Journal of Machining and Forming Technologies

Journal of Machining and Forming Technologies

(Parte 1 de 2)

In:Journal of Machine andForming TechnologiesISSN:1947-4369 Volume 3, Issue 1/2, p. 1 ?12' 2011 Nova Science Publishers, Inc


RogØrio Fernandes Brito1*, Joªo Roberto Ferreira1, Solidônio Rodrigues de

Carvalho2and Sandro Metrevelle Marcondes de Lima e Silva1 1Mechanical Engineering Institute-IEM, Federal University of ItajubÆ-UNIFEI,

Campus Prof. JosØ Rodrigues Seabra, Av. BPS, 1303, bairro Pinheirinho,

CEP: 37500-903, ItajubÆ, MG, BRAZIL 2School of Mechanical Engineering-FEMEC, Federal University ofUberlândia-

UFU, Campus Santa Mônica, Av. Joªo Naves de vila, 2160, bairro Santa Mônica, CEP: 38400-902, Uberlândia, MG, BRAZIL

Abstract This paper is concerned with the effect of variations in the thickness of the tool coating on the heat transfer in cemented carbide tool substrates ( ISO grade K10)

Titanium nitride ( TiN) and aluminum oxide ( Al2O3 ) with thickness values of 1 and 10 µm were used as coatings. In order to increase tool life and reduce costs, the thermal parameters of the turning operationare investigated aiming a more uniform temperature distribution in the cutting zone. Boundary conditions by convection and heat flux are known, as well as the thermophysical properties of the tool and coating involved in the numerical analysis. Two commercial softwares were used and the proposed methodology was validated experimentally under controlled conditions. Design of experiments ( DoE ) was used to identify the optimal parameters in order to obtain themaximum temperature between the tool substrate and the coating. The cutting tool temperature distribution is discussed and a thermal analysis on the influence of the coating is presented. Finally, the results are discussed and compared with data available in the published literature.

Keywords: TiN and Al2O3coatings, cutting tool, DoE, finite volume method, heat transfer.

1. Introduction

Machining processes generate enough heat to deform the materials involved: the chip and the tool. This level of heat is a factor that strongly influences tool performance. Friction

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RogØrio Fernandes Brito,Joªo Roberto Ferreiraet al.2 wear and heat distribution affect the temperature on both the chip and tool. In order to increase tool life, its surface can be coated with materials having thermal insulation features that reduce tool wear. Hence, the influence of coatings on heat transfer and friction wear is an area of study that deserves an in-depth investigation.

A review on the literature suggests that most orthogonal metal cutting simulations are designed for uncoated cemented carbide tools. In recent years, however, an opposite trend has emerged, using single and multiple coatings. In Marusichet al.( 2002 )

, for example, the authors carry out a simulation using a numerical model based on the Finite

Element Method ( FEM )

. The Thirdwave AdvantEdgefisoftware isused to simulate the chip-breakage of coated and uncoated tools. With multi-layered coatings, one result shows a temperature reduction, for the tool substrate, of 100° C. Grzesik ( 2003 ) studied the cutting mechanisms of several coated cemented carbide tools and found that the toolchip contact area and the average temperature of the tool-workpiece interface change according to the coating. Grzesik did not explain, however, whether the coatings are able to thermally insulate the substrate.

offer the first comprehensive assessments, using FEM, of an orthogonal cutting model for multi-layer coated cemented carbide tools. In their model, the authors analyze, both individually and as a group, the thermal

properties of thefollowing three layers: titanium carbide, ( TiC ) , aluminum oxide ( Al2O3 ) and titanium nitride ( TiN )

. They consider a layer with equivalent thermal properties.

Their results indicate that the fine width coatings of an Al2O3intermediary layer do not significantly alter the temperature gradients for a steady state between the chip and the worked with the qualification of the tribological system ᰀwork material-coated cemented carbide cutting tool-chip. ?Their aim was to better understand the heat flux generated during the turning operation. Their methodology, applied to several coatings deposited on cemented carbide inserts, showed that the coatings possess no significant influence on the substrate thermal insulation.

analyzed how several coatings on a cutting tool influenced heat transfer. They performed this analysis with an analytical model of their own. For actual cutting conditions, the authors carry out an experimental work on turning AISI 1035 steel to examine the behavior of different coated inserts. Their results showed that, while the other coatings fail to significantly modify the thermal field, the Al2O3coating yielded a slight reduction on the heat transferred to the tool.

used FEM to simulate the performance of polycrystalline cubic boron nitride ( PCBN ) tools when turning AISI 4340 steel, Coelho use titanium aluminum nitride ( TiAlN ) and aluminum chromium nitride ( AlCrN ) coated and uncoated cutting tools. The simulations performed indicate that, regardless of the coating, the temperature on the tool-chip interface was approximately 800° C with an flank wear was absent.

