Finite Element Analysis of Workpiece Temperature during Surface Grinding of Inconel 718 Alloy

Transcription

1 5 th International & 26 th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12 th 14 th, 2014, IIT Guwahati, Assam, India Finite Element Analysis of Workpiece Temperature during Surface Grinding of Inconel 718 Alloy Chahat Sharma 1, Sudarsan Ghosh 2, Prabal Talukdar 3 1 M.Tech student, Department of Mechanical Engineering, IIT Delhi, New Delhi, India sharmachahat20@gmail.com 2 Associate Professor, Department of Mechanical Engineering, IIT Delhi, New Delhi, India sudarsan.ghosh@gmail.com 3 Associate Professor, Department of Mechanical Engineering, IIT Delhi, New Delhi, India prabal@mech.iitd.ac.in Abstract This paper presents an approach for development of a three dimensional model to find out the temperature variation during grinding of Inconel 718 material. Model is developed using finite element analysis. Method of discretization is used to find out transient temperature variation. In this time step and sub step method is used for loading the movable heat source. Model is developed using FEM software ANSYS and then compared to the experimentally measured values using embedded thermocouple method. Keywords: Inconel 718, Temperature, Grinding, Finite element method 1. Introduction Grinding of aerospace materials such as Inconel 718 and Ti-6Al-4V is very difficult because of their poor thermal properties. These materials have low thermal conductivity which results in low rate of heat dissipation and consequently increases the workpiece temperature rapidly. This leads to the thermal damages in the form of workpiece burn, tensile residual stresses etc., Thermal damage to the Inconel workpiece during grinding is a major concern. By predicting the range of temperature and controlling the same appropriately during the grinding process, the thermal damages can be minimized. Using single step analytical models, temperature prediction during grinding cannot be done precisely. Hence extensive numerical solutions are required such as finite element method (FEM). In order to address the thermal issues related to grinding of Inconel 718, in this work a 3D thermal model has been developed and finite element analysis has been carried out to determine the temperature variation in grinding zone during surface grinding operation. This model is based on Jaeger analysis in grinding temperature distribution, in this analysis grinding wheel is assumed as heat source [1]. This heat is equally distributed over the workpiece surface. This heat source moves with the speed of workpiece along the surface. A commercial software ANSYS was used as a tool for the study. Rectangular distribution was taken for the heat flux entering into the grinding zone [3]. The material properties at different appropriate temperatures are also considered during implementation of this 3D model. 2. Thermal modelling of grinding 2D models give good analysis for simple workpiece geometries, in 2D model assumption is made that workpiece has an infinite width. But for thin plate this assumption does not applicable. For thin plate three dimensional thermal models is required for accurate analysis. 3D model also consider the convective cooling of side walls. This analysis is based on Jaeger s model [1], according to which heat source is considered as a physical quantity moving on the workpiece. Model is created by incrementally stepping the heat flux on the surface of workpiece elements. Size of elements is chosen on the basis of contact length

2 Finite Element Analysis of Workpiece Temperature during Surface Grinding of Inconel 718 Alloy 3. Finite Element Model 3.1. Geometric model In analysis of temperature distribution in grinding two factors mainly affect the distribution, heat flux distribution and shape of heat source zone. In this study rectangular heat source and plane heat source zone is considered. Heat source is moving along the surface of workpiece. Method of discretization is used in FEM analysis to study continuous grinding process. Time step and sub step methods is used in which a thin material layer is immediately separated from workpiece after each time step. Time step is time for which heat flux is stationary between successive time steps and can be found from the following relation Figure1- Schematic of Wheel-workpiece engagement Contact length can be calculated from the relation [2], lc=(a d s ) 1/2 Here d s is the diameter of grinding wheel and a is the depth of cut. The heat flux can be given by following relation, = Here is the percentage of heat flux entering into the workpiece, V s the wheel speed and, F t is the tangential force per unit area of the workpiece. Percentage of the heat flux entering the workpiece can be calculated by following relation, Time step = Accuracy of temperature distribution depends on time step. As time step decreases, accuracy of distribution increases. For this element size should be small but this will increase the simulation time for the process Workpiece meshing As temperature gradient is large on top surface of a ground specimen, so mesh size should be refined in top section to increase the accuracy and to reduce the simulation time Fig. 2 depicts the meshed workpiece. =1 Where U ch is the energy required for formation of chips, it is constant for a specific material in grinding, and U is the total specific grinding energy required for grinding, can be calculated from the relation, = Here V w is the work speed and, it is the speed of the moving heat source. Tangential force Ft per unit width can be calculated from the relation, = Here P t is the power per unit width of the workpiece, it is measured during the testing of the different grinding wheels. So heat flux can be calculated by these relations. Fig. 2- Meshed workpiece 3.3. Workpiece material properties Thermal properties like thermal conductivity and heat capacity greatly affect the temperature distribution in grinding. These properties change as temperature in grinding changes. However in past studies, the thermal properties of workpiece were taken either temperature independent or linearly variable to temperature. In this 420-2

