rus
DOI: 10.22184/1993-7296.FRos.2021.15.1.30.44
The second part continues to review the domestic and foreign articles on the topic of dissimilar metal laser welding, in particular, titanium with aluminum, aluminum with copper and other most common metal pairs. Based on the analysis of scientific articles, it has been established that when welding titanium and aluminum butt joint, it is effitient to offset the laser beam to aluminum (ultimate strength 168–180 MPa), while it is effitient to influence the laser beam from the side of titanium in overlapping welding. The offset of the laser beam and welding modes significantly affect the thickness of the IML, which can be reduced to 2–6 microns with butt welding. When welding aluminum and copper, the laser beam needs to be biased towards the aluminum, both for overlapping and butt welding. The main operational property of the aluminum-copper compound is electrical conductivity, which directly depends on the thickness and composition of the IML. The technologies of welding titanium and magnesium, steel and copper, and other pairs of metals are also considered.
The second part continues to review the domestic and foreign articles on the topic of dissimilar metal laser welding, in particular, titanium with aluminum, aluminum with copper and other most common metal pairs. Based on the analysis of scientific articles, it has been established that when welding titanium and aluminum butt joint, it is effitient to offset the laser beam to aluminum (ultimate strength 168–180 MPa), while it is effitient to influence the laser beam from the side of titanium in overlapping welding. The offset of the laser beam and welding modes significantly affect the thickness of the IML, which can be reduced to 2–6 microns with butt welding. When welding aluminum and copper, the laser beam needs to be biased towards the aluminum, both for overlapping and butt welding. The main operational property of the aluminum-copper compound is electrical conductivity, which directly depends on the thickness and composition of the IML. The technologies of welding titanium and magnesium, steel and copper, and other pairs of metals are also considered.
Теги: dissimilar metals intermetallic layer laser welding mechanical properties microstructure weldability интерметаллидный слой лазерная сварка механические свойства микроструктура разнородные металлы свариваемость
Dissimilar Metal
Laser Welding.
Review
Part 2
S. V. Kuryntsev 1, I. N. Shiganov 2
A. N. Tupolev Kazan National Research Technical University – KAI, Kazan, Russia
N. E. Bauman Moscow State Technical University, Moscow, Russia
The second part continues to review the domestic and foreign articles on the topic of dissimilar metal laser welding, in particular, titanium with aluminum, aluminum with copper and other most common metal pairs. Based on the analysis of scientific articles, it has been established that when welding titanium and aluminum butt joint, it is effitient to offset the laser beam to aluminum (ultimate strength 168–180 MPa), while it is effitient to influence the laser beam from the side of titanium in overlapping welding. The offset of the laser beam and welding modes significantly affect the thickness of the IML, which can be reduced to 2–6 microns with butt welding. When welding aluminum and copper, the laser beam needs to be biased towards the aluminum, both for overlapping and butt welding. The main operational property of the aluminum-copper compound is electrical conductivity, which directly depends on the thickness and composition of the IML. The technologies of welding titanium and magnesium, steel and copper, and other pairs of metals are also considered.
Keywords: laser welding, dissimilar metals, weldability, microstructure, intermetallic layer, mechanical properties
Received on: 14.09.2020
Accepted on: 10.12.2020
1. Laser welding of aluminum alloys with titanium alloys
As it already has been mentioned in the first part of the review (See Photonics Russia. 2020;14(6):492−506), according to the quantitative analysis of articles on the Scopus abstract database over the past 4 years, the titanium + aluminum pair is second to steel + aluminum pair in the spread of laser welding for dissimilar metals. Attempts to join bimetallic adapters from alloys based on titanium and aluminum by heat pressing [1, 2] and other types of mechanical and thermomechanical classes of welding have been undertaken for a long time. The resulting connections, in the form of adapters, were used for the manufacture of fuel, oil and temperature-controlled systems of aircraft and successfully passed bench tests. However, there are no data on thermal welding of titanium and aluminum with the same positive results. Laser welding, due to its precision of action and high cooling rates, can positively affect metallurgical and thermal deformation processes when welding titanium and aluminum, the physical properties of which differ significantly. Just as in the case of welding steel and aluminum, overlap and butt joints are most common.
The authors of [3] study the effect of laser beam offset on titanium alloy VT‑20 during butt welding with aluminum alloy 1461 and the effect of post-weld heat treatment on the mechanical properties of joints. The laser beam is offset by 0 mm, 0.5 mm, 1 mm, heat treatment is carried out at a temperature of 490 °C, 540 °C, 590 °C, for 4 and 6 hours. As a result of the research, the authors conclude that the maximum values of the tensile strength have the samples obtained with a offset of the laser beam by 1 mm (168 MPa), the minimum without offset 0 mm (75 MPa). An increase in the temperature and time of heat treatment significantly reduces the mechanical properties, which can be explained by significantly different coefficients of thermal expansion, at a sufficiently high heat treatment temperature (490–590 °C). The influence of low-temperature heat treatment on the thickness of the intermetallic layer (IML) and on the mechanical properties of welded joints of titanium and aluminum are studied by the authors [4].
