An investigation of ball burnishing process on CNC lathe using finite element analysis
Introduction
Inherent irregularities and surface defects such as tool marks and surface roughness are formed on the work piece surface after conventional machining processes such as turning and milling. These irregularities cause friction and surface damage which leads to low product life, poor metallurgical properties and overall poor product quality. To overcome these defects, conventional surface finishing processes such as grinding, honing and lapping are adopted. These processes essentially depend on chip removal to attain the desired surface finish and the skill and experience of the operator in handling the process also plays a role. To resolve both these problems, burnishing process is applied for better surface finish on the post machined components due to its chip-less and relatively simple operations. Ball burnishing which is also called ballizing process, is a replacement for other bore finishing operations such as grinding, honing, or polishing. A ball burnishing tool consists of one or more over-sized balls that are pushed through a hole. Burnishing is a cold working surface finishing process which is carried out on material surfaces to induce compressive residual stresses and to enhance surface qualities. The improvements in surface qualities include reduction in surface roughness, increase in surface hardness, improvements in grain size, wear-resistance, and fatigue and corrosion resistance. Burnishing tool typically consists of a hardened sphere which is pressed onto/across the part being processed which results in plastic deformation of asperities into valleys. Burnishing is also used to get close tolerance in areas like automobile, aircraft, defense, machine tool, hydraulic and pneumatic equipment and home appliances. The finite element method (FEM) is a numerical technique for finding approximate solutions of partial differential equations (PDE) as well as of integral equations. The solutions approach is based either on eliminating the differential equation completely or rendering the PDE into an approximating system of ordinary differential equations, which are then numerically integrated using standard techniques such as Euler's and Runge-Kutta method. Finite element modeling of chip formation process can be considered as a promising approach to study the cutting process, allowing reduction of experimental cost. It provides information on some of the difficulties to measure variables such as the temperature, energy, or stress and thus, it contributes to improve general understanding of chip formation process. Three kinds of mechanical formulation can be used. Eulerian formation, in which the grid is not attached to the material, is computationally efficient but needs updated free chip geometry. Langrangian formulation, in which the grid is attached to the material, requires to update the mesh or to use a chip separation criterion to form a chip from the work piece. An alternative method is to use arbitrary Langrangian Eulerian (ALE) formulation. In this case, the grid is not attached to the material and it can move, avoid distortion and update the free chip geometry. Mohammadpour et al. developed [1] two-dimensional finite element model for orthogonal cutting of AISI 1045 and also the numerical solution using the implicit FEM. The effects of cutting speed and feed rate on surface and subsurface residual stresses induced after orthogonal cutting are fully investigated and reported. The results of the stress distribution from simulation and experiment are found to be increasing with respect to cutting speed and feed rate. Yen et al.[2] established 2D and 3D FEM models for hard roller burnishing process. The simulation results (i.e. surface deformation and residual stress) are evaluated and compared with initial hard turned and burnished surfaces. The predicted residual stress is validated with the experimental data [2]. El-Tayeb et al. investigated the burnishing process on aluminum 6061 with interchangeable adapter for both roller and ball burnishing process. The impact of different burnishing parameters like burnishing speed, burnishing force and burnishing tool dimension on the surface qualities and tribological properties were investigated. It was found that burnishing speed of 330 rpm and burnishing force of 212 N were capable of improving surface roughness as much as 40% [3]. Partchapol developed the finite element analysis of ball burnishing process to study the change in different properties of work material. The impact of feed rate, flow stress and ball diameter on the surface properties were studied and a detailed explanation was provided [4]. Hamadache et al. studied the plastic deformation of structural Rb40 steel when ball and roller burnishing were performed. They also investigated the roughness, hardness and wear resistance on the Rb40 steel [5]. Bouzid et al. exclusively investigated the change in surface roughness of AISI 1042 after burnishing. The author used a finite element model, in which the elastic-plastic behavior of the piece was taken into account, to determine the material displacement. The experimental and simulation values were compared and found to be in good correlation [6].Yung–Chang Yen studied the change in residual stress values after hard-turning and roller burnishing process. The corresponding experimental results are compared with the developed 2D and 3D FEM models for roller burnishing process. The experimental and simulated values are validated. DEFORM 2D & 3D software are used to develop the FEM model [7]. Hassan et al. investigated the change of surface properties for ball-burnishing brass components. A mathematical model called surface response methodology is established to correlate the most pronounced parameters, i.e. the burnishing force and the number of tool passes, with the surface finish. The model helps in decreasing the number of experiments to be done and it predicts the optimum surface properties. The optimum, predicted by the surface response methodology, is surface finish of 0.172 mm when burnishing at a force of 203 N with two passes of the burnishing tool is provided [8]. Hassan investigated the effects of milling operations of ball burnishing process, the change in properties such as surface roughness and hardness of the work using different lubricants. Free machining brass and cast Al ± Cu alloy were used as work piece materials [9]. John et al. investigated the properties of Aluminum 63,400 and effectively used surface response methodology to obtain the optimal parameters for getting better surface properties [10].