reports an experimental study on the wear characteristics of electroless nickel-phosphorus ( Ni-P ) coatings sliding against steel. Sahoo ( 2009 ) optimized the coating process parameters aiming minimum wear. The optimization based on L27 orthogonal design, takes into account four process parameters: bath temperature, concentration of nickel source solution, concentration of reducing agent, and annealing temperature. The author observed that the two most

Thermal Analysis in TiNand Al2O3Coated ISO K10 Cemented Carbide &3 significant factors influencing the wear characteristics of electroless Ni-P coating were the annealing and bath temperatures.

The aim of the present work is to numerically analyze how the coatings oncutting tools influence heat transfer during the cutting process. It is intended to verify the thermal and geometrical parameters of the coated tool, striving for a more adequate temperature distribution in the cutting region. In order to obtain the cutting tool temperature field, ANSYS CFXfiAcademic Research software v.12 was used. Additionally, a cutting tool with a single coating layer was used, as reported by Rechet al.( 2005 )

. In this work, eight cases were analyzed, all with cutting tools having a single layer

. Different coating materials were investigated with two types of heat fluxes used on the tool-chip interface.

The design of experiments ( DoE ) is used owing to the fact that it is the most economical and accurate method for performing process optimization. The DoE accelerates the understanding on the influence of the process parameters by determining which variables are critical to the process and at which level. This investigation required the evaluation of the effects of three variables ( Montgomery, 2000)

. To ascertain the key relationships among them,DoEwas used to find the beststudied case of each simulation carried out. The temperature fields on the cutting tools were thus obtained. Finally, a numerical analysis of the thermal influence of these coatings is presented.

2. Problem Description

Figure 1 presents the thermal modelfor heat conduction in a cutting tool and the regions for imposing boundary conditions. The tool geometry, within the computational domain, is represented respectively byΩ1andΩ2, the coating solids of height h, the cutting tool substrate of height H, and interface C between the coating and the substrate. Only one type of material was considered for the cutting tool with dimensions 12.7x12.7x4.7 m, with a nose radius R=0.8 m and heat flux region S2with an area of approximately 1.424 mm2. The coating thickness values adopted were: h=1 and 10 µm.

( b ) Figure 1.Coated cutting tool: (a) interface detail and (b) heat flux region

RogØrio Fernandes Brito,Joªo Roberto Ferreiraet al.4

At room temperature, the thermal parameters of the materials investigated ( substrate

and coating) were as follows: ISO K10 cemented carbide tool with density =14,900

, specific heat capacityCp=200 thermal

, TiN coating with =

. Figures 2a and 2b show one of the meshes formed by hexahedral elements and used

in the numerical simulation. Figure 2c shows a typical contact area (A) on the tool-chip interface and the area used in the numericalsimulation of the present work (

, the following cutting conditions were used: cutting speed ofvc=209.23 m/min, feed rate off= 0.138 m/rot, and cutting depth ofp= 3.0 m.

Typical hexahedral mesh used.

Partial detail of theS2heat flux region withAarea in red color.

( c) Video image of theS2contact area on the chip-workpiece-tool interface (

Carvalho et

Figure 2.Non-structured mesh ( a) , mesh detail (b)

, image of the flux area(c)

Thermal Analysis in TiNand Al2O3Coated ISO K10 Cemented Carbide &5

2.1. Boundary Conditions

The present analysis assumed the following hypotheses: three-dimensional geometrical domain; transient regime; absence of radiation models; thermal properties, such as ,k, andCpare uniform and the temperatures are independent for the coating layer and the substrate body; there is a perfect thermal contact and no thermal resistance contact between the coating layer and the substrate body; the boundary conditions of the heat flux are uniform and the time is variable; the boundary conditions of the heat transfer coefficienthand room temperatureT"?are constant and also known; there is internal heat generation neither on the coating layer nor on the substrate body.

The heat diffusion equation is subject to two types ofboundary conditions: imposed time-varying heat flux inS2and constant convection inS1of the cutting tool. The initial temperature conditions are described for the thermal states of the substrate and coating

3.Numerical Method The solution of the continuity, momentum, and energy equations uses the Fluid

Dynamics Calculus using the Finite Volume Method ( FVM ) with Eulerian scheme for the spatial and temporal discretization of the physical domain, using a finite number of control volumes ( Versteeg and Malalasekra, 2007 and Löhner, 2008)

. Through this method, the control volume elements follow the Eulerian scheme with unstructured mesh ( Barth and Ohlberger, 2004)

. Using this approach, the transport equations may be integrated by applying the Gauss Divergence Theorem, where the approximation of surface integral is done with two levels of approximation. Firstly, the physical variables are integrated into one or more points on the control volume faces. Secondly, this integrated value isapproximated in terms of nodal values. This approximationrepresents, with second order accuracy, the average physical quantityof

. More details on the concepts involved inFVMmay be found in Barth and Ohlberger ( 2004 )

,where the authors explore discretization techniques, integral approximation techniques, convergence criteria, and calculus stability.

