Patent Publication Number: US-2019168336-A1

Title: Friction welding process

Description:
This application is based upon and claims the benefit of priority from British Patent Application Number 1614566.6 filed 26 Aug. 2016, the entire contents of which are incorporated by reference. 
     FIELD OF THE DISCLOSURE 
     The present disclosure relates to a friction welding process and particularly, but not exclusively, to a linear friction welding process, together with a weld stub geometry for use with the method. 
     BACKGROUND TO THE DISCLOSURE 
     Linear friction welding (LFW) is a solid state welding process for joining regular and irregular sections of metallic or non-metallic materials either welded to themselves or each other. 
     Welds are produced by linear oscillation, at a given frequency, of one part against the other while the parts are pressed together by a forge force applied to the interface. 
     During the LFW process, the components are locally heated at the contact zone by the friction force resulting from the combination of relative oscillatory motion and the forge force. As the temperature at the contact zone increases, the material becomes highly plastic, and flash is extruded from the weld zone under the action of the oscillatory motion and the forge force. 
     The continued application of the forge force during the LFW process causes the components to become closer together in a direction normal to that of the oscillatory motion. This length reduction occurs as long as the component material behaves in a plastic manner. 
     When the components have reached the desired length reduction (known as the burn-off distance) the oscillation amplitude is ramped-down to zero, and the parts are hot-forged together by the forge force for a predetermined time whilst the weld cools. 
     In a conventional LFW process, the components are formed with planar weld joint surfaces. This arrangement has been shown to retain contaminants in the weld joint even after appreciable burn-off, as shown in  FIG. 1 . In the prior art arrangement of  FIG. 1 , the weld stubs  1 , 2  have a weld interface  6 . The centre-line of the weld interface is indicated as  4 . As the weld stubs  1 , 2  are forced together during the LFW process material is ejected from the joint as flash  3 . The centre-line  4  down the length of the material approximately divides the regions where material is ejected to the right and left of the joint. 
     Any contaminants  5  sitting on or close to the centre-line  4  may not be ejected from the weld between stubs  1  and  2  and are therefore not extruded into the flash  3  and instead remain in the weld joint. These retained contaminants may compromise the weld integrity. 
     Edge breakaway is a welding feature characterised by deformation of the edge of the LFW stub such that a large cold chunk of parent material breaks off releasing local constraint to plastic flow and allowing material to be preferentially drawn across the weld. This potentially compromises weld integrity by exposing the full weld joint to atmospheric contamination formed at the weld interface during heating, for example by forming hard alpha particles in titanium alloys. 
     Deformation of the edge of the LFW stub can compromise optimum conditions for the extrusion and ejection of contamination from the weld. In extreme circumstances, the deformed stub corners may detach, further compromising optimum material flow conditions. This deformation and detachment of the stub corners may occur symmetrically or asymmetrically. 
       FIGS. 2A to 2C  illustrates a schematic progression of a typical LFW process showing how deformation or detachment of the stub corners occurs when one of the weld stubs  1  is provided with an angled face geometry  10 . This tapering region at the weld interface expels contaminants from the interface  6  by changing the flow regime from that which would be expected by the use of planar surfaces. This can prevent the inclusion of contaminants in the weld interface  6  by generating an increased quantity of flash  3 . However, as the angled face stub  1  digs into the opposing flat faced stub  2  (shown in  FIG. 2B ), some of the material from the weld interface  6  is deformed at the edges  12 , 14  of the weld interface  6 . 
     As the LFW process progresses, this deformed material  12 , 14  detaches from the weld stub  2  and is ejected into the flash  16 , 18 . 
     The conditions that lead to deformation and detachment of the stub corners may be exacerbated by welding two workpieces of dissimilar alloy materials. 
