Using a functional approach in solving problems improve performance waterjet equipment

The paper shows the influence of parameters of the calibration tube jet-abrasive devices on the quality of the process of waterjet cutting critical parts. The results of modeling the formation of two-phase flow and its movement in the calibration tube. Determined that the effects wear ductal fluid of the tube varying intensity and character. In this regard there is the need for functional-oriented approach to the choice of means to ensure the desired geometric parameters of the tube. It is concluded that the decrease in the intensity of ductal Shot material of the tube and thus maintain its geometric parameters is possible by the use of suitable protective coatings , thickness and physical and mechanical characteristics are selected on the basis of Value attributes according to the intensity and type of abrasive loading surface.


Introduction
The appearance and widespread using in industry of new structural materials, especially composites, and new high-tech products, makes the need for new methods of treatment.One of these methods is hydroabrasive cutting.The use of hydroabrasive cutting justifies itself especially where use of traditional methods does not give a satisfactory quality.Additional costs for works or reducing of the production rate go into oblivion.An additional advantage of this method is its cleanliness and environmental friendliness.Modern systems of hydroabrasive cutting provide continuous optimization of production and quality in manufacturing.The need for high processing results requires sustainability and compactness of jet-abrasive stream.Using of hydroabrasive processing enables significantly improve the quality of machined surface and cutting performance, which requires the development of promising new layout tools for combined treatment.Among the trends of hydroabrasive treatment the improving of the accuracy and efficiency of processing equipment is allocated.Effectiveness can be enhanced by increasing fluid pressure (600 MPa) and the number of concurrent working heads [1,2].However, increasing of working pressure leads to intensification of systems of wear of hydrocutting equipment, including calibration tubes (Fig. 1), which in turn reflected in the cost of processing.

Objective
Calibration tube is one of the most important elements of hydrocutting system, which affects the technological and economic characteristics of the cutting.Industrial observations suggest that most functional failures in the implementation of hydroabrasive cutting occurs due to sudden changes in geometry of jet-forming elements (nozzle and calibration tube).The tube is exposed to constant wear, resulting in internal diameter increases, which determines the wear tube.Usually firmness of tube presents 10...15 hours, and its cost arrives at 20...50 $ [2].
In this regard, an urgent task is improving the stability of calibration tube using the functional-oriented approach.To determine the peculiarities of wear of calibration tube, the impact of movement abrasive grains and species destruction are analyzed.

The research
It is known that in the motion of two-phase flow "liquidsolid particles" through the calibration tube there is chaotic action on the walls of the tube of some abrasive particles at different angles of attack and with different power of hit.Abrasive destruction of surface of the tube depends on the nature of action of the abrasive grains on its surface.Depending on the direction of action of the abrasive jet to the surface there are the following schemes of action: the destruction with a shock jet , when the angle of attack α = 90 (Fig. 2 a), with sliding jet, when α = 0 (Fig. 2 b), with oblique jet, when 0 <α <90 (Fig. 2 c) [3,4].

