2021 Volume 62 Issue 3 Pages 386-395
A 7075-T651 aluminum alloy was successfully welded by tungsten inert gas welding using commercial ER5183 wire and Al–5Mg–1Zn–0.3Sc–0.15Zr welding wire containing different contents of Mn. The microstructure, corresponding mechanical properties, and the corrosion behavior of the welded joint were investigated by microscopy methods, tensile tests, and corrosion tests. The results revealed that Al3(Sc,Zr) particles were distributed in a small amount in the welds and refined the primary α-Al grain. The addition of Mn gives a strong interaction of improving fracture strength, micro-hardness, and corrosion resistance reducing the harmful influence of iron, as a result, the properties of the welded joint were enhanced. And the increase of Mn content improves the tensile strength and hardness of welds, which could be attributed to the refinement of eutectic T phase by Mn. Potentiodynamic polarization tests revealed that the corrosion resistance of welds increased with the increase of Mn content. Compared with the welded joint using commercial filler, the welded joint with Al–Mg–Zn–Sc–Zr–Mn filler exhibited a higher resistance to stress corrosion cracking (SCC). In conclusion, Al–Mg–Zn–Sc–Zr–Mn filler can be using for welding 7XXX aluminum alloys.
Fig. 9 Schematic diagram of corrosion for weld joint suffering the potentiodynamic polarization tests.
High-strength thermally strengthened weldable alloys based on the 7xxx alloys have widely been used in structural applications of aerospace industries due to its excellent mechanical properties, and 5183 aluminum alloy wire has been extensively used in the welding of 7075 aluminum alloy.1,2) However, available commercial 5183 wires were unable to meet the welding requirements, due to its lower strength of the welded joint. On the other hand, the comparison and systematic research on the synergistic effect of Sc, Zr, and Mn micro-alloying additions on the mechanical properties and microstructures of 5183 wires with amount of Zn are not available.
Ying Deng indicated that the Al–Zn–Mg alloy which adds minor Sc and Zr has finer grains in the fusion boundary than the aluminum alloy without them, leading to a higher mechanical property in TIG welded joints.3) The reports on trace amount to add Sc on microstructure and mechanical properties of 2055 aluminum alloy wrote found that the same precipitated species were observed before and after Sc addition, which is significant for the mechanical properties of alloys.4) V.G. Davydov reported that, in the case of making joint scandium and zirconium additions, it is expedient to keep Sc and Zr content within ranges of 0.07–0.30% and 0.07–0.15%, respectively and Mn does not react with scandium and its role in scandium-bearing aluminum alloys is similar to that which it plays in commercial aluminum alloys.5) Peng Yongyi observed that the Al–Mg–Mn alloys, which added minor Sc and Zr, decreasing the content of Mn shows great combination of properties and exfoliation corrosion resisting property at reasonable annealing practice.6) Norman compares the AA7075 weld joint with Al–4Mg–2.8Zn–0.8Sc–0.1Ti–0.15Zr filler wire and the current commercial filler wires 5087, 5180, and 5039, the result shows the former have very high tensile strengths and much higher ductilities than can be obtained with any available commercial fillers.7)
Of all micro-alloying additions to aluminum alloy, Sc offers the best potential for developing new lightweight structural materials with great mechanical properties, desirable welding performance, and excellent corrosion and yield strength, when Mn gives a great potency for improving fracture strength and micro-hardness, reducing the brittleness of materials and the harmful influence of iron.8,9) Some transition metals, such as Cr, Sc, Ti, etc., can be added to aluminum alloy, among which Sc is the most efficient additive, the combined additions of Sc and Zr offer the greatest potential, because of the formation of extremely fine coherent secondary Al3(Sc, Zr) particles with L12 structure which can effectively inhibit recrystallization and improve the property of alloy.10–12) Based on the experimental welds, a filler wire composition of Al–5Mg–1Zn–0.3Sc–0.15Zr, was selected for further welding trials. In the main alloying elements of aluminum alloy (Mg, Zn, Li, Cu, Si), only Zn, Mg, and Li do not react with scandium. Therefore, it is very useful to add trace elements to Al–Mg–Zn alloys. Mg has the effect of solution strengthening in aluminum alloys, which can improve the work hardening rad of the material. However, with the increase of Mg content, there will be more precipitations at the grain boundary, thus reducing the corrosion resistance of the alloy. Hence, in this experiment, element Mn was added to improve the corrosion resistance of the alloy.