     SUMMARY 
     According to a first aspect of the present disclosure there is provided a workpiece for use with a friction welding process, the workpiece comprising a weld surface,
         wherein the weld surface comprises a central ridge surface extending along the weld surface, the central ridge surface being flanked on either side respectively by a first pyramidal surface and a second pyramidal surface, the first pyramidal surface subtending a first pyramidal angle with the central ridge surface, and the second pyramidal surface subtending a second pyramidal angle with the central ridge surface, the first pyramidal surface being further flanked by a third pyramidal surface, and the second pyramidal surface being further flanked by a fourth pyramidal surface, the third pyramidal surface subtending a third pyramidal angle with the central ridge surface, and the fourth pyramidal surface subtending a fourth pyramidal angle with the central ridge surface, and each of the third pyramidal angle and the fourth pyramidal angle being less than 90°.       

     By providing the weld surface with the combination of first and second pyramidal surfaces, and third and fourth pyramidal surfaces, the workpiece can be subjected to an LFW process and the resulting welded joint does not have trapped contaminants at a centre region of the joint, and also the welded joint does not suffer from deformation and detachment at the edges of the joint. 
     The double pyramidal geometry of the weld stub prevents the generation of material deformation and detachment by providing additional lateral support to the edges of the weld interface. This additional support prevents the material ejected from the weld interface from deforming and detaching from the weld stub. 
     Optionally, the first pyramidal angle is equal to the second pyramidal angle. 
     Making the first pyramidal angle equal to the second pyramidal angle provides for a symmetrical sectional geometry for the portion of the workpiece that is closest to the contact zone. This ensures that weld material is ejected symmetrically from the contact zone during the weld process. 
     Optionally, the third pyramidal angle is equal to the fourth pyramidal angle. 
     Making the third pyramidal angle equal to the fourth pyramidal angle provides for a symmetrical sectional geometry for the portion of the workpiece that is furthest from the contact zone. This ensures that weld material is ejected symmetrically from the contact zone and the welded joint does not suffer from deformation and detachment at edges of the joint. 
     Optionally, the central ridge surface has a lateral width of between approximately 1 mm and 5 mm. 
     Keeping the lateral width of the central ridge less than 5 mm more effectively provides for the ejection of surface contaminants from the weld zone. However, in alternative arrangements of the disclosure the central ridge surface may have a lateral width of up to approximately 9 mm. 
     Optionally, each of the first pyramidal angle and the second pyramidal angle is between approximately 6° and 12°. 
     Keeping the first pyramidal angle and the second pyramidal angle within the range of approximately 6° and 12° provides a balance between ensuring the elimination of surface contaminants from the weld zone, and minimising the volume of material that must be ejected from the joint as flash during the weld process. 
     Optionally, each of the first pyramidal angle and the second pyramidal angle is between approximately 6° and 30°. 
     Keeping the first pyramidal angle and the second pyramidal angle within the range of approximately 6° and 30° provides a balance between ensuring the elimination of surface contaminants from the weld zone, and minimising the volume of material that must be ejected from the joint as flash during the weld process. 
     Optionally, each of the third pyramidal angle and the fourth pyramidal angle is between approximately 30° and 65°. 
     The selection of the third pyramidal angle and the fourth pyramidal angle as being between approximately 30° and 65° ensures that there is sufficient mechanical support at the stub corners to avoid deformation and detachment conditions from developing 
     Optionally, each of the third pyramidal angle and the fourth pyramidal angle is between approximately 30° and 90°. 
     The selection of the third pyramidal angle and the fourth pyramidal angle as being greater than 65° and less than approximately 90° provides a balance between additional mechanical support at the stub corners to minimise deformation and detachment conditions from developing, and minimising the addition of material to the workpiece, which subsequently may have to be machined away after the friction welding process. 
     Optionally, the weld surface is curvilinear. 
     When using the LFW process to join blades to a disc, for example to form a bladed disk, the weld surface is curvilinear. 
     Optionally, the workpiece is formed from titanium or nickel alloys. 
     The process of deformation and detachment can expose the weld joint to atmospheric contamination formed at the weld interface during heating and compromise optimum conditions for the extrusion and ejection of contamination from the weld joint. A particular example of this being the formation of hard alpha particles in titanium alloys. 
