CN112941439B - Heat treatment method for regulating and controlling mechanical property of SLM (selective laser melting) titanium alloy static and dynamic load and anisotropy - Google Patents

Abstract

The invention discloses a thermal treatment method for regulating and controlling the static and dynamic load mechanical property and anisotropy of SLM titanium alloy. And (3) performing heat treatment of circulation spheroidizing annealing and solid solution aging on the SLM Ti-6Al-4V formed piece to obtain a two-state structure. After heat treatment, the plasticity of the sample piece is greatly improved (the elongation rate reaches 18.35%), the mechanical property exceeds the standard of a forged piece, the stability of the sample is improved, and the anisotropy of the plasticity is greatly reduced; the fracture toughness of the sample piece after the circulating spheroidizing annealing and the solution aging heat treatment is 85.5MPa, and the anisotropy is less than or equal to 15 percent; when the strain amplitude is more than or equal to 0.9%, the low cycle fatigue performance of the sample piece after heat treatment is higher than that of the forged piece; because the crack propagation path in the binary structure is longer than that of forgings; when the strain amplitude is 0.7-0.9%, the low cycle fatigue performance of the alloy is equivalent to that of a forged piece; when the strain amplitude is less than or equal to 0.7 percent, the low cycle fatigue performance of the forging is less than that of a forging.

Description

Controlling static and dynamic mechanical properties of SLM titanium alloy and anisotropic heat treatment method

technical field

The invention belongs to the technical field of metal laser additive manufacturing, and the manufactured titanium alloy structural parts can be applied in the fields of aerospace, chemical equipment, medical construction, and shipbuilding industries, especially industries that have certain requirements on static and dynamic load mechanical properties and anisotropy In the field of manufacturing, it has perfected and supplemented the heat treatment system of selective laser melting (SLM) titanium alloys, and provided a new method for the processing and manufacturing of aerospace titanium alloy structural parts.

Background technique

SLM additive manufacturing technology is developed on the basis of selective laser sintering technology and is one of the most popular near-net-shaping methods in recent years. SLM technology slices the model through the built-in slicing software of the system, and then uses a high-energy laser beam to melt the alloy powder layer by layer, and finally accumulates it into a three-dimensional part, which has high material utilization rate and is suitable for parts with complex cavities. SLM technology can directly prepare parts with good mechanical properties and high density, which makes up for the shortcomings of low utilization rate and difficult processing of traditional titanium alloy materials, and promotes the development of titanium alloys in aerospace and other fields.

Titanium alloy has the characteristics of high specific strength, good corrosion resistance and strong heat resistance. It is widely used in the aerospace field. It is one of the preferred materials for important components such as aero-engine fans, compressor discs, blades and landing gear. Ti-6Al-4V is widely used in many fields and is a classic (α+β) dual-phase alloy. During the SLM forming process, the sample is rapidly heated and cooled, which will produce coarse β columnar crystals, resulting in poor plasticity and toughness matching. The titanium alloy manufactured by SLM has high strength, but its plasticity and toughness are relatively low, and the same forming process The anisotropy of mechanical properties of the formed parts is quite different.

Many scholars have studied and explored the heat treatment process of forged titanium alloys. Heat treatment can improve the matching degree of plasticity and toughness. Due to the particularity of the forming method and cooling method of laser additive manufacturing, the structure of SLM TI-6AL-4V forming parts is similar to that of forgings. The structure is very different, so it is necessary to explore a heat treatment process suitable for SLM TI-6AL-4V formed parts to improve its comprehensive properties.

Cyclic spheroidizing annealing heat treatment is performed by spheroidizing annealing near the Tβ temperature, and then repeated cycles to break the columnar grain boundaries, reduce the grain aspect ratio, thereby reducing anisotropy and eliminating internal stress; but cyclic spheroidizing annealing will cause The grains are coarsened, and the strength decreases greatly, so the comprehensive mechanical properties are relatively low; therefore, on the one hand, by strictly controlling the holding time of the cyclic annealing, the grain coarsening phenomenon during the annealing process is reduced, and on the other hand, in the cyclic annealing process After the chemical annealing heat treatment and the solution aging treatment, the grains grown after annealing can be refined again, thereby reducing their anisotropy and improving their comprehensive mechanical properties.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a heat treatment method for regulating the dynamic and static mechanical properties and anisotropy of SLM TI-6AL-4V titanium alloy. The coexistence of lath α-phase and needle-like α-phase transforms into two-state structure. Compared with the sheet-like structure, the two-state structure has higher yield strength, plasticity, thermal stability and fatigue strength. Compared with the equiaxed structure, the dual-mode structure has higher permanent strength, creep strength and fracture toughness and lower crack growth rate, and the invention can improve the dynamic and static load mechanical properties of the material while reducing the material in all directions. opposite sex.

