Forging and heat treatment method for a high-strength and high-toughness titanium alloy

Abstract

The invention discloses a forging and heat treatment method for a high-strength and high-toughness titanium alloy, relating to the technical field of titanium alloys. The method comprises the following steps: (1) annealing a TC4 titanium alloy ingot in a region higher than the β-phase transformation temperature and air-cooling; (2) performing multi-directional forging on the titanium alloy ingot obtained in step (1), controlling the upsetting and drawing times per pass to be 3-4 times, and the upsetting and drawing forging ratio per time to be not less than 1.5, and forging a total of 4-10 passes; (3) performing cyclic heat treatment on the forged blank after multi-directional forging, with the number of cycles being 3-10 times and air-cooling; (4) performing two-step annealing treatment on the specimen after the cycle ends and air-cooling. Through the forging and heat treatment method, an equiaxed structure with fine grains can be obtained, so that the TC4 titanium alloy forgings have excellent strength and toughness.

Description

Technical Field

[0001] The present invention relates to the technical field of titanium alloys, and particularly relates to a forging technology and heat treatment method for preparing high-strength and high-toughness titanium alloys. Background Art

[0002] Due to their high specific strength, good corrosion and wear resistance, excellent biocompatibility, and excellent high-temperature creep resistance, titanium alloys are important metallic structural materials in the fields of aerospace, ordnance industry, biomedical applications, etc. In recent years, with the development of the aerospace and ordnance equipment industries, the demand for high-strength and low-density aviation materials has become increasingly urgent. Therefore, developing titanium alloys with high tensile strength at high temperatures, good high-temperature stability, excellent fatigue strength, and fracture toughness is an important research direction at present.

[0003] TC4 titanium alloy has high specific strength, a relatively wide working temperature range, and excellent corrosion resistance. It is a medium-strength α+β dual-phase titanium alloy and is one of the preferred materials for manufacturing aircraft engine fans, compressor disks, and blades, etc. Titanium alloys belong to lightweight and difficult-to-deform structural materials. During the hot working process, they have high deformation resistance, a narrow deformation temperature window, and are difficult to machine. For medium-sized titanium alloy plates, it is more difficult to control the compositional uniformity of the ingot and the compositional, microstructural, and property uniformity of the forging billet. Due to the limitations of equipment capabilities and forging technologies, the microstructural and property uniformity of the forging billet processed by traditional forging methods is poor and cannot meet the requirements. The multi-directional forging technology is a process in which the loading direction continuously rotates and changes during the deformation process, which is equivalent to forging the billet in different directions multiple times. Compared with the traditional unidirectional deformation process, the multi-directional forging technology enables recrystallized grains to be generated not only at the original grain boundaries but also in large quantities inside the grains, greatly increasing the grain refinement effect and being an effective means for preparing large-sized ultrafine-grained structural materials.

[0004] After forging and processing titanium alloy bars, there are large residual stresses, and appropriate annealing, solution treatment, and aging heat treatment methods are still required to eliminate or reduce the residual stresses in order to improve their strength, toughness, fatigue strength, etc. However, general annealing treatment is likely to cause grain growth, resulting in a decrease in the strength of the material. The cyclic heat treatment technology is relatively less applied in titanium alloys. It is a process of repeatedly heating and cooling the material in the two-phase region, and is expected to improve the microstructure of titanium alloys after forging and processing, enabling them to obtain relatively excellent mechanical properties. Summary of the Invention

[0005] The object of the present invention is to provide a forging and heat treatment method for high-strength and high-toughness titanium alloys, so that the prepared TC4 titanium alloy forgings have an equiaxed microstructure with fine grains, and the residual stresses in the forgings are significantly reduced, thereby enabling the forgings to have high tensile strength, plasticity, and fracture toughness.

[0006] To achieve the above object, the present invention provides a forging and heat treatment method for a high-strength and high-toughness titanium alloy, which comprises the following steps:

[0007] (1) Anneal the TC4 titanium alloy ingot at a temperature 20 - 150°C higher than the β-phase transformation temperature, hold for 0.5 - 2 h and then air-cool;

[0008] (2) Subject the titanium alloy ingot treated in step (1) to multi-directional forging to obtain a forging blank;

[0009] (3) Perform cyclic heat treatment on the forging blank after multi-directional forging, with the number of cycles being 3 - 10 times and each holding time being 10 - 30 min; after the cycle ends, perform two-step annealing treatment on the specimen and then air-cool.

