FINITE ELEMENT ANALYSIS OF HASTELLOY C-22HS IN END MILLING

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1 Journal of Mechanical Engineering and Sciences (JMES) e-issn: ; Volume, , December 0 FKM, Universiti Malaysia Pahang FINITE ELEMENT ANALYSIS OF HASTELLOY C-HS IN END MILLING K. Kadirgama, M.M. Rahman, A.R. Ismail and R.A. Bakar Faculty of Mechanical Engineering Universiti Malaysia Pahang 6600 Pekan, Pahang, Malaysia Phone: ; Fax: kumaran@um.edu.my ABSTRACT This aer resents the finite element analysis to study the stress distribution of nickel based sueralloy HASTELLOY C-000 in end milling oeration. A commercial finite element software was used to develo modeling and analysis the distribution of stress comonents in the machined surface after end milling of HASTELLOY C-HS using coated carbide tools. The friction interaction along the tool-chi interface was modeled with Coulomb friction law. It was found that the stress had lower values under the cut surface and increased gradually near the cutting edge. Keywords: Finite element analysis, stress, Milling INTRODUCTION In many industries, nickel-base alloys reresent an imortant segment of structural materials. Critical comonents made of these alloys are relied uon to function satisfactorily in corrosive services. Corrosion-resistant high alloy castings are often the subject of major concern because failures of cast comonents have led to significant downtime costs and oerating roblems (Strenkowski and Carroll, 985). Over the years, the nickel- chromium-molybdenum / tungsten alloys have roven to be among the most reliable and cost effective materials for aggressive seawater alication and excellent resistance to localized corrosive attack (itting, crevice corrosion). Among these alloys, Hestelloy C-tyes (C, C-4, C-76, and C-) are used to serve the above mentioned uroses. As these alloys are commonly subject to further machining after casting, it becomes very vital to have an idea about the change in roerties imarted to the machined surfaces after such cutting oerations as end milling. For this reason, finite element methodology is used in this study to determine the machined surface stress characteristics.in the ast decade, finite element method based on the udated- Lagrangian formulation has been develoed to analyze metal cutting rocesses (Strenkowski and Carroll, 985; Strenkowski and Mitchum, 987; Shih et al., 990). Several secial finite element techniques, such as the element searation (Komvooulos and Erenbeek, 99; Shih and Yang, 993), modeling of worn cutting tool geometry (Komvooulos and Erenbeek, 99; Shih and Yang, 993; Shih, 995), mesh rezoning (Komvooulos and Erenbeek, 99; Ueda and Manabe, 993), friction modeling (Strenkowski and Carroll, 985; Strenkowski and Mitchum, 987; Komvooulos and Erenbeek, 99), etc. have been imlemented to imrove the accuracy and efficiency of the finite element modeling. Detailed work-material modeling, which includes the couling of temerature, strain-rate, and strain hardening effects, has been alied to model material deformation (Shih et al., 990; Shih and Yang, 993; Shih, 995). An early analytical model for redicting residual stresses was roosed by Okushima and 37

