CN111168069A

CN111168069A - A heat treatment method that can effectively improve the strength and toughness of LAM TC4 and reduce anisotropy

Info

Publication number: CN111168069A
Application number: CN202010130236.2A
Authority: CN
Inventors: 王普强; 张安峰; 王豫跃; 吴梦杰; 齐振佳; 霍浩
Original assignee: Xian Jiaotong University
Current assignee: Xian Jiaotong University
Priority date: 2020-02-28
Filing date: 2020-02-28
Publication date: 2020-05-19
Anticipated expiration: 2040-02-28
Also published as: CN111168069B

Abstract

The invention discloses a heat treatment method which can effectively improve the strength and toughness of LAM TC4 and reduce anisotropy. Due to the layer-by-layer accumulation principle and rapid heating and cooling in the laser additive manufacturing process, it is easy to form a coarse β-column crystal in epitaxial growth, and the inner crystal is generally acicular α'martensite and α Widmanderin structure, resulting in the mechanical properties of LAM TC4. The properties have obvious anisotropy, and the plastic toughness is poor. Therefore, the present invention adopts the method of repeated cyclic heat treatment + solid solution aging heat treatment to transform the LAM TC4 deposited state structure into a dual state composed of coarsening equiaxed primary α phase, primary α lath and secondary precipitation α phase in the transformed β matrix Structure, in which the primary α lath and the secondary precipitated α phase are criss-crossed and distributed in a basket shape; in this way, the anisotropy is reduced, and the plasticity and toughness of the alloy are effectively improved while ensuring the strength. The comprehensive mechanical properties of heat-treated LAM TC4 (strength anisotropy≤4%; fracture toughness anisotropy≤9%) are better than forgings of the same material.

Description

Heat treatment method capable of effectively improving toughness of LAM TC4 and reducing anisotropy

Technical Field

The invention relates to application of LAM titanium alloy in the manufacturing fields of aerospace, biological navigation, vehicle high-speed rail and the like, in particular to the industrial manufacturing application field with corresponding requirements on reducing the anisotropy and improving the obdurability of the LAM TC4 titanium alloy.

Background

the TC4 titanium alloy has excellent corrosion resistance, higher specific strength and yield ratio, and is widely applied to the industries of aerospace, navigation, biomedical treatment and the like at present, the TC4 alloy component manufactured by adopting laser additive has the advantages of low cost, short period, high performance and the like, however, due to the characteristic of rapid heating and cooling in the LAM process, the LAM TC4 alloy deposition structure is greatly different from the structure of the traditional TC4 alloy cast forging piece in the aspects of size, shape, distribution and the like, the traditional TC4 titanium alloy (as-cast) can obtain the structure forms including widmanchurian structure, basket structure, two-state structure, equiaxed structure and the like by means of hot processing (such as hot forging) and the like, and the recently developed near β forging structure, different structure forms, different phase proportions and distribution play a decisive role in the mechanical properties of the alloy, the LAM TC4 alloy deposition microstructure mainly comprises coarse beta columnar crystals penetrating through the whole cladding layer and primary-grown alpha lath structures of crystal boundaries, the improved widal-shaped microstructure forms, the composition of the columnar subgrains, the different phase forms, the proportion and the distribution play a decisive role in the mechanical properties of the alloy, the alloy deposition microstructure forms are greatly influenced by the processing parameters, the three-state microstructure of needle-shaped microstructure, the alloy deposition microstructure of the alloy, the alloy is greatly reduced by the high mechanical properties of the high-shaped microstructure of the traditional TC4, the traditional TC4 alloy, the high-shaped microstructure of the high-shaped microstructure, the high-shaped microstructure is obviously reduced by the high-shaped.

Disclosure of Invention

The invention aims to overcome the existing problems and provide a heat treatment method capable of effectively improving the toughness and reducing the anisotropy of the LAM TC4 titanium alloy, and the method is characterized in that compared with the existing heat treatment method for eliminating the anisotropy and improving the toughness of the LAM TC4 titanium alloy, the strength of the material after heat treatment is reduced to a smaller extent, the anisotropy is reduced, and the ductility and toughness are improved.

