Multi-element composite object

A multi-element composite object composed from first, second, and third metal components is provided, wherein the first metal and the third metal are weld incompatible. The multi-element composite object includes a first component fabricated from a first metal. A second component, fabricated from a second metal, is brazed to the first component A third component, fabricated from a third metal, is inertia welded to the second component . The first metal may be provided as a titanium alloy, e.g. a TiNi alloy. The second metal may be provided as low-carbon mild or alloy steel. The third metal may be provided as alloy steel, e.g., 9310 nickel alloy steel. In an embodiment, the multi-element composite object is a gear assembly, with the first element of the gear assembly object being a shaft and the third element of the gear assembly being a gear member with hardened teeth surfaces. The first and second components can be mechanically keyed together via an anti-rotational element. The anti-rotational element can be provided as a pin-in-groove arrangement or a twist-fit arrangement A method of making a multi-metal composite object including a first component fabricated from a first metal, a second component fabricated from a second metal, and a third component fabricated from a third metal, wherein the first metal and the third metal are weld incompatible, is also disclosed. The first step of the method includes mechanically keying the first component to the second component. Net, the first component is brazed to the second component. Finally, the third component is welded to the second component. Where the first metal is a Ti alloy and the second metal is low-carbon steel, the step of brazing the first component to the second component can include brazing using a brazing material such as Ag and Cu. Where the third component is heat-treated steel, the assembly can be stress-relieved after inertia welding at a temperature sufficiently low so as not to degrade the heat-treated properties of the third component.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY-SPONSORED
 RESEARCH AND DEVELOPMENT
 None.
 CROSS-REFERENCE TO RELATED APPLICATIONS
 None.
 FIELD OF THE INVENTION
 The invention relates generally to multi-element composite objects. In
 particular, the invention relates to multi-element composite objects
 fabricated from weld-incompatible metals.
 DESCRIPTION OF RELATED ART
 Objects employing combinations of different metals, so-called "composite
 objects" are hardly new. For example, archaeologists have discovered
 artifacts combining iron and bronze components that date from before the
 birth of Christ. One basic premise present in these primitive
 metallurgical developments has carried through the centuries: the
 desirable characteristics of different metals may be combined by their use
 in a single object.
 One modern application of this premise is in the aircraft industry. It is
 well-recognized that every gram of weight that can be removed from an
 aircraft will pay large dividends by way of reduced fuel consumption,
 increased performance, or increased payload. Thus, a constant theme in the
 manufacture of aircraft components is the need to reduce the weight of
 every component while maintaining or increasing component strength and
 structural integrity.
 One area in which this theme is illustrated abundantly is in the
 fabrication of gears for use in aircraft. It is typical of such structures
 that the gear teeth, splines, and bearing races call for materials that
 have hard surfaces to resist wear, contact fatigue, and bending fatigue.
 By contrast, gear web and shaft portions are free of such requirements,
 and are therefore prime candidates for achieving weight reduction.
 In order to meet these goals, steel alloy gears with titanium alloy webs
 and shafts have been proposed. Unfortunately, traditional welding and
 casting methods are practically ineffective for joining these alloys
 together. One solution to this problem is described in U.S. Pat. No.
 5,492,264 to Wadleigh et al., which is incorporated by reference herein.
 The '264 disclosure is directed to a composite object formed by using
 inertia welding to join first and second dissimilar elements together via
 a third, mutually-compatible interlayer metal element. The method for
 forming the composite object involves inertia welding the interlayer to
 the first element, then inertia welding the second element to the
 interlayer. In an illustrative embodiment, the composite object is a
 multi-metal element composite gear, web, and shaft. In a preferred
 embodiment, a hardened steel alloy gear is inertia welded to an aluminum
 interlayer, which is in turn inertia welded to a titanium alloy shaft.
 While this solution overcomes many of the longstanding problems described
 herein, it is not without possible shortcomings itself. One area for
 improvement is in the nature of the interlayer. Despite the fact that
 aluminum is used extensively in the aircraft industry, there is an
 impression of vulnerability associated with some aluminum components. This
 impression is that, while aluminum is a very desirable material for
 airframes, it is unsuitable for powerplant and transmission applications.