2 Finite element analysis of HASTELLOY C-HS in end milling Kakino (97), in which residual stresses were related to the cutting force and temerature distribution during machining. In another analytical model a relation was made between residual stresses and the hardness of the workiece (Wu and Matsumoto, 990). Shih and Yang (993) conducted a combined exerimental/comutational study of the distribution of residual stresses in a machined workiece. Liu and Guo (000) used the finite element method to evaluate residual stresses in a workiece. They also observed that the magnitude of residual stress reduces when a second cut is made on the cut surface. Liu and Barash (98) measured the residual stress on the workiece subsurface with consideration of tool flank wear. Their findings indicated that under the condition of a lower cutting seed, the mechanical load had a greater imact on residual stress, while the thermal effect become the major factor affecting residual stress at higher cutting seed. Lee and Shaffer (95) roosed a shear-angle model based on the sli-line field theory, which assumes a rigid-erfectly lastic material behavior and a straight shear lane. Kudo (965) modified the sli-line model by introducing a curved shear lane to account for the controlled contact between the curved chi and straight tool face. Henriksen (95) conducted a series of tests to understand residual stresses in the machined surface of steel and cast iron arts under various cutting conditions. Kono et al. (980) and Tonsoff et al. (995) revealed that residual stresses are deendent on the cutting seed. Matsumoto et al. (986) and Wu and Matsumoto (990) observed that the hardness of the workiece material has a significant influence on the residual stress field. Konig et al. (993) showed that friction in metal cutting also contributes to the formation of residual stresses. FINITE ELEMENT MODEL The finite element model is comosed of a deformable workiece and a rigid tool. The tool enetrates through the workiece at a constant seed and constant feed rate. The model assumes lane-strain condition since generally deth of cut is much greater than feed rate. The finite model used in this study is based on the commercial finite element software. The software, called Thirdwave AdvantEdge uses six-noded quadratic triangular elements by default. AdvantEdge is an automated rogram and it is enough to inut rocess arameters to make a two-dimensional simulation of orthogonal cutting oeration. Thermal boundary conditions for Thirdwave AdvantEdge are given as:. The heat is generated due to heavy lastic work done on the workiece. Its formula is given in Eq. (): P m f W R () where W P is the rate of lastic work, f is the fraction of lastic work converted into heat (assumed to be 0.9), m is the mechanical equivalent of heat (taken as ) and is the density of workiece material (8.6 g/cm 3 ). The heat is generated due to friction between the chi and the rake face of the tool according to Eq. (). q F V m () fr r 38

3 Kadirgama et al. / Journal of Mechanical Engineering and Sciences (0) where F fr is the friction force, V r is the relative sliding velocity between tool and chi and m is the mechanical equivalent of heat (m = ) 3. The generated frictional heat is distributed to chi and tool according to Eq. (3): Q Q chi tool k k chi tool chi tool c c chi too l (3) where Q chi is the heat given to the chi, Q tool is the heat given to the tool, k is the conductivity, is the density and c is the heat caacity. Figure. Model for milling. Certain assumtions are made to simulate the comlex rocedure of metal cutting with FEM. These assumtions are used to define the roblem to be solved as well as to aly the boundary and loading conditions:. The cutting seed is constant.. The width of cut is larger than the feed (lane strain condition), and both are constant. 3. The cutting velocity vector is normal to the cutting edge. 4. The workiece material is a homogeneous olycrystalline, isotroic, and incomressible solid. 5. The workiece is set at a reference temerature of 0 C at the beginning of simulation. 6. The machine tool is erfectly rigid and no influence of machine tool dynamics on machining is considered. 7. Constant friction at tool-chi interaction and tool-workiece interaction. Figure shows the Thirdwave AdvantEdge model for milling oeration and Figure shows an examle of visual simulation of residual stresses induced after milling. 39

4 Finite element analysis of HASTELLOY C-HS in end milling Figure. Model for residual stress WORKPIECE AND TOOL MATERIAL MODELING The workiece material used for simulation is HASTELLOY C-HS and the cutting tool is carbide coated with TiALN and 0 o rake angle. Every one ass (80mm), the simulation was stoed. AdvantEdge uses an analytical formulation for material modeling. In a tyical machining event, in the rimary and secondary shear zones very high strain rates are achieved, while the remainder of the workiece deforms at moderate or low strain rates. In order to account for this, Thirdwave AdvantEdge incororates a stewise variation of the rate sensitivity exonent: f o where is the effective von Mises stress, m, if (4) t f is the flow stress, is the accumulated lastic strain, o is a reference lastic strain rate, m is the strain-rate sensitivity exonents, and t is the threshold strain rate which searates the two regimes. In calculations, a local Newton Rahson iteration is used to comute o according to the low rate equation, and switches to the high rate equation if the result lies above t. f, which is used in Eq. (5) is given as: f 0 T () 0 where T is the current temerature, 0 is the initial yield stress at the reference temerature T 0, 0 is the reference lastic strain, n is the hardening exonent and T is the thermal softening factor. In the resent study, it is assumed that the tool is not lastifying. Hence, it is considered as an absolutely rigid body n 40

