numerical simulations of 3d tool geometry effects on ... · pdf filejianwen hu received his...

490 Int. J. Mechatronics and Manufacturing Systems, Vol. 2, No. 4, 2009

Copyright © 2009 Inderscience Enterprises Ltd.

Numerical simulations of 3D tool geometry effects on deposition residual stresses in diamond coated cutting tools

Anderson Renaud, Jianwen Hu, Feng Qinand Y. Kevin Chou* Mechanical Engineering Department, 290 Hardaway Hall, 7th Ave., The University of Alabama, Tuscaloosa, AL 35487, USA E-mail: [email protected] E-mail: [email protected] E-mail: [email protected] E-mail: [email protected] *Corresponding author

Abstract: Deposition residual stresses in diamond-coated cutting tools significantly impact the coating adhesion and machining performance. In this study, Computer-Aided Design (CAD) software was used to create the solid model of coated tools, and further exported to Finite Element Analysis (FEA) software for 3D simulations of residual stresses generated. Design of experiments approach was employed to systematically investigate the tool geometry effects. Major results are summarised as follows. • the cutting edge radius is the most significant factor • for a 5 µm edge radius, the radial and circumferential normal stresses

( r and ) are about 1.5 GPa in tension and 3.7 GPa in compression, respectively.

Keywords: cutting tool; diamond coating; residual stresses; tool geometry.

Reference to this paper should be made as follows: Renaud, A., Hu, J., Qin, F. and Chou, Y.K. (2009) ‘Numerical simulations of 3D tool geometry effects on deposition residual stresses in diamond coated cutting tools’, Int. J. Mechatronics and Manufacturing Systems, Vol. 2, No. 4, pp.490–502.

Biographical notes: Anderson Renaud received his Bachelor’s Degree in Mechanical Engineering in 2008 from the University of Alabama. He currently works with Southern Company.

Jianwen Hu received his Bachelor’s Degree in 2001 and Master’s Degree in 2003, both in Mechanical Engineering from Wuhan University of Technology. He is currently a PhD student in Mechanical Engineering Department at the University of Alabama. His research areas are in machining, materials, and mechanics.

Feng Qin graduated from Zhejiang University at China with a Master’s Degree in Mechanical Engineering in 2004 and a Bachelor’s Degree in 1999 from China University of Petroleum, also Mechanical Engineering. His research and work involve machining, computer-aided manufacturing.

Numerical simulations of 3D tool geometry effects 491

Y. Kevin Chou is an Associate Professor in Mechanical Engineering Department at The University of Alabama. He received his PhD in Industrial Engineering from Purdue University. His teaching and research interests include design and manufacturing, material, metrology, tribology, and surface engineering.

1 Introduction

Numerous surface engineering technologies such as Chemical Vapor Deposition (CVD) have been widely applied for wear resistant applications including cutting tools for machining (Kustas et al., 1997; Grzesik et al., 2002; Bouzakis et al., 2004). Diamond coatings, owing to its unique properties and wide applications, have been attempted, as an alternative to costly synthetic Polycrystalline Diamond (PCD) tools, to machine advanced materials. Surveys of CVD diamond coating tool performance have been conducted (Hu et al., 2008) and show that there are mixed results of CVD diamond tool performance. The majority reported that wear resistance of CVD diamond tools is still distant to PCD counterparts.

Coating delamination is the major failure mode of CVD diamond coating (Amirhaghi et al., 2001). Chou and Liu (2005) also demonstrated that coating delamination at the tool flank can be of catastrophic nature and is the tool-life limiting factor for CVD diamond tools. High stresses and/or degraded adhesion during machining result in coating failure and rapid tool wear. In CVD processes, the mismatched thermal strains between coating and substrate materials generate high stresses in the coated tool and stress discontinuity at the interface. For CVD diamond tools, diamond coating, with a smaller thermal expansion coefficient, endures a compressive residual stress and the tungsten carbide (WC) substrate receives a stress in tension. Residual stresses in diamond coating tools depend upon substrate materials, surface treatment, and deposition temperatures, etc. Using literature data: 2.5 and 5.5 µm/(mK) of thermal expansion coefficients for CVD diamond and WC, and 1200 GPa and 0.07 for elasticity and Poisson’s ratio of diamond, respectively, a deposition temperature of 800°C can generate a biaxial stress as high as 3.0 GPa in compression in the coating.