The beam is offset, as in the previous work, on titanium (by 1 mm), long-term heat treatment of the obtained welded joints is carried out for 336 h and 138 h at temperatures of 350 °C and 450 °C, respectively. The thickness of the IML in the obtained welded joints was in the range of 30–50 microns, depending on the heat input during welding. The authors draw the following conclusions, heat treatment at a temperature of 350 °C does not affect the thickness and structure of the IML, the tensile strength (90–100 MPa) is comparable to the maximum without heat treatment (100–110 MPa). Whereas after heat treatment at a temperature of 450 °C, the thickness of the IML increased, and the tensile strength decreased to 55–75 MPa.
The study of the effect of laser beam offset on 5A06 aluminum alloy in the range of 300–700 µm in butt welding with titanium alloy Ti6Al4V is described in [5], the thickness of butt-welded sheets is 1.5 mm. The maximum mechanical properties during tensile testing (183 MPa) of welded joints were obtained under the following welding modes: beam offset – 500 microns, welding speed – 11 mm / s, laser radiation power – 1130 W, IML thickness under these modes is about 3 microns. The work pays attention to a thorough study of the fracture after tensile tests, since it consists of 3 parts: the central part fractured along the titanium / weld line (brittle fracture), upper and lower parts destroyed along the weld seam (ductile fracture). The upper and lower parts of the weld are formed as a result of wetting the titanium edges with molten aluminum (Fig. 1 a, b), and have a phase composition that is significantly different from the central part. Fig. 1 c, d shows SEM images of the microstructure of regions C and A, respectively, shown in Fig. 1b, it can be seen how significantly the thickness of the IML and the microstructure of the welded seam differ, depending on the depth of the welded seam. The authors conclude that the mechanical properties depend not only on the thickness and type of the IML, but also on the area and volume of titanium wetting with liquid aluminum.
The effect of the influence of the offset of the laser beam on aluminum and the modes of oscillation of the beam when welding with titanium butt joint is studied by the authors of [6]. The specified welding parameters affect the energy distribution, microstructure and mechanical properties of the joints. As in the previous work, the offset of the laser beam by 100–200 µm and the parameters of the beam oscillations significantly affect the microstructure of the joint. Also, the microstructure of the joints differs significantly in depth, the closer to the heating source, the greater the thickness of the IML. The maximum values of the ultimate strength at rupture are about 170 MPa, with an IML thickness of ~2 μm.
Features of the structure of welds obtained by laser welding of titanium alloy VT6S and aluminum alloy 1424 are described by the authors [7]. In contrast to the above works [5, 6], the authors use rather high welding speeds of 70–100 mm / s and offset the laser beam onto the aluminum alloy by 200 µm. When using a welding speed of 100 mm / s, a thinner IML is observed, the authors conclude that in order to form an IML with a smaller thickness, it is necessary to reduce heat input by increasing the indent on the aluminum alloy, increasing the welding speed and reducing the laser radiation power.
In work [8], the authors study the overlapping welded joints of plates made of titanium and aluminum, exposure to a laser beam is performed from the titanium side, a pulsed laser (300 W) is used, welding is performed with overcrossing points [9], the main variable parameters are the point diameter and laser pulse. The authors pay special attention to the study of the region of interfacial crack initiation, since this is an important indicator of the properties of the welded joint. It was revealed that the probable place of crack initiation and propagation is the surface between different IMLs (TiAl and TiAl3) with a high level of dislocation density (Fig. 2). The results show that IML with different stoichiometric composition (TiAl and TiAl3, etc.) will be the weak point of the welded joint, in which a crack will nucleate, while IML of constant composition will be more stable in terms of mechanical properties.
The observation of the onset of fracture not only in the brittle IML, but also in the HAZ of aluminum during shear tests of overlap joints is described by the authors in [10, 11]. A study of the influence of heat input and exposure to a laser beam from the side of aluminum and from the side of titanium was carried out, it was found that the most optimal effect is from the side of titanium. Welding modes and heat input significantly affect not only the phase composition due to the cooling rates, but also the diffusion processes and, as a result, the distribution of chemical elements over the cross section of the weld from dissimilar metals, which also affects the phase composition [12].
It should be emphasized that in works [5, 6, 8], a relatively small speed for laser welding is used, 11–17 mm / s, while in works [7, 10, 11], the welding speed is used, which is several times higher than that indicated (100–150 mm / s), however, due to the greater offset of the laser beam on aluminum, the thickness of the IML is approximately in the same range (2–6 microns).
