WO1996022854A1

WO1996022854A1 - Energy beam joining process producing a dual weld/braze joint

Info

Publication number: WO1996022854A1
Application number: PCT/US1996/000361
Authority: WO
Inventors: Harry Francis Miller; Jian Liu; Leroy William Myers, Jr.
Original assignee: Dresser-Rand Company
Priority date: 1995-01-23
Filing date: 1996-01-11
Publication date: 1996-08-01
Also published as: AU4752596A

Abstract

Two components (42, 44) are bonded together by placing a layer of braze filler metal between and in contact with the faying surfaces, and then directing a high energy beam (46) through one of the components, through the faying surfaces and the layer of brazer filler metal, and into the second component so as to form a melt-through weld bond (53) centrally located between two braze bonds (54, 55). Relative movement between the high energy beam and the components can be provided so that the high energy beam is always perpendicular to the portion of the faying surfaces in which the weld bond is currently being formed. The two components can be an impeller disc and an impeller cover.

Description

ENERGY BEAM JOINING PROCESS PRODUCING A DUAL WELD/BRAZE JOINT

FIELD OF THE INVENTION

The invention relates to a process for joining two members by brazing/welding, and to a brazed/welded product.

BACKGROUND OF THE INVENTION 5 The American Welding Society defines brazing as a

"group of welding processes that produces coalescence of materials by heating them to the brazing temperature in the presence of a filler metal having a liguidus above 450°C (840°F) and below the solidus of the base metal. The filler 0 material is distributed between the closely fitted surfaces of the joint by capillary action." The base metal or base material is the material that is welded, brazed, soldered or cut. Thus brazing differs from welding in that in brazing, the intention is to melt only the filler metal and not the base materials, while both filler metals and base metals are melted during welding. Also, in brazing the temperature of the entire assembly to be brazed is generally raised to a point where the filler metal becomes molten and fills the joint gap between the base materials, while in welding only the filler metal and a small portion of the base materials immediately adjacent the joint are heated sufficiently to become molten.

In brazing, wetting of the base materials by the filler metal is required in order to provide intimate contact between them sufficient to develop the desired bonding. Upon establishing the wetting conditions, capillary forces produce flow of the molten filler metal so as to fill the joint gap. In most brazing operations, the molten filler metal interacts with the solid base materials to establish a metallurgical bond when the filler metal solidifies. Traditionally, brazed joints have a disadvantage in that in general they are subject to embrittlement and fracture without warning, in contrast to most metal welds which are ductile and bend or vibrate for a significant time before fracturing.

Parts which are to be bonded along a curved joint present a problem for the brazing process wherein the entire assembly is heated to a temperature at which all of the filler metal becomes molten, as flow of the molten filler metal due to gravity can cause a maldistribution of the molten filler metal in the joint. In such situations, the only force attempting to maintain the molten filler metal in the joint is capillary surface tension. In order for the capillary surface tension to be effective, the faying surfaces would have to be machined very accurately to minimize the gap between the faying surfaces. However, such machining, even if practical, would be very expensive. When attempts were made to braze an impeller cover, such as illustrated in FIG. 6, to the impeller vanes, such as illustrated in FIG. 5, difficulties were encountered in that the cover tended to separate from the vanes in overspeed tests.

The American Welding Society defines electron beam welding as a "welding process that produces coalescence with a concentrated beam, composed primarily of high velocity electrons, impinging on the joint. The process is used without shielding gas and without the application of pressure." The electron beam is formed under high-vacuum conditions in a diode or triode electron gun and is emitted in the form of a collimated beam. The beam of electrons passes through the field of an electromagnetic focusing coil (magnetic lens) , wherein the diameter of the electron beam is reduced (focused) to produce a very small, high-intensity beam spot at the workpiece. For example, the beam spot can be in the range of 0.010 to 0.050 inch in diameter, with a power density of about 10⁶ W/in². Such spot intensity is sufficient to vaporize almost any material, forming a hole of metal vapors. As the electron beam travels through the metal vapors easier than through the solid metal, the depth of penetration continues to increase such that the vapor hole penetrates deeper into the workpiece. Penetration can be increased by placing the focal point below the surface of the workpiece and/or by increasing the power density. When this vapor hole is advanced along a joint, a continuous weld is produced as the material at the leading edge of the vapor hole melts and flows around the sides of the vapor hole to the trailing edge, thereby filling in the trailing edge and then solidifying to produce the continuous weld. One of the advantages of electron beam welding is the ability to produce welds that are deeper and narrower than arc welds, e.g. a depth to width ratio of greater than 30:1 is possible with electron beam welding. The ability to achieve a high weld depth-to-width ratio eliminates the need for multiple- pass welds which would be required in arc welding. Another advantage of electron beam welding is that it can be accomplished with a total energy input that is lower than that required in arc welding. Electron beam welds can be extremely narrow, for example, the width of an electron beam weld in 0.5 inch stainless steel can be 0.04 inch. Such narrow weld widths require very accurate positioning of the electron beam as even a small misalignment would allow an electron beam, intended to be directed in the plane of the joint, to miss the joint completely. If desired, a deflection coil can be provided to cause the beam to move in a variety of oscillation patterns to increase the effective width of the weld. Almost any metal can be welded in one pass with the electron beam welding process, including the super-alloys, the refractory metals, the reactive metals, and the stainless steels.

