Friction welding

Abstract: In one aspect the invention provided a friction welded component comprising at least two structural support members (2, 6) spaced apart and connected by a plurality of first (4) and second (12) studs. Each of the first studs are friction welded at one end to a first of the support members (2) and at the opposite end thereof to one end of a respective second stud. Each of the second studs are friction welded to a respective first stud and a second of the support members along a continuous annular friction welded joint (8, 10, 14) having a frusto-conical configuration. Another aspect of the invention provides a friction welding method comprising the steps of: friction welding a plurality of first studs (4) at one end thereof to a first structural support member (2); positioning a second structural member (6) having a plurality of apertures (10) adjacent the opposite ends of the first studs with the apertures positioned in register with the said first studs; and, friction welding a plurality of second studs (12) to the said second support member with each of the second studs extending through a respective aperture and being friction welded along a continuous friction weld surface (14) to the second support member and a respective first stud.

Description: This invention relates to friction welding and in particular concerns a method of friction welding a gaz turbine engine casing and such a product. The invention is dedicated to the manufacture of gas turbine aero-engine casings which hitherto have been manufactured from titanium, nickel or steel forgings. The manufacture of thin walled components such as aero-engine casings from metal forgings is particularly expensive in terms of material wastage and machining time. Typically ninety five per cent of forging material is removed during the machining of an engine casing forging. This low material utilisation is a consequence of the forging process since the size of load bearing reinforced features such as bosses and the like on the casing surface determine the wall thickness of the forging that is necessary for correct material flow during the forging process. For example, a forging for an engine casing having a 25 mm (1 inch) diameter boss on its surface requires a minimum wall thickness of at least 25mm to ensure correct material flow in the region of the boss during forging. Bosses are a common feature on gas turbine aero-engine casings since they are used extensively for mounting pipes and vane spigots, for example, which pass through apertures in the casing. For instance, the intermediate compressor casing for the V2500 engine manufactured by International Aero Engines comprises over two hundred bosses for locating and supporting the radially outer spigots of the variable geometry compressor vanes. Boss diameters of 25 mm or more are common on casings having a wall thickness in the region of 2-5 mm and the resultant material wastage and machining time adds considerably to the manufacturing cost of thin-walled engine casing structures and adds significantly to the lead time of the machined component. One proposal to address this problem has been to manufacture gas turbine aero-engine casings from sheet material using highly accurate fusion welding techniques such as electron beam welding. However, fusion welded bosses have a number of drawbacks particularly in terms of joint strength, mechanical integrity and the cost associated with non-destructive (NDI) weld inspection. In this respect fusion welded bosses are usually unsuitable for gas turbine aero-engine casing applications and the manufacture of casings from forgings has hitherto been preferred. Friction welding has also been proposed for joining bosses to engine casings manufactured from thin sheet material. This application is considered to represent the most relevant state of the art. Friction welding is the welding method of choice in many welding applications since parent material strength can be achieved at the weld joint with little or no heat affected zone. Attempts at friction welding bosses to thin walled structures such as engine casings have not been wholly successful however, since the high forging loads generated cannot always be supported by the thin walled casing when the casing material becomes plastic during the welding process. This results in the boss element punching through the thin walled casing, in a process known as "burn through", before a satisfactory weld is achieved. These constraints limit the size of boss that can be welded to a casing of a specified wall thickness. This is a significant obstacle where large diameter bosses are required, for instance on gas turbine areo-engine compressor casings where large diameter vane spigot bosses are used for supporting the aerodynamic loads that act on variable geometry compressor vanes. US-A-3 679 570 described a base plate assembly for an electrolytic cell in which a steel base plate is joined to an aluminium extractor plate. A plurality of aluminium studs are friction welded at one of their ends to the surface of the steel plate. The extractor plate comprises a plurality of apertures having the same spacing as the first studs on the steel plate so that when the extractor plate is located on the studs the openings are in register with the ends of the studs. The openings have a smaller diameter than the studs. Second aluminium studs of approximately the same diameter as the first studs are friction welded to the opposite side of the extractor plate so that they are forged through the respective openings in the plate to make a friction weld with the extractor plate and the end of the first studs. According to an aspect of the invention there is provided a friction welded gas turbine engine casing according to claim 1. It is to be understood that the term "stud" is used throughout this description to refer to any stud like component suitable for friction welding to form a "boss" on the surface of a structural member. The above aspect of the invention readily enables bosses in the form of studs to be friction welded to thin walled engine casing structures since