JP2009539620A - Creep forming and stress relaxation of metal bars - Google Patents

Abstract

  The thermal creep stretch wrap forming method comprises heating a metal rod (16) to a forming temperature within a temperature range suitable for its creep deformation, and the metal rod at a strain rate of 0.05 inch / inch / second or less. Applying a stretching force to the die, and winding the metal rod around a die (12), preferably a die (12) having a thermally and / or electrically insulating work surface (34, 36); including. Stretching force is generally applied until a strain in the range of 0.5% to 15.0% is obtained. The metal rod (16) is most preferably a titanium alloy having a forming temperature in the range of 0.45 to 0.60 of its melting temperature. The wound metal bar is held in place and its temperature is generally maintained within that temperature range for 5 to 120 minutes for stress relaxation. Preferably, the metal rod (16) is held substantially at the forming temperature throughout the process. Thermal insulation (62) around the die and metal bar reduces heat loss from the metal bar.

Description

BACKGROUND OF THE INVENTION TECHNICAL FIELD This invention relates to hot creep-stretch forming of metal parts, most particularly elongated metal bars. More specifically, the invention relates to hot stretch-wrap forming of titanium, titanium alloys, and similar metals that are generally difficult to stretch-wrap. In particular, the present invention relates to hot stretch wrap forming of metal foam using a die having a thermally and electrically insulated work surface.

2. BACKGROUND INFORMATION This invention relates to hot stretch wrap forming of elongated metal parts formed at high temperatures, particularly titanium alloy parts produced by extrusion, forging, rolling, machining, or a combination of these processes. Titanium alloys are widely used as aerospace materials due to their superior mechanical and corrosion properties and relatively light weight. However, forming titanium alloys is generally difficult and it is well known that titanium alloys must be heated to significant temperatures in order to accurately form such parts. Titanium alloys are highly desirable for use in aircraft contoured structural members, but there is an economically viable and preferred method of forming such contoured members. Due to the absence, the molding of such structural members is very limited. With the desire for lightweight, high-strength structural components, such as high performance airplane chords, the demand for such components is increasing.

  One process currently available for forming elongated titanium parts is known as “bump forming”. This process involves heating the elongated part in a furnace to a predetermined temperature, at which point the part is removed from the furnace and placed on a forming block of a forming press. This press applies a bending force that causes local deformation of the part. Since the temperature of the part drops rapidly during molding, the resistance to molding is significantly increased. Thus, bump molding requires repeated heating cycles to complete the molding process, which is time consuming and expensive. In addition, the bending moment resulting from bump molding creates tensile stresses on the part of the part above the neutral axis and compressive stresses below the neutral axis, thereby causing cracks and wrinkles in the part, respectively. . If there is a large stress gradient in the part, it will be difficult to control the geometry of the molded part. In addition, the local deformation caused by the complex stress state of the part facilitates the generation of large residual stresses inside the part and requires an off-line stress relaxation process using expensive equipment. Bump molding also has the disadvantage that it does not have a guide tool for obtaining the required contour without relying on trial and error methods. For example, it is difficult to maintain the structural integrity of the cross section along an angle such as between the flanges. Post-thermal sizing has been proposed to increase the dimensional consistency of molded parts. Finally, bump molding is not suitable for computer simulation.

Although the general concept of hot stretch wrap forming has been known for some time, the prior art known methods are not suitable for economically forming parts of titanium alloys or other materials that are difficult to form. . U.S. Pat. No. 2,952,767 to Maloney discloses an apparatus for stretch wrapping an elongated bar that is heated by resistance heating and within a die assembly. And wound around a mold heated by a conventional heating element. The main problem with this configuration is the electrical shunting effect that occurs between the heated die and the metal part as the heated die and metal part come into contact with each other. This leads to local overheating and constriction of the parts.