     According to a second aspect of the present disclosure there is provided a method of linear friction welding, the method comprising the steps of:
         providing a first workpiece and a second workpiece, the first workpiece comprising a first weld surface, and the second workpiece comprising a second weld surface, and at least one of the first workpiece and the second workpiece comprising a workpiece according to the first aspect;   positioning the first workpiece adjacent to the second workpiece, with the first weld surface being in engagement with the second weld surface;   reciprocating the first workpiece and the second workpiece against one another such that at least one of the first weld surface and the second weld surface moves relative to the other of the first weld surface and the second weld surface, such that a temperature at the first and second weld surfaces increases to create a weld interface; and   stopping the reciprocating and allowing the first workpiece and the second workpiece to cool to weld the first workpiece and the second workpiece together.       

     However, when a pyramidal geometry is used for one of the weld stubs then the problem of deformation and detachment has been shown to occur. 
     By providing the weld surface with the combination of first and second pyramidal surfaces, and third and fourth pyramidal surfaces, the workpiece can be subjected to an LFW process and the resulting welded joint does not have trapped contaminants at a centre region of the joint, and also the welded joint does not suffer from deformation and detachment at edges of the joint. 
     Optionally, the step of providing a first workpiece and a second workpiece, the first workpiece comprising a first weld surface, and the second workpiece comprising a second weld surface, and at least one of the first workpiece and the second workpiece comprising a workpiece according to the first aspect, comprises the step of:
         providing a first workpiece and a second workpiece, the first workpiece comprising a first weld surface, and the second workpiece comprising a second weld surface, and each of the first workpiece and the second workpiece comprising a workpiece according to the first aspect.       

     In an alternative embodiment of the method each of the first workpiece and the second workpiece comprises respectively a first weld surface and a second weld surface, and each of the first weld surface and the second weld surface has a double pyramidal geometry according to the first aspect of the disclosure. 
     Optionally, the first workpiece is formed from a first material having a first strength parameter, and the second workpiece is formed from a material having a second strength parameter, and a first ratio is defined between the first pyramidal angle of the first workpiece and a corresponding one of the first pyramidal angle and second pyramidal angle of the second workpiece, and a second ratio is defined between the second pyramidal angle of the first workpiece and the other of the first pyramidal angle and second pyramidal angle of the second workpiece, and each of the first ratio and the second ratio is a function of a third ratio between the first strength parameter and the second strength parameter. 
     As described above, the selection of the first and second pyramidal angles assists in the ejection of surface contaminants from the weld zone in the flash. However, when the first workpiece and the second workpiece are formed from different materials it may be necessary to provide the first workpiece with different first and second pyramidal angles to those on the second workpiece in order to ensure that the resulting friction weld is fully formed across the weld zone. 
     Consequently, it may be necessary to adjust the first and second workpiece geometry such that a ratio between the first and second pyramidal angles on the first workpiece, and the corresponding first and second pyramidal angles on the second workpiece, corresponds to a ratio between a strength parameter of the first and second workpiece materials. 
     Optionally, the first workpiece is formed from a first material having a first strength parameter, and the second workpiece is formed from a material having a second strength parameter, and a first ratio is defined between the third pyramidal angle of the first workpiece and a corresponding one of the third pyramidal angle and fourth pyramidal angle of the second workpiece, and a second ratio is defined between the fourth pyramidal angle of the first workpiece and the other of the third pyramidal angle and fourth pyramidal angle of the second workpiece, and each of the first ratio and the second ratio is a function of a third ratio between the first strength parameter and the second strength parameter. 
     As outlined above, the third pyramidal angle and the fourth pyramidal angle provide the outer edge regions of the workpiece with increased mechanical support and thus reduces the possibility of deformation or detachment of the workpiece corners. 
     In a situation where the first workpiece and the second workpiece are formed from dissimilar materials, the material characteristics for the first workpiece will differ from the material characteristics for the second workpiece. This difference in material characteristics between the first workpiece and the second workpiece will result in an asymmetric upsetting behaviour during the friction welding process between the first workpiece and the second workpiece. If both the first workpiece and the second workpiece have the same geometry then the resulting friction weld will be asymmetric across the weld interface, for example with a harder workpiece ‘burrowing’ into a softer workpiece and so producing a poor quality welded joint. 
