Microstructure and properties of TLP diffusion wel

2022-08-03
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Microstructure and properties of TLP diffusion welding of TC4 alloy

titanium alloy is more and more widely used in aviation, aerospace and other industrial fields because of its high specific strength, specific stiffness, oxidation resistance and corrosion resistance. As an important structural material, connection is one of the essential key manufacturing technologies for titanium alloy structural parts. For some structures with complex structures or requiring large-area welding, brazing and diffusion welding have unique advantages. Therefore, brazing and diffusion welding of titanium alloy are widely used in aerospace and other high-tech fields, such as supersonic aircraft stiffness panels, hollow fan blades of turbofan engines, space shuttle anti heat pads Space vehicle grid wing, etc. However, due to the very active chemical properties of titanium alloy, titanium and most metals form highly brittle intermetallic compounds, so its brazed joints have an insurmountable brittleness. On the 12th, the strength material giant kostron put into operation two new high-tech production lines in the Shanghai production base. The application of solid-phase diffusion welding in complex structures is limited because of the pressure requirements. The transient liquid phase diffusion welding technology combines the advantages of brazing and solid-phase diffusion welding. It does not need high pressure and can realize high-strength non brittle connection. It is an ideal process for titanium alloy connection

in this study, the representative TC4 titanium alloy was used, and the titanium based amorphous foil strip was used as the interlayer alloy to break the market monopoly. The transient liquid phase (TLP) diffusion welding process, the microstructure and properties of the joint and the distribution of elements were studied, tested and analyzed

1 test materials and methods

1.1 test materials

base metal for test is δ= 2.0mm TC4 titanium alloy plate. The intermediate layer alloy used for the test is bti35zrcuni amorphous foil strip with a thickness of 0.04mm

1.2 test method

TLP diffusion welding of lap and butt specimens was carried out with TC4 plate with a thickness of 2.0 mm. The intermediate layer alloy was clamped between the connecting surfaces, and the lap length was less than 2.0 mm; Grind the connection interface. The sample is put into the stainless steel fixture and then welded in the furnace to ensure good contact of the connecting surface; The welding specification is 940 ℃, 15 min ~ 4 h, and the vacuum degree during welding is better than 1 × 10 -2 Pa。 After welding, the specimen is cut into strip tensile specimens by wire cutting, and the joint performance is tested. The microstructure of the joint and the distribution of elements in the joint were observed, tested and analyzed by means of metallographic microscope, scanning electron microscope and electron probe microanalysis

2 test results and analysis

2.1 joint structure of TC4 alloy TLP diffusion welding

Figure 1 shows the joint structure of TC4 alloy with bti35zrcuni interlayer alloy at 940 ℃ for different holding times. When holding for 15min, it is actually brazed joint structure, and the joint area is mainly eutectic structure. After 1 ~ 4H heat preservation, the microstructure in the weld zone of the joint is obviously widened, the eutectic structure is reduced and completely disappeared, and a typical sheet is formed α+β Organization. With the increase of holding time, β The phase content decreases, the weld width increases, and no trace of the original assembly gap can be seen. The weld width has reached about 0.25mm, far wider than the original 0.05 mm assembly gap. as everyone knows, α+β After deformation, titanium alloy is cooled at different holding temperatures to obtain different microstructure [1]. At 940 ℃, the matrix α Equiaxed phase recrystallization α And a certain degree of growth occurred, forming a slightly coarser equiaxed structure than the original grain; However, in the weld zone, the brazing filler metal contains a certain amount of Cu, Ni, etc β Stabilize the elements so that the β The transition temperature decreases, that is, when the temperature is kept at 940 ℃, the weld zone has completely entered β Phase zone, flake obtained after furnace cooling α+β Organization, basket organization. Compared with the equiaxed structure, the basket structure has poor plasticity, but still has better strength and fracture toughness α+β It is allowed to exist in titanium alloys