5 Kadirgama et al. / Journal of Mechanical Engineering and Sciences (0) Workiece Material The roerties of the workiece material (Hastelloy C-HS) are shown in Tables and. Meanwhile cutting tool roerties and simulation are shown in Table 3 and 4. Table. Chemical comosition for Hastelloy C-HS. Ni Cr Mo Fe Co W Mn Al Si C B BAL (%) Table. Physical roerties of Hastelloy C-HS at room temerature. Proerties Value Density (g/cm 3 ) 8.6 Thermal Conductivity (W/m. o C).8 Mean Coefficient of Thermal Exansion (m/m. o C).6 Thermal Diffusivity (cm /s) Secific Heat (J/kg. o C) 4 Young Modulus (GPa) 3 Table 3. Cutting tool roerties Code name Comosition (%) % Co % WC %Cr3C %TaC %TiC %Nbc KC50M Table 4. Simulation conditions Coating PVD TiAlN Thickness m) 3.5 Simulation No. Cutting seed Feedrate Axial deth

6 Finite element analysis of HASTELLOY C-HS in end milling RESULTS AND DISCUSSION von Mises stress, V, is used to estimate yield criteria for ductile materials. It is calculated by combining stresses in two or three dimensions, with the result comared to the tensile strength of the material loaded in one dimension. Von Mises stress is also useful for calculating the fatigue strength (Schey, 000). Von Mises stress in three dimensions is exressed as (Schey, 000): 3 3 V (3) where,, 3 are the rincial stresses. Figure 3 shows the von Mises stress for simulation no.9 (Cutting seed 80 m/min, feedrate 0.5 mm and axial deth.0 mm) after 80 mm. Most of the tensile V aear at the cutting tool edge. Based on Von Mises criterion, it states that failure occurs when the energy of distortion reaches the same energy for yield/failure in uniaxial tension. Mathematically, this is exressed as (Schey, 000), (4) The yield strength and ultimate tensile strength for the coated carbide cutting tool used in this simulation are 600 MPa and 800 MPa resectively. Then von Mises stress at at region 9 is 4488 MPa, which is higher than the yield strength and ultimate tensile strength of the coated cutting tool. This strees can cause ermanent damage to the cutting tool since this stress is beyond the ultimate tensile strength and yield strength. Cutting seed, feedrate, and axial deth for this simulation is very high and this cause the high stress at the cutting edge, since high cutting seed, feedrate, and axial deth can cause high force in milling (Kadirgama and Abou-El-Hossein, 005; Alauddin et al., 998). The radial deth for every simulation is 3.5 mm. This factor also contributes to higher stress. At region, at the cutting tool and chi contact, the von Mises stress is 50 MPa, where the yield strength and ultimate strength of the workiece are 359 MPa and 759 MPa. The workiece start to deform since the stress is above its yield strength. Figure 4 shows von Mises stress for simulation no. 3 (Cutting seed 00 m/min, feedrate 0. mm/rev, axial deth.5 mm). The stress at cutting tool edge (region 5) is 345 MPa. The Von mises stress is lower comared to that in simulation run no. 9. Even though the stress still higher than yield strength and ultimate tensile strength, but the damage should be not severe comared to that of simulation no.9. At region 3, the stress for the contact oint cutting tool and chi is 577 MPa. This value is almost the same as in simulation no.9. From the von Mises stress distribution as shown in Figure 3 and 4, most of the tensile V locate at the edge of the cutting tool. The stress distribution also show the stress is lower at under the cut surface and increases gradually near the cutting edge. High force is needed at the tool edge for workiece enetration, and this is indirectly increase the stress at the tool edge. This distribution of the stress is same for both cases. The velocity vectors for simulation no.9 as shown in Figure 5 around the tool ti, clearly show the lastic flow of the material araound the cutting 3 3 y 4

7 Kadirgama et al. / Journal of Mechanical Engineering and Sciences (0) edge. The same trend of flow also was observed by Movahhedy et al., 000). Figure 6 shows the 3D icture for von Misses stress distribution for simulation no.9. Table 3 show the average value von Mises stress at cutting tool edge for every simulation that already run. This value will be investigated through statistical method to find the relationshi between variables (cutting seed, feedrate and axial deth) with resonse (von mises Stress). Figure 3. Von mises stress for simulation no. 9 Figure 4. von Mises stress for simulation no. 3 43

