Investigation of the effect of ultrasound during explosion welding on the formation of the structure and properties of the aluminum-steel composite

The paper presents the results of experimental studies of the effect of ultrasonic vibrations during explosion welding on the formation of the structure and properties of the connection of steel with aluminum. It is shown that the introduction of ultrasound during explosive loading has a significant effect on the structure and properties of the aluminum-steel compound. The chemical composition of the areas of the melted metal is considered. The results of the tensile strength of aluminum+steel composite layers are presented. It has been experimentally established that the introduction of additional ultrasound energy in the process of explosive loading of an aluminum-steel compound contributes to a significant reduction in the amount of molten metal at the junction boundary throughout the studied range.


Introduction
Steel and aluminum have a very limited range of explosion weldability, which most researchers associate with the difference in their mechanical and thermophysical properties [1][2][3].The main difficulty in welding steel with aluminum by explosion is associated with the appearance of intermetallic phases at the junction boundary [2][3][4][5][6][7][8][9][10][11].The structure of the melted sections and the amount of intermetallides in the melts depend on the welding modes [2].In low-intensity welding conditions, the areas of the melted metal are a mechanical mixture of a supersaturated solid solution of iron in aluminum with inclusions of FeAl3 intermetallic [2,3].The intensification of welding modes leads to an increase in the volume of molten metal, an increase in the lifetime of the metal in the liquid phase and the number of intermetallides in the melts, which ultimately leads to the formation of melts consisting entirely of Fe2Al5 and FeAl3 intermetallides [2,11].
In order to avoid the unfavorable structure of the areas of the melted metal, as well as to increase the heat resistance of the joints between the main materials, intermediate layers of metals are introduced that do not form brittle intermetallides with steel and aluminum.The disadvantages of this method include: the need to use expensive non-ferrous metals as a layer (silver, chromium [10], niobium [9], etc.); the need to apply a layer to the surface of at least one of the plates, or the complexity of assembly with simultaneous welding of all layers; as well as the need to choose explosion welding modes that guarantee the integrity of the interlayer during welding [9-10, 12, 15].
Difficulties in welding steel with aluminum arise with large thicknesses of welded parts.It is shown in [7] that an increase in the thickness of the cast aluminum when it is welded with steel leads to a narrowing of the weldability area and a shift of the optimal collision velocity towards lower values.
According to the authors, this is due to the effect of shear stresses, which manifests itself in the form of a double inflection on the shear deformation plot in the aluminum layer.
The technology of welding plates with a profiled contact surface according to the "dovetail" type has proven itself well for welding thick-sheet rolled products [5][6][7].Surface profiling leads to minimization of the volume of intermetallic interlayers at the junction boundary [5], and the compounds obtained in this way are used for operation at elevated temperatures [14].At the same time, the disadvantages of this technology include the need for additional technological operations for applying grooves on the surface of the steel part, as well as the pronounced structural heterogeneity of the joints on the protrusions and depressions of the grooves [13].
Previously, the team of authors in works [16][17][18][19][20] developed a technique for obtaining high-quality composites of a new type based on high-speed collision of metal plates under conditions of loading by a sliding detonation wave with simultaneous impact on the colliding system of ultrasonic waves.
The results of the conducted studies show the feasibility of using ultrasound during explosive loading, which is expressed in improving the quality of the resulting compounds and expanding the area of weldability of the metals being joined.The change in the characteristics of the junction zone indicates that the energy dissipation after the collision occurs by different mechanisms, therefore, the energy balance of the junction formation process changes.Thus, this created the prerequisites for conducting research on the formation of composite materials under explosion welding conditions with simultaneous exposure to high-frequency vibrations on colliding elements.

