Method for manufacturing object made of alpha-beta titanium alloy and object manufactured by method

Abstract

The invention relates to a method for producing an object made of an alpha-beta titanium alloy, in particular a shaped metal sheet made of an alpha-beta titanium alloy. According to the invention, the following method steps are provided in a method of this type: a) preheating an object to a preheating temperature; b) setting the proportion of the beta phase in the structure of the object; c) cooling the object to a forming temperature; d) shaping the object at the shaping temperature. Furthermore, the invention relates to an object produced using such a method.

Description

Technical Field

[0001] The present invention relates to a method for producing an object made of an alpha-beta titanium alloy, in particular a method for producing a shaped metal sheet made of an alpha-beta titanium alloy.

[0002] Furthermore, the invention relates to an object produced using this method. Background Art

[0003] Titanium alloys are relatively expensive materials that nevertheless exhibit many beneficial properties, which is why, despite their high cost, these alloys are advantageously used in specific technical fields. These include, for example, advanced technologies in automotive engineering and aerospace, but also applications in the medical field, such as implants.

[0004] The significant advantages of titanium alloys are their excellent strength properties, which are close to those of quenched and tempered steels, and their high thermal stability, which allows components made of this type of alloy to be used even at higher temperatures (e.g., those found in automotive engineering or aerospace applications). Titanium alloys also have superior corrosion resistance to many other materials, which is why they are often used in the aforementioned implants. Furthermore, from a performance perspective, titanium alloys also have a significantly lower specific density than steel, which provides an ideal performance range for many applications.

[0005] At room temperature and up to 882°C, pure titanium exists as close-packed hexagonal α titanium. At 882°C, the close-packed hexagonal α titanium transforms into body-centered cubic β titanium. This transition point is known as the β phase. Titanium alloys can exhibit these same transition points, but at different temperatures. By adding suitable elements, particularly chromium, copper, iron, manganese, molybdenum, niobium, vanadium, and / or tantalum, the β phase, which is inherently thermodynamically stable only at higher temperatures, can be stabilized down to room temperature. This means that α-β titanium alloys can be obtained by adding suitable alloying elements. The proportion of the β phase contributes to formability and also leads to changes in characteristic material parameters. Therefore, properties can also be controlled based on the proportion of the β phase. The β phase is of particular interest due to the extremely high strength values ​​that can be achieved using it.

[0006] In order to produce near-net-shape objects, as in other areas of metallurgy, it is usually necessary to properly form the objects from a given titanium alloy. For this reason, superplastic forming, which is relatively slow and therefore also energy-intensive compared to hot deep drawing, has become the mainstream in forming metal sheets in recent decades.

[0007] Hot deep drawing at temperatures of approximately 650°C or higher requires a relatively high energy input for heating the material, and expensive tool materials (e.g., nickel alloys) are used, which is disadvantageous. Furthermore, compared to superplastic forming, the limit drawing ratio is limited, and there is little ability to adjust the properties of the finished product. On the other hand, superplastic forming is slow and requires high energy consumption. Summary of the Invention

[0008] On this basis, the object of the present invention is to provide a method of the type mentioned in the introduction which allows the properties of an object made of an alpha-beta titanium alloy to be set in a targeted manner with an optimized energy input.

[0009] Another object of the invention is to propose a correspondingly manufactured object.

[0010] The method-related object can be achieved if the following method steps are provided in a method of the type mentioned in the opening paragraph:

[0011] a) Preheat the object to the preheating temperature;

[0012] b) setting the proportion of β phase in the structure of the object;

[0013] c) cooling the object to forming temperature;

[0014] d) forming the object at a forming temperature.

