Vol. 24 No. 2 2022 3 EDITORIAL COUNCIL EDITORIAL BOARD EDITOR-IN-CHIEF: Anatoliy A. Bataev, D.Sc. (Engineering), Professor, Rector, Novosibirsk State Technical University, Novosibirsk, Russian Federation DEPUTIES EDITOR-IN-CHIEF: Vladimir V. Ivancivsky, D.Sc. (Engineering), Associate Professor, Department of Industrial Machinery Design, Novosibirsk State Technical University, Novosibirsk, Russian Federation Vadim Y. Skeeba, Ph.D. (Engineering), Associate Professor, Department of Industrial Machinery Design, Novosibirsk State Technical University, Novosibirsk, Russian Federation Editor of the English translation: Elena A. Lozhkina, Ph.D. (Engineering), Department of Material Science in Mechanical Engineering, Novosibirsk State Technical University, Novosibirsk, Russian Federation The journal is issued since 1999 Publication frequency – 4 numbers a year Data on the journal are published in «Ulrich's Periodical Directory» Journal “Obrabotka Metallov” (“Metal Working and Material Science”) has been Indexed in Clarivate Analytics Services. We sincerely happy to announce that Journal “Obrabotka Metallov” (“Metal Working and Material Science”), ISSN 1994-6309 / E-ISSN 2541-819X is selected for coverage in Clarivate Analytics (formerly Thomson Reuters) products and services started from July 10, 2017. Beginning with No. 1 (74) 2017, this publication will be indexed and abstracted in: Emerging Sources Citation Index. Journal “Obrabotka Metallov” (“Metal Working & Material Science”) has entered into an electronic licensing relationship with EBSCO Publishing, the world's leading aggregator of full text journals, magazines and eBooks. The full text of JOURNAL can be found in the EBSCOhost™ databases. Novosibirsk State Technical University, Prospekt K. Marksa, 20, Novosibirsk, 630073, Russia Tel.: +7 (383) 346-17-75 http://journals.nstu.ru/obrabotka_metallov E-mail: metal_working@mail.ru; metal_working@corp.nstu.ru
OBRABOTKAMETALLOV Vol. 24 No. 2 2022 4 EDITORIAL COUNCIL EDITORIAL COUNCIL CHAIRMAN: Nikolai V. Pustovoy, D.Sc. (Engineering), Professor, President, Novosibirsk State Technical University, Novosibirsk, Russian Federation MEMBERS: The Federative Republic of Brazil: Alberto Moreira Jorge Junior, Dr.-Ing., Full Professor; Federal University of São Carlos, São Carlos The Federal Republic of Germany: Moniko Greif, Dr.-Ing., Professor, Hochschule RheinMain University of Applied Sciences, Russelsheim Florian Nürnberger, Dr.-Ing., Chief Engineer and Head of the Department “Technology of Materials”, Leibniz Universität Hannover, Garbsen; Thomas Hassel, Dr.-Ing., Head of Underwater Technology Center Hanover, Leibniz Universität Hannover, Garbsen The Spain: Andrey L. Chuvilin, Ph.D. (Physics and Mathematics), Ikerbasque Research Professor, Head of Electron Microscopy Laboratory “CIC nanoGUNE”, San Sebastian The Republic of Belarus: Fyodor I. Panteleenko, D.Sc. (Engineering), Professor, First Vice-Rector, Corresponding Member of National Academy of Sciences of Belarus, Belarusian National Technical University, Minsk The Ukraine: Sergiy V. Kovalevskyy, D.Sc. (Engineering), Professor, Vice Rector for Research and Academic Affairs, Donbass State Engineering Academy, Kramatorsk The Russian Federation: Vladimir G. Atapin, D.Sc. (Engineering), Professor, Novosibirsk State Technical University, Novosibirsk; Victor P. Balkov, Deputy general director, Research and Development Tooling Institute “VNIIINSTRUMENT”, Moscow; Vladimir A. Bataev, D.Sc. (Engineering), Professor, Novosibirsk State Technical University, Novosibirsk; Vladimir G. Burov, D.Sc. (Engineering), Professor, Novosibirsk State Technical University, Novosibirsk; Aleksandr N. Gerasenko, Director, Scientifi c and Production company “Mashservispribor”, Novosibirsk; Sergey V. Kirsanov, D.Sc. (Engineering), Professor, National Research Tomsk Polytechnic University, Tomsk; Aleksandr N. Korotkov, D.Sc. (Engineering), Professor, Kuzbass State Technical University, Kemerovo; Evgeniy A. Kudryashov, D.Sc. (Engineering), Professor, Southwest State University, Kursk; Dmitry V. Lobanov, D.Sc. (Engineering), Associate Professor, I.N. Ulianov Chuvash State University, Cheboksary; Aleksey V. Makarov, D.Sc. (Engineering), Corresponding Member of RAS, Head of division, Head of laboratory (Laboratory of Mechanical Properties) M.N. Miheev Institute of Metal Physics, Russian Academy of Sciences (Ural Branch), Yekaterinburg; Aleksandr G. Ovcharenko, D.Sc. (Engineering), Professor, Biysk Technological Institute, Biysk; Yuriy N. Saraev, D.Sc. (Engineering), Professor, Institute of Strength Physics and Materials Science, Russian Academy of Sciences (Siberian Branch), Tomsk; Alexander S. Yanyushkin, D.Sc. (Engineering), Professor, I.N. Ulianov Chuvash State University, Cheboksary
