Method for producing an object made of an alpha-beta titanium alloy, and object produced using said method

Abstract

The invention relates to a method for producing an object made of an a-ß titanium alloy, in particular in order to produce a formed sheet metal made of an a-ß titanium alloy. According to the invention, the following steps are carried out in such a method: a) pre-heating the object to a pre-heating temperature; b) setting the ß-phase proportion in the structure of the object; c): cooling the object to a forming temperature; and d) forming the object at the forming temperature. The invention additionally relates to an object produced using such a method.

Description

[0001] Method for producing an object from an a-ß-titanium alloy and object produced thereby

[0002] The invention relates to a method for producing an object from an a-ß-titanium alloy, in particular for producing a formed sheet from an a-ß-titanium alloy.

[0003] Furthermore, the invention relates to an object produced by such a method.

[0004] Titanium alloys are relatively expensive materials that offer many favorable properties, which is why they are used advantageously in certain technological areas despite their high costs. These include, for example, advanced technologies in automotive engineering and aerospace, as well as applications in the medical field, such as implants.

[0005] A significant advantage of titanium alloys is their excellent strength properties, which can approach those of tempered steels and are so temperature-resistant that components made of such alloys can also be used at higher temperatures, such as those found in automotive or aerospace applications. Corrosion resistance is also superior to many other materials, which is why titanium alloys are frequently used for the aforementioned implants. In addition, their specific density is significantly lower than that of steel, making them ideal for many applications.

[0006] Pure titanium exists at room temperature and up to 882 °C as hexagonal α-titanium. At 882 °C, the hexagonal α-titanium transforms into body-centered cubic β-titanium. This transition point is referred to as β-transus. Titanium alloys can exhibit similar transition points, but at different temperatures. By alloying with suitable elements, particularly chromium, copper, iron, manganese, molybdenum, niobium, vanadium, and / or tantalum, the β-phase, which is thermodynamically stable only at higher temperatures, can be stabilized down to room temperature. This means that by alloying with suitable alloying elements, an α-β-titanium alloy can be obtained. A proportion of β-phase promotes formability and also leads to modified material properties. Thus, the properties can be controlled depending on the proportion of the β-phase. The β-phase is particularly interesting because exceptionally high strength values ​​can be achieved with the β-phase.

[0007] As in other metallurgical fields, the production of a near-net-shape object usually requires a suitable forming process from a given titanium alloy. In recent decades, superplastic forming, a relatively slow and therefore energy-intensive process, has prevailed over hot deep drawing in sheet metal forming.

[0008] Hot deep drawing at temperatures of approximately 650 °C or higher requires a relatively high energy input for heating the tool and the use of expensive tool materials such as nickel alloys, which is a disadvantage. Furthermore, the limiting drawing conditions are limited compared to superplastic forming, and the possibilities for adjusting the properties of the finished product are virtually nonexistent. Superplastic forming, on the other hand, is slow and associated with high energy consumption.

[0009] Based on this, it is the object of the invention to provide a method of the type mentioned at the outset which enables a targeted adjustment of the properties of an object made of an a-ß-titanium alloy with optimized energy use.

[0010] A further object of the invention is to provide an object manufactured accordingly.

[0011] The procedural task is achieved if the following process steps are provided for in a process of the type mentioned at the outset: a) preheating the object to a preheating temperature; b) setting a ß-phase proportion in a structure of the object; c) cooling the object to a forming temperature; d) forming the object at forming temperature.

[0012] A method according to the invention offers several advantages: Firstly, the intended preheating and the setting of a proportion of ß-phase at a preheating temperature or in a suitable temperature range can lay the foundation for the later mechanical properties of the formed, in particular deep-drawn, object. By selecting a preheating temperature and a holding time at the preheating temperature or in a specific temperature range, in each case below ß-transus, the proportion of the ß-phase can be adjusted. At the same time, a suitable grain size is set, whereby coarse grain is avoided. During the subsequent cooling of the object to the forming temperature, part of the ß-phase transforms again, but another portion can remain in the microstructure.This changes the strength and hardening behavior, among other things because this portion remains in the metastable state or as a transformation product of a transformed ß-phase in the structure. The selected sequence of process steps also means that higher tensile stresses can be applied, so that, for example, in the production of a pot from a corresponding α-ß-titanium alloy, crack formation occurs later than with the state of the art. This also makes it possible to achieve larger limit drawing ratios. This is particularly important when tensile or tensile-compression forming processes such as stretch drawing or deep drawing are used as a forming step, which is often the case. Finally, the energy consumption is also reduced compared to a conventional forming process at elevated temperatures, such as hot deep drawing.Although the starting material made of the α-β titanium alloy, especially a corresponding sheet, must be preheated to a higher temperature than in hot deep drawing, the forming process can be carried out at lower temperatures, resulting in overall energy savings. Less expensive tools can also be used, as they require less temperature resistance.

