Abrasive Waterjet Cutting Of Polymer Composites

ABRASIVE WATERJET CUTTING OF POLYMER COMPOSITES Pankaj Tambe and Mukul Shukla Mechanical Engineering Department, MNNIT Allahabad, 211004, India E-mail: [email protected] Abstract. Cutting of composites by traditional and non-traditional methods like sawing, laser beam cutting etc. has many disadvantages. One of these is that almost all the traditional cutting processes involve the dissipation of heat into the workpiece. This serious shortcoming has been overcome by jetting technologies. Hence there has been a great interest for improving the machining of composite materials particularly in the aerospace industry. 1. INTRODUCTION At present, more than 50,000 constructional materials are known. This number tends to increase rapidly. Most new materials are “man made” and designed by material scientists to obtain specified properties. Many new materials consist of more than one component, which make them difficult to machine or separate by traditional methods. Manufacturers will be confronted with the need to adapt existing technologies or to develop new technologies for the specific behaviour of these new materials. Because of its characteristics, Abrasive Water Jet (AWJ) fits this trend extremely well. Polymer matrix composites are being used increasingly in various applications due to their superior physical and mechanical properties. However, there are a number of problems associated with the processing of these kinds of materials. The main difficulty encountered when machining composites using traditional techniques and some nontraditional techniques such as laser cutting is the high heat dissipation into the workpiece material that can cause charring, thermal cracks and other surface and subsurface defects on these components. The other problems associated with these cutting techniques include delamination and fiber pull out from the composites material, and the rapid wear of the tools used in conventional cutting due to the hard and abrasive fibers in these composites. Therefore, it is usually necessary to use wear resistant and expensive cutting tools such as coated or un-coated cemented carbide, cubic boron nitride (cBN) and diamond impregnated tools for cutting these materials using conventional cutting. Laser-cutting technology has been found to yield large burr formation, dimensional inaccuracy due to thermal distortion, a heat-affected (or burnt) zone, and even a fire hazard for these heat sensitive materials [1]. Abrasive Water Jet cutting (AWJC) technique is well suited for processing composite materials because of reduced interface temperature, low mechanical loading and ease of operation. 2. ABRSIVE WATERJET SYSTEM A conventional AWJ cutting system normally includes four major modules: an intensifier pump, providing high-pressure water; an abrasive delivery system and a cutting head producing the

abrasive waterjet; a computer controlled manipulator, which effectuates the desired motion of the cutting head; and a catcher, which dissipates the remaining jet energy after cutting. A typical system is schematically given in Fig. 1. [2].

Figure 1. General layout of an abrasive type waterjet cutting system. 3. PHYSICS OF AWJ MACHINING PROCESS The widely accepted explanation of the physics of the material removal process for ductile and homogenous materials is that of Hashish [3, 4] who investigated the AWJ cutting process using a high-speed camera to record the material removal process in a plexiglass sample. He found that the material removal process is a cyclic penetration process that consists of two cutting regimes. He termed these “cutting wear zone” and “deformation wear zone”, following Bittar’s erosive theory (Figure 2).

Figure 2. Wear cutting and deformation cutting mode

Figure 3. Intergranular network model

The material removal process in brittle materials is viewed as a brittle fracture process. Zeng and Kim [5] used an energy approach for the derivation of their plastic elastic model for AWJ cutting of brittle materials. They assumed the total material removal of a single impact as the sum of the removal due to a plastic flow and brittle intergranular fracture. Investigation conducted into the

material removal mechanism in refractory ceramics [6] indicated that in the smooth cutting region, material removal is by simultaneous cutting of the matrix and inclusion (transgranular), which could be due to the availability of higher specific jet energy at the top of the kerf, and in the rough cutting region the material removal process is by the removal of the binding matrix followed by the washing of the inclusion grain (intergranular) (Figure 3). A scanning electron microscopy (SEM) analysis of Phenolic Fabric Polymer Matrix Composites cut surfaces reveals that the erosive process for the matrix material (resin) involves shearing and ploughing as well as intergranular cracking. Shearing or cutting was found to be the dominant process for cutting the fibres in the upper cutting region, but the fibres are mostly pulled out in the lower region[7]. 4. SURFACE CHARACTERIZATION OF AWJ MACHINING The general characteristics of surface machined with abrasive waterjet are discussed in terms of surface texture and surface integrity [3]. These include: Surface Texture: Surface waviness (Figure 4), Kerf width and taper (Figure 5), Burr formation, Surface finish and Lay. Surface Integrity: Particle deposition, Delamination (Figure 6), Gouging, Microstructural transformation, Microcracking, Chipping, Hardness alteration, Heat affected zones and Residual stresses.