The technical scheme of the present invention is achieved by regulating the static and dynamic mechanical properties of the SLM titanium alloy and the anisotropic heat treatment method, and changing the structure of the SLM TI-6AL-4V from the coexistence structure of lath α phase and needle α phase to double The β-transformation matrix has a certain content of equiaxed α-phase and lath α-phase distributed on the β-transformed matrix, and the TI-6AL-4V formed parts with the best matching of strength-plasticity-toughness-low cycle fatigue are obtained, and the comprehensive mechanical properties are higher than forgings. Standard; has excellent plasticity, the plasticity anisotropy is greatly reduced compared to the deposited state, and the fracture toughness anisotropy is ≤15%; the low-cycle fatigue performance of the heat-treated sample is high under the condition of a large strain range (≥0.9%) For forgings, the heat treatment method includes the following steps:

1.1 Cyclic spheroidizing annealing, put the SLM TI-6AL-4V sample into a tubular atmosphere furnace with an argon atmosphere, and heat it up to 910-930 ° C with the furnace, keep it in this temperature range for 9-11 minutes, and continue to cool down with the furnace To 790-810 ℃, keep the temperature in this temperature range for 25-35 minutes, then cool down to 540-560 ℃ with the furnace for the first cycle; then heat up immediately, start the second cycle, cycle 4 or 5 times in total, and finally the sample is cooled in the furnace ;

1.2 Solution treatment, the SLM TI-6AL-4V sample after cyclic spheroidizing annealing is heated to 910-930 ℃ with the furnace in an argon atmosphere tube atmosphere furnace, kept for 50-70 min, and air-cooled to room temperature;

1.3 Aging treatment, the SLM TI-6AL-4V formed parts after cyclic spheroidizing annealing and solution treatment are heated to 540-560 ℃ with the furnace in an argon atmosphere tubular atmosphere furnace, kept for 230-250 min, and air-cooled to room temperature .

The samples used in the experiment are TI-6AL-4V alloy obtained by selective laser melting process, and the dimensions of the formed samples are Φ8×45mm, 45×8×8mm, 40×18×38mm, 72×14×14mm, respectively.

The vacuum degree of the heat treatment furnace is 10-2-10-3Pa, and the temperature difference of the effective working area of the furnace temperature is controlled within ±5℃; the heat treatment sequence is first cyclic spheroidizing annealing, then solution treatment, and finally aging treatment.

The cooling rate of furnace cooling is 4-6°C/min, and the cooling rate of air cooling is 100-200°C/min.

The heating rate of the sample heat treatment is controlled at 9-11°C/min; the cooling rate during the cycle is 4-6°C/min.

It is a continuous cyclic heat treatment, which needs to be cooled to room temperature before solid solution treatment, and then reheated for aging treatment after solid solution air cooling to room temperature. The resulting tissue is a two-state tissue.

After heat treatment, the room temperature mechanical properties of the SLM TI-6AL-4V sample exceed the requirements of GJB2744A-2007 standard. And the sample strength anisotropy ≤ 5%, plastic anisotropy ≤ 10%, fracture toughness anisotropy ≤ 15%; when the strain amplitude ≥ 0.9%, its low cycle fatigue performance is higher than forgings; when the strain amplitude is 0.7% %-0.9%, its low-cycle fatigue performance is comparable to that of forgings; when the strain amplitude is less than or equal to 0.7%, its low-cycle fatigue performance is lower than that of forgings.

In the invention, the coarse beta columnar crystals are broken by adjusting the heating temperature, the holding time, the number of cycles, and the cooling method, and the size and distribution of the new crystal grains are regulated; the laser is fundamentally weakened due to the four times of heating and heat preservation in the two-phase region. The high internal stress brought about by additive manufacturing and the dislocation packing brought about by the rapid heating and rapid cooling forming method can greatly reduce the anisotropy of the material and improve the static and dynamic mechanical properties of the SLM TI-6AL-4V formed parts . The mechanical properties of the heat-treated samples are more stable; by the method of the present invention, the tensile strength reaches 946MPa, the elongation is as high as 18.3% (1.5 times the elongation in the deposited state), and the tensile mechanical properties all exceed the forgings. The fracture toughness reaches 85.25MPa·m0.5, and when the strain amplitude is ≥0.9%, its low-cycle fatigue performance is higher than the forging standard.