[0015] The method according to the present invention offers several advantages. First, the provided preheating and setting the proportion of β phase at the preheating temperature or within a suitable temperature range lay the foundation for the subsequent mechanical properties of the formed, particularly deep-drawn, object. The proportion of β phase can be adjusted by selecting the preheating temperature and the holding time at the preheating temperature (respectively below the β transformation point) or within a specific temperature range. This simultaneously sets a suitable grain size, avoiding coarse grains. When the object is subsequently cooled to the forming temperature, some of the β phase transforms again, while another portion remains in the structure. This alters the strength and hardening behavior, particularly because this portion remains in the structure in a metastable state or as a transformation product of the transformed β phase. The selected sequence of method steps also enables the application of higher tensile strains, so that, for example, during the production of a can body made of a corresponding α-β titanium alloy, crack formation occurs only later than in the prior art. Consequently, a greater limiting draw ratio can be achieved. This is particularly important when using stretching or stretch / compression forming methods (such as stretch drawing or deep drawing), which are often conventional. Finally, compared to typical forming methods at higher temperatures, such as hot deep drawing, energy input is also reduced. While the starting material made of α-β titanium alloys (especially the corresponding metal sheets) must be preheated to a higher temperature than in hot deep drawing, forming is possible at lower temperatures, resulting in overall energy savings. More economical tools can also be used, as they do not need to be thermally stable.

[0016] In particular, the method according to the invention can also be well applied on an industrial scale. Typically, after a preheating step, for example in a furnace or along a heating section, the object to be formed (e.g., a metal sheet) needs to be transported to a forming station. The cooling step to the forming temperature can be combined with the transport of the metal sheet, resulting in a time-optimized process even in an industrial process sequence. The method according to the invention can be used for any desired object, in particular for structural components for aerospace. For example, the method according to the invention can be used to manufacture aircraft noses or other components for aircraft, in particular deep-drawn aircraft components, such as control boxes.

[0017] In the method according to the invention, suitable titanium alloys are used, in particular those containing elements that can stabilize the β phase (in particular, for example, chromium, copper, iron, manganese, molybdenum, niobium, vanadium and / or tantalum) as alloying components. In this case, aluminum and / or oxygen can also be provided as α-stabilizing elements. A preferred alloy is the titanium alloy Ti-6Al-4V (Ti64), which is common in many applications.

[0018] The preheating temperature is typically selected so that it is at least 100°C, preferably at least 150°C, and in particular at least 180°C above the forming temperature, for example, 200°C to 450°C or 500°C above the forming temperature. For energy efficiency reasons, the maximum possible temperature difference between the preheating temperature in step a) and the forming temperature in step c) can be beneficial. Typical preheating temperatures range from 50°C below the β transformation point of the α-β titanium alloy to 260°C below the β transformation point of the corresponding α-β titanium alloy. Preferably, the preheating temperature can be 155°C to 260°C below the β transformation point of the α-β titanium alloy, in particular 170°C to 225°C below the β transformation point of the α-β titanium alloy. For example, for the Ti6Al4 alloy, the preheating temperature is within a temperature window of 165°C to 220°C below the β transformation point. Typically, the preheating temperature can be in the temperature range of 680°C to 1100°C, preferably 700°C to 950°C, and in particular 720°C to 900°C. Once the preheating temperature is reached, a brief temperature overshoot into the beta phase may also occur in order to initiate the transition to the beta phase. The holding time is generally set so that the object is completely heated through and the desired structural state has appeared. Therefore, the holding time also depends on the preheating temperature. Higher preheating temperatures generally require shorter holding times. For metal sheets with a thickness of less than 5 mm, for example a thickness of 3 mm or less, the holding time is in the range of 1 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 below the target preheating temperature, in particular 3.5°C, for example 2.5°C. Heating to the preheating temperature can be carried out within 5 minutes, preferably less than 3 minutes, for example 2.5 minutes or less.