Figure 4. Surface waviness

Figure 5. Kerf taper

Figure 6. Delamination

3.1 Kerf characteristics Three unique cutting zone are often formed on materials machined with AWJ including an initial damage zone at jet entry, a mid-kerf smooth cutting zone, and a rough cutting zone adjacent to the jet exit [8]. The size of each of these three regions is dependent on the jet energy which is a function of cutting parameter and target material. The degree of initial damage width and depth at jet entry may be minimized with low standoff distances. Extension of smooth cutting region may be accomplished with high jet pressure, large grit size and lower traverse speeds which increases the inherent energy of the jet. Waviness patterns formed on the kerf wall as a result of inadequate cutting energy. The degree of waviness may be reduced or eliminated with combinations of high pressure, large grit sizes and moderate traverse speed.

3.2 Striation formation The sources of striation formation are classified into three groups; namely the nature of the step formation inherent to a jet cutting process, the dynamic characteristics of the water jet, and the vibration of the machining system. Chen et al. [9], proposed that striations are formed by the variation of the distribution of particle kinetic energy with respect to the cut surface. All the factors, which have an effect on the magnitude and distribution of the particle kinetic energy, contribute to the striation patterns and result in striated irregularities. Chao and Geskin [10] used spectral analysis and found that the

structure dynamics of the traverse system correlated with the cut surface striation, and machine vibration was the main cause of striation in AWJ cutting. 3.3 Surface roughness Increase in the traverse speed will be associated with an increase in the surface roughness since greater traverse speed allows less overlapping machining action on the cut surface and fewer abrasive particles impinging on the surface[11]. Lemma et al. [12] obtained improvements in surface quality as measured by Ra values, achieved using oscillation cutting 3.4 Delamination Delamination is a failure in the bonding between layers in layered material processing. It is seen as a crack perpendicular to the grain of the materials and affects the overall workpiece integrity. This phenomenon is greatly dependent on the properties of the composite materials being processed, particularly the bonding strength between the layers, and the waterjet properties in the cutting front. Hashish [3] postulated that at a certain depth within a kerf, the abrasive waterjet deflects and will penetrate between the layers causing delamination when the jet pressure is sufficiently high. Capello et al. [13] proposed that excessive loading on the material owing to factors such as workpiece vibration causes failure in bonding or delamination in the lower region of the cutting. Delamination occurs only when the jet is unable to cut through the workpiece [14]. 3.4 Burr Formation Loose hairline burrs (fibres) were formed mostly at the bottom of the cutting front. It has been found that at higher water pressure, burrs are almost absent from the specimen so that a clear exit edge can be formed. At a lower pressure, however, burrs were clearly visible at the exit edge. Hairline burrs were also found on the specimens when the cutting was performed with a higher traverse speed. It may be deduced that burr formation is dependent on the jet energy level at the bottom region of the cut. [7] 4. MACHINABILITY NUMBER Machinability Number (Nm) is the material characteristics parameter, which is inversely proportional to the ‘Erosion Resistance’[15]. It should have a unique value for certain workpiece material. The value of the material’s Machinability Number is determined experimentally with a standard AWJ Kerf cutting test and the following relation:

Nm =

ChD0.618 u 0.866 Pw1.25 m 0.687 m 0.343 w

---- (1)

Where, (h) depth of cut, (D) focusing nozzle diameter, (u) traverse speed, (Pw) water pressure, (mw) water flow rate, (m) abrasive flow rate, (C) constant. The value of Machinability Number for different material’s is: (Figure 7) Hardened Steel

80.4

304 Stainless Steel

81.9

316 Stainless Steel

83.1

Mild Steel

87.6

Copper

110

Titanium

115

Zinc Alloy

136 213

6061-T6 Aluminum

322

Granite

490

Lead White Marble

535

Nylon

538 596

Glass

690

Plexiglass

879

Graphite

985

Polypropylene

0

500

1000

1500

Machinability Number

Figure 7. Machinability numbers of selected engineering materials. 4.1 Machinability Number Application The most important application is to select the optional AWJ process parameters. Typically most process parameters are set using preset value of traverse speed. The optional traverse speed can be predicted using the following equation: 1.15

⎛ f N P1.25 m 0.687 m 0.343 ⎞ w u =⎜ a m w ⎟ CqhD 0.618 ⎝ ⎠

---- (2)

Where (q) quality level index, (fa) abrasive factor. The value of (q) has to be chosen between 1 to 5. The values of (q) for five different quality levels are defined as follows: Quality Levels q=1 q=2 q=3 q=4 q=5

Descriptions criteria for separation cuts. rough surface finish with striation marks at the lower half surface. smooth/rough transition criteria. slight striation marks may exist. striation free for most cases. Best surface finish.