Description of drawings

Fig. 1 is the heat treatment process flow chart that the inventive method carries out heat treatment in vacuum-assisted argon atmosphere furnace;

Fig. 2 is the microstructure diagram of SLM Ti-6Al-4V deposition state (a) and cyclic spheroidizing annealing + solid solution aging state (b) in the present invention;

Figure 3 is a comparison diagram of the anisotropy of different mechanical properties in the as-deposited state and cyclic spheroidizing annealing + solution aging.

Figure 4 is a comparison of the strain life curves of SLM Ti-6Al-4V alloy heat treatment and standard forgings;

Detailed ways

In order to more fully understand the technical content of the present invention, the technical solutions provided by the present invention are further introduced and described below with reference to the specific implementation process and the accompanying drawings.

Fig. 1 is the heat treatment process flow chart of the present invention, wherein the horizontal line part represents the heat preservation process, the number above the horizontal line represents the heat preservation time, the oblique line part represents the heating or cooling process, and the numbers next to it represent the heating or cooling rate.

Figure 2(a) shows the microstructure of SLM Ti-6Al-4V as deposited, which is composed of lath α phase and needle-like α phase; It can be seen that the coarse β columnar crystals disappear, and the structure is a dual-state structure composed of equiaxed α phase, lath α phase and β matrix phase.

Figure 3 is a comparison diagram of the anisotropy of cyclic annealing + solution aging and deposition state. It can be seen that after heat treatment, the anisotropy of elongation and section shrinkage of the sample has decreased to varying degrees. This is because the heat treatment Dislocation and internal stress are also eliminated while the grains are broken, so that the plasticity is improved and the structure is more uniform. Although the anisotropy of strength is slightly increased, it is less than 5%.

Figure 4 is a comparison of the strain life curves of heat-treated parts and forgings. It can be seen from the figure that when the strain amplitude is greater than or equal to 0.9%, the crack propagation path is longer than that of the forgings, which hinders the expansion of cracks, so the low cycle fatigue The performance is higher than that of forgings; when the strain amplitude is 0.7% to 0.9%, its low-cycle fatigue performance is equivalent to that of forgings; when the strain amplitude is less than or equal to 0.7%, its low-cycle fatigue performance is lower than that of forgings, which may be lower at lower strain amplitudes. The cyclic fatigue properties mainly depend on the comprehensive tensile properties.

Step 1: the particle size range of the titanium alloy raw material powder is 25 μm-65 μm;

Step 2: First, uniformly lay the titanium alloy raw material powder for laser additive manufacturing, design the three-dimensional model of the formed part, and then use the selective laser melting manufacturing process to form;

Step 3: The sample forming process parameters, the laser power is 280W, the spot diameter is 0.043mm, and the filling scanning speed is 1200mm/s; the contour is divided into two layers, the inner contour and the outer contour. The power is 150W, the outer contour scanning speed is 800mm/s, and the outer contour scanning power is 80W; the thickness of the powder layer is 0.03mm, the distance between two scans is 0.14mm, the filling is vertical, and the oxygen content in the working chamber is ≤1300ppm, The oxygen content is ≤600ppm after six hours of printing, and ≤300ppm after about ten hours;

Samples with sizes of Φ8×45mm, 45×8×8mm, 40×18×38mm, 72×14×14mm were formed respectively, and the TI-6AL-4V samples formed by SLM were cleaned with ultrasonic waves to remove the surface of the samples. impurities. Metallographic experiments were carried out to observe the microstructure of the samples, and heat treatment and tensile experiments, fracture toughness experiments, and low-cycle fatigue experiments were carried out.

Put the sample into the tubular atmosphere furnace with argon atmosphere, heat up to 910-930 ℃ with the furnace, keep it in this temperature range for 9-11min, continue to cool down to 790-810 ℃ with the furnace, and keep the temperature in this temperature range for 25 minutes. -35min; then cooled to 540°C-560°C with the furnace, which was recorded as the first cycle, then immediately heated up, and started the second cycle, a total of 4 cycles, and then cooled with the furnace.

For solution treatment, the completely cooled samples that have undergone cyclic spheroidizing annealing are raised to 910°C-930°C with the furnace in an argon atmosphere tubular atmosphere furnace, kept for 50-70 minutes, and air-cooled to room temperature.

For aging treatment, the sample drawn after cyclic annealing and solution treatment is raised to 540°C-560°C with the furnace in a tubular atmosphere furnace with argon atmosphere, kept for 230-250min, and air-cooled to room temperature.

