A heat treatment method for improving the fracture resistance of additive manufactured titanium alloys

Abstract

The present invention provides a heat treatment method for improving the fracture resistance of additive manufactured titanium alloys, belonging to the technical field of post-treatment of additive manufactured titanium alloys. Specifically, it includes the following steps: heating the additive manufactured β titanium alloy to 30 - 50 °C above the β transformation temperature and holding for 1.5 h for solution treatment, then cooling to 230 - 250 °C below the β transformation temperature and holding for 6 - 8 h for aging treatment, and furnace cooling to room temperature. By using the heat treatment method of the present invention, discontinuous grain boundary α phase can be obtained, and its existence can reduce the strain accumulation at the β grain boundaries during the deformation process and inhibit the crack propagation along the grain boundaries, thereby significantly improving the fracture resistance of the additive manufactured titanium alloys.

Description

Technical Field

[0001] The present invention belongs to the technical field of post-treatment of additive manufacturing titanium alloys, and particularly relates to a heat treatment method for improving the fracture resistance of additive manufacturing titanium alloys. Background Art

[0002] Due to advantages such as high specific strength, good corrosion resistance, and excellent biocompatibility, titanium alloys have become one of the most widely used alloys in the fields of aerospace, shipbuilding, and biomedicine. However, with the rapid development of these application fields, traditional processing methods are difficult to meet the forming requirements of some parts with high complexity and high precision. Metal additive manufacturing technology is a new forming technology based on the principle of layer-by-layer stacking manufacturing, which is suitable for forming complex structural parts with high forming accuracy, large size, and personalization. The emergence of metal additive manufacturing technology has greatly expanded the application scope of titanium alloy components in various fields. However, the unique forming process of additive manufacturing (such as having an extremely fast cooling rate, etc.) will cause the formed parts to have adverse factors such as large residual stresses in the as-deposited state. Therefore, post-print heat treatment is generally used to eliminate such adverse factors. Currently, most of the heat treatment methods in use mainly only consider the improvement of the strength and plasticity of additive manufacturing titanium alloys, but do not consider the improvement of their fracture resistance. For example, taking the commonly used selective laser melting formed TC4 titanium alloy as an example, its common post-treatment is annealing heat treatment (generally holding at 800 °C for 4 h and air cooling). Although this post-treatment can effectively eliminate adverse factors such as martensite and residual stresses formed by the extremely fast cooling rate during the selective laser melting forming process and improve the plasticity of the material, it is difficult to avoid the precipitation of grain boundary α phase during heat treatment. Grain boundary α phase is an α phase precipitated on the original β grain boundary. The appearance of grain boundary α phase after heat treatment will cause the titanium alloy to have intergranular brittle fracture during mechanical loading. Therefore, this annealing heat treatment method does not improve the fracture resistance of selective laser melting formed TC4 titanium alloy. And fracture resistance, as a main performance index for evaluating material performance, is also particularly important in practical applications. Summary of the Invention

[0003] Aiming at the problems existing in the above-mentioned prior art, the present invention provides a heat treatment method for improving the fracture resistance of additive manufacturing titanium alloys.

[0004] To achieve the above object, the present invention proposes the following technical solutions:

[0005] A heat treatment method for improving the fracture resistance of additive manufacturing titanium alloys, comprising the following steps:

[0006] Heat the additive manufacturing β titanium alloy to 30 - 50 °C above the β transformation temperature and hold for 1.5 h for solution treatment, then cool to 230 - 250 °C below the β transformation temperature and hold for 6 - 8 h for aging treatment, and furnace cool to room temperature.

[0007] Further, the β titanium alloy includes TC18, TB9 or TC4.

[0008] Further, during the solution treatment process, the heating rate is 10 °C / min.

[0009] Further, during the aging treatment process, the cooling rate is 2 - 5 °C / min.

[0010] Further, during the furnace cooling process, the cooling rate is 2 - 5 °C / min.

[0011] The present invention also provides a titanium alloy prepared by using the above heat treatment method.

[0012] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0013] In the common annealing heat treatment method of additively manufactured titanium alloys, the continuous grain boundary α phase that appears can lead to a significant reduction in the anti-fracture performance. Specifically, the continuous grain boundary α phase causes dislocation pile-up during the deformation process, resulting in the accumulation of strain at the continuous grain boundary α phase, and further triggering crack initiation and rapid propagation at the grain boundary α phase. In addition, in the β titanium alloy, during the formation of the grain boundary α phase, it absorbs the α-stable elements in the surrounding area, leading to the formation of a grain boundary precipitate-free zone in the surrounding area. There is a significant strength difference between the precipitate-free zone and the matrix precipitation zone, which can also cause intergranular fracture, thus reducing the anti-fracture performance of the β titanium alloy. However, the present invention mainly obtains a discontinuous / serrated grain boundary α phase through a novel heat treatment method, which plays an important role in improving the anti-fracture performance of additively manufactured titanium alloys. By using the heat treatment method of the present invention, a discontinuous grain boundary α phase can be obtained, and its existence can reduce the strain accumulation at the β grain boundary during the deformation process and inhibit the crack propagation along the grain boundary, thereby significantly improving the anti-fracture performance of additively manufactured titanium alloys.

