FIELD OF THE INVENTION 
     The present invention relates to a method and apparatus for superplastically forming a workpiece. The present invention relates in particular to a method and apparatus for superplastically forming a metal workpiece. 
     BACKGROUND OF THE INVENTION 
     It is known to superplastically form a workpiece by placing the workpiece in a die, within an autoclave, heating the die and workpiece up to a temperature at which the workpiece is superplastically formable and applying a pressure differential across the workpiece to superplastically form the workpiece to the shape of the die. 
     It is known from European patent application EP0703019A to superplastically form a workpiece using a laser to locally heat the workpiece up to a temperature at which the workpiece is superplastically formable and applying a pressure differential across the workpiece to superplastically form the workpiece to a required shape without the use of a die. 
     A problem with the above methods of superplastically forming a workpiece is that there is very little, or no, control of the thickness distribution of the resulting component or article. 
     SUMMARY OF THE INVENTION 
     Accordingly the present invention seeks to provide a novel method of superplastically forming a workpiece which reduces, preferably overcomes, the above mentioned problems. 
     Accordingly the present invention provides a method of superplastically forming a workpiece comprising placing the workpiece in a die, heating the whole of the workpiece up to a temperature at which the workpiece is superplastically formable, applying a pressure differential across the workpiece to superplastically form the workpiece to the shape of the die and heating and/or cooling the workpiece to provide a temperature distribution across the workpiece during the superplastic forming process to control the thickness of the workpiece. 
     Preferably the method comprises measuring the temperature of the workpiece and producing a temperature signal, analysing the temperature signal to determine the measured temperature distribution of the workpiece at least one stage in the superplastic forming process, maintaining a database of desired temperature distributions of the workpiece at different stages in the superplastic forming process, comparing the measured temperature distribution of the workpiece at the at least one stage in the superplastic forming process with the desired temperature distribution of the workpiece at the said stage in the superplastic forming process and controlling the heating means and/or the cooling means such that the temperature distribution of the workpiece is maintained at the desired temperature distribution of the workpiece or such that the temperature distribution of the workpiece is adjusted to more closely match the desired temperature distribution of the workpiece to control the thickness of the workpiece. 
     Preferably the method comprises heating the workpiece by directing at least one laser beam on the surface of the workpiece. Preferably the method comprises directing the at least one laser beam through at least one window in the die. Preferably the method comprises scanning the at least one laser beam across the surface of the workpiece. 
     Preferably the method comprises heating the workpiece by directing at least one infra red beam on the surface of the workpiece. Preferably the method comprises directing the at least one infra red beam through at least one window in the die. Preferably the method comprises scanning the at least one infra red beam across the surface of the workpiece. 
     Preferably the method comprises cooling the workpiece by directing a cooling inert gas on the surface of the workpiece. 
     The method may comprise heating and cooling the workpiece over a predetermined temperature range such that the workpiece is superplastically formed by phase transformation superplasticity. 
     Preferably the method comprises heating and/or cooling both sides of the workpiece. 
     Preferably the method comprises measuring the temperature of the workpiece by viewing the workpiece with a thermographic camera. 
     The present invention also seeks to provide a novel apparatus for superplastically forming a workpiece which reduces, preferably overcomes, the above mentioned problems. 
     The present invention also provides an apparatus for superplastically forming a workpiece comprising a die, heating means to heat the workpiece, cooling means to cool the workpiece, means to apply a pressure differential across the workpiece to superplastically form the workpiece to the shape of the die and processor means arranged to control the heating and/or cooling of the workpiece to provide a temperature distribution across the workpiece during the superplastic forming process to control the thickness of the workpiece. 