Vol. 24 No. 2 2022 5 CONTENTS OBRABOTKAMETALLOV TECHNOLOGY Timofeev S.P., Grinek A.V., Hurtasenko A.V., Boychuk I.P. Machining technology, digital modelling and shape control device for large parts..................................................................................................................... 6 Shlykov E.S. ,Ablyaz T.R.. Muratov K.R. Theoretical simulation of the process interelectrode space fl ushing during copy-piercing EDM of products made of polymer composite materials................................................ 25 Loginov Yu.N., Shimov G.V., Bushueva N.I. Deformations in the nonstationary stage of aluminum alloy rod extrusion process with a low elongation ratio.............................................................................................. 39 Sundukov S.K. Features of the superposition of ultrasonic vibrations in the welding process........................ 50 EQUIPMENT. INSTRUMENTS Podgornyj Yu.I., Martynova T.G., Skeeba V.Yu. On the issue of limiting the irregular motion of a technological machinewithin specifi ed limits.................................................................................................... 67 MATERIAL SCIENCE Burkov A.A., Kulik M.A., Belya A.V., Krutikova V.O. Electrospark deposition of chromium diboride powder on stainless steel AISI 304..................................................................................................................... 78 Gulyashinov P.A., Mishigdorzhiyn U.L., Ulakhanov N.S. Infl uence of boriding and aluminizing processes on the structure and properties of low-carbon steels........................................................................ 91 EDITORIALMATERIALS Guidelines for Writing a Scientifi c Paper ............................................................................................................ 102 Abstract requirements ......................................................................................................................................... 107 Rules for authors ................................................................................................................................................. 111 FOUNDERS MATERIALS 119 CONTENTS
OBRABOTKAMETALLOV Vol. 24 No. 2 2022 TECHNOLOGY Features of the superposition of ultrasonic vibrations in the welding process Sergey Sundukov * Moscow Automobile and Road Construction State Technical University (MADI), 64Leningradsky prospect, Moscow, 125319, Russian Federation https://orcid.org/0000-0003-4393-4471, sergey-lefmo@yandex.ru Obrabotka metallov - Metal Working and Material Science Journal homepage: http://journals.nstu.ru/obrabotka_metallov Obrabotka metallov (tekhnologiya, oborudovanie, instrumenty) = Metal Working and Material Science. 2022 vol. 24 no. 2 pp. 50–66 ISSN: 1994-6309 (print) / 2541-819X (online) DOI: 10.17212/1994-6309-2022-24.2-50-66 ART I CLE I NFO Article history: Received: 25 March 2022 Revised: 13 May 2022 Accepted: 15 May 2022 Available online: 15 June 2022 Keywords: Ultrasound Welding Vibrations Cavitation Microstructure Dendritic segregation Funding This research was funded by the Russian Science Foundation, grant number No. 21-79-00185, https://rscf.ru/project/21-79-00185/ Acknowledgements Research were partially conducted at core facility “Structure, mechanical and physical properties of materials” ABSTRACT Introduction. The main problem in obtaining welded joints is the nonuniform heating of the joint zone, which leads to differences in the structure and properties of the weld metal and the base metal. One of the ways to intensify the welding process is the use of ultrasonic vibrations. As a result of the analysis of methods for introducing ultrasonic vibrations into the melting zone, a method of superimposing vibrations on the elements to be welded was chosen for experimental studies. This method makes it possible to infl uence the welded elements throughout the entire welding cycle from the melt bath to complete crystallization of the metal. Methods. Experimental studies were carried out on plates made of carbon structural steel St3 (ASTM A568M, AISI 1017, DIN 17100) and aluminum deformable non-hardened alloy AMg4 (EN AW-5086, AW-AL Mg4, 5086). As a source of oscillations, a rod magnetostrictive oscillatory system was used, the end of which was rigidly fi xed on one of the welded plates. To determine the places of application of the oscillation source and the welding zone, a calculation method is proposed based on the equality of the resonant frequencies of the used oscillatory system and the natural frequency of bending vibrations of the welding component. It is shown that the optimal places for the application of vibrations and welding will be the antinodes of oscillations, which have the maximum amplitude. Welds were obtained by the method of semiautomatic gas metal arc welding. Results and Discussion. Microstructural study of obtained samples showed a signifi cant decrease in the proportion of dendritic segregation. The changes in the structure are the result of the effects that occur in the liquid melt when ultrasonic vibrations are introduced. The main effects are sound pressure, cavitation and acoustical streaming. The structure change mechanism consists in the dispersion of growing dendrites and crystallization nuclei under the action of shock waves and cumulative jets that occur when cavitation bubbles collapse. The formed fragments of dendrites are new crystallization nuclei that propagate through the melt pool under the action of acoustic currents. Then the process is repeated. The resulting effects affect the kinetics of the crystallization process – the degree of supercooling increases, the number of crystallization nuclei formed per unit time