The shape of the parts to be welded and the corresponding joint configuration are generally critical to the successful application of electron beam welding. In general, it is desirable that the electron beam be directed at the joint parallel to and in the plane of the faying surfaces to cause melting over the entire contact area, and that the joint has closely fitted faying surfaces to enable the narrow electron beam to fuse base metal on both sides of the joint. Thus, a butt joint to be electron beam welded is not open as in arc welding.

For a T joint, the recommended procedure has been to use either a single weld of the top surface of the column member to the bottom surface of the capital member with the electron beam being inclined to the joint at as small an angle as possible, or to weld one end surface of each of two half-capital members to opposite sides of the top portion of the column member with the electron beam being in the plane of the respective joint.

While it is possible to produce a T joint with the electron beam being located above the capital member and coaxial with the column member by melting a hole completely through the capital member into the column member, such melt-through or blind weld procedure has not been generally recommended. Not only does such melt-through procedure require beam energy for melting nonfunctional metal and result in the maximum confinement of molten metal, but it has a smaller joining area, i.e. less than the total contact area of the faying surfaces, in order to avoid molten base metal escaping from the joint. The unwelded portions of the joint reduce the strength of the joint, reduce fatigue life of the joint, and provide notch sensitivity and corrosion susceptibility.

Moreover, in joining complex structures it is not always possible to position an electron beam gun in the plane of the faying surfaces. For example, a compressor impeller can be formed in two parts, one being a disc with impeller vanes integrally formed thereon, such as illustrated in FIG. 5, and the other being an impeller cover, such as illustrated in FIG. 6. It is desirable that the cover be bonded to the distal edge of each impeller vane. However, an electron beam gun could not be positioned in the plane of the faying surfaces, thus necessitating a melt-through weld. When such an impeller cover has been bonded by prior electron beam welding processes to such an impeller vane with a melt-through weld, each weld was not coextensive with the faying surfaces, even when the electron beam welding process was followed by a gas tungsten arc (TIG) welding process. The TIG weld could be formed only along a portion of the edge of each joint due to the difficulties in accessing the underside of the impeller vanes. As the TIG weld is a fillet weld, the portions of the faying surfaces not occupied by the electron beam weld remained unbonded to each other, even in the areas where the TIG weld could be applied. In welding with fillets, it is very difficult to achieve fillets which are sufficiently small and of uniform dimensions such that the volume between the impeller vanes is not affected. While impellers which have been fabricated by a combination of electron beam welding and TIG welding have been found to be serviceable over long periods at the usually encountered maximum discharge pressures of approximately 1000 psi, it is now desirable to operate compressor impellers with a maximum discharge pressure as high as about 10,500 psi.

Sequential positioning of the electron beam on opposite sides of the joint for two successive welds is also not recommended because the second weld is fully constrained by the first weld and shows a strong tendency to crack. Thus, electron beam welding is generally limited to either a tight butt joint or a lap joint.

As a joint for electron beam welding ordinarily has closely fitted, abutting faces without V-grooves, an external source of filler metal is generally not used or needed in electron beam welding. However, when welding mild steel, highly deoxidized filler metal is sometimes used to deoxidize the molten metal and produce dense welds. Also, filler metal can be employed when electron beam welding dissimilar metals and alloys. The American Welding Society defines braze welding as a "welding process variation that uses a filler metal with a liquidus above 450°C (840*F) and below the solidus of the base metal. The base metal is not melted. Unlike brazing, in braze welding, the filler metal is not distributed in the joint by capillary action." For example, the American Welding Society defines flow welding as a "braze welding process variation that uses the heat from molten filler metal poured over the fusion faces" and electron beam braze welding as "Braze welding using an electron beam as the heat source."

Laser beam welding is similar to electron beam welding in that the focused high-powered coherent monochromatic light beam used in laser beam welding causes the metal at the point of the focus to vaporize, producing a column of vapor extending deep into the workpiece. The vapor column is surrounded by a liquid pool of molten metal which can be moved along the desired weld path to produce a continuous weld having a depth to width ratio greater than 8:1. Multi- kilowatt carbon dioxide gas laser systems can produce beams which can be focused to provide power densities of 6.5 MW/in² or greater.

The welding of joints having complex geometries, such as the distal edges of compressor impeller vanes to the adjacent impeller cover, present many problems. Due to small variations in welding conditions the impeller vanes can be distorted such that the spacing between blades can be altered, the pressure rise across the impeller blades can be affected, and the capacity of the compressor can be reduced. The axial thiclcness of the cover varies from a thick section at the radially innermost part of the cover to a thin section at the radially outermost part of the cover. The inner thick section is desirable in order to resist deflection of the impeller, while the outer thin section is desirable to reduce the resulting centrifugal forces experienced by the impeller. Such variation in thickness of the cover as well as the curved configuration of the impeller vanes and the narrowness of the space between impeller vanes make the formation of slot welds or internal fillet welds extremely difficult. Also, the use of slot welds or internal fillet welds can result in significant variations in the fluid passageway volumes between adjacent impeller vanes, affecting the pressure rise across the impeller vanes and the capacity of the compressor. Due to the close spacing of the impeller vanes and the annular curvature of the impeller cover, an electron gun cannot be positioned in the plane of the weld joint, and a melt- through weld results in unbonded areas which are susceptible to corrosion and cracking.

