenon has been introduced. Various parameters have been proposed for this purpose. These parameters are based on dimensional characteristics of the segmented chip and the strain distribution within the chip. A Finite Element based modelling has been developed to simulate the chip formation process in the case of machining aeronautical aluminium alloy AA2024-T351 with WC-Co based cutting tools. From the simulated chip morphologies, introduced chip segmentation parameters are assessed. The impact of the cutting speed and tool geometry on the chip segmentation intensity is clearly highlighted. The relevance of each parameter is discussed. Cutting force and contact length fluctuations with respect to the cutting speed variation when segmentation occurs are discussed and deeply analysed. A correlation between average cutting force reduction and segmentation intensity when the cutting speed increases as well as between chip formation process and cutting force oscillation has been established thanks to the introduced parameters, showing thus their usefulness.

A reliable method for predicting serrated chip formation in high-speed cutting: analysis and experimental verification

Serrated chip formation influences almost every aspect of a high-speed cutting (HSC) process. This paper aims to develop a reliable method to accurately predict such chip formation processes. To this end, a systematic finite element analysis was carried out and a series of HSC experiments were conducted on a heat treated AISI 1045 steel. It was found that the integrative use of the Johnson–Cook thermal-viscoplastic constitutive equation, Johnson–Cook damage criterion for chip separation, and the modified Zorev’s friction model can precisely predict the serrated chip formation in HSC without artificial assumptions. This advancement has removed the major barrier in the current machining investigations by numerical simulation. The present study also found that the tool rake angle has a significant effect on serrated chip formation. As the rake angle increases, the chip sawtooth degree and cutting forces decrease, but the chip segmentation frequency increases.

Cryogenic Machining of Hard-To-Machine Material, <i>AISI 52100</i>: A Study of Chip Morphology and Comparison with Dry Machining

Cryogenic cooling is a new emerging cooling application in machining processes. Quantitative understanding of the effects of cryogenic cooling on the machining performance is important for continued applications. This study focuses on cryogenic machining of hard-to-machine material, AISI 52100, particularly with an analysis of cooling-induced chip morphology, chip hardness and the effect of workpiece hardness, etc., as these measures reflect the material`s thermo-mechanical behavior during the plastic deformation. AISI 52100 steel, with different initial hardness values, is selected as the work material for orthogonal cutting under dry and cryogenic cooling conditions, and the results are compared. The findings of this study show that cryogenic cooling affects the chip formation process, and the associated hardness produced on the machined surface.

Mechanics of large strain extrusion machining and application to deformation processing of magnesium alloys

An analysis of the mechanics of large strain extrusion machining (LSEM), a constrained chip formation process, is presented for deformation processing of bulk alloys. The deformation field is shown to be narrowly confined and controllable, with attributes ranging from conventional deformation processing to severe plastic deformation. Controllable deformation parameters include strain/strain rate, hydrostatic pressure, temperature and deformation path. These attributes are highlighted in deformation processing of Mg AZ31B, an alloy of commercial significance but noted for its poor workability, into sheet and foil forms. Noteworthy features of the processing are suppression of segmentation, realization of a range of strains and deformation rates, engineering of microstructures ranging from conventional to ultrafine grained, and creation of sheet/foil from the bulk in a single step of deformation without pre-heating. Guidelines for realizing specific sheet attributes, and scalability of LSEM for production are analyzed and discussed.

Influence of the cutting edge rounding on the chip formation process: Part 1. Investigation of material flow, process forces, and cutting temperature

In order to meet the ever-increasing demand for high quality and low cost products, machining processes with geometrically defined cutting edges such as high speed cutting, or hard turning are being used. Due to the fact that cutting is accomplished through a physical interaction between the cutting edge and the workpiece, the characteristics of the cutting edge itself play a key role in influencing the machining process, which determines the product quality and the tool life. As a result, cutting edge design has attracted the focus of many researchers, and crucial improvements have been achieved. Rounded cutting edges have been found to improve the tool life, and the product quality. However, to better understand the impact of prepared cutting edges on the aspects of the machining processes, and to produce tailored cutting edges for specific load profiles, further investigation on the influence of the cutting edge design on the machining processes needs to be carried out. In this study, the effect of symmetrically and asymmetrically rounded cutting edge on the material in the vicinity of the cutting edge has been investigated using finite element simulation techniques. The results obtained from this investigation show that process forces and material flow under the flank face are mainly influenced by the micro-geometry Sα. However, the magnitude and the location of maximum nodal temperature are influenced by Sα as well as Sγ.

