Patent Application
20070261461
1. Technical Field
The present invention relates to hot stretch-wrap forming of metal parts, most particularly elongated metal bars. More particularly, the invention relates to hot stretch-wrap forming of titanium, titanium alloys and similar metals which are typically difficult to stretch-wrap form. In particular, the invention relates to hot stretch-wrap forming of a metal form using a die having a thermally and electrically insulated work surface.
2. Background Information
The present invention relates to the hot stretch-wrap forming of elongated metallic parts which are formed at high temperatures, and in particular parts made of titanium alloys which are manufactured by extrusion, forging, rolling, machining or a combination of these processes. Titanium alloys have been widely used as aerospace materials due to their excellent mechanical and corrosion properties in combination with being comparatively light weight. However, it is well-known that titanium alloys are difficult to form in general and require heating to a substantial temperature in order to properly form such parts. Titanium alloys are highly desirable for use in contoured structural members of an aircraft, but the formation of such structural members has been very limited due to the lack of a suitable and economically feasible method of forming such contoured members. The demand for such parts has increased with the desire for lightweight and high strength structural components such as chords in advanced airplanes.
One process currently available for forming elongated titanium parts is known as “bump forming”. This process involves the heating of an elongated part in the furnace to a predetermined temperature at which time the part is removed from the furnace and placed on forming blocks of a forming press. The press applies a bending force which results in a localized deformation of the part. The temperature of the part quickly decreases during formation and the resistance to forming thus significantly increases. Thus, bump forming requires repeated heating cycles to complete the forming process, which is time consuming and costly. In addition, the bending moment that results from bump forming causes tensile stresses in the section of the part above the neutral axis and compressive stresses below the neutral axis which lead respectively to cracks and wrinkles in the part. The considerable stress gradient within the part makes it difficult to control the geometry of the formed part. In addition, the localized deformation caused by the complex stress state of the part promotes the development of significant residual stresses therein, requiring an offline stress relief treatment with an expensive fixture. Bump forming also suffers from the lack of a guiding tool to achieve the required contour without resorting to a trial and error method. It is also difficult to maintain the structural integrity of the cross section, for example, along angles between flanges and the like. Post hot sizing has been suggested to improve the dimensional integrity of the formed part. Finally, bump forming is not amenable to computer simulation.
While the general concept of hot stretch-wrap forming has been known for some time, known prior art methods are not suitable for economically forming parts made of titanium alloys or other materials which are difficult to form. U.S. Pat. No. 2,952,767 granted to Maloney discloses an apparatus for stretch-wrap forming an elongated bar which is heated by resistance heating and wrapped around a metallic die heated by conventional heating elements within the die assembly. A major problem with this configuration is the electrical shunting effect that occurs between the heated die and the metal part as they contact one another, which leads to local overheating and necking of the part.
U.S. Pat. No. 4,011,429 granted to Morris et al. noted the above shunting effect and sought to overcome this problem by heating both the die and the elongated metal part via resistance by electrically connecting the die and the metal part in parallel and heating them with the same voltage. Unfortunately, this configuration is not practical because the parallel heating of the die and part requires a complex and prohibitively expensive configuration of the die. In addition, this method requires preheating of the part to a temperature substantially below its forming temperature while the die is heated to the forming temperature so that only the contacting portion of the part is brought up to the forming temperature upon contact with the die, which results in a non-uniform yield strength between the contacting and non-contacting portion of the bar. Because the deformation process is not uniform, it is extremely difficult to maintain the structural integrity of the formed part and to minimize the development of residual stresses.
The present invention addresses these and other problems as will be evident from the subsequent description.
The present invention provides an apparatus comprising a die having a work surface which is formed of at least one of a thermally insulative material and an electrically insulative material; wherein the work surface is adapted for wrapping a metal form there around during hot stretch wrap forming of the metal form; a set of first and second spaced jaws adapted for gripping the metal form; wherein the jaws are movable away from one another whereby the jaws are adapted to apply a stretching force to the metal form; and wherein at least one of the set and the die is movable relative to the other of the set and die between a pre-wrapping configuration and a post-wrapping configuration whereby the relative movement between the set and die is adapted to wrap the metal form around the work surface of the die.
The present invention further provides a method comprising the steps of heating a metal form; applying a stretching force to the heated metal form; and wrapping the heated metal form around a working surface which is formed of at least one of a thermally insulative material and an electrically insulative material.