  U.S. Pat. No. 4,011,429 issued to Morris et al. Describes the shunting action described above and connects the die and elongated metal parts in parallel electrically and heats them at the same voltage. By doing so, both the die and the metal part are heated by resistance to overcome this problem. Unfortunately, this configuration is not practical. This is because the parallel heating of the die and parts is prohibitively expensive and requires a complicated die configuration. In addition, this method requires preheating the part to a temperature substantially below its molding temperature, while the die is heated to the molding temperature so that only the contact portion of the part is When contacting the die, the temperature is raised to the molding temperature, and as a result, non-uniform yield strength is generated between the contact portion and the non-contact portion of the component. Because the deformation process is not uniform, it is extremely difficult to maintain the structural integrity of the molded part and minimize the occurrence of residual stresses.

  The present invention addresses these and other problems that will become apparent from the following description.

SUMMARY OF THE INVENTION The present invention includes a step of heating an elongated metal bar to a forming temperature within a temperature range suitable for creep deformation of the metal bar, and the heated metal bar is subjected to 0.05 inch / inch / second or less. Applying a stretching force at a strain rate of and a wrapping the heated metal bar around the die to form a wound metal bar.

  Like numbers refer to like parts throughout the drawings.

DETAILED DESCRIPTION OF THE INVENTION A hot stretch wrap forming apparatus of the present invention is shown generally at 10 in FIG. The apparatus 10 includes a die 12 and a pair of spaced-apart grippers 14 that heat a metal foam, shown as an elongated metal rod 16 adjacent to each end thereof. And is configured to clamp the metal foam to stretch the metal rod 16 when wound around the die 12. The apparatus 10 is particularly useful for thermal creep stretch forming of bars made of titanium alloys. Although not shown in the figure, each of the grippers 14 is attached to a swing arm that is well known in the art. Each of the grippers 14 is connected to a power source via a conductor or wire 20 and forms an electrical circuit for resistance heating of the metal bar 16. A plurality of heating elements 24 that are electrically connected to the power supply 18 via wires 22 can be inserted into the die 12 to heat the die 12. The die 12 has a void boundary surface 26 that defines a T-shaped die void 28 (FIG. 2). Surface 26 and void 28 have an arched configuration that extends from first end 30 of die 12 to second end 32 of die 12. The insulating first or inner layer 34 and the second or outer layer 36 are such that the first layer 34 is substantially continuous from the first end 30 to the second end 32 of the die 12. In this manner, the air gap 28 is disposed in contact with the surface 26 of the die 12. Similarly, the second layer 36 abuts the first layer 34 in a substantially continuous manner from the first end 30 to the second end 32. Layers 34 and 36 conform to surface 26 and thus have a generally T-shaped configuration. The second layer 36 defines a work surface 38 that abuts the metal rod 16 during the winding process. As shown in FIG. 2, the metal bar 16 has a T-shaped cross section, and this cross section has a meshing configuration with the T-shaped gap 28 and the work surface 38. The work surface 38 defines a T-shaped work space 40 in which the rod 16 is disposed during the stretch winding process.

  More specifically, each of first layer 34 and second layer 36 is most preferably formed of a thermally and electrically insulating material. Alternatively, if desired, one of layers 34 and 36 can be formed from a thermally insulating material and the other from an electrically isolated material. Although it is preferred to provide thermal and electrical insulation between the die 12 and the rod 16, it is contemplated that only a thermally insulating layer or only an electrically insulating layer may be used depending on the situation. .

  In the exemplary embodiment, layers 34 and 36 are formed of a flexible and heat resistant material. This allows layers 34 and 36 to easily conform to the shape of the die cavity. In addition, the use of such a flexible layer provides flexibility in positioning the layer prior to the winding process. For example, before inserting the metal rod into the die cavity, a layer may be disposed within the die cavity (as shown) and wrapped around part or all of the metal rod, or The insulating material can be pressed into the desired shape by simply floating between the gap and the metal rod, thereby inserting the metal rod into the gap. Layers 34 and 36 are typically refractory ceramic blankets. One such suitable ceramic blanket is sold under the name Kaowool. Such ceramic blankets generally provide both the thermal and electrical insulation properties described above and are formed of ceramic woven fibers. Such flexible blankets can also be easily removed from the die cavity or metal rod when they have deteriorated to the point where they are not useful for this purpose. Such a ceramic blanket is one form of desirable insulative material, but has the thermal and / or electrical insulation properties required for this invention and capable of withstanding the heat and pressure used during the winding process. Other suitable materials and / or coatings provided can be used.