[0014] Based on the idea of regulating the morphology of the grain boundary α phase, the present invention designs a novel multiple heat treatment method that can improve the anti-fracture performance of additively manufactured titanium alloys, providing a basis for the application of additively manufactured titanium alloys in various industries. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required to be used in the embodiments. Obviously, the following described drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0016] Figure 1 It is the heat treatment technical route of the present invention;

[0017] Figure 2 Microstructure morphology diagrams of selective laser melted TC18β titanium alloy for Example 1. Among them, a is the microstructure morphology diagram of the selective laser melted TC18β titanium alloy before heat treatment; b is the microstructure morphology diagram of the selective laser melted TC18β titanium alloy after heat treatment at 900 °C; c-d are the microstructure morphology diagrams of the selective laser melted TC18β titanium alloy after solution aging; c shows discontinuous grain boundary α phase; d shows serrated grain boundary α phase;

[0018] Figure 3 Microstructure morphology diagrams of selective laser melted TC4 titanium alloy for Example 2. Among them, a is the microstructure morphology diagram of the selective laser melted TC4 titanium alloy before heat treatment; b is the microstructure morphology diagram of the selective laser melted TC4 titanium alloy after heat treatment at 900 °C; c is the microstructure morphology diagram of the selective laser melted TC4 titanium alloy after solution aging. Detailed implementation manners

[0019] Now, various exemplary implementation manners of the present invention will be described in detail. This detailed description should not be regarded as a limitation of the present invention, but should be understood as a more detailed description of certain aspects, characteristics, and implementation schemes of the present invention.

[0020] It should be understood that the terms described in the present invention are only for describing specific implementation manners and are not used to limit the present invention. Additionally, for the numerical ranges in the present invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Each intermediate value within any stated value or stated range, as well as each smaller range between any other stated value or intermediate value within the stated range, is also included in the present invention. The upper and lower limits of these smaller ranges can be independently included or excluded from the range.

[0021] Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention pertains. Although the present invention only describes preferred methods and materials, any methods and materials similar or equivalent to those described herein can also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials related to the documents. In case of conflict with any incorporated document, the content of this specification shall prevail.

[0022] Without departing from the scope or spirit of the present invention, various improvements and changes can be made to the specific implementation manners of the description of the present invention, which are obvious to those skilled in the art. Other implementation manners obtained from the description of the present invention are obvious to those skilled in the art. The description and examples of this application are only exemplary.

[0023] Regarding the use of "comprising", "including", "having", "containing", etc. in this article, they are all open-ended terms, meaning including but not limited to.

[0024] "Room temperature" as described in the present invention refers to 25 °C.

[0025] β-titanium alloys have β-stabilizing chemical elements such as molybdenum, chromium, and vanadium. During normalizing or quenching, it is easy to retain the high-temperature β-phase to room temperature and obtain a metastable β single-phase structure, so it is called β-type titanium alloy. β-type titanium alloys can be heat-treated and strengthened, have relatively high strength, and good welding and pressure processing properties. The application of additive manufacturing in β-type titanium alloys can significantly improve the performance of β-type titanium alloys while forming in one step, thereby broadening their practical application fields. However, although the commonly used heat treatment methods for β-titanium alloys (such as TC18, TB9, etc.) can make α precipitates appear in the original β grains to improve strength, the improvement is limited because continuous grain boundary α-phase will inevitably appear at the grain boundaries, which will cause rapid crack propagation and poor fracture resistance.

[0026] The present invention mainly adopts a novel heat treatment method, based on the idea of regulating the morphology of grain boundary α-phase. Through this heat treatment method, it is possible to eliminate the adverse factors brought by the additive manufacturing forming process while improving the fracture resistance of additive manufacturing titanium alloys. Compared with the continuous grain boundary α-phase that appears in the traditional post-printing annealing heat treatment, this heat treatment method can form a discontinuous or serrated grain boundary α-phase. The existence of the discontinuous or serrated grain boundary α-phase makes it impossible to generate obvious stress concentration at the grain boundaries under mechanical loading conditions, and at the same time makes it difficult for cracks to propagate along the grain boundaries, significantly improving the fracture resistance of titanium alloys.