SUMMARY OF THE INVENTION

The present invention provides an improved method of bonding a first faying surface on a first workpiece of a base material to a second faying surface on a second workpiece of a base material, and an improved bonded product. A method in accordance with the invention comprises: providing an assembly of the first and second workpieces with the first and second faying surfaces in opposition to each other and with at least one layer of braze filler metal positioned between and in contact with the first and second faying surfaces; directing a high energy beam at the assembly so that the high energy beam passes through a first portion of the first workpiece, exiting through the first faying surface, and enters a first portion of the second workpiece through the second faying surface, the high energy beam being of sufficient intensity to melt the first portion of the first workpiece and the first portion of the second workpiece to thereby form molten base material while also providing sufficient heat to an immediately laterally adjacent portion of the at least one layer of braze filler metal to melt the immediately adjacent portion, without forming a burn-out opening of molten base material in any of the first workpiece, the second workpiece, and the at least one layer; and permitting the first portion of the first workpiece and the first portion of the second workpiece to cool sufficiently for the thus formed molten base material to solidify to form a weld bond between a first portion of the first faying surface and a first portion of the second faying surface and for the immediately laterally adjacent portion of the at least one layer to solidify to form a braze bond between a second portion of the first faying surface and a second portion of the second faying surface without having melted the second portion of the first faying surface or the second portion of the second faying surface.

The first and second faying surfaces can extend along an elongated joint which comprises a longitudinally extending intermediate width portion located between first and second longitudinally extending edge portions, and relative movement between the high energy beam and the assembly can be provided so that a continuous weld bond is formed to occupy the intermediate width portion and continuous braze bonds are formed to occupy the first and second longitudinally extending edge portions.

In a preferred embodiment, the second workpiece comprises a discoidal member having a plurality of impeller vanes positioned thereon, with each of the impeller vanes having a distal edge surface which is remote from the discoidal member. The first workpiece comprises an impeller cover having an internal surface facing and conforming to the distal edge surfaces. A layer of braze filler metal is positioned between and in contact with each distal edge surface and the facing portion of the internal surface of the cover. Relative movement between the high energy beam and the assembly is provided so that the high energy beam travels along the exterior surface of the cover immediately above each distal edge surface.

The present invention provides an improved article of manufacture which comprises a first component having a first faying surface, a second component having a second faying surface; the first and second components being positioned with the first and second faying surfaces in opposition to each other, a weld bond between a first portion of the first faying surface and a first portion of the second faying surface, and at least one braze bond between a second portion of the first faying surface and a second portion of the second faying surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a T-joint formed by a conventional brazing process;

FIG. 2 is a cross-sectional view of a T-joint formed by a conventional electron beam welding process;

FIG. 3 is a cross-sectional view of a preform assembly of two members positioned to be joined along faying surfaces to form a T-joint, having a tape of brazing metal positioned between the faying surfaces; FIG. A is a cross-sectional view of the preform assembly of FIG. 3 after it has been subjected to a braze/EBW process in accordance with the present invention to form a bond between the two workpieces;

FIG. 4B is a cross-sectional detail view of the resulting bond between the two workpieces of the preform assembly;

FIG. 5 is a plan view of an impeller disc having impeller blades integrally formed thereon by milling, which is suitable for use in a gas compressor; FIG. 6 is a plan view of a cover for the impeller disc of FIG. 5;

FIG. 7 is a side elevational view, partly in cross- section of a preform assembly comprising the cover of FIG. 6 positioned on the impeller disc of FIG. 5; FIG. 8 is a simplified perspective view of an electron beam welding machine which can be employed in the practice of the present invention; and

FIGS. 9A, 9B, and 9C are simplified sequential perspective views of the preform assembly of FIG. 7 at three different times in the brazing/welding of a single vane on the impeller disc to the cover. DETAILED DESCRIPTION

FIG. 1 illustrates the formation of a conventional electron beam melt-through weld, wherein the faying surface 11 of a first, upper workpiece 12 (i.e., closest to the electron gun) is bonded to the faying surface 13 of a second, lower workpiece 14 (i.e., remote from the electron gun) . Each of the workpieces 12 and 14 is formed of a suitable base material which can be bonded by electron beam welding. In this illustration the upper workpiece 12 is positioned so that the portion thereof being welded extends at least generally horizontally with the faying surface 11 being on the bottom side of the upper workpiece 12, while the lower workpiece 14 is positioned below the upper workpiece 12 and extends at least generally vertically with the faying surface 13 being the upper end surface of the lower workpiece 14 so that the faying surface 13 contacts and is coextensive with the faying surface 11. Thus, the upper workpiece 12 constitutes the capital member of a T-weld product, while the lower workpiece 14 constitutes the column member of the T-weld product.