Application of FEM simulation of chip formation to stability analysis in orthogonal cutting process

Models for chatter prediction in machining often use a mechanistic force model that calculate the force as the product of a material dependent cutting constant and chip area. However, in reality, the forces are the result of complex interaction between the tool and the chip, and are affected by many factors. The effects of these complex, and often nonlinear, factors on the machining dynamics may only be included in chatter prediction if the chip formation process is simulated concurrently with simulation of the machining dynamics. In this paper, finite element simulation of the chip formation process is combined with simulation of chatter dynamics and the inter-relationship between the chip formation process and the chatter phenomenon is investigated. Mesh adaptation technique is used to simulate the chip formation within an FEM elastoplastic analysis with dynamic effects and frictional contact. The combined modeling predicts the occurrence of process damping at low cutting speeds, which other models are generally unable to predict.

Hardness-Based Flow Stress for Numerical Simulation of Machining Inconel 718 Alloy

The phenomenological models for material flow stress and fracture, typically used in the Finite Element simulations of Inconel 718 alloy during machining processes, are often deemed to represent only certain metallurgical material states. In contrast, these models are not suitable to describe the constitutive behaviour of the workpiece for different metallurgical states (i.e., annealed, aged, etc.) and, consequently, different hardness values. Since the description of the material behaviour requires correct formulation of the constitutive law, new flow stress models which include also the hardness effect should be developed and, accordingly used, for computer simulation of machining Inconel alloy. This paper describes the development of a hardness-based flow stress and fracture models for machining Inconel 718 alloy, which can be applied for a wide range of work material hardness. These models have been implemented in a non-isothermal viscoplastic numerical model to simulate the influence of work material hardness on the chip formation process. The predicted results are being validated with experimental results available in literature. They are found to satisfactory predict the cutting forces, the temperature, the shear angle and the chip morphology from continuous to segmented chip as the hardness values change.

Distribution of unit forces on the tool nose rounding in the case of constrained turning

The development of force models is important in understanding the effects of tool edge geometry where the tool edge radius is of similar value to the depth of cut. In this paper it is demonstrated that both the rounding of the tool edge and the rounding of the tool nose affect the chip formation process and the quality of the machined surface.The distribution of unit forces on the nose of a tool is presented in detail. The main assumption in all the considerations was constant temperature conditions in the cutting zone, maintaining the thermo-mechanical properties of the work material at a constant level. The experiment was conducted for free and non-free turning, and in both cases the temperatures and forces were measured.For free cutting the distribution of unit forces on the tool edge rounding was investigated—the minimal thickness of cut and characteristic equations for forces Fc and Ff were determined. Then the unit forces distribution was recalculated for the case of non-free turning with rounded tool nose. The summed forces for non-free turning were compared with measured values, for verification of the cutting force model.The distribution of unit forces along the tool nose revealed nonlinearities influencing tool and machined surface integrities. The unit forces acting on the rake face were established to determine the length of the active part of the tool where the conditions for chip formation were favorable. Distribution of unit forces acting on the rake face in the reverse direction indicated the conditions in which a chip was pushed away from the transient surface.The distribution of unit forces acting on the flank face in the feed direction was found to determine the machined surface quality. The range of negative values of unit forces defined the conditions for moving away of the non-machined material (under the condition hj<hmin) in a direction opposite to the feed. This work material was found to be responsible for the effect of material side flow.