increases, and the rate of its growth decreases. Changes in the structure of the weld metal lead to an increase in the quality of the welded joint, which reduces welding deformations, increases the tensile strength and signifi cantly increases ductility. For citation: Sundukov S.K. Features of the superposition of ultrasonic vibrations in the welding process. Obrabotka metallov (tekhnologiya, oborudovanie, instrumenty) = Metal Working and Material Science, 2022, vol. 24, no. 2, pp. 50–66. DOI: 10.17212/1994-6309-2022-24.250-66. (In Russian). ______ * Corresponding author Sundukov Sergey K., Ph.D. (Engineering), Associate Professor Moscow Automobile and Road Construction State Technical University (MADI) 64 Leningradsky prospect, 125319, Moscow, Russian Federation Tel.: 8 (926) 369-19-70, e-mail: sergey-lefmo@yandex.ru Introduction Welding is a key method to produce permanent joints in various machine-engineering fi elds. Creating stable bonds between atoms or molecules of surfaces to be joined using heating or surface plastic deformation ensures a high-quality joint of both homogeneous and heterogeneous metals and alloys and its joints with non-metallic materials [1]. Fusion welding dominates today among the existing welding types. The primary issue of this welding type is irregular heating of parts to be connected [2]. Due to the crystallization of molten and mixed base
OBRABOTKAMETALLOV TECHNOLOGY Vol. 24 No. 2 2022 and fi ller metals, the weld has a cast structure. A partial melting zone of the base metal is located near the fusion boundary, which is followed by a heat-affected zone characterized by a structure changed by temperature taking place with increasing the distance from the center of the welding zone [3]. Due to structural differences, the transitions between the considered zones are accompanied by changes in mechanical properties, which is especially pronounced in the transition over the fusion boundary, which is a weak point of the weld joint. Along with structural non-uniformity, welding issues also include residual stresses, welding deformations and weld porosity [4–7]. Various methods are used today to avoid these drawbacks that can be classifi ed into those applied during and after welding. The methods used during welding include strain balancing by means of a reasonable sequence of weld passing, creating initial distortions and rigid fi xing the elements to be welded. The methods used after welding include weld heat treatment, mechanical leveling of structures, thermal leveling, and surface plastic deformation (SPD) [8]. Another effi cient method to minimize the consequences of these drawbacks is the vibration treatment of metal in a molten state [9–10]. This method was proposed in 1950 as applicable to crystallizing metal by Chernov in order to improve the ingot structure after casting. Vibrations increase the homogeneity of ingots by dispersing the growing dendrites [11–12]. To ensure effi cient action on the structural formation of the weld, the crystallization of which is several times faster, it is reasonable to use high-frequency vibrations of ultrasonic frequency, which will make it possible to exert a signifi cant impact in a limited time interval. There are the following methods of using ultrasonic vibrations during welding: – applying vibrations to the electrode [13]; – applying vibrations to the non-consumable electrode [14]; – transferring vibrations to the gas burner body [15]; – transferring vibrations to non-weldable structural elements [16]; – using the arch as a source of ultrasonic radiation [17]. The studies considering these methods show a positive effect on the welding process and weld structure. In particular, depending on the method, the depth of penetration of the base metal can be increased, the porosity of the weld can be decreased, the conditions for the transfer of molten metal drops from the electrode to the workpiece can be improved, the microstructure of the weld can be refi ned, the proportion of dendritic segregation in the weld metal can be decreased, and the mechanical properties can be improved [18–22]. More detailed results can be found in overview papers on this subject [23, 24]. The effect of ultrasonic machining on the structure of the crystallizing weld metal has a clear positive effect. Nevertheless, these technologies are not widely used in welding processes, for example, as compared to ultrasonic SPD that is applied for post-treatment of welds [25-27]. This can be explained by a number of reasons: 1. Additional equipment is required: ultrasonic generator and vibration system. 2. Complex organization of the process related to coordination between welding conditions and acoustic process parameters of ultrasonic machining. 3. It is preferable to use more complex and larger magnetostrictive transducers requiring forced cooling since piezo-ceramic ones lose its effi ciency at high temperatures. 4. Increased power consumption for the welding process. Despite the diffi culties, opportunities of using ultrasonic vibrations make the development of these technologies attractive. This paper describes the research of applying ultrasonic vibrations on the elements to be welded and selecting the vibration application spot and the welding zone.