3 5 th International & 26 th All India Guwahati, Assam, India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12 th 14 th, 2014, IIT study non-linear material properties are used to find more accurate simulation. Polynomial interpolation method is used to find out these material properties which show a good agreement to real values. Table 1-Thermal properties of Inconel 718 at different temperatures Temperature ( C) Conductivity (W/(m C)) Specific heat capacity (J/(kg C)) Experiment To verify the analytical and numerical model experiments has been conducted on Chevalier Model Smart-H1224 two axis CNC surface grinding machine with aluminium oxide wheel. Forces were measured with Kistler 9257B dynamometer. In these experiments 30 gauge K-type thermocouple is used to measure the temperature. Embedded thermocouple technique is used to measure the temperature. In this method thermocouple is fixed inside a blind hole. Thermocouplee is fixed with grinding surface using high temperaturee epoxy. During grinding, thermocouple junction is exposed and direct contact of grinding surface to thermocouple is achieved. This thermocouple is directly connected to the data acquisition device and this DAQ device is interfaced with National Instrument Lab view software to get temperature variation Boundary conditions The boundary in simulation changes as heat source moves on the surface of workpiece. Heat flux is loaded on the rectangular surface with each time step and moves to next rectangular surface in next time step. For current time step, the result of last time step is used as the initial condition. Coolant effect on upper surface and side surface is simulated by convective boundary condition. Bottom surface is considered as adiabatic surface as small amount of grinding fluid goes onto this surface and there is little heat going out from this surface. Initial temperature of workpiece is taken as 20 C 4. Simulation and result of transient temperature variation Based on the surface grinding condition as given in Table 2, simulation is done by ANSYS software. Basically there is three step involved to solve this problem. These steps are pre-processor, solution of linear equation and post-processoparameters given as inputs in the pre-processor step. step. All required Type of element is also selected in this step. In this model hexahedron element solid70 is selected. One pass of grinding process is completed in 0.5 sec. The whole process is divided into 50 steps of time. So each time step is of 0.01 sec. every time step is divided into 5 substeps to improve accuracy and precision in model. By loop iteration and loading step by step, 50 times, temperature distribution for workpiece is obtained. Fig. 3 -Schematic drawing of grinding temperature measurement 420-3

4 Finite Element Analysis of Workpiece Temperature during Surface Grinding of Inconel 718 Alloy Table 2- Different parameters and conditions in grinding parameters Data Workpiece material Inconel 718 Workpiece size mm 3 Grinding wheel speed (V s ) 28 m/s Workpiece speed (V w ) 8 m/min Grinding wheel dia. (d) 250 mm Depth of cut (a) 0.02 mm Base temperature (T ) 20 C Convection coefficient (h) 20 kj /m 2 s C Tangential force (F t) N/mm Heat flux (q) W/mm 2 6.Results Figure 4 represents three dimensional temperature distribution at time of 0.2 sec for depth of cut of 0.02 mm.average heat flux to the workpiece, q w, is W/mm 2. Maximum temperature location and isothermal can be easily observed from figure 4 (a). (a) (b) Fig 4- Temperature field developed when grinding Inconel 718 with grinding wheel at time t=0.2 sec for a depth of 0.02 mm. (a)top view (b) front view Figure 5 shows the variation of surface temperature with distance from the edge of workpiece. Different curves present the variation of temperature at different location of workpiece for different time steps. In fig. 5 variation is presented for different depth of cut of 0.02 mm and figure 6 represent variation for depth of cut of 0.05 mm. As seen from figure that nature of variation of temperature is almost same for both depth of cut, only difference between two curves is rise of maximum temperature in workpiece