Almeida et al. (2005) experimentally investigated edge preparations of diamond coated tools in machining WC. Three types of edge conditions, i.e., up-sharp, chamfer, and hone, were tested. The edge conditions were found significant to the machining forces, wear pattern, and tool life, etc. The authors further reported that the coating delamination occurred first at the honed tool, despite lower deposition-stress concentrations. Novak et al. (1999) studied the behaviour of TiN coatings on high-speed steel substrates with strongly curved surfaces (edge of tools). The authors reported that sharp edges result in serious irregularity across the coating, e.g., build-up and ball-shaped parts, and adhesion between the coating and substrate is rather poor, and cutting can results in spontaneous coating disintegration.

Diamond coating tools have a potential to be a cost-effective alternative to PCD tools in machining advanced materials. There have been numerous research efforts on new diamond coating technologies and applications. However, current practices to fabricate coating tools use off-the-shelf products as substrates and the tool edge geometry has not been integrally studied. To effectively use diamond coating tools, it is necessary

492 A. Renaud et al.

to understand stress modifications, in particular, around the cutting edge, due to the deposition for different geometries.

The deposition stresses reported in the literature is for nominal biaxial stress conditions. Around any geometric changes, the local stress fields will be altered. An earlier studied investigated the deposition stress altered by the cutting edge radius in 2D conditions. FE simulations, plane-stress assumption, were developed and applied (Hu et al., 2007). Figure 1 shows typical stress contours (normal component, parallel to the coating surface) after the deposition. The area away from the edge has a uniform compressive stress around 4.0 GPa in the coating and 0.7 GPa of tension in the substrate. Note that the simulated bulk coating stress, 4.0 GPa departs from the analytical biaxial stress, 3.0 GPa, because of the plane-stress assumption. It is also observed that around the edge, the stress distributions alter considerably.

Figure 1 An example of deposition stress distribution for 2D plane stress conditions (MPa)(see online version for colours)

Source: Hu et al. (2007)

In practical machining operations, the area of a tool that is engaged in chip removal is the corner area. It is well-known that the tool tip geometry strongly impact the machining performance and tool wear (Schimmel et al., 2000, 2002). A typical cutting tool corner has complex geometry, clearly 3D in nature as shown in Figure 2, a Scanning Electron Microscopic (SEM) image of a common cutting insert. Further, the addition of coating will change the tool geometry, in particular the edge radius. Stress augmentations in coated tools, which are due to the change of geometry, affect the coating-substrate interface behaviour. It is hypothesised that the tool tip geometric parameters may impact the stress levels increased by the geometry change. However, it is not clear which parameters are dominant.

The objective of this research is to investigate the tool geometry effects, both macro- and micro-levels, on the deposition residual stresses in diamond coated cutting tools. The substrate geometry was constructed using CAD software (Pro/Engineer) and FEA software (ANSYS) was used to model and analyse the tool stress conditions. To systematically investigate the tool geometry effects, the design of experiment approaches was employed with 4 factors and two levels. Analysis of variance was conducted to identify factors, and interactions between factors if any, significant to the deposition residual stresses around the tool corner area.


Figure 2 An SEM image of a cutting tool corner area

2 Numerical simulations

2.1 Tool geometry model

Diamond coating tools were modelled with 3D geometry using Pro/Engineer software. The substrates studied were common disposable-type inserts with various geometries. The substrate had 3.18 mm thick with different shapes, but a constant inscribed circle of 12.7 mm diameter. Four parameters, namely, the edge radius, the relief angle, the corner radius, and the corner angle, were varied. The diamond coating on the substrate has a uniform thickness, 30 µm, at the rake and the relief faces, extending to at about 0.9 mm from the substrate bottom. Figure 3 illustrates an example of a coated insert model.

Figure 3 An example of a coated tool geometry model (see online version for colours)

The CAD models of the tool (substrate and coating) were then imported into FEA software, ANSYS, for mechanical simulations. Due to the symmetry, only a partial model is needed for analysis, generally a quarter, e.g., Figure 4, for a 35°


diamond-shaped insert. The element used for structural analysis was Solid45, which is a tetrahedron of 4 nodes, 6 edges and 4 faces. Meshing was generated in the coating first using the default setting and the edge area was refined to have about 0.2 µm of minimum element sizes, Figure 5. The substrate was then meshed using the default setting.