     It is therefore necessary to adjust the first and second workpiece geometry such that a ratio between the third and fourth pyramidal angles on the first workpiece, and the corresponding third and fourth pyramidal angles on the second workpiece, corresponds to a ratio between a strength parameter of the first and second workpiece materials. 
     In one arrangement, the ratio between the third and fourth pyramidal angles on the first workpiece, and the respective third and fourth pyramidal angles on the second workpiece may be determined on the basis of the relative upset of each of the first and second workpieces when the first workpiece and the second workpiece have the same geometry. 
     Optionally, the strength parameter is selected from the group consisting of flow stress, yield stress and ultimate tensile stress. 
     As outlined above, empirical relative upset data may be used to determine the ratio between the third and fourth pyramidal angles on the first workpiece, and the respective third and fourth pyramidal angles on the second workpiece. Alternatively, this ratio may be determined using an analytical modelling technique. Such techniques use the material parameters such as flow stress, yield stress or ultimate tensile stress to model the flow behaviour of the material during the friction welding process. 
     Optionally, the step of providing a first workpiece and a second workpiece, the first workpiece comprising a first weld surface, and the second workpiece comprising a second weld surface, and at least one of the first workpiece and the second workpiece comprising a workpiece according to the first aspect, comprises the step of:
         providing a first workpiece and a second workpiece, the first workpiece comprising a first weld surface, and the second workpiece comprising a second weld surface, the first workpiece comprising a workpiece according to the first aspect, and the second weld surface comprising a central surface being flanked on either side respectively by a first flank surface and a second flank surface, the first flank surface subtending a first flank angle with the central surface, the second flank surface subtending a second flank angle with the central surface, and each of the first flank angle and the second flank angle being less than 90°.       

     In this arrangement, the first workpiece has a double pyramidal workpiece geometry as detailed above, and the second workpiece has planar central surface flanked on either side respectively by first and second flank surfaces. 
     In the same way as outlined above for the double pyramidal geometry, the first and second flank surfaces provide the outer edge regions of the second workpiece with increased mechanical support and thus reduces the possibility of deformation or detachment of the second workpiece corners. 
     According to a third aspect of the present disclosure there is provided a pair of friction welding workpieces for use with a friction welding process, comprising a first workpiece and a second workpiece, wherein the first workpiece comprises a first weld surface and the second workpiece comprises a second weld surface, the first weld surface comprising a central ridge surface extending along the weld surface, the central ridge surface being flanked on either side respectively by a first pyramidal surface and a second pyramidal surface, the first pyramidal surface subtending a first pyramidal angle with the central ridge surface, and the second pyramidal surface subtending a second pyramidal angle with the central ridge surface, the first pyramidal surface being further flanked by a first side surface, and the second pyramidal surface being further flanked by a second side surface, each of the first and second side surfaces being normal to the central ridge surface, the second weld surface comprising a central surface being flanked on either side respectively by a first flank surface and a second flank surface, the first flank surface subtending a first flank angle with the central surface, the second flank surface subtending a second flank angle with the central surface, and each of the first flank angle and the second flank angle being less than 90°, and the first weld surface is positioned in conformal engagement with the second weld surface. 
     In this arrangement, the first workpiece and the second workpiece may each be formed from the same material. There may be design limitations on a lateral width of the first workpiece that prevents the geometry of the first workpiece from including third and fourth pyramidal surfaces. In other words, the first workpiece is provided with first and second side surfaces that are normal to the central ridge surface. In this arrangement, the second workpiece may be provided with angled first and second flank surfaces. These first and second flank surfaces act to provide additional lateral support to the edges of the weld interface. In this way, the first and second flank surfaces act to prevent deformation and detachment at the edges of the weld interface. 
     Optionally, the central ridge surface has a lateral width of between approximately 1 mm and 5 mm. 
     Keeping the lateral width of the central ridge less than 5 mm more effectively provides for the ejection of surface contaminants from the weld zone. 
     Optionally, each of the first pyramidal angle and the second pyramidal angle is between approximately 6° and 30°. 
