Heat Partition Coefficient Research Articles

Transient Thermal Analysis of Wheel-Mounted Brake Disc of Gray Cast Iron

Determining how to accurately calculate the thermal loads and boundary conditions are the most critical issues in the thermal analysis of the brake disc. The heat flux acting on the disc surface was got by the uniform wear theory with consideration of the cover angle of pad. The result shows that: the heat flux determined by uniform wear is not only related to the braking time but also radial distance; the convective heat transfer coefficients will decrease by the raise of the time; the cover angle of pad increases at first and then decreases, while the heat partition coefficient is in the opposite situation; the results determined by uniform wear with the consideration of the cover angle of pad is more close to that calculated by energy balance law.

Partition of friction heat between sliding semispaces due to adhesion-deformational heat generation

Analytical expressions of heat-partition coefficient and contact temperatures for two sliding semispaces with account for adhesion-deformational heat generation and contact heat exchange have been obtained. The rate of deformational heat generation is assumed to decay exponentially with increase of distance from the interface. It has been shown that heat-generation configuration and the intensity of contact heat exchange have impact on heat partition only within a transient interval. The features of perfect thermal contact have been analyzed. Perfect thermal contact implies variation of heat partition in time. Heat partition and contact temperature for a semispace, sliding over a semispace with a constant temperature, have been studied. Adhesion-deformational heat generation results in a change of the direction of surface heat flow.

Characterisation of friction and heat partition coefficients at the tool-work material interface in cutting

The development of cutting simulation still requires an improvement in the understanding of the frictional phenomena at the tool-work material interface. This paper introduces a method for a fast identification of friction and heat partition models, based on a special tribometer able to simulate wide ranges of contact pressures and sliding velocities, similar to those occurring along the tool-work material interface in cutting. The method is applied for a wide spectrum of work materials and lubrication conditions. Combined with an analytical post-treatment, this set-up provides a modelling of the frictional behaviour that may improve significantly thermal aspects in cutting simulations.

Characterization of Friction and Heat Partition Coefficients during Machining of a TiAl6V4 Titanium Alloy and a Cemented Carbide

This article aims at characterizing the frictional behavior of a TiAl6V4 alloy and a carbide tool under extreme conditions corresponding to those occurring at the cutting tool–work material interface. A specially designed open tribometer was used to characterize the macroscopic friction coefficient, heat partition coefficient, and adhesion in the contact versus sliding velocity and contact pressure. It has been shown that titanium leads to intense adhesion, which seems to be even more intensive with high contact pressure and high sliding velocity, which limits the local sliding movement at the interface (stuck layer). However, the tribometer provides the evolution of an apparent friction coefficient and a macroscopic heat partition coefficient related to the shearing of titanium between the adhesive layer and the bulk material. An increase in sliding velocity or contact pressure induces a small decrease in the apparent friction coefficient as well as the heat partition coefficient. It has been shown that adhesion is thermally activated by a combination of contact pressure and sliding velocity, which leads to a threshold effect. Furthermore, the application of an emulsion showed a small decrease in the apparent friction coefficient associated to a decrease in adhesion. Finally, this work provides quantitative data on the apparent friction and heat partition coefficients versus sliding velocity and contact pressure that can support the development of macroscopic cutting models for titanium alloys.

CHARACTERIZATION OF FRICTION COEFFICIENT AND HEAT PARTITION COEFFICIENT BETWEEN AN AUSTENITIC STEEL AISI304L AND A TiN-COATED CARBIDE CUTTING TOOL

This work aims at characterizing the tribological behavior of an AISI304L austenitic steel and a TiN-coated carbide tool under extreme conditions corresponding to the one occurring at the cutting tool-work material in dry machining. A specially designed open tribometer has been used to characterize the friction coefficient, heat partition coefficient and adhesion in the contact versus sliding velocity and contact pressure. It has been shown that sliding velocity is the most influential parameter. An increase of sliding velocity induces a great decrease of friction coefficient, whereas contact pressure seems to play a secondary role on the friction coefficient. Adhesion is minimized when sliding speed increases. Heat partition coefficient decreases with sliding velocity, which tends to decrease the ratio of heat transmitted to cutting tools when sliding velocity increases. Finally, this work provides quantitative data of friction and heat partition coefficients versus sliding velocity and contact pressure in order to be implemented in any analytical or numerical cutting model.