Fig. 2. Scheme of action of the abrasive jet to the surface
If on the flat surface of the material affects the flow of hard abrasive particles that fly at the speed vu at an angle α to the surface, each share with a punch, elastically deforms the work surface and slip on it with friction.Assumed that the normal component of velocity v0 makes only elastic deformation of the material, and tangent v0 is partially or completely extinguished and does the work of cutting, engaging in frictional contact with the surface.With the reduced turns out that the largest processing performance theoretically should be at an angle of attack α = 45, which is confirmed by many authors [5].The intensity of wear in the abrasive flow is defined as the result of multiple impacts of solid particles on the surface wear at different angles of attack.Initial period of destruction of the metal is characterized by the introduction of abrasive particles in the surface layer at some depth, the second -of continuous movement of particles of the material along surface layer at some distance at which the displacement of microvolumes of the metal in layer occurs in the direction of introduction of particles and their separation from the array.
In implementing of the abrasive particle in the surface layer of metal in a free kick there is deformation near the contact zone, resulting in this layer there is a complex inhomogeneous stress-strain field with a variable limit.Stresses and strains arising from the introduction of abrasive particles in the metal depend on complex factors that characterize the parameters of the flow of particles and resistance of the metal to elastic-plastic deformations.Elastic and plastic deformations will develop in the contact zone, which will assist to metal collapse under the particle in the radial direction and subsequent tangential changes in the direction of motion of the relative particles to the surface.Depth introduction of the particle and its tangential displacement associated with the mechanical properties of the abrasive and of material of wearing surface, particles size, and deformation of the metal.
At small angles of attack due to the predominance of tangential velocity impact, the main process of destruction of the surface layer is the tangential displacement microvolumes of metal in the direction of implementation, i.e. microcutting.At the angles of the attack close to 90º, mechanism of destruction of the surface layer of metal in a stream of abrasive particles takes a polydeformational striking character due to the predominance of the normal velocity.Destruction of structural materials under the action of abrasive particles contained in the flow of fluid is very complex and may be semifluid, brittle, polydeformational or acquires fatigue character and is complicated with phenomenon of cavitation.
Thus, the parameters of the internal profile of tube should ensure the following conditions of flow: 1) the minimum thickness of the boundary layer flow inside the tube, turbulent layer of output free jet, which reduces the thickness and the interaction of jet with the environment; 2) reducing of the possibility of separation of boundary layer of jet, helping to reduce excitation of central flow; 3) reducing of the possibility of cavitation, which provides an exception of formations of low pressures inside the tube to avoid formation of bubbles and destruction of tube.
The physical and mechanical properties of the material, which facing the particle, also have impact on the efficiency of processing.The theoretical dependence given in [5] help to assess of the impact angle of attack on the intensity of wear only qualitatively.Therefore, to analyze the phenomena that occur in calibration tube during the formation of liquid abrasive flow the software package flow vision was used.When modeling the problem of detection of conditionality of diagrams load on the cross-section of calibration tube with microhermetic parameters of jet erosion was treated, establish the role and activity of wave processes occurring in streams and exert force on the inner diameter of the calibration tube.
The core package is a unit of the numerical solution of equations of fluid motion in the orthogonal coordinate system (Navier-Stokes equation ( 1)) that for certain initial and boundary conditions defined by the user, provides a diagram of distribution of velocities and dynamic pressures at the point of contact with the jet or other surface: where fr, fφ, fzvoltages of mass forces along the respective axes; r, φ, z; vkinematic viscosity; components of the strain rates of elementary volume of fluid (the same name derived describes the strain rate compression or stretching of line elements of the selected volume, and dissimilarthe rate of change of the angles between them); ρmass density of fluid; vr, vφ, vzspeeds of the elementary volume of the respective coordinates.
Found stresses values of mass forces along the respective axes provided continuity determines mass force acting on the volume of liquid, which is considered: where Welementary volume.
Initial conditions are determined by geometrical profile nozzle orifice, by the pressure of liquid pb and its properties.Boundary conditions determine the leakage of jet to the surface, which is an internal cylindrical surface of the calibration tube.Mass transfer was determined by (3) based on the Stokes equation, which determines the strength of resistance in a stream of particles: where μrate of fluid flow through the nozzle; dsdiameter of tube.Three-dimensional model of free jet was created to determine the parameters of the system and using the software package flow vision was determined the pressure and velocity in the jet.For the model studies was considered the problem of leakage of jet fluid that flows out of the hole diameter of 1 mm.In order to determine pressure and velocity of fluid in the jet the problem with free surface was created and solved.
Were given the following physical parameters: -the initial parameters; -basic values -temperature 273 K, pressure 101 000 Pa; -parameters of the model -the density of liquid 1000kh/m 3 ; -liquid 0 -clear water; -adaptation of the grid up to level 2. Boundary conditions for the elements (walls, entrance, exit) and the degree of adaptation for each of them are given.Speed jet at the inlet is 300 m/s.Initial mesh with 25652 cells where 21168 of them solidified it in the area of the exit from nozzle and at the surface of leakage is given.The general model parameters and time course of the calculation in the range from 0 to 5 • 10 -5 are given, the calculation is standard.The calculation results derived in the form of diagrams (Fig. 3, a, b) fill -pressure in the range of 3,5•10 5 Pa to 7,579•10 7 Pa; rateisolines, from 0 to 300 m/s [6].Researches showed that most loaded from the point of view of abrasive wear are places of entrance of liquidabrasive mixture and on an exit from a tube (Fig. 3).In these areas transversal pressure caused by wave processes maximally influences on the mobile particles of abrasive.
where Тduration of wear; Пconcentration of abrasive particles in a liquid; dsize of particles; Ктcoefficient of hardness equal to the ratio of hardness of material to hardness of abrasive particles; Кф'coefficient that describes the shape of the particles; vvelocity of abrasive particles in the moment of impact with the details surface; αangle of velocity vector of particle to wearing surface (angle of attack); χcoefficient characterizing the decrease of mechanical properties of material resulting softening physical-chemical action environment.

