Summary
To solve the problem of poor joint performance caused by the fragile compound layer at the aluminum/titanium interface, nanosecond lasers have been used to treat the surface of titanium alloys through cross-linking and linear treatment, which changes the surface micromorphology . Next, laser brazing of the 6061 aluminum alloy and the TC4 titanium alloy was carried out.
The study showed that the spreadability of the filler metal improves significantly with decreasing point spacing. Laser surface texturing treatment can effectively improve the shape of the weld surface, and the network treatment was more effective than the linear treatment.
The texturing treatment has a minor impact on the types of interfacial compounds, which are brittle Ti-Al compounds, mainly affecting the growth direction and morphology of the compounds in the pits. After dot matrix treatment, the tensile load of the aluminum/titanium laser welded joint increased from 5% to 21%.
Pitting, which resulted from the texturing treatment, effectively blocked crack propagation, while the linear treatment had a negligible effect on the properties of the aluminum/titanium joints.
The study highlights the need to improve the wetting effect of molten solder, while ensuring the wetting of dissimilar metals and improving the mechanical properties of joints. This will be the main focus in the next stage of the research.
Preface
The aluminum/titanium composite structure has high specific strength, excellent corrosion resistance, economic and energy-saving benefits, and ease of processing. As a result, it has significant potential for application across a wide range of industries, including aerospace, shipbuilding and automobile manufacturing.
Airbus, for example, employs a titanium plate frame with aluminum ribs for the seat guide rail and welds aluminum alloy sheets to titanium alloy tubes to manufacture engine room radiators. In the automotive sector, Germany has developed an aluminum/titanium composite exhaust system that is 40% lighter than traditional steel exhaust systems.
The aluminum/titanium different material structure meets the rigorous demands of modern industry in terms of energy conservation, emissions reduction and performance retention. Consequently, the technology connecting the two has attracted a lot of attention.
However, the physical and chemical properties of aluminum alloy and titanium alloy are quite different, making it difficult to control the thickness of the brittle compound during welding, which poses a challenge for reliable bonding between the two materials. This limitation has hampered the application of aluminum alloy and titanium alloy composite components.
The rapid development of laser welding technology has led to its widespread use in modern industry. Laser fusion brazing has enabled precise control of heat input and effective regulation of interface compounds, making it an attractive option for connecting aluminum and titanium plates.
Since the mechanical properties of different metal joints between aluminum and titanium and the wetting and spreading effect of welds are related to interface compounds, researchers have conducted extensive research to improve these properties by adding alloying elements and regulating heat input.
On the one hand, the wettability of the weld metal significantly influences the performance of the joint. For example, Cui Qinglong found that by adjusting the welding parameters when welding TC4 titanium alloy and 5A06 aluminum alloy, the optimal wettability of the filler metal can significantly improve the tensile strength of aluminum/titanium non-metal joints .
On the other hand, the type, morphology and distribution of interfacial compounds play a decisive role in the mechanical properties of the joints. However, controlling the interface structure using conventional methods can be very challenging.
In this study, laser surface texturing was used to treat titanium plates. By improving the wettability of the filler metal on the titanium surface and regulating the morphology and distribution of the interface reaction layer, the bonding quality of aluminum/titanium dissimilar metals was improved, resulting in joints with good mechanical properties.
The study revealed the influence of laser texturing on the weld shape, mechanical properties and microstructure of the aluminum/titanium laser fusion brazing interface.
1. Test materials and methods
The test specimens are composed of plates made of TC4 titanium alloy and 6061 aluminum alloy, both measuring 100 mm x 50 mm x 1.5 mm.
The 6061 aluminum alloy is rolled and its chemical composition is presented in Table 1, while the composition of the TC4 titanium alloy is available in Table 2.
For the filler wire, ER4043 (AlSi5) aluminum silicon welding wire with a diameter of 1.2 mm is chosen. See Table 3 for its chemical composition.