As in the case of welding steel and aluminum, when welding titanium and aluminum, the thickness and composition of the intermediate between the two metals, the IML is a problem area of the welded joint. In butt welding, the laser beam offset technique is also common, in most cases the beam is displaced to aluminum, the offset value depends on the thickness of the workpieces being joined, and the IML thickness depends on the welding modes and is in the range of 2–6 microns. The use of a offset of the laser beam on titanium leads to the formation of an IML that is many times larger in thickness, the mechanical properties of such compounds are 40–50% lower than those obtained by offset of the beam on aluminum.
2. Laser welding of aluminum alloys with copper
The lattice parameters and the size of copper and aluminum atoms are similar, the type of crystal lattice is the same, the lattice constants are comparable, therefore, aluminum and copper form solid substitution solutions of limited solubility, the limiting solubility of copper in aluminum is 2.2 at.%, and various intermetallic phases (Al2Cu – θ, AlCu – η2, Al3Cu4 – ζ1, Al4Cu9 – δ, Al4Cu9 – γ1) are formed [13].
As in welding aluminum with titanium or with iron, the main types of joints are butt and overlap. Due to the fact that aluminum and copper are the most common conductors of electric current, pure alloys with minimal electrical resistance are mainly used in welding. As it was said in the first part of the review, pure copper and aluminum, mainly copper, have a low coefficient of absorption of laser radiation and have high thermal conductivity values; therefore, laser welding of this pair of metals is complicated.
As a rule, mechanical types of welding are used to join this pair of plastic metals, such as cold, explosion, rolling, ultrasonic. Due to the fact that the laser beam, as a metalworking tool, is multifunctional and sometimes a minimum metallurgical contact or microcontact is required from the connection of copper and aluminum, and high mechanical characteristics are not required, laser welding is used to connect them. A sufficient number of works are devoted to the study of compounds, in which, mainly, the types of intermetallics and their influence on the values of electrical resistance are studied. It should be noted that the electrical resistance of intermetallic compounds is 5–8 times higher than that of copper and aluminum, and the mechanical properties during shear tests are in the range of 100–120 MPa.
The authors of [13] study which of the formed intermetallic phases is more brittle and leads to a deterioration in mechanical properties. Welding is performed with an overlap when exposed to the beam from the aluminum side. The authors conclude that mainly cracks originate in the AlCu and Al4Cu9 phases, the chemical composition of the fracture surface is 61.6–62.7% copper, the rest is aluminum, which corresponds to the Al4Cu9 phase.
In [14], the influence of oscillations and offset of a laser beam on aluminum in the range 0–400 µm is studied. Welding is carried out in a vacuum at reduced atmospheric pressure, the type of joint is butt, after preparation of the samples, the electrical resistance is measured. It is shown that the ratio of the melting of the materials to be joined is an important factor in obtaining a high-quality joint, which can be successfully controlled by offsetting the laser beam to aluminum. The thickness of the layer of interaction between copper and aluminum, obtained with optimal welding conditions and a beam offset of 300 microns on aluminum, is 80 microns, the thickness of the intermetallic layer is 8–13 microns, the electrical properties of the joint did not deteriorate significantly.
The study of the influence of the parameters of sinusoidal oscillations by a laser beam perpendicular to the welding direction on the mechanical properties and electrical resistance is described by the authors [15], welding is performed with an overlap, the thickness of the sheets being joined is 1 mm, the laser beam was applied from the aluminum side. The authors consider the geometry of the weld (width and depth of penetration into copper) as a function of the beam oscillation parameters (amplitude and frequency), then consider the copper content in the weld and electrical resistance as a function of the geometry of the weld. As a result of the research carried out, the following conclusions are drawn. To ensure the minimum electrical resistance of the jointed workpieces with a thickness of 1 mm when overlapping welding, a joint width of at least 1 mm is required (Fig. 3). When welding with a laser beam without transverse oscillations, the joint width is 0.2–0.3 mm, with transverse oscillations 1–1.2 mm due to an increase in the width of the weld pool on the aluminum side. The minimum electrical resistance and maximum mechanical properties can be obtained at the maximum width to depth ratio of the weld (>4) (Fig. 3), while the penetration depth into copper must be at least 0.2 mm. Also, based on the correlation of mechanical properties and electrical resistance, the authors propose to use the measurement of electrical resistance as a method of non-destructive testing of a welded joint.
A method for monitoring the process of laser welding of copper and aluminum and controlling the offset of the laser beam based on observing the spectrum of a specific wavelength for aluminum (394.4 and 396.1 nm) and for copper (578 nm) was proposed by the authors [16]. In view of the fact that the offset of the laser beam on one of the metals being welded is a factor that significantly affects the formation of the intermetallic layer, the development of methods for controlling micro-offsets is an urgent task. The physical basis of the method is the change in the wavelength of the emission spectrum of the plasma torch, as a result of the offset of the laser beam to one of the components.