     Preferably the apparatus comprises means to measure the temperature of the workpiece and to produce a temperature signal, a processor arranged to analyse the temperature signal to determine the measured temperature distribution of the workpiece at at least one stage in the superplastic forming process, a database of desired temperature distributions of the workpiece at different stages in the superplastic forming process, the processor being arranged to compare the measured temperature distribution of the workpiece at the at least one stage in the superplastic forming process with the desired temperature distribution of the workpiece at the said stage in the superplastic forming process, the processor being arranged to control the heating means and/or the cooling means such that the measured temperature distribution of the workpiece is maintained at the desired temperature distribution of the workpiece or such that the temperature distribution of the workpiece is adjusted to more closely match the desired temperature distribution of the workpiece to control the thickness of the workpiece. 
     Preferably the heating means comprises at least one laser gun arranged to direct a laser beam on the surface of the workpiece. Preferably the die comprises at least one window and the laser gun is arranged to direct the at least one laser beam through at least one window in the die. Preferably there are means to scan the at least one laser beam across the surface of the workpiece. 
     Preferably the heating means comprises at least one infra red lamp arranged to direct at least one infra red beam on the surface of the workpiece. Preferably the die comprises at least one window and the infra red lamp is arranged to direct the at least one infra red beam through at least one window in the die. Preferably there are means to scan the at least one infra red beam across the surface of the workpiece. 
     Preferably the cooling means comprises means to direct a cooling inert gas on the surface of the workpiece. 
     Preferably the heating and/or cooling means are arranged on both sides of the workpiece. 
     Preferably the means to measure the temperature of the workpiece comprises a thermographic camera. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be more fully described by way of example with reference to the accompanying drawings in which: 
         FIG. 1  shows an apparatus for superplastically forming a workpiece according to the present invention. 
         FIG. 2  is graph showing thinning of an axis-symmetric workpiece during superplastic forming using a constant temperature. 
         FIG. 3  is graph showing thinning of an axis-symmetric workpiece during superplastic forming using a temperature gradient. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An apparatus  10  for superplastically forming a workpiece  16  is shown in FIG.  1 . The apparatus  10  comprises a back plate  12  and a die  14  which are arranged to clamp the periphery  18  of the workpiece  16  to form sealed chambers  20  and  22  with the back plate  12  and the die  14  respectively. 
     The back plate  12  comprises at least one window, in this example two windows  11  and  13  to allow radiant heating beams to impinge upon the surface  26  of the workpiece  16 . 
     The die  14  comprises at least one window, in this example three windows  28 ,  30  and  32  to allow radiant heating beams to impinge upon the surface  24  of the workpiece  16 . The die  14  has one or more further windows  34  and  35  to allow radiant heat from the surface  24  of the workpiece  16  to leave the die  14 . 
     At least one infra red lamp, in this example three infra red lamps  36 ,  38  and  40  are provided to direct infra red beams through the windows  28 ,  30  and  32  respectively onto the surface  24  of the workpiece  16 . The infra red lamps  36 ,  38  and  40  are supplied with electrical power from a source of electrical energy  42  via electrical leads  44 . The infra red lamps  36 ,  38  and  40  are arranged to provide a broad, defocused, beams to heat large areas of the surface  24  of the workpiece  16 . 
     At least one laser gun, in this example two laser guns  46  and  48  are provided to direct laser beams through the windows  28  and  32  respectively onto the surface  24  of the workpiece  16 . The laser guns  46  and  48  are arranged to provide narrow, focused, beams to heat small areas of the surface  24  of the workpiece  16 . In this example mirrors  54  and  56  are provided to direct the laser beams from the laser guns  46  and  48  respectively through the windows  28  and  32  respectively. The mirrors  54  and  56  are gimballed to allow the mirrors  54  and  56  to rotate about two perpendicular axes. The mirrors  54  and  56  allow the laser beams to be scanned across the surface  24  of the workpiece  16 . The mirrors  54  and  56  may be moved by galvanometers. 