Table 1 Chemical compositions of 6061（wt%）
| Al | You | mg | Yes | Faith | Ass |
| Rem. | 0.15 | 0.80-1.20 | 0.40-0.80 | 0.70 | 0.15-0.40 |
Table 2 Chemical compositions of TC4（wt%）
| You | Al | V | Faith | W | N | H | O |
| Rem. | 5.50-6.80 | 3.50-4.50 | 0:30 | 0.10 | 0.05 | 0.01 | 0.20 |
Table 3 Chemical compositions of ER4043（wt%）
| Al | Yes | Faith | Ass | You | Zn | mg | Mn |
| Rem. | 5:00 | 0.80 | 0:30 | 0.20 | 0.10 | 0.05 | 0.05 |
Before welding, use a chemical cleaning method to remove the oxide film on the surface of the aluminum plate. Use a 6% to 10% NaOH aqueous solution at 40°C to 60°C for approximately 7 minutes for alkaline cleaning.
Then, immerse the specimen in 30% HNO3 for approximately 3 minutes to neutralize and undergo photochemical treatment, removing any gray or black ash hanging on the surface. To clean the titanium plate, use a HCl-HF solution (3:1).
For welding test, use the YLS-6000 IPG fiber laser, and the test platform is shown in Figure 1a. Based on previous research, the test parameters are set as follows: laser power 2000 W, defocus amount +20 mm, welding speed 0.5 m/min, wire feed speed 5 m /min and shielding gas (99.9% Air) flow rate of 10 L/min.
Figure 1b illustrates the aluminum/titanium laser brazing process using the continuous light emission method.
The base metal was covered with a titanium plate on top and an aluminum plate on the bottom, with a lapping width of 5 mm.
Two groups of titanium alloy-based materials were treated with a low-power laser, one group being subjected to matrix texturing and the other to linear texturing.
For matrix texturing, the spacing between points was varied with values of 0.8 mm, 1.0 mm and 1.2 mm. On the other hand, for linear texturing, the linear spacing was varied with values of 0.2 mm, 0.4 mm and 0.6 mm. The linear processing direction was parallel to the welding direction.
Fig.1 Laser welding-brazing equipment and Al/Ti schematic
After treatment, regular grooves and depressions form on the surface of the titanium plate, as depicted in Fig. 2. Fig. 3 illustrates the three-dimensional morphology of grooves and grooves observed under the ultra-depth-of-field microscope. 、
As shown in Figure 3, the number of cavities and grooves generated per unit area varies under different spacing between points and lines. However, the depth and diameter (width) of the pits and grooves remain constant. This indicates that the smaller the spacing, the greater the increase in the surface area of the titanium plate.
Fig.2 Laser surface texturing of titanium alloy
Fig.3 3D morphology of titanium alloy with laser surface texture
After welding, cut it perpendicular to the weld and process it into a tensile specimen measuring 50 mm x 10 mm for testing the mechanical properties of the joint.
To ensure accurate test results, shims should be added to both ends of the aluminum and titanium plates during testing to prevent torque or deflection during the pulling process.
The metallographic samples must be polished and the weld microstructure characterized using an optical microscope (OM), scanning electron microscope (SEM) and energy dispersive spectrometer (EDS).
2. Test results and analysis
2.1 Effect of different texturing methods on weld formation
The roughness of the titanium alloy substrate increases with smaller interlattice spacing in laser texturing. This, in turn, increases capillarity, which promotes direct propagation of the weld metal.
Figure 4 illustrates the macroscopic morphology of aluminum/titanium laser brazing weld under different grid spacing. There is a significant difference in weld formation between different experimental groups.
Without texturing, the brazing filler metal does not spread well, resulting in poor weld formation. The weld metal is not moistened during solidification, forming a large wetting angle and a weak spreading effect.
However, after texturing treatment, the weld formation is significantly improved, resulting in a good wetting and spreading effect, leading to continuous and stable weld formation.
Figure 4e displays the statistical results of the wetting angle and spreading width of the filler metal under different lattice spacings. As the spot spacing decreases, the wetting angle gradually decreases and the spreading effect of the weld metal improves.
The enhancement effect is more significant with a smaller dot spacing. This is mainly due to the capillary effect of the network, which promotes the spread of the molten weld, resulting in better weld formation.
