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(11) | EP 0 194 827 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Superplastic forming |
| (57) A hollow vessel can be made by fabricating a preform from welded together sheets
of superplastically deformable metal and then expanding the preform to the required
shape of the vessel but the necessary weld lines can be a source of weakness. Herein,
a vessel is made by superplastic deformation of a preform made by machining a hollow
space within a solid piece of metal or by extruding a tubuler preform and blanking
off at least one of its ends. |
[0001] This invention relates to the manufacture of hollow articles such as fluid containers and pressure vessels, by the superplastic expansion of a preform made of superplastically deformable metal.
[0002] It is known in the art to form hollow articles by expanding preforms fabricated Lsuperplastic material in sheet form. In such methods, one, two or more sheets of superplastic material are fusion welded together around their peripheries to form an envelope which is then superplastically expanded against a mould tool to form a structure. The applicants have made vessels in this way and it is found that the local microstructures resulting from the fusion welding do not exhibit superplastic properties to any great degree. In addition, the presence of a seam is thought to degrade the structural integrity of the container and moreover the finished product will have an external flange remaining adjacent the seam which will need to be machined away if the container is to have a smooth outer surface.
[0003] According to one aspect of the invention, there is provided a method of making a hollow article, for example a fluid container or pressure vessel which method includes making a pre-form by machining a hallow space within a solid piece of superglastically deformable metal and then superplastically expanding the pre-form to the required shape of said article by applying pressure within said hallow space while maintaining the pre-form at a temperature permitting superplastic deformation of said metal.
[0004] According to a second aspect, there is provided a method of producing a container for fluid, said method including the steps of:-
(i) selecting a solid blank of a superplastic metal material,
(ii) foinung an elongate blind passage in said blank thereby to create a preform element,
(iii) introducing said preform into a mould tool,
(iv) expanding said preform superplastically under selected conditions of temperature and pressure to conform to said mould tool.
[0006] Conveniently, said method further includes the steps of attaching a hollow plug to the mouth of said blind passage by welding, and the step of introducing pressurised fluid into said passage via said plug, thereby to expand said preform.
[0008] The invention also extends to a pressure vessel whenever manufactured in accordance with the above method.
[0009] All of the prior art examples of structures known to the applicants are formed from a preform which is fabricated from sheet or plate material. The Applicants themselves previously thought it inpossible, or at least highly undesireable, to use any material other than sheet because it was thought necessary that the material to undergo superplastic forming should have generally isotropic properties (which can be developed in sheet) rather than the unidirectional properties of bar which arise as a consequence of its forming process. For convenience, the phrase "superplastic material" is used to described material capable of undergoing superplastic deformation.
[0010] By way of example only, an embodiment of pressure vessel manufactured in accordance with this invention will now be described in detail, reference being made to the accoupanying drawings, in which:
Figure 1 is a schematic section view of a preform element,
Figure 2 is a schematic section view of the preform element located in the mould tool prior to forming,
Figure 3 is a schematic section view of the pressure vessel formed by expansion of the preform element, and
Figure 4 is a sectioned view of another preform element located in its mould tool prior to forming.
[0011] Referring to the drawings, the preform conprises a main body portion 1 which is formed by cutting a length of rod and drilling or machining it to form a blind bore. The rod is hot-rolled, centreless ground pickled and produced by normal metallurgical methods from the alloy Ti 6Al 4V.
[0012] An end plug 2 is turned from the same rod material as the main body portion 1 and a spout 3 is formed of the same material. The main body portion 1, end plug 2 and spout 3 are thoroughly degreased and then electron beam welded together to form in combination a preform 4.
[0013] The preform is cleaned and degreased and then placed in a mould tool conprising mould tool portions 5, 51. The mould is formed of mild steel and prior to insertion of the preform the interior of the mould is degreased, spray coated with Yttria release agent in a solvent base, and then baked in an oven to drive off the solvent.
[0014] The assembled mould tool with the preform inside is placed in a stainless steel muffle box 6 which is subsequently sealed by welding, but having inlets for shielding gas and for forming gas. The muffle box and its contents are then placed in a cold press. Low pressure gas is allowed to flood the muffle box via inlet 7 and the cold press is then heated electrically to a temperature of 930°Ct5°C and allowed to stabilize.
[0015] Once the temperature of the preform has reached the superplastic forming temperature (typically between 900°C and 950°C) argon gas at pressure is admitted to the preform via spout 3. The pressure is controlled to increase gradually over a period from atmospheric to the forming pressure (typically 200-400 psi) whereupon this pressure is maintained for a period to suit the superplastic forming strain rate, and then released (the whole process taking approximately three hours).