 Aluminum components introduce a temperature limitation of approximately
 350.degree. F., above which the metal begins to thermally soften. Although
 gearboxes do not ordinarily operate at temperatures above 350.degree. F.,
 there is a military requirement for gearboxes to function for one hour
 after all the oil is drained out. The reason for this damage tolerance is
 to allow the crew sufficient time for escape and egress after the gearbox
 has been punctured, typically by gunfire or other ordnance. In the
 civilian world, it is extremely rare for aircraft to be subjected to
 gunfire, even when such aircraft are operating over high-crime urban
 areas. However, particularly in the case of helicopters, the same
 manufacturers make aircraft for civilian and military use. Consequently,
 the design approach (and many standard components such as gearboxes) are
 common to both applications.
 It can be seen from the foregoing that the need exists for multi-metal
 composite objects, and methods for their manufacture, that meet weight and
 strength objectives without sacrificing emergency operating capabilities.
 SUMMARY
 The present invention achieves these and other objects by providing a
 multi-element composite object composed from first, second, and third
 metal components, wherein the first metal and the third metal are weld
 incompatible. The multi-element composite object includes a first
 component fabricated from a first metal. A second component, fabricated
 from a second metal, is brazed to the first component. A third component,
 fabricated from a third metal, is inertia welded to the second component.
 The first metal may be provided as a titanium alloy, e.g. a TiNi alloy.
 The second metal may be provided as steel, e.g., low-carbon alloy or mild
 steel. The third metal may be provided as alloy steel, e.g., 9310 nickel
 alloy steel.
 In an embodiment, the multi-element composite object is a gear assembly,
 with the first element of the gear assembly object being a shaft and the
 second element of the gear assembly being a gear member with hardened
 teeth surfaces. The first and second components can be mechanically keyed
 together via an anti-rotational element. The anti-rotational element can
 be provided as a pin-in-groove arrangement or a twist-fit arrangement.
 A method of making a multi-metal composite object including a first
 component fabricated from a first metal, a second component fabricated
 from a second metal, and a third component fabricated from a third metal,
 wherein the first metal and the third metal are weld incompatible, is also
 disclosed. The first step of the method includes mechanically keying the
 first component to the second component. Next, the first component is
 brazed to the second component. Finally, the third component is welded to
 the second component. Where the first metal is a Ti alloy and the second
 metal is low-carbon mild or alloy steel, the step of brazing the first
 component to the second component can include brazing the steel component
 to the Ti alloy component using a brazing material selected from a group
 consisting of Ag and Cu. Where the third component is heat-treated steel,
 the inertia weld joint between the second and third components may be
 stress-relieved at a temperature sufficiently low so as not to degrade the
 heat-treated properties of the third component after inertia welding the
 third component to the second component.
 The features of the invention believed to be patentable are set forth with
 particularity in the appended claims. The invention itself, however, both
 as to organization and method of operation, together with further objects
 and advantages thereof, may be best understood by reference to the
 following description taken in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
 While the invention is susceptible of embodiment in many different forms,
 there are shown in the drawings, and will herein be described in detail,
 exemplary embodiments, with the understanding that the present disclosure
 is to be considered as illustrative of the principles of the invention and
 not intended to limit the invention to the exemplary embodiments shown and
 described.
 A multi-metal composite object 10 is shown in FIG. 1. The multi-metal
 composite object 10 is illustrated as what is commonly known as a torsion
 test coupon. The multi-metal metal composite object 10 includes a first
 component 12, a second component 14, and a third component 16. The first
 component 12 is fabricated from a first metal selected to provide a
 desired characteristic, for example, the component 12 may be fabricated
 from a titanium alloy, such as Ti-6Al-4V, to provide reduced weight. The
 third component 16 is fabricated from a second metal selected to provide a
 different desired characteristic, for example, the component 16 may be
 fabricated from a steel alloy, such as carburized alloy 9310, to provide
 strength and surface hardness.