[0019] The forming temperature is advantageously selected within the range of 350°C to 570°C, preferably 355°C to 570°C, particularly 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 surface, but can also differ within the interior of the sheet metal with greater sheet thicknesses. Within this temperature range, a suitable combination of workpiece ductility and hardenability is achieved, and tool steel suitability is good compared to nickel-based alloys. The structure can be tailored by utilizing the preheating conditions (preheating temperature, preheating time, and cooling), the forming temperature, and the deformation rate (including additional cooling). This is determined by the position of the cooling curve in the time / temperature transformation diagram (TTT diagram). Forming can be combined with subsequent cooling, resulting in a partial transformation of the β phase into a thin sheet made of α phase and a retained metastable β phase, or even a martensitic transformation. In addition to high strength, this also allows for advantageous uniform elongation at appropriate temperatures. However, if desired, subsequent cooling to room temperature can be carried out such that the cooling curve passes through the range or nose of the TTT diagram where the beta phase transforms to the alpha phase to a considerably more pronounced extent.

[0020] The present invention provides for cooling the object from the preheating temperature or allowing it to cool to the forming temperature within a predetermined time. Advantageously, cooling occurs relatively quickly. It has proven advantageous if the object cools from the preheating temperature to the forming temperature within 60 seconds, preferably within 30 seconds, and in particular within less than 20 seconds, or if the object is allowed to cool to the forming temperature at a cooling rate of less than 50 K / s. Depending on the size of the object, cooling to the forming temperature can occur passively or, if necessary, actively by cooling with air or a fluid medium. For metal sheets with a thickness of less than 5 mm, in particular less than 4 mm, passive cooling using ambient air may be sufficient. If passive cooling is provided for a suitable metal sheet thickness, that is, if the metal sheet is allowed to cool without active cooling, this can be used to transport the metal sheet from the furnace to the forming tool. This results in particularly efficient process management, as step c) occurs while the object (in particular the metal sheet) is being transported from the heating device (e.g., furnace) to the forming tool. Cooling to the forming temperature then takes place within 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 takes place passively, in that the finished product is allowed to cool to room temperature. However, active cooling is also possible. After cooling to room temperature, further processing steps can be carried out in order to bring about a further increase in strength, such as further heat treatments, such as heat treatment or deep cooling; and / or further forming or material removal processes, as well as methods for surface treatment. However, the mechanical properties achieved also allow the strength-increasing heat treatment to be omitted, provided that the forming is carried out below the martensite start temperature and / or a corresponding structural state is present after the forming after cooling; annealing is then sufficient to reduce the residual stresses.

[0021] It is preferred to deform the object rapidly compared to forming in a superplastic forming process. Advantageously, the strain rate is 0.001s -1 to 4s -1 , especially 0.005s -1 to 2.0s -1 , for example, 0.005s -1 to 1.5s -1 .

[0022] Forming can be performed above the martensite start temperature of the β phase, which is advantageous for the formation of beneficial structures. However, the forming temperature can also be lowered to the martensite temperature window to enable forming with more economical tools. s ) and the martensite termination temperature of the β phase (M f). Forming at temperatures below the martensite finish temperature of the β phase is also possible within the scope of the present invention. Therefore, the forming temperature is related to the preheating temperature, which determines whether forming occurs above or below the martensite start temperature of the β phase.

[0023] The forming is preferably carried out by stretch forming or stretch / compression forming. These methods are known to the skilled person and are clearly defined in DIN 8585 (stretch forming) and DIN 8584 (stretch / compression forming). Preferably, the sheet thickness of the metal sheet is less than 5 mm, in particular 3 mm or less. In particular, the sheet thickness can be in the range of 1.3 mm to 2.6 mm. Deep drawing is preferred, but press hardening or stretch drawing can also be used. If metal sheets are used, rolled starting materials are used. It may prove beneficial to apply a lubricant to the object before and / or during forming, which lubricant promotes forming, in particular in the case of deep drawing. For example, the lubricant sold under the trade name Commercially available lubricants are used as lubricants (www.henkel.de). For example, the lubricant is preferably applied to the metal sheet together with the solvent in multiple coating steps. Between the coating steps, a heat treatment at a temperature in the range of 35° C. to 90° C. can take place in order to remove individual components, such as the solvent.