[0010] Further, the TC4 titanium alloy ingot can be prepared by triple melting in a vacuum consumable arc furnace.

[0011] Further, the multi-directional forging in step (2) is as follows: control the initial forging temperature at 950 - 1100°C, the final forging temperature at 750 - 850°C, the strain rate at 1 mm / s, the upsetting-drawing forging ratio per time is not less than 1.5, and the reheating method is used between each pass to control the number of upsetting-drawing times per pass at 3 - 4 times, and a total of 4 - 10 passes are forged.

[0012] Further, the cyclic heat treatment in step (3) is as follows: select the upper limit temperature of the cyclic heat treatment to be 20 - 40°C below the β-phase transformation temperature, hold for 5 - 60 min to maximize the dissolution of the α-phase; the lower limit temperature of the cyclic heat treatment is 600 - 800°C, hold for 10 - 60 min to maximize the precipitation of the α-phase. The heating and cooling rates of the cyclic heat treatment are both controlled at 4 - 6°C, the number of cycles is 3 - 10 times, and then air-cool after completion.

[0013] Further, the two-step annealing treatment of the specimen in step (3) is as follows: the first annealing process is to hold at 600 - 900°C for 0.5 - 2 h and then air-cool, and the second annealing process is to hold at 400 - 700°C for 1 - 3 h and then air-cool.

[0014] The advantages of the present invention are as follows: By adopting the multi-directional forging combined with the cyclic heat treatment process technology, first, the TC4 titanium alloy ingot is annealed at a temperature 20 - 150°C higher than the β-phase transformation temperature to obtain a lamellar structure in the first step; in the second step, the TC4 titanium alloy ingot is subjected to multi-directional forging, controlling the number of upsetting-drawing times per pass at 3 - 4 times and the upsetting-drawing forging ratio per time not less than 1.5, and a total of 4 - 10 passes are forged. The multi-directional forging can break the grain boundaries of the lamellar structure and cause the α-phase to undergo recovery and recrystallization, thereby achieving the effect of refining the grains; finally, the cyclic heat treatment is used to further eliminate the residual stress introduced by the multi-directional forging, stabilize the structure of the TC4 titanium alloy forging and strengthen the performance of the forging, so that the forging has excellent strength and toughness. Detailed implementation manners

[0015] The present invention will be further described and explained below in conjunction with specific implementation manners. The described embodiments are merely illustrative of the present disclosure and do not delimit the scope of limitation. The technical features of each implementation manner in the present invention can be combined accordingly without conflict.

[0016] Example 1:

[0017] A TC4 titanium alloy ingot with dimensions of (Ф180mm×300mm) is annealed at 1000°C, air-cooled after holding for 2h; the annealed titanium alloy ingot is subjected to multi-directional forging, controlling the initial forging temperature at 1000°C, the final forging temperature at 850°C, the strain rate at 1mm / s, and the temperature is replenished by reheating between each pass. The number of upsetting and drawing operations per pass is 4 times, the upsetting and drawing forging ratio is 2, and a total of 6 passes of forging are carried out; the forged billet after multi-directional forging is subjected to cyclic heat treatment, the upper limit temperature is 960°C, and the holding time is 30min; the lower limit temperature is 700°C; the holding time is 20min. The number of cycles is 6 times. The specimens after cyclic heat treatment are subjected to two-step annealing treatment. The first annealing process is air-cooling after holding at 900°C for 2h, and the second annealing process is air-cooling after holding at 600°C for 1h.

[0018] Tests show that the average grain size of the Ti-6Al-4V titanium alloy forgings obtained in Example 1 is 300nm. According to GB / T 228.1-2010 "Metallic materials - Tensile testing - Part 1: Method of test at room temperature" and GB3075-82 "Metallic materials - Axial fatigue testing method", the yield strength of the forgings reaches 1200MPa, the tensile strength reaches 1350MPa, and the fatigue limit reaches 650MPa.