Numerical Simulation of High Speed Machining of Alloy Cast Iron

High speed milling of hard alloy steels utilized in dies and molds is a highly demanding operation. The finite element model was developed to investigate the high speed machining of alloy cast iron which is used in auto panel dies. The modified Johnson-cook constitutive model was used to model the complex dynamic material behavior, a damage evaluation law based on Cockroft and Latham model was used to simulate the ductile fracture of alloy cast iron. The crack initiation and propagation was simulated explicitly using an explicit FEM code. Simulation results showed that the chip morphology transited from continuous to saw-tooth chip with increasing cutting speed, cutting force decreased when increasing the cutting speed, which provide a useful understanding of chip formation process in high speed machining of alloy cast iron.

Investigation of Thermal Effects on Machining Chatter Using FEM Simulation of Chip Formation

Chatter vibration is a major cause of limitations in increasing material removal rate in machining. Machining chatter is caused by the interaction between tool and work-piece. Various nonlinear phenomena in the machining process affect the occurrence of chatter. Finite element simulation of chip formation allows for incorporation of various factors in the chip formation process. It can therefore be used to simulate the occurrence of chatter in practical conditions and to predict the conditions that lead to stable cutting. In this paper, a one degree of freedom dynamic model of the cutting tool is used to simulate the interaction of machining dynamics with the thermo-mechanical chip formation in orthogonal conditions. In this way, the effects of various phenomena such as friction, plastic deformation, heat generation and process damping on the occurrence of chatter may be investigated. In particular, the effect of heat rise in the cutting zone is investigated in this paper, which has not been reported in the literature before. It is shown how rising the temperature in the cutting zone can affect occurrence of chatter. Simulation results are presented and stability lobe diagram is obtained from simulation.

Experimental Investigation on Chip Deformation in Drilling 1Cr18Ni9Ti

It is difficult to study the chip deformation due to the complexity of the chip formation process in drilling. The chip deformation has a direct effect on the drilling forces, drilling temperature and surface quality. The austenitic stainless steel 1Cr18Ni9Ti belongs to the hard-to-cut material, so it is necessary to investigate the chip deformation of 1Cr18Ni9Ti in drilling. An experimental investigation of the chip transformation process on the cutting edges using quick-stop of the drilling processes is carried out. Results indicate that the chip deformation increases with the increment of drilling velocity and decreases with the increment of the distance to chisel edge on cutting edge and the feed rate in drilling 1Cr18Ni9Ti; the chip deformation decreases with the increment of drilling velocity, and decreases with the increment of the distance to chisel edge on cutting edge and the feed rate in drilling AISI 1045; the chip deformation in drilling 1Cr18Ni9Ti is larger than that in drilling AISI 1045.

A Study of Non-uniqueness During the Inverse Identification of Material Parameters

The Finite Element simulation of machining has been instrumental in developing a deeper understanding of the chip formation process. One of the problems in obtaining results which are valid over a large range of cutting conditions has been finding appropriate material parameters. The Johnson-Cook material model has been one of the most widely used models for machining simulations as it can describe material behaviour for different strains, strain-rates and temperatures. It has been challenging to obtain the Johnson-Cook parameters from experiments which are valid over the large range of conditions during machining. Whereas during machining strains larger than 200% and strain-rates of over 105 s-1 are observed, in experiments it is only possible to reasonably obtain a strain of 50% and strain-rate of 103 s-1 [1]. Inverse identification of the Johnson-Cook parameters from machining experiments has been proposed as an alternative method, so that the parameters are valid over large ranges [2–5]. One of the problems encountered in this technique is the non-uniqueness of parameter sets which give rise to similar chip shapes and cutting forces even if the material parameters are widely different. In this paper, the non-uniqueness of parameters has been studied with respect to varying strain-rates and temperatures. In order to do so, the Johnson-Cook parameter space is explored systematically to find parameter sets with closely matching stress-strain curves. The influence of plastic heat on the non-uniqueness is studied as well. A large pool of non-unique parameters can be reduced by widely changing the cutting conditions, for instance the rake angle.