Materials and methods
The following basic materials were used for the research: technically pure A5 grade aluminum and structural steel of the VSt3sp grade.An aluminum plate with thicknesses of 3.7 and 3.9 mm acted as a throwing plate, and a steel plate with a thickness of 4 mm served as a stationary plate.Explosion welding was performed according to a plane-parallel scheme.Since the throwing scheme can have a significant impact on the structure and properties of compounds during explosion welding of dissimilar materials, additional experiments were conducted to study not only the direct (aluminum is thrown) loading scheme, but also the reverse (steel is thrown) loading scheme.In each series of experiments, several pairs of compounds were examined, consisting of a sample welded with ultrasound exposure and a control sample welded by explosion without ultrasound exposure.The parameters of high-speed collision of welded metals were calculated using the EW Calc application software package.Explosion welding with the influence of ultrasound was performed according to a scheme with counter-directional propagation of ultrasonic vibrations.
Ultrasonic vibrations in a fixed plate were created using piezoceramic transducers with resonant frequencies in the range from 16 to 24 kHz, and an ultrasonic generator UZG4-2, with a maximum output power of up to 2 kW.The vibrations of the transducer were brought to the stationary plate using a waveguide from the end of the detonation, so that the effect of ultrasound was maintained throughout the welding process.The supply of ultrasonic vibrations to the fixed plate was started 10-30 seconds before the start of welding.The mounting scheme of the waveguide to a fixed plate is shown in Fig. 1,  b.A fixed plate (3) was attached to a waveguide (7), and installed on a chipboard base (1).Gaps (2) were installed on the base, on which the throwing plate (4) and the explosive charge (5) were located.In order to reduce the influence of edge effects associated with lateral expansion the detonation products and the pressure pulse drop, the dimensions of the charge and the thrown plate from all sides exceeded the dimensions of the fixed plate.Assembly for welding of control plates welded without ultrasound was carried out in the same way.
The amplitude of the standing wave was measured using a Vibro Flex Neo laser vibrometer (VFC-I-110).The measurements showed that at a given ultrasound power, the RMS value of the amplitude near the antinodes was 6 ± 1 microns.At the opposite end of the plate from the fixing point of the transducer, the amplitude decreased slightly and amounted to 5 ± 1 microns.
To conduct research, samples were cut out of various parts of explosion-welded plates for the manufacture of micro-grinds and mechanical tests.The properties and quality of explosion-welded composites were evaluated by changes in the strength of the layers on separation, the amount of molten metal and wave parameters at the metal junction boundary of the near-seam zone.

