In order to solve the problem that the brittle compound layer at the interface of aluminum/titanium joint leads to poor joint performance, nanosecond laser was used to treat the surface of titanium alloy with lattice and linear treatment to change the surface micro morphology, and laser brazing of 6061 aluminum alloy and TC4 titanium alloy was carried out.
The results show that the spreading ability of filler metal is significantly enhanced with the decrease of point spacing.
Laser surface texturing treatment can effectively improve the weld surface shape, and the effect of linear treatment is weaker than that of lattice treatment;
Texturing treatment has little effect on the types of interfacial compounds, which are brittle Ti Al compounds, mainly changing the growth direction and morphology of compounds in the pits;
The tensile load of the aluminum/titanium laser welded joint was increased by 5%~21% after dot matrix treatment.
The pit after texturing treatment blocked the crack propagation well, while the linear treatment had little effect on the properties of the aluminum/titanium joints.
It is pointed out that the key work in the next step is how to improve the wetting effect of molten solder under the premise of texturing treatment, and ensure the wetting of dissimilar metals while improving the mechanical properties of joints.
Aluminum/titanium composite structure has high specific strength, good corrosion resistance, economic and energy-saving advantages, and is easy to process.
It has broad application prospects in aerospace, shipbuilding, automobile manufacturing and other fields.
In the aviation field, Airbus uses the titanium plate aluminum rib structure as the seat guide rail, welding aluminum alloy blades to titanium alloy tubes to make the engine room radiator;
In the automotive industry, Germany has developed an aluminum/titanium composite exhaust system, which is 40% lighter than the steel exhaust system.
The aluminum/titanium dissimilar material structure meets the strict requirements of modern industry for energy conservation, emission reduction and performance retention, so the connection technology of the two has attracted extensive attention.
However, the physical and chemical properties between aluminum alloy and titanium alloy are quite different, and the thickness of the brittle compound is difficult to control during welding, which causes great difficulties for the reliable connection between the two materials, limiting the application of aluminum alloy and titanium alloy composite components.
With the rapid development of laser welding technology, it has been widely used in modern industry.
The connection of aluminum plate and titanium plate by laser melting brazing can accurately control the heat input and effectively regulate the generation of interface compounds, so it has attracted much attention.
Because the mechanical properties of aluminum/titanium dissimilar metal joints and the wetting and spreading effect of welds are related to the interface compounds, relevant scholars have conducted a lot of research on improving the mechanical properties of aluminum/titanium welded joints by adding alloy elements and regulating heat input.
On the one hand, the wettability of weld metal has an important influence on joint performance.
Cui Qinglong found that when welding TC4 titanium alloy and 5A06 aluminum alloy, by changing the welding parameters, the wettability of filler metal is optimal, which can significantly improve the tensile strength of aluminum/titanium dissimilar metal joints;
On the other hand, the type, morphology and distribution of interfacial compounds play a decisive role in the mechanical properties of joints, but it is very difficult to carefully control the interface structure by using conventional methods.
In this study, laser surface texturing was used to treat titanium plates.
By improving the wettability of filler metal on the titanium surface and regulating the morphology and distribution of the interface reaction layer, the connection quality of aluminum/titanium dissimilar metals was improved, and the joints with good mechanical properties were obtained.
The influence of laser texturing on the weld shape, mechanical properties and interface microstructure of aluminum/titanium laser fusion brazing was revealed.
1. Test materials and methods
The test materials are TC4 titanium alloy and 6061 aluminum alloy plates, both of which are 100 mm×50 mm×1.5 mm in size.
6061 aluminum alloy is in rolled state, and the chemical composition is shown in Table 1, while TC4 titanium alloy is shown in Table 2.
ER4043 (AlSi5) aluminum silicon welding wire with a diameter of 1.2 mm is selected as the filler wire. See Table 3 for its chemical composition.