OBRABOTKAMETALLOV Vol. 24 No. 2 2022 TECHNOLOGY Methods Materials The materials widely used in welding have been selected for the study: carbon structural steel of commercial quality St3 and deformable non-heat-treatable aluminum and magnesium alloy AMg4. Welding plates were cut from a sheet workpiece 4 mm thick. Filler materials were fi ller wires suitable for welding the selected materials: Sv08Kh2GS wire for St3, ER5356 wire for AMg4. The wire diameter was 0.8 mm. The chemical compositions of the materials and wires are given in Tables 1 and 2. Ta b l e 1 Chemical composition of AMg4 alloy and ER5356 wire Alloy Element Fe Si Mn Cr Ti Cu Be Mg Zn Al % <0.4 <0.4 0.5–0.8 0.05–0.25 0.02–0.1 <0.05 0.0002–0.005 3.8–4.8 <0.2 the rest Filler wire % <0.1 <0.25 0.55 0.12 0.12 – – 5.0 – the rest Ta b l e 2 Chemical composition of steel St3 and wire Sv08Kh2GS Steel Element C Si Mn Ni Cr Cu S P As Fe % 0.14–0.22 0.15–0.3 0.4–0.65 <0.3 <0.3 <0.3 <0.05 <0.04 <0.08 the rest Filler wire % <0.1 0.6–0.85 1.4–1.7 <0.025 1.8–2.2 <0.025 <0.015 <0.013 – the rest Design of experiment and equipment The research was carried out in two stages in accordance with the design given in Fig. 1. The fi rst stage included welding the seam onto plates 4 mm thick and 30 mm wide followed by detecting changes in the structure of the welding zone. The second stage included welding of two plates and tension testing of the joint. The plate length was measured based on the distribution of vibration along it (as described below). To excite vibrations in the weld zone, an ultrasonic rod vibration system comprising a magnetostrictive transducer 6 and waveguide 5 made of titanium alloy was attached to a plate 1 via threaded connection 4. The waveguide diameter was equal to the plate width (30 mm). A UZG 2.0/22 ultrasonic generator with frequency and amplitude adjustment was used to supply power to the vibration system. These functions maintain stable vibrations in conditions of increasing temperature and changing the volume of the plate that occur during welding. Before welding, the plate surface was cleaned using a disc metallic brush and degreased. This was followed by ultrasonic machining and welding. Ultrasound was turned off when the weld joint cooled down to 100 C, so that all phase transformations occurred under the infl uence of vibrations. The weld was obtained by semi-automatic welding in shielding gases. Table 3 lists equipment and welding modes for St3 and AMg4.
OBRABOTKAMETALLOV TECHNOLOGY Vol. 24 No. 2 2022 Ta b l e 3 Equipment and welding modes Welding conditions Material St3 AMg4 Welding unit type MIG 235 MIG 215AL PULSE Welding current Iweld, A 60 125 Electrical polarity reversed reversed Voltage Uweld, V 28 22 Wire speed, Vwire m/min 1.9 15.2 Shielding gas CO2 Ar Shielding gas fl ow, l/min 8 17.5 Welding time, s 12 2 Fig. 1. Design of experiment: 1 – plate; 2 – weld; 3 – welding torch; 4 – bolt; 5 – waveguide; 6 – magnetostrictive transducer One of the most important aspects in welding with superimposed vibrations is to determine the place of its application to the plate and also the place of welding, where a stable ultrasonic effect is ensured. Determination of vibration application and weld passing spots when using AMg4 According to the applied design of ultrasonic vibration application, the vibration system is a source of bending vibrations in the case of its normally oriented location. In terms of process application, the optimal spot to apply vibrations will be one of the antinodes of the own vibrations of the plate. In this case, the resonance frequencies of the vibration system and the plate should be aligned. Since a commercially available ultrasonic vibration system having a certain resonance frequency f is used in this research, the calculation is done based on the need to ensure the equal resonance frequency fp of the plate.