Identification of a Friction Model at the Tool-Chip-Workpiece Interface in Dry Machining of a AISI 1045 Steel With a TiN Coated Carbide Tool

The characterization of frictional phenomena at the tool-chip-workpiece interface in metal cutting remains a challenge. This paper aims at identifying a friction model and a heat partition model at this interface during the dry cutting of an AISI1045 steel with TiN coated carbide tools. A new tribometer, based on a modified pin-on-ring system, has been used in order to reach relevant values of pressures, temperatures, and sliding velocities. Additionally a 3D Arbitrary Lagrangian Eulerian model (A.L.E.) numerical model simulating the frictional test has been developed in order to extract local parameters around the spherical pin, such as average contact pressure, average local sliding velocity, and average contact temperature, from experimental macroscopic measurements. A large range of sliding velocities [0.083–5 m/s] has been investigated. It has been shown that friction coefficient and heat partition coefficient are mainly dependant on local sliding velocity at the interface. Three friction regimes have been identified. These experimental and numerical results provide a better understanding of the tribological phenomena along the tool-chip-workpiece interfaces in dry machining of an AISI 1045 steel with a TiN coated carbide tool. Finally a new friction model and heat partition model has been developed for implementation in a numerical cutting model.

Characterization of friction properties at the workmaterial/cutting tool interface during the machining of randomly structured carbon fibers reinforced polymer with carbide tools under dry conditions

Carbon fiber reinforced polymers (CFRP) are increasingly employed within the aerospace industry, particularly within the aircraft sector. However, machining of fiber reinforced composites can be quite complex, first due to the inherent heterogeneity resulting from the reinforcements/matrix assembly and second due to the presence of high modulus/high strength fibers. Therefore, a lot of Finite Element models have been developed in order to understand their material removal mechanisms. Among the scientific issues faced by these works, the identification of friction coefficients between CFRP and cutting tool materials remains a strategic field of research. This paper aims at characterizing the friction properties between composite and cutting tool materials. More precisely, the paper focuses on the context of a randomly structured CFRP, called HEXTOOL™, machined with a carbide tool under dry conditions. The specific tribological conditions during machining of such heterogeneous materials are discussed in the paper, especially the configuration of the tribosystem (‘opened tribosystem’). The great lack of friction coefficient is mainly due to the absence of relevant tribometers simulating the tribological conditions occurring in cutting. This paper presents the development of a new tribometer designed to simulate conditions corresponding to machining of randomly structured CFRP materials. It provides quantitative values of friction coefficient and heat partition coefficient depending on sliding velocities. This work has revealed that friction coefficients are very low in dry regime compared to those obtained in metal cutting. Moreover, experimental results confirm that friction coefficient decreases from 0.25 to 0.1 when sliding velocity increases. Finally this works establishes that a TiN layer deposited on carbide tools is not able to modify friction properties.

Experimental characterization of friction coefficient at the tool–chip–workpiece interface during dry cutting of AISI 1045

This paper presents the application of an experimental set-up able to simulate similar tribological phenomena as the ones occurring at the tool–chip–workpiece interface in metal cutting. Especially, this system enables to reach contact pressures up to 3GPa and sliding velocities between 0 and 300m/min. In addition to classical measurements of friction coefficients, the system provides information about the heat flux transmitted to pins, which enables to estimate the heat partition coefficient along the interface. This system has been applied to the characterization of the tool–chip–workpiece interface during dry cutting of an AISI 1045 steel with TiN coated carbide tools. It has been shown that the sliding velocity is the most influential parameter whereas contact pressure has only a limited influence. However, three friction regimes can be distinguished. In the first regime (low sliding velocity), friction coefficient is almost constant whereas heat flux transmitted to cutting tools is proportional to sliding velocity. In the second regime (intermediate sliding velocity), friction coefficient decreases very significantly with sliding velocity whereas heat flux remains almost constant. In the third regime (high sliding velocity), friction coefficient is not affected by sliding velocity whereas heat flux transmitted to the cutting tools increases again.

Effects of Lubrication Mode on Friction and Heat Partition Coefficients at the Tool–Work Material Interface in Machining

The quantification of friction coefficient along the tool–work material interface in machining remains an issue in tribology. This article aims at identifying the evolution of friction coefficient for a large range of sliding velocity during the machining of an AISI 4140 steel (290 HB) with a TiN-coated carbide tool. The influence of various lubricants (straight oil or emulsion) and lubrication modes (flow or mist) is investigated and compared to a dry sliding situation (dry machining). It has been shown that, in dry machining, the friction coefficient decreases with the sliding velocity until reaching a lower limit around 0.2. On the contrary, the presence of a straight oil significantly decreases friction coefficients to a value around 0.1. Emulsion enables a significant decrease of friction coefficient to around 0.2 for low sliding velocities, whereas its action is absent for higher sliding velocities. Oil mist exhibits an intermediate behavior. Finally, it has been shown that all kinds of lubrication lead to a large decrease of heat flux transmitted to cutting tools, even if heat flux is almost similar irrespective of the nature of oil and of its application mode. Straight oils decrease the heat flux transmitted to cutting tools due the decrease of friction coefficient, whereas they are not able to modify the heat partition coefficient at the interface. On the contrary, emulsions limit the transmission of heat flux to cutting tools due to their lower heat partition coefficient, even if their influence on friction is more limited.