[0027] The specific technical solution is as follows: A heat treatment method for improving the fracture resistance of additive manufacturing titanium alloys, comprising the following steps:

[0028] Heat the additive manufacturing β-titanium alloy to 30 - 50 °C above the β transformation temperature and hold for 1.5 h for solution treatment, so that all continuous α-phases can be completely transformed into β-phases. This treatment will also release the residual stress introduced by additive manufacturing and avoid deformation during high-temperature heat treatment;

[0029] Then cool down (furnace cooling) to 230 - 250 °C below the β transformation temperature and hold for 6 - 8 h for aging treatment, so that part of the β-phase is re-transformed into α-phase, and then furnace cool to room temperature. The whole cooling process is a process in which the β-phase is re-transformed into discontinuous α-phase. The intermediate holding treatment is to make the β-phase fully decompose into stable and fine α-phases and make these α-phases evenly distributed.

[0030] This technical route is as Figure 1As shown. This process will also cause the continuous grain boundary α-phase to become a discontinuous grain boundary α-phase again.

[0031] In some preferred embodiments, the β-titanium alloy includes TC18, TB9 or TC4.

[0032] In some preferred embodiments, during the solution treatment process, the heating rate is 10 °C / min.

[0033] In some preferred embodiments, during the aging treatment process, the cooling rate is 2 - 5 °C / min.

[0034] In some preferred embodiments, during the furnace cooling process, the cooling rate is 2 - 5 °C / min.

[0035] The present invention also provides a titanium alloy prepared by using the above heat treatment method.

[0036] The titanium alloy selected in the embodiment of the present invention is an ultra-high strength titanium alloy (referring to a titanium alloy with a room temperature tensile strength of about 1100 MPa, generally near-β titanium alloy and metastable β titanium alloy, but its plasticity is generally poor).

[0037] Example 1

[0038] Using a common selective laser melting formed TC18 β-titanium alloy as raw material, its β transformation temperature is 850 °C, and the heat treatment is carried out in a Kejing OTF-1200X tube-type heat treatment furnace.

[0039] 1) Heat the tube-type heat treatment furnace to 900 °C at a heating rate of 10 °C / min, and put a 100 mm×10 mm×10 mm selective laser melting formed TC18 β-titanium alloy sample into the tube-type heat treatment furnace and hold for 1.5 h for solution treatment;

[0040] 2) Then cool the furnace to 600 °C at a speed of 3 °C / min, and continue to hold at this temperature for 8 h;

[0041] 3) Finally, cool the furnace to room temperature at a speed of 3 °C / min again.

[0042] Figure 2 It is the microstructure morphology diagram of the selective laser melting TC18 β-titanium alloy for Example 1. Among them, Figure 2 a in is the microstructure morphology diagram of the selective laser melting TC18 β-titanium alloy before heat treatment; Figure 2 b in is the microstructure morphology diagram of the selective laser melting TC18 β-titanium alloy after heat treatment at 900 °C. It can be found that at this time, the continuous grain boundary α-phase exists at the grain boundary; Figure 2In , c shows the microstructure morphology of the selective laser melted TC18 β-titanium alloy after solution aging. By observing its microstructure, obvious discontinuous and serrated grain boundary α-phases can be observed.

[0043] Example 2

[0044] Using the common selective laser melted TC4 titanium alloy as the raw material, its β-transformation temperature is 1000 °C, and the heat treatment is carried out in a Kejing OTF-1200X tube heat treatment furnace.

[0045] 1) Heat the tube heat treatment furnace to 1050 °C at a heating rate of 10 °C / min, and put a 100 mm×10 mm×10 mm selective laser melted TC4 titanium alloy sample into the tube heat treatment furnace for solution treatment by holding for 1.5 h.

[0046] 2) Then cool the furnace to 750 °C at a rate of 3 °C / min and continue to hold at this temperature for 8 h.

[0047] 3) Finally, cool the furnace to room temperature at a rate of 3 °C / min.

[0048] Figure 3 It is the microstructure morphology diagram of the selective laser melted TC4 titanium alloy in Example 2. Among them, Figure 3 a in shows the microstructure morphology of the selective laser melted TC4 titanium alloy before heat treatment. It can be found that the initial microstructure state is hard and brittle martensite, and no grain boundary α-phase is observed at the grain boundary; Figure 3 b in shows the microstructure morphology of the selective laser melted TC4 titanium alloy after heat treatment at 1050 °C. It can be found that at this time, continuous grain boundary α-phases exist at the grain boundary, and the martensite in the grains has decomposed into equilibrium α-phases; Figure 3 c in shows the microstructure morphology of the selective laser melted TC4 titanium alloy after solution aging. By observing its microstructure, obvious discontinuous grain boundary α-phases can be observed.