In order to improve the mechanical properties and corrosion resistance of the welded joint, the relationship between the microstructure characteristics, mechanical behavior, and corrosion behavior of 7075 aluminum alloy welded joints with different Sc, Zr, and Mn additives between with commercial ER5183 wires were studied in this dissertation.
In this experiment, sheets of 4 mm thick 7075 aluminum alloys were welded by tungsten inert gas (TIG) welding and four kinds of aluminum alloy with and without Sc, Zr, and Mn were used as filler metal. The chemical composition used in the alloy experiment is shown in Table 1. The resistance furnace, intelligent temperature controller, thermocouple, automatic stirrer, and Tjs-3000 intelligent ultrasonic processor V5.0 was used in the present study as foundry equipment, and Panasonic welding robot and AC TIG welding machine were used as welding apparatus.
According to the burning loss rate, the alloy needed for casting was prepared. Figure 1(a) exhibits the manufacturing processes of the composite filler metals. Pure aluminum with a purity of 99.94% put in alumina crucible melting in a resistance furnace, which the preheated intermediate alloy added in. The liquid aluminum was stirred evenly by an automatic stirrer and ultrasonic processor. After removed gas and slag, the liquid aluminum was raised to the casting temperature and poured into the preheated steel membrane, which can obtain a cylindrical ingot with a diameter of 12 mm by removing the mold. The homogenization treatment of the ingot casting was carried on at 470°C for 10 h and 520°C for 4 h, and rolled to a diameter of 3 mm as shown in Fig. 1(b), and then the finished welding wire was obtained, by cleaning and polishing process. The welding wire and the surface of base metal were cleaned before welding. After wiping the base metal with acetone or ethanol, clean the welding area with a metal brush. Table 2 lists the parameters of TIG welding.
Schematic illustration of the fabrication of microalloying of Sc, Zr and Mn fillers: (a) microalloying alloy preparation; (b) multi-pass rolling.
Microstructure and texture were characterized by scanning electron microscopy (SEM), X-ray diffractometer (XRD), transmission electron microscopy (TEM), and electron backscatter diffraction (EBSD). After welding, metallographic samples were cut in the direction perpendicular to the weld sheets. Metallographic specimens were prepared by conventional polishing followed by chemical etching with Keller reagent, and the metallographic structure was observed under the type ica-DMi8 metallographic micros. TEM specimens were prepared in the cross-section of the fusion zone of welded sheets by ion thinning and electrolysis double spraying, and their micromorphology was observed under jem-2100f tem. Electron back scattered diffraction (EBSD) observation was performed using a Zeiss GeminiSEM 300 emission gun scanning electron microscope equipped with EBSD system. Prior to examination with EBSD, the samples were electro-polished at 30 V using a mixture of 10 ml HClO4 + 90 ml C2H5OH solution after mechanical grinding and polishing. EBSD data were subsequently analyzed using Channel5 software.
Tensile mechanical properties (σb, σ0.2, δ5) were tested on SHIMADZU-AG201 electron tensile tester. Vickers microhardness was measured under 50 KN loads along the transverse cross-section of the joints by using a LECO-AMH43 digital micro-sclerometer.
Through electrochemical polarization measurement in 3.5% NaCl aqueous solution, the corrosion behavior of the specimen has been characterized. Electrochemical tests were carried out at room temperature using a standard three-electrode cell consisting of a platinum counter electrode, a single working electrode, and a saturated calomel reference electrode (SCE) on a GHI-760 instrument. The working electrode was an aged sample with an area of 1 cm2 from the fusion zone of the joints. The Tafel curves were recorded at a sweep rate of 0.001 V·s−1.13) Using the Tafel extrapolation method, the corrosion current density evaluated. To ensure reproducibility, all of the electrochemical parameters were repeated at least three tests. Stress corrosion cracking (SCC) susceptibility was evaluated through slow strain rate testing (SSRT). The specimens were prepared perpendicular to welded direction, with a thickness of 4 mm, a width of 4.5 mm, and a gauge length of 20 mm. The SSRTs were carried out in an aqueous 3.5 wt% NaCl solution at room temperature on a slow strain rate tensile machine (WDML-10) at the strain rate of 2 × 10−6 s−1. Subsequently, the fracture surfaces of the SSRT samples were evaluated on a Hitachi S-4800 scanning electron microscope (SEM, Hitachi, Tokyo, Japan), operating at 5 kV.