[0013] The method according to the invention is particularly well-suited for industrial applications. Typically, after the preheating step in a furnace or along a heating line, the object to be formed, such as a sheet metal, must be transported to a forming station. The cooling step to the forming temperature can be combined with the transport of the sheet metal, resulting in a time-optimized process even in industrial processes.

[0014] A method according to the invention can be applied to any object, in particular structural components for aerospace. For example, a method according to the invention can be used to produce aircraft noses or other components for aircraft, in particular deep-drawn aircraft components, such as control boxes.

[0015] In a process according to the invention, suitable titanium alloys are used, in particular those containing elements as alloying constituents that can stabilize the β-phase, such as chromium, copper, iron, manganese, molybdenum, niobium, vanadium, and / or tantalum. Aluminum and / or oxygen can also be used as α-stabilizing elements in this context. A preferred alloy is the titanium alloy Ti-6AI-4V (Ti64), which is commonly used for many applications.

[0016] The preheating temperature is usually chosen so that it is at least 100 °C, preferably at least 150 °C, in particular at least 180 °C, for example 200 °C to 450 °C or 500 °C, above the forming temperature. For reasons of energy efficiency, the largest possible temperature difference between the preheating temperature in step a) and the forming temperature in step c) can be advantageous. Typical preheating temperatures are in a temperature range from 50 °C below ß-transus of the α-β-titanium alloy to 260 °C below ß-transus of the respective α-β-titanium alloy. The preheating temperature can preferably be 155 °C to 260 °C below ß-transus, in particular 170 °C to 225 °C below ß-transus, of the α-β-titanium alloy. For the alloy Ti6AI4, for example, the preheating temperature lies in the temperature window from 165 °C to 220 °C below the ß-transus temperature.In general, the preheating temperature can be within a temperature range of 680 °C to 1100 °C, preferably from 700 °C to 950 °C, in particular from 720 °C to 900 °C. Even when the preheating temperature is reached, the temperature can briefly overshoot into the ß-phase in order to initiate the transformation into the ß-phase. The holding time is usually set so that the object is completely heated through and the desired microstructure has been established. The holding time also depends on the preheating temperature. Higher preheating temperatures generally require shorter holding times. For sheets with a thickness of less than 5 mm, for example 3 mm or less, the holding times are in the range of one minute to 30 minutes, preferably in the range of 2 minutes to 15 minutes, in particular in the range of 3 minutes to 10 minutes.The holding time begins as soon as the object temperature is 5 °C, especially 3.5 °C, for example 2.5 °C, below the target preheating temperature. Heating to the preheating temperature can take place within 5 minutes, preferably less than 3 minutes, for example 2.5 minutes or less.

[0017] A forming temperature is expediently selected in the range from 350 °C to 570 °C, preferably 355 °C to 570 °C, in particular 360 °C to 550 °C, for example 375 °C to 535 °C. The temperature of the tool used for forming is set according to the desired forming temperature. The forming temperature of the object generally corresponds at least approximately to the tool temperature, particularly at the contact surfaces, but can also deviate from this in the case of greater sheet thicknesses inside the sheet. In this temperature range, a suitable combination of ductility and work hardening capacity of the workpiece and the applicability of tool steels that are favorable compared to nickel-based alloys is achieved. The microstructure is specifically adjusted via the preheating conditions (preheating temperature, preheating time and cooling), forming temperature and forming speed including further cooling.This depends on the position of the cooling curve in the time-temperature transformation diagram (TTT diagram). The forming process, including subsequent cooling, can be carried out in such a way that the ß-phase partially transforms into lamellae of ß-phase and a remaining metastable ß-phase, or even martensitic. In addition to high strength, advantageous uniform elongation can also be achieved at the right temperature. However, the subsequent cooling to room temperature can, if necessary, be carried out in such a way that the cooling curve passes through the region, or the nose, in the TTT diagram where the ß-phase transforms significantly more into the ß-phase.

