Brazing and soldering alloy wires

Abstract

Brazing alloy wire formed from a composite comprising a sheath of at least one ductile first phase and a core comprising particles of a different composition to the sheath, in which: the sheath has an annealing temperature in degrees K the particles have a melting point at least 20% above the annealing temperature of the sheath the particles have a size distribution in which 25% by weight or less comprise particles less than 25 m in size the particles are discrete.

Claims

1. A brazing alloy wire formed from a composite comprising a sheath of at least one ductile first phase and a core comprising particles of a different composition to the sheath, wherein the sheath has an annealing temperature in degrees K; the particles have a melting point at least 20% above the annealing temperature of the sheath; the particles have a size distribution in which 25% by weight or less comprise particles less than 25 m in size and the particles are capable of flowing during a drawing process; the particles are discrete; the particles have a free metal content of less than 10% by volume; and the overall composition of the composite is equivalent to a low ductility alloy composition.

2. The brazing alloy wire as claimed in claim 1, in which the particles have an aspect ratio of less than 4:1.

3. The brazing alloy wire as claimed in claim 2, in which the particles have an aspect ratio of less than 2:1.

4. The brazing alloy wire as claimed in claim 3, in which the particles have an aspect ratio of less than 3:2.

5. The brazing alloy wire as claimed in claim 1, in which the particles have a melting point at least 30% above the annealing temperature of the sheath.

6. The brazing alloy wire as claimed in claim 5, in which the particles have a melting point at least 40% above the annealing temperature of the sheath.

7. The brazing alloy wire as claimed in claim 1, in which the particles have a free metal content of less than 5% by volume.

8. The brazing alloy wire as claimed in claim 1, in which the particles have a convexity greater than 0.7.

9. The brazing alloy wire as claimed in claim 8, in which the particles have a convexity greater than 0.8.

10. The brazing alloy wire as claimed in claim 9, in which the particles have a convexity greater than 0.9.

11. The brazing alloy wire as claimed in claim 1, in which the low ductility composition is an alloy selected from the group comprising aluminum based alloys, magnesium based alloys, copper based alloys, silver based alloys, gold based alloys, palladium based alloys, nickel based alloys, and cobalt based alloys.

12. The brazing alloy wire as claimed in claim 1 in which the brazing alloy wire is an annealed wire.

13. The brazing alloy wire as claimed in claim 1, in which the wire is in the form of a ring.

14. The brazing alloy wire as claimed in claim 1, in which the wire has a diameter of less than 1 mm.

15. The brazing alloy wire as claimed in claim 14, in which the wire has a diameter of less than 0.5 mm.

16. The brazing alloy wire as claimed in claim 15, in which the wire has a diameter of less than 0.25 mm.

17. A method, for forming a brazing alloy wire as claimed in claim 1 comprising: a) providing a preform of a composite comprising a sheath of the at least one ductile first phase and a core comprising particles of a different composition to the sheath, wherein the sheath has an annealing temperature in degrees K; the particles have a melting point at least 20% above the annealing temperature of the sheath; the particles have a size distribution in which 25% by weight or less comprise particles less than 25 m in size and the particles are capable of flowing during a drawing process: the particles are discrete; the particles have a free metal content of less than 10% by volume: and the overall composition of the composite is equivalent to a low ductility alloy composition; and b) in two or more reduction steps, reducing the preform in diameter while increasing the length of the preform to form the wire; and in which one or more annealing steps are provided between the two or more reduction steps to restore ductility to the sheath.

18. The method as claimed in claim 17, in which the two or more reduction steps comprise one or more swaging steps.

19. The method as claimed in claim 17, in which the two or more reduction steps comprises one or more roll drawing steps.

20. The method as claimed in claim 17, in which the two or more reduction steps comprises one or more deep drawing steps.

21. The method as claimed in claim 17, in which the two or more reduction steps comprises a combination of two or more different reduction steps selected from the group swaging, roll drawing and deep drawing.

22. The method as claimed in claim 17, in which the material of the preform is degassed prior to the one or more reduction steps.

23. The method as claimed in claim 17, in which the one or more ductile first phases are present in proportions sufficient to permit drawing of the composite to a reduction in diameter of 5% or more.

24. The method of furnace brazing comprising joining parts to form an article, wherein the brazing employs the wire as claimed in claim 1.

25. The brazing alloy wire as claimed in claim 1, wherein the overall composition of the composite is equivalent to a low ductility alloy composition selected from the group: aluminium based alloys, magnesium based alloys, silver based alloys, gold based alloys, palladium based alloys, nickel based alloys, and cobalt based alloys.

26. The brazing alloy wire as claimed in claim 1, wherein the overall composition of the composite is equivalent to a low ductility alloy composition comprising: 97.6% Cu and one or more of (0-35)% Ag (0-35)% Zn (0-20)% Cd (0-5)% Ni (0-5)% Mn in which CuAg CuZn; and Cu+Ag+Zn+Cd+Ni+Mn99.5% balance impurities.