The heat treatment furnace is a quartz tube atmosphere electric furnace. The oxygen content in the argon atmosphere during the heat treatment is less than 10ppm, the heating rate of the heat treatment is 9-11°C/min, and the cooling rate is 4-6°C/min.

The microstructure and morphology of SLM TI-6AL-4V alloy are mainly composed of lath α phase and acicular α' phase. The two-state structure composed of the strip α phase and the β matrix phase reduces dislocations and internal stress, thereby reducing its anisotropy and obtaining good strong-plastic matching forming parts.

In the present invention, the laser additive manufacturing Ti-6Al-4V alloy is subjected to the heat treatment shown in Fig. 1, and the tensile properties at constant temperature are tested. The strength and plasticity both exceed the national standard, and the strength anisotropy does not exceed 5%, and the plasticity is anisotropic. The opposite sex is less than 10%. The anisotropy of fracture toughness is less than 15%.

The Ti-6Al-4V powder with a particle size of 25-65 μm was placed in a drying oven and dried at 100 °C for 60 min to remove the moisture in the powder.

Table 1: Elemental content of each component of the powder

Determine the laser additive manufacturing procedure and scanning path, prepare the titanium alloy forming substrate, perform surface treatment on the substrate, remove surface burrs and oxide layers, polish to bright, and remove surface impurities with acetone.

Fix the titanium alloy substrate on the worktable, adjust the position of the laser head, open the air outlet of the processing chamber, pass in high-purity argon to exhaust the air, and then control the oxygen content through the gas circulation system. When the oxygen content is lower than 1300ppm, the laser is turned on and the selective laser melting deposition is performed.

During the printing process, the computer controls the laser beam to irradiate the designated area, and the powder in the designated area melts and solidifies rapidly. After printing one layer, the forming substrate drops to the designated height, and at the same time, the scraper lays a new layer of TI-6AL-4V alloy powder. In this way, after the printing is completed, the substrate is raised, and the excess powder is swept out, and the SLM-shaped titanium alloy workpiece can be obtained.

The parts are cut from the substrate by a laser wire cutting machine, and they are divided into horizontal and vertical parts according to the spatial position of the maximum side length and the deposition direction. The maximum side length is perpendicular to the deposition direction. The deposition direction is called vertical sample.

In the present invention, the Ti-6Al-4V alloy sample of the deposited state, the specific steps of the heat treatment process are as follows:

The first step is to evenly disperse the samples in the porcelain crucible or quartz boat to ensure that each part is heated evenly and the heating area is the largest;

In the second step, place the porcelain crucible or quartz boat with the shaped parts in the middle of the glass tube of the high-temperature electric furnace, and close the warehouse door;

The third step, turn on the vacuum pump until the relative vacuum in the glass tube is about -0.1MPa, turn off the vacuum pump, and then pass in argon gas, close the argon gas valve after the pressure in the glass tube is approximately equal to atmospheric pressure, turn on the vacuum pump again, and repeat this three times , and then turn off the vacuum pump, at this time the glass tube is in a near-vacuum environment;

Open the inlet valve and slowly let in argon gas. When the pressure in the glass tube is slightly higher than atmospheric pressure, open the outlet valve to ensure that a stable and uniform argon flow slowly passes through the glass tube.

The fifth step is to debug the heat treatment program as shown in Figure 1, turn on the heating switch, and perform heat treatment.

Step 6: After the program is over, the samples need to be cooled to below 200 ℃ with the furnace, and then the samples are taken out for air cooling; for air cooling, the crucible or quartz boat needs to be quickly pulled out from one end of the glass tube, and the samples are evenly dispersed in the The wire mesh in the air ensures that the actual conditions of air cooling of each sample are consistent; finally, the heat treatment furnace is closed.

The tensile properties of the Ti-6Al-4V formed parts after heat treatment are tested. The tensile test standard is GB/T 228.1-2010. The normal temperature tensile test samples are 5 horizontal and 5 vertical. Fracture toughness test samples are a group of 5, and the tensile test standard is GB/T-4161-2007. There are 15 groups of samples for low cycle fatigue, and the test standard used is GB/T15248-2008.

The test results of the room temperature tensile properties of the experimental Ti-6Al-4V deposition state and heat treatment state are shown in Table 2, and the fracture toughness test results are shown in Table 3.

Table 2 The test results of the room temperature tensile properties of Ti-6Al-4V as-deposited and heat-treated.