Other works on laser welding of aluminum and copper consider microwelding of electrical contacts [17–20], the effect of welding modes and filler material on the microstructure and mechanical properties [21–23], theoretical studies of the optimal offset of a heat source based on the phase diagram [24].
3. Features of laser welding of titanium alloys with magnesium alloys
As mentioned in the first part of the review, according to the prevalence of laser welding for dissimilar metals, after welding aluminum and copper, there are such pairs as titanium and nickel, steel and titanium, steel and nickel, magnesium and titanium, steel and copper. Of which, from the point of view of the promising application and high mechanical characteristics, one can single out titanium and magnesium, steel and copper pairs.
Methods and technologies for butt and overlap welding of titanium and magnesium are rather complicated; in most of the studies considered, the deposition of copper or nickel coatings by electrolytic deposition on titanium is used. The main technological parameter, in addition to the modes of welding and offset of the laser beam, is the thickness of the deposited coating (Fig. 4).
The paper [25] describes the effect of the thickness of the deposited copper coating (range 10.8–28.2 μm) on the microstructure and mechanical properties of the butt joint of titanium and magnesium alloys. The mechanism of joint formation, which depends on the thickness of the copper coating and the area of the weld (lower or upper part), is divided into metallurgical (Fig. 5 a) and mechanical (Fig. 5 b). From the presented data, it can be seen that, at a coating thickness of 19.7 μm, an even line of fusion and transition from titanium to the weld is observed, while at a coating thickness of 24.9 μm, the transition line is rough and uneven and, in addition to metallurgical contact, mechanical interaction of the titanium surface and filler material is observed.
Also, the authors of this work have other publications [26–31] on welding titanium and magnesium, based on a similar approach to the organization of the experiment and research methods. In these works, the influence of the thickness of the electrodeposited coating, the coating material (copper, nickel, their combination), laser welding modes is considered, the method of laser conductive laser welding is studied [9, 32], which is a promising area of research due to the significant difference in the melting temperatures of titanium and magnesium.
Fig. 6a shows a diagram of the process of interaction of the filler material with the welded metals. Based on the analysis of the microstructure, the crystallization mechanism of the weld metal, at the interface between titanium coated with copper magnesium-based filler material, with a coating thickness of 19.7 μm is shown in Fig. 6b. A similar mechanism of crystallization and formation of various phases at a coating thickness of 24.9 μm is shown in Fig. 6 in (upper part of the seam) and Fig. 6 g (bottom of the seam). As can be seen from the data presented, due to the larger amount of copper (coating thickness 24.9 μm), a large number of phases are formed, leading to the formation of microcracks. Also, the phase composition of the upper and lower parts of the weld with a coating of 24.9 μm differs significantly, due to the larger volume of molten metal in the upper part and, accordingly, lower cooling rates. The Ti3Al phase has the lowest enthalpy of formation; therefore, this phase is easily crystallized in comparison with the phases containing copper (AlCu2Ti, Ti2Cu, Mg2Cu). The optimum coating thickness (19.7 μm) ensures the mechanical properties of the welded joint at the level of 85% of the mechanical properties of the magnesium alloy.
4. Laser welding of steels with copper
A small number of publications in recent years have been devoted to laser welding of steels with copper and copper-based alloys, due to the fact that these metals weld well and the main problem in laser welding is the high degree of reflection of laser radiation from copper. This problem is solved in several ways: by offsetting the laser beam to steel and preparing the edges when welding small thicknesses [33], by exposing steel to a certain angle when welding thicknesses of more than 2 mm (Fig. 7 a, b) [34], the use of grooving that changes the plane of the joint when it is impossible to tilt the laser beam [35], the use of a defocused beam or lead-in plate [36, 37].
The above laser welding methods make it possible to obtain welded joints with mechanical properties at the level of pure copper. The data obtained by the authors of [34] are consistent with the experimental results obtained in [37], with the optimal offset of the laser beam on the steel, it is possible to obtain a welded joint, which, in tensile tests, will fail along the HAZ of copper (Fig. 7 c, d, e). Due to the thermal effect, a coarse-grained structure is formed in the HAZ of copper, which has less mechanical properties than a weld, in which copper is strengthened by iron and alloying elements of steel, for example, nickel and manganese.
Conclusion
The following conclusions can be drawn from the described results of work carried out by domestic and foreign researchers.
When welding titanium and aluminum butt join, the preferred technique is to offset the laser beam to the aluminum, while in overlapping welding, it is advisable to act with the laser beam from the titanium side.
Laser welding of aluminum and copper is best performed using the technique of offsetting the beam to aluminum, both in butt welding and in overlapping welding, due to the greater degree of reflection of laser radiation and the higher thermal conductivity of copper.
