SIMULATION OF ABRASIVE WATERJET MACHINING (AWJM)

XXXVIII Sympozjon ,,Modelowanie w mechanice’’ 1999 Ashraf I. HASSAN, Jan KOSMOL, Katedra Budowy Maszyn, Politechnika ląska, Gliwice SIMULATION OF ABRASIVE WATERJET MACHINING (AWJM) Abstract: Erosion of ductile materials by abrasive waterjet machining (AWJM) is still a complex phenomenon. This paper presents a first attempt to simulate the abrasive waterjet machining (AWJM) process using the finite element method (FEM) in order to determine the workpiece response in this erosive wear process. Additionally, deformations occurring in the workpiece material in the vicinity of the cutting interface as a result of the high speed AWJ impact could be obtained. The results indicate that the finite element method is a useful tool in the prediction of the AWJ deformations. Streszczenie: Erozja plastycznych materiałów poprzez obróbkę strumieniem wodno- ciernym (AWJM) ciągle należy do bardzo złożonych zjawisk . W referacie przedstawiono pierwszą próbę symulacji obróbki strumieniem wodno- ciernym z zastosowaniem metody elementów skończonych (MES) w celu okre lenia zachowania się przedmiotu obrabianego w procesie zużycia ciernego. Możliwe będzie także okre lenie odkształcenia przedmiotu w okolicach działania strumienia wodno- ciernego o bardzo dużej prędko ci. Wyniki symulacji pokazują, że metoda elementów skończonych jest użytecznym narzędziem w przewidywaniu deformacji przedmiotu poddanego obróbce AWJM. 1. INTRODUCTION Abrasive waterjet machining (AWJM) has many advantages over other existing conventional and non conventional machining processes. The most important of these advantages include: low cutting forces, low heat generation, the ability to machine a wide variety of workpiece materials such as mild steel, alloy steels, titanium [1], ceramics [2], metal 82 Ashraf I. HASSAN, Jan KOSMOL and ceramic matrix composites [3], polymeric matrix composites [4] and glass. Although the process has been increasingly applied in recent years, it has a more limited use, compared to laser and plasma beam machining, because of the high capital and running costs and more importantly the relatively lack of the understanding of the exact nature of the occurring erosion mechanisms. A complete review of the erosion mechanisms suggested for AWJM was conducted by Hassan and Kosmol [5]. Two cutting mechanisms dominate in AWJM: cutting wear, found in the upper smooth part of the kerf, and deformation wear, found in the lower striated part of the kerf. In the cutting wear zone, each abrasive particle strikes the workpiece at a shallow angle of impact and works as an undefined cutting tool which removes material by shear in such a way similar to chip formation in metal cutting operations [6-9]. This micromachining mechanism, initially suggested by Finnie [6], was confirmed by a scanning electron microscopic study which revealed that some particles leave material piled up at the sides or the end of the crater. This raised material is presumably removed easily by subsequent particles [10]. In the deformtion wear zone, each abrasive particle strikes the workpiece at a large angle of impact and causes plastic deformation in the workpiece material [6-9]. These two mechanisms are independent on the type of the material being machined [4,8,11] but the only difference lies in the length of the cutting wear zone, which is larger in ductile materials and shorter in brittle materials such as ceramics and polymeric composite materials [4]. A time-dependent computer model to simulate the dynamic erosion of the workpiece by AWJM was developed by Corcoran et al [12]. This model traces the abrasive particle action from exiting the mixing tube to the erosion of the workpiece surface. Up to date, there has been no attempt at analyzing the AWJM using the powerful tool of the finite element method. A preliminary effort of modeling the impact of a solid surface by a pure waterjet using a finite element model was carried out by Hassan and Kosmol [13]. Using this model, they were able to predict the shape of the cutting kerf, workpiece deformations and workpiece stresses. The main drawback of the model is that it could not be used in the prediction of the depth of cut because it is linear elastic. It is believed that the powerful tool of the finite element method will clear some of the mysteries of the process and will contribute in explaining the nature of the erosion process. This will enable easier prediction of the process performance. The objective of this paper is to conduct a finite element analysis of the abrasive waterjet machining process in order to determine the workpiece response in this erosive wear process regarding deformations occurring in the workpiece material in the vicinity of the cutting interface. This aims at accurate prediction of the kerf geometry. 