     Keeping the first pyramidal angle and the second pyramidal angle within the range of approximately 6° and 30° provides a balance between ensuring the elimination of surface contaminants from the weld zone, and minimising the volume of material that must be ejected from the joint as flash during the weld process. 
     Optionally, the reciprocating motion is a linear reciprocating motion. 
     In one arrangement the relative motion between the first workpiece and the second workpiece is a linear reciprocating motion. 
     In other arrangements, the relative motion between the first workpiece and the second workpiece is nonlinear, such as a sinusoidal reciprocating motion or an elliptical reciprocating motion. 
     Optionally, the weld surface is curvilinear. 
     Optionally, each of the first workpiece and the second workpiece is formed from a titanium alloy. 
     Optionally, the first workpiece is a rotor, and the second workpiece is a rotor blade. 
     Optionally, the rotor is a fan disc, and the blade is a fan blade. 
     Optionally, the rotor is a compressor disc or a compressor drum, and the blade is a compressor blade. 
     According to a fourth aspect of the present disclosure there is provided a computer program that, when read by a computer, causes performance of the method according to the second aspect of the present disclosure. 
     According to a fifth aspect of the present disclosure there is provided a non-transitory computer readable storage medium comprising computer readable instructions that, when read by a computer, cause performance of the method according to the second aspect of the present disclosure. 
     According to a sixth aspect of the present disclosure there is provided a signal comprising computer readable instructions that, when read by a computer, cause performance of the method according to the second aspect of the present disclosure. 
     Other aspects of the disclosure provide devices, methods and systems which include and/or implement some or all of the actions described herein. The illustrative aspects of the disclosure are designed to solve one or more of the problems herein described and/or one or more other problems not discussed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       There now follows a description of an embodiment of the disclosure, by way of non-limiting example, with reference being made to the accompanying drawings in which: 
         FIG. 1  shows a schematic sectional view of a linear friction welded joint according to the prior art; 
         FIGS. 2A, 2B and 2C  shows schematic views of a linear friction welded joint, according to the prior art, in which one stub portion has a single pyramidal geometry, and illustrating the problem of deformation and detachment of the joint corners; 
         FIG. 3  shows a schematic perspective view of a weld stub according to a first embodiment of the present disclosure; 
         FIG. 4  shows a sectional view on the weld stub of  FIG. 3 ; 
         FIG. 5  shows a schematic perspective view of two opposing weld stubs, each according to a first embodiment of the present disclosure, and illustrating the orientation of axial and lateral motion; 
         FIG. 6  shows the two opposing weld stubs of  FIG. 5  in contact with one another; 
         FIG. 7  shows a schematic perspective view of a rotor and a rotor blade embodying a weld stub according to the disclosure of  FIGS. 3 to 6 ; 
         FIG. 8  shows a schematic sectional view of two opposing weld stubs, each according to a second embodiment of the present disclosure; 
         FIG. 9  shows a schematic sectional view of two opposing weld stubs, each according to a third embodiment of the present disclosure; 
         FIG. 10  shows a schematic sectional view of two opposing weld stubs, each according to a fourth embodiment of the present disclosure; and 
         FIG. 11  shows a schematic sectional view of two opposing weld stubs, each according to a fifth embodiment of the present disclosure. 
     
    
    
     It is noted that the drawings may not be to scale. The drawings are intended to depict only typical aspects of the disclosure, and therefore should not be considered as limiting the scope of the disclosure. In the drawings, like numbering represents like elements between the drawings. 
     DETAILED DESCRIPTION 
     Referring to  FIGS. 3 to 5 , a workpiece for use with a friction welding process, according to a first embodiment of the disclosure is designated generally by the reference numeral  100 . 
     The workpiece  100  takes the form of a weld stub  100  and comprises a weld surface  110 . The weld surface  110  comprises a central ridge surface  120  extending along the weld surface  110 . The central ridge surface  120  extends linearly across a lateral width  122  of the weld surface  110 . In the illustrated embodiment the central ridge surface  120  has a lateral width  122  of 4 mm. The central ridge surface  120  is flanked on either side respectively by a first pyramidal surface  130  and a second pyramidal surface  140 . 