 It is frequently the case that metals having widely differing
 characteristics have molecular compositions that render them difficult to
 join together, i.e., weld incompatible. This is true with titanium alloys
 and steel alloys generally, and with Ti alloy Ti-6Al-4V and carburized
 steel alloy 9310 specifically. Accordingly, the second component 14 is
 provided as a connecting member between the first component 12 and the
 third component 16. The second component 14 is fabricated from a material
 that can be joined to the weld-incompatible materials of the first
 component 12 and the third component 16. In the present example, the
 second component 14 can be fabricated from a low-carbon mild steel or
 low-carbon alloy steel, such as 1018 or 9310 steel. It Is believed that
 alloy steel or mild steel having a carbon content below around 0.25 would
 be suitable.
 As shown in FIG. 1, the first component 12 is joined to the second
 component 14 by brazing, for example, by vacuum/inert gas brazing with
 silver alloy or copper alloy brazing material. In order to provide
 additional rotational strength, one or more keys, here shown as
 cross-sectionally square steel keys 18, may be provided. The keys 18 fit
 into corresponding grooves 20, 22 in the first component 12 and the second
 component 14. The first component 12, second component 14, and keys 18 are
 assembled, and then brazed completely along all interfaces. It the first
 and second components were not mechanically keyed together, the torsional
 operating stresses would be borne by the braze material alone. The
 provision of mechanical keying causes the torsional operating stresses to
 be at least partially transferred to the keys themselves, through which
 the load passes.
 Once the first component 12 is joined to the second component 14, a
 low-carbon steel to alloy steel weld is performed between the second
 component 14 and the third component 16. This weld is performed in
 accordance with the inertia welding and friction welding set forth in U.S.
 Pat. No. 5,492,264. The carbon content of the second component 14 must be
 low enough so as to avoid the formation of brittle, untempered martensite
 in the weld region.
 Unfavorable metallurgical changes may occur to the titanium alloy at
 approximately 1800.degree. F. In order to avoid such changes, the brazing
 step is carried out at a temperature slightly lower than 1800.degree. F.
 For example, if silver alloy braze is used, brazing is performed at a
 maximum temperature of 1700.degree. F. which then becomes the temperature
 limitation of the component. This compares rather favorably with the
 350.degree. F. limitation associated with the aluminum interlayer
 multi-metal composite gear technology. The elevated brazing temperature
 does, however, produce an annealed or normalized structure in the
 low-carbon steel connecting member.
 Although the lower strength condition of the intermediate steel component
 would be of inadequate hardness for a gear or bearing surface, the
 composite component as described is more than adequate as a structural
 member. The large load path cross-section results in stress amplitudes in
 the intermediate component that are well within safe levels for
 non-heat-treated materials.
 Except in a very localized region of the weld, the inertia weld does not
 adversely affect the properties of the heat-treated third component 16.
 Consequently, the third component 16 can be quenched and tempered (and any
 features formed thereon, such as gear teeth, can be formed and gas
 carburized), prior to the welding operation. The treated third component
 16 would remain unaffected, and retain the advantages of having been
 heat-treated, during subsequent service of the multi-metal composite
 object 10. As an additional measure, the assembly can be heated to around
 300.degree. for several hours in order to relieve any localized internal
 stresses that may be caused by inertia welding. This stress-relief
 operation does not adversely affect the heat-treated properties of the
 carburized features (such as bearing races or gear teeth) on the third
 component 16.
 Turning now to FIG. 2, a gear assembly 20 is shown which embodies the
 principles of the present invention. The gear assembly 20 includes a shaft
 22 fabricated from a titanium alloy, such as Ti-6Al-4V. Disposed at each
 end of the shaft 22 is a gear member 24 having a plurality of gear teeth
 26.
 As can be seen in FIG. 3, a connecting member 28 is provided between the
 shaft 22 and each gear member 24. The connecting member 28 is provided
 with grooves 30 which correspond in number and configuration with grooves
 32 provided on a cylindrical extension 34 of an end of the shaft 22. The
 grooves 30, 32 are adapted to receive keys 36 when the subassembly
 including shaft 22, intermediate connecting member 28, and keys 36 are
 assembled prior to brazing. The connecting member 28 is also provided with
 a frustoconical surface 38, which corresponds in configuration to a
 frustoconical surface 40 on the gear member 24, as shown in FIG. 4.
