Patent Publication Number: US-11660654-B2

Title: Super plastic forming apparatus and method

Description:
FIELD OF THE INVENTION 
     This invention relates to a super plastic forming apparatus and method. 
     BACKGROUND OF THE INVENTION 
     There is a demand for thin and strong metal components in a variety of complex shapes, particularly in the aerospace and automotive industries. 
     One method of forming such components involves heating the material to a high temperature (around 1,000° C., but varying depending on the particular material) so that it enters a “plastic” state, in which it can be easily formed and moulded to the required shape. The temperature which results in the plastic state is referred to in the art as the “transition temperature”, and the method is known as “super plastic forming” (SPF). 
     In addition, when sheets of the material are heated to their transition temperature, they can be bonded together by the application of a load such as a clamping force. This bonding process is known as “diffusion bonding” (DB), and can provide a homogenous joint between the sheets. 
     In one example method, components—particularly substantially hollow components—are formed by placing two or more layers of material in a die or mould. The layers are heated to the transition temperature and then clamped at a plurality of bonding zones, so that the layers are diffusion bonded at the bonding zones. High pressure gas, such as Argon, is then introduced between the layers, forcing apart the layers in the non-bonded areas. As the layers are forced apart, the material conforms to the shape of the mould, thereby resulting in a component of the desired shape. This method allows for the accurate production of complex shapes, and the use of more than two layers enables the formation of thin internal section walls. 
     Typically, the components are removed from the mould or die immediately after the forming process is complete (i.e. when the material is still in a plastic state), so that production is maintained at a relatively high speed. However, difficulties can arise during the removal of the component from the mould and during the subsequent cooling of the component. 
     In particular, thin sections of material (for example 0.5 mm to 1 mm thick) having a large surface area (for example over 1 m 2 ) are susceptible to distortion. Furthermore, during cooling, and particularly between the transition temperature and a temperature at which the material stabilises (hereinafter referred to as the “stabilisation temperature” and typically around 550° C., but varying depending on the particular material), any differential between the internal and external pressures acting on the newly-formed component can result in relatively large forces acting on the component. Accordingly, distortion may result. 
     A further difficulty occurs during cooling, in that any oxygen entering the internal cavities of the component causes oxidisation of the material. 
     It is an aim of the present invention to address at least some of the above difficulties, or other difficulties which will be appreciated from the description below. It is a further aim of the present invention to provide apparatuses and methods which allow for the rapid, accurate and reliable production of super plastically formed components. 
     SUMMARY 
     According to a first aspect of the present invention, there is provided a cooling apparatus for a component formed by super plastic forming comprising:
         a gas source configured to supply a gas to an interior space of the component via a gas inlet;   a gas outlet configured to allow the gas to exit the interior space, and   a gas column connected to the gas outlet and configured to compensate for changes in an external pressure acting on the component.       

     Preferably, the gas is an inert gas. More preferably, the gas is argon. Preferably the gas source is configured to supply the gas until the component cools to a stabilisation temperature. Advantageously, the inert gas prevents the oxidisation of the interior space of the component during cooling. 
     Preferably the gas inlet is a pipe or tube connected to an inlet hole in the component. More preferably, the inlet hole is a pre-existing hole through which gas was introduced during super plastic forming. 
     Preferably, the component is formed from a metal. More preferably, the component is formed from titanium. The component may instead be formed from aluminium. 
     Preferably, the gas column is a substantially vertically aligned structure, wherein a, preferably vertical, height of the gas column is greater than a width of the gas column. Preferably, the gas column is at least partially filled with a gas, preferably the same gas as is supplied by the gas source. Preferably, a weight of the gas in the gas column exerts a pressure in a downward direction. Preferably, the gas column acts to ensure that an internal pressure, preferably exerted by the gas on the interior space, is substantially equal to the external pressure acting on an exterior surface of the component. Preferably, the external pressure is an ambient atmospheric pressure. 