     At least one infra red lamp, in this example two infra red lamps  94  and  96  are provided to direct radiant heat through the windows  11  and  13  heat onto the surface  26  of the workpiece  16 . The infra red lamps  94  and  96  are supplied with electrical power from a source of electrical energy  100  via electrical leads  98 . The infra red lamps  94  and  96  are arranged to provide a broad, defocused, beams to heat large areas of the surface  26  of the workpiece  16 . 
     An inlet pipe  58  and valve  60  allow the supply of inert gas, for example argon, into the chamber  20  defined between the back plate  12  and the workpiece  16  and an outlet pipe  62  and valve  64  allow the removal of the inert gas from the chamber  20 . An inlet pipe  66  and valve  68  allow the supply of inert gas, for example argon, into the chamber  22  defined between the die  14  and the workpiece  16  and an outlet pipe  70  and valve  72  allow the removal of the inert gas from the chamber  22 . 
     A greater pressure of inert gas is supplied to the chamber  20  than the chamber  22  to cause the workpiece  16  to be superplastically formed to the shape of the die  14 . 
     On or more thermographic, thermal, cameras  82  and  83  are arranged to view the surface  24  of the workpiece  16  through the windows  34  and  35 . The thermographic cameras  82  and  83  send temperature signals to the processor  86  via electrical leads  84  and  85 . 
     The processor  86  is arranged to control the superplastic forming process. The processor  86  comprises a database  88 , or model, of the temperature distribution of the workpiece  16  at different stages of the superplastic forming process. Each desired temperature distribution of the workpiece  16  stored in the database  88  of the processor  86  has a desired temperature gradient across the workpiece  16 . The temperature gradient controls the thickness of the resulting component or article. The processor  86  comprises an analyser  90 , which analyses the temperature signals produced by the thermal cameras  82  and  83  to determine the measured temperature distribution of the workpiece  16  sequentially at each stage in the superplastic forming process. The processor  86  comprises a comparator  92 , which sequentially compares the measured temperature distribution of the workpiece at each stage in the superplastic forming process with the desired temperature distribution of the workpiece  16  stored in the database  88  at the corresponding stage in the superplastic forming process. 
     The processor  86  is arranged to control the heating of the surface  24  of the workpiece  16  provided by the infra red lamps  36 ,  38  and  40  by controlling the amount of electrical energy supplied by the source of electrical energy  44 . 
     The processor  86  is arranged to control the heating of the surface  24  of the workpiece  16  provided by the laser guns  46  and  48  via electrical leads  50  and  52  respectively. 
     The processor  86  is arranged to control the heating of the surface  26  of the workpiece  16  provided by the radiant heaters  94  and  96  by controlling the amount of electrical energy supplied by the source of electrical energy  100 . 
     The processor  86  is arranged to control the supply of inert gas into and out of the chamber  20  by adjusting the valves  60  and  64  via the electrical leads  74  and  76  respectively. The processor  86  is arranged to control the supply of inert gas into and out of the chamber  22  by adjusting the valves  68  and  72  via the electrical leads  78  and  80  respectively. 
     In operation the periphery  18  of a workpiece  16  of the desired material, for example a titanium alloy 6 wt % Al, 4 wt % V and the balance Ti, is clamped between the back plate  12  and the die  14 . 
     The processor  86  allows the infra red lamps  94  and  96  and the infra red lamps  36 ,  38  and  40  to heat the whole of the workpiece  16  up to a temperature at which the workpiece  16  is capable of superplastic deformation. In the case of the above mentioned titanium alloy, Ti 6Al 4V, the temperature is at least 760° C., preferably at least 850° C. 
     During the superplastic forming process the at least one thermographic camera  82  and  83  sequentially supply temperature signals indicating the temperature at all points on the surface  24  of the workpiece  16  at different stages in the superplastic forming process to the processor  86 . The analyser  90  of the processor  86  sequentially analyses the temperature signals to determine the measured temperature distribution of the workpiece  16  at the different stages in the superplastic forming process. The analyser  90  preferably analyses the temperature signals to determine the temperature distribution in three dimensions. Either there are two cameras  82  and  83  to provide a stereoscopic view of the surface  24  of the workpiece  16  or a single camera has a split field of view. The comparator  92  of the processor  86  sequentially compares the measured temperature distribution of the workpiece  16  at each stage in the superplastic forming process with the desired temperature distribution of the workpiece  16 , stored in the database  88 , at the corresponding stage in the superplastic forming process. 