8 Finite element analysis of HASTELLOY C-HS in end milling The deformation direction Figure 5. The velocity vectors for simulation no.9. Cutting edge Figure 6. 3D icture for Von Misses stress distribution for simulation no. 9 CONCLUSION In the milling oeration, cutting seed, feedrate and axial deth lay the major role in roducing high stresses. The Von Mises stress distribution also show the stress is lower at under the cut surface and increase gradually when come at cutting edge. The highest comressive xx aear at the cutting edge. Most of the tensile V aear at the cutting tool edge. The stress distribution also show the stress is lower at under the cut surface and increases gradually near the cutting edge. High force is needed at the tool edge for workiece enetration, and this is indirectly increasing the stress at the tool edge. This distribution of the stress is same for both cases. 44

9 Kadirgama et al. / Journal of Mechanical Engineering and Sciences (0) REFERENCES Alauddin, M., Mazid, M.A., EL Baradi, M.A. and M.S.J.Hashmi, Cutting forces in the end milling of Inconel 78. Journal of Materials Processing Technology,77(998), Henriksen, E.K. 95. Residual stresses in machined surfaces. Transactions ASME Journal of Engineering for Industry, 73: Kadirgama, K., Abou-El-Hossein, K.A Force Prediction Model for Milling 68 tool Steel Using Resonse Surface Methodology. American Journal of Alied Sciences, (8): -7. Komvooulos, K. and Erenbeek, S.A. 99. Finite Element Modelling of orthogonal cutting, ASME. Engng Ind. 3: 53. Konig, W., Berktold, A. and Koch, K.F Turning versus grinding a comarison of surface integrity asects and attainable accuracy. Annals of the CIRP, 4(): Kono, Y., Hara, A., Yazu, S., Uchida, T. and Mori, Y Cutting erformance of sintered CBN tools, Cutting tool materials. Proceedings of the International Conference, American Society for Metals, Ft. Mitchell, KY, Kudo, H Some new sli-line solutions for two-dimensional steady state machining, International Journal of Mechanical Science 7: Lee, E.H. and Shaffer, B.W. 95. The theory of lasticity alied to a roblem of machining, Journal of Alied Mechanics, 8: Liu, C.R. and Barash, M.M. 98. Variables governing atterns of mechanical residual stress in a machined surface. Transactions of the ASME, Journal of Engineering for Industry, 04: Liu, R. and Guo, Y.B Finite element analysis of the effect of sequential cuts and tool-chi friction on residual stresses in a machined layer, International Journal of Mechanical Sciences, 4: Matsumoto, Y., Barash, M.M. and Liu, C.R Effects of hardness on the surface integrity of AISI 4340 steel. Transactions of the ASME, Journal of Engineering for Industry, 08: Movahhedy, M., Gadala, M.S. and Altintas, Y Simulation of the orthogonal metal cutting rocess using an arbitrary Lagrangian- Eulerian finite element method. Journal of Materials Processing Technology,03: Okushima, K. and Kakino, Y. 97. The residual stresses roduced by metal cutting, Annals of the CIRP, 0(): 3 4. Schey, J. A Introduction to Manufacturing Processes. McGraw Hill, 3 rd ed. Shih, A.J Finite Element Simulation of Orthogonal Metal Cutting, ASME. Engng Ind. 7: 84. Shih, A.J. and Yang, H.T Exerimental and finite element redictions of residual stresses due to orthogonal metal cutting. Int. I. Numer. Meth. Engng. 36, 487. Shih, J. and Yang, H.T.Y Exerimental and finite element redictions of the residual stresses due to orthogonal metal cutting, International Journal for Numerical Methods in Engineering, 36: Shih,A.J., Chandrasekar, S. and Yang, H.T Fundamental issues in machining, ASME PED, 43:. Strenkowski, J.S. and J. T. Carroll, J.T A Finite Element Model of Orthogonal Metal Cutting, ASME. Eng. Ind. 07:

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