Table 1 Chemical compositions of 6061（wt.%）
Table 2 Chemical compositions of TC4 （wt.%）
Table 3 Chemical compositions of ER4043 （wt.%）
Before welding, use chemical cleaning method to remove the oxide film on the surface of aluminum plate, use 6%~10% NaOH aqueous solution at 40~60 ℃ for alkaline cleaning for about 7 min, then put the test piece into 30% HNO3 for about 3 min for neutralization and photochemical treatment, remove the gray or black hanging ash on the surface, and use HCl-HF (3:1) solution to clean the titanium plate.
The welding test adopts IPG YLS-6000 fiber laser, and the test platform is shown in Fig. 1a.
According to the previous research, the test parameters are set as follows: laser power 2000 W, defocusing amount+20 mm, welding speed 0.5 m/min, wire feeding speed 5 m/min, shielding gas (99.9% Ar) flow rate 10 L/min.
The aluminum/titanium laser brazing process is shown in Figure 1b. The continuous light emitting method is adopted.
The base metal is lapped with titanium plate on the top and aluminum plate on the bottom, and the lapping width is 5 mm.
Two groups of titanium alloy base materials were treated with dot matrix texturing and linear texturing respectively by low-power laser.
The dot spacing of dot matrix texturing was 0.8 mm, 1.0 mm and 1.2 mm respectively, and the linear spacing of linear texturing was 0.2 mm, 0.4 mm and 0.6 mm respectively.
The direction of linear processing was parallel to the welding direction.
Fig.1 Laser welding-brazing equipment and schematic of Al/Ti
After treatment, regular grooves and pits are formed on the surface of titanium plate, as shown in Fig. 2.
Fig. 3 shows the three-dimensional morphology of pits and grooves observed under the ultra depth of field microscope.
It can be seen from Fig. 3 that under the condition of different spacing between points and lines, the number of pits and grooves generated per unit area is different, but the depth and diameter (width) of pits and grooves are the same, that is, the smaller the spacing, the greater the increase of titanium plate surface area.
Fig.2 Laser surface texturing of titanium alloy
Fig.3 3D morphology of laser surface textured titanium alloy
After welding, cut it perpendicular to the weld, and process it into a 50 mm×10 mm tensile sample for joint mechanical property test.
During the test, add shims at both ends of the aluminum plate and titanium plate to prevent torque or deflection during the tensile process from affecting the test results.
The metallographic samples were polished, and the weld microstructure was characterized by 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 smaller the lattice spacing of laser texturing, the greater the roughness of titanium alloy substrate, and the greater the roughness, the more obvious the capillarity, which can promote the forward spread of weld metal.
The macroscopic morphology of aluminum/titanium laser brazing weld under different lattice spacing is shown in Fig. 4.
There is a large gap in weld formation between different experimental groups.
It can be seen that the brazing filler metal does not spread well without texturing, the weld formation is poor, the weld metal is not wetted when solidification, the wetting angle formed is large, and the spreading effect is poor;
After the texturing treatment, the formation is obviously improved, the wetting and spreading effect is good, and the continuous and stable weld formation is formed.
The statistical results of the wetting angle and filler metal spreading width under different lattice spacing are shown in Fig. 4e.
With the decrease of the dot spacing, the wetting angle gradually decreases, and the spreading effect of weld metal gradually improves.
The smaller the dot spacing, the more obvious the improvement effect.
This is mainly due to the capillary effect of the lattice to promote the spread of molten solder, thereby improving the weld formation.
Fig.4 Weld appearances of Al/Ti joint produced at different spot spacings
The macro morphology of aluminum/titanium laser brazing weld at different linear intervals is shown in Fig. 5.
The weld wetting angle and solder spreading width corresponding to different straight line spacing are shown in Fig. 5e.
As the straight line spacing decreases, the wetting angle does not change significantly, the weld spreading ability increases slightly, and the improvement effect of solder spreading ability is weaker than that of lattice processing.