Fig.4 Weld appearances of the Al/Ti joint produced at different point spacings
Figure 5 shows the macromorphology of aluminum/titanium laser brazing weld at different linear intervals.
The corresponding values for the weld wetting angle and weld spreading width at different straight line spacings are shown in Figure 5e.
As the spacing between straight lines decreases, the wetting angle remains relatively unchanged, while the solder's spreadability increases slightly. However, the effect of improving solder spreading ability is weaker than that of network processing.
This implies that the energy barrier created by the slot treated with straight lines is greater than that of grid processing. As a result, it hinders the movement of the molten weld metal. Furthermore, the edge of the groove has a clamping effect on the three-phase line, thus inhibiting further spreading of the molten metal.
Fig.5 Appearance of the Al/Ti joint weld produced under different linear spacings
2.2 Effect of different texturing methods on tensile properties
The test results of tensile properties of joints under different texturing modes are presented in Fig. 6, all of which fail at the interface.
The tensile load of the joint without texturing treatment was 2345N.
Matrix treatment improved joint performance by 5% to 21%, while aluminum/titanium joint performance was not affected by linear treatment.
The analysis reveals that the dot matrix treatment resulted in a lower weld joint contact angle, larger weld width and greater mechanical bite effect, leading to a significant improvement in the tensile strength of the dot matrix treatment sample.
However, the linear treatment made it more difficult to spread the molten filler metal, resulting in an insignificant difference in the spreading effect and tensile properties.
Fig.6 Joint tensile test results
2.3 Effect of different texturing methods on interface structure
The microstructural characteristics of the fusion welded joint interface after matrix texturing treatment are shown in Fig.
The microstructure of the joints after dot matrix treatment is similar to that of untreated joints, as dot treatment is minimal and most of the cross-sectional morphology does not show dot treatment points.
According to literature research, the interface composite layer generated at the interface after texturing treatment no longer has a smooth distribution in the cavities and grooves. Instead, it is distributed in a zigzag pattern across the interface.
This pattern increases the effective interface connection area while improving mechanical mosaicism, leading to better mechanical properties of the joint.
However, due to the large temperature gradient caused by local laser heating, the microstructures of the weld toe b and the intermediate irradiation zone c are different.
Fig. 7d illustrates that the thickness of the reaction layer in the weld tip area is thin, and the line scan results show an enrichment of the Si element, which can be speculated to be the TiAlSi phase.
On the other hand, the thickness of the reaction layer in the middle irradiation zone is about 30 μm, and the scan results indicate that it is a brittle TiAl phase with 55.69% Al, 44.22% Ti and 0.08% Mg.
Fig.7 Interface microstructure of the Al/Ti joint with matrix texturing
The interface structure characteristics of fusion welded joints with linear treatment are shown in Fig.
When the laser acts on the joint, the filler metal melts and fills the grooves in the titanium plate by capillary action and its own fluidity.
It was found that the compounds formed in the treated wells in a straight line near the weld tip in zone b and laser irradiation zone e. The direction of its growth was inconsistent with the direction of the matrix (see Fig. 8c), which could play a role in inhibiting crack growth.
The tissue in the area directly irradiated by the laser is thicker.
The energy spectrum results indicate that point b contains 60.93% Al, 38.73% Ti and 0.33% Mg, while point e contains 4.16% Al, 25.19% Ti and 0.65% Mg.
It is inferred that the brittle intermetallic compound is the TiAl3 phase, and the continuous interfacial brittle compound may be the source of interface failure.
Fig.8 Microstructure of Al/Ti joint interface with linear texturing processing
After analyzing the above microstructure observation results, it is evident that the dot array and linear texturing have a minimal effect on the interface morphology. Furthermore, the interface generates continuous reaction products.
Due to the high brittleness of the interface compound, an untreated interface can become a source of cracking. These cracks can continue to expand into the flat, brittle layer of the composite, ultimately leading to fracture of the joint.
Although the interface compound layer will also produce cracks after texturing, the base metal and interface compound will be serrated. As a result, when microcracks extend to the serrated edge, they will be blocked, inhibiting further expansion of the crack and preventing brittle fracture of the joint.