Figure 2 shows that there is almost no difference in X-ray diffraction between the first three welded joints, which consist of α (Al) and T [Mg32(Al,Zn)49] phases. In the welded joint using the welding wire 3, besides the T phase, the diffraction peak from Al6(Mn,Fe) phase can also be observed in X-ray diffraction patterns. In addition, primary Al3(Sc,Zr) particles cannot be detected by X-ray diffraction due to their low contents. Compared with the weld joint with 5183 wire, the microalloying joint moves to the right by a certain distance, which because the atomic radius of Mn is smaller than that of Al, while the Mn dissolved into Al matrix leads to the lattice distortion.
X-ray diffraction patterns of 7075 aluminum alloy welded joint.
Figure 3 shows backscattered scanning electron images of various microstructural zones of welded joints containing microalloyed fillers with different Sc, Zr, and Mn contents on the plane cross-section perpendicular to the welding direction. Coarse and continuous white second phases can be observed in four microstructural zones. To further determine the composition of the corrosion products on the surface of the samples, EDS detection was performed at the specified location in Fig. 3. According to EDS results, the intermetallic is identified as Mg Zn and Cu enriched compounds, which can be regarded as phase T [Mg32 (Al, Zn)49] combined with XRD results. This phase along grain boundary forms a brittle layer, leading to a decrease in the toughness and strength of the weld center. The results of EDS analysis also indicate that the white quadrilateral polygons particles with a size of 50∼80 µm in weld joint 3 shown in Fig. 3(d) were Al6(Mn, Fe) phases.14)
Backscattered scanning electron images of weld joints with different fillers containing: (a) wire 5183, (b) wire 1, (c) wire 2, (d) wire 3.
Obviously, Mn is dissolved in the aluminum matrix at first, and with the increase of Mn content, manganese begins to enrich and form a new phase. The precipitation probability of Al6(Mn,Fe) increases due to the grain refinement of Sc and Zr elements. Therefore, when the Mn content of the welding wire is 0.6%, Al6(Mn,Fe) phase appeared in the welded joint. Compared with the matrix, Al6(Mn,Fe) can act as a local cathode, which reduces the corrosion resistance of Al–Mg alloys.15) Compared with TIG welding with the commercial welding wire, the eutectic phase at grain boundary is more dispersed and discontinuous.
The bright-field TEM image shows the microstructure of two samples (Fig. 4). No obvious fine precipitated phase in the weld center corresponding to the commercial welding wire, but lots of tiny particles are uniformly dispersed in the weld corresponding to the welding wire containing 0.4% (mass fraction) Mn, and its size is between 20 nm and 40 nm. It shows a SAD pattern showing characteristic superlattice spots of these precipitates in Fig. 4(b). They clearly show in this figure that the presence of Ashby–Brown contrast for the bright-field images and the superstructure reflections confirms that this particle is Al3(Sc,Zr) particle with an Ll2 cubic crystal structure, coherent with the Al matrix.16–18) The fine second phase, namely Al3(Sc,Zr) particle, was dispersed in the grains, had a strong pinning effect on dislocations and sub-grain boundaries as showed in Fig. 4(d), which could significantly inhibit recrystallization.
TEM images of the weld metal zone of welded joint using different welding wires, (a) bright-field TEM image, wire 5183; (b)–(d) bright-field TEM image and selected area diffraction pattern, [200]Al projection, wire 2.
EBSD technology was performed to display the grain orientation distribution. Figure 5 shows the microstructure of the welded joint using different welding wires. The characteristic distribution of grain boundaries is summarized in Table 3. The high angle grain boundaries (HAGBs) with a misorientation more than 15° are indicated by black lines. The low angle grain boundaries (LAGBs) with a misorientation ranging from 2°–15° are depicted by white lines. There were essentially no grain boundaries with a misorientation less than 2°.
Orientation maps of welded joint using different welding wires and misorientation angle distribution, (a) 5183, (b) wire 1, (c) wire 2, (d) wire 3.