Similar numbers refer to similar parts throughout the drawings.
The hot stretch-wrap forming apparatus of the present invention is indicated generally at 10 in
First or inner and second or outer layers 34 and 36 of insulation serve as a die face cover which is positionable between die face 26 and metal bar 16 during the hot stretch-wrap process. Layers 34 and 36 are shown disposed within cavity 28 with first layer 34 abutting surface 26 of die 12 in a substantially continuous manner from first end 30 to second end 32 of die 12. Second layer 36 likewise abuts first layer 34 in a substantially continuous manner from first end 30 to second end 32. Layers 34 and 36 conform to surface 26 and thus are of a generally T-shaped configuration. Second layer 36 defines a working surface 38 which abuts metal bar 16 during the wrapping process. As shown in
More particularly, each of first and second layers 34 and 36 is most preferably formed of a thermally and electrically insulative material. Alternately, one of layers 34 and 36 may be formed of a thermally insulative material and the other may be formed of an electrically insulated material if desired. While it is preferred to provide thermal and electrical insulation between die 12 and bar 16, it is contemplated that only a layer of thermal insulation or only a layer of electrical insulation may be used depending on the circumstances.
Referring to
In the exemplary embodiment, layers 34 and 36 are formed of a flexible refractory material. This allows layers 34 and 36 to easily conform to the shape of the die cavity. In addition, the use of such flexible layers allows for versatility in positioning the layers prior to the wrapping process. For example, priorto insertion of the metal bar into the die cavity, the layers may be disposed within the die cavity (as shown), wrapped around a portion or all of the metal bar, or simply suspended between the cavity and the metal bar so that insertion of the metal bar into the cavity presses the insulation material into the desired shape. Layers 34 and 36 are typically refractory ceramic blankets. One such suitable ceramic blanket is sold under the name Kaowool. Such ceramic blankets typically provide both the thermal and electrical insulative properties previously described and are formed of woven ceramic fabric or fibers. These flexible blankets are also easily removed from the die cavity or the metal bar when degraded to a degree such that they are no longer useful for the present purpose.
While such ceramic blankets are one form of a desirable insulative material, other suitable materials may be utilized which provide the thermal and/or electrical insulative properties needed for the present invention and which are capable of withstanding the heat and pressure utilized during the wrapping process.
The operation of apparatus 10 is described with reference to
The electrically insulative property of layer 34 and/or 36 prevents the electrical shunting between bar 16 and die 12 which was discussed in the Background section of the present invention. In addition, the thermal insulative property of layer 34 and/or 36 minimizes or eliminates the creation of hot spots in bar 16 which might otherwise be caused by die 12 when it is heated, and especially if not uniformly heated. The thermal insulative property also allows for the use of die 12 either without heating die 12 or heating die 12 at a substantially reduced level compared to known prior art configurations.
Thus, apparatus 10 provides a configuration for resistively heating metal bar 16 without creating shunting problems or hot spots during the wrapping operation. This is highly beneficial due to the fact that the resistive heating of metal bar 16 provides for uniform heating throughout the metal bar. Preferably, metal bar 16 is heated to a particular temperature range, is stretched at a relatively low strain rate and is held at the temperature range throughout the process and for a holding period after the wrapping process in order to allow formation of the metal bar in its final form substantially without springing back and substantially without undesirable tensile or compressive stresses within the formed part. Longitudinal stretching may continue during the holding period if desired. The uniform temperature of bar 16 is preferably maintained throughout the stretching and wrapping process and during any holding period, even if die 12 is not independently heated or is heated to a temperature substantially below the forming temperature of bar 16. Insulation layers 34 and/or 36 greatly facilitate the ability to maintain this uniform temperature due to the corresponding reduction in heat loss from bar 16 and prevention of the hot spots previously discussed. Apparatus and methods for several preferred aspects of the invention are described in greater detail in the copending patent application entitled Method And Apparatus For Creep Forming Of And Relieving Stress In An Elongated Metal Bar, which is filed concurrently herewith and incorporated by reference herein.
In the foregoing description, certain terms have been used for brevity, clearness, and understanding. No unnecessary limitations are to be implied therefrom beyond the requirement of the prior art because such terms are used for descriptive purposes and are intended to be broadly construed.
Moreover, the description and illustration of the invention is an example and the invention is not limited to the exact details shown or described.