In short, the serrated interface formation of laser texturing reduces the chances of large-scale crack propagation in the brittle composite layer, thereby improving the mechanical properties of the joint.
The SEM morphology of the aluminum/titanium fracture surface under dot matrix treatment is shown in Figure 9 .
It can be seen that part of the weld metal in the fracture, especially the dents after texturing treatment, adhered to the titanium substrate during stretching, resulting in regular “bumps” on the surface, as shown in Figure 9a. This indicates that the lattice treatment effectively improved the adhesion of the joint.
Power spectrum analysis identified that crater adhesion is the weld metal formed after weld melting (#1: Al content 98.39%, Ti content 0.46%, Mg content 1.15% ). Furthermore, the titanium substrate well is surrounded by Ti-Al compounds (#2: 38.56% Al, 60.32% Ti, 1.12% Mg), as shown in Figure 9d.
These findings indicate that when a fracture occurs, the interface crack does not pass through the pit interface but instead cuts the weld metal in the pit. This suggests that the pit effectively blocks crack growth and improves joint performance.
These results provide valuable information for future research.
Fig.9 Morphology of the joint fracture surface with point texturing
Figure 10 shows the SEM morphology of the aluminum/titanium fracture surface after linear treatment.
As seen in Figures 10b and 10d, after linear texturing treatment, some weld metal remains on the titanium substrate at the fracture surface of the joint.
Analysis of the energy spectrum reveals that the metal in the pit is filler metal (#1: Al content 69.19%, Ti content 1.68%, Mg content 0.94%, Si content 21.52 %), which is surrounded by Ti-Al reaction products (#2: Al content 33.28%, Ti content 55.18%, Mg content 1.81%).
Thus, the grooves created by linear treatment play a crucial role in preventing the propagation of cracks at the interface.
However, the mechanical properties of the joint did not improve significantly due to limited wetting and spreading of the molten solder.
Fig.10 Morphology of the fracture surface of the joint with linear texturing
In summary, different texturing methods can have varying effects on weld metal wettability, mechanical properties, and joint microstructure.
After undergoing matrix texturing treatment, the filler metal is able to flow into the grooves and grooves during welding. This is due to the capillary effect, which facilitates the propagation of the filler metal on the titanium surface, resulting in better mechanical properties.
On the other hand, linear texturing treatment has no significant effect on solder spreading. Grooves parallel to the weld generate an energy barrier that prevents further spreading of the molten weld.
However, both texturing methods can increase the interface connection area, and the interface compound will become serrated, which can inhibit the propagation of large-scale cracks.
It should be noted that linear texturing treatment does not have a significant effect on improving the filler metal spreading area and mechanical properties.
3. Conclusion
(1) Laser surface texturing can significantly improve the formation of weld surfaces.
After dot matrix treatment, the wetting angle decreased from 98° to a minimum of 62°. The improved weld wettability, due to capillarity, resulted in a decrease in the weld metal wetting angle and an increase in the weld spreading width.
Dot matrix treatment is more effective than linear texturing treatment in increasing weld wettability, and the improvement is more significant with smaller dot spacing.
(2) Matrix texturing treatment can significantly improve the tensile properties of the joint, increasing the tensile load by 21% compared with an untreated joint.
The matrix texturing treatment improves the wettability of the weld and increases the effective area of the joint, while the holes formed in the network block the propagation of cracks.
Although linear texturing treatment can also prevent cracking, it does not significantly improve joint wettability and propagation, leading to no significant improvement in joint performance.
(3) Texturing treatments have little effect on the type of intermetallic interface, which are all brittle Ti-Al compounds. The brittle and continuous intermetallics at the interface form a source of cracks.
However, texturing treatment increases the effective connection area of the interface and changes the morphology of the interface composite. The growth orientation of the compound formed by texturing treatment is different from that of the continuous compound without substrate treatment. The composite layer changes from a straight distribution to a zigzag distribution, which inhibits crack expansion, reducing the possibility of large-scale crack growth in the interface composite.
(4) The following research focuses on how to further improve the wetting effect of molten solder, under the premise of texturing, to improve the mechanical properties of joints and ensure the wetting of dissimilar metals.
