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6METHODS FOR REMOVING DEFECTS ON THE SURFACE OF PARTS IN
THE PROCESS OF STAMPING
Y. Y. Khusanov A. M. Yakupov
Fergana polytechnic institute Andijan Machine-Building Institute
ABSTRACT
In modern machine-building production, one of the main problems solved in the manufacture of parts for various purposes is to ensure the high quality of their working surfaces. Among the various technological methods of finishing, hardening methods are widely used. One of the most commonly used methods at present is surface plastic deformation (SPD). The SPD method is easy to implement, economical, productive, provides the formation of low roughness, a given depth and degree of hardening, as well as residual stresses, fine-grained structure and other quality indicators of the surface layer of machined parts.
Keywords: stamp, technology, plastic, deformation, scientific and methodological, quality, zone.
INTRODUCTION
The formation of the quality of the surface layer is mainly carried out at the finishing operations. One of the highly economical and productive processing methods is surface plastic deformation (SPD). The use of SPD makes it possible to reduce roughness, obtain the required microprofile of the surface, strengthen the surface layer with a given degree, obtain favorable residual stresses, etc. etc. The level of quality indicators achieved as a result of processing, in turn, causes an increase in fatigue strength, contact endurance, wear resistance of rubbing surfaces, an increase in contact stiffness, and an increase in corrosion resistance. For this reason, PPD has found wide application in many branches of engineering production in the manufacture of parts for various purposes, made of cast iron, non-ferrous metals and even hardened steels.
In the field of surface plastic deformation, a large number of both theoretical and experimental research results have been obtained.
Through the efforts of many well-known scientists, a significant contribution has been made to the development of the theory of surface plastic deformation, among which: Azarevich G. M., Alekseev P. G., Barats Ya. I., Braslavsky V. M., Drozd M. S., Zhasimov M. M. , Ilyushin A. A., Ishlinsky A. Yu., Konovalov E. G., Kragelsky I. V., Kudryavtsev I. V., Matalin A. A., Matlin M. M., Papshev D. D., Pinegin S. V., Proskuryakov Yu. G., Reznikov A. P., Ryzhov E. V., Sidyakin Yu. I., Smelyansky V. M., Suslov A. G., Shakhov V. I., Chepa P. A., Shkolnik L. M., Schneider Yu. S., Yaroslavtsev V. M. et al.
The scientific results achieved so far make it possible in many cases to reasonably assign processing modes and make a choice of rational design parameters of the processing deforming tool. However, in view of the complexity of the RPM process and a large number of factors that affect the productivity of the quality of the machined surface, there are difficulties in choosing their optimal combination, and in some cases there are conflicting information that does not allow unambiguously assessing the degree of influence of certain processing parameters on the RPM process. .
The transformation of the qualitative state of the surface of the part is carried out as a result of plastic deformation, which consists in the irreversible forced movement of individual atoms or their groups, is a complex process, the study and management of which is complicated by the following factors:
• Without exception, all metals are alloys containing soluble or insoluble impurities in varying amounts and are characterized by a heterogeneous structure.
• The complexity of studying the processes of plastic deformation of a metal is due to the fact that, for given initial mechanical properties, the value of the resistance of the metal to plastic deformation is continuously changing, since at the same time there are changes in the mechanical properties of the surface layer caused by hardening. These changes are heterogeneous in nature. Therefore, when calculating the parameters of metal treatment modes by pressure, it is necessary to first experimentally establish a functional dependence characteristic of a given metal, which relates its resistance to plastic deformation with the amount of deformation.
• The phenomena that characterize the process of plastic deformation are determined by the structure and properties of the metal being processed.
• All metals have a polycrystalline structure, i.e. they are a set of soldered crystallites-grains of irregular shape, anisotropic in mechanical, chemical and physical properties.
• The mechanism of cold and hot plastic deformation is different. Cold deformation is characterized by shear deformation and bending of the spatial lattice. Processes such as diffusion plastic deformation and recrystallization, which are characteristic of hot deformation, are practically absent during cold deformation.
• During surface plastic deformation, only the surface layer of the part material is plastically deformed.
There are static, shock, vibration and ultrasonic 1111D 72]. With static methods, the tool, working fluid or medium acts on the surface to be treated with a certain constant force, while there is a smooth and consistent movement of the focus of influence over the entire surface. With impact methods, the tool or working bodies repeatedly and consistently act on its entire part, while the impact force each time changes from zero to a certain maximum value. Static SPD methods, as a rule, provide a
lower roughness with a favorable microroughness shape, and compared to some impact processing methods, static methods allow achieving a greater degree of hardening [1].
As working bodies, deforming elements of various configurations and sizes, balls, shot, deforming elements, the working surface of which is a body of revolution, are used. PPD can be performed simultaneously by several processing methods (combined PAP) or sequentially by several methods (combined PAP).