5. CHALLENGES AND SCOPE OF FUTURE DEVELOPMENT 5.1 Development of Machinability Index for Polymer Composites Up till now, no study is reported to determine the Machinability Number of Polymer Composites. There is a need to develop the Machinability Index for Polymer Composites so as to use the AWJ for cutting Polymer Composites in various industries, specially Aerospace in India. 5.2 Selection of optimal cutting parameters In general, it has been found that an increase in water pressure and abrasive mass flow rate and a decreases in nozzle traverse speed and standoff distance result in an increase in the depth of cut and smooth depth of cut and a decrease in kerf taper and surface roughness. It has also been found that water pressure and nozzle traverse speed are more dominant in affecting the depth of jet penetration and the depth of upper smooth zone than abrasive mass flow rate and standoff distance. Although Machinability Number helps to predict the traverse speed in a certain range for a specific material depend upon the pressure, thickness, abrasive flow rate etc. at the selected quality level. Based on these studies, recommendations have been made for selecting the optimum cutting parameters for the materials under consideration. The optimal parameters can be selected by using Neural Network/Fuzzy Logic/Genetic Algorithm. 5.3 Dealing with newer class of composites Expert system (ES) based software is to be developed bases on semi-empirical model so as to achieve the following objective: 1. To determination of Machinability Number. 2. Based upon the Machinability Number, Design of Experiments (DOE) module select the range for process parameters to be varied. 3. Feed the measured cut surface data in a Parameters Selection module so as to find out the optimal process parameters.

This expert system can be integrated into CNC-Controller and linked to CAD/CAM software. 5.4 Development of an advanced controller The block diagram of the new generation of AWJ cutting system equipped with different levels of closed loop feedback control system is shown in Figure 8. as the vision for the future. Even though few of these controllers such as position and speed controller developed and the pressure controller is being developed, the challenge for the future lies in integrating the total system. Once this objective is met, AWJ could be ideal for flexible manufacturing environment.

Figure 8. Block diagram of AWJ cutting control system hardware 6. CONCLUSION This paper gives the overview of cutting of Polymer Matrix Composites using AWJ as well as the challenges and scope of future developments. The research oriented towards understanding the physics of the process has been very vital for the operations and system study. Towards the realization of an effective and optimum control of the AWJ process performance, the relationship between AWJ process parameters and process output needs to be understood in greater detail. REFERENCES [1] Lemma, E., Chen, F. L., Siores, E. and Wang, J., Study of cutting fiber-reinforced composites by using abrasive water-jet with cutting head oscillation, Composite Structures, 57, pp 297–303, 2002 [2] Mustafa, K. K., Processes and apparatus developments in industrial waterjet applications, International Journal of Machine Tools & Manufacture, 42, pp 1297–1306, 2002 [3] Hashish, M., Characteristics of surfaces machined with abrasive waterjets, J Eng Mater Technology, 113, pp 354–362, 1991 [4] Hashish, M., A modelling study of metal cutting with abrasive waterjets, J Eng Mater Technology, 106, pp 88–100, 1984

[5] Zeng, J., and Kim, J., Development of an abrasive waterjet kerf cutting model for brittle materials, Jet cutting technology, Kluwer Acad. Press, Dordrecht, pp 483-501, 1992 [6] Momber, A., Kovacevic, R., and Eusch, I., Cutting refractory ceramics with abrasive waterjets, Proc. of 8th American waterjet conference, pp 229-244, 1995 [7] Wang, J., A abrasive waterjet machining of polymer matrix composites cutting performance, erosive process and predictive models, International Journal of Advanced Manufacturing Technology, 15, pp 757-768, 1999 [8] Ramulu, M., and Arola, D., A study of kerf characteristics in abrasive waterjet machining of graphit/epoxy laminates, Transaction of the ASME, 118, pp 256-265, 1996 [9] Chen, F. L., Siores, E., Morsi, Y., and Yang, W., A study of surface striation formation mechanisms applied to abrasive waterjet process, Proceedings of the CIRP, pp. 570–575, 1997 [10] Chao, J., and Geskin, E. S., Experimental study of the striation formation and spectral analysis of the AWJ generated surfaces, J of Mat. Proc. Tech, 141, pp 213–218, 2003 [11] Wang, J., A machinability study of polymer matrix composites using abrasive waterjet cutting technology, Journal of Materials Processing Technology, 94, pp 30–35, 1999 [12] Lemma, E., Chen, F. L., Siores, E., and Wang, J., Study of cutting fiber-reinforced composites by using abrasive waterjet with cutting head oscillation, Composite Structures, 57, pp 297–303, 2002 [13] Capello, E., Manno, M., and Semeraro, Q., Delamination in waterjet cutting of multilayer composite materials: A predictive model, Proceedings of 12th International conference on jet cutting technology, Rouen, France, pp 463-476, 1994 [14] Wang, J., and Guo, D. M., A predictive depth penetration model for abrasive waterjet cutting of polymer matrix composites, Journal of Material Processing Technology, 131, pp 390394, 2002 [15] Zeng J, Kim T J, R J Wallace, Quantitative evaluation of machinability in abrasive waterjet machining, Precision Machining, ASME PED, 58, pp. 169-179, 1992