The structure and microhardness of Cu-Ta joints produced by explosive welding were studied. It was found that, during explosive welding, an intermediate layer
One of the most essential problems of modern materials science is the development of reliable joints of dissimilar metals with significantly different physical and mechanical properties [
Below we consider in detail a Cu-Ta system characterized by nearly zero mutual solubility of the components in the solid state [
This paper deals with the structure and microhardness of welds produced by explosive welding of copper and tantalum plates followed by heat treatment.
For explosive welding we used plates of technically pure copper (99.98% Cu) and tantalum (99.95% Ta). The materials used are significantly different in structure, density, melting point, and thermal conductivity in (Table
Some properties of copper and tantalum.
Features | Cu | Ta |
---|---|---|
Crystal lattice | fcc | bcc |
Density, g/cm3 | 8,93 | 16,6 |
Melting point, °C | 1084 | 2996 |
Recrystallization temperature, °C | 300 | 1300 |
Thermal conductivity, W/m·K | 390 | 52.1 |
Explosive welding of copper and tantalum plates arranged in parallel (Figure
Schematic diagram of explosive welding with parallel arrangement of copper and tantalum plates.
The thermal stability of the Cu-Ta joint was determined by annealing samples at temperatures in the range of 100 to 900°C. The dwell time in the furnace was 1 h. A characteristic feature of tantalum is its active interaction with the gases that make up the air. For this reason, the welded joints were annealed in a vacuum furnace at 10-6 Pa.
Structural studies were performed on transverse sections. For metallographic studies and microhardness measurements, laminated composite specimens were cut along the direction of shock-wave propagation. Metallographic sections were prepared by conventional techniques comprising grinding and polishing. The structure of copper was revealed by etching in an aqueous solution of ferric chloride and hydrochloric acid.
Metallographic studies were carried out using a Carl Zeiss Axio Observer Z1m microscope. Subtle structural changes in plastically deformed materials were studied using a Tecnai G2 20 TWIN transmission electron microscope. The objects of study were foils. The foils were prepared by a technique combining cutting of samples with an electric spark machine, mechanical thinning to a thickness of 100
Microhardness of the samples was measured before and after heat treatment with a Wolpert Group 402 MVD microhardness tester. The load on the diamond indenter was 0.245 N. The measurement procedure involved the creation of a track of indentations perpendicular to the weld in the direction from the copper layer to the tantalum layer. The indenter indentation were spaced at 50
The structure and the scheme of the explosively welded Cu-Ta joint in a longitudinal section are shown in Figure
General view (a) and schematic diagram (b) of Cu-Ta joint produced by explosive welding.
Due to the high degree of dispersion of the resulting mixture, it is difficult to determine the size and shape of individual elements of the structure in the layer by scanning electron microscopy. For this reason, a more detailed structural analysis of the mixing regions was performed using transmission electron microscopy. The main factors determining the structural features of the material are the high rates and amounts of strain in the surface layers of the plates, high heating temperatures of tantalum and copper in the contact area, and the presence of a cumulative sheet of fine particles of the interacting materials.
In general, the structure of the intermediate heterophase layer can be defined as a mixture of fine copper and tantalum. The matrix material in the layer, characterized by continuity, is predominantly copper. A lot of dark rounded particles of tantalum can be seen against the background of the bright copper matrix. Tantalum particles in copper have sizes of
Structure of the intermediate layer with a heterophase structure after explosive welding of copper and tantalum plates (a–e) and after heating for 1 hour at 900°C (f, g). (e) is a scanning electron micrograph, and the other images are transmission electron micrographs.