     If the comparator  92  determines that the measured temperature distribution of the workpiece  16  differs from the desired temperature distribution of the workpiece  16  at a particular stage in the superplastic forming process, then the processor  86  sends signals to control the amount of heating provided by the infra red lamps  36 ,  38  and  40 , the infra red lamps  94  and  96  and the laser guns  46  and  48  and sends signal to the valves  60 ,  64 ,  68  and  70  to control the amount of cooling provided by the inert gas flowing through the chambers  20  and  22 . 
     The processor  86  opens, or closes, the valves  60 ,  64 ,  68  and  72  to increase, or decrease, the flow of cooling inert gas to maintain the minimum temperature of the workpiece  16  at the temperature required for superplastic forming. In addition the processor  86  allows selected ones of the infra red lamps  36 ,  38  and  40  and infra red lamps  94  and  96  to direct infra red radiation over large regions of the workpiece  16 , which were at a temperature below the desired temperature. These large regions are those regions in which the temperature energy deficit multiplied by the area is the largest. The processor  86  allows the laser guns  46  and  48  to direct the laser beams at small regions of the workpiece  16 , which have a relatively high temperature gradient. 
     The processor  86  also controls the valves  60 ,  64 ,  68  and  72  such that there is pressure difference between the chambers  20  and  22  to superplastically form the workpiece  16  to the shape of the die  14 . 
     The heating intensity may be reduced towards the end of the superplastic forming process and the pressure of the inert gas in the chamber  20  is increased to force the workpiece  16  into the corners of the die  14  to reduce the processing time and save energy and costs. 
       FIG. 2  shows the thickness distribution of an axis-symmetric workpiece  16  during conventional superplastic forming. During conventional superplastic forming the workpiece  16  is superplastically formed with a constant temperature across the workpiece  16 . It is clear that the workpiece  16  and thus the resulting component, or article, is thicker at its periphery with a gradual reduction in thickness to a minimum thickness at the centre of the workpiece  16 . 
       FIG. 3  shows the thickness distribution of an axis-symmetric workpiece  16  during superplastic forming according to the present invention. During superplastic forming according to the present invention the workpiece  16  is superplastically formed with a temperature gradient applied across the workpiece  16 . It is clear that in this example the workpiece  16  and thus the resulting component, or article, has a more uniform thickness and reduced thinning. 
     As an example of the present invention the superplastic forming of an axis-symmetric workpiece is described. A temperature gradient is provided between the periphery  18  of the workpiece and the centre of the workpiece  16 . In particular the periphery  18  of the workpiece  16  is heated to a higher temperature than the centre of the workpiece  16 , such that the thinning of the centre of the workpiece  16  is reduced. 
     The advantages of the invention are that the thinning, thickness, or strain distribution of the workpiece  16  may be precisely controlled, by controlling the heating and/or cooling of different regions of the workpiece  16 . This advantage enables thinner, and hence lower cost, workpieces to be used because the workpiece is thinned only in the required regions and in a more uniform manner within those regions. Alternatively, by making the deformation more uniform, and hence reducing the peak strain, greater mean strain rates may be applied to the workpiece  16 , leading to a reduction in manufacturing time and costs. In addition the localised heating of the workpiece  16  by the infra red beams and laser beams means that the back plate and die are not heated and therefore the back plate and die are lower cost and easier to manufacture. 