OBRABOTKAMETALLOV Vol. 24 No. 2 2022 TECHNOLOGY The amplitude frequency response of the vibration system (Fig. 2) was taken at the end of the waveguide using a dial indicator. The resonance frequency is f = 21,800 Hz. Fig. 2. Amplitude-frequency characteristic PMS-2.0-22 A differential equation for the bending vibrations of the plate [28]: 2 2 4 2 0 0 4 2 2 2 0. m m m d d dx c c dx Where m is the vibration amplitude, 0 is the angular frequency of self-induced vibrations, x is the plate coordinate in the longitudinal direction, c is the propagation rate of longitudinal vibrations, is the cross-section inertia radius. / . I S Where I is the moment of inertia relative to the axis, S is the cross-section area. For the rectangular plate being used (30×4 mm): 3 / 0.0012. 12 bh bh If the condition of 2 2 0.05 l is met (0.0006 for the case under consideration), the rotary inertia can be neglected, and the equation of steady-state vibrations looks as follows: 2 4 0 4 2 0. m m d dx c This equation was solved by Krylov (1936): 1 2 3 4 , m x x x x C A C B C C C D where C1, C2, C3, C4 are the constants of integration that are found from boundary conditions: ( ) cos( ) / 2, x A ch kx kx ( ) sin( ) / 2, x B sh kx kx
OBRABOTKAMETALLOV TECHNOLOGY Vol. 24 No. 2 2022 ( ) cos( ) / 2, x C ch kx kx ( ) sin( ) / 2. x D sh kx kx To fi nd the constants, the following equations for derivatives should be used: 1 2 3 4 ( ), m x x x x k C D C A C B C C 2 1 2 3 4 ( ), m x x x x k C C C D C A C B 3 1 2 3 4 ( ). m x x x x k C B C C C D C A . The factor k is the wave multiplier depending on the material properties and vibration frequency: 2 4 , m k EI (1) where E is the Young modulus of the waveguide material (E=71 GPa for AMg4), m is the waveguide weight per unit of length (m = b · h· l · ρ = 0.03×0.004×1×2,670 = 0.320 kg/m for the case under consideration), angular frequency 2 p f , where p f is the resonance frequency of the self-induced vibrations of the plate. To determine the nature of vibration propagation depending on the plate fi xing conditions, let’s use the algorithm described by Bulgakov (1954). Boundary conditions for this algorithm are written in expanded form, which results in heterogeneous equations relative to the constants. To avoid the constants being zero, the determinant made for the equation system coeffi cients should be equal to zero. The calculation scheme is given in Fig. 3. The ultrasonic vibration application spot x should be selected, so that in the welding zone lweld there is a maximum amplitude of vibrations. For these fi xing conditions (free plate ends on both sides): for lsc = 0 and for lsc = lp: 0 m and 0 m , constants C3 = 0 and C4 = 0. Substituting these values into the solution of the vibration equation, the following frequency equation is resulted: ï ï ( ) cos( ) 1 ch kl kl The roots of the equation are: ï / 2, kl n (2) where n =1, 2, 3... Let’s express the coeffi cient k from equation (2) and equate it to equation (1): 2 4 ï / 2 . n m l EI Taking into account that 2 p f one can obtain an equation to fi nd the plate length depending on the vibration frequency (3): ï 2 4 / 2 . (2 ) ï n l m f EI (3) Fig. 3. Scheme for calculating bending vibrations: lp is the length of the plate, x is the place of ultrasonic vibrations application, lweld is the place where the weld is applied
OBRABOTKAMETALLOV Vol. 24 No. 2 2022 TECHNOLOGY Let’s calculate from the condition fp ≈ 21,800 Hz for various n and include the calculation values in Table 4. Ta b l e 4 Dependence of the resonant length of the plate on n at frequency of 21,800 Hz n 1 2 3 4 5 6 7 8 lp 0.031 0.052 0.072 0.093 0.114 0.134 0.155 0.176 k 151.6 Thus, the size of the plate, which provides vibrations at a resonant frequency of 21,800 Hz, corresponds to the 7th vibration mode and is 155 mm (this size was chosen for research). The coeffi cient k allows associating the frequency and propagation rate of bending vibrations CB: B , k C where B 2 904.5 C f c . Where E c is the rod rate of longitudinal vibrations (5,157 m/s for AMg4). If the rate and frequency are known, the bending wavelength can be found (4): B B p 41.3 mm C f . (4) In this manner, when ultrasonic vibrations are communicated, the plate length fi ts / 3.75 p B l bending waves. Taking into account that plate ends are free and can’t have zero vibrations, let’s build the diagram of vibration distributions over the plate (Fig. 4). Vibration nodes where the amplitude is equal to zero are located at the half-wave length with a shift by 1/8 of the wave length x1 = (B/2) I + B/8 (I = 0, 1, 2…), and antinodes with the maximum amplitude are located at 1/4 of the wave length from nodes x2 = (B/2) I + B/8 – B/4. The weld locations and ultrasonic vibration application points should be selected according to distance x2. Fig. 4. Distribution of vibrations over the welded plate