5 5 th International & 26 th All India Guwahati, Assam, India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12 th 14 th, 2014, IIT Fig 5-Variation of surface temperature with distance when grinding Inconel 718 for a depth of cut of 0.02 mm Fig 6- experimental result for depth of cut of 0.02 mm energy imparted to ground surface and kinetic energy imparted to chips. In figure 7 variation of surface temperature with different depth of cut is shown. As seen from figure for Inconel 718 material when depth of cut changes from 0.02mm to 0.05 mm,maximum temperatue of surface changes from 780 C to 1050 C. It is because for large depth of cut large specific energy is required for cutting. Because as we increases depth of cut adhesive effect between workpiece material and wheel increases, Due to increase in depth of cut more work material particles will adhere to the wheel and thus lead to an increase in the frictional surface area. Due to this more energy is required for material removal. Fig 7-Variation of surface temperature with distance when grinding Inconel 718 for a depth of cut of 0.05 mm Figure 5 shows the temperature distribution obtained by the experiments for depth of cut of 0.02 mm. It is found from figure that temperature profile obtained with proposed model well matches the result obtained from experiments. The error between experiments and numerical temperature profile is may be likely caused by bead size of thermocouple which affects the temperature. Heat sink source may be other reason for difference in temperature profile which is neglected in numerical simulation, such as residual Fig 8- Variation of maximum surface temperature with depth of cut 420-5

6 Finite Element Analysis of Workpiece Temperature during Surface Grinding of Inconel 718 Alloy 7. Conclusion In this study super alloy Inconel 718 is chosen which has poor machinability. A 3 D Finite element method is employed to simulate grinding temperature. From the simulations and also keeping in mind the application of this material, the following conclusions can be drawn. 1. Three dimensional temperature variation and maximum temperature can be calculated with the help of this model when force or power is known. 2. HAZ zone of workpiece can be determined easily using temperature variation field. 3. The simulated results have been validated by experimental observations made through embedded thermocouple method. 4. Possible damage in workpiece due to large heat can be predicted easily without measuring temperature which is time taking and often expensive. 5. It is also can be seen from figure 4 that maximum temperature rise take place at back side of wheel, so more coolant should be applied in this region to reduce temperature. So reduction in temperature can be done in two ways, either reduce depth of cut or change the grinding parameters for Inconel 718 grinding. References- 1. Jaeger, J.C (1942), Moving heat sources of heat and the temperature at sliding contacts, Proceedings of the Royal Society of New South Wales, Vol. 76, pp Mao, C., Zhou, Z. X. (2003), Analysis and FEM simulation of temperature field in wet surface grinding, materials and manufacturing processes, Vol. 25, pp Mahdi, M., Zhang, L. (1995), The finite element thermal analysis of grinding processes by ADINA, Computers and Structures, Vol. 56, pp Paul, S., Chattopadhyay, A.B. (1995), A study of effects of cryo-cooling in grinding, International Journal of Machine Tools and Manufacture, Vol. 35, pp Mamalis, A.G., Kundra, J., Manolakos, D.E., Gyani, K., Markopoulos, A. (2003), Thermal modelling of surface grinding using implicit finite element techniques, International Journal of Advanced Manufacturing Technology, Vol. 219, pp Wang, L., Qin, Y., Liu, Z.C., Ge, P.Q., Gao, W. (2003), Computer simulation of a workpiece temperature field during the grinding process, Proceedings of theinstitution of Mechanical Engineers Part B:Journal of Engineering Manufacture, Vol. 217, pp Biermann, D., Schneider, M. (1997), Modeling and simulation of workpiece temperature in grinding by finite element analysis, Machining Science and Technology, Vol. 1, pp Aguiar, A., Monteiro, A., Natal, R., M.P. Lages (2005), Experimental and FEM study of the influence of the grinding stone one the temperature field during superficial grinding. 9. Jin, T., Stephenson, D.J. (1999), Three dimensional finite element simulation of transient heat transfer in high efficiency deep grinding, Annals of the CIRP, Vol. 53, pp Anderson, D., Warkentin, A., Bauer, R. (2008), Experimental validation of numerical thermal models for shallow and deep dry grinding, Journal of Materials Processing Technology, Vol. 204, pp Rowe, W.B. (2001), Thermal analysis of high efficiency deep grinding, International Journal of Machine Tools and Manufacture. 12. Malkin, S. (2004), Burning limit for surface and cylindrical grinding of steels, Annals CIRP 27(1), pp Guo, C., and Malkin, S. (1999), Energy partition and cooling during grinding, 3rd International Machining and Grinding Conference, Society of Manufacturing Engineers. 14. Shaw, M. C., and Vyas, A. (1994), Heat affected zones in grinding steel, Annals CIRP 43(1), pp Zhang, L., Mahdi, M. (2004), Applied mechanics in grinding IV. The mechanism of grinding induced phase transformation, International Journal of Machine Tools and Manufacture, Vol. 35, pp