[0019] Comparative Example 1:

[0020] The difference from Example 1 is that the forged billet after multi-directional forging is directly subjected to two-step annealing treatment without cyclic heat treatment. Tests show that the average grain size of the obtained Ti-6Al-4V titanium alloy forgings is 460nm, the yield strength of the forgings is 1060MPa, the tensile strength is 1170MPa, and the fatigue limit reaches 560MPa, which is much smaller than that of Example 1.

[0021] Comparative Example 2:

[0022] The difference from Example 1 lies in the different process of cyclic heat treatment for the forged billet after multi-directional forging. The upper limit temperature of the cyclic heat treatment is 1020 °C, with a holding time of 30 min; the lower limit temperature is 700 °C; the holding time is 20 min. The number of cycles is 2. Tests show that the average grain size of the obtained Ti-6Al-4V titanium alloy forgings is 380 nm, the yield strength of the forgings is 1120 MPa, the tensile strength is 1240 MPa, and the fatigue limit reaches 590 MPa, which is much smaller than that of Example 1.

[0023] Example 2:

[0024] A TC4 titanium alloy ingot with dimensions of (Ф180 mm × 300 mm) is annealed at 1050 °C, air-cooled after holding for 1 h; the annealed titanium alloy ingot is subjected to multi-directional forging, controlling the initial forging temperature at 1050 °C, the final forging temperature at 800 °C, the strain rate at 1 mm / s, and the re-heating method for temperature compensation is adopted between each pass. The number of upsetting and drawing operations for each pass is 4 times, and the upsetting and drawing forging ratio is 2. A total of 8 passes of forging are carried out; the forged billet after multi-directional forging is subjected to cyclic heat treatment, with the upper limit temperature at 950 °C, holding for 30 min; the lower limit temperature at 750 °C, holding for 10 min, and the number of cycles is 8. The specimens after cyclic heat treatment are subjected to two-step annealing treatment. The first annealing process is air-cooling after holding at 850 °C for 2 h, and the second annealing process is air-cooling after holding at 650 °C for 1 h.

[0025] Tests show that the average grain size of the Ti-6Al-4V titanium alloy forgings obtained in Example 2 is 250 nm. According to GB / T 228.1-2010 "Metallic materials - Tensile testing - Part 1: Method of test at room temperature" and GB3075-82 "Metallic materials - Axial fatigue testing method", the yield strength of the forgings reaches 1250 MPa, the tensile strength reaches 1400 MPa, and the fatigue limit reaches 680 MPa.

[0026] Comparative Example 3:

[0027] The difference from Example 2 is that the titanium alloy ingot is not pre-annealed, and is directly subjected to multi-directional forging and subsequent cyclic heat treatment. Tests show that the average grain size of the obtained Ti-6Al-4V titanium alloy forgings is 380 nm, the yield strength of the forgings is 1140 MPa, the tensile strength is 1280 MPa, and the fatigue limit reaches 610 Mpa, which is much smaller than that of Example 2.

[0028] Example 3:

[0029] The TC4 titanium alloy ingot with dimensions of (Ф180mm×250mm) was annealed at 1050°C, air-cooled after holding for 2h; the annealed titanium alloy ingot was subjected to multi-directional forging, controlling the initial forging temperature at 1030°C, the final forging temperature at 830°C, the strain rate at 1mm / s, using the method of reheating for temperature compensation between each pass, the upsetting and drawing times for each pass were 3 times, the upsetting and drawing forging ratio was 1.8, and a total of 8 forging passes were carried out; the forged billet after multi-directional forging was subjected to cyclic heat treatment, the upper limit temperature was 950°C, holding for 20min; the lower limit temperature was 730°C, holding for 30min. The number of cycles was 4 times. The specimens after cyclic heat treatment were subjected to two-step annealing treatment. The first annealing process was air-cooling after holding at 900°C for 2h, and the second annealing process was air-cooling after holding at 650°C for 2h.