Figure 4 A tool with a partial model to be analysed (see online version for colours)

Figure 5 An example of meshed edge area of a coated tool model (see online version for colours)

2.2 Deposition stress modelling

For deposition-stress simulations, static structural analysis with thermal strains considered was conducted. A uniform deposition temperature of 800°C was set as the initial condition and a room temperature of 25°C was set as the final temperature. Since both diamond and WC have high melting points and are of brittle nature with limited plastic deformation, linear-elastic material models independent of temperatures were used. The elasticity, Poisson’s ratio, and thermal expansion coefficient of diamond (Heath et al., 1986) and WC (Amirhaghi et al., 2001) used were 1200 GPa, 0.07, 2.5 µm/(m⋅K), and 620 GPa, 0.24, 5.5 µm/(m⋅K), respectively. The boundary conditions used in the mechanical analysis included two symmetric planes and a fully constrained point at the bottom corner of the partial model, where two symmetric planes intersect, Figure 5. After the model setup, structural analysis was executed to obtain displacement, strain, and stress data.


It has been reported that coating interface stresses are crucial to the tool performance, in particular, related to coating-substrate adhesion integrity. Thus, the cross-section on the symmetric plane (also bisecting the tool corner) was targeted to extract stress data along the interface. The extracted interface stress data was then transformed into the local polar coordinate to evaluate the normal and shear stresses along the cutting edge interface. Figure 6 is a schematic drawing representing the interfacial stresses including three components: the radial normal stress ( r), the circumferential normal stress ( ),and the shear stress ( r ) that are a function of the relative location, in Figure 6, or nondimensional unit from 0 to 1, 0 being beginning of curvature at rake.

Figure 6 A schematic drawing showing the interfacial stress components around the edgeradius area

2.3 Parametric study

It is assumed that the tool geometry may significantly affect the deposition residual stresses. To systematically investigate the tool geometry effects, a test matrix, determined using the design of experiments approach, includes four factors and two levels with a full factorial design. The four factors include the edge radius, the relief angle, the corner radius, and the corner angle (related to the insert shape), shown Figure 7. Table 1 lists the parameter and corresponding values at different levels tested.

Figure 7 A schematic illustrating four factors studied (see online version for colours)


Table 1 Factors and levels used in the parametric study

A B C D

Edge radius Relief angle Corner radius Corner angle Factor re (µm) Ae (°) rn (mm) An (°)

High 50 11 1.6 90 LevelLow 5 5 0.4 35

Each case was modelled using Pro/Engineer and analysed in ANSYS software. The interface stresses, all three components, were then extracted and compared between different tool geometries. The maximum values of the interface stresses of each case were recorded for statistical analysis. Analysis of variance (ANOVA) was used to identify factors and interactions between factors significant to the interface stresses.

3 Results and discussion

Figure 8 shows the stress contours (both x and y components, two orthogonal axes parallel to the surface) in a diamond coated tool. The tool geometry employed in this specific case is 50 µm edge radius, 11° relief angle, 1.6 mm corner radius, and 35° corner angle, corresponding to the VPG424 insert specification. For both components, the stress away from the edge and corner in coating is about 3.0 GPa in compression, which is consistent with the result from the biaxial stress analysis, 3 GPa in compression. Note that the nominal stress is lower than the previous study (about 4.0 GPa) because of the plane-stress condition assumed in the 2D analysis. Figure 9 shows a closed-up view of the stress distribution ( y component) around the tool edge. It can be observed that considerable stress alterations, values and distributions, occurs around the tool edge.

Figure 8 Examples of stress contours: (a) x and (b) y components, in a diamond coated tool(see online version for colours)

(a)


Figure 8 Examples of stress contours: (a) x and (b) y components, in a diamond coated tool(see online version for colours) (continued)

(b)

Figure 9 Stress distribution around the cutting tool edge (see online version for colours)

Figure 10 compares stress contours, first principal component, around the tool corner area for two different edge radii (50 µm and 5 µm) and two different corner radii (1.6 mm and 0.4 mm). Away from the cutting edge, the stress values are virtually the same for all three cases. However, around the cutting edge, significant stress concentrations occur due to the change of the tool geometry. It is noted that the sharp edge (5 µm) has a higher-level stress concentration, ~1.6 GPa vs. ~0.9 GPa for 5 µm vs. 50 µm edge radius. In addition, a sharper edge also results in much severe stress gradients around the edge area. On the other hand, examining the area around the cutting edge of two different tool corner radii: (a) and (c) of Figure 10, it can be noted that the corner radius only marginally affects the deposition residual stresses in diamond coated tools.