     In a further embodiment of the present invention the laser guns  46  and  48  are scanned across the surface  24  of the workpiece  16  such that the temperature of the workpiece  16  at each point on the surface  24  is allowed to oscillate over a predetermined temperature range. Additionally, the inert gas may be directed against the surfaces  24  and  26  of the workpiece  16  to increase the cooling rate to increase the oscillation rate. The oscillation of the temperature over the predetermined temperature range causes phase transformation superplasticity to occur in the workpiece  16 . The predetermined temperature range is arranged to extend to a temperature greater than a phase transformation temperature of the workpiece  16  and to a temperature less than the phase transformation temperature of the workpiece  16 . In phase transformation superplasticity the relative amounts of the different phases in an alloy changes to allow superplastic deformation to occur under conditions of constant load. This enables a small amount of superplastic deformation to occur at each temperature cycle. The phase transformation superplasticity enables materials, which do not exhibit superplasticity to be superplastically formed, for example welded metals, welded alloys, coarse grain metals or alloys or metal matrix composites. The rate of deformation of the workpiece  16  may be constrained by the rate at which the workpiece  16  may be cycled. The response time for the workpiece  16  is proportional to the square of the thickness of the workpiece  16 . The response time for the workpiece  16  depends upon the heat transfer coefficient and the temperature of the inert gas. Thus heating and cooling both surfaces  24  and  26  of the workpiece  16  increases the rate at which the workpiece  16  may be cycled. 
     The titanium alloy consisting of 6 wt % Al, 4 wt % V and the balance Ti, comprises alpha and beta phases below a temperature of about 950° C. and comprises only beta phase above a temperature of about 950° C. For the titanium alloy consisting of 6 wt % Al, 4 wt % V and the balance Ti, the temperature may be cycled between 760° C. and 980° C. or between 880° C. and 1020° C. to obtain the phase transformation superplasticity, or between other suitable temperatures one of which is above and one of which is below the phase transformation temperature. 
     For a 1 mm thick workpiece of titanium alloy 6 wt % Al, 4 wt % V and the balance Ti, the heating time is 0.33 seconds for single surface heating and 0.1 second for double surface heating. 
     The present invention has been described with reference to mirrors to direct the laser beam onto the workpiece, but it may be equally possible to arrange each laser gun in a carriage and to move the carriage in two mutually perpendicular directions. Alternatively, each laser beam may be directed using an optical fibre. 
     Although the present invention has been described with reference to laser guns directing laser beams on only one surface of the workpiece it is equally possible to arrange for laser guns to direct laser beams on both surfaces of the workpiece. 
     Any suitable number of laser guns may be provided. 
     Any suitable number of infra red lamps may be provided. 
     Although the present invention has been described with reference to laser guns and infra red lamps heating the workpiece, other suitable heaters may be provided to heat the workpiece without heating the die and/or back plate. 
     The present invention is applicable to the superplastic forming of aluminium, aluminium alloys, magnesium, magnesium alloys, lead/tin alloys, iron alloys, nickel alloys for example IN718, metal matrix composite material, either fibre reinforced or particle reinforced, and ceramics.
A method of superplastically forming a workpiece comprises placing the workpiece ( 16) in a die ( 14) and heating the workpiece ( 16) to a temperature at which the workpiece ( 16) is superplastically formable. Pressure is applied to the workpiece ( 16) to superplastically form the workpiece ( 16) to the shape of the die ( 14). The temperature of the workpiece ( 16) is measured and is analysed to determine the measured temperature distribution of the workpiece ( 16) at stages in the superplastic forming process. A database of desired temperature distributions of the workpiece ( 16) at different stages in the superplastic forming process is kept. The measured temperature distribution of the workpiece ( 16) at each stage in the superplastic forming process is compared with the desired temperature distribution of the workpiece ( 16) at the stage in the superplastic forming process and heating means and/or cooling means ( 36, 38, 40, 46, 48, 58, 60, 66, 68, 94, 96) are controlled such that the temperature distribution of the workpiece ( 16) is adjusted to more closely match the desired temperature distribution of the workpiece ( 16) to control the thickness of the workpiece ( 16).