     The first pyramidal surface  130  subtends a first pyramidal angle  132  with the central ridge surface  120 . The second pyramidal surface  140  subtends a second pyramidal angle  142  with the central ridge surface  120 . The first and second pyramidal surfaces  130 , 140  together with the central ridge surface  120  together define an upper pyramidal width  146 , which in this embodiment has a value of 8 mm. 
     The first pyramidal surface  130  is further flanked by a third pyramidal surface  150  on a distal side of the first pyramidal surface  130  from the central ridge surface  120 . The second pyramidal surface  140  is further flanked by a fourth pyramidal surface  160  on a distal side of the second pyramidal surface  140  from the central ridge surface  120 . The third and fourth pyramidal surfaces  150 , 160  together with the central ridge surface  120  together define a lower pyramidal width  166 , which in this embodiment has a value of 14 mm. 
     The third pyramidal surface  150  subtends a third pyramidal angle  152  with the central ridge surface  120 . The fourth pyramidal surface  160  subtends a fourth pyramidal angle  162  with the central ridge surface  120 . 
     This arrangement of a central ridge surface  120  flanked on opposing sides by first and second pyramidal surfaces  130 , 140  that are in turn flanked on opposing sides by third and fourth pyramidal surfaces  150 , 160  provides a double pyramidal sectional geometry to the workpiece  100 . 
     In the arrangement shown in  FIGS. 3 to 5 , the first pyramidal angle  132  is equal to the second pyramidal angle  142 . In this arrangement, the first and second pyramidal angles  132 , 142  are each acute angles and have a value of between 8° and 12° relative to the central ridge surface  120 . 
     In the arrangement shown in  FIGS. 3 to 5 , the third pyramidal angle  152  is equal to the fourth pyramidal angle  142 . In this arrangement, the third and fourth pyramidal angles  142 , 152  are each acute angles and have a value of between 30° and 65° relative to the central ridge surface  120 . 
       FIG. 5  shows a schematic perspective view of a weld joint comprising a first workpiece  100  and a second workpiece  102 . In this arrangement, each of the first workpiece  100  and the second workpiece  102  comprises the features described above in relation to the workpiece  100  shown in  FIGS. 3 and 4 . 
     The first workpiece  100  and the second workpiece  102  are brought together such that the central ridge surface  120  of each workpiece  100 , 102  are aligned and in contact with one another, defining a weld interface  170 . 
     A linear friction welding process is then initiated by applying a compressive force normally across the contact between the central ridge surfaces  120  of each of the first and second workpieces  100 , 102 , whilst also providing relative reciprocating motion between the first and second workpieces  100 , 102 . This follows conventional linear friction welding process operation and the details of this operation will not be discussed further here, being well known to a skilled person. 
     In the arrangement illustrated in the figures, the reciprocating motion is in the lateral direction as indicated by feature  190  in  FIG. 5 . 
     A workpiece according to a second embodiment of the disclosure is illustrated in  FIG. 8 . In this arrangement, the first workpiece  100  is formed from a first material having a first hardness value, and the second workpiece  102  is formed from a second material having a second hardness value, where the first hardness is less than the second hardness. For example, the first workpiece  100  may be formed from a first titanium alloy and the second workpiece  102  may be formed from a second titanium alloy, where the first titanium alloy has a higher hardness than the second titanium alloy. 
     Both the first workpiece  100  and the second workpiece  102  have a double pyramidal sectional geometry as outlined above in relation to the first embodiment of the disclosure. 
     Each of the first workpiece  100  and the second workpiece  102  comprises a central ridge surface  120 A, 120 B having a lateral width  122 A, 122 B that is flanked on either side respectively by a first pyramidal surface  130 A, 130 B and a second pyramidal surface  140 A, 140 B. In this embodiment the central ridge surface  120 A has a lateral width  122 A of 4 mm, and the central ridge surface  120 B has a lateral width  122 B of 2 mm. 