It is evident that the α-Al grain size was considerably refined in the weld joints with the addition of Sc, Zr, and Mn to the filler wires as Fig. 5 and Table 3 show. Weld joint welded by 5183 wire exhibited a mixed structure containing coarse elongated grains and a small amount of fine equiaxed grains (Fig. 5(a)), when the microalloyed weld (welded by wire 2) mainly contained fine equiaxed grain structures (Fig. 5(b)), which were formed by the coupling of heterogeneous nucleation by Al3(Sc, Zr) particles with recrystallization.12,19)
We can conclude that compared with the sample with wire 5183, the proportion of LAGBs in the sample with wire 2 increased from 10.22% to 56.76%, and the grain boundary angle of 2–5 accounted for a larger proportion as shown in Fig. 5(e). Concomitantly, the average grain size dropped rapidly. As summarized in Table 3, the addition of rare earth Sc and Zr refine primary α-Al phases, as a result of the Inhomogeneous nucleation by Al3(Sc, Zr) particles, the grains in the welds prepared with Al–Mg–Zn–Sc–Zr–Mn filler wires were significantly refined. It can also be found in Table 3 that, with the increase of Mn content, the number of large-angle grain boundaries first decreases and then increases slightly, which means the refining effect of α-Al grains is lower by the complex addition of excessive Mn.
3.2 Mechanical propertiesThe microhardness distributions of the TIG joint are across the transverse cross-section illustrated in Fig. 6. It can be seen that the TIG weld joint is composed of fusion zone (FZ) and heat-affected zone (HAZ). The lowest micro-hardness values are located at the weld center, and it doesn’t fluctuate very much. With the increase of the distance away from the weld center to the base metal, the hardness decreases slightly in the HAZ, and then both increase significantly to 145 HV in the base metal. Obviously, with the increase of Sc, Zr, and Mn content, the hardness values in the weld zone increased. In the weld zone, the solution of Sc, Zr, and Mn leads to a solid solution strengthening, which results in a higher hardness compared with commercial filler wire. When Mn content in welding wire is less than 0.6% (mass fraction), the hardness of the weld center increases with the increase of Mn content, but when the content of Mn increases further, the hardness decreases with the increase of manganese content. In the TIG welding process, the eutectic T phase in the weld center gradually changed from continuous distribution to discontinuous distribution owing to the increase of Mn element, making the fine grains strengthened.
Microhardness distributions of the weld joints produced with different wire.
The tensile properties, such as yield strength, tensile strength, elongation, joint failure position, weld joint coefficient, were evaluated (as shown in Table 4). Compared with the reference weld, the ultimate tensile strength and ductility of the microalloyed weld are significantly improved. The tensile strength and elongation of welding seam welded with wire 2 welding wire were 10.4% and 56.7% higher than those of the commercially available 5183 welding wire, respectively, while the yield strength was almost the same.
The strength of microalloyed welds can be improved by adding Sc and Zr. The primary Al3(Sc,Zr) particles formed in the weld center can refine the grain, and the fine particles precipitated can stabilize the substructure, hinder dislocation movement, and further improve the strength of the alloy.20) There are two main mechanisms of particle reinforcement: one is the shear strengthening mechanism of tiny particles, which include ordered strengthening, coherent strengthening, and modulus mismatch strengthening; the other is the Orowan bypass mechanism for large particles. With the increase of particle size, the first step is to start the cutting mechanism. When the particle size is larger, the Orowan mechanism starts. According to the references, for the Al–Sc–Zr alloy, the critical particle diameter from the cutting mechanism to the bypassing mechanism is 4∼6 nm, which means that the particle strengthening mechanism in this study is principally the Orowan mechanism.21,22)
In general, as the decrease of grain size, the volume fraction of grain boundaries increases, so local plastic deformation is limited by the grain boundaries. It is universally acknowledged that the yield strength of metals increases with the refinement of grains according to the Hall-Petch relationship:23)
| \begin{equation} \Delta \delta\text{y} = \mathrm{KyD}(-1/2) \end{equation} | (1) |
A polarization curve is another method that can reveal the corrosion behavior of alloys. The results of the potentiodynamic polarization tests and the Open circuit potential (OCP) measurements of all specimens are illustrated in Fig. 7; their kinetic parameters, the corrosion potential, and corrosion density are listed in Table 5. The characteristics of the four curves are almost the same that passivation zones exist; the anodic and cathodic branches are asymmetric, indicating the same polarization behavior in all samples.
