Fiber-reinforced composite workpiece surface quality improvement in machining by milling-cutter with opposite cutting edges using SPH-method simulation

Fiber-reinforced composites (FRC), among them unidirectional ones (UD-FRC), are widely used in practice. Such composite workpieces are often machined. One of the problems of this machining is interfacial debonding between fibers and matrix in the area under machined surface [1]. Preliminary investigations show that the main reason of this fact is high strength of fibers in comparison with the matrix one. So under cutting edge these fibers bend much greater than matrix and debonding is occurred [2]. This peculiarity requires special cutting tools.

It is well known that most of famous tool producers develop special alternate design for this purpose [3–6]. Nevertheless, the review of these cutting tools shows that no one of them significantly differs from tools for metal cutting except some geometrical edge characteristics. Among other tools end mills with alternate arrangement interrupted helix edges on a cylindrical surface are used (fig. 1). Many of such tools significantly improve composite workpiece surface quality, but do not solve the problem of preservation of fiber-matrix adhesion completely. Such end mills have interrupted helix edge with tool cutting edge angle nearly equal to 45°. The corresponding small feed per tooth gives deformed zone location near a tooth corner and significant fibers bend, as well as orthogonal cutting. Publications about surface quality improvement in machining of composites appear constantly and this fact means that the problems still remain [7].

Among other things, tool designers note that orthogonal cutting is widely discussed in the publications due to its relative simplicity [8]. At the same time, they notice that oblique cutting is more effective for fiber-reinforced composite workpieces [9, 10]. N. Naresh at al. [10] report that the increasing of tool cutting edge inclination from 25° to 45° degrades machined surface roughness from Ra1,2 to Ra1,8.

In master's thesis by G. Gudimani developed at Wichita State University in 2005 more than 50 photographs with machined surface zone of UD-FRC workpieces were presented. All structures were shown depending on tool cutting edge inclination and angle between fiber axis vector and the direction of primary motion. Nevertheless, these photographs and author conclusions show that the monotonic relation between tool cutting edge inclination and surface quality is not observed.

Fig. 1. End mill form SECO [5]

Such contradictory information demands an additional study of influence of the mentioned above angle on machined surface roughness. This investigation may be a base of further tools improvement if their construction is changed. The method used by authors is based on numerical calculation. Now, popular finite element method (FEM) is very often used to calculate homogeneous and heterogeneous work piece cutting processes.

In general, two large groups of such calculations may be distinguished, i.e. macro-modelling and micro-modelling [17–19]. Micro-modelling includes fiber, matrix and interfacial layer (fiber-matrix interface) description. Previously the authors of the presented paper used FEA to calculate strain-stress in UD-FRC cutting zone [20]. Micro-model simulation was executed as 3D task with fibers, matrix and interfacial layers. However, obtained results did not conform to experiment pictures perfectly.

There are many publications with positive examples of cutting zone modeling using smooth particle hydrodynamics method (SPH). However, these examples refer to homogeneous material cutting only [21–23].

For this reason, an attempt of micro-modeling of UD-FRC cutting zone is made with the use of SPH method. The main goal of the research is an estimation of new cutting wedge geometry to decrease fibers-matrix interfacial debonding. The practical result should be a new tool design for UD-FRC work- pieces processing.

• 2.    Problem statement and assumptions

• 3.    Calculation results

  Cutting simulation is executed with SPH software un i t of computer program ABAQUS. Steel fibers at 25–450 °C and strain rate equal up to 600 c –1 have the foll o wing parameters: the yield stress a = 1 150 MPa, the strain-hardening constant b = 739 MPa, the strain rate constant с = 0,014, the work hardening exponent n = 0,26, the thermal softening exponent m = 1, 0 3, m elting temperature T m = 1723 K [24, 25]. Matrix material has a = 352 MPa, b = 440 MPa, c = 0,0083, n = 0,42, m = 1, T m = 520 K. Frictional coefficient is equal to 0,3 everywhere [26].

The model source data are: three-dimensional approach; free orthogonal and oblique cutting of composite workpiece. Parallelepiped workpiece has sizes 0,112 × 0,08 × 0,045 mm. A tool is represented as a wedge with parameters: rounded cutting edge radius ρ = 0,001 mm, tool orthogonal rake γ = 15°, tool orthogonal clearance α = 7° and tool cutting edge inclination λ = 45°. Cutting speed vector is directed along workpiece axis; cutting speed Vc = 0,15 m/s; cutting depth t = 0,01 mm. Composite fibers are located at an angle to the horizon equal to ψ = 90°. Fiber diameter equals to 0,014 mm, distance between fiber axes equals to 0,018 mm.