[0030] Tests showed that the average grain size of the Ti-6Al-4V titanium alloy forgings obtained in Example 3 was 320nm. According to GB / T 228.1-2010 "Metallic materials - Tensile testing - Part 1: Method of test at room temperature" and GB3075-82 "Metallic materials - Axial fatigue testing method", the yield strength of the forgings reached 1180MPa, the tensile strength reached 1320MPa, and the fatigue limit reached 630MPa.

[0031] Comparative Example 4:

[0032] The difference from Example 3 was that the titanium alloy ingot was not subjected to multi-directional forging and subsequent cyclic heat treatment. Tests showed that the average grain size of the Ti-6Al-4V titanium alloy forgings obtained was 600nm, the yield strength of the forgings was 900MPa, the tensile strength was 1080MPa, and the fatigue limit reached 530MPa, which was much smaller than that of Example 3.

[0033] Comparative Example 5:

[0034] The difference from Example 3 was that the processes of multi-directional forging of the titanium alloy ingot and cyclic heat treatment of the forged billet were different. For the multi-directional forging of the titanium alloy ingot, the initial forging temperature was 1150°C, the final forging temperature was 900°C, the strain rate was 1mm / s, the method of reheating for temperature compensation was used between each pass, the upsetting and drawing times for each pass were 2 times, the upsetting and drawing forging ratio was 1.8, and a total of 2 forging passes were carried out; for the forged billet after multi-directional forging, cyclic heat treatment was carried out, the upper limit temperature was 950°C, holding for 10min; the lower limit temperature was 850°C, holding for 20min. The number of cycles was 3 times. Tests showed that the average grain size of the Ti-6Al-4V titanium alloy forgings obtained was 450nm, the yield strength of the forgings was 1080MPa, the tensile strength was 1190MPa, and the fatigue limit reached 570MPa, which was much smaller than that of Example 3.

[0035] Conclusion: By adopting the multi-directional forging combined with cyclic heat treatment process technology, the microstructure of TC4 titanium alloy can be optimized and improved, mainly manifested as the refinement of grains, and at the same time the residual stress in the forgings is eliminated, ultimately enabling the forgings to have excellent strength and toughness.

[0036] The above-described embodiments merely represent several implementation manners of the present invention. The description thereof is relatively specific and detailed, but it should not be construed as a limitation to the scope of the patent of the present invention. For those of ordinary skill in the art, without departing from the concept of the present invention, several deformations and improvements can still be made, and these all belong to the protection scope of the present invention.

Claims

1. A forging and heat treatment method for a high-strength and high-toughness titanium alloy, characterized in that It includes the following steps: (1) Anneal the TC4 titanium alloy ingot at a temperature 20 - 150 °C higher than the β-phase transformation temperature, hold for 0.5 - 2 h and then air cool; (2) Subject the titanium alloy ingot treated in step (1) to multi-directional forging to obtain a forging blank; (3) Conduct cyclic heat treatment on the forging blank after multi-directional forging, with the number of cycles being 3 - 10 times and each holding time being 10 - 30 min; after the cycle ends, conduct two-step annealing treatment on the specimen and then air cool; The multi-directional forging described in step (2) is: control the initial forging temperature at 950 - 1100 °C, the final forging temperature at 750 - 850 °C, the strain rate at 1 mm / s, the upsetting-drawing forging ratio per time not less than 1.5, use the furnace return temperature compensation method between each pass, control the number of upsetting-drawing times per pass at 3 - 4 times, and a total of 4 - 10 forging passes; The cyclic heat treatment described in step (3) is: select the upper limit temperature of the cyclic heat treatment to be 20 - 40 °C below the β-phase transformation temperature and hold for 5 - 60 min; the lower limit temperature of the cyclic heat treatment is 600 - 800 °C and hold for 10 - 60 min; the heating and cooling rates of the cyclic heat treatment are both controlled at 4 - 6 °C, the number of cycles is 3 - 10 times, and air cool after completion; The two-step annealing treatment of the specimen described in step (3) is: the first-step annealing process is to hold at 600 - 900 °C for 0.5 - 2 h and then air cool, and the second-step annealing process is to hold at 400 - 700 °C for 1 - 3 h and then air cool.