The high intensity electron beam 15 enters the upper workpiece 12 at an upper surface 16 of the workpiece 12, passes completely through the upper workpiece 12, exiting through the first faying surface 11, and then enters the lower workpiece 14 through the second faying surface 13. The electron beam 15 enters but does not pass through the second workpiece 14. The portion of the material of the upper workpiece 12 through which the electron beam 15 passes and a portion of the material of the lower workpiece 14 through which the electron beam 15 passes are heated locally by the electron beam 15 to a temperature above the melting point of these materials, so that a keyhole 17 of molten weld material is formed which extends from the upper surface

16 through the upper workpiece 12 and into the lower workpiece 14. The width of this keyhole 17 of molten weld material is sufficiently less than the width of each of the faying surface 11 of the upper workpiece 12 and the faying surface 13 of the lower workpiece such that the formation in either of the workpieces 12, 14 of a "burn-out" or "melt- through" hole through which molten weld material could escape by gravity, is avoided. In general, apart from the site 18 of the entry of the electron beam 15 into the upper surface 16 of the upper workpiece 12, it is desirable that the distance between the keyhole 17 of molten weld material and the closest external workpiece surface be at least 0.020 inch. The heat from the keyhole 17 of molten weld material can be sufficient to effect a thermal treatment of the portion 19 of the upper workpiece 12 surrounding the keyhole

17 without melting the material in portion 19. Similarly, the heat from the keyhole 17 of molten weld material can be sufficient to effect a thermal treatment of the portion 21 of the lower workpiece 14 surrounding the keyhole 17 without melting the material in portion 21. However, since the material in portion 19 and the material in portion 21 are not melted, there is no bond between the area of the faying surface 11 which is part of the portion 19 and the area of the faying surface 13 which is part of the portion 21. The result is a gap or unbonded area 22 between the lower workpiece 14 and the upper workpiece 12 on each side of the keyhole 17. Such gaps 22 reduce the achievable strength of the joint, increase the potential for corrosion, and increase the potential for stress cracking.

The welded joint of FIG. 1 can involve the formation of a single keyhole 17, as in spot welding. However, in general, the keyhole 17 will be initially formed closely adjacent to one end of the joint and then moved along the joint, forming an elongated weld path, until just prior to the opposite end of the joint. In such situation, the illustration in FIG. 1 is of the width of the joint, and represents the structure of the joint through the length of the joint other than the initial and final portions of the joint which are not subjected to melting. If desired, a backing, which will not melt at the operating temperatures of the electron beam 15 and thus will not be bonded by the weld, can be placed at each end of the joint so that the initial and final keyholes can intersect the respective end surfaces of the workpieces 12 and 14. Also, where post-weld machining is feasible, the weld path can be initiated in a starting tab at one end of the joint and concluded in a runoff tab at the other end of the joint and then these tabs can be machined off after the welding is completed. However, such machining of tabs is not always possible in weld joints having a complex geometry. After the completion of the formation of the weld, the upper surface layer, e.g. approximately 0.125 inch depth, of the upper workpiece 12 can be grit blasted and then machined off to eliminate any scaling or weld surface irregularities which occurred during the welding operation.

FIG. 2 illustrates the formation of a conventional brazed joint, wherein the faying surface 31 of a first workpiece 32 is bonded to the faying surface 33 of a second workpiece 34 through a layer 35 of braze filler metal. In this illustration the upper workpiece 32 and the lower workpiece 34 are positioned so that the faying surfaces 31 and 33 extend at least generally horizontally with the braze filler metal 35 being positioned between and in contact with the faying surfaces 31 and 33. The first workpiece 32 can be positioned on top of or beneath the second workpiece 34. 16

The horizontal orientation of the joint during the time period that the braze filler metal is molten is desirable in order to minimize the possibility of gravity causing molten braze filler metal to drain from the joint. It is possible to position the faying surfaces at an orientation other than horizontal if a backing is placed on the lower side of the joint so that molten braze filler metal is retained within the joint. The braze filler metal 35 can be prepositioned between the faying surfaces 31 and 33, particularly when a horizontal orientation of the joint is employed. An external source of braze filler metal 35 can be employed where there is sufficient capillary action for the molten braze filler metal to flow into and completely fill the joint. In general, the brazed bond is effected by placing workpieces 32 and 32 and the braze filler metal 35 in the desired position within an oven, and then the temperature in the oven is raised sufficiently to cause the braze filler metal 35 to melt without melting any portion of either of the workpieces 32 and 34. The resulting joint comprises a continuous layer of solidified braze filler metal 35 between and in contact with, and preferably coextensive with, the faying surfaces 31 and 33, without any weld bond between the faying surfaces 31 and 33.

FIG. 3 illustrates a preform assembly 40 for the formation of a bond in accordance with the present invention between a faying surface 41 of a first, upper workpiece 42 and the faying surface 43 of a second, lower workpiece 44. Each of the workpieces 42 and 44 is formed of a suitable base material which can be bonded by either electron beam welding or brazing. In this illustration the upper workpiece 42 is positioned so that the portion thereof currently being bonded extends at least generally horizontally with the faying surface 41 being on the bottom side of this portion of the upper workpiece 42, while the lower workpiece 44 is positioned below the upper workpiece 42 and extends at least generally vertically with the faying surface 43 being the upper end surface of the lower workpiece 44 so that the faying surface 43 is aligned with and coextensive to the faying surface 41. A foil 45 of braze filler metal is positioned between and in contact with, and at least substantially coextensive to, the faying surfaces 41 and 43. The foil 45 is preferably secured to the faying surface 43 by suitable means, e.g., spot welding, prior to the workpiece 42 being placed in its illustrated position. Thus, the upper workpiece 42 constitutes the capital member of a T-weld product, while the lower workpiece 44 constitutes the column member of the T-weld product. The conventional wisdom for T-welds is that the ratio of the thickness of the capital member to the thickness of the column member should be less than 4:1. However, with the present invention, satisfactory T-weld products can be produced where the ratio of the thickness of the capital member to the thickness of the column member is 8:1 or higher.