4. Numerical Validation

The commercial software used here was validated extensively by comparing this work ?s results of with those obtained in experimental and numerical investigations. For example, we compared our software ?s numerically obtained temperatures with those obtained, both numerically and experimentally, by Carvalhoet al.( 2006 )

. The largest deviation was 6.07%.In most of the simulated cases, the number of nodal points was 501,768 and the number of hexahedral elements was 481,500.

RogØrio Fernandes Brito,Joªo Roberto Ferreiraet al.6

5.Results and Discussion

Eight caseswere selected to investigate the temperature distribution for a time intervalt. The main goalis to study the influence of heat flux and variations in the coating thickness on the heat flux.

Table 1. Numerical results obtained from the temperature values after 63.14 s.


Coating/ Thickness

Heat flux

Chip-Tool Temperature

Coating- Substrate Temperature

Temperature Difference

Detail of the two temperature monitoring points.

Position of temperature monitoring.

Figure 3. Temperature monitoring points located on and under the 10 µm TiN coating layer.

Table 1 shows temperature values obtained after cutting for 63.14 s on the chip-tool interfaces. This was determined using the ANSYS

Thermal Analysis in TiNand Al2O3Coated ISO K10 Cemented Carbide &7

CFXfiAcademic Research software, v.12. For the10 µm coating with fluxq2 ?(t)

( TiN coated K10 substrate) had the highest calculated temperature difference. For 1 µm coating thickness with fluxq1 ?(t) , case 7 ( Al2O3 coated K10 substrate) had the lowest calculated temperature difference.

Figure 3shows the two points where temperature was monitored during the simulation. For the coordinates on the tool substrate-coating interface:x=1.5 m,y=0.25 m andz=10 µm and on the coating:x=1.5 m,y=0.25 m andz=0 m. Case 04

, with a thickness of 10 µm, exhibited the greatest temperature decrease,

. Figures 4a and 4b show, respectively, heat against cutting time.

2.5 Heat flux rate

Figure 4.Heat rate and heat flux utilized in the present work.

Figures 5 through 8 show the simulation results for coated cutting tools. The influence of heat flux and coating thickness on the temperature fields on the chip-tool coating-substrate interfaces can thus be assessed. It can be seen that owing to the fact that the coating did not affect temperature reduction, it possesses negligible influence on thermal insulation.

with TiN coating - 1 m - 1 q"

K10 substrate surface with TiN coating - 1 m - 10 q"

K10 substrate surface

( b ) Figure 5.Effect of heat flux variation on temperature( TiN coatingwith 1 µm thick)

RogØrio Fernandes Brito,Joªo Roberto Ferreiraet al.8

) with TiN coating - 10 m - 1 q"

K10 substrate surface with TiN coating - 10 m - 10 q"

K10 substrate surface

( b ) Figure 6. Effect of heat flux variation on temperature ( TiN coatingwith 10 µm thick) Case 05 - K10 (C6

) with Alumina coating - 10 m - 1 q"

K10 substrate surface with Alumina coating - 1 m - 1 q"

K10 substrate surface

( b ) Figure 7.Effect of Al2O3coating thickness variation on temperature withq1 ?(t) heat flux:

1 µm thick. Case 07 - K10 with Al2O3 coating - 1 m - 10 q"

K10 substrate surface Case 08 - K10 with Al2O3 coating - 10 m - 10 q"

K10 substrate surface

( b ) Figure 8.Effect of Al2O3coating thickness variation on temperature withq2 ?(t) heat flux:

Thermal Analysis in TiNand Al2O3Coated ISO K10 Cemented Carbide &9 Figures 9a, 9b and 9c show the temperature fields at instant 63.14 s on the top and

bottom of the insertand the heat flux surface, respectively, for case 4 (

TiN coating with

Figure 9.Temperature fields on: (a) top and (b) bottom and ( c) heat flux surface

5.1.Design ofExperiment ( DoE ) TheDoEwas configured with 3 factors ( thickness, heat flux and coating material) at two aiming to determine their influence on the temperature field. The coatings were TiN and Al2O3, the thickness values were1 µm and 10 µm and the heat flux values were 1 and 10 times ( Montgomery, 2000)

(Parte 1 de 2)