Characterization of friction coefficient and heat partition coefficient between an AISI4140 steel and a TiN-coated carbide – influence of (Ca, Mn, S) steel's inclusions

This work aims at characterizing the influence of Ca—Mn—S inclusions in an AISI4140 steel (280 HB) during friction against a TiN-coated carbide under extreme conditions. A specially designed open tribometer has been used to characterize the friction coefficient and heat partition coefficient at the contact under specific extreme contact conditions simulating those occurring at the tool—chip—workpiece interface in dry cutting. It has been shown that sliding velocity is the most influential parameter. A great decrease of the friction coefficient and heat partition coefficient is observed under high sliding velocities. Moreover, it has been revealed that inclusions lead to a great decrease of friction at low sliding speeds, irrespective of contact pressure. On the contrary, under high sliding velocities, there is no significant influence of inclusions. Additionally, it has been observed that inclusions are not able to modify the heat partition coefficient at the interface. Finally, this work provides quantitative data of the friction coefficient and heat partition coefficient versus sliding velocity and contact pressure in order to be implemented in any analytical and numerical model.

Friction at high sliding speed of WC-6Co pin versus steel disc AISI 1045: estimation of the contact temperature

Cemented carbide based cutting tools remain widely used in machining processes for their wear resistance. Knowledge and modelling of wear processes are of prior importance to get models to predict and increase the cutting tool service life. The present paper deals with some results of a research work that study relationships between wear and temperature in the case of pin-on-disc tribological experiments. Topic of the paper is focussed on the estimation of the pin/disc contact temperature by coupling experimental measurements and computational methods. Friction experiments are conducted with WC-6Co pins against steel discs made of an AISI 1045 grade. Furthermore, WC-6Co pins are instrumented with two type-K thermocouples. A large sliding velocity range is considered in the study: from 100m/min up to 600m/min. The present paper is focussed on the 600m/min velocity. During these tribological tests, tangential forces and thermocouple temperatures are measured and monitored. From these values, the heat flux in the WC-6Co pin is estimated by two different ways. On one hand by considering the unidirectional Fourier law, on the other hand by estimating the heat partition coefficient [1] between the disc and the pin. The heat partition coefficient is determined from a physical approach based on the consideration of the mechanical power dissipated in the contact. In both cases, the heat transfer in the pin is then modelled by finite element methods. It is necessary to perform numerical analysis to estimate the pin/disc interface temperature because of the impossibility to measure it directly during friction tests. Results of experiments and of numerical simulations are compared.

Estimation of thermal contact parameters at the interface of two sliding bodies

The knowledge of heat transfer between sliding solids has determinant applications in many industrial sectors (motor, railway, space...). The heat flux generated by friction increases the temperatures, which can create an excessive deformation of these solids, leading to the damaging of surfaces. The thermal coupling at the interface of two solids can be described either by a model of 'perfect contact' which assumes the equality of surface temperatures of solids, or by a model of 'imperfect contact' where a difference of temperatures is introduced at the interface due to surfaces irregularities. The modelling of heat transfer between two sliding solids introduces three macroscopic parameters: hcg (the thermal conductance), a (the local heat partition coefficient) and ϕg (the generated flux). Some numerical approaches of the simultaneous identification of these parameters have been developed with an assumption of constant parameters. This article presents the first estimation of the three parameters for the dry real sliding contact.

Temperature and division of heat in a pin-on-disc frictional device—Exact analytical solution

A 3D analytical solution is proposed in this paper to determine the temperature and the partition coefficient of heat flux generated by friction in a pin-on-disc tribometer. This solution is valid without restriction on the rotational velocity of the disc. It is given under an explicit form, thus avoiding any iterative calculation. A validation is performed by comparing the results with those available for a circular heat source in rectilinear motion. Thermal maps of the disc are presented for different values of its rotational velocity. The evolution of the temperature and the heat partition coefficient are presented and analysed as a function of the disc velocity and the cooling conditions.

