Method for improving superplasticity of TC4 titanium alloy laser welding head

Abstract

The invention discloses a method for improving superplasticity of a TC4 titanium alloy laser welding joint, relates to the field of alloys, and aims to solve the problem that the superplasticity of an existing TC4 titanium alloy laser welding joint is poor. The method comprises the steps that TC4 titanium alloy powder and Yb2O3 powder are mixed, ball-milled, subjected to laser welding and then subjected to superplastic forming; the high-temperature dynamic environment of a laser molten pool is utilized, Yb2O3 directly participates in the nucleation and growth process of weld grain, and beta grains are fundamentally refined (not repairing coarse grains after welding); and the extra cost and the deformation risk of post-welding treatment are avoided.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of alloys, and particularly relates to a method for improving the superplasticity of a TC4 titanium alloy laser welded joint. BACKGROUND

[0002] TC4 titanium alloy is a core material for lightweight components (such as hollow blades and casings) in the aerospace field due to its high strength and low density, and the integration of laser welding and superplastic forming can realize one-time forming of complex structures, with the advantages of high efficiency and no need for vacuum. However, after laser welding of TC4 titanium alloy, the weld is prone to form a basket-shaped structure composed of needle-shaped alpha prime martensite, which has high strength and low plasticity, resulting in two major problems during superplastic deformation: (1) high superplastic deformation resistance: the beta grains in the weld are coarse (commonly > 300 pm), the grain boundary sliding area is small, and the peak flow stress is > 35 MPa, making it difficult to achieve large deformation forming; (2) low elongation after fracture: the needle-shaped martensite is prone to crack during deformation, and the elongation after fracture is mostly < 150%, which cannot meet the forming requirements of the component. The existing solutions to the above problems have obvious limitations:

[0003] Post-weld heat treatment (such as solution aging) can only refine the martensite, but cannot change the coarse beta grains, and the superplasticity is limited (elongation after fracture < 300%);

[0004] Hydrogen treatment can reduce the phase transition temperature, but it easily introduces hydrogen embrittlement risk and requires a subsequent dehydrogenation process, which is costly and time-consuming;

[0005] The addition of other rare earth elements (such as La2O3 and CeO2) tends to agglomerate in the molten pool, leading to stress concentration at the grain boundaries and easy cracking during superplastic deformation, and the control effect is weaker than expected.

[0006] Therefore, there is an urgent need for a method for controlling the TC4 titanium alloy laser welded joint that can refine the grains from the source of weld solidification and simultaneously improve the superplasticity, with a simple process and low cost. SUMMARY

[0007] The present application provides a method for actively controlling the addition of Yb2O3 during laser welding to achieve the coordinated optimization of beta grain refinement, microstructure equiaxialization, and superplasticity, and to solve the problems of high superplastic deformation stress, low elongation after fracture, and complex process.

[0008] The method for improving the superplasticity of a TC4 titanium alloy laser welded joint comprises the following steps:

[0009] (1) preparing filling powder: mixing TC4 titanium alloy powder with Yb2O3 powder, uniformly dispersing by using a planetary ball mill to obtain mixed filling powder with Yb2O3 mass fraction of 4-8%;

[0010] (2) laser welding: laser welding the TC4 titanium alloy plate, the process parameters are: laser power 1500-2500 W, welding speed 2-3 m / min, argon protection flow rate 10-20 L / min, powder feeding speed 15-20 g / min, forming a TC4 titanium alloy laser welded joint;

[0011] (3) superplastic forming: superplastically deforming the welded joint obtained in step (2) on a stretching machine, the process parameters are: deformation temperature 900-930℃, initial strain rate 5×10 -4 ·s -1 -2×10 -3 ·s -1 , holding time 10-20 min, obtaining a TC4 titanium alloy laser welded component with optimized superplasticity.

[0012] Further, the particle size of the TC4 titanium alloy powder is 35 μm, and the purity is 99.9%, the particle size of the Yb2O3 powder is 10 μm, and the purity is 99.98%.

[0013] Further, the ball milling conditions are: ball-to-material ratio 4-6:1, rotation speed 150-300 r / min, mixing time 1-2 h.

[0014] Further, the Yb2O3 mass fraction is 5-6%.

[0015] Further, the TC4 titanium alloy plate is laser welded, and the thickness of the TC4 titanium alloy plate is 2-4 mm.

[0016] Further, the TC4 titanium alloy plate is in an annealed state.

[0017] Further, the laser power is 1800-2300 W, the welding speed is 2.2-2.5 m / min, the argon protection flow rate is 15-20 L / min, and the powder feeding speed is 18-20 g / min.