The composite has the following properties. Fibers are made of structural alloy steel STEEL 4340 (Russian analogy – 40XH2MA) and have the such properties: density 7800 kg/m³, Young’s modulus E = 210 GPa, fracture strain 0,95. Matrix material is aluminum alloy AL6061-T6 (Russian analogy – AD33) with the density 2700 kg/m³ [16]. The fiber and matrix material behavior model is Johnson-Cook hardening model. Workpiece is completely fixed by its lower edge (Fig. 2).

Fig. 2. Theoretical model (orthogonal cutting – on the left, oblique one – on the right)

Calculation results for orthogonal and oblique cutti n g are shown in Fig. 3 a nd 4. One can see that oblique cutting has minimal fibers bend in the cutting speed vector direction. Accordingly, fibers-matrix debonding is less in this case. Thus it can be confirmed that the usage of tools w ith large cutting edge inclination in UD-FRC machining is reasonable.

Fig. 3. Orthogonal (above) and oblique (below, turned around vertical axis) cutting. Image is located at right angle to tool major cutting edge

Fig. 4. Orthogonal cutting (on the left) and oblique one (on the right). Isometric view

Since maximal fiber and matrix deformations are o b served in normal to m a jor cutting e dge direction, so is reasonable to decrease such deformations. One of the ways to obtain it is to use the opposite cutting process by symmetrically located wedge. Furt h ermore, to exclude undesired excessive chip packing in a zone between the adjacent cutting edges the last one should be shifted behind the first one.

It gives the possibilities to remove chip between wedges on the one hand, and to o r ganize opp o site fiber and matrix motion in front of the cutting edge on the other hand. Considering the fact that t h e cutting speed is very high, material inertia in cutting zone may pr e serve its deformation a f ter the first tooth action to opposite influence from the second one. Such cascading effect from interleaved teeth gives fiber deformation decrease and fiber-matrix debonding reduction.

This cutting mode is also calculated, deformation images are shown in the Fi g . 5. It turne d out that deformation in front of the second tooth is less than defor m ation in front of single one. Undesir e d excessive chip packing in a zone between the adjacent cutting edges is not observed. On this basis a new milling cutter design with tool major cutting edges located as a chevron on a cylindri c al surface with alternate arrangement is suggested. As one can see in the Fig. 6, in contrast to the p r ototype, tool cutting edge angle equals to 90°. This milling cutter should be manufactured by prelimina r y cutting-out of helix flutes in opposite directions. The perfect orthogonal rakes should be provided in this case. Further, major flanks should be grinded.

Fig. 5. Oblique cutting. Two opposite located wedges

Fig. 6. Milling cutter with rectangular shape tooth in the tool reference plane (to compare, milling cutter from SECO on the right side)

• 4.    Conclusions

• 1.    Unidirectional fiber-reinforced composite workpie c e surface quality improvement is obtainable in the case of fiber and matrix deformation decreasing by using oblique cutting and opposite alternate arrangement of major cutting edges.

• 2.    Alternate arrangement of major cutting edges excludes undesired excessive chip packing in a zone between the adjacent cutting edges. Material inertia in cutting zone can preserve its deformation after the first tooth to opposite influence from the second one. Such cascading e f fect from i n terleaved teeth gives fiber deformation decrease and fiber-matrix debonding reduction.

• 3.    SPH method is sufficient for micro-modeling of U D -FRC workpiece cutting process.

• 5.    Discussion and application

Although studies have established qualitative assessment of composite fiber bending, it is essential to calculate fiber-matrix interface damage length along fiber orientation. The usage of SPH method has not given appropriate values of these characteristics. It is necessary to crowd particles set and to increase model size, that nowadays is limited by computer possibilities (simulation is executed by computer with Core i7-4790k with 8 threads processor during several hours).

Considering the fact that this simulation is not verified by experiment yet, the adequacy of obtained results is not sufficient. The distance between opposite adjacent cutting edges should be calculated more precisely. This distance depends on composite properties as well as on cutting parameters. Moreover, a question about overlapping of tool major cutting edges should be resolved. So, the results of proposed investigation should be admitted as preliminary results.