Welding of titanium and magnesium should be carried out using thin layers (10–50 microns) of copper or nickel on titanium by electrolytic deposition, which helps to minimize the formation of intermetallic phases leading to the propagation of microcracks. When applying a coating of optimal thicknesses and optimal welding conditions, the mechanical properties of the welded joint are provided at the level of 85% of the mechanical properties of the magnesium alloy.
Contribution of authors
Kuryntsev S. V. – concept, translation and analysis of material, work with graphic part, results processing.
Shiganov I. N. – discussion, suggestions and comments, material analysis.
Conflict of interests
The authors declare that they have no actual or potential conflict of interests.
Authors
Kuryntsev S. V., Cand. of Sciences (Economics), A. N. Tupolev Kazan National Research Technical University – KAI, Kazan, Russia.
Shiganov I. N., Dr. of Science (Engineering), Professor, N. E. Bauman Moscow State Technical University., Moscow, Russia.
Laser Welding.
Review
Part 2
S. V. Kuryntsev 1, I. N. Shiganov 2
A. N. Tupolev Kazan National Research Technical University – KAI, Kazan, Russia
N. E. Bauman Moscow State Technical University, Moscow, Russia
The second part continues to review the domestic and foreign articles on the topic of dissimilar metal laser welding, in particular, titanium with aluminum, aluminum with copper and other most common metal pairs. Based on the analysis of scientific articles, it has been established that when welding titanium and aluminum butt joint, it is effitient to offset the laser beam to aluminum (ultimate strength 168–180 MPa), while it is effitient to influence the laser beam from the side of titanium in overlapping welding. The offset of the laser beam and welding modes significantly affect the thickness of the IML, which can be reduced to 2–6 microns with butt welding. When welding aluminum and copper, the laser beam needs to be biased towards the aluminum, both for overlapping and butt welding. The main operational property of the aluminum-copper compound is electrical conductivity, which directly depends on the thickness and composition of the IML. The technologies of welding titanium and magnesium, steel and copper, and other pairs of metals are also considered.
Keywords: laser welding, dissimilar metals, weldability, microstructure, intermetallic layer, mechanical properties
Received on: 14.09.2020
Accepted on: 10.12.2020
1. Laser welding of aluminum alloys with titanium alloys
As it already has been mentioned in the first part of the review (See Photonics Russia. 2020;14(6):492−506), according to the quantitative analysis of articles on the Scopus abstract database over the past 4 years, the titanium + aluminum pair is second to steel + aluminum pair in the spread of laser welding for dissimilar metals. Attempts to join bimetallic adapters from alloys based on titanium and aluminum by heat pressing [1, 2] and other types of mechanical and thermomechanical classes of welding have been undertaken for a long time. The resulting connections, in the form of adapters, were used for the manufacture of fuel, oil and temperature-controlled systems of aircraft and successfully passed bench tests. However, there are no data on thermal welding of titanium and aluminum with the same positive results. Laser welding, due to its precision of action and high cooling rates, can positively affect metallurgical and thermal deformation processes when welding titanium and aluminum, the physical properties of which differ significantly. Just as in the case of welding steel and aluminum, overlap and butt joints are most common.
The authors of [3] study the effect of laser beam offset on titanium alloy VT‑20 during butt welding with aluminum alloy 1461 and the effect of post-weld heat treatment on the mechanical properties of joints. The laser beam is offset by 0 mm, 0.5 mm, 1 mm, heat treatment is carried out at a temperature of 490 °C, 540 °C, 590 °C, for 4 and 6 hours. As a result of the research, the authors conclude that the maximum values of the tensile strength have the samples obtained with a offset of the laser beam by 1 mm (168 MPa), the minimum without offset 0 mm (75 MPa). An increase in the temperature and time of heat treatment significantly reduces the mechanical properties, which can be explained by significantly different coefficients of thermal expansion, at a sufficiently high heat treatment temperature (490–590 °C). The influence of low-temperature heat treatment on the thickness of the intermetallic layer (IML) and on the mechanical properties of welded joints of titanium and aluminum are studied by the authors [4].
The beam is offset, as in the previous work, on titanium (by 1 mm), long-term heat treatment of the obtained welded joints is carried out for 336 h and 138 h at temperatures of 350 °C and 450 °C, respectively. The thickness of the IML in the obtained welded joints was in the range of 30–50 microns, depending on the heat input during welding. The authors draw the following conclusions, heat treatment at a temperature of 350 °C does not affect the thickness and structure of the IML, the tensile strength (90–100 MPa) is comparable to the maximum without heat treatment (100–110 MPa). Whereas after heat treatment at a temperature of 450 °C, the thickness of the IML increased, and the tensile strength decreased to 55–75 MPa.
