Features of the manufacturing technology of a multilayer tool with an ordered texture

. The auto-oscillatory process in metal cutting is analysed in order to understand the nature of chip formation in turning and to evaluate the importance of ensuring the stability of the technological system in machining. The method of multilayer tooling with ordered texture is selected as one of the promising directions for solving this problem. Microfractographic analysis is carried out on fractures of specimens cut in transverse and longitudinal directions. The character of crack propagation at fracture by the ductile intragrain mechanism is established. The design of a holder of a penetrating cutter with inserts having a directed texture is proposed on the basis of studies of a multilayer tool with an ordered


Introduction
Vibrations arising in the machining process significantly complicate the operation of technological equipment operating in an automated cycle, cause premature tool wear, machine tool, and fixture failures.Dynamic stability of the technological system and reduction of vibrations that arise in the cutting process are the key to the stability of chip formation, which is especially important in the case of automation of the technological process [1,[3][4][5][6][7].Thus, the task of ensuring the stability of the technological system by reducing the level of auto-vibration is one of the most important in the field of blade machining, which becomes especially important in the finishing of products on automatic and numerically controlled machines [2,[8][9][10][11][12].
Several fundamental studies [13,[20][21][22][23][24] have been devoted to the study of metal cutting auto-oscillations in order to understand the nature of chip formation processes in turning and to evaluate the importance of ensuring the stability of the technological system in machining.Due to the analysis and classification of methods and techniques for ensuring the dynamic stability of the technological system, it can be stated that, to a greater or lesser extent, all these methods allow for increasing the stability of the technological system.However, at present there is no universal method that allows to effectively combat vibrations arising during turning.When finishing turning, the most vulnerable in terms of dynamic stability is the "tool" subsystem [14,[16][17][18][19].The technological system is affected by an external periodic force that causes an oscillatory process with a frequency equal to the frequency of the perturbing force, or complex periodic processes due to the nonlinear properties of the system, as a consequence of which there are natural damped and forced oscillations, parametric excited oscillations, and auto-oscillations of the system.The intensity of forced oscillations is especially high under resonance modes conditions, which are inadmissible as operating modes in finishing machining on metal cutting machines [15,[21][22][23][24][25][26].

Problem statement
One of the promising directions for solving this problem is the development of multilayer tools with ordered texture.The choice of the optimal composition of the structure of metallic materials should be made taking into account anisotropy, which will ensure vibration resistance during tool operation.
The most common assessment of anisotropy is made by the ratio of strength, ductility and impact toughness in different directions relative to the rolling direction.Valuable information on anisotropy can be obtained by analysing macro and micro fractures of differently oriented specimens.
Most real structural materials are characterised by anisotropy of mechanical properties, which is caused by the peculiarities of their internal structure and production technology [27,34].To a large extent, the inhomogeneity of structures is formed at the stage of ingot crystallisation in the form of dendritic liquation, depending on the chemical composition, steelmaking technology, the method of deoxidation and modification, and the size of the cast ingot, which along with the casting temperature determines the solidification rate of ingot cooling.The cast structure is characterised by large crystals of primary crystallisation, along the boundaries of which there are interlayers enriched with impurities and nonmetallic inclusions [28].
Deformation (forging and rolling) of the cast structure leads to crushing of crystallites and pulling of their intercrystalline interlayers containing non-metallic inclusions in the direction of the most intense metal flow.At a sufficiently large degree of deformation, nonmetallic inclusions form a macrostructure banding, which leads to anisotropy of metal properties.At the micro level, the anisotropy of properties of plastically deformed polycrystals is associated with three types of textures: mechanical, dislocation and crystallographic textures [29].The crystallographic texture causes fundamental effects on all types of anisotropy of crystalline bodies [30][31][32][33].