Fig. 5. Graph of velocity and pressure under leakage of jet on the walls of the calibration tube
Total volume of material seized in the interaction with the walls of hydroabrasive stream calibration tube will be: hole depth п and its length а as a function of process parameters: where mmass of abrasive particles; vn, vanormal and tangential component of particle impact velocity with the treated surface; Ra, HB, bparameters of roughness, hardness and strength of surface; zngranularity of abrasive particles; Tpconstant, taking into account the inertia of the process microcutting; kn, kaconstant coefficients.
Since the calibration tube takes hydrodynamic and mechanical loading from fast moving abrasive particles inserted in the flow of fluid from the nozzle cut, the reason for its withdrawal from the calibration aperture Dk growth over the limit and split of individual elements, thus changing the geometry formed by grooves cut (Fig. 6).This damage is caused with wearing phenomena.It is logical increase firmness of the calibration tubes by causing protective coatings.The use of special wear-resistant coatings can significantly improve the time of stability of materials in aggressive environments, improve the stability of the process due to less intensive changes in the dynamics of initial geometrical parameters of the channel [7].However, currently there are no clear guidelines regarding the type of coatings that can be used in this practice and rational technologies application of such coatings on the inner surface of the channel.

Fig. 6. The dependence of change of the calibration opening on run-in time
To address the question of whether the use of certain surface coatings on hydroabrasive resistance the samples with size 40x10x5 mm from titanium alloy VK8 were tested, which is instrumental material of elements of hydroabrasive processing, as no cover, so with a vacuum-plasma coating TiN, with the nitrided layer and the combined coverage of the nitrided layer and TiN (Fig. 7).Modes of coating are given in Table 1.To microelectronic studies of these, coating were deposited considering functional-oriented approach to flat plates, which were established for research under certain angles to leakage jet (Fig. 8).
Functionally-oriented approach involves the production of calibration tube by special technology, which is based on strict topologically oriented implementation of the required set of algorithms of technological performance of processing in required micro and macro zones and parts of the product that meet the functional requirements of their operation in each of its zone.