Polarization curves and open circuit potentials versus time curves for the four samples.
On account of the variation in the corrosion potential (Ecorr), the corrosion current density (Icorr), and the reaction kinetics, it is noticeable that the contents of Sc, Zr, and Mn affect the polarization characteristics. The microalloyed welded joints have lower Icorr values, while the Icorr values without microalloyed welded joints are higher, so the anodic kinetics rate of the latter is higher. In the Sc, Zr, and Mn microalloyed weld joint, the corrosion potential moved to positive and showed passive behavior. With the increase of Mn content in the weld, the corrosion potential of the weld increases gradually. However, when the contents of Mn exceed a certain level, the addition of Mn will not increase the corrosion potential but decrease it. For aluminum alloys, the more positive the Ecorr value, the better the corrosion resistance. The negative shift in the open-circuit potential of Al alloys indicates higher electrochemical activity.25) And the greater the fluctuation of open-circuit potential, the greater the corrosion sensitivity, which can partly indicate the corrosion resistance of all the alloys. Generally speaking, there is a positive correlation between the Icorr and corrosion resistance.26) Therefore, as shown in Fig. 7, the corrosion trend of different welded joints was in order from large to small: wire 5183 ≈ wire 3 > wire 1 > wire 2.
In order to explain the mechanism of the potentiodynamic polarization tests, the backscattered scanning electron images and the Schematic diagram of corrosion of weld joint suffering was shown in Fig. 8 and Fig. 9, respectively. It can be seen in Fig. 8 that the localized corrosion progress started from the eutectic areas, especially the T phase which is distributed continuously along the grain boundary. Therefore, the precipitation and distribution of T phase are the main factors affecting the electrochemical properties of materials.
Backscattered scanning electron images of weld joint suffering the potentiodynamic polarization tests.
Schematic diagram of corrosion for weld joint suffering the potentiodynamic polarization tests.
It is well known that T phases act as anodic during the potentiodynamic polarization testing because of the high Mg contents. As shown in Fig. 3, the continuous and large T phases were obtained in weld joints without microalloyed. When corrosion occurs, the T phase would dissolve first, as shown in Fig. 8 and Fig. 9, after the T phase fell out, some corrosion pits remained and corrosion would continue along this path. In the weld joint with commercial filler, the T phase existed in the form of irregular and coarse second phase, which increases local corrosion. However, the form of T phases was restricted when the Sc, Zr, and Mn were introduced into the weld joints, which indicates that micoralloyed of the weld wires can improve the corrosion resistance of the weld joints. Moreover, with the increase of Mn, the T phase gradually changes from the continuous coarse phase distributed along the grain boundary to a small spherical phase. Thus, the refinement of T phase improves the electrochemical performance of welded joints. The harmful effects of iron in the composition of commercially pure aluminum reduced with the addition of Mn and part of Mn were solidly soluble in the Al matrix, which increases the corrosion potential of Al, thus reducing the corrosion rate.27) Compared with the matrix α (Al), the Ecorr of Al6(Mn,Fe) was higher, which made Al6(Mn,Fe) into local cathodes concerning the matrix.28) Therefore, adding excessive Mn in Al–Zn–Mg–Sc–Zr filler would promote the precipitation probability of Al6(Mn,Fe), which reduces the corrosion resistance of the alloy.
The slow strain rate test (SSRT) investigated the stress corrosion cracking of the weld joint. Figure 10(a) depicts the stress-strain curves of the welded joint for the slow strain rate testing (SSRT) test of the welded joint at 3.5 wt% NaCl solution, and Fig. 10(b) characterizes the tensile properties of the weld joints with commercial 5183 filler or self-made filler wires containing 0.2, 0.4, and 0.6 wt% Mn at air. The stress-strain curves at solutions showed almost the same trend as the engineering stress-strain curves at air. During SSRT, the ultimate tensile strength (UTS) and the time to failure of the welds with separate wires summarized in Table 6.