[0024] The method according to the present invention is preferably used for alloys of the following composition but not limited thereto:

[0025] - Aerospace-approved alloys, in particular those approved according to MMPDS (Metallic Material Property Data Manual), such as (guideline values ​​for the beta transformation point in brackets)

[0026] Ti-6Al-4V(995℃)

[0027] Ti-6Al-6V-2Sn(946℃)

[0028] Ti-4Al-2.5V-1.5Fe (971℃)

[0029] Ti-4.5Al-3V-2Fe-2Mo (900℃)

[0030] Ti-6Al-2Sn-4Zr-2Mo (935℃)

[0031] Ti-8Al-1Mo-1V(1050℃)

[0032] -Other alloys, such as

[0033] Ti-3Al-2.5V(935℃)

[0034] Ti-6Al-7Nb (1010℃)

[0035] Another object of the present invention is achieved by an object produced according to the method of the present invention. This object is characterized by high strength and a favorable fine-grained structure obtained by the provided process steps. The average grain size of the structure, measured according to the standardized linear intercept method, is approximately 1 μm to 10 μm, preferably 3 μm to 8 μm. The structure is generally substantially free of coarse grains. The proportion of β phase in the final object is preferably between 5% and 80% by volume, in particular between 10% and 70% by volume, more particularly between 15% and 60% by volume, for example between 17% and 50% by volume. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Additional features, advantages and effects of the present invention are derived from the exemplary embodiments described below. In the accompanying drawings to which reference is made:

[0037] Figure 1 Schematic diagram showing a heating curve (left) and subsequent cooling curve (right) for forming below the martensite start temperature;

[0038] Figure 2 Shows something like Figure 1 Schematic diagram with different cooling curves for forming above the martensite start temperature and in the range of martensite formation;

[0039] Figure 3 shows a diagram with a temperature progression during the method according to the invention;

[0040] Figure 4 A graph showing the exponential hardening coefficient measured at a measuring temperature of 500° C. as a function of preheating temperature and cooling time is shown;

[0041] Figure 5 Graphs illustrating the deformation behavior of metal sheets with different pretreatments are shown. DETAILED DESCRIPTION

[0042] exist Figure 1 and Figure 2 In FIG. 1 , the concept of the method according to the invention is schematically shown. The temperature course during heating is shown on the left, while the temperature course in the TTT diagram is shown on the right. Figure 1As can be seen on the left, the object to be formed is heated to a temperature below the β transformation point, at which a certain proportion of the β phase appears. Heating can be performed relatively quickly, for example within 120 seconds. The object is then maintained at the preheating temperature in a targeted manner until it reaches the desired temperature and is heated through without causing excessive grain growth. This ensures a uniform structure, including the necessary grain size, before forming.

[0043] Then, according to Figure 1 and Figure 2 In the TTT diagram on the right side of the figure, the object is cooled to the desired forming temperature. Cooling is carried out relatively quickly, for example within a time window of 3 to 60 seconds. In particular, cooling can be carried out while the metal sheet is transported from the furnace to the forming station. Cooling typically occurs quickly, for example within 60 seconds. Cooling occurs passively during transport and during the holding time in the tool before forming begins. The transport time (including the holding time in the forming tool until forming) is within the general scope of the present invention and is therefore typically less than 30 seconds, preferably between 0.5 and 30 seconds, for example, 1 to 30 seconds, and in particular 2.5 to 15 seconds. Therefore, in addition to other parameters (such as sheet thickness), the transport time has a decisive influence on cooling. Forming is then carried out, after which the formed object is allowed to cool without active cooling, or alternatively, is subjected to controlled cooling until the object reaches room temperature. The object can then undergo further processing steps, such as further heat treatment, or processing steps, such as material removal processing.

[0044] exist Figure 1 The process of a typical hot deep drawing is also schematically shown on the left (bottom curve) in the figure. Here, the object is heated to the forming temperature, formed at this temperature, and then allowed to cool. This illustration is not to scale, as the forming temperature for hot deep drawing is approximately in the range of 650°C.