     The first pyramidal surface  130 A, 130 B subtends a first pyramidal angle  132 A, 132 B with the central ridge surface  120 A, 120 B. The second pyramidal surface  140 A, 140 B subtends a second pyramidal angle  142 A, 142 B with the central ridge surface  120 A, 120 B. In this embodiment the first pyramidal angle  132 A is equal to the first pyramidal angle  132 B and, in turn, is equal to each of the second pyramidal angles  142 A, 142 B. In this embodiment, the first and second pyramidal angles  132 A, 132 B; 142 A, 142 B subtend an angle of 14° relative to the respective central ridge surface  120 A, 120 B. 
     Each first pyramidal surface  130 A, 130 B is further flanked by a third pyramidal surface  150 A, 150 B on a distal side of the first pyramidal surface  130 A, 130 B from the central ridge surface  120 A, 120 B. Each second pyramidal surface  140 A, 140 B is further flanked by a fourth pyramidal surface  160 A, 160 B on a distal side of the second pyramidal surface  140 A, 140 B from the central ridge surface  120 A, 120 B. 
     The third pyramidal surface  150 A, 150 B subtends a third pyramidal angle  152 A, 152 B with the central ridge surface  120 A, 120 B. The fourth pyramidal surface  160 A, 160 B subtends a fourth pyramidal angle  162 A, 162 B with the central ridge surface  120 A, 120 B. 
     In this embodiment, the third pyramidal angle  152 A is equal to the fourth pyramidal angle  162 A, and each subtends an angle of 40° relative to the central ridge surface  120 A. Additionally, the third pyramidal angle  152 B is equal to the fourth pyramidal angle  162 B, and each subtends an angle of 70° relative to the central ridge surface  120 B. 
     The higher hardness of the second workpiece  102  relative to that of the first workpiece  100  means that the third and fourth pyramidal angles  152 B, 162 B of the second workpiece  102  can be greater than the corresponding third and fourth pyramidal angles  152 A, 162 A of the first workpiece  100 . This is because the higher hardness of the second workpiece  102  requires less mechanical support to prevent deformation or detachment of the sub corners. 
       FIG. 9  illustrates a third embodiment of the workpiece of the present disclosure. The workpiece of  FIG. 9  has the same double pyramidal cross-sectional geometry as has been described above in relation to the second embodiment of the workpiece. 
     The embodiment of  FIG. 9  differs from the embodiment of  FIG. 8  only in respect of the first and second pyramidal angles  132 A, 142 A. In the embodiment of  FIG. 9 , the first pyramidal angle  132 A is equal to the second pyramidal angle  142 A, and each subtends an angle of 25° relative to the central ridge surface  120 A. 
     As explained previously, the third and fourth pyramidal surfaces ( 150 A, 150 B; 160 A, 160 B) prevent the generation of material deformation and detachment at the weld interface by providing additional lateral support to the edges of the weld interface. This additional mechanical support prevents the material ejected from the weld interface from deforming and detaching from the weld stub. 
     Since the first and second materials have different hardness values to one another it is necessary to provide the first workpiece  100  with a different value for the third and fourth pyramidal angle to that of the second workpiece  102 . 
     The determination of the third and fourth pyramidal angles  152 A, 152 B; 162 A, 162 B for each of the first and second workpieces  100 , 102  can be determined from the relative upset between the first and second workpieces  100 , 102 . In other words, by knowing the upset behaviour of each of the first and second materials, for example for a standard geometry it is possible to determine the magnitudes the third and fourth pyramidal angles  152 A, 152 B; 162 A, 162 B for each of the first and second workpieces  100 , 102 . 
     As an example, if the first workpiece  100  is formed from a harder material than the second workpiece  102 , then the third and fourth pyramidal angles  152 A; 162 A for the first workpiece  100  will be greater (i.e. closer to 90°) than the corresponding third and fourth pyramidal angles  152 B;  162 B for the second workpiece  102 . 
     While the second and third embodiments (illustrated in  FIGS. 8 and 9 ) relate to situations in which the first and second workpiece  100 , 102  are formed from materials having different hardness, the workpiece geometry of the present disclosure may equally be applied to situations in which the first workpiece  100  and the second workpiece  102  are formed from materials having the same hardness. For example, the first workpiece and the second workpiece may each be formed from a titanium alloy. 
