CN111168069B - Heat treatment method capable of effectively improving toughness of LAM TC4 and reducing anisotropy - Google Patents
Heat treatment method capable of effectively improving toughness of LAM TC4 and reducing anisotropy Download PDFInfo
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Abstract
The invention discloses a heat treatment method capable of effectively improving the toughness of LAM TC4 and reducing the anisotropy. Due to the principle of layer-by-layer accumulation in the laser additive manufacturing process and the reason of rapid heating and cooling, coarse beta columnar crystals in epitaxial growth are easily formed, and acicular alpha' martensite and alpha Weishi structures are generally formed in the crystals, so that the LAM TC4 has obvious anisotropy in mechanical property and poor plasticity and toughness. Therefore, the invention adopts a method of repeated cycle heat treatment and solid solution aging heat treatment to convert the LAM TC4 deposition structure into a dual-state structure consisting of a primary alpha phase, a primary alpha lath and a secondary precipitated alpha phase in a converted beta matrix, wherein the primary alpha lath and the secondary precipitated alpha phase are criss-cross and distributed in a basket shape; therefore, the anisotropy is reduced, and the plasticity and the toughness of the alloy are effectively improved while the strength is ensured. The comprehensive mechanical properties (strength anisotropy is less than or equal to 4 percent, and fracture toughness anisotropy is less than or equal to 9 percent) of the heat-treated LAM TC4 are better than those of forgings made of the same material.
Description
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 and high 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 the 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 sedimentary structure is greatly different from the traditional TC4 alloy forge piece structure in the aspects of size, shape, distribution and the like. The conventional TC4 titanium alloy (as-cast) can obtain a microstructure morphology comprising widmannstatten structure, basket structure, bimodal structure, equiaxed structure and the like by means of hot working (such as hot forging) and the like, and a recently developed near-beta forged tri-modal structure. The different structure forms, the proportions and the distribution of the different phases 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 alpha laths of grain boundaries, the composition of sub-microstructures in the columnar crystals is complex and is greatly influenced by processing parameters, generally acicular alpha' martensite, alpha Weishi structures and the like, so that the strength is high, the ductility and toughness are poor, and the mechanical property anisotropy is obvious, the microstructure structure of the LAM TC4 alloy is changed by carrying out heat treatment on the LAM TC4 alloy part, and the LAM TC4 alloy deposition microstructure is an effective means for improving the mechanical property of the LAM TC4 alloy at present, however, the heating temperature of the existing heat treatment method capable of eliminating the anisotropy and improving the toughness is relatively close to or even exceeds the beta transition temperature, so that the alloy structure is serious, and the strength is greatly reduced while the anisotropy and the toughness are reduced.
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 the focused 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 an alpha + 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 converted beta 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, heating the deposition tensile sample and the fracture toughness sample from room temperature to 790-810 ℃ under the argon atmosphere environment condition, preserving the temperature for 30min, then heating to 910-930 ℃ and preserving the temperature for 10min, cooling 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 the samples to room temperature in air;
5 particularly emphasizes that under the argon atmosphere environment condition, the oxygen content is lower than 10ppm, and the first cycle in the cyclic annealing process is repeated, namely the heating rate is 8-10 ℃/min from room temperature to 790-810 ℃, and then from 790-810 ℃ to 910-930 ℃; the heating rate of the later cycles from 540 ℃ and 560 ℃ to 790 ℃ and 810 ℃ is 4-6 ℃/min, and the heating rate of the later cycles from 790 ℃ and 810 ℃ to 910 ℃ and 930 ℃ is 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 an alpha + 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 converted beta 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:
provides a heat treatment method which can effectively improve the toughness of LAM TC4 and reduce the anisotropy. 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 beta columnar crystals which penetrate through the whole cladding layer and are epitaxially grown are formed, and needle-shaped alpha' martensite, alpha Weishi structures and the like are generally formed in the crystals, so that the LAM TC4 titanium alloy has obvious anisotropy in mechanical property and poor plasticity and toughness. Aiming at the problems, the invention adopts a method of repeated cycle heat treatment and solid solution aging heat treatment to convert LAM TC4 sedimentary microstructure into a bimodal microstructure consisting of coarsened equiaxial primary alpha phase, primary alpha laths and secondary precipitated alpha phase in a converted beta matrix, wherein the primary alpha laths and the secondary precipitated alpha phase are criss-cross and distributed in a basket shape, so that the anisotropy of the alloy on the mechanical property is reduced, and the toughness of the alloy is improved; so that the alloy obtains better comprehensive mechanical property. The method provides a guiding function for improving the performance of titanium alloy parts (especially for aerospace application), and brings obvious economic benefits for the field of laser additive manufacturing of titanium alloys.