[0045] like Figure 1 As shown on the right, the starting temperature of martensite formation during cooling (martensite start temperature M s ) and the final martensite termination temperature (M f ), and then forming is performed below the corresponding temperature window. The result is as follows Figure 1 The structure shown on the right is made of α phase and grains of transformed β phase composed of metastable β phase and α′ phase. Figure 2 As shown on the right side of the figure, it is also possible to manage the cooling so that it moves to the nose of the TTT diagram. Thus, a different structure can be obtained in which the β phase exists together with the α phase. Therefore, the structure formation can be controlled by targeted temperature management during cooling. Figure 2 As shown in the figure on the right, the forming temperature can therefore also be above the martensitic window.

[0046] exist Figure 3 In FIG. 1 , temperature profiles can be seen, such as are typically used for forming a metal sheet into an object for producing a corresponding object made of an alpha-beta titanium alloy.

[0047] Figure 4 The hardening index is shown as a function of the preheating temperature. To determine the values ​​described, the -1 The tensile test was carried out at a constant strain rate.

[0048] Figure 5 The graph shows the rheological behavior of a material pretreated according to the invention compared to a non-pretreated material at 500° C. Clearly, the pretreated material exhibits significantly better rheological behavior.

[0049] In the test, after pre-treatment of the blank, a sheet with a thickness of about 1.5 mm was used to apply lubricant (100 ml in 100 ml distilled water) on both sides. Acceptable parts can be produced from the alpha-beta titanium alloy Ti6Al4V by deep drawing (after cleaning the sheet with isopropyl alcohol). Therefore, a typical preheating time is 10 minutes. The pressing depth ranges from 20 mm to 61 mm. The pressing speed is 15 mm / s. The forming temperature ranges from 390°C to 500°C. After preheating in the furnace, the sheet is formed within 30 seconds and passively cooled during transport from the furnace to the forming tool for this step.

[0050] The method according to the invention therefore offers the possibility of setting the structure and strength, wherein higher limiting drawing ratios can be achieved with a relatively low overall energy requirement.

Claims

1. A method for producing an object made of an alpha-beta titanium alloy, in particular a method for producing a shaped metal sheet made of an alpha-beta titanium alloy, comprising the following steps: a) Preheat the object to the preheating temperature; b) setting the proportion of β phase in the structure of the object; c) cooling the object to forming temperature; d) forming the object at a forming temperature.

2. The method according to claim 1 , wherein the object is heated to a preheating temperature of at least 100° C., preferably at least 150° C., in particular at least 180° C. above the forming temperature, for example a preheating temperature of 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 the temperature range of 50°C below the β transformation point of the α-β titanium alloy to 260°C below the β transformation point of the α-β titanium alloy, preferably 155°C to 260°C below the β transformation point of the α-β titanium alloy, and in particular 170°C to 225°C below the β transformation point of the α-β titanium alloy.

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

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

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

7. The method according to any one of claims 1 to 6, wherein the object is moved at a speed of 0.001s. -1 to 4s -1 , especially 0.005s -1 to 2.0s -1 , for example, 0.005s -1 to 1.5s -1 Strain rate forming.

8. The method according to any one of claims 1 to 7, wherein the forming is performed at a temperature above the martensite start temperature (M s ) implementation.

9. The method according to any one of claims 1 to 7, wherein the forming is performed at a temperature below the martensite termination temperature (M f ) implementation.

10. The method according to any one of claims 1 to 7, wherein the forming is carried out at the martensite start temperature (M s ) and the martensite termination temperature of the β phase (M f ) is implemented within the temperature range between .

11. The method according to any one of claims 1 to 10, wherein the forming is performed by stretch forming or stretch / compression forming.

12. A method according to any one of claims 1 to 11, wherein a lubricant is applied to the object before and / or during forming.

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

14. The method according to claim 13, wherein: The thickness of the metal sheet is less than 5.0 mm, in particular less than 4.0 mm, preferably less than 3.5 mm, for example less than 3 mm.

15. An object, in particular a deep-drawn aircraft component, obtainable according to any one of claims 1 to 14.

Images