27. The brazing alloy wire as claimed in claim 1, wherein the overall composition of the composite is equivalent to a low ductility alloy composition comprising: 97.6% Cu and one or more of (0-40)% Au (0-20)% Ni (0-20)% Pd (0-20)% Mn (0-5)%Ti in which CuAu CuNi CuPd CuMn and Cu+Au+Ni+Pd+Mn+Ti99.5% balance impurities.

28. A brazing alloy wire formed from a composite comprising a sheath of at least one ductile first phase and a core comprising particles of a different composition to the sheath, wherein the sheath has an annealing temperature in degrees K; the particles have a melting point at least 20% above the annealing temperature of the sheath; the particles have a size distribution in which 25% by weight or less comprise particles less than 25 m in size and the particles are capable of flowing during a drawing process; the particles are discrete; wherein the overall composition of the composite is equivalent to a low ductility alloy composition, wherein the wire has a diameter of less than 1 mm.

29. A brazing alloy wire formed from a composite comprising a sheath of at least one ductile first phase and a core comprising particles of a different composition to the sheath, wherein the sheath has an annealing temperature in degrees K; the particles have a melting point at least 20% above the annealing temperature of the sheath; the particles have a size distribution in which 25% by weight or less comprise particles less than 25 m in size and the particles are capable of flowing during a drawing process; the particles are discrete; wherein the overall composition of the composite is equivalent to a low ductility alloy composition, wherein the wire is a longitudinally seamless wire.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further features of the invention are apparent from the claims and are exemplified in the following with reference to the drawings in which:

(2) FIG. 1 is a schematic section of a brazed joint between a flange and a notched tube;.

(3) FIG. 2 is a schematic section of a brazed joint between a notched flange and a tube;

(4) FIG. 3 is a schematic section of a preform for reduction in diameter to produce a brazing wire to the invention;

(5) FIG. 4 shows schematically an apparatus for degassing a core powder component in the preform of FIG. 3;

(6) FIG. 5 shows schematically steps of a wire forming process that may be used to reduce the preform of FIG. 3 into a wire; and

(7) FIG. 6 shows schematically a section of a preform for use in preparing a cored wire.

DETAILED DESCRIPTION OF THE INVENTION

(8) The present invention encompasses: brazing alloy wire formed from a composite comprising a sheath of at least one ductile first phase and a core comprising particles of a different composition to the sheath the method of drawing a composite comprising a sheath of at least one ductile first phase and a core comprising particles of a different composition to the sheath to form such a brazing alloy wire a furnace brazing process using such brazing alloy wire brazed parts formed from such a process.

(9) The particles of a different composition to the sheath may comprise a brittle alloy, element, or compound and may comprise one or more intermetallic compounds.

(10) The brazing alloy wire may be provided in the form of a ring or shape adapted to fit in a groove in one or more of pieces to be brazed.

(11) The furnace brazing may be vacuum brazing.

(12) For furnace brazing, typical wire diameters are 0.010 to 0.040 inch [0.25-1 mm] but sizes outside this range are usable.

(13) The low ductility alloy compositions may be low ductility alloys comprising one or more of the following compositions, which are based on compositions shown as brazing alloys in the AWS Brazing Handbook as being brazing filler metals, but restricted to those alloys that are of low ductility.

(14) The present invention is not restricted to these alloys [which are for example only] and the present invention is predicted to make new alloy compositions feasible.

(15) Typical Alloy Compositions.

(16) Aluminum Based Alloys:

(17) Low ductility alloy compositions comprising: 59.5-97.5% Al and one or more of (0-20)% Si (0-10)% Cu (0-5)% Mg (0-2)% Bi (0-1)% Fe (0-0.5)% Zn (0-0.5)% Mn (0-0.5)% Cr (0-0.5)% Ti in which Al+Si+Cu+Mg+Bi+Fe+Zn+Mn+Cr+Ti99.5% balance impurities.

(18) Magnesium Based Alloys:

(19) Low ductility alloy compositions comprising: 67.5-99% Mg and one or more of (0-20)% Al (0-5)% Zn (0-5)% Mn (0-1)% Cu (0-1)% Si in which Mg+Al+Zn+Mn+Cu+Si99.5% balance impurities.

