Features of mechanical processing of aluminum alloys

. The issue of ensuring the technological quality of aluminum alloy products is considered. The problems that arise during the processing of aluminum alloys are described in detail. The results of various studies aimed at solving the described problem are presented. The author proposes to consider the method of magnetic abrasive machining as one of the promising methods for processing complex products.


Introduction
Aluminum and aluminum alloys are among the most commonly used light metal materials because they offer a number of different distinctive mechanical and thermal properties: low specific gravity, high ductility, corrosion resistance, high electrical conductivity [1].In addition, they are relatively easy to shape, especially with the help of mechanical cutting [2].In fact, aluminum and its alloys are considered to be easily workable materials compared to other light materials, such as titanium and magnesium alloys [3,4].
However, it is known that alloying aluminum with various elements, such as magnesium, manganese, copper, silicon, etc., entails a change in the workability of the alloy [5].This change is described by such negative factors associated with the high viscosity of aluminum alloys as the formation of a build-up on the front surface of the cutting tool, the appearance on the treated surface of a layer with increased microhardness (riveting), which makes it difficult to achieve a given surface quality, leads to overheating, jamming, breakage of the cutting tool [6].The size of the build-up and the rivet is affected by the cutting modes, the forces arising during the cutting process, the geometry of the cutting tool, its durability.

Research object
Based on the listed features of processing aluminum alloys, various methods of abrasive processing were considered as a method of final processing [7,8].With the current technical progress, a wide range of enterprises have an urgent need for such a method of final surface treatment of the workpiece, which will provide a low surface roughness Ra (up to 0.01 microns) [9].There are a number of methods of final surface treatment of the workpiece associated with the use of abrasives: abrasive treatment, treatment using magnetic rheological fluid as an abrasive and magnetic abrasive treatment using magnetic abrasive powder [10][11][12][13].
Solving problems related to ensuring high surface quality and precision machining in the machine-building industry has always been one of the first and priority tasks.Their solution is especially relevant in the conditions of constantly increasing technical requirements for mechanical engineering products, which is due to the improvement of technologies and the expansion of the range of materials used.A significant part of all technological operations related to shaping falls on mechanical processing.
According to the criterion of achieving roughness, aluminum alloys can be attributed to hard-to-process materials.The high roughness of the treated surface of these materials is associated with their basic mechanical properties -high ductility and toughness, which leads to the formation of elemental chips, significant stretching of metal grains in the direction of chip separation and vibration of the technological system.However, from a practical point of view, this issue has been solved by increasing the cutting speed and selecting the optimal coolant composition [14].

