Pulsed Laser Assisted Micromilling for Die/Mold Manufacturing Tuğrul Özel* Dept. of Industrial and Systems Engineering Rutgers University Piscataway, NJ 08854 USA ozel@rci.rutgers.edu *Corresponding author Abstract Laser assisted machining is an alternative to conventional machining of hard and/or difficult-to-process materials which involves pre-heating of a focused area with a laser beam over the surface of the workpiece to cause localized thermal softening along the path of the cutting action. The main advantage that laser assisted machining has over conventional machining is the increased material removal rate and productivity. Laser assisted micromilling is a scaled down derivative of laser assisted machining assuming that the process effectiveness potentially exists at the meso/micro scale. It is well-known that continuous-wave (c.w.) lasers generate a wide and deep heat affected zone, and can cause microstructure alterations, potentially making laser assistance counter-productive at the meso/micro scale. The novel use of a pulsed laser in assisting micromilling enables processing of die/mold metal alloys that are typically hard and/or difficult-to-process in micro scale, while reducing the heat affected zone. A fairly innovative technique is introduced by thermally softening only the focused microscale area of the work material with induced heat from a pulsed laser, and material removal is performed immediately with micro mechanical end milling. The focus of this paper is to present a fundamental understanding of the pulsed laser assisted micromilling (PLAM), in particular, to investigate the influence of pulsing on microscale localized thermal softening by coupling with the finite element simulation of the micromilling process. Experiments and Finite element method-based process simulations for micromilling of AISI 4340 steel with and without the laser assistance are conducted to study the influence of the pulsed laser thermal softening on the reduction in cutting forces and its influence on the temperature rise in the cutting tool. Frank Pfefferkorn Department of Mechanical Engineering University of Wisconsin Madison, Wisconsin 53706 USA pfefferk@engr.wisc.edu 1. Introduction The demand for miniaturized meso-(100 µm-10 mm)/ micro-(0.1-100 µm) products with high aspect ratios and superior surfaces has been rapidly increasing in aerospace, automotive, biomedical, optical, and micro-electronics packaging industries. Micro mechanical machining has gained popularity due to its high flexibility and capability in direct and rapid manufacturing of miniaturize metal parts with complex features [1]. Laser assisted machining (LAM) is a manufacturing process that has been investigated as an alternative to conventional machining of hard and/or difficult-to-process materials [2-5]. Laser assisted machining involves pre-heating of a focused area with a laser beam (often a c.w. laser) over the surface of the workpiece to cause localized heating and thermal softening along the path of the cutting action. The main advantage that laser assisted machining has over conventional machining is the increased material removal rate, productivity, and tool life. Micro end milling has been investigated and analytical and mechanistic models have been developed for better process planning [6-9]. In laser assisted micro end milling, cutting edge geometry produces ploughing and excessive plastic deformations in thermally softened work material resulting in unsuccessful cutting with side burrs. Preliminary studies of laser assisted micro mechanical machining have demonstrated some benefits to the cutting forces, however, its effect on surface integrity and tool life is still unresolved [10-11]. Lasers are usually categorized as two groups: continuous wave (c.w.) and pulsed lasers. Among pulsed lasers, short-pulsed (microsecond (µs) and nanosecond (ns)) lasers are of interest in the current research. As Copyright 2007 by ASME