The technical scheme of the invention is realized by the following steps:

the preparation method of the TC4 titanium alloy comprises the following steps:

1.1 putting TC4 powder into a vacuum drying furnace, heating to 120 ℃, drying for 2h, and then adding into a powder feeder;

1.2 fixing the substrate on a water-cooling workbench in a work box;

1.3 closing the door of the working box, filling argon with the purity of 99.9 percent into the box, gradually discharging the air in the box through the continuous filling and discharging of the argon, starting a purification system in the working box when the oxygen content is reduced to be below 1000ppm, and reducing the oxygen content in the box to be below 100ppm through the circulating filtration;

1.4, setting a laser scanning path and additive manufacturing process parameters through a numerical control system, starting a TC4 alloy deposition process after the oxygen content is lower than 100ppm, starting a water cooling module, and cooling a laser head and a workbench;

1.5 introducing a laser beam to melt and deposit the synchronously fed TC4 powder on the substrate;

1.6 depositing TC4 alloy blocks with the size not less than 150 multiplied by 100mm by continuously and quantitatively and uniformly lifting a laser head;

1.7 the as-deposited alloy mass of LAM TC4 was sampled by wire cutting in sampling directions parallel to the deposition direction (V) and perpendicular to the deposition direction (H), respectively, and cut into tensile specimens having dimensions of 48X 8mm and fracture toughness specimens having dimensions of 40X 18X 38 mm;

2, the substrate used in 1.2 is a rolled TC4 titanium alloy or a pure titanium substrate;

3, the laser used in 1.5 is a solid-state fiber laser, the diameter of a focused light spot is 5-6mm, and the laser power is 2800-3200W.

4, carrying out repeated cycle annealing and solution aging heat treatment on an LAM TC4 sedimentary state sample consisting of α + beta sheet layer, a Weishi alpha cluster and a part of equiaxed primary alpha phase in an argon atmosphere protective heat treatment furnace, so that the sedimentary state structure is converted into a dual-state structure consisting of coarsened equiaxed primary alpha phase, primary alpha laths and secondary precipitated alpha phases in a conversion β matrix, wherein the primary alpha laths and the secondary precipitated alpha phases are criss-crossed and distributed in a basket shape:

4.1, repeating cycle annealing, namely heating the deposition tensile sample and the fracture toughness sample from room temperature to 790-810 ℃ under the argon atmosphere environment condition, preserving heat for 30min, then heating to 910-930 ℃ and preserving heat for 10min, cooling the furnace to 540-560 ℃ (the first cycle), and then immediately heating to start the second cycle; repeating the circulation for 4 times, cooling the furnace to be lower than 300 ℃, taking out and air-cooling;

4.2, carrying out solution treatment, namely heating the tensile sample and the fracture toughness sample subjected to the repeated cycle annealing from room temperature to 910-930 ℃ under the argon atmosphere environment condition, respectively preserving heat for 1h and 2h, taking out and air-cooling to room temperature;

4.3, aging treatment, namely heating the tensile sample and the fracture toughness sample subjected to the solution treatment from room temperature to 540-560 ℃ under the argon atmosphere environment condition, respectively preserving heat for 4h and 5h, and then cooling in air to room temperature;

5 particularly emphasizes that under the condition of argon atmosphere, the oxygen content is lower than 10ppm, and the first cycle in the cyclic annealing process is repeated, namely the temperature is heated from room temperature to 790-810 ℃, and then the temperature is heated from 790-810 ℃ to 910-930 ℃ with the heating rate of 8-10 ℃/min; heating from 540-560 ℃ to 790-810 ℃ in the latter cycles at a heating rate of 4-6 ℃/min and from 790-810 ℃ to 910-930 ℃ at a heating rate of 8-10 ℃/min;

6, cooling along with the furnace after repeated circulating annealing, wherein the cooling rate is 4-6 ℃/min;