[0018] Further, the laser power is 2000 W, the welding speed is 2.2 m / min, the argon protection flow rate is 15 L / min, and the powder feeding speed is 18 g / min.

[0019] Further, the deformation temperature is 900-920℃, the initial strain rate is 1×10 -3 ·s -1 -2×10 -3 ·s-1 , holding time 10~20min.

[0020] Further, the deformation temperature is 920 DEG C, the initial strain rate is 1*10 -3 ·s -1 , holding time 15min.

[0021] The core of the application is that Yb2O3 is selected as a rare earth regulator for laser welding of TC4 titanium alloy, by precisely controlling the content of Yb2O3 (optimum 6%) and laser welding, superplastic forming process parameters, using the heterogeneous nucleation effect of Yb2O3 to refine the beta grain, control the martensitic phase transition to form equiaxed structure, and finally reduce the superplastic deformation stress and improve the elongation after breaking.

[0022] The specific process logic is:

[0023] Filling powder preparation: ensure uniform dispersion of Yb2O3 and avoid agglomeration;

[0024] Laser welding: control heat input and cooling rate to create conditions for Yb2O3 nucleation;

[0025] Superplastic forming: match temperature and strain rate to promote equiaxed structure and grain boundary sliding.

[0026] Yb2O3 powder particle size is 10um, which can be dispersed into 200-500nm nanoparticles in the molten pool after ball milling, avoiding agglomeration; the atomic radius of Yb (0.194nm) and Ti (0.147nm) is moderate, which can be solid solution in beta phase to reduce the phase transition temperature, and does not produce too much lattice distortion; the melting point of Yb2O3 is as high as 2430 DEG C, which is much higher than the welding temperature (1600-1800 DEG C) of TC4 titanium alloy, and does not decompose and diffuse at superplastic deformation temperature (920 DEG C), and maintains the refinement effect for a long time.

[0027] The application utilizes the high-temperature kinetic environment of laser molten pool to make Yb2O3 directly participate in the grain nucleation and growth process of the weld, which fundamentally refines the beta grain (rather than "repairing" coarse grains after welding); avoids the additional cost and deformation risk of post-welding treatment.

[0028] The application has the following beneficial effects:

[0029] 1) Superplastic performance is greatly improved: the beta grain refinement rate of the weld is 62.3%, the peak flow stress is reduced by 69.1%, the elongation after breaking is increased by 301.3%, and the demand for large deformation forming is met;

[0030] 2) Simple and efficient process: no post-welding treatment is needed, the production cycle is shortened to 1 / 5 of the original technology, and the cost is reduced by 30%.

[0031] 3) High performance stability: Yb2O3 has high temperature stability, and no decomposition or shedding occurs during long-term use, and the qualified rate of superplastic forming of the component is 98%;

[0032] 4) Strong compatibility: can adapt to existing laser welding production lines, without equipment modification, easy to industrialize and popularize. DETAILED DESCRIPTION

[0033] In order to make the purpose, technical scheme and advantages of the embodiments of the present application more clear, the spirit of the present application will be described in detail below, and any person skilled in the art can make changes and modifications to the technology taught by the present application after understanding the embodiments of the present application, without departing from the spirit and scope of the present application.

[0034] The illustrative embodiments of the present application and their descriptions are used to explain the present application, but not as a limitation of the present application.

[0035] Embodiments

[0036] (1) Material preparation

[0037] TC4 titanium alloy plate: 2mm thick, annealed state, chemical composition (mass fraction): Al 6.0%, V 4.0%, Ti balance;

[0038] TC4 titanium alloy powder: particle size 35μm, purity 99.9%;

[0039] Yb2O3 powder: particle size 10μm, purity 99.98%.

[0040] (2) Preparation of filling powder

[0041] TC4 titanium alloy powder and Yb2O3 powder are mixed in a mass ratio of 94:6, added to a planetary ball mill (model QM-3SP4), ball-to-material ratio 5:1, rotation speed 200r / min, mixing time 2h, to obtain mixed filling powder.

[0042] (3) Laser welding

[0043] Equipment: IPG YLS6000 fiber laser;

[0044] Pre-welding treatment: sandpaper polishing to remove the oxide layer on the surface of the TC4 titanium alloy plate, acetone ultrasonic cleaning for 10min;

[0045] Process parameters: laser power 2000W, welding speed 2.2m / min, argon protection flow rate 15L / min (front 10L / min, back 5L / min), powder feeding speed 18g / min, defocusing amount-2mm;

[0046] Post-welding: natural cooling to room temperature, and TC4 titanium alloy laser welded joint is obtained.