bosses having cross-sections smaller than would otherwise be necessary may be friction welded to thin walled engine casing structures to support relatively high loads. The linking members increase the bending stiffness of the welded structure in the region of the bosses which enables smaller diameter bosses to be used. This readily permits load supporting bosses to be friction welded to thin walled structures in applications where hitherto it has not been possible because of constraints such as "bum through" due to the higher forging loads associated with larger diameter studs or bosses. The continuous annular friction welded joint readily enables the weld zone between the welded components to be minimised since only limited local frictional heating is required to form the welded joint. In this aspect of the invention the first and second studs may be considered to constitute the web and the casing and the linking members the respective flanges of an I-section beam. Preferably, the said continuous annular weld has a divergent cross-section. This readily enables the first and second studs to be joined along the single continuous surface and thereby minimise the cross-section of the welded joint between the studs and linking members. In preferred embodiments, the continuous annular weld diverges in the direction from the engine casing towards the linking members. Preferably, the continuous annular weld has a frusto-conical configuration. This readily enables rotary friction welding methods such as inertia welding to be used to join the first and second studs to each other and the linking members. In preferred embodiments, the second stud protrudes from the linking members in the direction away from the engine casing. Preferably, each pair of welded first and second studs include a throughbore extending through the engine casing, the respective linking member or members and the said studs. In this respect the welded bosses so formed may be used to support load bearing members passing through the engine casing. In preferred embodiments, the engine casing comprises a primary structural support member and the linking members comprise a secondary reinforcement member. In preferred embodiments, the linking members provide a containment structure for containing projectiles piercing the said primary casing. Thus the linking members may comprises a layered structure for containment purposes, for example for containing high energy projectiles such as fragments of rotating components or the like piercing the engine casing. The friction welded component may further comprise a containment casing spaced apart and substantially parallel to the primary casing. This can further improve the above mentioned advantages. According to another aspect of the invention there is provided a method of friction welding a gas turbine engine casing according to claim 10. The continuous weld surface proposed by this aspect of the invention readily enables a localised joint having a relatively small cross-section to be formed between the components since frictional heating, forging and welding occurs in a uniform manner over the whole of the weld contact surfaces. An advantage of using friction welding to attach the bosses in this embodiment is the relatively low amount of heat input into the fabricated structure which can minimise distortion, buckling etc of sheet metal casings, for example. Preferably, the step of friction welding the second studs comprises the step of simultaneously welding the second stud to the respective first stud and aperture along the continuous friction weld surface of the second stud. In preferred embodiments, the end of the first studs to be welded to the second studs are provided with a recess for receiving the end of the second stud to be welded. Preferably, each recess and respective adjacent aperture define a continuous surface which contacts the continuous weld surface of the second stud. In preferred embodiments, the said continuous surface of the recess and aperture and the continuous surface of the second stud have corresponding frusto-conical configurations. Preferably, the step of friction welding the said second studs to the said first studs comprises rotary friction welding. Embodiments of the invention will now be more particularly described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a cross-section view showing a pair of fabricated bosses friction welded to a section of a circular cylindrical gas turbine engine casing; Figure 2 is a cross-section view similar to that of Figure 1 showing a further strengthening component to be friction welded to the bosses of Figure 1; Figure 3 is a cross-section view similar to that of Figures 1 and 2 showing the strengthening component welded to the bosses; Figure 4 is an perspective view of an aero-engine casing strengthened according to an embodiment of the invention. Referring to the illustrated example in Figures 1 to 3, where the same parts have the same reference numerals, part of a circular cross-section cylindrical gas turbine engine casing 2 is shown. In the drawings of Figures 1 to 3 only part of the circumference of the engine casing is shown, with the illustrated cross-section being in the plane normal to the longitudinal axis of the casing. The casing 2 comprises a relatively thin sheet of metal material to which a plurality of upstanding circular cross-section cylindrical bosses 4, in the form of a first set of metal studs, are attached to the casing at one end by friction welding and protrude radially outwards from the outer surface of the casing. In the embodiment shown it is preferred that the circular section cylindrical bosses 4 are attached by rotary friction welding. In the drawings of Figures 1 to 3 only one pair of bosses is shown, however it is to be understood that a one or a plurality of circumferential rows of bosses may extend around the radially outer surface of the casing, for example for supporting variable geometry compressor vane spigots or other components located in throughbores (not shown) passing through the bosses and the casing. The bosses and casing may be drilled to