[0049] Test Example 1

[0050] Process the samples heat-treated in Example 1 and Example 2, and use a lathe to machine them into tensile samples that meet the ASTM E8 standard. Specifically: the diameter of the parallel section is 4 mm, the gauge length is 20 mm, the standard tensile rate is 0.0001 / s, and the experiment is carried out on an INSTRON 5892 device. Machine them into specimens with dense tensile (CT) dimensions that meet the ASTM E 399-72 standard, with a thickness of 25 mm, for fracture toughness testing. The test is carried out on an INSTRON8001 machine at a constant speed of 1 mm / min.

[0051] The specific tensile test results show that: for the additively manufactured TC18 β-titanium alloy in Example 1, the tensile strength increased from 850 MPa without heat treatment to 1350 MPa after solution heat treatment, an increase of about 70%. The fracture toughness of the additively manufactured TC18 β-titanium alloy increased from 40 MPa·m 1 / 2 without heat treatment to 97 MPa·m 1 / 2 after solution heat treatment, an increase of about 140%. This indicates a significant enhancement in the fracture resistance. For the additively manufactured TC4 titanium alloy in Example 2, the tensile strength changed from 1085 Mpa without heat treatment to 1018 MPa after solution heat treatment (this is due to the decomposition of the hard and brittle martensite in the as-deposited state into the equilibrium α-phase), and there was no significant change in the tensile strength. However, its plasticity increased from 7.5% without heat treatment to 19.0% after solution heat treatment. Considering the overall results, it shows a significant improvement in the fracture resistance. In addition, the fracture toughness of the additively manufactured TC4 titanium alloy increased from 79 MPa·m 1 / 2 without heat treatment to 111 MPa·m 1 / 2 after solution heat treatment, an increase of about 42%. This also indicates that its fracture resistance is enhanced after heat treatment.

[0052] Comparative Example 1

[0053] Same as Example 2, except that the raw material was replaced with a conventionally formed titanium alloy (cast TC4 titanium alloy).

[0054] It was found that the fracture resistance of the sample processed by this method was 53 MPa·m 1 / 2 , and the tensile strength was 930 MPa.

[0055] Comparative Example 2

[0056] Same as Example 1, except that: 1) the tubular heat treatment furnace was heated to 600 °C at a heating rate of 10 °C / min, and a 100 mm × 10 mm × 10 mm selective laser melting formed TC4 titanium alloy sample was placed in the tubular heat treatment furnace and held for 1.5 h for solution treatment;

[0057] 2) Then it was furnace cooled to 900 °C at a rate of 3 °C / min and held at this temperature for 8 h;

[0058] 3) Finally, it was furnace cooled to room temperature at a rate of 3 °C / min.

[0059] It was found that the fracture resistance of the sample processed by this method was 65 MPa·m 1 / 2 , and the tensile strength was 1280 MPa.

[0060] Comparative Example 3

[0061] Same as Example 1, except that the heat preservation time in step 1) is 3 h.

[0062] It is found that the fracture resistance of the sample processed by this method is 78 MPa·m 1 / 2 , and the tensile strength is 1120 MPa.

[0063] Comparative Example 4

[0064] Same as Example 2, except that the heat preservation time in step 2) is 5 h.

[0065] It is found that the fracture resistance of the sample processed by this method is 76 MPa·m 1 / 2 , and the tensile strength is 990 MPa.

[0066] Comparative Example 5

[0067] Same as Example 2, except that in step 3), the furnace is cooled to room temperature at a rate of 10 °C / min.

[0068] It is found that the fracture resistance of the sample processed by this method is 101 MPa·m 1 / 2 , and the tensile strength is 1005 MPa.

[0069] The above are only the preferred embodiments of the present invention, and are not intended to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principles of the present invention shall be included in the protection scope of the present invention.

Claims

1. A heat treatment method for improving the fracture resistance of additive manufacturing titanium alloy, characterized in that, It includes the following steps: Heat the additive manufacturing β-titanium alloy to 30-50 °C above the β transformation temperature and hold for 1.5 h for solution treatment, then cool to 230-250 °C below the β transformation temperature and hold for 6-8 h for aging treatment, and furnace cool to room temperature; The β-titanium alloy includes TC18, TB9 or TC4; During the solution treatment process, the heating rate is 10 °C / min; During the aging treatment process, the cooling rate is 2-5 °C / min; During the furnace cooling process, the cooling rate is 2-5 °C / min.

2. A titanium alloy prepared by the heat treatment method according to claim 1.

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