This indicates that the energy barrier generated by the groove treated with straight line is greater than that of lattice processing, which hinders the movement of molten weld metal, and the edge of groove has a pinning effect on the three-phase line, inhibit the further spreading of molten metal.
Fig.5 Weld appearance of Al/Ti joint 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 shown in Fig. 6, which are all broken at the interface.
Without texturing treatment, the tensile load of the joint is 2345N.
The performance of the joint is improved by 5%~21% after the dot matrix treatment, while the performance of the aluminum/titanium joint is not affected by the linear treatment.
The analysis shows that the contact angle of the welded joint is smaller, the weld width is larger, and the mechanical bite effect is greater under the dot matrix treatment, so the tensile strength of the dot matrix treatment sample is significantly improved;
However, it is more difficult to spread molten filler metal with linear treatment, which makes the spreading effect little different and then leads to little difference in tensile properties.
Fig.6 Results of tensile test on joints
2.3 Effect of different texturing methods on interface structure
The microstructure characteristics of the interface of the melt brazed joint after dot matrix texturing treatment are shown in Fig. 7.
The microstructure of the joints after dot matrix treatment is similar to that of the untreated ones.
This is because the dot treatment is very small, and most of the cross section morphology does not observe the dot treatment pits.
According to literature research, the interface compound layer generated at the interface after texturing treatment no longer presents a smooth distribution at the pits and grooves, but is zigzag distributed along the interface.
On the one hand, it increases the effective connection area of the interface, on the other hand, it enhances the mechanical mosaicism.
The mechanical properties of the joint are improved.
Due to the large temperature gradient caused by local laser heating, the microstructures of weld toe b and middle irradiation zone c are different.
It can be seen from Fig. 7d that the thickness of the reaction layer in the weld toe area is thin, and the line scanning results show that there is enrichment of Si element, which may be speculated to be Ti Al Si phase.
The thickness of the reaction layer in the middle irradiation zone is about 30 μm, and the scanning results show that it is a brittle TiAl phase (55.69% Al, 44.22% Ti, 0.08% Mg).
Fig.7 Interface microstructure of Al/Ti joint with dot matrix texturing
The interface structure characteristics of fusion brazed joints with linear treatment are shown in Fig. 8.
Under the action of laser, the filler metal melts and fills the grooves on the titanium plate by capillary action and its own fluidity.
It was found that compounds were formed in the pits treated in a straight line near the weld toe in zone b and laser irradiation zone e, and their growth direction was inconsistent with the direction of the matrix (see Fig. 8c), which would also play a certain role in inhibiting the crack growth.
The tissue of the laser direct irradiation area is thicker.
The results of energy spectrum show that point b is 60.93% Al, 38.73% Ti and 0.33% Mg;
The e point is 4.16% Al content, 25.19% Ti content, 0.65% Mg content.
It is inferred that the brittle intermetallic compound is TiAl3 phase, and the continuous interfacial brittle compound may be the source of interface failure.
Fig.8 Interface microstructure of Al/Ti joint with linear texturing processing
From the above microstructure observation results, it can be seen that dot matrix and linear texturing have little effect on the interface morphology, and continuous reaction products are produced at the interface.
Because of the high brittleness of the interface compound, it is easy to become a crack source on the untreated interface and continue to expand in the flat brittle compound layer, eventually leading to the fracture of the joint.
Although the interface compound layer after texturing will also produce cracks, the base metal and the interface compound are serrated.
When the microcrack extends to the serrated edge, it will be blocked, so as to inhibit the further expansion of the crack and cause brittle fracture of the joint.
That is to say, the serrated interface formed by laser texturing reduces the possibility of large scale crack propagation in the brittle compound layer, so it can improve the mechanical properties of the joint.
The SEM morphology of aluminum/titanium fracture surface under dot matrix treatment is shown in Fig. 9.