The apparatus 10 further includes a thermal insulating cover 62 that substantially holds the die 12 and the metal bar 16 when the bar 16 is clamped between the gripper 14 and the gripper 14 in its initial position. Enclose. The cover 62 includes a trapezoidal upper wall 64 that extends from the front wall 66 to the rear wall 68 and from the first side wall 70 to the second side wall 72. The cover 62 further includes a lower wall 74 (FIG. 2) having substantially the same shape as the upper wall 64. The cover 62 includes a door 76, which includes a part of the upper wall 64 and is hinged to the other part of the upper wall 64 via a hinge 78. Instead of this hinged connection design, a retractable design can be used. The door 76 also includes a front wall 66 and a portion of the lower wall 74. The door 76 is movable between a closed position shown in FIGS. 1 to 4 and an open position shown in FIG. Each of the walls 64, 66, 68, and 74 includes an outer support wall 80, typically made of metal, and a thermally insulating inner layer 82. The side walls 70 and 72 may also include a thermally insulating support wall 80 and layer 82, respectively, but the walls 70 and 72 are configured to move the metal bar 16 from the initial position of the metal bar 16 to a completed position. Note that Thus, the side walls 70 and 72 can be either fully open, partially open, or movable between an open position and a closed position, thereby providing a metal rod. 16 can move from the initial position to the completed position in the open position. Thus, for example, the side walls 70 and 72 can be hinged to the top wall 64 to move between such open and closed positions. Instead of these side walls, ceramic fiber or blanket flexible curtains can also be used. In the exemplary embodiment, the insulating layers 82 of the upper wall 64 and the lower wall 74 abut the upper and lower surfaces of the die 12 respectively, while the insulating layer 82 of the door 76 is outward from the die 12. Spaced apart from each other to define a space defining an initial position of the metal bar 16 therebetween. The cover 62 facilitates that the heating of the die 12 and the metal rod 16 can be controlled separately. The description and display of the cover is not limited to a hinged design. For example, other methods and designs using retractable sliding covers with or without a flexible curtain of sidewall ceramic blankets or fibers are not excluded from the scope of this invention.

  The general operation of the device 10 will be described with reference to FIGS. Referring to FIG. 1, the power source 18 is activated to pass a current through the metal bar 16 to resistively heat the metal bar 16 to a desired predetermined temperature. Once this temperature is reached, the gripper 14 applies a stretching force as indicated by arrow A, i.e., a longitudinal tensile force or strain. On the other hand, the die 12 may or may not be heated depending on the specific situation. If the die 12 can be heated, the power source 18 can be activated to resistively heat the die 12 via the wire 22 and the heating element 24. Regardless of whether the die 12 is heated or not, the gripper 14 is then moved toward the die 12 and the bar 16 is moved into the working space 40 indicated by arrow B in FIGS. Alternatively, or in combination, the die 12 may be moved to facilitate relative movement between the die 12 and the gripper 14. Next, the gripping tool 14 is moved as shown by the arrow C in FIG. This results in an arched configuration. While the metal bar 16 is wound around the die 12, a longitudinal stretching force is continuously applied to the bar. In this way, the gripping tool 14 shifts from the configuration before winding of the device 10 shown in FIG. 1 to the configuration after winding shown in FIG.

  The electrical insulation properties of layers 34 and / or 36 prevent electrical shunting between rod 16 and die 12 as discussed in the background section of this invention. In addition, due to the thermal insulation properties of layers 34 and / or 36, a rod that would otherwise be caused by die 12 when it is heated, particularly when die 12 is not uniformly heated, is provided. The formation of hot spots within 16 is minimized or eliminated. This thermal insulation property also allows the use of the die 12 either to heat the die 12 at a substantially lower level compared to known prior art configurations or not to heat the die 12. To.