[0016] The mould is then allowed to cool to a temperature sufficiently low to allow handling and the muffle box and its contents are removed from the press and disassembled to allow removal of the formed component.
[0017] It is emphasised that in the above example the ranges of values of temperature and pressure are relevant only for the example under consideration. Appropriate parameters for other preforms can be calculated from knowledge of the flow stress of the superplastic material concerned and assessment of the internal pressure required to generate an approximately equivalent hoop stress.
[0018] In the above example Ti 6A1 4V is the material employed; however, it is believed that the above described method has applications with other superplastic material.
[0019] In design of an appropriate preform, it is the ratio of wall thickness to internal diameter that is the critical factor in determining the forming pressure cycle. The preform may be shaped internally and externally by conventional machining methods, to form, for example, external lugs or internal ribs.
[0020] In the above example, the mould tool comprises two portions of mild steel. It can however be formed of stainless steel and indeed this is preferred for it obviates the need for a muffle box.
[0021] The rod from which the body portion l, plug 2 and spout 3 of the preform 4 are made can be produced by hot rolling as mentioned earlier or alternatively could be made by a forging process. As a further possibility, the body portion 1 could be produced by extrusion as an open-ended hollow cylinder which is then fitted with plug 2 and spout 3 at one end while a solid blanking plug (not shown) is welded in place at its other end.
[0022] For the best results, the length of rod and/or the details of the process (ie the particular parameters of the hot rolling, forging or extrusion process) is or are selected so that the metallurgical grain dimensions of the wall of the preform are not too elongated in any one direction, ie axially or circumferentially.
[0023] Particularly where the article to be produced is to have one or more fairly sharp corners, such as the peripheral corners where the cylindrical part of the completed pressure vessel shown in figure 3 meets the domed end faces of the vessel, it may be preferred to so control the application of the pressure within the preform that the major part of the expansion of the preform occurs by a superplastic deformation process while the remainder, ie the final expansion of the preform into the sharp comers of the mould, occurs by a creep deformation process. For example, while maintaining the preform at its superplastic deformation temperature throughout the process, the pressure applied within the preform can be controlled to maintain a strain rate within the preform material in or around the range 10 to 10-4, this giving the superplastic deformation stage of the process, and then the pressure is maintained constant, or very gradually increasing, at a value producing a strain rate of around 10 for quite a large proportion (around one third say) of the overall deformation process time to produce the final creep deformation stage.
[0024] Figure 4 illustrates the manufacture of another hollow article, nameley a two-compartment pressure vessel for use in the manufacture of cold dry air. The preform comprises two cylindrical pieces of rod 41 and 42 each of which has been drilled from one end to form a blind hole therein. A radial hole is also drilled in each piece and the two are joined by a length of titanium alloy tube 43 fixed by welding into the respective radial holes. The open end of the hole in the piece 41 is closed by welding a solid titanium alloy plug 44 therein while the open end of the hole in piece 42 is fitted with a plug 2 and spout 3 of figure 1. Both pieces 41 and 42 are then superplastically expanded by the application of pressure to spout 46 so that they take up the shape of the mould 47. After expansion, plug and spout 45 and 46 are replaced by a solid plug or possibly the spout 46 is sinply sealed off and exit and entry holes (not shown) for the air to be processed are drilled in the originally closed ends of the pieces 41 and 42.
1. According to one aspect of the invention, there is provided a methoc of making
a hollow article, for exanple a fluid container or pressure vessel, which method includes
making a preform by machining a hollow space within a solid piece of superplastically
deformable metal and then superplastically expanding the preform to the required shape
of said article by applying pressure within said hollow space while maintaining the
preform at a temperature permitting superplastic deformation of said metal. // 2.
A method of making a hollow article, for example a fluid container or pressure vessel,
which method includes making a tubular preform by an extrusion process, closing off
at least one end of the interior of the preform, and then superplastically expanding
the preform to the required shape of said article by applying pressure within said
interior while maintaining the preform at a temperature permitting superplastic deformation
of said metal.
3. A method of producing a container for fluid, said method including the steps of:-
(i) selecting a solid blank of a superplastic metal material,
(ii) forming an elongate blind passage in said blank thereby to create a preform element,
(iii) introducing said preform into a mould tool,
(iv) expanding said preform superplastically under selected conditions of tenperature and pressure to conform to said mould tool.