The prospects for the development of SPD are due to the following features and advantages of this type of processing: the preservation of the initial volume of metal during shaping processes ensures its significant savings; preservation of the integrity of metal fibers, hardening of its surface layer, accompanying cold plastic deformation, creation of significant compressive residual stresses during shaping, finishing and, especially, hardening processing.
METHODS
The study of the elastoplastic deformation process involves, at the first stage, the establishment of the geometric parameters of the contact zone, since the physical and mechanical processes and the qualitative transformation of the surface layer occur within it. As a rule, when studying the geometric parameters of the contact zone, one parameter is determined - the contact area, assuming that it is the dominant indicator that determines the results of PPD processing.
Let us further agree to consider any body of revolution as a deforming element; in a particular case, a specified deforming element will be called a roller for simplicity.
The contact between the deforming element and the part, in addition to the contact area, is characterized by other parameters. These include: change in the half-width of the contact along its length (the equation of the contact contour line), the maximum half-width and length of the contact, the change in the depth of penetration of the deforming element along the line of its maximum loading, the volume of metal displaced from under the deforming element, the surface area of the contact (the area of the part deforming element in contact with the surface of the part).
These geometric parameters determine the nature of the distribution of strains and stresses over the contact area, the deformation force, and directly depend on the geometric parameters of the deforming elements, the dimensions and type of the surface of the part (shaft, hole, plane), the mechanical properties of the metal being machined, and the depth of penetration of the deforming element into the machined surface.
The determination of the geometrical parameters of the contact zone during SPD in the vast majority of calculated dependencies refers to deforming elements that have the simplest shape of the working surface: a ball, a torus, a cylinder, a right circular cone, or a combination of these surfaces. The shape of the contact in these cases is an ellipse or rectangle. When determining the geometry of the contact, there is no methodological
unity of the solution of this problem, which makes it difficult to comprehensively analyze the obtained dependencies. Along with this, there is the problem of finding the optimal shape and size of the deforming elements that provide the best processing conditions from the point of view of simultaneously ensuring the accepted criteria for productivity and quality of the surface layer. The urgency of solving this problem is obvious.
Therefore, in order to analyze the influence of the geometric parameters of the deforming elements and technological factors of processing on the productivity and quality of the surface layer, it is necessary to establish a universal mathematical relationship between the design parameters of the deforming elements, the dimensions and type of the surface to be machined, the technological factors of processing, the geometric parameters of the contact zone and the surface quality of the part during processing. PPD.
Obviously, all the parameters of the contact zone listed above are determined through the law of change in the half-width of the contact along its length and the law of change in the depth of penetration of the deforming element along the line of maximum load, which in turn depend on the radius of the workpiece, the shape and size of the deforming element, and also from the depth of its penetration into the part. When solving the problem, it is necessary to make some assumptions.
Theoretical and experimental studies have shown that the deforming elements during SPD can be considered sufficiently rigid with an accuracy of several percent. This assumption simplifies the development of a mathematical model, since it does not require taking into account the elastic deformations of the deforming element for the contact geometry. We also assume that there is no seizure between the surfaces of the deforming element and the part, the material of the part is elastic-plastic. During deformation, the material of the part is continuous and has no gaps, the deforming element has a hardness much higher than the workpiece [3].
When solving the problem, it is necessary to take into account that the axes of the deforming rollers during processing are rotated relative to the axis of the part by the angle of penetration and self-tightening. The penetration angle is necessary to create a drop-shaped contact, and the self-tightening angle helps to reduce axial forces by moving the deforming element along a helical line on the surface of the part, and in some cases is used to ensure self-tightening of the tool.
The difference between the static introduction of a deforming element during elastic deformation of bodies with the same mechanical properties and elastic-plastic deformation when it rolls along an elastic-plastic surface is that in the first case (Fig. 2.1, a) the contact is a flat figure, since the bodies are deformed in the same way and have practically the same mechanical properties. In the second case (Fig. 1), a deforming element with a high hardness is introduced into the part to a certain depth,
the value of which is determined by the magnitude of the applied force, the dimensions of the deforming elements and the mechanical properties of the deformable surface. Let us calculate the deformation of the deforming element when it is compressed by two opposite forces. The decrease in the diameter of a cylindrical deforming element located between two faces compressing it, taking into account contact and general deformations, is determined by the formula [4].
1 - u2
A D = 4 P--7—(0.41 + InnE
2D
v
P-D
p
where P-forces acting on the roller from opposite sides; ^-Poisson's ratio; E-modulus of elasticity; Dp-diameter of the squeezed cylinder
Rice. 1 Features of deformation of two cylindrical bodies: a) - with identical elastic properties of bodies, b) - with elastic-plastic deformation of a part by a solid
deforming element
The results of calculations using this formula are shown in the graphs (Fig. 2). The relative deformation of the rollers when they are loaded depends little on the diameter, but only on the loading force and does not exceed 3% at the maximum values assigned under production conditions (up to 30 kN). Consequently, the deforming element during SPD can be assumed to be sufficiently rigid and the change in its radius during deformation can be ignored.