7 the heating rate of the solid solution and aging treatment is 8-10 ℃/min, and the air cooling rate is 100-200 ℃/min;

8, the heating furnace is a quartz tube furnace, the furnace door is sealed after the sample is put into the tube furnace, the vacuum degree is pumped by a vacuum pump until the vacuum degree reaches 10^-2After Pa, closing a vacuumizing valve, introducing high-purity argon to ensure that the air pressure in the furnace is balanced with the atmosphere again, repeating the vacuumizing process for 3 times, closing the vacuumizing valve, introducing argon with the purity of 99.9 percent to ensure that the air pressure in the furnace is slightly higher than the atmospheric pressure, opening an exhaust valve of the heating furnace to ensure that the argon in the furnace is discharged into the atmosphere at an extremely low flow rate, and keeping the air pressure in and out of the furnace balanced;

the microstructure of the 9LAMTC4 titanium alloy consists of α + beta sheet layer, a Weishi alpha cluster and a part of equiaxial primary alpha phase, and a dual-state structure consisting of a coarsened equiaxial primary alpha phase, primary alpha laths and a secondary precipitated alpha phase in a transition β matrix is formed after heat treatment, wherein the primary alpha laths and the secondary precipitated alpha phases are criss-cross and distributed in a basket shape;

the invention has the technical effects that:

in the process of manufacturing the TC4 titanium alloy by laser additive manufacturing, due to the principle of layer-by-layer accumulation and the reason of rapid heating and cooling, coarse β columnar crystals which penetrate through the whole cladding layer and grow epitaxially are formed, needle-shaped α' martensite, α Weishi structures and the like are generally formed in the crystals, so that the mechanical property of the LAM TC4 titanium alloy has obvious anisotropy, and meanwhile, the ductility and toughness are poor.

Drawings

FIG. 1 is a heat treatment process diagram of the heat treatment in a vacuum assisted argon atmosphere furnace according to the method of the present invention;

FIG. 2 is a laser additive manufacturing TC4 alloy as-deposited microstructure employed in the method of the present invention;

fig. 3 shows the microstructure of the as-deposited TC4 alloy after heat treatment in the laser additive manufacturing method according to the present invention.

Detailed Description

The invention is further described below in connection with a specific additive manufacturing process.

In the present invention, the LAM TC4 alloy as-deposited sample was subjected to the repeated cycle annealing + solution aging heat treatment as shown in fig. 1:

repeating the cycle annealing for the first cycle: heating the deposition-state tensile sample and the fracture toughness sample from room temperature (8-10 ℃/min) to 790-810 ℃ under the argon atmosphere environment condition, preserving heat for 30min, then heating (8-10 ℃/min) to 910-930 ℃ and preserving heat for 10min, and then furnace cooling to 540-560 ℃; and a second circulation: heating from 540-560 ℃ to 790-810 ℃ (heating rate is 4-6 ℃/min), preserving heat for 30 minutes, then heating from 790-810 ℃ to 910-930 ℃ (heating rate is 8-10 ℃/min), preserving heat for 10 minutes, and then furnace cooling to 540-560 ℃; and a third cycle: the same as the second cycle; and a fourth cycle: in the same second circulation, then furnace cooling is carried out until the temperature is lower than 300 ℃, and then the furnace is taken out;

preferred are (as shown in fig. 1): repeating the cycle annealing for the first cycle: heating the deposition-state tensile sample and the fracture toughness sample from room temperature (8-10 ℃/min) to 800 ℃ under the argon atmosphere environment condition, preserving heat for 30min, then heating (8-10 ℃/min) to 920 ℃, preserving heat for 10min, and then cooling the furnace to 550 ℃; and a second circulation: heating from 550 ℃ to 800 ℃ (the heating rate is 4-6 ℃/min), preserving heat for 30 minutes, then heating from 800 ℃ to 920 ℃ (the heating rate is 8-10 ℃/min), preserving heat for 10 minutes, and then furnace cooling to 550 ℃; and a third cycle: the same as the second cycle; and a fourth cycle: in the same second circulation, then furnace cooling is carried out until the temperature is lower than 300 ℃, and then the furnace is taken out; respectively heating the tensile sample and the fracture toughness sample subjected to the repeated cycle annealing from room temperature to 910-930 ℃ under the argon atmosphere environment condition, respectively preserving heat for 1h and 2h, taking out the tensile sample and the fracture toughness sample, and air-cooling to room temperature (solution treatment); heating the tensile sample and the fracture toughness sample after the solution treatment from room temperature to 540-560 ℃ respectively under the argon atmosphere environment condition, preserving heat for 4h and 5h respectively, and then cooling in air to room temperature (aging treatment);