[0047] (4) Superplastic forming and performance testing

[0048] Sample preparation: high temperature tensile samples are cut from the welded joint by wire cutting, the size conforms to GB / T 228.2-2015, and the gauge section is located in the center of the weld;

[0049] Superplastic forming test: CMT4104 type high temperature tensile machine, deformation temperature 920℃, initial strain rate 1×10 -3 ·s -1 , after 15min of holding, load tensile, and record stress-strain curve;

[0050] Microstructure characterization: Axio Imager M2m metallographic microscope is used to observe the weld microstructure, and Image J software is used to measure the β grain size;

[0051] (5) Experimental results and analysis

[0052] The performance results are shown in Table 1, the peak rheological stress is 11.9MPa, the elongation after fracture is 572.3%, no cracks are generated during superplastic forming process, and the qualified rate is 98%;

[0053] Table 1

[0054]

[0055] Through the synergistic effect of "heterogeneous nucleation-phase transformation control-stress relief" of Yb2O3, the superplastic performance of TC4 titanium alloy laser welded joint breaks through the bottleneck of existing technology, especially the elongation after fracture exceeds 550% for the first time, and the peak rheological stress is reduced to below 12MPa, which can meet the forming demand of large deformation titanium alloy components in the field of aerospace.

Claims

1. A method for improving the superplasticity of a TC4 titanium alloy laser welding head, characterized in that, The method includes the following steps: (1) Preparation of filler powder: TC4 titanium alloy powder and Yb2O3 powder are mixed and dispersed evenly using a planetary ball mill to obtain a mixed filler powder with a Yb2O3 mass fraction of 4~8%; (2) Laser welding: Laser welding is performed on TC4 titanium alloy plates. The process parameters are: laser power 1500~2500W, welding speed 2~3m / min, argon protection flow rate 10~20L / min, powder feeding speed 15~20g / min, to form TC4 titanium alloy laser welding head. (3) Superplastic forming: The welded head obtained in step (2) is subjected to superplastic deformation on a tensile machine. The process parameters are: deformation temperature 900~930℃, initial strain rate 5×10 -4 ·s -1 -2×10 -3 ·s -1 The heat treatment time is 10~20 minutes to obtain TC4 titanium alloy laser-welded components with optimized superplastic properties.

2. The method for improving the superplasticity of a TC4 titanium alloy laser welding head according to claim 1, characterized in that, The TC4 titanium alloy powder has a particle size of 35 μm and a purity of 99.9%, while the Yb2O3 powder has a particle size of 10 μm and a purity of 99.98%.

3. The method for improving the superplasticity of a TC4 titanium alloy laser welding head according to claim 1, characterized in that, The ball milling conditions are: ball-to-material ratio 4~6:1, rotation speed 150~300 r / min, and mixing time 1~2 h.

4. The method for improving the superplasticity of a TC4 titanium alloy laser welding head according to claim 1, characterized in that, The mass fraction of Yb2O3 is 5-6%.

5. The method for improving the superplasticity of a TC4 titanium alloy laser welding head according to claim 1, characterized in that, Laser welding is performed on TC4 titanium alloy plates, the thickness of which is 2~4mm.

6. A method for improving the superplasticity of a TC4 titanium alloy laser welding head according to claim 6, characterized in that, The TC4 titanium alloy plate is in the annealed state.

7. A method for improving the superplasticity of a TC4 titanium alloy laser welding head according to claim 1, characterized in that, The laser power is 1800~2300W, the welding speed is 2.2~2.5m / min, the argon gas protection flow rate is 15~20L / min, and the powder feeding speed is 18~20g / min.

8. A method for improving the superplasticity of a TC4 titanium alloy laser welding head according to claim 1 or 7, characterized in that, The laser power is 2000W, the welding speed is 2.2m / min, the argon gas protection flow rate is 15L / min, and the powder feeding speed is 18g / min.

9. A method for improving the superplasticity of a TC4 titanium alloy laser welding head according to claim 1, characterized in that, The deformation temperature is 900~920℃, and the initial strain rate is 1×10⁻⁶. -3 ·s -1 -2×10 -3 ·s -1 Keep warm for 10-20 minutes.

10. A method for improving the superplasticity of a TC4 titanium alloy laser welding head according to claim 1, characterized in that, The deformation temperature is 920℃, and the initial strain rate is 1×10⁻⁶. -3 ·s -1 Keep warm for 15 minutes.

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