FIBER-REINFORCED COMPOSITE WORKPIECE SURFACE QUALITY IMPROVEMENT IN MACHINING BY MILLING-CUTTER WITH OPPOSITE CUTTING EDGES USING SPH-METHOD SIMULATION

Unidirectional fiber-reinforced composite workpieces are often machined in practice. During such cutting process stronger fibers are bended more than weaker matrix and an interfacial debonding between them is occurred under machined surface. It deteriorates working properties of composite and so debonding reduction is a vital technological problem. One of the perspective ways to diminish such debonding is to substitute the orthogonal cutting by the oblique one. This decision is confirmed by known practice of milling cutter design. Milling cutters with the continuous helix edges and with interrupted ones with a large tool cutting edge angle are often used. In this case fiber deformations in a cutting zone are significant. To diminish such deformations the hypothesis about efficient use of opposite cutting edges location with a large tool cutting edge inclination is assumed. This hypothesis is checked by using of SPH method with micro-simulation of cutting of metal-matrix composites (steel fibers and aluminum alloy matrix). Johnson-Cook approach is used for both materials. Obtained qualitative deformation pictures confirm this hypothesis. On this basis a new milling cutter design with tool major cutting edges located as a chevron on a cylindrical surface with alternate arrangement is suggested. Computer simulation of such edges operation shows minimal fibers deformations in cutting zone. At the same time, undesired excessive chip packing in a zone between the adjacent cutting edges is not observed. The obtained results are preliminary considering the limited computer model size and no experiment verification made.


Introduction
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][4][5][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 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][18][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][22][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 workpieces processing.

Problem statement and assumptions
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/m 3 , 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/m 3 [16].The fiber and matrix material behavior model is Johnson-Cook hardening model.Workpiece is completely fixed by its lower edge (Fig. 2).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.

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

site located wed ooth in the tool CO on the right
that nowadays is limited by computer possibilities (simulation is executed by computer with Core i7-4790k with 8 threads processor during several hours).