Simulation of a single interaction of an abrasive particle with the surface of a part during blasting

. It is shown that when modeling a single interaction of an abrasive particle with the surface of a part in the process of collision under air (liquid) pressure, it is necessary to take into account the specifics of impact-abrasive wear, when the separation of a wear particle is preceded by metal destruction. In the process of schematization of the contact interaction during abrasive blasting, some commonality with the shot blasting process and a number of assumptions typical for grinding, when micro cutting with a single abrasive grain is considered, are taken into account.


Introduction
Abrasive blasting of metal surfaces of parts refers to grinding with a free abrasive, the totality of which is directed to the surface to be treated at a certain angle α (angle of attack) and under pressure p of compressed air or as part of an anti-corrosion liquid (hydro abrasive blasting). It is known that the grinding process has much in common with the process of abrasive wear during friction, but there are a number of distinctive features: 1) the working surface of the tool is much rougher compared to the surface of the abrasive body; 2) grinding grains have high hardness, heat resistance and wear resistance with relatively high brittleness; 3) high intensity of metal removal per unit time, and the resulting chips during grinding are much larger in relation to friction wear products.
The systematization of the force contact of the abrasive with the wear surface is based primarily on the types of friction -sliding friction, rolling friction, impact of the abrasive with the metal surface, transformation of the geometric and physical and mechanical properties of the surface layer of the metal. This systematization has signs of universality. So, for example, under conditions of sliding friction, the nature of the force interaction of a single abrasive particle with the wear surface is close to when, instead of a separate abrasive particle, a certain protrusion acts on the friction surface, imitating the case of fixing the particle on the contact (grain on the grinding wheel).
Regardless of the difference in the fundamental schemes of the interaction of abrasive particles with the wear surface, they have an element of commonality, which consists in the fact that in each case the separation of the wear particle is preceded by the destruction of the metal, i.e. mechanical (abrasive) wear is observed.
The specificity of abrasive blasting of metal surfaces is that the sliding friction of abrasive particles is preceded by an impact with a destructive effect, thereby causing shock-abrasive wear. With this type of wear [1,2], the direct penetration of a solid particle into the metal under the action of a shock pulse creates a depression in the form of a dimple on its surface, which approximately copies the geometry of the particle.

Methods
The polydeformation process inherent in abrasive blasting due to the many single introductions of particles at each successive impact forms a kind of macro profile on the wear surface in the form of alternating dimples and bridges between them without characteristic marks of directional orientation, typical for abrasive wear during sliding friction. The abrasive particle at the initial moment of dynamic contact upon impact must overcome the resistance of the metal to this penetration, which is possible only if the hardness and strength are higher than those of the metal. Depending on the angle of attack α of the abrasive particle, regime parameters and physical and mechanical properties of the processed (wearable) material, abrasive grains, having penetrated to a certain depth due to the kinetic energy reserve and elastic aftereffect, the contact zones can form short risks in the form of small scratches, i.e. . to carry out micro cutting in the process of sliding along the formed surface [3,4].
Abrasive blasting is characterized by simultaneous and multiple impacts of abrasive grains having different angles of attack within the flow falling on the metal surface in the form of a so-called solid particle jet. The essence and thermodynamics of contact interaction during the impact of deforming and cutting particles on the treated surface depend on many factors: the physical and mechanical properties of the contacted materials, the size and speed of impact of solid particles, the pressure of the working medium (air, liquid), processing time, angle of attack, density flow [5][6][7]. Due to the complexity and polydeformation nature of dynamic contact during processing with free abrasive particles, schematization is necessary for this interaction of solids in order to build a mathematical model of the abrasive blasting process.
When drawing up the schematization of the contact interaction for abrasive blasting, some common features with the shot blasting process [8][9][10][11][12] were taken into account and a number of assumptions were made that are typical for grinding when micro cutting with a single abrasive grain is considered. So, the schematization of abrasive blasting is based on the following assumptions. 1. From the flow of a jet of solid particles, we select one abrasive grain and assume that it hits the surface of the body being treated with an average flow velocity ʋ at a given angle of attack α, moreover, some of the abrasive particles fall on the surface at an angle close to 900. The particle is introduced into the body, followed by sliding. 2. From the flow of a jet of solid particles, we select one grain of abrasive and assume that it hits the surface of the body being treated with an average flow velocity ʋ at a given angle of attack α, moreover, some of the abrasive particles fall on the surface at an angle close to 900. The particle is introduced into the body, followed by sliding. 3. To describe the deformation process in dynamic contact, we simulate in the form of a single-act collision of a rigid non-deformable solid particle of a spherical shape. The assumption of non-deformability of abrasive particles is acceptable due to their increased hardness and strength. 4. The surface of the machined (wear) part is assumed to be smooth and the deformable body (surface layer) is represented as an elastic half-space. The legitimacy of this assumption is determined by the fact that the dimensions of the plastic imprint (hole) and traces of micro cutting (scratching) are significantly smaller than the dimensions of the body. 5. The processed material is considered homogeneous and isotropic, which is fundamental in the theory of elasticity and plasticity. 6. The intensity of impacts of abrasive particles on any treated area remains constant with the same specific gravity. 7. The probability of hitting a treated area with an area ΔS of two blows in a very small but finite period of time is negligible compared to one blow.