direct laser machining (i.e., ablation of material) is not the goal of this work the ultra-short pulse lasers that produce clean holes with very little heat affected zone or debris are not ideal for local preheating without melting. However, these lasers are very promising for the future applications in precision micromachining and thermal processing of materials with minimal damage. The main parameters in pulsed laser processing are: (1) Laser spot size and beam quality: Beam quality is measured by energy, the focus ability, and the homogeneity. If the beam is not of a controlled size, the laser-affected region may be larger than desired size with excessive slope in the sidewalls. (2) Peak power: The peak power must be able to soften the workpiece, but not strong enough to cause direct ablation. There exist optimum values of laser beam intensity such that the extremely localized material softening will occur. Fig.1 Laser-assisted micro-milling process (3) Pulse duration: Theoretically, the pulse duration should not be longer than the thermal relaxation time for thermal diffusion to the cutting zone. The short pulse duration can maximize peak power and minimize thermal diffusion to the surrounding bulk work material, leading to localized heating. (4) Pulse repetition rate: Ideally, the pulse rate will coincide with the rate at which the cutting edge (e.g., flute) engages the workpiece. This would result in heat only being delivered when cutting is occurring and allowing the maximum dwell time between cuts. An increased pulse rate will also enable a higher feedrate but may be limited by the upper pulse rate of the laser. If the pulse rate were too low, the energy would diffuse out of the preheating zone and be wasted in heating the bulk of the workpiece. A higher pulse rate can limit the time for conduction and retain thermal energy near the cutting zone making the process more efficient. 2. Laser Assisted Micro End Milling In laser assisted micro end milling, short-pulsed lasers produce a very small heat affected zone when compared to c.w. or long-pulsed lasers (see Fig. 1), enabling extremely localized laser heating in the microscale. Localized microscale heating reduces thermal damage to the bulk material. This damage is often detrimental and is a limiting factor when high precision micro end milling is required. The ultimate goal of this research is to investigate the fundamental aspects of material removal mechanism, laser-material interaction as a thermal process, and influence of laser pre-heating of the workpiece at micro/nano scales. For this purpose, a test bed that consists of pulsed laser and high precision 3-axis fast Fig.2 Experimental set-up for pulsed laser assisted micro end milling system at Rutgers University positioning system with high-speed spindle for the laser-assisted micro end milling experiments is used (Fig 2). The system has been equipped with an ultra precision frequency controlled high-speed spindle (up to 80,000 rev./min with less than 0.5 µm runout from NSK ). This system is capable of utilizing micro end milling cutters with 3.175 mm (1/8 ) shank size and as small as 25 µm in diameter. An interface and process planning software to couple the micro-machining system with a ns pulsed Nd:Yag laser (5 ns, 1064 nm) that is attached to a vision system has also been developed. 2
Fig. 3 Experimental set-up for micro end milling The experimental test bed integrated with laser system has a central computer control, which controls the movement of stages for translating the workpiece under the micro-milling cutter and focused laser spot. For maintaining the proper vertical location to adjust the focus, a laser pulse control system adjusting the pulse repetition rate and maintaining the proper pulse duration as the speed of the workpiece movement s change is implemented. A CCD-camera based vision system is used for proper positioning and for in-situ monitoring of the laser assisted operation. The pulsed laser system (Nd: YAG) has the following characteristics: Laser wavelength: 1064 nm Focused laser spot area: 1 mm 2 Total energy at exit port: 485 mj Total power at exit port: 4.85 Watts Peak power: 100-70 MW Pulse duration: 5-7 ns Pulse repetition rate: 10 Hz 3. Finite Element Model of Laser-Assisted Micro Milling Fundamentally, the metal cutting process can be considered as a deformation process where deformation is highly concentrated in a small zone. Thus, chip formation in milling process can also be simulated using Finite Element Method (FEM) techniques developed for deformation processes. The main advantage of using such an approach is to be able to predict chip flow, cutting forces, and especially a distribution of tool temperatures and stresses for various cutting conditions. In this section, simulation of the micro-milling process is presented. FEM-based commercially available software, DEFORM-2D, was used for the process simulations. An FEM