It can be found that some weld metal at the fracture, especially the dents after texturing treatment, has adhered to the titanium substrate when stretching, forming regular “bumps” on the surface, as shown in Fig. 9a, which shows that the lattice treatment has played a good role in improving the joint adhesion.
It is identified by energy spectrum analysis that the crater adhesion is the weld metal formed after the solder melting (# 1: Al content 98.39%, Ti content 0.46%, Mg content 1.15%);
The titanium substrate pit is surrounded by Ti Al compounds (# 2: 38.56% Al, 60.32% Ti, 1.12% Mg), as shown in Fig. 9d.
This shows that when fracture occurs, the crack at the interface does not pass through the pit interface but cuts the weld metal at the pit.
It can be seen that the pit effectively blocks the crack growth and improves the joint performance.
This also provides an important reference for further research.
Fig.9 Fracture surface morphology of joint with dot texturing
The SEM morphology of aluminum/titanium fracture surface under linear treatment is shown in Fig. 10.
It can be seen from Fig. 10b and 10d that in the joint fracture surface after linear texturing treatment, some weld metal at the groove remains on the titanium substrate.
The results of energy spectrum show that the fracture metal at the pit is filler metal (# 1: Al content 69.19%, Ti content 1.68%, Mg content 0.94%, Si content 21.52%);
It is surrounded by Ti Al reaction products (# 2: Al content 33.28%, Ti content 55.18%, Mg content 1.81%).
Therefore, the grooves obtained by linear treatment also play an important role in joint fracture, preventing crack propagation on the interface.
The mechanical properties of the joint have not been significantly improved, which is mainly due to the limited wetting and spreading of the melted solder.
Fig.10 Fracture surface morphology of joint with line texturing
To sum up, different texturing methods have different effects on the wettability of weld metal, mechanical properties and microstructure of joints.
After the dot matrix texturing treatment, the filler metal flows into the pits and grooves during welding.
Due to the capillary effect, the filler metal is easier to spread on the titanium surface, and the mechanical properties are improved more;
Compared with the dot matrix treatment, the linear texturing treatment has no obvious effect on the solder spreading, and the energy barrier generated by the groove parallel to the weld prevents the further spreading of the molten solder.
However, both of the two texturing methods will increase the interface connection area, and at the same time, the interface compound will become serrated, which will inhibit the large-scale propagation of cracks.
The linear texturing treatment has no obvious effect on the improvement of filler metal spreading area and mechanical properties.
(1) Laser surface texturing can obviously improve the weld surface formation.
After the dot matrix treatment, the wetting angle decreased from 98 ° to the lowest 62 °. Driven by capillarity, the wettability of solder was improved, the wetting angle of weld metal decreased, and the spreading width of solder increased.
The improvement of wettability of solder by dot matrix treatment is better than that by linear texturing treatment, and the smaller the dot spacing is, the more significant the improvement is.
(2) Matrix texturing treatment can greatly improve the tensile properties of the joint, and the tensile load of the joint is increased by 21% compared with that of the untreated joint.
On the one hand, the matrix texturing treatment improves the wettability of solder and increases the effective joint area;
On the other hand, the pits formed in the lattice form well block the crack propagation.
Although the linear texturing treatment can also play a good role in preventing cracks, the wettability and spread of the joint have not been significantly improved, resulting in no significant improvement in joint performance.
(3) Different texturing treatments have little effect on the type of interface intermetallics, which are all brittle Ti-Al compounds.
The continuous brittle intermetallics at the interface form a crack source;
However, the texturing treatment increases the effective connection area of the interface and changes the morphology of the interface compound.
The growth orientation of the compound formed at the pit of the texturing treatment is different from that of the continuous compound without the substrate treatment, and the compound layer that changes from straight to zigzag distribution inhibits the expansion of cracks, reducing the possibility of large-scale crack growth in the interface compound.
(4) The following research focuses on how to further improve the wetting effect of molten solder under the premise of texturing, so as to improve the mechanical properties of joints and ensure the wetting of dissimilar metals.