  This method will be described more specifically with reference to FIGS. FIG. 6 is a schematic stress / temperature map for commercially pure titanium having a particle size of 0.1 mm. Here, the number on the left represents the normalized shear stress (σs / μ), the number on the right represents the shear stress at 20 ° C. (MN / m 2), and the number on the upper side represents the temperature expressed in ° C. The lower number represents the in-phase temperature (T / Tm), where T is the temperature of the metal expressed in ° K, and Tm is the melting temperature of the metal expressed in ° K. FIG. Harold J. Frost and Michael F. "Deformation-Mechanism Maps: The Plasticity and Creep of Metals and Ceramics" by Michael F. Ashby, Pergamon Press, 1st edition (1982) (October year), taken from Figure 6.10 on page 50. This figure, made by Frost and Ashby, shows that there are certain conditions under which such titanium can be deformed by plastic deformation, by diffusion deformation, or by creep molding without the occurrence of plastic deformation or diffusion deformation Indicates to do. Such creep deformation occurs mainly in the region indicated by 84 in FIG.

  Applicants have identified the rod as a specific material associated with such creep deformation in order to produce a hot-stretch wound titanium alloy rod having a high quality that overcomes the stress problems of prior art methods. It was judged that it should be treated under the conditions. As shown in FIG. 7, plasticity controlled deformation (block 86) results in poor metal flow and work hardening, which leads to molding difficulties. In the other limit, the diffusional flow controlled deformation (block 88) results in a high rate of creep deformation, which leads to creep failure. In addition, oxidation and surface contamination during diffusion deformation results in alpha-case titanium products. In contrast, creep molding within certain parameters (block 90) eliminates the adverse effects of plastic deformation and diffusional deformation and produces the uniform deformation required to produce the desired high quality product. .

  More specifically, the metal rod 16 is heated to a forming temperature in the range of 0.45 to 0.60 Tm (° K) (about 650-925 ° C. or 1202-1690 ° F.) to reduce the temperature of the metal rod 16. While maintained within the above range, it is pre-stretched from 0.5% to 3.0% strain at a controlled strain rate of less than 0.05 inches / inch / second. This strain rate is preferably in the range of 0.00005 to 0.005 inches / inch / second. Strain is defined as the difference between the stretched length and the original length of the bar 16 (or part thereof) divided by the original length of the bar or part thereof, respectively. In Ti-6Al-4V, which is a preferred titanium alloy, the molding temperature is in the range of 1250-1450 ° F. Next, under the same conditions, the metal rod 16 is creep drawn and wound around the layers 34, 36 and the die surface 26. The above treatment minimizes the residual stress in the metal bar 16. However, once the curved metal rod 16 of FIG. 4 is generated by the stretch wrapping step, the gripper 14 holds the metal rod 16 in this curved configuration against the layers 34, 36 and the die surface 26. On the other hand, the temperature of the metal rod 16 is continuously maintained over a holding period in the range of 5 minutes to 120 minutes to relieve any residual stress that may have occurred during the stretching and winding process. This inherent retention period depends on the stress relaxation properties of the metal from which the rod 16 is formed. Depending on the particular situation, a longitudinal stretching force may or may not be applied during this holding period.

  Most preferably, the metal rod 16 is maintained at a substantially uniform temperature (forming temperature) throughout the creep pre-stretching, creep-stretch wrap forming, and holding period. Once the metal bar 16 has been heated to this forming temperature, the temperature during these steps generally varies from the forming temperature to 30 ° C. or less, preferably 15 ° C. or less. The temperature of the metal bar 16 can be maintained throughout each of these steps by simply heating the metal bar 16, but the use of the thermal insulation cover 62 allows this process with reduced energy consumption. Greatly promote. The use of one or more layers 34, 36 having thermal insulating properties also helps prevent heat loss from the metal rod 16, and thus, during processing, particularly, the layers 34, 36 surround the metal rod 16. Helps maintain that temperature when fully wound on. In some cases, maintaining a substantially uniform temperature over the duration of a predetermined holding period is achieved only by the warming provided by the cover 62 and / or layers 34, 36 without further heating of the metal rod 16. Can be done. The uniform temperature of the rod 16 is generally maintained even when the die 12 is not heated separately or even to a temperature substantially below the molding temperature of the rod 16.