Results and discussion
In the contact external force action of a hard abrasive particle on the treated surface, the wear mechanism during sliding is manifested, in which two successive stages can be distinguished (Figure 1). The first stage is characterized by the impact of the abrasive particle on the treated surface and ends with its penetration into the thin surface layer of the metal to a certain depth h. A necessary condition for implementation is the superiority in hardness and strength of the abrasive particle over the metal of the treated surface, as well as sufficient kinematic and dynamic conditions for the contact of solids. At the second stage, the abrasive particle, having penetrated to a certain depth, performs translational motion and forms a wear surface, while carrying out a complex of complex interrelated processes: plastic deformation, micro cutting (scratching), elastic displacement, etc. Ultimately, the features of these phenomena during contact interaction are mechanism of wear of the surface layer of metal in the abrasion zone.
The external force action of a single abrasive particle on the workpiece surface is inevitably accompanied by its deformation and further formation of local fracture centers with the separation of wear particles. Depending on the intensity of the force factor, the deformation in the contact zone of the abrasive particle with the metal can be elastic or plastic if the intensity of normal stresses qi exceeds the physical (or conditional) yield strength of the material being processed σт (σ0,2).
Since abrasive wear is characterized by continuous and in many cases significant removal of metal from the friction surface, then, taking into account the final result of the action of an abrasive particle, one should keep in mind mainly plastic deformation. A complex deformation mechanism in the zone of movement of an abrasive particle along the friction surface is predetermined by the complex shape, geometry, and particle size, i.e. on the surface of one abrasive particle there can be areas with different cutting properties, which creates heterogeneity of deformation processes during micro cutting.
In the process of grinding the grains of the circle, mass micro cutting is carried out -the scratching of the surface layer of the material being processed, therefore, the study of the operation of a separate grinding grain is reduced primarily to the study of the mechanism of the process of scratching the material. The scheme of micro cutting (scratching) of a material by a rounded grain cutting element [13] does not fundamentally differ from the classical scheme of free and non-free cutting, adopted in the theory of metal cutting [14,15].
On Fig. 2 shows a diagram of the process of micro cutting with a single free abrasive having a rounded cutting element (rounding radius ρ) and coming into contact with the material being processed after impact under compressed air pressure. Let us consider the case of micro cutting during the translational movement of the scratching element, which has a rounded apex of radius ρ, which is affected by the external impact force Pud. Expanding the force Rud. into the components Рz and Ру, we establish that the force Рz cuts off the chips, and the force Ру presses the scratching element against the surface to be machined. The rounding of the scratching element provides its high mechanical strength, large actual cutting angles.
In the process of scratching, plastic deformation of the metal occurs in front of the scratching element in the zone A1, A2, on the sides of it (in the zone L1, L2) and below the cut line in the zone H1, H2 (Fig. 2). An increase in the thickness of the removed layer a leads to an increase in the volume of metal involved in plastic deformation in all indicated directions: if a2>a1, then we have A2>A1, L2>L1, H2>H1.
The translational movement of the scratching element, accompanied by continuous chip removal, is possible under the action of shearing (shearing) stresses τ in the shear plane, which are greater than the true resistance of the material being processed to shear (shear) shear: τ≥τshear, where shear is the shear yield strength of the material being processed [16]. In this way, the plastic deformation that occurs during the contact interaction of the abrasive particle with the metal surface of the workpiece is fully responsible for the quality and condition of the thin surface layer, characterized by a set of geometric and physicalmechanical parameters.
In metals, the process of plastic deformation is mainly carried out by sliding, carried out by the movement in the slip plane of individual imperfections (violations) of the spatial crystal lattice -dislocations [17,18].
At present, the theory of dislocations is widely spread due to its universality and allows solving a wide range of problems in plasticity, fracture and strength of metals, thermal physics and thermodynamics. The undoubted advantage of the theory of dislocations is that it connects the micro-and macro-representations of the process of plastic deformation of metals through such parameters as the shear modulus, Poisson's ratio, Burgers vector, dislocation density, normal and shear stresses, and yield strength.