Results and discussion
The results of metallographic studies have shown that ultrasound has a significant effect on the formation of the structure and properties of steel-aluminum composites during explosive loading.
In the entire studied range of values of the energy expended on plastic deformation, all the obtained compounds had a wave-free boundary with a layer of molten metal along the contact surface.At the same time, according to [1], the wave-free boundary is typical for aluminum-steel compounds, which is explained by the large difference in strength properties between the materials.During explosion welding of control samples (without ultrasound) at energy values W2 = 0.66 MJ/m 2 , the joint zone had an uneven border, the depressions of which were filled with solid fused sections on the steel side (Fig. 2, a).Steel particles of various sizes were present inside the fused sections.When welding by explosion with the influence of ultrasound with the same energy input, the joint zone was significantly different and was almost straight, the layer of the melted metal had the same thickness along the entire length of the joint (Fig. 2, b).
The chemical composition of the detected layer was similar both for the control sample and for the one welded by explosion with ultrasound exposure.Thus, aluminum as the most fusible material of the pair prevailed in the composition, and the iron content, as a rule, did not exceed 10 atomic%.The structure of the molten metal layer was a finely dispersed mechanical mixture of a supersaturated solid solution of iron in aluminum with inclusions of FeAl3 intermetallic [3].
With an increase in the energy of plastic deformation to W2 = 0.9 MJ/m 2 , the effect of ultrasound on the structure and properties of the compound decreases somewhat.In both samples, the junction zone is an almost flat interface with a similar thickness and structure of the molten metal layer.An increase in the W2 energy during welding was accompanied by a slight increase in the iron content in the molten metal zone to 15-20 atomic%.
With an increase in energy costs for plastic deformation to W2 = 1.1 MJ/m 2 , the junction zone of the control sample was an uneven boundary, and in some areas the formation of a wave profile with a length of about 450 microns and a span of about 35 microns was observed.When welding by explosion with the influence of ultrasound, the junction zone had an almost rectilinear boundary with a much smaller thickness of the molten metal layer.The structure of the molten metal layer in both samples was similar, and, as in the previous pairs of samples, it was a mechanical mixture of FeAl3 intermetallic particles with an aluminum matrix.The reason for the appearance of intermetallic particles, apparently, was the interaction of molten aluminum with iron particles carried out by the cumulative flow.This is supported by the shape of the particles, as well as the presence of unreacted iron sites inside intermetallic particles.A thin layer of pure intermetallic compound was found at the boundary of the molten metal with steel, the iron content of which, according to chemical analysis, is 25 atomic%, which corresponds to the FeAl3 compound.Transverse shrinkage cracks were found inside this layer.
With a further increase in energy input to W2 = 1.2 MJ/m 2 in both samples, the junction zone is an uneven interface, and the formation of a wave profile was also observed in some areas.When welding by explosion with the influence of ultrasound, the thickness of the molten metal layer is significantly less.Separately, it should be noted that at the above values of the energy expended on plastic deformation, the chemical composition of the areas of the melted metal in the studied and control samples differs.Thus, in the studied samples, the iron content in the area of the melted metal was still in the range of 15-20 atomic%, while in the control samples, the chemical composition of the area of the melted metal is uneven, the iron content reached 35 atomic%.
A similar pattern was observed at values W2 = 1.4 MJ/m 2 and W2= 1.6 MJ/m 2 with the only difference that in the control samples the chemical composition of the area of the melted metal is uneven, the iron content increased to 45 atomic %.The results of the study of the effect of the explosive loading scheme showed that the effect of ultrasound on the structure and properties of the compound is somewhat reduced.In both samples, the junction zone is an unstable wave formation.An increase in the W2 energy during welding was accompanied by a slight increase in the iron content in the molten metal zone to 15-20 atomic%.
To numerically estimate the amount of molten metal, the specific area of the molten metal in relation to the joint length was measured (which is equivalent to the average thickness of the molten metal interlayer).The results showed that when welding by explosion with the influence of ultrasonic vibrations, the amount of molten metal in the joint zone in the entire energy range under study turned out to be significantly less than with the usual method of explosion welding.Thus, at an energy W2 = 1.1 MJ/m 2 , specific area in the control samples was 20.4 mm 2 /mm, while in the studied samples obtained by explosion welding with ultrasound exposure -10.8 mm 2 /mm.
The results of the conducted mechanical tests showed that when welding by explosion with ultrasound, all the obtained compounds were equally strong, while when welding by explosion, only the samples obtained at values W2 = 1.6 MJ/m 2 did not have equal strength despite the presence of such a significant amount of molten metal at the boundary did not affect the strength of the separation of layers.This is probably due to the structure of the molten metal layer and the presence of a plastic aluminum matrix, which allows relaxing stresses during cooling of the melt without cracking.At the same time, in the aluminum-steel samples obtained by explosion welding with ultrasound exposure, the tear-off strength of the layers over the entire range of changes in the energy expended on plastic deformation is higher compared to control samples without ultrasound exposure.Thus, at W2 = 0.66 MJ/m 2 , the average value of the layer separation strength in aluminum-steel samples obtained by explosion welding with ultrasound exposure was about 104 MPa, while in control samples the strength value was significantly less -86 MPa.
The study of the influence of the frequency of ultrasonic vibrations on the mechanical properties of explosion-welded joints showed that with an increase in f, an increase in the separation strength of layers is first observed, reaching a maximum value in the frequency range of 20-22 kHz.A further increase in the frequency f of ultrasonic vibrations leads to a decrease in strength due to an increase in the volume of the melted metal in the joint zone.Previously conducted studies [17,18] have shown that due to the formation of undesirable melted metal at the boundary of the welded joint, electrical resistance increases sharply.

Conclusions
Ultrasound has a significant impact on the formation of the structure and properties of steel-aluminum composites during explosive loading.In the entire studied range of values of the energy expended on plastic deformation, all the obtained compounds had a wave-free boundary with a layer of molten metal along the contact surface.
When welding by explosion under the influence of ultrasonic vibrations, the amount of molten metal in the joint zone in the entire energy range under study turned out to be significantly less than with the usual method of explosion welding.
In the samples of aluminum with an article obtained by explosion welding with the influence of ultrasound, the tear strength of the layers over the entire range of changes in the energy expended on plastic deformation is higher compared to control samples without the influence of ultrasound.
It has been experimentally established that the introduction of additional ultrasound energy during explosive loading makes it possible to improve the quality of the connection of aluminum with steel, manifested in changes in the structure of the connection zone, an increase in strength, and a significant decrease in the amount of molten metal compared with explosion welding without the use of ultrasound.

a b Figure 2 .
Microstructure of the connection zone of an aluminum-steel sample obtained at an energy W2 = 0.66 MJ/m 2 : aexplosion welding (control sample); bexplosion welding with ultrasound.