Chip Prediction in Finite Element Modeling of the Chip Formation Process during Cutting Ti-6Al-4V Alloy

Based on the theory of adiabatic shearing, the forming process of a serrated chip during cutting Ti-6Al-4V titanium alloy was analyzed by comparing the results of the finite element (FE) calculations with the cutting experiments. The results show that the equivalent stress, equivalent strain and equivalent strain rate within a ribbon chip varied a little, but they varied a lot within a serrated chip. Moreover, the effect of cutting speed on equivalent strain rate is greater than on the equivalent stress and equivalent strain within a serrated chip. It can also be found from the results that there are small gaps between the simulation results and experimental results for the chip thickness and sawtooth height, while there is a big gap for saw-tooth pitch. This means that the simulation model has its limitations for accurate simulation of micro-geometric shape of a chip during cutting the Ti-6Al-4V titanium alloy, and further research remains to be done.

Modeling and interpretation of fiber orientation-based failure mechanisms in machining of carbon fiber-reinforced polymer composites

The development and implementation of a microstructure-based finite element model for the machining of carbon fiber-reinforced polymer composites is presented. A new approach to interfacial modeling is introduced where the material interface is modeled using continuum elements, allowing failure to take place in either tension or compression. The model is capable of describing the fiber failure mode occurring throughout the chip formation process. Characteristic fiber length in the chips, and machining forces for microstructures with fibers orientated at 0°, 45°, 90°, and 135° are examined. For model validation purposes, the model-based machining performance predictions are compared to the machining responses from a set of orthogonal machining experiments. A parametric study is presented that identifies a robust tool geometry, which minimizes the effects of fiber orientation and size on the machining forces.

Cutting force variability as a consequence of exchangeable cleavage fracture and compressive breakdown of wood tissue

In the present study, the conditions of chip propagation or fracture in orthogonal oblique cutting of beech wood (Fagus silvatica) in the 90°–0° direction for a type-I chip has been investigated. The force required for orthogonal wood cutting is pronouncedly variable, which is the consequence of exchangeable different ways of material breakdown. The chip formation process is discontinuous because of interrupted splitting of the material in front of the cutting tool. A 10-mm-thick specimen was cut at a rake angle of 31° and 42° with chip thicknesses ranging from 0.1 to 0.3 mm. The chip segment length increased with the chip thickness. A chip of varying length and thickness was modelled using the finite element method. For each case, the bending or compressive stress in the chip and the stress intensity factor at the crack tip was calculated. The segment length of the chip can be calculated by taking into account the condition that a crack propagates when the stress intensity factor KI at the crack tip equals the critical stress intensity factor KIC, and the bending or compressive stress σx in the chip is smaller than the strength σu. Good agreement between the calculated and the measured values was observed. The chip segment length can change considerably already with small changes in the bending strength and critical stress intensity factor. This large sensitivity is also confirmed by the fluctuation of the measured chip segment lengths by as much as 400%.

Analysis of the Surface Residual Stress in Grinding Aermet100

Grinding induces residual stresses, which can play an important role on the fatigue of the component. In general, residual stresses in a ground surface are primarily generated due to three effects: thermal expansion and contraction during grinding, plastic deformation caused by the abrasive grains of the wheel and phase transformations due to high grinding temperature. It was found that thermal expansion and plastic deformation in the grinding process were the major causes of residual stresses. In this paper, an analysis model for the calculation of residual stresses induced by a surface grinding process on an ultrahigh-strength steel (Aermet100) workpiece is presented. Firstly, the stress distribution induces by thermal expansion was obtained base on the transient heat conduction equation and the thermal properties of Aermet100. All the calculations were based on the moving heat source solution which was modeled as a uniformly distributed, 2D heat source moving across the surface of a half-space, found in Carslaw and Jaeger. The results show that the near surface residual stress is predominantly tensile and that the magnitude of this stress increases with increasing heat flux values. Secondly, the plastic deformation caused by the abrasive grains of the wheel was simulated base on the grain-workpiece interaction. The chip formation process and the material removal mechanisms can be examined using the micro-scale approach. The results show that the residual stress induced by the grinding force itself is generally compressive which is smaller than the residual tensile stress induced by thermal stress. Therefore, the residual stress brought about by grinding operation is generally a tensile stress. This paper offers an insight into the mechanism understanding of thermal and mechanical residual stresses induced by surface grinding. Key words: grinding, residual stress, grain