In addition, the heterophase layer was experimentally found to contain microvolumes in which the matrix material is tantalum and copper is in the form of separate islands. A transmission electron micrograph of a structure of this type is shown in Figure
The mechanism of formation of the intermediate heterophase layer can be described as follows (Figures
Direction of the jet in the collision of a copper plate (top) and an aluminum plate (bottom) simulated using a molecular dynamics model [
X-ray photograph of copper plate colliding with different velocities (the upper plate at a velocity
It should be emphasized that the surface of the joined plates is rough. The size of the majority of particles in the gap between the joined plates (
The impact of the jet fragments on the surface layers of the tantalum plate leads to the formation of a mixing zone and a cloud of fine particles of a mixture of copper and tantalum. Most of the tantalum particles in the mixing zone are randomly distributed in copper. Note that the melting points of copper and tantalum differ by almost a factor of three (1084°C and 2996°C). Tantalum particles remain in the solid state under such collisions. At the same time, microvolumes of copper with ordered arrangement of tantalum particles are also observed. The tantalum clusters shown in Figure
Banded clusters of tantalum in copper.
The thermal stability of the Cu-Ta joint was evaluated by heating the samples with subsequent analysis of structural changes and microhardness of the materials. The heating temperature was in the range of
Increase in the heating temperature was accompanied by structural transformations, leading to strength degradation of the copper and tantalum plates and the heterophase layer located between them. Structural analysis revealed the most pronounced changes in the copper plate. In the initial state (before welding), the grain size was 22
Dislocation structure (a) and twins of deformation origin (b–d) in a copper plate at a distance of 50
The most notable changes in the strength properties of the plastically deformed materials occur upon heating to values close to the recrystallization temperature. After heating to 500°C, the copper plate has no indications of recrystallization. During heating to 600°C, the recrystallization of copper is nonuniform. Recrystallization occurs in islands spaced at 2000
Results of microhardness measurements of thermally untreated samples of welded joints and those annealed at various temperatures are presented in Figures
Effect of heating temperature on the microhardness of copper (1), tantalum (2), and the intermediate layer with a heterophase structure (3).
Microhardness of the intermediate layer and its adjacent regions after explosive welding of copper and tantalum plates and subsequent heating of the weld at different temperatures. (1) without heat treatment, (2) heating to 200°C, (3) 300°C, (4) 500°C, (5) 600°C, (6) 800°C, and (7) 900°C.
The observed effect of hardening of the material is probably due to the formation of a highly dispersed mixture and strain hardening of tantalum. Despite the large amount of plastic strain in the surface layer of the copper plate, strain hardening does not play a significant role in this case. Rapid heating of local microvolumes of the materials leads to melting of copper and eliminates the dislocations structure formed in the zone of dynamic interaction of the plates. The strain hardening effect is eliminated not only by melting but also upon reaching a temperature leading to recrystallization processes. These processes are typical of explosive welding of metallic materials [
In samples heated to 500°C, the microhardness in the zone of mixing of tantalum and copper does not change significantly. This indicates that the heterophase structure is thermally stable in this temperature range. Heating to 600°C or above results in a marked decrease in the microhardness of the material. After annealing at 900°C, the microhardness of the intermediate layer with mixed structure becomes equal to the microhardness of the tantalum plate (~150 HV). In contrast to this layer, the temperature dependence corresponding to tantalum shows no changes. This is due to the fact that the recrystallization temperature of tantalum is more than 300°C higher than the maximum temperature of annealing of the welded joint. Increasing the annealing temperature of copper to 900°C leads to an almost twofold decrease in its microhardness (from 130 to 75 HV) due to relaxation processes in dynamically deformed zones and the formation of a more equilibrium structure than that of the initial one [
During explosive welding in the joint area of tantalum and copper, plates form an intermediate layer which has a heterophase structure and consist of a mixture of fragments of dissimilar materials. The matrix material is predominantly copper. Tantalum in copper is in the form of isolated particles. Less common are microvolumes in which the matrix is tantalum with copper particles embedded in it. Based on the sizes of the dispersed-phase particles observed in the intermediate layer, the material can be classified as a highly dispersed system. The size of tantalum particles is predominantly in the nanometer range (~
Copper-tantalum bimetal was obtained by explosive welding. The intermediate layer contains tantalum particles of A mechanism for the formation of the layer based on the formation of a fragmented cumulative jet is proposed. The thermal stability of the Cu-Ta joint is maintained up to 500°C. Upon heating to 900°C, the microhardness of the intermediate layer decreases from 280 HV to 150 HV.