Figure 10 Stress contour comparison, first principal component, between different tool geometries: (a) 50 µm re and 1.6 mm rn, (b) 5 µm re and 1.6 mm rn, (c) 50 µm reand 0.4 mm rn (see online version for colours)

(a)

(b)

(c)


Figure 11 compares the interfacial deposition stresses around the edge, for two different edge radii and two different corner radii. The abscissa in Figure 11 is the distance, normalised by the arc length of the rounded edge, from where the curve begins at the rake (Figure 6). First, high stress concentrations due to the edge sharpness can be quantified. For the radial normal stress ( r), the maximum reduces from 1.4 GPa for 5 µm re to 0.8 GPa for 50 µm re. The large edge radius also shows smooth stress gradients along the edge. For the circumferential normal stress ( ), stress reductions by the edge hone were from –3.7 GPa for 5 µm re to –2.4 GPa for 50 µm re. For the shear stress component ( r ),the stress magnitude is smaller and reductions at a large radius are also evident, from 0.9 GPa for 5 µm re to 0.4 GPa for 50 µm re. Moreover, it is further confirmed that the corner radius has minor effects on the interface stresses, less than 10% difference between 0.4 mm and 1.6 mm corner radii.

Figure 11 Edge radius effects on interfacial stresses: (a) radial normal stress; (b) circumferential normal stress and (c) shear stress (see online version for colours)

(a)

(b)


Figure 11 Edge radius effects on interfacial stresses: (a) radial normal stress; (b) circumferential normal stress and (c) shear stress (see online version for colours) (continued)

(c)

The maximum or minimum values of three interface-stress components from all 16 studied cases are listed below, Table 2. The most noticeable result is that the edge radius has the foremost effects to all three stress components in all cases. On the other hand, the effects of other factors are not clear. In order to determine if any other tool geometry parameters affect the interface stress, ANOVA using Minitab was conducted. Table 3 below summarises the ANOVA results; a factor is considered important if the associated P value is less than 0.005. It is unambiguously concluded that the edge radius is the most significant factor. In addition, the corner radius is of secondary importance to r,and the relief angle is of secondary importance to . Moreover, no high-order interactions between factors are observed.

Table 2 Maximum or minimum stress values of all simulated cases (Parameters A, B, C and D were defined in Table 1)

Factor (ABCD)/Level Max r (GPa) Min (GPa) Max r (GPa)

HHHH 0.92 –2.46 0.38 HLHH 0.88 –2.78 0.42 HHLH 0.99 –2.45 0.37 HHHL 0.80 –2.41 0.46 HLLH 0.96 –2.53 0.46 HLHL 0.85 –2.58 0.37 HHLL 0.88 –2.56 0.37 HLLL 0.85 –2.54 0.40 LHHH 1.41 –3.80 1.00 LLHH 1.47 –4.03 1.06 LHLH 1.53 –3.88 1.04 LHHL 1.43 –3.67 0.94


Table 2 Maximum or minimum stress values of all simulated cases (Parameters A, B, C and D were defined in Table 1) (continued)

Factor (ABCD)/Level Max r GPa Min GPa Max r GPaLLLH 1.54 –4.00 1.10 LLHL 1.46 –3.99 1.01 LHLL 1.53 –3.85 0.99 LLLL 1.53 –4.00 1.04

Table 3 ANOVA results of interface stress values, three components (Parameters A, B, Cand D were defined in Table 1)

P value Factors σr σθ τrθ

A 0.000 0.000 0.000 B 0.744 0.002 0.068C 0.002 0.793 0.415 D 0.031 0.346 0.131 R-sq, % 98.95 98.99 98.96

4 Conclusions

Tool geometry effects on diamond coated cutting tools were studied using 3D FEA modelling to systematically analyse the geometry parameter effects on the deposition stresses. The deposition stress was simulated (3D) considering thermal strain and temperature changes. The interfacial stresses around the cutting tip were particularly analysed and compared. Design of experiments, includes four factors (edge radius, relief angle, corner radius, and corner angle) and 2 levels with a full factorial design was employed to systematically evaluate tool geometry effects on deposition stresses. Analysis of variance was performed to quantitatively reveal the significant factors and interactions between the factors that dominate the stress concentrations. The results are summarised as follows:

• The coating in the bulk surface has an isotropic ~3.0 GPa compressive stress, which is consistent with the result of biaxial stress analysis.