Features of aluminum alloy processing
In general, the main problems in the machining of aluminum alloys are the occurrence of significant thermal deformations, leading to a decrease in the accuracy of the parts obtained during dry (without the use of coolant) finishing, as well as a sufficiently low roughness of the treated surface.The low roughness is explained by intensive build-up and sticking formation when cutting aluminum alloys.It is known that a feature of aluminum alloys is their increased tendency to frictional transfer of the processed material to the tool blade, which leads to the formation of an unstable build-up, which greatly affects the quality of the treated surface (Fig. 1) [15].The instability of chip and mold formation associated with the negative effect of the build-up is characteristic of aluminum alloys for three reasons.The first reason is the special role of the hardening factor during deformation inherent in all soft alloys [16].The second reason is the high thermal conductivity of aluminum alloys, which smooths the dependence of the cutting temperature on the cutting speed and helps to expand the range of the mode in which the growth exists [17].The third reason is an abnormally high ratio of the mechanical properties of the layers of aluminum oxides and aluminum alloy, which causes a negative gradient of mechanical properties in depth in the contact zone in an environment containing an oxidizer.An analysis of existing scientific papers devoted to the research of grinding processes of aluminum alloy workpieces has shown that traditional grinding methods due to instantaneous salting of the abrasive wheel are ineffective [18].During the grinding process, a continuous process of destruction and smoothing of the surface is carried out.Moreover, local metal outbursts resulting from previous operations are partially or completely covered with softened metal on the performed operation.Practically micro-destructions, snatches and nadirs cannot be avoided.The surface layer is structurally always heterogeneous.Also, the authors of many studies believe that the reduction of the roughness of the polished surface can be achieved by various methods, the choice or combination of which determines the success in solving the task [19][20][21][22][23]. But, unfortunately, the degree of influence of one or another technological factor on the roughness of the treated surface has not yet been clarified.In general, the grinding process has limited use for processing aluminum alloy parts, since in this case, the abrasive tool is quickly salted and the treated surface is partially melted.
It is possible to solve the problem by applying ultrasonic vibrations to the cutting tool during turning operations, which should eliminate the formation of a build-up on the front surface of the tool blade and ensure the required surface roughness quality (Fig. 2) [24,25].Fig. 2. Device for processing holes in aluminum alloy products: 1 -magnetostrictive transducer, 2rod, 3 -tool holder, 4 -workpiece, 5 -ultrasonic vibration concentrator, 6 -boring cutter, 7 -ultrasonic vibration generator, 8 -axis of rotation of the workpiece, 9 -amplitude of vibrations in the direction of feed, 10 -feed direction of the boring cutter [23] The ultrasonic vibration concentrator, pressed to the side surface of the cutter, generates the energy of traveling waves from the point of contact of its tip with the boring cutter to the cutting zone in a double amplitude of vibrations in the direction of the cutter feed.The cutter under ultrasonic action, in accordance with the parameters of the cross section and the tensile strength, generates the energy of traveling waves from the point of contact of the resonant waveguide with the cutter to the cutting zone in the minimum amplitude mode in the direction of the cutter feed.
The energy of the traveling waves along the rod of the holder does not allow the formation of an outgrowth when the chips descend along the front surface of the tool and, accordingly, the adhesion of layers of the outgrowth to the workpiece surface to be processed, which has a positive effect on the quality of processing and improves the roughness of the surface of the part.
The rigidity and accuracy of positioning used in technological equipment operations have a great influence on the quality of the formation of the surface layer of aluminum alloy parts.Currently, quality parameters are provided through the use of modern high-performance machine tools with numerical control.This equipment allows for high-speed machining HSM (High Speed Machining) [26].
Its distinctive feature is the high cutting speed, at which the temperature in the chip formation zone increases significantly, the material of the workpiece becomes softer and the cutting forces decrease, which allows the tool to move with a large working feed.The achievement of the HSM effect is due to structural changes in the material in the chip separation zone.This is due to the formation of plastic deformations occurring at a high rate.With an increase in the rate of deformation, the cutting forces initially increase, and then, with the achievement of a certain temperature in the chip formation zone, they begin to significantly decrease.At the same time, the contact time of the cutting edge with the workpiece and chips is so short, and the chip separation rate is so high that most of the heat generated in the cutting zone is removed along with the chips, and the workpiece and the tool do not have time to heat up significantly.This effect is already known earlier.The processing of aluminum and its alloys using HSM has many features related to the characteristics of the equipment and the accuracy of control programs for HSM trajectories.Based on many studies of processing processes and HSM programming, specialists [27] have developed various recommendations related directly to the choice of equipment, cutting modes, selection and positioning of cutting tools.
In addition, the advantage of using anodic-mechanical processing (Fig. 3) [28] is currently known, with the help of which it is possible to obtain parts of higher quality, as well as to provide a significant increase in the efficiency and wear resistance of the cutting tool.However, anodic-mechanical milling is not yet widespread.It is also known that during finishing and finishing operations of anodomechanical processing, cutting schemes involving the sequential action of a mechanical and then an electrochemical component are the most effective [29,30].This method of processing is based on a combination of electrocontact interaction of the tool and the workpiece (mechanical destruction or shaping of metal surfaces produced simultaneously with heating or melting of these surfaces by electric current) and a galvanic process (in this case, it is the anodic dissolution of metal from the treated surface) [31].The moving tool not only supplies current and removes the softened metal, but also, due to vibration, contributes to the emergence of many intermittent contacts necessary for the formation of arc discharges [21].Electrocontact treatment can be performed in both air and liquid media.Processing performance increases almost linearly with increasing voltage and power of the power supply.Particular attention should be paid to the process of magnetic abrasive treatment, as it is gradually gaining popularity among manufacturers of high-precision products, as it is able to provide a surface roughness of Ra from 0.01 to 0.4 microns, and it can be used for processing both magnetic and non-magnetic materials.
The magnetic abrasive treatment is based on the phenomena of the magnetic field and magnetic induction, which is a force characteristic of the magnetic field.The magnetic field can be created by permanent magnets or electromagnets, which are arranged relative to each other in an order depending on the method of magnetic abrasive treatment used.
The magnetic field is necessary for the formation and retention of the magnetic abrasive brush at the working poles of the magnetic abrasive installation, the action of magnetic induction on the magnetic particles of the magnetic abrasive brush, due to which the tangential Ft and normal Fn forces acting on the abrasive particles are formed.When an abrasive particle held by magnetic particles comes into contact and presses against the surface of the workpiece due to the action of the normal force Fn, it scratches the surface of the workpiece due to the action of the tangential force Ft, producing material removal [22,25].Thus, a qualitative surface layer of the part is formed.
The mechanism of formation of a magnetic abrasive brush is a complex system, therefore, when calculating the forces acting on an abrasive particle, researchers of the magnetic abrasive treatment process have adopted certain assumptions.Firstly, all abrasive and magnetic particles are spherical objects, equally oriented relative to the surface of the workpiece.Secondly, the diameter of the particles is the same and constant for all particles of the same type, the particles are arranged relative to each other without voids.Thirdly, the magnetic flux density is distributed evenly throughout the workspace.
It should be noted that the density of the magnetic flux and the uniformity of the magnetic field are among the key factors influencing the formation of a high-quality surface layer.The distribution of the magnetic field in the treatment area determines the shape and stiffness of the magnetic abrasive brush.The characteristics of the magnetic field in the working gap are the density and magnitude of the magnetic flux F, its gradient.They depend on the shape, size, material of which the working poles are made, the voltage or current supplied to the coils, and the relative position of the working poles relative to each other [7,9].
The magnetic flux acts on the abrasive particle by applying a magnetic force to the magnetic particles that hold the abrasive particle.Due to this effect, the normal force Fn and the tangential force Ft are formed, which are responsible for pressing the abrasive particle into the workpiece and removing the material.