       FIGS. 10 and 11  respectively show fourth and fifth embodiments of the present disclosure in which the first workpiece  200 , 300  and the second workpiece  202 , 302  are formed from materials having the same hardness. 
       FIG. 10  illustrates a fourth embodiment of the present disclosure, in which a first workpiece  200  is formed from a first material, and a second workpiece  201  is formed from a second material, with the first and second materials having the same hardness. 
     The first workpiece  200  comprises a central ridge surface  220  having a lateral width  222  that is flanked on either side by a first pyramidal surface  230  and a second pyramidal surface  240 . The first pyramidal surface  230  subtends a first pyramidal angle  232  with the central ridge surface  220 , and the second pyramidal surface  240  subtends a second pyramidal angle  242  with the central ridge surface  220 . In this embodiment, each of the first pyramidal angle  232  and the second pyramidal angle  242  is 10°. 
     The first pyramidal surface  230  is further flanked by a third pyramidal surface  250  on a distal side of the first pyramidal surface  230  from the central ridge surface  220 . The second pyramidal surface  240  is further flanked by a fourth pyramidal surface  260  on a distal side of the second pyramidal surface  240  from the central ridge surface  220 . 
     The third pyramidal surface  250  subtends a third pyramidal angle  252  with the central ridge surface  220 . The fourth pyramidal surface  260  subtends a fourth pyramidal angle  262  with the central ridge surface  220 . In this embodiment, the third pyramidal angle  252  is equal to the fourth pyramidal angle  262 , and each subtends an angle of 40° relative to the central ridge surface  220 . 
     The second workpiece  202  comprises a central surface  224  having a lateral width  226  of approximately 24 mm, flanked on either side respectively by a first flank surface  234  and a second flank surface  236 . Each of the first flank surface  234  and the second flank surface  244  subtends a corresponding first flank angle  235  and second flank angle  245  relative to the central surface  224 . In this arrangement, the first flank angle  234  is equal to the second flank angle  245  and has a value of 40°. 
       FIG. 11  illustrates a fifth embodiment of the present disclosure, in which a first workpiece  300  is formed from a first material, and a second workpiece  301  is formed from a second material, with the first and second materials having the same hardness. 
     The first workpiece  300  comprises a central ridge surface  320  having a lateral width  322  that is flanked on either side by a first pyramidal surface  330  and a second pyramidal surface  340 . The first pyramidal surface  330  subtends a first pyramidal angle  332  with the central ridge surface  320 , and the second pyramidal surface  340  subtends a second pyramidal angle  342  with the central ridge surface  320 . In this embodiment, each of the first pyramidal angle  332  and the second pyramidal angle  342  is 10°. 
     The first pyramidal surface  330  is further flanked by a first side surface  350  on a distal side of the first pyramidal surface  330  from the central ridge surface  320 . The second pyramidal surface  340  is further flanked by a second surface  360  on a distal side of the second pyramidal surface  340  from the central ridge surface  320 . 
     Each of the first side surface  350  and the second side surface  360  is oriented at 90° to (i.e. normal to) the central ridge surface  320 . 
     The second workpiece  302  comprises a central surface  324  having a lateral width  326  of approximately 24 mm, flanked on either side respectively by a first flank surface  334  and a second flank surface  336 . Each of the first flank surface  334  and the second flank surface  344  subtends a corresponding first flank angle  335  and second flank angle  345  relative to the central surface  324 . In this arrangement, the first flank angle  334  is equal to the second flank angle  345  and has a value of 40°. 
     The workpieces and the method of the disclosure may be applied to a variety of linear friction welding scenarios.  FIG. 7  illustrates an example of such an application in which a rotor  400  is joined with a rotor blade  410 . A plurality of fan rotor blades  410  may then be joined to the rotor  400  to thereby form a bladed fan disk  402 . In the alternative, a plurality of compressor blades  412  may be joined to a compressor drum  406  to form a compressor disk  404 . 
     Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein. 
     The foregoing description of various aspects of the disclosure has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure to the precise form disclosed, and obviously, many modifications and variations are possible. Such modifications and variations that may be apparent to a person of skill in the art are included within the scope of the disclosure as defined by the accompanying claims.