the LAMTC4 titanium alloy sedimentary microstructure is composed of α + β sheet layers, Weishi β 1 bundles and a part of equiaxial primary β 2 phases (shown in figure 2), primary β 3 laths and equiaxial primary α phases are coarsened after repeated cyclic annealing, secondary α phases are precipitated in a converted β 0 matrix after solid solution and aging, and the sedimentary microstructure is finally converted into a dual microstructure composed of the coarsened equiaxial primary α phases, primary α laths and secondary precipitated α phases in the converted β matrix, wherein the primary α laths and the secondary precipitated α phases are criss-cross and distributed in a basket shape (shown in figure 3), so that the plastic toughness of the alloy is improved, the anisotropy is reduced, and an effective means is provided for reducing the mechanical property anisotropy and improving the plastic toughness of the LAM TC4 alloy on the premise that the tensile strength is higher than 1000 MPa.

1, setting the process parameters of the laser additive manufacturing forming TC4 titanium alloy:

the granularity of TC4 powder is 50-150 mu m, the laser power is 2800-3200W, the diameter of a laser spot is 5-6mm, the scanning speed is 700-900mm/s, the powder feeding amount is 6-10g/min, the Z-axis lifting amount delta Z is 0.5-0.6mm, and the overlapping ratio is 40-50%;

2, forming and additive manufacturing of a titanium alloy entity on a rolled titanium alloy substrate with the thickness of 220 multiplied by 170 multiplied by 10 mm:

in an argon atmosphere protection work box with oxygen content lower than 100ppm, TC4 (chemical components: Al6.32wt.%, V4.06wt.%, Fe0.076wt.%, O0.13wt.%, N0.01wt.%, H0.001wt.%, C0.007wt.%, and the balance Ti) powder fed synchronously is melted and deposited on a 220X 170X 10mm rolled titanium alloy substrate by a laser beam generated by a solid-state laser, and continuous melting deposition is carried out by continuous quantitative lifting of a laser head to prepare a LAMTC4 deposition state alloy block body with the size of 200X 160X 100mm and the inner part composed of an α + beta sheet layer, a Weishi α cluster and a part of equiaxial primary α phase;

3 in the invention, the LAM TC4 alloy deposition state sample formed on the rolled titanium alloy substrate with the thickness of 220 multiplied by 170 multiplied by 10mm is subjected to repeated cycle annealing and solution aging heat treatment as shown in figure 1;

4 the equipment required by the method for manufacturing the TC4 alloy with good matching between strength and ductility and toughness in the heat treatment state comprises the following steps:

(1) a solid state fiber laser providing a laser beam;

(2) the argon gas supply gas circuit, the argon gas protection working box body and the oxygen circulation filtering system avoid the oxidation of titanium alloy in the additive manufacturing process and the heat treatment process;

(3) the laser additive manufacturing mechanical system realizes an additive manufacturing process;

(4) the numerical control system is used for realizing the setting of relevant parameters of additive manufacturing, setting an additive manufacturing path and controlling the forming precision;

(5) the synchronous powder feeder is used for synchronously feeding TC4 powder to realize the continuous deposition and manufacturing process of the alloy;

(6) a vacuum pump and a quartz tube type heat treatment furnace to realize the heat treatment process of the LAM TC4 alloy.