  Thus, this process produces undesirable tensile or compressive stresses that are substantially free of springback and can cause breakage 92 and buckling 94, respectively, shown in exaggerated manner in the prior art metal rod 96 of FIG. The final form (FIG. 9), which does not substantially occur in the molded part, is produced in a hot creep stretch wound metal rod 16 (FIG. 9). Since this process substantially eliminates residual stress in the metal bar 16, there is no need for post-molding stress relaxation processing off-line of the metal bar 16, thus substantially reducing manufacturing time and manufacturing cost substantially. Lower. Yet another aspect of the present invention is the “Method And Apparatus For Hot Forming Elongated” filed concurrently with this specification and incorporated herein by reference. This is described in more detail in a co-pending patent application entitled “Metalic Bars”.

The molding parameters and the results of creep molding of the three extrudates will be described with reference to FIG. 10 and the table included below. FIG. 10 shows an L-shaped extrudate or extruded rod 98 having a first leg 100 and a second leg 102 extending vertically therefrom. Bar 98 represents each of the three tested extrudates discussed below, and more specifically, extrudate 1, extrudate 2, and extrudate 3. The rod 98 has a full width W1 along the first leg 100 and an overall thickness T1 along the leg 102. Leg 100 has a thickness T2, and leg 102 has a width W2. Each of the tested extrudates was formed with Ti6AL-4V. In each of the tested extrudates, the width W1 before molding is 1.610 inches, the thickness T1 is 2.260 inches, the width W2 is 0.350 inches, and the thickness T2 is 0.780. It was inches. For each extrudate, the cross-sectional area of rod 98 was 1.791 square inches. The scope of the invention is not limited to any of the dimensions or shapes presented above for illustration. The ratio of overall width to overall thickness is generally in the range of 1: 1 to 10: 1.

  As shown in Table 1, Extrudate 1 was molded using a modified prior art process using a die heated to a temperature approximately equal to the molding temperature of Extrudate 1. During heating, a ceramic wool blanket was used as a partition between the die and the extrudate 1 to prevent diversion effects on the extrudate 1. Thereafter, the current through the extrudate 1 was interrupted for resistance heating, and the ceramic wool partition was removed just prior to the winding process.

  Extrudates 2 and 3 were molded using a non-heated die facing the ceramic wool and silica sheath. All of these extrudates were heated to the molding temperature by electrical resistance heating before and during the winding process. After molding was complete, the extrudates were held in place with continued resistance heating under the ceramic wool blanket. As can be seen from Table 1, the extrudate 1 formed according to the deformed prior art shows that there was substantial movement of the parts after stress relaxation, while the extrudates 2 and 3 were after pre-stretching and forming The minimum movement by holding was shown.

  As can be seen from Table 2, there is no substantial difference in tensile properties between the molded and non-molded portions of the extrudate. In addition, there is substantially no difference in microstructure, alpha case, and structural uniformity between the molded and non-molded portions of the extrudate. Accordingly, creep molding does not produce substantial changes in properties or microstructure compared to the original extruded and annealed bar.

  In the above description, certain terminology has been used for the sake of brevity, clarity and understanding. Beyond the requirements of the prior art, no unnecessary limitations should be implied therefrom. This is because such terms are used for illustrative purposes and are intended to be interpreted broadly.

  Further, the description and illustration of the invention is an example, and the invention is not limited to the exact details shown or described.

It is a schematic plan view of the thermal creep stretch molding apparatus of this invention, and is a figure which shows the thermally and electrically insulating material in the space | gap of die | dye, and the metal bar before winding around the said die | dye. FIG. 2 is a cross-sectional view taken along line 2-2 of FIG. 1, showing two layers of insulating material in the die cavity. FIG. 3 is a cross-sectional view similar to FIG. 2, showing that the metal rod is inserted into the gap of the die with the insulating material separating the metal die and the metal rod. FIG. 5 is a view similar to FIG. 1 showing the gripper moving from the initial position of FIG. 1 to the completed position of FIG. FIG. 4 is a view similar to FIG. 3, showing the coated door in the open position and the stretch wound metal bar removed from the die gap. FIG. 3 is a schematic stress / temperature map for commercially pure titanium, the left number indicates normalized shear stress, the right number indicates shear stress at 20 ° C., and the upper number indicates The temperature indicated in ° C. is shown, and the lower number is a diagram showing the common mode temperature. It is the schematic which shows the area | region of a plastic deformation, the area | region of a uniform deformation | transformation, and the area | region of a diffusion deformation. 1 is a schematic plan view of a prior art metal rod showing buckling and breakage caused by stress generated during hot stretch wrap forming. FIG. FIG. 9 is a view similar to FIG. 8 showing a metal rod formed by the hot stretch winding method of the present invention, the metal rod having buckling and breaking of the prior art metal rod of FIG. FIG. It is sectional drawing of the metal bar which has a L-shaped structure.