While the width of the foil 45 can be coextensive with the width of the faying surface 43 of the lower workpiece 44, it is presently preferred that the width of the foil 45 be slightly greater than the width of the faying surface 43 of the lower workpiece 44, as illustrated in FIG. 3, so as to provide an amount of braze filler metal for forming a fillet at the intersection of a side surface of lower workpiece 44 with the bottom surface of workpiece 42, as illustrated in FIG. 4A. However, the width of the foil 45 should not be greater than the width of the faying surface 43 to the extent that the marginal edge portions of the foil 45 would not be melted during the brazing/welding process. On the other hand, the width of the foil 45 can be slightly less than the width of the faying surface 43 of the lower workpiece 44 so long as the thickness of the foil 45 is sufficient to provide braze filler metal for filling out the remainder of the joint between the workpieces and preferably for forming the desired fillets.

The preform assembly 40 is positioned in the work chamber of an electron beam welding machine which provides for relative movement of the electron beam and the preform assembly 40 so that the electron beam 46 can move along the desired weld path. The relative movement can be accomplished by moving the preform assembly 40 with respect to the electron beam, by moving the electron beam with respect to the preform assembly, or by moving both the electron beam and the preform assembly simultaneously. However, the movement of the preform assembly 40 with respect to a stationary electron beam gun is presently preferred for weld paths having complex curves as this permits the portion of the faying surfaces currently undergoing electron beam melting to be maintained in a generally horizontal orientation. The high intensity electron beam 46 enters the upper workpiece 42 at an upper surface 47 of the workpiece 42, passes completely through the upper workpiece 42, exiting through the first faying surface 41, passes through the foil 45 of braze filler metal, and then enters the lower workpiece 44 through the second faying surface 43. The electron beam 46 enters but does not pass through the second workpiece 44.

A portion of the material of the upper workpiece 42 through which the electron beam 46 passes, a portion of the braze filler metal foil 45 through which the electron beam 46 passes, and a portion of the material of the lower workpiece 44 through which the electron beam 46 passes are heated locally by the electron beam 46 to a temperature above the melting point of these materials, so that a keyhole 48 of molten weld material is formed which extends from the upper surface 46 through the upper workpiece 42 and the braze filler foil 45 and into the lower workpiece 44. Thus, the molten weld material contains material from the upper workpiece 42, material from the braze foil 45, and material from the lower workpiece 44. The width of this keyhole 48 of molten weld material is sufficiently less than the width of each of the faying surface 41 of the upper workpiece 42, and the faying surface 43 of the lower workpiece 44 such that the formation in either of the workpieces 42, 44 or the braze foil 45 of a "burn-out" hole through which molten weld material could escape by gravity, is avoided. In general, apart from the site 49 of the entry of the electron beam 46 into the upper surface 47 of the upper workpiece 42, it is desirable that the distance between the keyhole 48 of molten weld material and the closest external workpiece surface be at least 0.020 inch. The heat from the keyhole 48 of molten weld material is sufficient to raise the temperature of the portion 51 of the upper workpiece 42 surrounding the keyhole 48 above the melting temperature of the braze foil 45 without melting the material in portion 51. Similarly, the heat from the keyhole 48 of molten weld material is sufficient to raise the temperature of the portion 52 of the lower workpiece 44 surrounding the keyhole 48 above the melting temperature of the braze foil 45 without melting the material in portion 52. Although the material in portion 51 and the material in portion 52 are not melted, there is a bond between the area of the faying surface 41 which is part of the portion 51 and the area of the faying surface 43 which is part of the portion 52 as a result of the foil 45 of braze filler metal being melted throughout its lateral extent. The result is a bond which is at least coextensive with the faying surfaces 41 and 43, with the bond comprising an electron beam weld bond 53 along the longitudinally extending central width portion of the faying surfaces 41 and 43 and a braze bond

54, 55 along the longitudinally extending side edge portions of the faying surfaces 41 and 43, as illustrated in FIG. 4A. For example, a braze foil 45 having a thickness of 3 mils can be employed to form braze bonds 54, 55 having a typical thickness of 0.3 mil, thereby providing adequate molten braze filler metal for forming the fillets at the intersection of the side surfaces of the lower workpiece 44 with the bottom surface of workpiece 42, as illustrated in FIG. 4A. The absence of open gaps between the faying surfaces increases the achievable strength of the joint while reducing the potential for corrosion and stress cracking.

The process of the present invention can be utilized to bond any two or more workpieces of the same or differing materials, so long as the materials of the workpieces are capable of being electron beam welded and are capable of being brazed. The process is particularly applicable to low to medium carbon steels, low alloy steels, martensitic stainless steels, austenitic stainless steels, nickel-based alloys, aluminum alloys, precipitation hardened stainless steels, titanium alloys, etc.

Any suitable braze filler metal can be employed in the process of the invention. In general, gold, gold alloys, nickel, nickel alloys, aluminum, and aluminum alloys are considered to be suitable. However, the alloys should be at least substantially free of any component which would adversely affect either the weld bond or the braze bond. Gold and gold alloys are presently preferred. Due to lower cost and similar melting temperatures to the gold alloys, nickel alloys are very attractive. However, experiments with nickel alloys containing boron to lower the melting temperature have revealed cracks in both the weld bond and in the braze region. Thus, the alloys should be free of detrimental materials such as boron.