[0018] Within the scope of the invention, it is provided that the object is cooled or allowed to cool from a preheating temperature to a forming temperature within a predetermined period of time. The cooling advantageously takes place relatively quickly. It has proven advantageous if the object is cooled from a preheating temperature to a forming temperature within 60 s, preferably within 30 s, in particular in less than 20 s, or is allowed to cool to a forming temperature at a cooling rate of less than 50 K / s. Depending on the dimensions of the object, cooling to the forming temperature can take place passively or, if appropriate, actively by cooling with air or a liquid medium. For sheet metal with a thickness of less than 5 mm, in particular less than 4 mm, cooling can take place passively using ambient air, which is sufficient.If passive cooling is provided for a suitable sheet thickness, i.e. the sheet is allowed to cool without active cooling, this can be used for transporting the sheet from the furnace to a forming tool. This results in a particularly efficient process, since step c) occurs when the object, in particular a sheet, is transported from a heating device such as a furnace to a forming tool. Cooling to the forming temperature then takes place in the time span from the start of transport to the start of forming in the forming tool. After the actual forming process, further cooling to room temperature usually follows. This further cooling to room temperature usually occurs passively, by allowing the finished object to cool to room temperature. Active cooling is also possible.After cooling to room temperature, further processing steps can be performed to further increase strength, such as further temperature treatment, such as heat treatment or deep freezing, and / or further forming or machining processes, as well as surface treatment procedures. However, the achieved mechanical properties also allow for the omission of strength-enhancing heat treatment, provided the forming takes place below the martensite initiation temperature and / or a corresponding microstructure is established after cooling after forming; in this case, annealing is sufficient to reduce residual stresses.

[0019] The object is preferably deformed rapidly compared to forming in a superplastic forming process. Advantageously, the strain rate is 0.001 1 / s to 4 1 / s, in particular 0.005 1 / s to 2.0 1 / s, for example 0.005 1 / s to 1.5 1 / s.

[0020] Forming can be carried out above a martensite start temperature of the ß-phase, which can be advantageous for favorable microstructure formation. However, the forming temperature can also be lowered into the martensite temperature range in order to be able to carry out forming with more economical tools. Thus, forming in the temperature range between a martensite start temperature (M s ) of the ß-phase and a martensite finish temperature (Mf) of the ß-phase. Forming below a martensite finish temperature of the ß-phase is also possible within the scope of the invention. The forming temperature is related to the preheating temperature, which determines whether the forming takes place above or below the martensite start temperature of the ß-phase.

[0021] Forming is preferably carried out by tensile or tensile compression forming. These processes are known to those skilled in the art and specified in DIN 8585 (tensile forming) and DIN 8584 (tensile compression forming). Sheets with a thickness of less than 5 mm, in particular 3 mm or less, are preferably formed. Sheet thicknesses can in particular be in the range of 1.3 mm to 2.6 mm. Deep drawing is preferred, but press hardening or stretch forming can also be used. If sheet metal is used, rolled starting material is used. It can prove advantageous to apply a lubricant to the object before and / or during forming to promote forming, particularly during deep drawing. Examples of lubricants used include commercially available lubricants sold under the brand name Bonderite® (www.henkel.de).The lubricant is applied to metal sheets, for example, together with a solvent, preferably in several coating steps. Between the coating steps, a temperature treatment in the range of 35 °C to 90 °C can be carried out to remove individual components such as solvents.