(a), Stress–strain curves of the welded joints with and without microalloyed during slow strain rate testing (SSRT) in 3.5 wt% NaCl solution. (b), Strength and Elongation values of weld joints in air.
As is seen, all of the samples exhibit higher UTS, elongation, and the time to failure in welded joints with microalloyed wire than those without. The ultimate tensile strength (UTS) of the weld joints prepared with filler wire 1 is 363 MPa, whereas the value of the weld joints without microalloyed wire is only 278 MPa. And with the change in welded joint with different wire, the welded joint tensile strength exhibits the following trend: wire 5183 < wire 3 < wire 1 ≈ wire 2.
In aging reinforced alloys, the strength or hardness of the materials is mainly determined by the dissolution, coarsening, and precipitation of the aging reinforced phase. It is well accepted that grain boundary precipitates (GBPs), like T phase, and discontinuous distribution are beneficial to improving the SCC resistance of Al–Zn–Mg–Cu alloy.29) It can be seen that the refinement or dissolution of T phases in the Al matrix of weld joints leads to the decrease of SCC sensitivity, which causes by the Mn content. In the welds without microalloyed, the T phases distribute continuously along the grain boundaries, compared with microalloyed weld joint, which leads to a weakening in SCC sensitivity.
To explain the mechanism of the Slow strain rate tests, the fractographies of the 7075 welded joints suffered from SSRT testing in solution were observed by SEM. It can be seen from the Fig. 11 that the fractography of the welding joint welded by the commercial 5183 wire was covered by a few large dimples and extensive distribution of facets, while the one welded by Al–5Mg–1Zn–0.3Sc–0.15Zr–xMn has obvious characteristics of ductile fracture such as dimples and micro-void (Fig. 11(b)–(d)). Furthermore, obvious tearing ridges and tearing planes appear in weld joint, as shown in Fig. 11(c) and (d).
Fractographic pictures of the weld joint using different filler: (a) wire 5183, (b) wire 1, (c) wire 2, (d) wire 3.
With the increase of Mn content, the fracture dimple size decreases gradually and the hard points progressively disperse and grow. According to reports, there is a corresponding relationship between the decrease of the dimple size and increase of the strength. The smaller the dimple size, the higher the strength of the corresponding weld. Small-angle grain boundaries are beneficial to the improvement of corrosion resistance, because the corrosion cracks easily cross the grain boundary with a large Angle, while it is difficult to cross the grain boundary with a small angle.30) Moreover, grain refinement provides a smoother slip mode, reducing the sensitivity to stress corrosion cracking.31) It can be seen from Table 3 that a larger proportion of grain boundaries with small Angle and smaller grain sizes can be obtained by adding the proper amount of Mn. From this we can infer, the SCC susceptibility of the welds decreased as an appropriate amount of Mn was introduced into the weld, which is consistent with the experimental results.
The strength and corrosion resistance of the 7000 series (Al–Zn–Mg–Cu) alloys seldom co-exist and are often trade-off with each other.29) The simultaneous improvement in tensile strength and corrosion resistance in the microalloyed welds can be attributed to the changes in the microstructure of the weld joints. It is acknowledged that the shape, size, and distribution of the second phase and the grain size of the matrix phase have a significant influence on the mechanical properties of alloys.32,33) In this study, the mechanisms of strength and ductility and corrosion behavior can be attributed to the following factors: (i) the formation of hard Al3(Sc,Zr) particles formed in the HAZ increased dislocation density;34,35) (ii) the size of grain decreases in the weld joint with microalloying (Fig. 5); (iii) Solid solution enhancement of Mn; and (iv) refined or dissolved of T phases in the weld joints by Mn addition (Fig. 3).
In this paper, the TIG welding joint welded by self-made Al–Mg–Zn–Sc–Zr–Mn microalloy welding wire. And the microstructure, corresponding mechanical properties, and the corrosion behavior of welds were investigated. The main results of the current study are as follows:
Hence, for the TIG welding of aluminum and its alloys, the Al–Mg–Zn–Sc–Zr–Mn wires with appropriate Mn content can be applied as novel fillers.
The authors are grateful to the Science and Technology Major Project of Guangxi, China (No. AA17129005), and Collaborative Innovation Center for Exploration of Nonferrous Metal Deposits and Efficient Utilization of Resources, Guilin University of Technology, China.