Fig. 1 TLP joint microstructure of TC4 alloy at different holding times 200 ×

Fig.1The microstructures of the joints bonded for different time

(a) 940℃/15 min ; (b) 940℃/1h ; (c) 940℃/2h ; (d). The distribution of Ti, Zr, Cu and Ni in TC4 alloy joints was measured and analyzed by electron probe microanalysis. During the test, the average composition of one point shall be measured at a certain distance from the center of the weld to the base metal, and the spot diameter is 10 μ m. And draw the measured results into a curve, as shown in Figure 2. From the element distribution curve, it can be seen that the element distribution in the joint has obvious regularity. With the increase of holding time, Zr, Cu, Ni and other elements diffuse farther and farther, and their content in the center of the weld is also lower and lower. After 4H diffusion, the maximum content of Cu and Ni in the center of the weld is reduced to 1.63% and 0.99% respectively, which is far lower than that in the middle layer alloy. It can be seen that increasing the holding time at this temperature is very beneficial to the homogenization of the components in the joint area and the reduction of the content of harmful elements. According to the document [2], when the Cu content is lower than 5.4% in the lid diffusion welding of titanium alloy with Cu as the interlayer alloy, the joint has good toughness. However, the highest Cu content in the joint obtained from this test is lower than 2% and the Ni content is lower than 1%. The fracture photos of the TLP joint welded under the 940 ℃/4h specification shown in Figure 3 (b) show that the fracture at the weld is a small dimple, indicating that the joint is ductile fracture, and the joint has relatively satisfactory toughness

Fig. 2 element distribution in TC4 alloy TLP joint

Fig. 3 joint fracture morphology

(a) base metal fracture at lap fillet 200 ×; (b) 940 ℃/4h butt joint weld fracture 2000 ×

2.3 tensile properties of joints

the shear and tensile properties of TLP diffusion welded joints of TC4 alloy using lap and butt joints were tested. The lap length of the shear joint was about 2.0mm. The results are shown in Table 1. Table 1 Mechanical Properties of TC4 alloy TLP diffusion welded joint

it can be seen from table 1 that all lap specimens are broken at the base metal lap fillet, which is caused by improper selection of joint form. For the lap joint, on the one hand, the structural form causes the stress concentration at the lap joint; on the other hand, due to the large amount of intermediate layer alloy, solder accumulation occurs at the lap joint fillet. Because the solder layer at the accumulation is thick and the total amount of Cu and Ni is large, it diffuses to the base metal at this place and makes the base metal brittle. Fig. 3 fracture photos also show that about 0.2mm brittle zone is generated at the lap fillet. Although all the lap joints tested were broken from the base metal, the actual strength of the weld could not be measured, but it is certain that the shear strength is greater than 400MPa. In order not to weaken the performance of the base metal, the amount of intermediate layer alloy must be controlled to avoid forming too large rounded corners

the butt joint data in Table 1 basically reflects the actual strength of the joint. The joint is a brazed structure with low strength when holding for 15min. The fracture analysis shows that the fracture is a brittle fracture. With the increase of holding time, the Cu and Ni elements in the joint diffuse more and more far, the chemical composition in the joint gradually homogenizes, the joint strength increases, and the brittleness decreases. The strength of TLP joint at 940 ℃/4h reached 907mpa, which was close to the level of base metal. Fracture analysis showed that the joint had good toughness

3 conclusion

(1) bti35zrcuni amorphous foil interlayer alloy can be successfully used for TLP diffusion welding of titanium alloy

(2) the obtained TLP junction tissue is α+β After 4H diffusion, the weld zone is blue structure, and the width of lamellar structure is much larger than the original assembly gap

(3) with the increase of holding time, the element distribution of TLP joint tends to be uniform. After 4H diffusion, the maximum content of Cu and Ni decreases to 1.63% and 0.99% respectively

(4) under the specification, the tensile strength of TLP joint is greater than 900MPa, and the toughness of the joint increases with the increase of holding time. (end)

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