[0022] A process according to the invention is preferably used for alloys of the following compositions, but is not limited thereto:

[0023] - alloys approved for aviation, in particular alloys approved according to MM PDS (Metallic Materials Properties Data Handbook), such as (guide values ​​for ß-transus in brackets) Ti-6AI-4V (995 °C) Ti-6AI-6V-2Sn (946 °C) Ti-4AI-2.5V-1.5Fe (971 °C) Ti-4.5AI-3V-2Fe-2Mo (900 °C) Ti-6AI-2Sn-4Zr-2Mo (935 °C) Ti-8AI-1Mo-1V (1050 °C) - other alloys such as

[0024] Ti-3AI-2.5V (935 °C)

[0025] Ti-6AI-7Nb (1010 °C)

[0026] A further object of the invention is achieved by an object produced by a method according to the invention. The object is characterized by high strength and an advantageously fine-grained structure, which is achieved by the provided process steps. The average grain size of the structure, measured according to the standardized line-section method, is approximately 1 μm to 10 μm, preferably 3 μm to 8 μm. The structure is generally essentially free of coarse grains. The proportion of β-phase in the final object is preferably between 5 vol.% and 80 vol.%, in particular between 10 vol.% and 70 vol.%, in particular 15 vol.% to 60 vol.%, for example 17 vol.% to 50 vol.%.

[0027] Further features, advantages, and effects of the invention will become apparent from the following exemplary embodiments. Reference is made to the drawings, which show:

[0028] Fig. 1 is a schematic representation of a heating curve (left) and a subsequent cooling curve (right) for a forming process below the martensite start temperature;

[0029] Fig. 2 is a schematic representation analogous to that in Fig. 1 with different cooling curves for forming above the martensite start temperature or in the area of ​​martensite formation;

[0030] Fig. 3 is a diagram showing a temperature profile during a method according to the invention;

[0031] Fig. 4 shows a representation of a measured exponential hardening coefficient depending on a preheating temperature and cooling time at a measuring temperature of 500 °C;

[0032] Fig. 5 is a diagram illustrating the deformation behavior of differently pretreated sheets.

[0033] The concept of a method according to the invention is shown schematically in Fig. 1 and Fig. 2. On the left, the temperature profile during heating is shown, whereas on the right the temperature profile is shown in a TTT diagram. As can be seen particularly on the left-hand side of Fig. 1, an object to be formed is heated to a temperature below ß-transus, at which a certain proportion of ß-phase is established. The heating can take place relatively quickly, for example within 120 s. The object is then deliberately held at the preheating temperature until the object has reached the desired temperature and is thus thoroughly heated without causing excessive grain growth. This ensures a homogeneous microstructure including a required grain size before forming.

[0034] The object is then cooled to a desired forming temperature in accordance with the TTT diagrams on the right-hand side in Fig. 1 and Fig. 2. Cooling takes place comparatively quickly, for example within a time window of 3 s to 60 s. In particular, cooling can take place while a sheet is being transported from a furnace to a forming station. Cooling usually takes place quickly, for example within 60 s. Cooling takes place passively during transport and the residence time in the tool before forming begins. In the general context of the invention, a transport time, including a residence time in the forming tool until forming, is usually less than 30 s, preferably between 0.5 s and 30 s, for example 1 s to 30 s, in particular 2.5 s to 15 s. The transport time therefore determines the cooling alongside other parameters such as sheet thickness.

[0035] Subsequently, the forming process takes place, after which the formed object is allowed to cool without active cooling or, alternatively, is subjected to controlled cooling until it reaches room temperature. The object can then be subjected to further treatment steps, such as further heat treatments or processing steps such as machining.

[0036] In Fig. 1, on the left side (lower curve), the process of a conventional hot deep drawing is also schematically shown. The object is heated to a forming temperature, formed at this temperature, and then allowed to cool. This illustration is not scaled, as the forming temperature for hot deep drawing is approximately in the range of 650 °C. As can be seen on the right side of Fig. 1, the starting temperature for martensite formation (martensite starting temperature M s) and a final martensite finish temperature (Mf) are passed through and then forming takes place below the corresponding temperature window. The result is a microstructure as shown in Fig. 1 on the right, namely grains from an α-phase and transformed β-phase, which consists of a metastable β-phase and an α'-phase. However, it is also possible, as indicated in Fig. 2 on the right, to cool the material in such a way that it moves into the nose of the TTT diagram. This makes it possible to achieve a different microstructure in which the β-phase exists alongside the α-phase. In this way, microstructure formation can be controlled by targeted temperature control during cooling. In this case, a forming temperature can also lie above the martensite window, as shown in Fig. 2 on the right.