OBRABOTKAMETALLOV TECHNOLOGY Vol. 24 No. 2 2022 In this manner, for experimental studies, the vibration system end is fi xed at the distance of 7/8B, which corresponds to 36 mm, and the weld is located at the distance of 15/8B, which corresponds to 77.5 mm and is in the middle of the plate. As a result of similar calculations for steel St3, the plate with the length of 130 mm was selected, the vibration system fi xing point is 30 mm, and the weld location is 65 mm. The same dimensions were used during the second stage of the research, e.g., welding of two plates and defi ning the mechanical properties of the joints. The plate was cut in the middle, and then the plate parts were welded on the ends, so that 0.5 mm remained between it, with no edge preparation. In these conditions, the vibrations are transmitted to the second plate via welding points along ends and the distribution nature of the vibrations remains the same. The welding time was 3.5 sec for AMg4 and 16 sec for St3. Defi ning the structure and properties After welding the seam, specimens were cut out of plates for further examination of the surface. Specimens were selected so that the surface under examination is the cross-section in the middle of the weld. The micro- and sub-microstructure were studied. The specimens were prepared for analysis by pouring with protacryl, with microsections obtained after its cooling. The microstructure was examined using a METAM RV-22 metallographic microscope (AO LOMO, Saint Petersburg). After welding of two plates, the obtained joints were examined for defl ection caused by metal shrinkage. The joints corresponding in size to XII specimens under GOST 6996-66 were tested for tensile. A contour measuring station model 220 (AO Proton, Zelenograd) that is intended to measure geometric parameters of products of various shapes was used to measure defl ection. The operation of the device is based on the principle of feeling the irregularities of the measured surface using a feeler gage with an inductive sensor by moving the gage along the measured surface and then converting the resulting mechanical vibrations of the gage into a digital signal. Further, the necessary measurements are carried out in the program for processing the surface profi le. Tensile tests were carried out using a UTS-110M-50-0U tensile machine designed to measure the specifi ed value of the force during mechanical tests in the tension or compression mode of the structural materials specimens. Results and Discussion Effects of ultrasonic parameters on the nature of vibrations To evaluate the nature of vibrations during the experiment, the distribution of vibrations over the plate was imaged by applying a sodium hydrocarbonate powder over it (Fig. 5). When ultrasound is turned on, the powder is distributed over the plate in accordance with the vibration amplitude: it is displaced from the antinode zone and accumulated in the nodes. The resonance frequency was f = 21,800 Hz, which is 700 Hz less than the calculated one (3.2 % error). This is explained by mechanical losses when converting longitudinal vibrations of the source into bending vibrations of the plate and by the fact that the calculation was done in case of pointed application of vibrations, and the surfaces contact area in the studies equals the area of the waveguide end having the diameter of 30 mm. Since the frequency is constant, the processing conditions were defi ned by the change in the vibration amplitude. Three types of ultrasonic vibrations were compared: low-amplitude (m = 3–4 μm); intermediate (m = = 9–10 μm) and high-amplitude (m = 13–15 μm). The analysis of vibration distribution shows that the zones of maxima and minima of the amplitude are irregular in shape, which is related to a complex nature of the plate vibration: apart from bending vibrations
OBRABOTKAMETALLOV Vol. 24 No. 2 2022 TECHNOLOGY Fig. 5. Visualization of oscillation propagation along a plate made of AMg4 alloy in the longitudinal direction (for which the calculation was done), there are transverse bending vibrations that lead to certain rounding of vibration nodes, which is especially prominent in the nodes most distant from the fi xing point in low-amplitude and intermediate conditions. There are also longitudinal vibrations transmitted from the waveguide, the proportion of which increases with increasing power. In high-amplitude conditions, longitudinal vibrations dominate over the others, which, in combination with high amplitude, leads to the powder completely sliding from the plate (the fi rst moments of that process are shown in Fig. 6). Bending vibrations are predominant at 200 W and 350 W, and antinode zones and vibrations nodes are well defi ned on the plates. In low-amplitude conditions, vibrations nodes and antinode zones are less pronounced, since the powder is less displaced from the vibration zone and, accordingly, the node zones are much wider. When measuring the half-wave, its length varies from 19.7 to 21.1 mm, and the vibrations are irregular along the plate width. For example, at the section with the half-wave of 19.7 mm, the antinode is located in the central zone on the one side and along the ends on the other side. In intermediate conditions, the pattern considerably corresponds to the estimate indicators. The vibration zones are very prominent; the distance between vibration nodes is almost the same along the plate length and is 20.8 mm. If the calculation is done using equation (4) for frequency 21,100 Hz, the half-wave length is λB/2 = 21 mm, the error is 1%. The difference of 200 Hz does not play a key role in selecting the welding location since the antinode z one width in this case is more than the nodes and the shift of 0.2 mm does not affect the variation nature in the welding zone. In high-amplitude conditions, vibrations occur along the entire plate length due to the predominating radial component. There are zones of maximum and minimum vibrations, which, by location, correlate with other modes. The plate picture (Fig. 5) shows that the powder on the closer end slides down from the maximum amplitude zone faster than from the minimum zone. An optimal welding location (the bold line in Fig. 5) for low-amplitude and intermediate conditions is 78.5 mm from the left plate end, which is 1 mm longer than the calculated length lw = 77.5 mm. In highamplitude conditions, welding can be done in any place.