     Preferably, the gas column comprises an opening at an upper portion thereof, so that the external pressure acts on an upper surface of the gas in the gas column. Preferably, the opening is open to the atmosphere. More preferably, the opening is at an uppermost point of the gas column. Preferably, the gas in the gas column has a higher density than air. Advantageously, excess internal pressure in the component is vented via the gas column. Advantageously, an increase in the external pressure is compensated for by an increase in internal pressure in the component caused by the weight of the gas in the gas column. 
     Preferably, the height of the gas column is calculated based on a volume of the interior space of the component. Preferably, the height of the gas column is calculated based on an expected upper limit of the atmospheric pressure. Preferably, the height of the gas column is calculated based on an expected lower limit of the atmospheric pressure. Preferably, the height of the gas column is calculated based on a change in a density of the gas during cooling. Advantageously, the height of the gas column may be adjusted for the cooling of different components. 
     Preferably, the apparatus further comprises a control valve, configured to control the exit of the gas from the apparatus. More preferably, the control valve is connected to the gas outlet  112 . 
     Preferably, the gas column is connected to the gas outlet at a position between the component and the control valve. 
     According to a second aspect of the present invention, there is provided a pressure equalising device for a cooling apparatus comprising:
         a gas outlet connectable to an interior space of a component, and   a gas column connected to the gas outlet and configured to compensate for changes in an external pressure acting on the component.       

     Further preferred features of the components required in the device of the second aspect are defined hereinabove in relation to the first aspect and may be combined in any combination. 
     According to a third aspect of the present there is provided a super plastic forming apparatus for forming a component comprising:
         a heating means configured to heat a plurality of sheets of material, each sheet comprising at least one bonding zone;   a mould corresponding to the desired shape of the component and configured to receive the plurality of sheets;   a first gas source configured to introduce a first gas between the plurality of sheets, so that the plurality of sheets are forced apart in areas other than those corresponding to the at least one bonding zone, so as to conform to the shape of the mould;   a second gas source configured to supply a second gas to an interior space of the component via a gas inlet;   a gas outlet configured to allow the second gas to exit the interior space, and   a gas column connected to the gas outlet and configured to compensate for changes in an external pressure acting on the component.       

     Further preferred features of the components required in the apparatus of the third aspect are defined hereinabove in relation to the first and second aspects and may be combined in any combination. 
     According to a fourth aspect of the present invention, there is provided a method of cooling a component formed by super plastic forming comprising:
         supplying a gas to an interior space of the component;   allowing the gas to exit the interior space via an outlet, and   compensating for changes in an external pressure acting on the component using a gas column connected to the outlet.       

     Further preferred features of the components required in the method of the fourth aspect are defined hereinabove in relation to the first, second and third aspects and may be combined in any combination. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to the accompanying diagrammatic drawings in which: 
         FIG.  1 ( a )  is a cross sectional view of an example super plastic forming process involving two layers; 
         FIG.  1 ( b )  is a cross sectional view of an example super plastic forming process involving three layers; 
         FIG.  2    is a schematic view of an example cooling apparatus for a component formed by super plastic forming, and 
         FIG.  3    shows a flowchart of an exemplary method of cooling a component formed by super plastic forming. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1 ( a )  illustrates a method of super plastic forming a component  10 , showing the materials before and after forming, and the resulting component  10 . In  FIG.  1 ( a ) , the component  10  is formed from two sheets of material: an upper sheet  11   a  and a lower sheet  11   b . In one example, the sheets  11  are formed from titanium. In further examples, the sheets  11  are formed from aluminium. 
     A plurality of bonding zones  16  are defined on the sheets  11 , at which the sheets  11  are to be diffusion bonded together. In one example, the bonding zones  16  are defined by applying a coating  14  to either or both of the sheets  11  in the areas of the sheets  11  which are not to be diffusion bonded. Particularly, the coating  14  is applied to the surfaces of the sheets  11  which are disposed facing each other during the forming process. In other words, the coating is applied to either or both of the lower surface of the upper sheet  11   a  and the upper surface of the lower sheet  11   b . The coating  14  therefore acts as a mask, defining the bonding zones  16 . In one example, the coating  14  is applied to the or each sheet  11  using screen printing. 