model is designed as shown in Fig. 4 for micro-milling of AISI 4340 steel. A Johnson-Cook workpiece material model is used for rigid-perfectly plastic deformation analysis. The finite element model accounts for strain hardening, thermal softening and elastic recovery effects of work material with Johnson-Cook constitutive model under high strain, strain-rate and temperate conditions. In the Johnson-Cook model (Eq. 1), the constant A is yield strength of the material at room temperature and ε represents the plastic equivalent strain. The strain rate &ε is normalized with a reference strain rate & ε 0. Temperature term in the J-C model reduces the flow stress to zero at the melting temperature of the work material, leaving the constitutive model with no temperature effect. n [ + B( ε ) ] & ε T T σ = A 1 + C ln 1 & ε 0 Tmelt T room room m (1) Johnson-Cook work material model parameters for AISI 4340 steel and thermo mechanical properties for work and tool materials are given in Tables 1 and 2. The tool material is tungsten carbide with 6-8% cobalt binder (WC-Co). Table 1: Johnson-Cook material model constants Material AISI 4340 A (MPa) B (MPa) n C m T melt (ºC) 792.0 510.0 0.26 0.014 1.03 1520 Table 2: Thermo mechanical properties of work and tool materials AISI WC-Co 4340 Density (g/cm 3 ) 7.85 15.7 Modulus of elasticity (GPa) 205 650 Poisson s ratio 0.29 0.25 Specific heat capacity 0.475 0.26 (J/g-ºC) Thermal conductivity 44.5 28.4 (W/m-K) Thermal expansion (µm/m-ºc) 12.3 5.2 Finite Element simulations are conducted for the cutting conditions of 80 m/min surface cutting speed, 10 µm feed per tooth, with a micro end mill geometry of 0.635 mm diameter and 3 µm tool edge radius. In the FEM model, a constant friction factor of 0.65 at the chip-tool-workpiece contacts is used. 3
(a) ( o C) ( o C) (b) Fig. 4 Finite Element simulation of micro-milling of AISI 4340 steel: (a) no laser assistance, (b) with laser assistance The fully developed continuous chip was simulated at a tool rotation angle of 53 in micro-milling of AISI 4340 steel as shown in Fig. 4a under the aforementioned cutting conditions. Temperatures in the cutting zone are predicted around 100-150 C for micro-milling of AISI 4340 steel with no laser assistance and around 350-400 C at the same cutting conditions with laser assistance. It is assumed that the laser pre-heating has provided temperature rise of 400 C around the undeformed chip zone. These temperatures are very low when compared to the temperatures in conventional milling conditions primarily due to the very small chip loads. However, the specific cutting forces are very large when compared to the conventional milling conditions. 4. Micro-Milling Experiments In this study, micro-milling experiments using flat-bottom micro end mills are conducted by taking slot cuts (full immersion) at a constant axial depth of cut and spindle speed for AISI 4340 steel. The experiments are repeated for laser-assisted micro-milling. The summary of the experimental conditions is given in Table 3. Laser assistance parameters are given in Table 4. Table 3: Micro-end milling experiment parameters Material AISI 4340 steel 2- Flute Carbide End Mill Tool with 30 0 helix angle Tool diameter (mm) 0.635 Axial depth of cut (mm) 0.127 Spindle speed (rpm) 40,000, 60,000, 80,000 Cutting speed (m/min) 79.8, 119.7, 169.65 Feed per tooth (µm) 1.27, 2.54, 5.08 A micro end mill with 0.635 mm tool diameter with 2-flutes is used with varying feed per tooth to investigate the effect of feed rate on the cutting forces generated. The cutting forces were acquired using a piezo-electric dynamometer and charge amplifier (Kistler, models 9257B and 5010) with an estimated uncertainty of ±0.2 N. The (x, y, and z) global axis forces have been recorded at 2667, 4000 and 5333 Hz for the spindle speed of 40,000, 60,000, and 80,000 rpm, respectively, with a PC-based data acquisition system and Kistler DynoWare software. The force signals at each channel of the dynamometer are sampled with twice the tooth passing frequency; hence four samples were collected per rotation. Feed and normal forces for 40,000, 60,000 and 80,000 rpm spindle speeds at various feeds per tooth (1.27 µm, 2.54 µm, 5.08 µm) were measured for micro-end milling of AISI 4340 steel. Measured mean resultant forces are given in Fig. 5 for no laser assistance and laser-assisted micro-milling. The cutting forces for PLAM are larger than for un-assisted machining at low feed rates and lower at higher feed rates. This may be due to material hardening caused by pulsed laser assistance at lower feed rates while at higher feed rates the laser assistance becoming more effective. There was no comparison made with the FE simulation results. Table 4: Laser assistance parameters Laser pre-heating distance 100 mm Spindle tilt angle 0 degrees Laser focal spot size 0.3 mm Pulse duration 7 ns Focal distance 20 mm 4