OBRABOTKAMETALLOV TECHNOLOGY Vol. 24 No. 2 2022 Further studies were done in intermediate conditions, since compared with others, it makes it possible to obtain a stable distribution of vibrations over the plate. The results of preliminary experiments showed a better effect on the weld structure. Low-amplitude conditions show almost no effect, and high-amplitude conditions may cause heavy splashing of dropping metal (ultrasonic spatter) and a signifi cant number of pores. Microstructural changes The application of ultrasonic vibrations during welding leads to changes in the microstructure of the weld (Figs. 6 and 7). a b Fig. 6. Microstructures of the weld metal of steel St3: a – without vibrations applied; b – with vibrations applied а b Fig. 7. Microstructures of the fusion zone of the AMg4 alloy: a – without vibrations applied; b – with vibrations applied The vibrations cause a decreased proportion of dendritic segregation for steel St3 and a decreased height of the dendrite zone for alloy AMg4. A different nature of effects is associated with a longer steel crystallization time as compared to aluminum, which allows vibrations to have a greater effect. Microstructure changes are caused by effects occurring in the molten metal when ultrasonic vibrations are applied to it. Phenomena having a signifi cant effect on crystallization kinetics include sound pressure, cavitation and acoustic streams.
OBRABOTKAMETALLOV Vol. 24 No. 2 2022 TECHNOLOGY Introducing vibrations into the system increases its free energy that characterizes the conversion of molten components from a liquid to a solid phase [28]. The total change in Gibbs energy ∆Gtot will be as follows: tot us, G S V G E where S is the total surface area of crystals, σ is the surface tension between the liquid metal and crystal, V is the nucleus volume, ΔG is the difference of Gibbs energy of metal in liquid and solid states, Eus is the energy of introduced ultrasonic vibrations. Eus means the kinetic energy imparted to the formed crystallization nuclei: 2 2 us (2 ) , 2 m m f E where m is the nuclei weight, f is the vibration frequency, and m is the vibration amplitude. As a result of a change in the energy balance, the work required for the formation of a stable nucleus increases, which leads to a decrease in the crystallization start temperature. The highest effect in the structural formation when applying vibrations is caused by cavitation, which includes the formation, growth, and collapse of bubbles, which is accompanied by an increase in pressure and temperature, the instantaneous values of which can reach several hundred MPa and several thousand degrees [29–37]. Shock waves and cumulative jets associated with that process disperse the formed nuclei. First of all, dendrites are fragmentized since they are the fi rst to start growing from the fusion boundary, which in this case is a surface that emits vibrations. Dispersed particles of dendrites will be new crystallization nuclei that will grow and then will be broken by cavitation. Acoustic streams occurring in the processed melt improve heat and weight transfer in the melt before crystallization begins. After the fragmentation of dendrites, the fl ows distribute new nuclei over the molten pool, some of which go into the active cavitation zone and are dispersed once again. Since all molten components in both liquid and solid phases move due to vibrations, acoustic streams, shock waves and cumulative jets, more complex conditions are created for the attachment of liquid phase atoms to nuclei. The weld crystallization process when applying vibrations can be schematically shown as follows (Fig. 8). Zone I: when the metal crystallizing without vibrations is cooled to the liquidus point Tliq, the fi rst dendrites start to grow. When applying vibrations, nuclei formation is not yet started due to an increase in the Gibbs energy. Cavitation and acoustic streams contribute to the uniform mixing of molten components in the liquid phase. Fig. 8. Crystallization scheme of the weld
OBRABOTKAMETALLOV TECHNOLOGY Vol. 24 No. 2 2022 Zone II: during further cooling without vibrations, dendrite growth continues and new dendrites are formed. Dendrites start forming along with vibrations. Due to specifi c features of the distribution of cavitation bubbles, last ones are primarily accumulated in places of small irregularities that are the zone of dendrites in this case. Zone III: dendrite growth continues without vibrations and nuclei start forming in the remaining volume of the molten pool. The collapse of cavitation bubbles results in the dispersion of dendrites, the fragments of which are carried by acoustic streams deep into the weld. These pieces are crystallization nuclei and at the same time the areas attracting