     The sheets  11  are heated to the transition temperature of the material using a suitable heating means. For example, the heating means may comprise a platen which is heated using an electrical resistance heating system. When the sheets enter a plastic state which allows them to be easily moulded and formed, as well as allowing them to be diffusion bonded. In examples where the sheets  11  are formed from titanium, the transition temperature is around +925° C. In examples where the sheets  11  are formed from aluminium, the transition temperature is around +495° C. 
     The heated sheets  11  are then placed in a mould  30  or a die. The mould  30  defines one or more recesses  31 , which correspond to the desired shape of the finished product  10 . A clamp  20  applies a force at one or more clamping points  21  to secure the heated sheets  11  in the mould. 
     Once secured within the mould  30 , a gas is introduced at high pressure between the sheets  11 . The gas is supplied by a suitable gas source. In one example, the gas is an inert gas. In one example, the gas is argon. In one example, the gas is introduced via a small, needle-like tube inserted between the sheets  11 . In one example the gas is introduced between the sheets at a pressure of approximately 6 Megapascals (60 bars). 
     The gas forces apart the sheets  11  in the non-bonded areas (i.e. the areas where the coating  14  has been applied). Accordingly, the sheets  11  effectively inflate within the mould  30 , with the gas causing the sheets  11  to conform to the shape of recesses  31  of the mould  30 . The resulting component  10  comprises one or more internal spaces or cavities  15  in the non-bonded areas between the sheets  11 . 
       FIG.  1 ( b )  shows a similar method of super plastic forming a component  10 . However, in  FIG.  1 ( b ) , the component  10  is formed from three sheets  11 : an upper sheet  11   a , a lower sheet  11   c , and a middle sheet  11   b  disposed between the upper sheet  11   a  and the lower sheet  11   c.    
     In the example shown in  FIG.  1 ( b ) , bonding zones  16  are defined on the surfaces of the sheets  11  which are disposed facing another one of the sheets  11 . In other words, the coating  14  is applied to either or both of the lower surface of the upper sheet  11   a  and the upper surface of the middle sheet  11   b . The coating  14  is also applied to either or both of the lower surface of the middle sheet  11   b  and the upper surface of the lower sheet  11   c.    
     The sheets  11  are heated and clamped in the mould  30  in a similar way to that described above with reference to  FIG.  1 ( a ) . Again, the gas is introduced at high-pressure between the sheets  11 , and the outer sheets  11   a  and  11   c  conform to shape of the mould  30 . The gas causes the middle sheet  11   b  to form internal sectional walls within the component  10 , defining a plurality of cavities  15  therebetween. 
     It will be understood by those skilled in the art that the number of sheets  11 , the number and position of the bonding zones  16 , and the shape of the mould  30  may be varied according to the desired shape and internal structure of the resulting component. 
     In one example, the component  10  is formed using a super plastic forming apparatus comprising the heating means, the mould and the gas source. 
     Once formed, the component  10  is removed from the mould  30  whilst still hot, and then cooled using a cooling apparatus  100 , which is described below with reference to  FIG.  2   . It will be understood that, in further examples, the super plastic forming apparatus comprises the cooling apparatus  100 . In such examples, the component  10  may be cooled in the mould  30 , rather than after removal from the mould  30 . 
     The cooling apparatus  100  comprises a gas source  110 , a gas inlet  111 , a gas outlet  112  and a gas column  120 . 
     The gas source  110  is configured to supply a gas to the interior space  15  of the component  10  via the gas inlet  111 . In one example, the gas is an inert gas. In one example, the gas is argon. In one example, the gas inlet  111  is a pipe or tube connected to an inlet hole in the component  10 . The inlet hole may be the same inlet through which gas was introduced at high-pressure between the sheets  11  during the above-described super plastic forming. 