In a particular application, wherein the preformed assembly represented by FIG. 3, comprised steel workpieces 42, 44 and a three mil gold foil as foil 45, the resulting braze/EBW bond is represented in FIG. 4B. The gold braze filler metal melted prior to the formation of the weld bond 53. As gold does not form a molecular alloy with the iron of the steel workpieces, the molten gold tended to move outwardly as the steel at the central portions of the faying surfaces 41 and 43 was melted by the electron beam 46. Thus, the electron beam weld bond 53 comprised a central section 53a located between two transition sections 53b and 53c. The central section 53a was substantially pure steel; while each of transition section 53b, which joins the central section 53a to the braze bond 54, and transition section 53c, which joins the central section 53a to the braze bond 55, was a physical mixture of macroparticles of gold dispersed in steel. The braze bonds 54 and 55, which were substantially pure gold, had a nominal thickness of 0.3 mil. Thus, the amount of gold displaced from the central portion of the faying surfaces 41 and 43, the amount of gold displaced by the reduction in thickness from 3 mils to 0.3 mil, and the amount of gold provided by the marginal edges of the foil 45 which had originally extended beyond the sides of the workpiece 44, provided the necessary amount of gold to form excellent braze bonds and to form the fillets at each edge of the workpiece 44. As a weld bond is generally considered to be stronger than a braze bond, it is desirable for the braze filler metal to have this characteristic of not forming a molecular alloy with the base material, so that the central portion of the resulting braze/weld bond is a weld bond at least substantially free of braze filler metal.

Referring now to FIGS. 5 and 7, an impeller disc 61 has a discoidal member 62 having a central axis 63, and a plurality of vanes 64 integrally formed with the discoidal member 62, with the vanes 64 being positioned on the discoidal member 62 at spaced apart locations about the circumferential extent of the discoidal member 62, with the vanes 64 extending away from the discoidal member 62 in a direction at least substantially parallel to the central axis 63 and also extending generally radially outwardly away from the central axis 63, although in a complex curve. Each of the vanes 64 has a distal edge surface 65 remote from the discoidal member 62.

Referring now to FIGS. 6 and 7, an impeller cover 66 is an annular member having a large diameter first end 67, which when the cover 66 is mounted on the impeller disc 61 is close to the discoidal member 62, and a small diameter second end 68, which when the cover 66 is mounted on the impeller disc 61 is remote from the discoidal member 62. The cover 66 has a concave arcuate external surface 69 extending inwardly from the first end 67 to the second end 68, and a convex arcuate internal surface 71 extending inwardly from the first end 67 to the second end 68.

In order to bond the cover 66 to the impeller disc 61, it is desirable to form a continuous bond between the distal edge surface 65 of each vane 64 and the adjacent portion of the internal convex surface 71 of the cover 66, with each bond being coextensive with the distal edge surface 65. While any suitable electron beam welding machine can be employed in the practice of the present invention, FIG. 8 is a simplified perspective view of a large-chamber electron beam welding machine 80 by Precision Technologies, Inc., Enfield, Connecticut, which is useful in the practice of the present invention for bonding the cover 66 to the impeller disc 61.

The EB welding machine 80 includes an electron beam gun 81, which is positioned above the work chamber housing 82 with the longitudinal axis of the electron beam gun 81 extending at least generally vertically through the ceiling of the work chamber 83. The electron beam gun can be operated at either low-voltage (e.g. 60 kV) or high-voltage (e.g. 150 kV) . The upper section of the electron beam gun 81 contains the beam generating components while the lower section contains the focusing lens, deflection coil, and an optical viewing system. The beam generator is a triode having a voltage from 60 kV to 175 kV applied between a directly heated cathode and an annular anode, while the grid serves as a control electrode to regulate the electron flow. Immediately below the beam generator, the electron beam is accurately aligned with the optical axis of the magnetic focusing lens. The focusing lens focuses the electron beam as a high intensity beam on a very small site on the workpieces. The deflection coil can be employed to deflect the electron beam statically or dynamically in all directions from the optical axis. A vacuum valve enables the upper section of the electron beam gun 81 to be hermetically isolated from the lower section when either the work chamber 83 is vented for removal of the welded product or the upper section is opened for changing the filament. The work chamber 83, having a working volume in the range of approximately 700 to approximately 20,000 liters, has an opening 84 in one wall of housing 82 which can be closed by a sliding door 85. A runout platform 86 is positioned adjacent the opening 84 and is provided with rails 87 which are in alignment with similar rails 88 within the work chamber 83. A carriage 89, which supports the workpiece manipulator 91, is mounted for travel on rails 87 and 88. The workpiece manipulator 91 can consist of an X/Y coordinate table, with both axes driven by precisely regulated DC motors, for simple welds. The workpiece manipulator 91 can be a multi-axis workpiece manipulator by combining with the X/Y coordinate table one or more of a turning device, a tilting device, a swiveling device, and a lifting device for more complex welds. The 4-axis manipulator and the 5-axis manipulator are frequently employed, and a 6-axis manipulator can be utilized. A vacuum pumping system 92 provides the desired degree of vacuum for the work chamber 83, while a vacuum pumping system 93 is provided for the electron beam gun 81. A power supply 94 provides high voltage and auxiliary power for the electron beam gun 81. The welding machine is controlled by a CNC device 95, a control cabinet 96, and an operating panel 97.