Magnetic-abrasive finishing
Both mechanisms of magnetic field formation are investigated -with the help of an inductor (non-permanent magnets) or permanent neodymium magnets [1].In order to ensure shaping, accuracy and the required quality indicators, it becomes relevant to improve the known mechanisms of magnetic field formation [2,3].The assumption is substantiated that magnetic-abrasive treatment of a complex-profile surface between two flat pole tips is ineffective, since several zones with different processing intensity are formed during such treatment.As an example, the results of an experimental study of changes in the roughness of the treated surface of the ellipsoid of rotation due to magnetic abrasive treatment are given.The results of the experimental study showed that the intensity of processing of the complex profile surface of the ellipsoid of rotation differs in its upper and lower parts.A detailed examination of the processing allowed us to conclude that the magnetic abrasive powder adheres to the treated surface in the upper part of the ellipsoid better than in the lower (Fig. 4).The non-attachment of the magnetic abrasive powder to the treated surface in its lower part provides less intensive processing than in the upper part of the treated surface.Thus, "blind zones" are formed in which the quality of the treated surface in terms of roughness values turned out to be worse by 0.210 microns, and the roughness of the lower part of the treated surface was 0.995 microns.
At the same time, in the upper part of the treated surface of the ellipsoid, the processing was more intensive, since the magnetic abrasive powder was fully attached to the treated surface.The roughness of the upper part of the treated composite surface was 0.785 microns.
Thus, unequal values of the working gap negatively affect the intensity of processing of a complex-profile surface.To prevent the disparity of the values of the working gap, it is necessary to use devices with permanent magnets, the working surfaces of which will be parallel to the tangent to the surface, thus copying a complex surface.

Conclusion
The article is devoted to the issue of technological quality assurance of shaped surfaces of products made of corrosion-resistant aluminum alloy AMc.In the first part, an overview of the current state of the issue of the topic of the dissertation is carried out, the main requirements for electrical products having shaped surfaces made of aluminum alloy AMc, problems arising during the final machining of these surfaces are given.In the second part, global scientific achievements on the research topic are presented, the results of research by a number of scientists are presented, the main unsolved scientific problems are identified and a solution method is proposed -the use of the magnetic abrasive treatment method for the purpose of technological quality assurance of shaped surfaces.1.
Based on the results of theoretical and experimental studies of the possibility of magnetic abrasive treatment of composite surfaces made of aluminum alloy, the following conclusions can be drawn: 2. Products operating under the influence of aggressive media and subjected to fatigue loads need modern processing technology, which will ensure the requirements for qualitative indicators of surface condition (roughness); 3. Aluminum alloys are difficult-to-process materials, since in the process of their processing there are processes of caricature and build-up.The use of such a modern technology of final processing as magnetic abrasive treatment will avoid the occurrence of the above processes; 4. "Blind zones" are formed in which the quality of the treated surface in terms of roughness values turned out to be worse by 0.210 microns, and the roughness of the lower part of the treated surface was 0.995 microns.