H - horizontal direction sample; V - vertical direction sample

Table 3: TI-6AL-4V heat treatment fracture toughness test results

Table 4: Experimental results of low-cycle fatigue properties of Ti-6Al-4V as heat-treated

Claims (7)

1. The method is characterized in that the SLM TI-6AL-4V tissue is converted into a two-state tissue from a lath alpha phase and acicular alpha phase coexisting tissue, a certain content of equiaxial alpha phase and lath alpha phase are distributed on a beta conversion matrix, a TI-6AL-4V forming piece with strength-plasticity-toughness-low cycle fatigue best matching is obtained, and the comprehensive mechanical property is higher than the forging standard; the material has excellent plasticity, the anisotropy of the plasticity is greatly reduced relative to the deposition state, and the anisotropy of the fracture toughness is less than or equal to 15 percent; the low cycle fatigue performance of the heat treatment sample piece is higher than the standard of a forging piece under the condition that the strain amplitude is more than or equal to 0.9 percent, and the heat treatment method comprises the following steps:

1.1 circulating spheroidizing annealing, placing the SLM TI-6AL-4V sample piece into a tubular atmosphere furnace with an argon atmosphere, heating to 910-; then immediately heating, starting a second cycle, totally circulating for 4 times or 5 times, and finally cooling the sample piece in a furnace;

1.2 solid solution treatment, namely heating the SLM TI-6AL-4V sample piece subjected to the circulating spheroidizing annealing to 910-and 930 ℃ along with the furnace in a tubular atmosphere furnace in an argon atmosphere, preserving the temperature for 50-70min, and air cooling to room temperature;

1.3 aging treatment, namely heating the SLM TI-6AL-4V forming piece subjected to the circulating spheroidizing annealing and the solution treatment to 560 ℃ along with the furnace in a tubular atmosphere furnace in an argon atmosphere, preserving the temperature for 250min at 230 ℃ and air cooling to room temperature.

2. The method for regulating and controlling the static and dynamic load mechanical property and the anisotropic heat treatment of the SLM titanium alloy according to claim 1, characterized in that: the samples used in the experiment were TI-6AL-4V alloy obtained by a selective laser melting process, and the sizes of the formed samples were phi 8X 45mm, 45X 8mm, 40X 18X 38mm, and 72X 14mm, respectively.

3. The method for regulating and controlling the static and dynamic load mechanical property and the anisotropic heat treatment of the SLM titanium alloy according to claim 1, characterized in that: vacuum degree of the heat treatment furnace is 10^-2-10^-3Pa, controlling the temperature difference of the effective working area of the furnace temperature within +/-5 ℃; the heat treatment sequence is circulating spheroidizing annealing, then solid solution treatment and finally aging treatment.

4. The method for regulating and controlling the static and dynamic load mechanical property and the anisotropic heat treatment of the SLM titanium alloy according to claim 1, characterized in that: the cooling speed of furnace cooling is 4-6 ℃/min, and the cooling speed of air cooling is 100-.

5. The method for regulating and controlling the static and dynamic load mechanical property and the anisotropic heat treatment property of the SLM titanium alloy as claimed in claim 1, wherein the method comprises the following steps: controlling the heating rate of the sample piece during heat treatment to be 9-11 ℃/min; the cooling rate in the circulation process is 4-6 ℃/min.

6. The method for regulating and controlling the static and dynamic load mechanical property and the anisotropic heat treatment of the SLM titanium alloy according to claim 1, characterized in that: the method is continuously carried out with circulating heat treatment, and needs to carry out solution treatment after furnace cooling to room temperature, carry out aging treatment by reheating after solution air cooling to room temperature, repeatedly heat and crush coarse beta columnar crystals, refine crystal grains through solution aging, and finally obtain a structure with a two-state structure.

7. The method for regulating and controlling the static and dynamic load mechanical property and the anisotropic heat treatment of the SLM titanium alloy according to claim 1, characterized in that: after heat treatment, the room temperature mechanical property of the SLM TI-6AL-4V sample piece exceeds the requirement of GJB2744A-2007 standard, the elongation reaches 18%, the reduction of area reaches 47%, and the fracture toughness is 85 MPa.m^0.5The strength anisotropy of the sample piece is less than or equal to 5 percent, the plastic anisotropy is less than or equal to 10 percent, and the fracture toughness anisotropy is less than or equal to 15 percent; when the strain amplitude is more than or equal to 0.9 percent, the low cycle fatigue performance of the alloy is higher than that of a forged piece; when the strain amplitude is 0.7-0.9%, the low cycle fatigue performance of the alloy is equivalent to that of a forged piece; when the strain amplitude is less than or equal to 0.7 percent, the low cycle fatigue performance of the forging is less than that of a forging.

Images