cavitation bubbles. Zone IV: the growth of dendrites continues without vibrations; wherein non-dendritic nuclei grow and new dendrites are formed. Along with ultrasound, the cavitation activity goes down and the number of bubbles is reduced due to cooling and the associated increase in the molten metal viscosity. Dendrites and its fragments start to grow and new nuclei are formed. Bubble collapse continues having a dispersing effect. When the growth of dendrites and remaining nuclei continues without vibrations, the fi nal weld structure is formed. The effect of ultrasonic effects ceases when the melt reaches a high viscosity, and the dendrites and nuclei formed by this moment grow until complete solidifi cation. Thus, the introduction of ultrasonic vibrations reduces the crystallization start temperature, increases the number of nuclei formed, and decreases its growth rate. This results in a fi ne structure with a signifi cantly reduced proportion of dendritic segregation. Determination of weld joint properties The obtained changes in the microstructure lead to an increased weld joint quality. A weld joint obtained by applying vibrations and having a regular structure with decreased dendritic segregation has a lower shrinkage during cool-down, which reduces weld deformations. This causes a decreased defl ection of the joint (Fig. 9). a b Fig. 9. Geometrical parameters of a welded joint made of AMg4 alloy: a – without vibrations applied; b – with vibrations applied In the case of identical geometric parameters of the weld bead, as a result of shrinkage decreased for the St3 joint from 145ʹ to 21ʹ and from 124ʹ to 10ʹ for AMg4 joint. It means that elements welded with vibrations remain parallel, and without vibrations, the slope of one plate relative to the other will be ≈2.5 mm by 100 mm of length, which is especially critical for elongated welded structures. Tension tests of joints (Fig. 10) also show that the weld characteristics are improved. Applying vibrations results in a 5–10 % increased ultimate strength. The ultrasound had a higher effect on the plasticity of the weld metal, the elongation of which increases by 13–22 %.
OBRABOTKAMETALLOV Vol. 24 No. 2 2022 TECHNOLOGY Fig. 10. Changes in tensile strength and elongation of specimens during the tensile testing Conclusions As a result of the theoretical and experimental studies, the following conclusions can be drawn: 1. An optimal place for ultrasonic vibration application and welding the seam is one of the maximum amplitudes of self-induced bending vibrations of the welding plate. The weld joint length is selected based on the equality of the resonance frequency of self-induced vibrations and the frequency of the vibration source; 2. The nature of plate vibrations depends on the processing conditions defi ned by the vibration amplitude. The highest proportion of bending vibrations is achieved in intermediate conditions of vibrations; 3. The application of ultrasonic vibrations leads to changes in the weld metal microstructure, which is expressed in a signifi cantly reduced proportion of dendritic segregation; 4. The introduction of ultrasonic vibrations reduces the crystallization start temperature, increases the number of nuclei formed, and decreases its growth rate; 5. The mechanism of changing the microstructure is the dispersion of dendrites and crystallization nuclei when cavitation bubbles collapse. Dendrite fragments are the new nuclei of crystallization that propagate through the processed volume due to acoustic streams. Then the process is repeated; 6. In a welded joint, the formation of which was accompanied by the applying of ultrasonic vibrations, welding deformations decrease, and ultimate strength and plasticity increase. References 1. Wang H., Cen S. Research on microstructure and mechanical properties of CMT and MIG welded joints of A6N01 aluminum alloy. Journal of Physics: Conference Series, 2022, vol. 2185, iss. 1, p. 012051. DOI: 10.1088/1742-6596/2185/1/012051. 2. Sundukov S.K., Nigmetzyanov R.I., Fatyukhin D.S. Structure of the weld formed during the application of ultrasonic vibrations. Russian Metallurgy (Metally), 2021, vol. 13, pp. 1667–1672. DOI: 10.1134/ S0036029521130309. 3. Prikhodko V.M., Karelina M., Sundukov S., SukhodolyaA., Moiseev V. Improvement of operational properties of parts permanent joints with ultrasound technologies use. Journal of Physics: Conference Series, 2019, vol. 1353, iss. 1, p. 012081. DOI: 10.1088/1742-6596/1353/1/012081. 4. Babchenko N.V., Seliverstova O.V., Sundukov S.K., Fatyuhin D.S. Povyshenie ekspluatatsionnykh svoistv svarnykh shvov ul’trazvukovymi metodami [Improving operational properties weld the ultrasonic method]. Vestnik
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