Once the electron beam welding machine is in the operating state, the workpieces to be joined are loaded on the workpiece manipulator 91 with each pair of opposing faying surfaces in alignment with each other and with a strip of braze filler metal securely positioned between and in contact with each respective pair of opposing faying surfaces. The strip of braze filler metal can be secured between the respective pair of opposing faying surfaces by any suitable technique, with spot welding to the faying surface of one workpiece prior to the positioning of the second workpiece being presently preferred. It is desirable that the securing technique not introduce any contaminants into the joint which would interfere with either the weld formation or the braze bond formation. Then, the carriage 89 and the workpiece manipulator 91 are moved from the runout platform 86 to the work chamber 83, the work chamber door 85 is closed, the work chamber 83 is evacuated by the vacuum pumping system 92 to the desired degree of vacuum, the workpieces are positioned at the starting point with respect to the electron beam gun 81 by using the optical viewing system (not shown) , the welding parameters are inputted to the CNC device 95 and the operating panel 97, the electron beam is activated and the welding/brazing of the workpieces is initiated at the starting point, as illustrated in FIG. 9A. The workpiece manipulator 91 can vary the position of the workpieces about the various axes such that upon relative motion between the faying surfaces and the electron beam, the portion of the faying surfaces and braze foil currently being impinged by the electron beam is at least substantially perpendicular to the electron beam, as illustrated in FIGS. 9B and 9C for two points in time during the brazing/EBW of the distal edge surface 65 of a single vane 64 to the interior surface 71 of cover 66.

Although such molten weld material is not formed throughout the width of the faying surfaces, the thermal energy transferred to the portions of the first and second workpieces immediately adjacent to the instantaneous position of the electron beam is sufficient to raise the temperature of such portions above the melting temperature of the braze foil, thereby melting the portions of the braze foil which are laterally adjacent to the instantaneous position of the electron beam, such that as the electron beam moves away from a particular portion of the weld path, the molten weld material at that particular portion of the weld path solidifies and the laterally adjacent portions of the molten braze foil also solidify. Thus, the first and second workpieces are securely joined along the full length and width of their opposing faying surfaces, with the longitudinally extending central width portion of the joint being constituted by an electron beam weld and the two longitudinally extending edge portions of the joint being constituted by a braze bond. Such joints can be formed to be free of gaps or unbonded areas.

Upon completion of the welding, the work chamber 83 is vented to the atmosphere, the work chamber door 85 is opened, the carriage 89 and the workpiece manipulator 91 are moved from the work chamber 83 to the runout platform 86, and the welded product is removed from the workpiece manipulator 91. The CNC can be employed to automate the entire operating cycle, from the closing of the work chamber door 85 to the opening of the door 85 and the removal of the welded product.

Reasonable variations and modifications of the invention are possible within the scope of the foregoing disclosure and the appended claims to the invention. For example, a movable electron gun system can be employed instead of the workpiece manipulator and a stationary electron beam gun. A laser beam welding system could be employed instead of the electron beam welding system. However, for the present invention, the electron beam welding process is currently preferred over the laser beam welding process.

Claims

WE CLAIM:

1. A method of bonding a first faying surface on a first workpiece of a base material to a second faying surface on a second workpiece of a base material, said method comprising the steps of: providing an assembly of said first and second workpieces with said first and second faying surfaces being in opposition to each other and with at least one layer of braze filler metal positioned between and in contact with the first and second faying surfaces, directing a high energy beam at said assembly so that the high energy beam passes through a first portion of said first workpiece, exiting through said first faying surface, and enters a first portion of said second workpiece through said second faying surface, said high energy beam being of sufficient intensity to melt said first portion of said first workpiece and said first portion of said second workpiece to thereby form molten base material while also providing sufficient heat to an immediately laterally adjacent portion of said at least one layer of braze filler metal to melt said immediately adjacent portion, without forming a burn-out opening of molten base material in either of said first workpiece and said second workpiece, and permitting said first portion of said first workpiece and said first portion of said second workpiece to cool sufficiently for the thus formed molten base material to solidify to form a weld bond between a first portion of said first faying surface and a first portion of said second faying surface and for said immediately laterally adjacent portion of said at least one layer of braze filler metal to solidify to form a braze bond between a second portion of said first faying surface and a second portion of said second faying surface without having melted said second portion of said first faying surface or said second portion of said second faying surface.

2. A method in accordance with claim 1, wherein said at least one layer of braze filler metal is at least substantially coextensive with said first and second faying surfaces.

3. A method in accordance with claim 1, wherein said at least one layer of braze filler metal is a foil of a braze filler metal which is at least substantially coextensive with said first and second faying surfaces.

4. A method in accordance with claim 1, wherein said high energy beam is an electron beam.

5. A method in accordance with claim 1, wherein said high energy beam is directed at least substantially perpendicular to the portion of the first faying surface and the portion of the second faying surface being impinged by the high energy beam.

6. A method in accordance with claim 1, wherein each of said first and second faying surfaces extends along an elongated joint, wherein said elongated joint comprises a longitudinally extending intermediate width portion located between first and second longitudinally extending edge portions, wherein said weld bond is formed to occupy said intermediate width portion of said elongated joint, and wherein said braze bond is formed to occupy the first and second longitudinally extending edge portions of said elongated joint.