Cutting Tool Temperature Analysis in Heat-Pipe Assisted Composite Machining

Machining of advanced materials, such as composite, encounters high cutting temperatures and rapid tool wear because of the abrasive nature of the reinforcement phases in the workpiece materials. Ultrahard coatings, such as chemical vapor deposition diamond, have been used for machining such advanced materials. Wear of diamond-coated tools is characterized by catastrophic coating failure, plausibly due to the high stress developed at the coating-substrate interface at high temperatures because of very different elastic moduli and thermal expansion coefficients. Temperature reductions, therefore, may delay the onset of the coating failure and offer tool life extension. In this study, a passive heat-dissipation device, the heat pipe, has been incorporated in composite machining. Though it is intuitive that heat transfer enhanced by the heat pipe may reduce tool temperatures, the heat pipe will likely increase heat partitioning into the tool at the rake face, and complicate the temperature reduction effectiveness. A combined experimental, analytical, and numerical approach was used to investigate the heat-pipe effects on cutting tool temperatures. A machining experiment was conducted and the heat-source characteristics were analyzed using cutting mechanics. With the heat sources as input, cutting tool temperatures in machining, without or with a heat pipe, were analyzed using finite element simulations. The simulations encompass a 3-D model of a cutting tool system and a 2-D chip model. The heat flux over the rake-face contact area was used in both models with an unknown heat partition coefficient, determined by matching the average temperature at the tool-chip contact from the two models. Cutting tool temperatures were also measured in machining using thermocouples. The simulation results agree reasonably with the experiment. The model was used to evaluate how the heat pipe modifies the heat transport in a cutting tool system. Applying heat-pipe cooling inevitably increases the heat flux into the tool because of the enhanced heat dissipation. However, the heat pipe is still able to reduce the tool-chip contact temperatures, though not dramatically at current settings. The parametric study using the finite element analysis (FEA) models shows that the cooling efficiency decreases as the cutting speed and feed increase, because of the increased heat flux and heat-source area. In addition, increasing the heat-pipe volume and decreasing the heat-pipe distance to the heat source enhances the heat-pipe cooling effectiveness.

An experimental investigation of temperatures during conventional and CBN grinding

In many grinding applications, the material removal rate is constrained by the undesired thermal effects such as surface burn, tensile residual stresses, and micro-cracks on the ground parts. Thermal damage is a common productivity limitation factor for conventional grinding wheels largely employed in industry due to their convenient cost and known behavior. The development of superabrasive materials having high heat conduction coefficients allowed for higher material removal rates, pushing up the limits of productivity previously achieved with conventional wheels. This paper presents the results of a comparative investigation of maximum surface temperatures generated during the plunge grinding of 52100 steel using Al2O3 and CBN wheels. The experiments were conducted under wet as well as dry grinding conditions. The temperatures measured experimentally were compared to those determined analytically. A discussion relative to heat partition coefficients concludes this paper.

Effect of Internal Cooling on Tool-Chip Interface Temperature in Orthogonal Cutting

A numerical method to study the effect of internal heat sink variables on the tool-chip interface temperature in orthogonal cutting was presented. The analytical method is based on two main assumptions that the chip can be treated as a semi-infinite body with a moving heat source at the tool-chip interface and the tool can be treated as a semi-infinite body with a stationary heat source at the tool-chip interface and a uniform plane heat sink inside the semi-infinite body. An approach using the point heat partition coefficient is employed to obtain the tool-chip interface temperature distribution. The temperature distributions along the tool-chip interface with different heat sink intensities, heat sink distances from the tool-chip interface, and heat sink areas were presented. The effects of the heat sink intensity, heat sink distance from the tool-chip interface, and heat sink area on the tool-chip interface temperature were investigated. It was found that the internal cooling with a heat sink in the cutting tool could greatly affect the tool-chip interface temperature. Presented at the STLE Annual Meeting in Toronto, Ontario, Canada May 17-20, 2004 Review led by Jane Wang

FINITE DIFFERENCE METHOD-BASED SIMULATION OF TEMPERATURE FIELDS FOR APPLICATION TO ORTHOGONAL CUTTING WITH COATED TOOLS

ABSTRACT A finite difference method was proposed to model the effect of a variety of tool coatings on the magnitude and distribution of temperatures through the tool-chip contact region and the coating/substrate boundaries. For each workpiece-tool pair tested the intensity of uniformly distributed heat flux and relevant analytically obtained values of the heat partition coefficient were assumed to change with variations of cutting speed and the corresponding friction. In this case the simulation of an orthogonal machining of AISI 1045 steel was performed using special computing algorithm with elementary balances of induced energies (MBE). It is concluded that the temperature contours obtained reflect favorable the specific contact conditions existing at the tool-chip interface and substantial differences can arise from coating effects.