     The gas outlet  112  is configured to allow the gas to exit the interior space  15  of the component  10 . Accordingly, a stream of gas is passed through the component  10  during cooling. In one example, the stream of gas is supplied until the component has reached the stabilisation temperature. In an example where the component is formed from titanium, the stabilisation temperature is approximately +550° C. The supply of gas prevents air entering the interior space  15  of the component, thereby preventing oxidisation caused by the hot internal surfaces of the component  10  reacting with oxygen in the air. 
     The supply of the gas to the interior space  15  of the component  10  exerts a pressure on the interior space  15 , hereinafter referred to as the internal pressure. At the same time, an external pressure P acts on the outer surfaces of the component. Typically, this external pressure is the ambient atmospheric pressure. It will however be understood that the cooling apparatus may be situated in an environment where the external pressure is not the ambient atmospheric pressure, but is instead a different external pressure is maintained. 
     It will be understood that the external pressure P varies according to climatic conditions. If the internal pressure and the external pressure P are not equal during cooling, the component  10  may deform or distort. 
     The gas column  120  is connected to the gas outlet  112 , and acts to equalise the internal pressure and external pressure P. The gas column  120  is a substantially vertically aligned structure, having a height h greater than the width of the column. The vertical orientation of the gas column  120  results in gravity acting on the gas contained therein, the weight of the gas thereby providing a pressure in a downward direction. 
     In one example, the gas column  120  is filled with the same gas which is supplied by the gas source  110 . The gas column  120  is open to the atmosphere at an uppermost point  121 . The gas contained in the gas column  120  has a higher density than air, and so is not contaminated by the oxygen in the air, and nor does the gas in the column  120  escape. For example, argon gas has a density of approximately 1.6 kg/m 3  compared to air, which has a density of approximately 1.2 kg/m 3 . 
     In one example, the height h of the gas column  120  is calculated to achieve a desired pressure at the base of the column. 
     In one example, the apparatus  100  also comprises a control valve  130 . The control valve  130  is connected to the gas outlet  112 , and is configured to control the exit of the gas from the apparatus  100 . In one example, the gas column  120  is connected to the outlet  112  at a position between the component  10  and the control valve  130 . 
     In use, the component  10  is connected to the inlet  111  and the outlet  112 . The gas source  110  supplies gas to the interior space  15  of the component  10 , so that a gas stream is passed through the interior space  15 . The gas passes out of the component  10  via the outlet  112 , and out of the apparatus via the control valve  130 . 
     In use, any variation in the external pressure P is compensated for by the gas column  120 , thereby ensuring that the internal and external pressures remain substantially equal. If the external pressure P reduces, the excess internal pressure is vented from the upper end  112  of the column  120 . If, on the other hand, the external pressure P increases, the weight of the gas in gas column  120  acts to increase the internal pressure. Accordingly, the effects of variation in ambient pressure are minimised. 
       FIG.  3    shows a flowchart of an example method. 
     The method comprises a first step S 301  of supplying a gas to the interior space  15  of the component  10 . The method comprises a second step S 302  of allowing the gas to exit the interior space  15  via the outlet  112 . Accordingly, a stream of the gas is supplied to the interior of the component. The method further comprises a step S 303  of compensating for changes in atmospheric (i.e. external) pressure P using a gas column  120  connected to the outlet  112 . Accordingly, the internal and external pressures acting on the component remain substantially equal. 
     The above-described apparatuses and methods provide an advantageous method of cooling a component formed by super plastic forming. Such components, and particularly components having a large surface area compared to the thickness of the material from which they are formed, are susceptible to distortion during cooling caused by a difference in the internal and external pressures acting on the component. The above-described apparatuses and methods provide a simple and cost-effective way of compensating for changes in the external pressure acting on the component, for example due to changes in climactic conditions. Accordingly, the need for expensive, complicated and fragile control systems and valves is obviated. 
     Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference. 
     All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. 
     Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. 
     The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.