The study of the effect of laser beam offset on 5A06 aluminum alloy in the range of 300–700 µm in butt welding with titanium alloy Ti6Al4V is described in [5], the thickness of butt-welded sheets is 1.5 mm. The maximum mechanical properties during tensile testing (183 MPa) of welded joints were obtained under the following welding modes: beam offset – 500 microns, welding speed – 11 mm / s, laser radiation power – 1130 W, IML thickness under these modes is about 3 microns. The work pays attention to a thorough study of the fracture after tensile tests, since it consists of 3 parts: the central part fractured along the titanium / weld line (brittle fracture), upper and lower parts destroyed along the weld seam (ductile fracture). The upper and lower parts of the weld are formed as a result of wetting the titanium edges with molten aluminum (Fig. 1 a, b), and have a phase composition that is significantly different from the central part. Fig. 1 c, d shows SEM images of the microstructure of regions C and A, respectively, shown in Fig. 1b, it can be seen how significantly the thickness of the IML and the microstructure of the welded seam differ, depending on the depth of the welded seam. The authors conclude that the mechanical properties depend not only on the thickness and type of the IML, but also on the area and volume of titanium wetting with liquid aluminum.
The effect of the influence of the offset of the laser beam on aluminum and the modes of oscillation of the beam when welding with titanium butt joint is studied by the authors of [6]. The specified welding parameters affect the energy distribution, microstructure and mechanical properties of the joints. As in the previous work, the offset of the laser beam by 100–200 µm and the parameters of the beam oscillations significantly affect the microstructure of the joint. Also, the microstructure of the joints differs significantly in depth, the closer to the heating source, the greater the thickness of the IML. The maximum values of the ultimate strength at rupture are about 170 MPa, with an IML thickness of ~2 μm.
Features of the structure of welds obtained by laser welding of titanium alloy VT6S and aluminum alloy 1424 are described by the authors [7]. In contrast to the above works [5, 6], the authors use rather high welding speeds of 70–100 mm / s and offset the laser beam onto the aluminum alloy by 200 µm. When using a welding speed of 100 mm / s, a thinner IML is observed, the authors conclude that in order to form an IML with a smaller thickness, it is necessary to reduce heat input by increasing the indent on the aluminum alloy, increasing the welding speed and reducing the laser radiation power.
In work [8], the authors study the overlapping welded joints of plates made of titanium and aluminum, exposure to a laser beam is performed from the titanium side, a pulsed laser (300 W) is used, welding is performed with overcrossing points [9], the main variable parameters are the point diameter and laser pulse. The authors pay special attention to the study of the region of interfacial crack initiation, since this is an important indicator of the properties of the welded joint. It was revealed that the probable place of crack initiation and propagation is the surface between different IMLs (TiAl and TiAl3) with a high level of dislocation density (Fig. 2). The results show that IML with different stoichiometric composition (TiAl and TiAl3, etc.) will be the weak point of the welded joint, in which a crack will nucleate, while IML of constant composition will be more stable in terms of mechanical properties.
The observation of the onset of fracture not only in the brittle IML, but also in the HAZ of aluminum during shear tests of overlap joints is described by the authors in [10, 11]. A study of the influence of heat input and exposure to a laser beam from the side of aluminum and from the side of titanium was carried out, it was found that the most optimal effect is from the side of titanium. Welding modes and heat input significantly affect not only the phase composition due to the cooling rates, but also the diffusion processes and, as a result, the distribution of chemical elements over the cross section of the weld from dissimilar metals, which also affects the phase composition [12].
It should be emphasized that in works [5, 6, 8], a relatively small speed for laser welding is used, 11–17 mm / s, while in works [7, 10, 11], the welding speed is used, which is several times higher than that indicated (100–150 mm / s), however, due to the greater offset of the laser beam on aluminum, the thickness of the IML is approximately in the same range (2–6 microns).
As in the case of welding steel and aluminum, when welding titanium and aluminum, the thickness and composition of the intermediate between the two metals, the IML is a problem area of the welded joint. In butt welding, the laser beam offset technique is also common, in most cases the beam is displaced to aluminum, the offset value depends on the thickness of the workpieces being joined, and the IML thickness depends on the welding modes and is in the range of 2–6 microns. The use of a offset of the laser beam on titanium leads to the formation of an IML that is many times larger in thickness, the mechanical properties of such compounds are 40–50% lower than those obtained by offset of the beam on aluminum.
2. Laser welding of aluminum alloys with copper
The lattice parameters and the size of copper and aluminum atoms are similar, the type of crystal lattice is the same, the lattice constants are comparable, therefore, aluminum and copper form solid substitution solutions of limited solubility, the limiting solubility of copper in aluminum is 2.2 at.%, and various intermetallic phases (Al2Cu – θ, AlCu – η2, Al3Cu4 – ζ1, Al4Cu9 – δ, Al4Cu9 – γ1) are formed [13].