The above literature review shows the immense scope of development required in the surface characteristics developed by the ball burnishing process. This project deals with analyzing the surface characteristics generated on the surface of the AISI D3 Tool Steel work piece after the ball burnishing process. The effect of ball burnishing parameters like speed, feed, and force on the generation of the surface roughness, micro-hardness, residual stress and out of roundness has all been investigated. The workpieces are initially turned and the surface characteristics after turning and burnishing are compared. The initial turned work pieces have some unwanted surface irregularities that have been minimized by the ball burnishing process. The main interest of the machinist is surface roughness. Hence, FEM is used to predict the surface roughness of ball burnishing process. The finite element method based on DEFORM-2D software is used to predict the surface characteristic and also residual stress that is generated on the burnished work piece of AISI D3 Tool Steel.
Section snippets
Experimental work
The ball of burnishing tool is made up of tungsten carbide. The schematic view of the ball burnishing tool is shown in Fig. 1. The work piece material used is AISI D3 tool steel. The total length of the work piece is 150 mm and the diameter is 28 mm. The work piece is divided into 10 mm and considered as one section.
The chemical compositions of AISI D3 tool steel work pieces are as follows, in % of weight: C – 2.00 to 2.35%, Mn – 0.6%, Si-0.6%, Cr – 11.00 to 13.5 %, Ni – 0.3%, W – 1%, V-1.00%, Cu
Mathematical model, force and displacement calculation
The distance between the successive traces of the ball causes burnishing action on the surface. This burnishing action reduces irregularities of the surface and produces a smooth surface. The theoretical model of the burnishing action is given in Fig. 2 [6].
For small values of the feed (f) when compared to the ball radius (R2), the height (h) can be calculated as follows and is given in Eq. (1):
When calculating Ra, it was found that it varies linearly with Roughness factor Rt , (R
FEM modeling
For studying the plastic deformation behaviour of a given metal it is appropriate to consider uniform or homogeneous deformation conditions. The yield stress of a metal under uniaxial conditions as a function of strain (), strain rate (), and temperature (T) can also be considered as flow stress. The metal starts flowing or deforming plastically when the applied stress reaches the value of yield stress or flow stress.
Surface roughness modeling
The surface roughness generated after the turning operation is measured and the output data is used to develop the surface roughness pattern using the DEFORM-2D software. The surface roughness profiles are assumed to be sinusoidal in nature with distance between the two peaks kept as 0.83 µm and the average peak to valley height (Ra) as 1.3 µm.
Burnishing simulations in deform -2D
Finite element based simulation of ball burnishing process is carried out in the DEFORM-2D software. The normal displacement calculated is given to the ball so that it can penetrate the workpiece surface by the amount calculated. This in turn generates the equivalent static normal load which will be equal to the load applied by the burnishing tool during the experimental process, generated by the spring stiffness attached with the ball burnishing tool. The meshing of the part is then done by
Roughness measurement from deform-2D
Fig. 5 shows the enlarged section of the surface generated after the burnishing process in DEFORM -2D. Surface roughness measurement is taken by leaving the 0.1 mm distance from the left end, since the distortion will be maximum at the edges. The experimental values of the surface roughness obtained are compared with that of the values obtained from the simulations. The surface roughness values are obtained by measuring the workpiece at different points along the Y-direction and comparing it
Residual stress measurement
Residual stress is measured based on x-ray diffraction technique [11]. It is a non-destructive method. By this method, residual stress is measured quantitatively. X3000 software is used in the analyzer. It is based on solid state linear sensor technique that enables converting x-rays directly into electrical signals. The angular resolution of the machine is 0.029°/pixel, 512 pixels/12.8 mm.
Bragg's law is used to describe the diffraction of X-rays by a crystal. In Fig. 6(a), the incident X-rays
Results and discussions
From Figs. 9 and 10, it is seen that the surface roughness of Ra value decreased as feed rate is increased from 0.05 mm/rev to 0.1 mm/rev, correspondingly surface micro hardness and surface residual stress are increased. At feed rate of 0.1 mm/rev, the point contact of ball tool is widening compared to feed rate of 0.05 mm/rev. Hence, the asperities of the surface deform and surface finish is increased as well as micro hardness and residual stress. But with further increase in feed rate from 0.01
Out of roundness
Out of roundness is measured using ZEISS Contura G2 co-ordinate measuring machine. Out of roundness after turning process is 0.0197 mm and after burnishing is 0.0032 mm. The out of roundness is measured when the burnishing condition of force is at 220 N, feed at 0.05 mm/rev, speed at 1000 rpm, and number of pass at one. From Figs. 15 and 16, it is seen that out of roundness is less for burnishing process compared to turning process due to controlled burnishing force on the surface.
Conclusion
The effect of ball burnishing process on the AISI D3 Tool Steel using DX-160 CNC lathe was investigated. The input parameters for the burnishing process were speed, feed, force, number of pass and ball diameter. The output surface characteristics were surface roughness, micro-hardness, residual stress and out of roundness. The workpiece was initially turned and then the ball burnishing process was used to enhance the surface characteristics i.e. to minimize the surface roughness, to increase
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