 Fabrication of the gear assembly 20 may be understood by those of skill in
 the art with reference to the exploded view illustrated in FIG. 3 and the
 cross-sectional view of FIG. 4. The connecting member 28 and the extension
 34 of the shaft 22 are brought together with the grooves 30, 32 aligned,
 and the keys 36 are inserted therein. Next, this subassembly is brazed
 completely along all interfaces between the connecting member 28, the
 shaft 22, and the keys 36. Finally, the frustoconical surface 38 of the
 connecting member 28 is welded to the frustoconical surface 40 of the gear
 member 24 by inertia welding or friction welding.
 Turning next to FIG. 5, a gear assembly 42 is shown which also embodies the
 principles of the present invention. The gear assembly 42 includes a shaft
 44 fabricated from a titanium alloy, such as Ti-6Al-4V. Disposed at each
 end of the shaft 44 is a gear member 46 having a plurality of gear teeth
 48.
 As can be seen in FIG. 6, a connecting member 50 is provided between the
 shaft 44 and each gear member 46. The connecting member 50 is provided
 with grooves 52 which correspond in number and configuration with grooves
 54 on a cylindrical extension 56 on an end of the shaft 44. The grooves
 52, 54 are adapted to receive keys 58 when the gear assembly 42 is
 assembled. Additionally, the connecting member 50 is provided with arcuate
 cam slots 60, which extend from the grooves 52 and are adapted to receive
 cam followers 62 on the cylindrical extension 56 in a "bayonet"
 connection. The connecting member 50 is also provided with a frustoconical
 surface 64, which corresponds in configuration to a frustoconical surface
 66 on the gear member 46.
 Fabrication of the gear assembly 42 is as follows. The connecting member 50
 and the extension 56 of the shaft 44 are brought together with the cam
 followers 62 inserted into the grooves 52. The connecting member 50 and
 the shaft 44 are then rotated relative to one another, such that the cam
 followers 62 travel along the arcuate cam slots 60, and the grooves 52 are
 brought into alignment with the grooves 54. Next, the keys 58 are inserted
 into the aligned grooves 52, 54, and the subassembly is brazed completely
 along all interfaces between the connecting member 50 (including the cam
 followers 62), the shaft 44 (including the cam slots 60), and the keys 58.
 Finally, the frustoconical surface 64 of the connecting member 50 is
 welded to the frustoconical surface 66 of the gear member 46 by inertia
 welding or friction welding.
 Thus it is apparent that in accordance with the present invention, an
 apparatus that fully satisfies the objectives, aims, and advantages
 achievable in accordance with the principles of the present invention is
 set forth in the above exemplary embodiments. The present invention
 achieves the weight and strength of previously known technologies while
 allowing the gear assembly to operate for meaningful periods of time at
 temperatures in excess of 1000.degree. F. The process of joining the
 elements of the multi-metal composite object is accomplished such that
 service abuse limitations are those associated with the gear teeth, rather
 than the joining process itself, as is the case with single-material
 assemblies. This is accomplished while achieving significant weight
 reductions, perhaps in the order of 25-30%, depending on the particular
 component geometry and specifications.
 While the invention has been described in conjunction with the exemplary
 embodiments, it is evident that many alternatives, modifications,
 permutations, and variations will become apparent to those skilled in the
 art in light of the foregoing description. For example, the locations of
 the cam followers and slots could be reversed, or other materials used for
 brazing, or for the components themselves. It is also conceivable that the
 keying mechanisms could be eliminated entirely. The present invention
 could also find utility in applications other than gear assemblies, for
 example, in other rotational drive members. Accordingly, it is intended
 that all such alternatives, modifications, permutations, and variations to
 the exemplary embodiments can be made without departing from the scope and
 spirit of the present invention as set forth in the appended claims.