As in welding aluminum with titanium or with iron, the main types of joints are butt and overlap. Due to the fact that aluminum and copper are the most common conductors of electric current, pure alloys with minimal electrical resistance are mainly used in welding. As it was said in the first part of the review, pure copper and aluminum, mainly copper, have a low coefficient of absorption of laser radiation and have high thermal conductivity values; therefore, laser welding of this pair of metals is complicated.
As a rule, mechanical types of welding are used to join this pair of plastic metals, such as cold, explosion, rolling, ultrasonic. Due to the fact that the laser beam, as a metalworking tool, is multifunctional and sometimes a minimum metallurgical contact or microcontact is required from the connection of copper and aluminum, and high mechanical characteristics are not required, laser welding is used to connect them. A sufficient number of works are devoted to the study of compounds, in which, mainly, the types of intermetallics and their influence on the values of electrical resistance are studied. It should be noted that the electrical resistance of intermetallic compounds is 5–8 times higher than that of copper and aluminum, and the mechanical properties during shear tests are in the range of 100–120 MPa.
The authors of [13] study which of the formed intermetallic phases is more brittle and leads to a deterioration in mechanical properties. Welding is performed with an overlap when exposed to the beam from the aluminum side. The authors conclude that mainly cracks originate in the AlCu and Al4Cu9 phases, the chemical composition of the fracture surface is 61.6–62.7% copper, the rest is aluminum, which corresponds to the Al4Cu9 phase.
In [14], the influence of oscillations and offset of a laser beam on aluminum in the range 0–400 µm is studied. Welding is carried out in a vacuum at reduced atmospheric pressure, the type of joint is butt, after preparation of the samples, the electrical resistance is measured. It is shown that the ratio of the melting of the materials to be joined is an important factor in obtaining a high-quality joint, which can be successfully controlled by offsetting the laser beam to aluminum. The thickness of the layer of interaction between copper and aluminum, obtained with optimal welding conditions and a beam offset of 300 microns on aluminum, is 80 microns, the thickness of the intermetallic layer is 8–13 microns, the electrical properties of the joint did not deteriorate significantly.
The study of the influence of the parameters of sinusoidal oscillations by a laser beam perpendicular to the welding direction on the mechanical properties and electrical resistance is described by the authors [15], welding is performed with an overlap, the thickness of the sheets being joined is 1 mm, the laser beam was applied from the aluminum side. The authors consider the geometry of the weld (width and depth of penetration into copper) as a function of the beam oscillation parameters (amplitude and frequency), then consider the copper content in the weld and electrical resistance as a function of the geometry of the weld. As a result of the research carried out, the following conclusions are drawn. To ensure the minimum electrical resistance of the jointed workpieces with a thickness of 1 mm when overlapping welding, a joint width of at least 1 mm is required (Fig. 3). When welding with a laser beam without transverse oscillations, the joint width is 0.2–0.3 mm, with transverse oscillations 1–1.2 mm due to an increase in the width of the weld pool on the aluminum side. The minimum electrical resistance and maximum mechanical properties can be obtained at the maximum width to depth ratio of the weld (>4) (Fig. 3), while the penetration depth into copper must be at least 0.2 mm. Also, based on the correlation of mechanical properties and electrical resistance, the authors propose to use the measurement of electrical resistance as a method of non-destructive testing of a welded joint.
A method for monitoring the process of laser welding of copper and aluminum and controlling the offset of the laser beam based on observing the spectrum of a specific wavelength for aluminum (394.4 and 396.1 nm) and for copper (578 nm) was proposed by the authors [16]. In view of the fact that the offset of the laser beam on one of the metals being welded is a factor that significantly affects the formation of the intermetallic layer, the development of methods for controlling micro-offsets is an urgent task. The physical basis of the method is the change in the wavelength of the emission spectrum of the plasma torch, as a result of the offset of the laser beam to one of the components.
Other works on laser welding of aluminum and copper consider microwelding of electrical contacts [17–20], the effect of welding modes and filler material on the microstructure and mechanical properties [21–23], theoretical studies of the optimal offset of a heat source based on the phase diagram [24].
3. Features of laser welding of titanium alloys with magnesium alloys
As mentioned in the first part of the review, according to the prevalence of laser welding for dissimilar metals, after welding aluminum and copper, there are such pairs as titanium and nickel, steel and titanium, steel and nickel, magnesium and titanium, steel and copper. Of which, from the point of view of the promising application and high mechanical characteristics, one can single out titanium and magnesium, steel and copper pairs.
Methods and technologies for butt and overlap welding of titanium and magnesium are rather complicated; in most of the studies considered, the deposition of copper or nickel coatings by electrolytic deposition on titanium is used. The main technological parameter, in addition to the modes of welding and offset of the laser beam, is the thickness of the deposited coating (Fig. 4).