Deformation field in large-strain extrusion machining and implications for deformation processing

The deformation field in large-strain extrusion machining, a constrained chip formation process, is characterized using high-speed imaging and particle image velocimetry. The field is shown to be controllable and narrowly confined, with attributes ranging from conventional deformation processing to severe plastic deformation. Implications for deformation processing of sheets, engineering of microstructures in bulk forms and on machined surfaces, and study of large-strain deformation phenomena in metal alloys are highlighted.

A Finite Element Study of Chip Formation Process in Orthogonal Machining

This paper examines the plane strain 2D Finite Element (FE) modeling of segmented, as well as continuous chip formation while machining AISI 4340 with a negative rake carbide tool. The main objective is to simulate both the continuous and segmented chips from the same FE model based on FE code ABAQUS/Explicit. Both the adiabatic and coupled temperature displacement analysis has been performed to simulate the right kind of chip formation. It is observed that adiabatic hypothesis plays a critical role in the simulation of segmented chip formation based on adiabatic shearing. The numerical results dealing with distribution of stress, strain and temperature for segmented and continuous chip formations were compared and found to vary considerably from each other. The simulation results were also compared with other published results; thus validating the developed model.

Process Parameter and Cutting-Tool Geometry Study for Precision Face Turning of a Clock Dial

After the processing of a clock dial, the precision dimension and uniform distribution of the tool-trace pattern on the dial surface have a connection with luster image and attractiveness, which in turn would have an impact on the additional values and prices for a clock. Through a systematic investigation among the lathe structure, process parameter and cutting-tool geometry in advance, the total results indicated that the rigidity of the lathe structure and the precision of the slider movements are excellent and they had only a little effect on the surface-related quality for a dial face turning. Hence, the combination of process parameter and cutting-tool angle becomes more essential. End face turning simulation and experiment of a copper alloy were thus conducted in this paper, and the chip formation process and machined surface-related quality are investigated, respectively. The effects of cutting tool geometry and process parameter on the results of chip formation, surface rough, tool-trace pattern and luster uniformity are investigated, and these results are also compared with each other. The results show that when larger clearance and rake angles used in conjunction with a lower feed rate, no matter how much cutting speed was enhanced, the surface-related quality of a dial surface is not good. However, when these two larger angles used combined with a larger feed rate, the quality of a dial surface would slightly be improved. By using smaller clearance and rake angles along with the higher feed rate and cutting speed would obtain a better surface-related quality with uniform luster and attractiveness.

Characterization of the transition from ploughing to cutting in micro machining and evaluation of the minimum thickness of cut

When the machining process is miniaturized two process mechanisms, ploughing and chip formation, are essential and a critical cutting thickness needs to be exceeded so that not only ploughing will occur but chips will also be formed. The ploughing effect thereby influences the chip formation process, workpiece surface roughness, burr formation and residual stress state after processing and is therefore of great interest. In order to optimize the machining process a better understanding of the minimum thickness of cut is crucial. The changes in surface topography along the cutting track occurring during machining with a constant feed rate of the cutting tool were analyzed. The influence of the built-up edge phenomena on the micro machining process was investigated for normalized AISI 1045 using confocal white light microscopy and scanning electron microscopy. Furthermore the sin 2 ψ-method was applied in order to study the residual stress state in the workpiece surface induced by the machining process. Both surface layer properties investigated, surface roughness and residual stresses, show a characteristic transition indicating a change in the dominating process mechanisms. Based on these results a model is developed to determine the minimum thickness of cut. The minimum thickness of cut is found to significantly decrease with higher cutting velocities and to moderately increase with higher cutting edge radii. In addition a propagation of error for the values obtained with the model was performed, proving the quality of the model developed.