Surfaces generated with micro-milling with and without laser-assistance have also been inspected and compared. In both cases edge burrs were observed. However, the laser heated material exhibits more ductile behavior that facilitates the formation of edge burrs increasing the surface roughness and reducing surface quality. Surfaces generated with laser assisted micro-milling have indicated laser burns in the size of a few microns. Force (N) 70 60 50 40 30 20 10 Res. Force 60 krpm Res. Force 40 krpm Res. Force 80 krpm Res. Force LAM 40 krpm Res. Force LAM 60 krpm Res. Force LAM 80 krpm 0.3175 1.27 5.08 Feed rate (micron) Fig.5 Measured mean resultant forces at varying feed rate (µm/tooth) and rpm in micro-end milling of AISI 4340 steel. Chips on the surface Edge burrs Laser burns Fig. 6 Burrs and laser burns on the machined edges and surface. 5. Conclusions Laser-assisted micromilling is a scaled down derivative of the laser assisted machining assuming that the process effectiveness potentially exists at the meso/micro scale. This paper presents initial studies conducted in laser-assisted micro-end milling. While laser-assistance reduces the cutting forces, it also significantly affects the machined surface. Further, refinements to this current technology are critical to explore its potential for manufacturing of miniature dies/molds. 6. Acknowledgments The authors would like to acknowledge the financial support of Rutgers University and the University of Wisconsin-Madison. 7. References 1. Dornfeld, D., Min, S. and Takeuchi, Y., 2006, Recent advances in mechanical micromachining, Annals of the CIRP, 55/2, pp. 745-768. 2. Chryssolouris, G., Anifantis, N. and Karagiannis, S., 1997, Laser assisted machining: an overview, ASME, Journal of Manufacturing Science and Engineering, 119, pp. 766-769. 3. Lei, S., Shin, Y.C., and Incropera, F.P., 1999, Experimental investigation on thermomechanical characteristics in laser-assisted machining of silicon nitride ceramics, Proceedings of ASME, Manufacturing Science and Engineering, MED-Vol. 10, pp. 781-788. 4. Rozzi, J.C., Pfefferkorn, F.E., Shin, Y.C., and Incropera, F.P., 2000, Experimental Evaluation of the Laser Assisted Machining of Silicon Nitride Ceramics, ASME J. Manuf. Sci. Eng. 122, pp. 666-670. 5. Rozzi, J. C., Pfefferkorn, F. E., Incropera, F. P., and Shin, Y. C., 2000, Transient, three dimensional heat transfer model for the laser assisted machining of silicon nitride: I. Comparison of predictions with measured surface temperature histories, International Journal of Heat and Mass Transfer, 43, pp. 1409-1424. 6. Lee, K., and Dornfeld, D.A., 2002, An experimental study on burr formation in micro milling aluminum and copper, Transactions of NAMRI/SME, XXX, pp. 255-262. 7. Vogler, M.P., DeVor, R.E., and Kapoor, S.G., 2004, On the modeling and analysis of machining performance in micro-endmilling, Part I: Surface generation, ASME Journal of Manufacturing Science and Engineering, 126 (4), pp. 685-694. 8. Vogler, M.P., DeVor, R.E., and Kapoor, S.G., 2004, On the modeling and analysis of machining performance in micro-endmilling, Part II: Cutting force prediction, ASME Journal of Manufacturing Science and Engineering, 126 (4), pp. 695-705. 9. Dhanorker, A., and Özel, T., 2006, An experimental and modeling study on meso/micro end milling process, Proceedings of 2006 ASME International Conference on Manufacturing Science and Engineering, Paper No. 21127, October 8-11, 2006, Ypsilanti, MI, USA. 10. Jeon, Y., and Pfefferkorn, F., 2005, Effect of laser preheating the workpiece on micro-end milling of metals, Proceedings of 2005 ASME International Conference on Manufacturing Science and Engineering, Orlando, FL, USA. 11. Singh, R., and Melkote, S. N., 2005, Experimental characterization of laser-assisted mechanical micromachining (LAMM) process, Proceedings of 2005 ASME International Conference on Manufacturing Science and Engineering, Orlando, FL, USA. 5
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