This makes it possible to estimate the deformation of only the roller itself. As can be seen from the presented results, the deformation of the roller is insignificant, therefore, as stated in a number of literary sources, the deforming roller in analytical calculations can be taken almost rigid, that is, when determining the geometric parameters, its deformation cannot be taken into account.
Since all parameters of the contact zone: maximum length, width, contact area, volume and surface area of the contact zone are determined through the law of change
of the half-width of the contact along its length, the problem is reduced mainly to finding the contour line limiting the contact.
Determining the relationship between the geometric parameters of the deforming elements and the geometric parameters of the contact zone can be solved on the basis of direct and inverse problems.
When solving the direct problem, the initial data are the geometric parameters of the deforming element (maximum diameter and length, as well as the equation for changing the radius along the length of the contact that determines its shape), the depth of penetration into the machined surface, the dimensions and type of the part (hole, shaft or plane) , after which the geometric parameters of the contact are determined.
When solving the inverse problem, the shape and dimensions of the contact zone are specified, and then the geometric parameters of the deforming element are determined, which provide a given contact located on the surface of a part of a given type and size. When studying the SPD process, most authors use the solution of the direct problem [4]. The importance of the inverse problem lies in the fact that, first, the necessary rational shape and dimensions of the contact zone can be set, at which the required stress distribution over the contact area and quality indicators of the surface layer are achieved, and then, based on these data, the transition to the choice of design parameters of the deforming element and technological parameters is carried out. PPD modes. In addition, some questions can be more simply and rationally solved only on the basis of an inverse problem. For example, by setting a contact zone of the same size and shape, it is possible to investigate the influence of the shape and size of the deforming elements, the size and type of the treated surface on the deformation force and the quality of the surface layer.
One of the issues to be solved when determining the parameters of the contact zone is related to the identification of a correspondence between a static imprint with a simple indentation of a deforming element and an imprint that occurs when the deforming element rolls along an elastically deformable surface. Obviously, the conditions for the formation of the contact zone in both cases are somewhat different. This relationship cannot be established through the development of an appropriate mathematical model. However, it should be borne in mind that the chipping process is classified as a static process. This means that practically on the entire processing interval, with the exception of the initial short-term and final time intervals, the processing is stationary. Therefore, we can assume the constancy of the contact parameters over the entire period of processing time, and when developing dependencies for determining the parameters of the contact zone, consider the contact instantaneous and embedded in the part as with static indentation. The difference between contacts during static indentation and rolling of the roller on the machined
surface can be established on the basis of experimental studies of the change in the depth of penetration of the roller into the surface of the part in both cases.
CONCLUSIONS
1. The actual scientific problem of technological assurance of the quality of the surface layer is substantiated and solved, which consists in the development of the theory of processing machine parts with rollers and the creation on this basis of a scientific and methodological base covering the technological preparation of production, the manufacture of advanced processing tools and technological support.
2. A mathematical model has been developed for determining the parameters of the contact zone, taking into account the influence of the dimensions and type of the machined surface (hole, shaft, plane) and deforming elements of arbitrary shapes and sizes. Dependences are obtained to determine the change in the half-width of the contact on its length, contact area, contact surface area and volume of the contact zone. The relationship between the dimensions of the deforming elements and the parameters of the contact zone is determined on the basis of a direct and inverse problem. The ratio between the deforming elements used for shafts and holes is determined, providing equivalent indicators of the quality of the machined surface.
3. It has been established that at the same depth of penetration of the deforming element into the surface of the part, the dimensions of the contact area change significantly in the range of increasing machining diameters up to 120 mm. With an increase in the diameter of the shaft, the dimensions of the contact increase, and when machining a hole, they decrease. With an increase in the size of the deforming elements and a given diameter of the part, the dimensions of the contact increase both when machining a hole and when machining a shaft. A given contact can be obtained by different sizes of deforming elements. At the same time, the depth of penetration and the volume of the contact zone change. With a decrease in the radius of the deforming element, the volume of contact and the depth of penetration of the element increase and vice versa.
4. Studies of the relationship between the geometric parameters of the deforming elements and the contact zone have established that the area and volume of the contact zone do not unambiguously determine the formation of the quality of the surface layer.
5. A mathematical model has been developed for determining the kinematics of moving points of a deformable surface. The connection between the kinematics of moving points of a deformable surface and strains and stresses distributed over the contact area formed by an arbitrary deforming element is established.
6. Dependences are obtained to determine the radial and tangential forces, specific heat release and temperature per single contact through the distribution of strains and stresses in the contact.
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