provide the throughbores on completion of the subsequent fabrication steps described with reference to Figures 2 and 3. The engine casing 2 is strengthened by linking the radially outermost ends of the upstanding bosses together to form a modified 1-beam type structure. In one embodiment the bosses are interconnected by means of a plurality of link elements 6 which extend between adjacent bosses parallel to the outer surface of the engine casing 2. In one embodiment the links form a reinforcement ring around the engine casing linking a row of circumferentially spaced bosses. In other embodiments the link elements form more complex support structures comprising a network of interconnected links which connect a matrix of boss type nodes. The link members 6 are attached to the outermost ends of the bosses 4 by friction welding. As shown in Figure 2 each of the bosses 4 is prepared for friction welding with a weld preparation in the form of a conical blind bore 8 machined into the other end face of the boss, that is to say the end face facing radially outwards from the casing structure in Figures 1 and 2 . The conical blind bore 8 diverges from an apex point within the boss to an opening on the outermost end face of the boss. The link element 6 are also prepared for friction welding with a weld preparation in the form of a conical through bore 10 being provided at each position in the link element where it is to be joined to a respective boss. The pitch of the conical blind bores in a link element is the same as the pitch of the bosses on the casing which are to be joined by the link element. A link element may extend between two or more adjacent bosses as required. Each link element is positioned on the radially outermost end faces of the bosses it is to join so that the conical blind bores in the bosses are in register with the conical through bores in the link member. The ends of the bosses 4 are joined to the respective link elements by a second set of bosses 12, in the form of circular cross-section cylindrical studs. The second bosses 12 have the same or substantially the same cross-section as the first bosses 4 and include a weld preparation in the form of a conical protrusion 14 at one end thereof. The protrusion 14 is shaped to fit in the combined conical bores 8 and 10 such that the protrusion is in complete contact with the conical surfaces of the bores 8 and 10. In this way the mating contact surfaces on the bores 8 and 10 and surface of the protrusion 14 provide a continuous conical joint interface for friction welding the protrusion to the bores to form a single conical weld. The joint interface geometry is particularly suitable for friction welding by a rotary inertia welding process in this example since only local heating is required at the conical joint interface to join the respective components along continuous conical contacting surfaces. The other, non-welded, end of the second boss protrudes radially outwards from the outer surface of the link element in the welded component of Figure 3, however this may be removed by further machining if required. Figure 4 shows a section of a fabricated cylindrical casing in perspective view to illustrate a more complex matrix of load carrying link elements. In this embodiment a plurality of link elements 6 are welded together to provide a lattice structural support structure comprising a network of interconnected link elements, several of which intersect at and are mutually joined at the boss location. The lattice of link elements comprises a first plurality of circumferentially extending hoop elements 6a, a second plurality of clockwise, helically extending link elements 6b, and a third plurality of anti-clockwise, helically extending link elements 6c that are friction welded to the outer surface of a gas turbine engine casing as previously described. The link elements 6a, 6b, 6c extend substantially parallel to the other surface of the engine casing. The method of joining the link elements to the bosses 4 in Figure 4 is essentially the same as the method described above with reference to Figures 1-3. In this embodiment, however, there are up to three link members superimposed at each boss location. The respective through bores in the link elements for receiving the conical protrusion 14 are correspondingly sized and positioned over the blind bores in the bosses 4. The bosses 4 and 12, link elements 6 and casing 2 may be formed of any metal or metal alloy material suitable for friction welding. The material of the bosses nay be the same as that of the casing and the link elements. Different but compatible materials may also be used. For example the bosses and link elements may be steel and the casing nickel alloy. In another example the bosses and the casing may be titanium or a titanium alloy. Dissimilar materials such as titanium and steel, and titanium and nickel result in brittle inter-metallic alloys and are incompatible for friction welding purposes. The invention also contemplates other methods of friction welding, for example friction stir welding, in which case bosses having shapes other than circular cross-sections may be used. Further, in the embodiments described the step of forming the conical weld bores 10 in the link elements is separate from the step of forming the corresponding conical bores in the bosses. In a variation to the sequence of these method steps the link elements 6 may be positioned over the bosses 4 and the conical bores 8,10 formed in one machining operation which may be followed immediately by inserting and welding the second bosses 12 to the fabricated structure. In one embodiment, the individual link elements illustrated in Figure 4 may be replaced by a link element in the form of a secondary cylindrical engine casing structure spaced apart from and surrounding the main engine casing 2. In this embodiment the secondary casing structure not only stiffens and strengthens the main engine casing 2 as in the embodiments of Figures 1 to 4, but it also provides a containment structure for containing projectiles that may pierce the main casing in the event of an engine failure.