2. FINITE ELEMENT MODEL A complete description of the FE model, boundary conditions and the assumptions adopted is given in a previous study by the authors [14]. A total number of 900 elements were used to Simulation of abrasive waterjet machining (AWJM) 83 model the workpiece with size of each element of 0.2 * 0.2 mm. The workpiece material data is shown in Table 1 below and the cutting conditions are shown in Table 2. Table 1 Workpiece material data Property Material Young's modulus Poisson's ratio Density Yield stress Strain hardening modulus Carbon steel 207 E 3 MPa 0.3 7850 Kg/m3 207 MPa 34.5 E3 MPa Table 2 AWJM conditions Parameter Pressure AWJ mixing tube diameter Stand off distance 400 MPa 1 mm 0 The boundary conditions include a fixed support of the workpiece from the bottom as it is fixed on the table of the AWJ machine. The abrasive waterjet is allowed to move downwards perpendicular to the workpiece surface. In the present study the workpiece has both nonlinear material behaviour and geometric nonlinearity under the applied loads, hence a nonlinear analysis is required. The constitutive model used for the workpiece is chosen as elastic plastic Von Mises with linear strain hardening. The model was then analyzed on a PC workstation and the stresses within the workpiece material obtained using ALGOR Accupak/VE nonlinear dynamic stress analysis and event simulation [12], Version 3.18 WIN, which models the nonlinear behaviour by incrementing the load and updating the geometric stiffness matrix. It uses the total Lagangian fornmulation. 3. RESULTS AND DISCUSSION To better understand the AWJ erosion process, the interaction between the abrasive particle and the workpiece needs to be dynamically tracked at small time steps. The deformation pattern of the workpiece as a result of AWJ impact is shown in Fig. 1. There is a strong interaction between the AWJ and the highly deformable layer of the workpiece. This could be easily identified especilly at the cutting interface. It is apparent from the figure that the contact zone is undergoing extreme deformations at the moment of impact. This is consistent 84 Ashraf I. HASSAN, Jan KOSMOL with the results obtained by both Alder [16] for water drop impact and Hassan and Kosmol [13] for pure waterjet machining. In Fig. 1 (c), the workpiece is subject to the maximum deformation leading to plastic deformation. (a) t = 0.025 s (b) t = 0.25 s Simulation of abrasive waterjet machining (AWJM) 85 (c) t = 0.5 s Fig. 1 Deformation of the workpiece material under the action of the AWJ at different time steps CONCLUSIONS This paper presents a first trial to apply the finite element method to the field of abrasive waterjet machining. The following conclusions are relevent: 1. The finite element method is a useful tool in the analysis of the abrasive waterjet machining process. It can be used to provide an explanation of the complex erosion process. A finite element model has been successfully developed to predict the profile of the generated kerfs. 2. To better understand the AWJ erosion process, the interaction between the abrasive particle and the workpiece needs to be dynamically tracked at small time steps. 3. The deformation pattern of the workpiece shows a strong interaction between the AWJ and the highly deformable layer of the workpiece. At the cutting interface, the contact zone is undergoing extreme deformations at the moment of impact. 86 Ashraf I. HASSAN, Jan KOSMOL REFERENCES [1] Hashish M. : Cutting with Abrasive Waterjets, Mechanical Engineering, Vol. 106, pp. 60, March 1984. [2] Zeng J., Kim T. J. : An Erosion Model of Polycrystalline Ceramics in Abrasive Waterjet Cutting, Wear, Vol. 193, No. 2, pp. 207, 1996. [3] Hamatani G., Ramulu M. : Machinability of High Temerature Composites by Abrasive Waterjet, Transactions of the ASME, Journal of Engineering Materials and Technology, Vol. 112, pp. 381, Oct. 1990. [4] Hassan, A.I., Kosmol, J., Bursa J.: Recent advances in machining of polymeric composite materials by AWJM. Proceedings of the Conference: Polimery i kompozyty konstrukcyjne, Ustron, Poland, 6-10 October, 1998. [5] Hassan A. I., Kosmol J. : An Overview of Abrasive Waterjet Machining (AWJM), Prace Naukowe Katedry Budowy Maszyn, Politechnika Slaska, Nr. 2/97, pp. 181-205, Gliwice, 1997. [6] Finnie I. : Erosion of Surfaces by Solid Particles, Wear, Vol. 3, No. 3, pp. 87, 1960. [7] Hashish M. : A Modeling Study of Metal Cutting with Abrasive Waterjets, Transactions of the ASME, Journal of Engineering Materials and Technology, Vol. 106, No. 1, pp. 88, 1984. [8] Hashish M. : Visualization of the Abrasive-Waterjet Cutting Process, Experimental Mechanics, pp. 159, June 1988. [9] Hashish M. : A Model for Abrasive-Waterjet (AWJ) Machining, Transactions of the ASME, Journal of Engineering Materials and Technology, Vol. 111, pp. 154, Apr 1989. [10] Finnie I. : Some Reflections on the Past and Future of Erosion, Wear, Vol. 186-187, pp. 1, 1995. [11] Paul S., Hoogstrate A. M., Van Luttervelt C. A., Kals H. J. J. : Analytical Modeling of the Total Depth of Cut in the Abrasive Waterjet Machining of Polycrystalline Brittle Materials, Journal of Materials Processing Technology, Vol. 73, pp. 206, 1998. Simulation of abrasive waterjet machining (AWJM) 87 [12] Corcoran M., Mazurkiewicz M., Karlic P.: Computer simulation of an abrasive waterjet cutting process, 9th International conference on Jet Cutting Technology, Sendai, Japan, October, 1988, pp. 49-59. [13] Hassan, A.I., Kosmol, J.: A preliminary finite element model of waterjet machining (WJM). Proceedings of the Conference Ś III Forum prac badawczych “Kształtowanie cz esci maszyn przez usuwanie materiału", Koszalin, Poland, 1998, pp. 234 - 244. [14] Hassan, A. I., Kosmol, J.: A new model for abrasive waterjet machining, Proceedings of the International Conference on Water Jet Machining (WJM'98), krakow, Nov 1998. [15] ALGOR DocuTech Server 2.16-WIN, Nov 1997, ALGOR Software Inc. [16] Alder W. F. : Waterdrop Impact Modeling, Wear, Vol. 186-187, pp. 341, 1995.