Fig. 8. Installation of a test piece on desktop of hydrocutting machine LSK-5-400
For a comparative analysis all experimental studies were made at the same processing parameters, which were adopted: leakage pressure -280 MPa, volumetric concentration of abrasive in suspension -0,6 kg/min, processing time -10,5 minutes.Investigation of angle of attack for wear of samples were carried out using a garnet sand from grit 30 mesh as an abrasive in suspension.The distance between the initial section of ejecting adjutage of ink-jet apparatus and the sample was 80 mm.In Fig. 9 there are comparative diagrams of coating TiN and combined coating wearing, presented as the depth of damage layer, measured relative to the plane undamaged surface.Angle of leakage of samples is 15 0 (/12).Found that the dynamics of damage protective coatings is fundamentally different: coating TiN weared more rapidly and in 3 minutes damage reached its maximum.After that damage rate declined, and on the samples were observed area almost its complete removal.Further active destruction of substrate material began (Fig. 10).The of combined coating was more resistant to liquid cavitation abrasive wear (almost 3 times).This wear was evenly without forming complete detachment areas.In the following diagrams the dependence of the degree of damage is presented (like a deep hole that is formed on the surface (h) for a fluid time).Dependence of change of depth hole formed in function of the angle jet of leakage α is given.Impact angle of leakage boost on the speed of removal of coating was determined by measuring the penetration of the jet in the specimen at a time equal to 1 min.Measurements performed in the range of angles from 3 0 (/60) to 30 0 (a further increase in angle leads to complete destruction of coating and base and a groove about 3-4 mm).The adequacy of the obtained models -0,95.
According to the obtained data it is easy to conclude that most works better coverage at the tangent jet of leakage.The operation time in 1 min is not exponential, because this time is necessary to establish to ensure comparability of measurements of the dynamics of wear at the big corners of leakage (Fig. 11).The difference between the mechanisms of interaction between the abrasive particles to the surface of the tube along the canal and attempts to reach approximately the same low rate of erosion due to the development of modern functional approach discussed in the works O. Mykhaylova ( [8,9], etc.).Suggests the presence on the surface clearly deterministic function of makro zones, which require different properties and different ways of providing functions.These may be the appropriate coverage that satisfactorily be taken one or the other type of abrasive loading.However, the difficulty lies precisely in the fact that the diameter of the tube is not the same and rather smallfrom 4,2 mm at the inlet of the tube to 0,95-1,20 mm at its cut.
However, the use of coatings provides for a change in the design of the handset and change the material from which it is made.We suggest to use a calibration tube of fundamentally new design.Wear-resistant gauge tube to form a liquid abrasive jet cutting was made of two symmetrical parts, connected together by mechanical means, including covered clip, planted with the intention to form a longitudinal axial channel for passage of the working fluid from the abrasive surface and the longitudinal axial channel for the precedence of damage from exposure waterjet stream inflicted with durable fragmented layer, which is based on the functional-oriented approach is the adhesion of the film material, which the wear intensity for excellent action waterjet stream is the same, the length of the application layer (a fragment) is responsible length of the high intensity of the action of abrasive particles.According to functional-based approach product manufacturing, it should be implemented a number of features that can be provided by individual carriers of cheaper material or work that is the least costly.Therefore, in our opinion , the use of fragmented layer is more efficient than the production of the entire tube with a wear resistant, but also more expensive in material.
To verify the presented ideas and get real wear profiles 10 gauge tubing coated with patchy layers (Fig. 12) were produced and tested after a fixed lifetime (T = 10,0 hours).Tubes material was regenerated solid alloy of VK with burning created by electrocontact TiN coating thickness of 1,5 mm (factionalism 30/50 μm) at the point where the movement of abrasive grains was ordered (in zone 1), PVD-coated in Zone 2 (where channel sees mostly rolling effect of the abrasive particles rolling and PVD-coating of nitriding (N + TiN) at the end of the tube, where there is influence reflected from the surface treatment of the particles; nitride layer thickness -200 μm, coating TiN -10 μm.The results shows that the coating securely protects channel tube, embryos failed to precede the occurrence of intense destruction on the surface of the channel, while apparent space docking in different layers along the length of the channel, and the thickness of the PVD-coating of nitriding (N + TiN) 200 μm proved practically remove.However, the development of damage to the channel took place almost without disturbing the original diametric portion and quite evenly.Studies performed using laser jet set LSC-400-5.Thus, we can state that, along with other means of reducing the wear lodged in [8,9], and improve the stability of geometric parameters of channel function approach is effective and efficient.At the same time, the need for further analysis of options for implementation layers, and in the futureattempts to create an inverted gauge tubes, layers that build the channel (made of a material that can then be easily removed) to the outer surface .In this case, the obvious advantage is the formation of a surface of the channel, where carbides or nitride compounds can be oriented so that the surface was formed by low hydraulic resistance.For these studies appropriate involvement of Rapid Prototyping.Further work is currently being regenerated with replacement hard metal cheaper material , such as alloyed steel.However, in our opinion, identified patterns and approaches can be used in any critical inkjet devices.

Conclusions
A model of two-phase flow of high-speed waterjet cutting stream in ductal calibration of the tube allowed to prove the existence of certain areas in which great damage processes manifested channel based on what is proposed to use a functional approach and created a new design tube which implies the existence of fragmented functional layers on the surface of the channel.Experimental tests of the proposed type gauge tubes that have proven given consideration.Of all the parameters -Dk, σDk, γDk, λDk (Fig. 6), the application of functional coatings can reduce γDk, thereby increasing the lifetime of the tube to the predicted onset of failure as

Fig. 3 .Fig. 4 .
Fig. 3.The results of modeling of liquid-abrasive mixture with the walls of the calibration tube