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-phase 810 ℃ under the argon atmosphere environment condition, then, immediately heating (8-10 ℃/min) to 910-phase 930 ℃ and preserving heat for 10min, and then, cooling the sample to 540-phase 560 ℃; and a second circulation: raising the temperature from 540-560 ℃ to 790-810 ℃ (the temperature raising rate is 4-6 ℃/min), preserving the heat for 30 minutes, then raising the temperature from 790-810 ℃ to 910-930 ℃ (the temperature raising rate is 8-10 ℃/min), preserving the heat for 10 minutes, and then cooling the furnace 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 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, respectively preserving the heat for 4h and 5h, and then air-cooling to room temperature (aging treatment);
the LAMTC4 titanium alloy as-deposited structure consists of alpha + beta lamellae, Weishi alpha bundles and a part of equiaxed primary alpha phase (as shown in FIG. 2); coarsening primary alpha laths and equiaxed primary alpha phases after repeated cyclic annealing, precipitating secondary alpha phases in a transformed beta matrix after solution aging, and finally transforming a sedimentary microstructure into a dual-microstructure consisting of the coarsened equiaxed primary alpha phases, the primary alpha laths and the secondary precipitated alpha phases in the transformed beta matrix, wherein the primary alpha laths and the secondary precipitated alpha phases are criss-cross and distributed in a basket shape (as shown in figure 3); therefore, on the premise of ensuring that the tensile strength is higher than 1000MPa, the ductility and toughness of the alloy are improved, the anisotropy is reduced, and an effective means is provided for reducing the anisotropy of the mechanical property of the LAM TC4 alloy and improving the ductility and toughness of the LAM TC4 alloy.
1, setting the process parameters of the laser additive manufacturing forming TC4 titanium alloy:
the granularity of TC4 powder is 50-150 μm, the laser power is 2800-3200W, the diameter of a laser spot is 5-6mm, the scanning speed is 700-900mm/s, the powder delivery amount is 6-10g/min, the Z-axis lifting amount delta Z is 0.5-0.6mm, and the lap joint rate 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 alpha + beta sheet layer, a Weishi alpha cluster and a part of equiaxial primary alpha 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 an alpha + 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-; 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 (7)
1. A heat treatment method capable of effectively improving the toughness of LAM TC4 and reducing the anisotropy is characterized in that an LAM TC4 titanium alloy deposition state sample with an alpha + beta sheet layer, a Weishi alpha cluster and a part of equiaxial primary alpha phase is subjected to repeated cycle annealing and solid solution aging heat treatment in an argon atmosphere protection heat treatment furnace to obtain a dual-state structure consisting of equiaxial coarsening primary alpha phase, primary alpha laths and secondary precipitated alpha phase in a transformed beta matrix, wherein the primary alpha laths and the secondary precipitated alpha phase are criss-cross and distributed in a basket shape, a formed piece obtains better toughness and plasticity matching, and the anisotropy is reduced, and the heat treatment method comprises the following steps:
1.1, repeating cycle annealing, namely heating the deposition-state sample from room temperature to 790-560 ℃ under the argon atmosphere environment condition, keeping the temperature for 30min, then heating to 910-930 ℃, keeping the temperature for 10min, and then cooling the sample to 540-560 ℃ (first cycle); then immediately raising the temperature to start a second cycle; repeatedly circulating for 4 times, cooling the furnace to below 300 ℃, and taking out; coarsening primary alpha laths and equiaxial primary alpha phases, reducing the length-width ratio, eliminating stress and improving the tissue homogenization degree;
1.2, performing solution treatment, namely heating the tensile sample and the fracture toughness sample subjected to the repeated cyclic annealing from room temperature to 910-plus 930 ℃ under the argon atmosphere environment condition, respectively, keeping the temperature of the tensile sample for 1 hour, keeping the temperature of the fracture toughness sample for 2 hours, and then respectively taking out the tensile sample and the fracture toughness sample for air cooling to room temperature;
1.3 aging treatment, namely heating the tensile sample and the fracture toughness sample after the solution treatment from room temperature to 540-.