Results
Hot-rolled steel mechanical properties anisotropy study material was used as a sheet rolled from structural steel 34HN4MFDA of industrial smelting, ingots of which were forged with a degree of compression equal to 7 into 70 mm thick sheets.The temperature of rolling start was 1100...1200 o C, the temperature of rolling end 900 о C, cooling from the rolling temperature was carried out in air.The heat treatment of the sheet consisted of quenching from the temperature 850-880 о C into water and high tempering temperature 590…620 о C.
The test specimens were cut along the rolling direction (X), perpendicular to the rolling direction (Y) and perpendicular to the sheet plane (Z).Static tensile tests were performed on cylindrical specimens on an Instron-5000 machine, and impact bending (impact toughness) tests were performed on 10x10x55 mm specimens with a notch radius of 1 mm on a pendulum press with an impact energy reserve of 4635 Joule.The test results are given in Table 1.As can be seen from the above data, the samples cut across the fibre show lower mechanical properties during testing compared to the samples cut along the fibre.The degree of metal anisotropy in the U-X plane is 4...6% for strength properties and 11...15% for plastic properties.The fibre direction affects mainly the ductility and toughness of steel.Anisotropy is most significantly manifested in the direction perpendicular to the rolling direction Z-spatial anisotropy.The degree of anisotropy of samples perpendicular to the rolling plane in terms of strength is also not great, and is within 2...4%, while the relative elongation decreases by 3 times, and the narrowing of the cross-section by 6 times.
It is known that the test results of smooth specimens in many cases characterise average rather than local material properties [2].The presence of a stress concentrator and the dynamic nature of loading in impact bending tests often leads to results that differ from the results of static tests on smooth samples, and the failure may be more brittle.As impact toughness is both a function of strength and ductility, it is the most anisotropy-sensitive characteristic of metal [4].
The main reasons for the anisotropy of impact toughness of steel products are mechanical texture, mainly fibrousity, oriented arrangement of oxide, sulphide and oxysulphide inclusions and crystallographic texture (in low alloy steels).The anisotropy of impact toughness in the rolling plane does not exceed 14%, while the volumetric anisotropy is 63%, the impact toughness of transverse and vertical samples differs almost 3 times.
Since the direct influence on the impact toughness anisotropy is caused by non-metallic inclusions deformed in the rolling direction, we additionally determined the content of impurities in the steel (Table 2), including sulphur and oxygen, which completely pass into oxide and sulphide inclusions [5].The microstructure of the investigated steel revealed by pickling in 4% alcoholic nitric acid solution is shown in Fig. 1.After the final heat treatment, the microstructure represents a finely dispersed sorbitol of tempering, which is shown in Fig. 2.
The actual grain of austenitic steel, revealed according to GOST 5639-82 by etching in the heated supersaturated solid solution of picric acid with the addition of the surfactant "Syntol" to 70 о C, is shown in Fig. 1.The austenite grain is equiaxed and corresponds to number 8-9.As can be seen from the above figures, at the microstructural level the anisotropy of the metal is not manifested.Non-metallic inclusions in the investigated steel are shown in Fig. 1, from which it can be seen that the anisotropy of properties is enhanced by stretched after rolling lines of nonmetallic inclusions representing sulphide and oxide inclusions.Microfractographic analyses of the fractures were carried out on a Phillips PSEM-500 scanning electron microscope.Fractographic analysis allowed to establish that the surface of fracture samples cut in longitudinal X (a) and transverse Y (b) directions relative to the rolling direction has a ductile dimpled character, which corresponds to ductile intra-grain fracture (Fig. 3).The results of microfractographic analysis showed that in the fractures of specimens cut in transverse or longitudinal direction, crack propagation at fracture occurs by a ductile intra-grain mechanism.In the fractures of vertical samples, fracture occurred predominantly by clusters of non-metallic inclusions of various sizes and morphology.Fracture on the base metal free of non-metallic inclusions did not exceed 20% of the fracture area and was predominantly ductile intra-grain fracture.Separate small areas of quasi-brittle intra-grain fracture and brittle intra-grain fracture were found near eutectic manganese sulfides.
It is known that anisotropy of mechanical properties causes anomalous changes in the velocity of elastic waves, their propagation path and dissipation (damping) coefficient; this determines the possibility of using the anisotropy characteristic of most real structural materials to develop and create a design of a cutting tool holder with enhanced damping characteristics.

Development of a multilayer tool with ordered texture
To achieve the greatest damping effect, the disorientation of the strain texture in two neighbouring plates should be maximal.Then, when crossing the interface, the vibrational wave changes its direction, resulting in the dissipation of vibrational energy.If the strain texture disorientation value is small, the energy dissipation is negligible.Therefore, in the proposed technical solution, the plates of the holder should be orientated in such a way that when passing from one plate to another, the deformation texture changes to 90 10° in relation to the main (tangential) component of the cutting force acting on the holder.Fig. 4 shows the design of a holder for a through cutter with inserts having a directional texture.The plane of insert 1 has a vertical orientation with respect to the rolling direction, and its cross-section has a longitudinal orientation.The plane of plate 2 is orientated across the rolling direction, and the plane of its cross-section is orientated along the rolling direction.The plate 3 is cut so that its plane has a longitudinal orientation relative to the rolling direction.At the same time, the cross-sectional plane of the plate 3 has a transverse orientation with respect to the rolling direction.Thus, all the inserts have different deformation texture in their plane and in their cross-section, have different physical and mechanical, including damping, properties with respect to the action of the cutting force components loading the holder [1,[20][21][22][23].Under the action of the cutting force, maximum tensile stresses occur mainly in the upper layers of the holder σt, and in the lower reference ones -compressive stresses σcomp.
Non-metallic inclusions in the investigated steel are shown in Fig. 1 and 2, and it can be seen that the anisotropy of properties is enhanced by stretched after rolling lines of nonmetallic inclusions representing sulphide and oxide inclusions.Microfractographic analysis of fractures was carried out on a PSEM-500 scanning electron microscope (Philips).Fractographic analysis allowed to establish that the surface of fractures of the samples cut in longitudinal x (Fig. 3, a) and transverse y (Fig. 3, b) directions relative to the rolling direction has a ductile dimpled character, which corresponds to ductile intra-grain fracture.

Conclusions
Anisotropy of mechanical properties associated with the mechanical texture of deformation causes a change in the velocity of elastic waves, their propagation path, and dissipation coefficient (damping) in different directions; this determines the possibility of using the anisotropy characteristic of most structural materials to develop and create a design of a cutting tool holder with enhanced damping characteristics.
The use of vibration-resistant properties of holders with such characteristics is reasonable, first of all, for finishing and fine machining at high cutting speeds, with low stiffness of workpieces, variable, continuous or discontinuous machining surface, when there are high requirements to geometric accuracy of machined part profile and surface roughness.

Fig. 4 .
Fig. 4. Design of the holder of the through cutter with inserts having directional texture: 1 -plate with vertical rolling orientation; 2 -plate with transverse rolling orientation; 3 -plate with longitudinal rolling orientation; 4 -cutting plate; Py is radial cutting force; Pz is tangential cutting force; is tensile stress; is compressive stress.

Table 1 .
Mechanical properties of sheet steel in different directions

Table 2 .
Steel content of certain elements and impurities