5 the method in the above 4 concretely comprises the following steps:

(1) fixing rolled TC4 titanium alloy or pure titanium substrate with the size of 220 multiplied by 170 multiplied by 10mm on a workbench in an argon atmosphere protection working box body, closing a box door, introducing argon with the purity of 99.9 percent, opening an oxygen circulation filtering system, and reducing the oxygen content in the box to be below 100 ppm;

(2) placing TC4 powder (chemical components: Al6.32wt.%, V4.06wt.%, Fe0.076 wt.%, O0.13wt.%, N0.01wt.%, H0.001wt.%, C0.007wt.%, and the balance Ti) with a particle size of 50-150 μm into a vacuum drying oven, keeping the temperature at 120 ℃ for 2h for drying, and adding into a powder cabin of a powder feeder;

(3) setting laser scanning paths and additive manufacturing process parameters through a numerical control system;

(4) opening a solid-state laser to introduce a laser beam, starting an additive manufacturing process program, and starting a TC4 alloy additive manufacturing process, so as to prepare a LAM TC4 alloy sedimentary mass body with the size of 200 multiplied by 160 multiplied by 100mm and the interior composed of α + beta sheet layer, a Weishi alpha cluster and a part of equiaxed nascent alpha phase;

(5) respectively sampling LAM TC4 alloy deposition state blocks by linear cutting along the sampling direction (V) parallel to the deposition direction and the sampling direction (H) perpendicular to the deposition direction, cutting into tensile samples with the size of 48 multiplied by 8mm and fracture toughness samples with the size of 40 multiplied by 18 multiplied by 38mm, marking, putting into an argon atmosphere protection quartz tube type heat treatment furnace, closing the furnace door of the heat treatment furnace, vacuumizing by a vacuum pump until the vacuum degree reaches 10^-2After Pa, closing the vacuumizing valve, introducing argon with the purity of 99.9 percent to ensure that the pressure in the furnace is balanced with the atmosphere again, repeating the vacuumizing process for 3 times, closing the vacuumizing valve, introducing argon with the purity of 99.9 percent to ensure that the pressure in the furnace is slightly higher than the atmospheric pressure, opening an exhaust valve of the heating furnace to ensure that the argon in the furnace is exhausted into the atmosphere at an extremely low flow rate, and keeping the pressure in and out of the furnace balanced;

(6) turning on a power supply of the heat treatment furnace, setting heat treatment process parameters, turning on a heat treatment working switch to start heat treatment, and adopting a repeated circulation heat treatment mode preferentially recommended in the specific embodiment; then heating the tensile sample and the fracture toughness sample subjected to the repeated circulating heat treatment from room temperature (8-10 ℃/min) to 920 ℃, respectively carrying out solid solution for 1h and 2h, and then carrying out air cooling (the cooling speed is 100-200 ℃) to room temperature; then respectively heating the tensile sample and the fracture toughness sample after the solution treatment from room temperature to 550 ℃ again (8-10 ℃/min), aging for 4h and 5h respectively, taking out and air-cooling to room temperature;

6 the samples after the heat treatment were sampled in the horizontal direction (H) and the vertical direction (V) to carry out a tensile test and a fracture toughness test, and the test data are shown in Table 1. The strength of TC4 alloy samples in two different sampling directions is still kept at a higher level, which is higher than the national standard, the ductility and toughness are higher, the comprehensive mechanical property is better than that of a forged piece made of the same material, and the strength anisotropy is less than or equal to 4 percent; the anisotropy of fracture toughness is less than or equal to 9 percent.