[0037] Fig. 3 shows a temperature profile as it is typically used in the forming of a sheet metal object to produce a corresponding object from an a-ß-titanium alloy.

[0038] Fig. 4 shows the hardening exponent as a function of the preheating temperature. Tensile tests were conducted at a constant strain rate of 0.0075 1 / s to determine the values ​​shown.

[0039] Fig. 5 shows a diagram of the flow behavior at 500 °C for material pretreated according to the invention compared with untreated material. It is evident that the pretreated material exhibits significantly better flow behavior.

[0040] In tests, good parts could be produced by deep drawing for an α-β-titanium alloy Ti6AI4V with a sheet thickness of approximately 1.5 mm after blank pretreatment by applying a lubricant to both sides (100 ml Bonderite® in 100 ml distilled water, after cleaning the sheets beforehand with isopropanol). A typical preheating time was 10 minutes. The pressing depth was between 20 mm and 61 mm. The pressing speed was 15 mm / s. The forming temperature was between 390 °C and 500 °C. After preheating in the furnace, the sheets were formed within 30 seconds and cooled passively for this step during transport from the furnace to the forming tool. A process according to the invention thus offers the possibility of adjusting the microstructure and strength, whereby higher limit drawing ratios can be achieved, and this with a comparatively low overall energy requirement.

Claims

Patent claims 1 . A method for producing an object from an α-β-titanium alloy, in particular for producing a formed sheet from an α-β-titanium alloy, comprising the following method steps: a) preheating the object to a preheating temperature; b) adjusting a β-phase proportion in a microstructure of the object; c) cooling the object to a forming temperature; d) forming the object at the forming temperature.

2. Method according to claim 1, wherein the object is heated to a preheating temperature which is at least 100 °C, preferably at least 150 °C, in particular at least 180 °C, for example 200 °C to 450 °C or 500 °C, above the forming temperature.

3. The method according to claim 1 or 2, wherein the preheating temperature is in a temperature range of 50°C below ß-transus of the α-β-titanium alloy to 260°C below ß-transus of the α-β-titanium alloy, preferably 155°C to 260°C, in particular 170°C to 225°C, below ß-transus of the α-β-titanium alloy.

4. The method according to any one of claims 1 to 3, wherein the preheating temperature is in a temperature range from 680 °C to 1100 °C, preferably from 700 °C to 950 °C, in particular from 720 °C to 900 °C.

5. Method according to one of claims 1 to 4, wherein the object is formed at a forming temperature of 350 °C to 570 °C, preferably 355 °C to 570 °C, in particular 360 °C to 550 °C, for example 375 °C to 535 °C.

6. Method according to one of claims 1 to 5, wherein the object is cooled from a preheating temperature to a forming temperature within 60 s, preferably within 30 s, in particular in less than 20 s, or is allowed to cool to a forming temperature at a cooling rate of less than 50 K / s.

7. Method according to one of claims 1 to 6, wherein the object is formed at a strain rate of 0.001 1 / s to 4 1 / s, in particular 0.005 1 / s to 2.0 1 / s, for example 0.005 1 / s to 1.5 1 / s.

8. The method according to any one of claims 1 to 7, wherein the forming is carried out above a martensite start temperature (M s ) of the ß-phase.

9. The method according to any one of claims 1 to 7, wherein the forming is carried out below a martensite finish temperature (Mf) of the ß-phase.

10. The method according to any one of claims 1 to 7, wherein the forming is carried out in the temperature range between a martensite start temperature (M s ) of the ß-phase and a martensite finish temperature (Mf) of the ß-phase.

11. The method according to any one of claims 1 to 10, wherein the forming is carried out as tensile forming or tensile compression forming.

12. Method according to one of claims 1 to 11, wherein the object is subjected to a lubricant before and / or during the forming process.

13. The method according to any one of claims 1 to 12, wherein the object is made from a sheet metal.

14. The method according to claim 13, wherein the sheet has a thickness of less than 5.0 mm, preferably less than 4.0 mm, preferably less than 3.5 mm, for example less than 3 mm.

15. Object, in particular deep-drawn aircraft component, obtainable according to one of claims 1 to 14.

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