Claims (20)

Heating the elongated metal bar to a forming temperature within a temperature range suitable for creep deformation of the metal bar;
Applying a stretching force to the heated metal bar;
Winding the heated metal bar around a die to form a wound metal bar;
Holding the wound metal bar in a wound position while maintaining the temperature of the wound metal bar within the temperature range for at least 5 minutes.   The step of holding includes the step of holding the wound metal bar in a winding position while maintaining the temperature of the wound metal bar within the temperature range for at least 10 minutes. The method described in 1.   The step of holding includes the step of holding the wound metal rod in a winding position while maintaining the temperature of the wound metal rod within the temperature range for at least 20 minutes. The method described in 1.   The step of holding includes the step of holding the wound metal rod in a winding position while maintaining the temperature of the wound metal rod within the temperature range for at least 40 minutes. The method described in 1.   5. The step of holding includes the step of holding the wound metal rod in a winding position while maintaining the temperature of the wound metal rod within the temperature range for at least 60 minutes. The method described in 1.   The method of claim 1, wherein the step of holding includes positioning thermal insulation adjacent to the wound metal bar to reduce heat loss from the wound metal bar.   The step of winding includes heating the heated metal around a die surface of the die and a layer of thermally insulating material disposed between the die surface and the heated metal rod. The method of claim 6 including the step of winding the rod.   Winding the heated metal rod around the die surface of the die extending between opposing sides of the die; and further, the step of winding the die. Positioning the first and second layers of thermal insulation adjacent to each of the sides.   The method of claim 1, wherein the applying step comprises applying a stretching force to the heated metal bar at a strain rate of 0.05 inches / inch / second or less.   The step of applying includes applying a stretching force to the heated metal bar to a strain in the range of 0.5% to 15.0% at a strain rate of 0.05 inches / inch / second or less. Item 10. The method according to Item 9.   11. The method of claim 10, wherein the step of heating comprises heating the metal bar to a forming temperature within a temperature range of 0.45 to 0.60 Tm, where Tm is the melting point of the metal bar. The method of claim 9, wherein the applying step comprises applying a stretching force to the heated metal bar at a strain rate in the range of 0.00005 inches / inch / second to 0.005 inches / inch / second. Method.   The method of claim 1, wherein the heating step comprises heating the metal rod to a forming temperature within a temperature range of 650 ° C.-925 ° C. (1202 ° F-1690 ° F.).   The method of claim 1, wherein the step of heating comprises heating a metal rod formed of titanium to a forming temperature within a temperature range of 1250 ° F. to 1450 ° F.   The method of claim 1, wherein the maintaining comprises maintaining the metal rod at a temperature within 15.0 ° C. from the forming temperature for at least 5 minutes.   The step of maintaining includes maintaining the metal rod at a temperature within 30.0 ° C. from the forming temperature throughout the step of adding, winding, holding, and maintaining. The method of claim 1.   The step of heating comprises heating the metal bar to a substantially uniform forming temperature throughout the metal bar, and further, at a substantially constant temperature throughout the adding and winding steps. And maintaining the metal bar.   18. The method of claim 17, further comprising maintaining the metal rod at a substantially constant temperature throughout the step of maintaining and the step of maintaining.   The step of heating includes the step of resistively heating the metal rod through a current through the metal rod, wherein the step of wrapping the electrically insulating material layer heated the outer surface of the metal die. Wrapping the heated metal rod around the outer surface of the metal die in a state that separates from the metal rod and prevents electrical transmission between the metal rod and the metal die. Item 2. The method according to Item 1.   The step of winding includes winding the heated metal rod around a work surface formed of at least one of a thermally insulating material and an electrically insulating material. The method according to 1.

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