Thermal modeling for white layer predictions in finish hard turning

Part thermal damage is a process limitation in finish hard turning and understanding process parameter effects, especially, tool wear, on cutting temperatures is fundamental for process modeling and optimization. This study develops an analytical model for cutting temperature predictions, in particular, at the machined-surfaces, in finish hard turning by either a new or worn tool. A mechanistic model is employed to estimate the chip formation forces. Wear-land forces are modeled using an approach that assumes linear growth of plastic zone on the wear-land and quadratic decay of stresses in elastic contact. Machining forces and geometric characteristics, i.e. shear plane, chip–tool contact, and flank wear-land, approximate the heat intensity and dimensions of the shear plane, rake face, as well as wear-land heat sources. The three heat sources are further discretized into small segments, each treated as an individual rectangular heat source and subsequently used to calculate temperatures using modified moving or stationary heat-source approaches. Temperature rises due to all heat-source segments are superimposed, with proper coordinate transformation, to obtain the final temperature distributions due to the overall heat sources. All heat sources are simultaneously considered to determine heat partition coefficients, both at the rake face and wear-land, and evaluate the final temperature rises due to the combined heat-source effects. Simulation results show that, in new tool cutting, maximum machined-surface temperatures are adversely affected by increasing feed rate and cutting speed, but favorably by increasing depth of cut. In worn tool cutting, flank wear has decisive effects on machined-surface temperatures; the maximum temperature increases 2–3 times from 0 to 0.2 mm wear-land width. White layers (phase-transformed structures) formed at the machined-surfaces have been used to experimentally validate the analytical model by investigating tool nose radius effects on the white layer depth. The experimental results show good agreement with the model predictions. The established model forms a framework for analytical predictions of machined-surface temperatures in finish hard turning that are critical to part surface integrity and can be used to specify a tool life criterion.

A mixed solution for the three-dimensional temperature distribution in turning inserts using finite and boundary element techniques

It is an established fact that critical thermal aspects would have a significant influence on any metal cutting process. The possible improvement in the overall efficiency that might arise out of any refinements in the thermal fields would naturally attract the attention of the industry. At the same time, the very complex nature of the problem poses serious problems when the true influence of the critical parameters is sought for. While the numerous analytical, experimental and numerical investigations reported earlier show the interest and challenges associated with the process, it appears there is still some scope for further research and refinement. This paper presents another method of solving the thermal fields of metal cutting tools combining certain classical techniques suggested in the past with some relatively new methods of the continuum approach. The moving work piece and the chip are considered as one domain and the stationary cutting tool as another domain to simulate the material flow conditions. The iterative solution sufficiently takes care of the distribution of the primary and secondary heat sources and the need to assume a heat partition coefficient is eliminated. A mixed finite element and boundary element method finally enables the estimation of the cutting temperatures. While the thermal fields analysed in carbide turning inserts show considerable amount of agreement with previous data, the approach seems to be quite simple and reliable and can be readily applied to any metal cutting problem.

Finite difference analysis of the thermal behaviour of coated tools in orthogonal cutting of steels

Temperature measurement and prediction have been a major focus of machining for several decades but now this problem became more important due to the wider use of advanced cutting tool coatings. Practically, there is a lack of simulation programs for prediction of the temperatures in the cutting zone when machining with differently coated cutting tools. In all literature items cited the finite difference methods (finite difference approaches) were used to find the distribution of temperature inside the uncoated tool body or along the tool–chip interface for continuous (turning) and interrupted (milling) machining processes. The algorithm applied overcomes this limit. In this study, a special variant of the finite difference method called the method of elementary balances (MBE), in which difference equations are defined based on balances of energy produced for all discrete elements of the model, is proposed to predict the tool temperature fields in continuous (orthogonal) machining of AISI 1045 steel with uncoated and coated carbide tools. These predictions provide a detailed view of the changes in the two-dimensional thermal field inside the tool body as a function of cutting speed for the defined friction conditions and values of the heat partition coefficient. Special attention was paid to temperature distribution curves at the tool–chip and work–flank interfaces and possible sources of computed errors. The computed results were compared with the selected experimental values to verify the accuracy of the simulation technique used.