TABLE 1LAM TC4 alloy sample mechanical property test data

Claims

1. The heat treatment method that can effectively improve the strength and toughness of LAM TC4 and reduce anisotropy, it is characterized in that, is to have the LAM TC4 titanium alloy deposited sample with α+β lamellae, Widmanners α bundle and part of the equiaxed primary α phase , after repeated cyclic annealing + solution aging heat treatment in an argon atmosphere protection heat treatment furnace, a dual-state structure consisting of coarsening equiaxed primary α phase, primary α lath and secondary precipitation α phase in the transformed β matrix is obtained, Among them, the primary α lath and the secondary precipitated α phase are crisscrossed and distributed in a basket shape. The formed part obtains a good match of strength, toughness and plasticity, and the anisotropy is reduced. The heat treatment method includes the following steps:

1.1 Repeated cyclic annealing, the as-deposited sample was heated from room temperature to 790-810°C for 30 minutes under argon atmosphere, then heated to 910-930°C, and cooled to 540-560°C after holding for 10 minutes (the first cycle). ); then immediately heated up to start the second cycle; after repeating the cycle for 4 times, the furnace was cooled to less than 300 °C and taken out; the primary α lath and the equiaxed primary α phase were coarsened, the aspect ratio was reduced, and the stress was eliminated at the same time , the degree of tissue homogenization is improved;

1.2 Solution treatment, the tensile specimens and fracture toughness specimens after repeated cyclic annealing were heated from room temperature to 910-930 °C in an argon atmosphere, respectively, kept for 1 h and 2 h, and then taken out and cooled to room temperature;

1.3 Aging treatment, the solution-treated tensile samples and fracture toughness samples were heated from room temperature to 540-560 °C in an argon atmosphere, respectively, kept for 4 h and 5 h, and then taken out and cooled to room temperature; A uniform and fine secondary α phase will be precipitated in the β matrix.

2. The heat treatment method that can effectively improve the strength and toughness of LAM TC4 and reduce anisotropy as claimed in claim 1, characterized in that, under argon atmosphere environmental conditions, the oxygen content is lower than 10ppm, for the first cycle of repeated cycle annealing The process, that is, heating from room temperature to 790-810 °C, and then heating from 790-810 °C to 910-930 °C, the heating rate is 8-10 °C/min; the next three cycles are heated from 540-560 °C to 790-810 °C The heating rate was 4-6°C/min, and the heating rate from 790-810°C to 910-930°C was 8-10°C/min.

3. The heat treatment method for effectively improving the strength and toughness of LAM TC4 and reducing anisotropy as claimed in claim 1, characterized in that, after repeated cyclic annealing, the furnace is cooled with a furnace cooling rate of 4-6°C/min.

4. the heat treatment method that can effectively improve LAM TC4 toughness and reduce anisotropy as described in claim 1, it is characterized in that, solid solution and aging treatment heating rate are 8-10 ℃/min, and air cooling cooling rate is 100- 200°C/min.

5. can effectively improve LAM TC as described in claim 1 The heat treatment method that toughness reduces anisotropy, it is characterized in that, heating furnace is quartz tube furnace, after sample is put into tube furnace, furnace door is Closed, evacuated by a vacuum pump, when the vacuum degree reaches 10 ^-2 Pa, close the vacuum valve, and pass in argon with a purity of 99.9%, so that the air pressure in the furnace is re-balanced with the atmosphere, repeat the above vacuuming process three times, then close Vacuum the valve, pass argon into the furnace so that the pressure in the furnace is slightly higher than the atmospheric pressure, open the exhaust valve of the heating furnace, and discharge the argon in the furnace to the atmosphere at a low flow rate, and keep the air pressure inside and outside the furnace balanced.

6. the heat treatment method that can effectively improve LAM TC4 strength and toughness reduces anisotropy as described in claim 1, it is characterized in that, LAM TC4 titanium alloy deposition state structure is composed of α+β lamellae, Widmanners α bundle and part etc. Axial primary α phase composition, after heat treatment, a dual-state structure consisting of coarsening equiaxed primary α phase, primary α lath and secondary precipitation α phase in the transformed β matrix is formed, in which primary α lath and secondary precipitation α phase are formed. It is crisscrossed and distributed in a basket shape.

7. the heat treatment method that can effectively improve LAM TC4 strength and toughness reduces anisotropy as described in claim 1, it is characterized in that, the comprehensive mechanical property of heat treated state LAM TC4 titanium alloy is better than forgings of the same material, wherein strength anisotropy ≤4%; fracture toughness anisotropy ≤9%.