2. The heat treatment method for effectively improving the toughness and reducing the anisotropy of LAM TC4 as claimed in claim 1, wherein the oxygen content is less than 10ppm under the argon atmosphere condition, and the temperature rising rate for the first cycle of the repeated cycle annealing, i.e. heating from room temperature to 790-810 ℃, and then from 790-810 ℃ to 910-930 ℃, is 8-10 ℃/min; the heating rate of the last three cycles from 540-560 ℃ to 790-810 ℃ is 4-6 ℃/min, and the heating rate from 790-810 ℃ to 910-930 ℃ is 8-10 ℃/min.
3. The heat treatment process effective in increasing toughness and decreasing anisotropy of LAM TC4, as claimed in claim 1, wherein the annealing is repeated cycle annealing followed by furnace cooling at a furnace cooling rate of 4-6 ℃/min.
4. The heat treatment method for improving the toughness and decreasing anisotropy of LAM TC4 effectively as claimed in claim 1, wherein the temperature-raising rate of the solution and aging treatment is 8-10 ℃/min, and the air-cooling rate is 100-.
5. The heat treatment method for effectively improving the toughness and reducing the anisotropy of LAM TC4 as claimed in claim 1, wherein the heat treatment furnace is a quartz tube furnace, the furnace door is closed after the sample is placed in the tube furnace, the vacuum degree is pumped by a vacuum pump until the vacuum degree reaches 10-2After Pa, the vacuum-pumping valve is closed and pure water is introducedAnd (3) argon with the temperature of 99.9 percent enables the pressure in the furnace to be balanced with the atmosphere again, after the vacuumizing process is repeated for three times, the vacuumizing valve is closed, the argon is introduced to enable the pressure in the furnace to be slightly higher than the atmospheric pressure, the exhaust valve of the heating furnace is opened, the argon in the furnace is discharged into the atmosphere at a low flow rate, and the pressure balance of the inside and the outside of the furnace is kept.
6. The heat treatment method for effectively improving the toughness and reducing the anisotropy of LAM TC4 as claimed in claim 1, wherein the LAM TC4 titanium alloy as-deposited structure is composed of α + β sheets, Wei's α cluster and part of equiaxed primary α phase, and after heat treatment, a bimodal structure is formed which is composed of coarsened equiaxed primary α phase, primary α laths and secondary precipitated α phase in a transformed β matrix, wherein the primary α laths and the secondary precipitated α phase are criss-crossed and distributed in a basket shape.
7. The heat treatment method capable of effectively improving the toughness and reducing the anisotropy of the LAM TC4 as claimed in claim 1, wherein the comprehensive mechanical properties of the heat-treated LAM TC4 titanium alloy are better than those of a forged piece made of the same material, and the strength anisotropy is less than or equal to 4%; the anisotropy of fracture toughness is less than or equal to 9 percent.
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