Claims: A friction welded gas turbine aero-engine casing comprising a cylindrical sheet metal casing (2), a plurality of structural support members (4, 12) in the form of upstanding circular cross section cylindrical bosses disposed on the radially outer side of the cylindrical sheet metal casing (2), each boss comprising a respective first stud (4) and second stud (12), and a network of interconnected linking members (6a, 6b, 6c), the linking members (6a, 6b, 6c), being spaced apart from and connected to the sheet metal casing (2) by a plurality of said first studs (4), and interconnected with one another by a plurality of said second studs (12), each of the first studs (4) being friction welded at one end to the sheet metal casing (2) and at the opposite end thereof to one end of a respective second stud (12), whereby each of the second studs (12) is friction welded to a respective first stud (4) and at least one of the linking members (6) along a continuous annular friction weld. A friction welded gas turbine aero-engine casing, as claimed in Claim 1 wherein the said continuous annular weld has a divergent cross-section. A friction welded gas turbine aero-engine casing as claimed in Claim 2 wherein the continuous annular weld diverges in the radially outward direction of the engine casing (2). A friction welded gas turbine aero-engine casing as claimed in Claim 3 wherein the continuous annular weld has a frusto-conical configuration. A friction welded gas turbine aero-engine casing as claimed in any preceding claim wherein the second studs (12) protrude from the linking members (6a, 6b, 6c) in the radial direction away from the engine casing (2). A friction welded gas turbine aero-engine casing as claimed in any preceding claim wherein each pair of welded first and second studs (4, 12) include a throughbore extending through the engine casing (2) and the said studs (4, 12). A friction welded gas turbine aero-engine casing as claimed in any preceding claim wherein the linking members (6a, 6b, 6c) comprise an engine casing reinforcement structure. A friction welded gas turbine aero-engine casing as claimed in any preceding claim wherein the linking members (6a, 6b, 6c) provide a containment structure on the radially outer side of the engine casing (2) for containing projectiles piercing the casing. A friction welded gas turbine aero-engine casing as claimed in any preceding claim further comprising at least one containment casing spaced apart and substantially parallel to the said engine casing (2). A method of friction welding a gas turbine engine casing having cylindrical sheet metal casing (2), a plurality of structural support members (4, 12) in the form of upstanding circular cross-section cylindrical bosses disposed on the outer side of a sheet metal casing (2), each boss (4, 12) comprising a respective first stud (4) and a second stud (12), and a network of interconnected linking members (6a, 6b, 6c), the linking members (6a, 6b, 6c) being spaced from and connected to the sheet metal casing (2) by a plurality of the said first studs (4), and interconnected with one another by a plurality of said second studs (12); the said method comprising the steps of friction welding each of the first studs (4) at one of their ends to the sheet metal casing (2) and at the opposite thereof to one end of a respective second stud (12), whereby each of the second studs (12) is friction welded to a first stud (4) and at least one of the linking members (6a, 6b, 6c) by providing a continuous annular friction weld preparation contact surface extending through the said at least one linking member (6a, 6b, 6c) and into the end of the first stud (4). A method as claimed in Claim 10 further comprising the step of providing the end of the first studs (4) to be welded to the second studs (12) with a recess for receiving the end of the second stud (12) to be welded. A method as claimed in Claim 11 wherein the said continuous contact surface and the end of the second stud (12) have correspondingly shaped frusto-conical configurations. A method as clamed in any one of Claims 10 to 12 wherein the step of friction welding the said second studs (12) to the said first studs (4) comprises a rotary friction welding.

Inventors: BENN BRYAN LESLIE, FOSTER DEREK JOHN, BENN, BRYAN LESLIE, FOSTER, DEREK JOHN
Applicants: ROLLS ROYCE PLC
Application year: 2001
Main classification: B23K20
IPC codes: B23K20