7. A method in accordance with claim 1, wherein each of said first and second faying surfaces extends along an elongated joint, and further comprising providing relative movement between said high energy beam and said assembly so as to form a continuous bond between said first faying surface and said second faying surface along said elongated joint.

8. A method in accordance with claim 7, wherein said step of providing relative movement comprises maintaining the high energy beam at least substantially perpendicular to the portion of the first faying surface and the portion of the second faying surface currently being impinged by the high energy beam.

9. A method in accordance with claim 8, wherein said elongated joint comprises a longitudinally extending intermediate width portion located between first and second longitudinally extending edge portions, wherein said weld bond is formed to occupy said intermediate width portion of said elongated joint, and wherein said braze bond is formed to occupy the first and second longitudinally extending edge portions of said elongated joint.

10. A method in accordance with claim 9, wherein said weld bond and said braze bond extend along at least substantially all of the length of said elongated joint.

11. A method in accordance with claim 10, wherein said second workpiece comprises a discoidal member having a plurality of impeller vanes positioned thereon at spaced apart locations about the circumferential extent of said discoidal member, each of said impeller vanes having a distal edge surface which is remote from the discoidal member and which constitutes a second faying surface; wherein said first workpiece comprises an impeller cover having an internal surface with portions of said internal surface facing said distal edge surfaces and conforming to said distal edge surfaces, each of said portions of said internal surface constituting a first faying surface; wherein each distal edge surface and the respective facing portion of the internal surface form constitute a pair of faying surfaces and form an elongated joint extending along the radial length of a respective impeller vane; wherein a layer of braze filler metal is positioned between and in contact with a respective pair of faying surfaces; and wherein said step of providing relative movement between said high energy beam and said assembly comprises providing relative movement between said electron beam and said assembly so that said electron beam travels along each of the elongated joints.

12. A method in accordance with claim 11, wherein each layer of braze filler metal is a foil of a braze filler metal which is coextensive with the respective pair of faying surfaces.

13. A method in accordance with claim 1, wherein said second workpiece comprises a discoidal member having a plurality of impeller vanes positioned thereon at spaced apart locations about the circumferential extent of said discoidal member, each of said impeller vanes having a distal edge surface which is remote from the discoidal member and which constitutes a second faying surface; wherein said first workpiece comprises an impeller cover having an internal surface with portions of said internal surface facing said distal edge surfaces and conforming to said distal edge surfaces, each of said portions of said internal surface constituting a first faying surface; wherein each distal edge surface and the respective facing portion of the internal surface form constitute a pair of faying surfaces and form an elongated joint extending along the radial length of a respective impeller vane; wherein a layer of braze filler metal is positioned between and in contact with a respective pair of faying surfaces; and further comprising providing relative movement between said high energy beam and said assembly so that said high energy beam travels along each of the elongated joints.

14. An article of manufacture comprising: a first component of a base material, said first component having a first faying surface; a second component of a base material, said second component having a second faying surface; said first and second components being positioned with said first and second faying surfaces being in opposition to each other; a weld bond between a first portion of said first faying surface and a first portion of said second faying surface; and at least one braze bond between a second portion of said first faying surface and a second portion of said second faying surface.

15. An article of manufacture in accordance with claim 14, wherein said weld bond and said at least one braze bond are collectively coextensive with said first and second faying surfaces.

16. An article of manufacture in accordance with claim 14, wherein said second component comprises a discoidal member having a plurality of impeller vanes positioned thereon at spaced apart locations about the circumferential extent of said discoidal member, each of said impeller vanes having a distal edge surface which is remote from the discoidal member and which constitutes a second faying surface; wherein said first component comprises an impeller cover having an internal surface with portions of said internal surface facing said distal edge surfaces and conforming to said distal edge surfaces, each of said portions of said internal surface constituting a first faying surface; wherein each distal edge surface and the respective facing portion of the internal surface form constitute a pair of faying surfaces and form an elongated joint extending along the radial length of a respective impeller vane; and wherein each pair of faying surfaces has a weld bond between a first portion of the respective first faying surface and a first portion of the respective second faying surface, and at least one braze bond between a second portion of the respective first faying surface and a second portion of the respective second faying surface.

17. An article of manufacture in accordance with claim 16, wherein each respective weld bond and at least one braze bond are collectively coextensive with the respective pair of faying surfaces.

18. An article of manufacture in accordance with claim 17, wherein each pair of faying surfaces extends along a respective elongated joint, wherein each said elongated joint comprises a longitudinally extending intermediate width portion located between first and second longitudinally extending edge portions, wherein each weld bond occupies the intermediate width portion of the respective elongated joint, and wherein each braze bond occupies the first and second longitudinally extending edge portions of the respective elongated joint.

19. An article of manufacture in accordance with claim 14, wherein each of said first and second faying surfaces extends along an elongated joint, wherein said elongated joint comprises a longitudinally extending intermediate width portion located between first and second longitudinally extending edge portions, wherein said weld bond occupies said intermediate width portion of said elongated joint, and wherein said braze bond occupies the first and second longitudinally extending edge portions of said elongated joint.

20. An article of manufacture in accordance with claim 19, wherein said braze bond and said weld bond are collectively at least substantially coextensive with said first and second faying surfaces.