The paper [25] describes the effect of the thickness of the deposited copper coating (range 10.8–28.2 μm) on the microstructure and mechanical properties of the butt joint of titanium and magnesium alloys. The mechanism of joint formation, which depends on the thickness of the copper coating and the area of the weld (lower or upper part), is divided into metallurgical (Fig. 5 a) and mechanical (Fig. 5 b). From the presented data, it can be seen that, at a coating thickness of 19.7 μm, an even line of fusion and transition from titanium to the weld is observed, while at a coating thickness of 24.9 μm, the transition line is rough and uneven and, in addition to metallurgical contact, mechanical interaction of the titanium surface and filler material is observed.
Also, the authors of this work have other publications [26–31] on welding titanium and magnesium, based on a similar approach to the organization of the experiment and research methods. In these works, the influence of the thickness of the electrodeposited coating, the coating material (copper, nickel, their combination), laser welding modes is considered, the method of laser conductive laser welding is studied [9, 32], which is a promising area of research due to the significant difference in the melting temperatures of titanium and magnesium.
Fig. 6a shows a diagram of the process of interaction of the filler material with the welded metals. Based on the analysis of the microstructure, the crystallization mechanism of the weld metal, at the interface between titanium coated with copper magnesium-based filler material, with a coating thickness of 19.7 μm is shown in Fig. 6b. A similar mechanism of crystallization and formation of various phases at a coating thickness of 24.9 μm is shown in Fig. 6 in (upper part of the seam) and Fig. 6 g (bottom of the seam). As can be seen from the data presented, due to the larger amount of copper (coating thickness 24.9 μm), a large number of phases are formed, leading to the formation of microcracks. Also, the phase composition of the upper and lower parts of the weld with a coating of 24.9 μm differs significantly, due to the larger volume of molten metal in the upper part and, accordingly, lower cooling rates. The Ti3Al phase has the lowest enthalpy of formation; therefore, this phase is easily crystallized in comparison with the phases containing copper (AlCu2Ti, Ti2Cu, Mg2Cu). The optimum coating thickness (19.7 μm) ensures the mechanical properties of the welded joint at the level of 85% of the mechanical properties of the magnesium alloy.
4. Laser welding of steels with copper
A small number of publications in recent years have been devoted to laser welding of steels with copper and copper-based alloys, due to the fact that these metals weld well and the main problem in laser welding is the high degree of reflection of laser radiation from copper. This problem is solved in several ways: by offsetting the laser beam to steel and preparing the edges when welding small thicknesses [33], by exposing steel to a certain angle when welding thicknesses of more than 2 mm (Fig. 7 a, b) [34], the use of grooving that changes the plane of the joint when it is impossible to tilt the laser beam [35], the use of a defocused beam or lead-in plate [36, 37].
The above laser welding methods make it possible to obtain welded joints with mechanical properties at the level of pure copper. The data obtained by the authors of [34] are consistent with the experimental results obtained in [37], with the optimal offset of the laser beam on the steel, it is possible to obtain a welded joint, which, in tensile tests, will fail along the HAZ of copper (Fig. 7 c, d, e). Due to the thermal effect, a coarse-grained structure is formed in the HAZ of copper, which has less mechanical properties than a weld, in which copper is strengthened by iron and alloying elements of steel, for example, nickel and manganese.
Conclusion
The following conclusions can be drawn from the described results of work carried out by domestic and foreign researchers.
When welding titanium and aluminum butt join, the preferred technique is to offset the laser beam to the aluminum, while in overlapping welding, it is advisable to act with the laser beam from the titanium side.
Laser welding of aluminum and copper is best performed using the technique of offsetting the beam to aluminum, both in butt welding and in overlapping welding, due to the greater degree of reflection of laser radiation and the higher thermal conductivity of copper.
Welding of titanium and magnesium should be carried out using thin layers (10–50 microns) of copper or nickel on titanium by electrolytic deposition, which helps to minimize the formation of intermetallic phases leading to the propagation of microcracks. When applying a coating of optimal thicknesses and optimal welding conditions, the mechanical properties of the welded joint are provided at the level of 85% of the mechanical properties of the magnesium alloy.
Contribution of authors
Kuryntsev S. V. – concept, translation and analysis of material, work with graphic part, results processing.
Shiganov I. N. – discussion, suggestions and comments, material analysis.
Conflict of interests
The authors declare that they have no actual or potential conflict of interests.
Authors
Kuryntsev S. V., Cand. of Sciences (Economics), A. N. Tupolev Kazan National Research Technical University – KAI, Kazan, Russia.
Shiganov I. N., Dr. of Science (Engineering), Professor, N. E. Bauman Moscow State Technical University., Moscow, Russia.
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