• The cutting edge radius significantly causes the stress concentrations around the tool tip.

• More importantly, the edge radius impacts the coating-substrate interface stresses around the cutting edge. For a 5 µm edge radius, the radial normal stress ( r)increases from 0 at the top uniform surface to about 1.5 GPa in tension, and the circumferential normal stress ( ) increases from around 3.0 GPa in compression to over 3.7 GPa.

• ANOVA results confirm that the edge radius is the most dominant factor. In addition, the corner radius is of secondary importance to r, and the relief angle is of secondary importance to .


Acknowledgement

This research is supported by National Science Foundation, Grant No.: CMMI 0728228.

ReferencesAlmeida, F.A., Oliveira, F.J., Sousa, M., Fernandes, A.J.S., Sacramento, J. and Silva, R.F. (2005)

‘Machining hardmetal with CVD diamond direct coated ceramic tools: effect of tool edge geometry’, Diamond and Related Materials, Vol. 14, Nos. 3–7, pp.651–656.

Amirhaghi, S., Reehal, H.S., Wood, R.J.K. and Wheeler, D.W. (2001) ‘Diamond coatings on tungsten carbide and their erosive wear properties’, Surface and Coatings Technology,Vol. 135, Nos. 2–3, pp.126–138.

Bouzakis, K-D., Hadjiyiannis, S., Skordaris, G., Mirisidis, I., Michailidis, N., Efstathiou, K., Pavlidou, E., Erkens, G., Cremer, R., Rambadt, S. and Wirth, I. (2004) ‘The effect of coating thickness, mechanical strength and hardness properties on the milling performance of PVD coated cemented carbides inserts’, Surface and Coatings Technology, Vols. 177–178, pp.657–664.

Chou, Y.K. and Liu, J. (2005) ‘CVD diamond tool performance in composite machining’, Surface and Coatings Technology, Vol. 200, pp.1872–1878.

Grzesik, W., Zalisz, Z. and Nieslony, P. (2002) ‘Friction and wear testing of multilayer coatings on carbide substrates for dry machining applications’, Surface and Coatings Technology,Vol. 155, No. 1, pp.37–45.

Heath, P.J. (1986) ‘Properties and uses of amborite’, Industrial Diamond Review, Vol. 46, pp.120–127.

Hu, J., Chou, Y.K. and Thompson, R.G. (2007) ‘On stress analysis of diamond coating cutting tools’, Transactions of the North American Manufacturing Research Institute of SME,Vol. 35, pp.177–184.

Hu, J., Chou, Y.K. and Thompson, R.G. (2008) ‘Nanocrystalline diamond coating tools for machining of high-strength Al alloys’, International Journal of Refractory Metals and Hard Materials, Vol. 26, pp.135–144.

Kustas, F.M., Fehrehnbacher, L.L. and Komanduri, R. (1997) ‘Nanocoatings on cutting tools for dry machining’, Annals of CIRP, Vol. 46, No. 1, pp.39–42.

Novak, R., Kvasnicka, I., Novakova, D. and Mala, Z. (1999) ‘Study of hard PVD coatings on strongly curved surfaces’, Surface and Coatings Technology, Vol. 144, pp.65–69.

Schimmel, R.J., Endres, W.J. and Stevenson, R. (2002) ‘Application of an internally consistent material model to determine the effect of tool edge geometry in orthogonal machining’, Transactions of the ASME, Journal of Manufacturing Science and Engineering, Vol. 124, pp.536–543.

Schimmel, R.J., Manjunathaiah, J. and Endres, W.J. (2000) ‘Edge radius variability and force measurement considerations’, Transactions of the ASME, Journal of Manufacturing Science and Engineering, Vol. 122, pp.590–593.

numerical simulations of 3d tool geometry effects on ... · pdf filejianwen hu received his...

Documents