(20) Copper Based Alloys

(21) Low ductility alloy compositions comprising: 43.5-97.6% Cu and one or more of (0-35)% Ag (0-10)% P (0-10)% Sn (0-1)% Si in which Cu+Ag+P+Sn+Si99.5% balance impurities

(22) Low ductility alloy compositions comprising: 97.6% Cu and one or more of (0-35)% Ag (0-35)% Zn (0-20)% Cd (0-5)% Ni (0-5)% Mn in which CuAg CuZn; and Cu+Ag+Zn+Cd+Ni+Mn99.5% balance impurities

(23) Low ductility alloy compositions comprising: 97.6% Cu and one or more of (0-40)% Au (0-20)% Ni (0-20)% Pd (0-20)% Mn (0-5)% Ti in which CuAu CuNi CuPd CuMn and Cu+Au+Ni+Pd+Mn+Ti99.5% balance impurities

(24) Silver Based Alloys:

(25) Low ductility alloy compositions comprising: 90% Ag and one or more of (0-35)% Cu (0-30)% Zn (0-25)% Cd (0-15)% Sn (0-10)% Mn (0-10)% Ni (0-1)% Li in which AgCu AgZn AgCd and Ag+Cu+Zn+Cd+Sn+Mn+Ni+Li99. 5% balance impurities

(26) Low ductility alloy compositions comprising: 90% Ag and one or more of (0-50)% Cu (0-25)% Pd (0-15)% In (0-15)% Sn (0-1)% Ni in which AgCu AgPd In+Sn2% optionally In+Sn11% and Ag+Cu+Pd+In+Sn+Ni99.5% balance impurities

(27) Gold Based Alloys:

(28) Low ductility alloy compositions comprising: 99% Au and one or more of (0-30)% Ni (0-30)% Pd (0-20)% Mn (0-10)% Cr (0-10)% Ag (0-5)% B (0-5)% Ge (0-5)% P (0-5)% Si (0-5)% Ti (0-5)% Y in which AuNi AuPd AuMn B+Ge+P+Si+Ti0.5% and Au+Ni+Pd+Mn+Cr+Ag+B+Ge+P+Si+Ti+Y99.5 balance impurities

(29) Low ductility alloy compositions comprising: 99% Au and one or more of (0-50)% Cu (0-20)% Ag (0-20)% Sn (0-15)% In (0-15)% Ge in which AuCu Au>Ag Au>Sn Sn+In+Ge0.5% in which Au+Cu+Ag+Sn+In+Ge99.5% balance impurities.

(30) Palladium Based Alloys:

(31) Low ductility alloy compositions comprising: 99% Pd and one or more of (0-50)% Co (0-25)% Cr (0-10)% Si (0-5)% W (0-5)% Cu (0-5)% Fe (0-5)% Mo (0-5)% Nb (0-1)% B in which PdCo PdCr PdSi Si+B0.5%, optionally 5% Pd+Co+Cr+Si+W+Cu+Fe+Mo+Nb+B99.5% balance impurities.

(32) Low ductility alloy compositions comprising: 99% Pd and one or more of (0-50)% Ni (0-25)% Cr (0-10)% Si (0-5)% W (0-5)% Cu (0-5)% Fe (0-5)% Mo (0-5)% Nb (0-1)% B in which PdNi PdCr PdSi Si+B0.5%, optionally 5% Pd+Ni+Cr+Si+W+Cu+Fe+Mo+Nb+B99.5% balance impurities.

(33) Nickel Based Alloys:

(34) Low ductility alloy compositions comprising: 98% Ni and one or more of (0-40%)% Pd (0-30)% Cr (0-25)% Mn (0-20)% W (0-15)% Si (0-15)% P (0-5)% B (0-5)% Cu (0-5)% Fe (0-5)% Mo (0-5)% Nb in which NiPd NiCr NiMn NiW NiSi NiP B+Si+P0.5%, optionally 5% Pd+Ni+Cr+Mn+W+Si+P+B+Cu+Fe+Mo+Nb99.5% balance impurities.

(35) Cobalt Based Alloys:

(36) Low ductility alloy compositions comprising: 99% Co and one or more of (0-30)% Cr (0-25)% Mn (0-20)% W (0-15)% Si (0-15)% P (0-5)% B (0-5)% Cu (0-5)% Fe (0-5)% Mo (0-5)% Nb in which Co>Cr Co>Mn Co>W Co>Si Co>P B+Si+P0.5%, optionally 5% Co+Cr+Mn+W+Si+P+B+Cu+Fe+Mo+Nb99.5% balance impurities.

(37) The following are typical examples of the invention.

EXAMPLE 1

(38) As mentioned above, a AuSn alloy will potentially become brittle with a 1% addition of Sn due to the formation of a gold rich intermetallic (Au10Sn) phase which consumes a large fraction of the gold (the ductile phase) to form the brittle intermetallic phase.

(39) A typical gold-tin alloy might have a composition of 85 wt % Au 15 wt. % Sn and would not be ductile enough to be formed into a wire. For this AuSn alloy an intermetallic powder may be produced [for example an AuSn.sub.2 intermetallic] and combined with a gold rich ductile material to produce a composite of the intermetallic and the ductile phase.

(40) For example the AuSn.sub.2 intermetallic is encapsulated into an Au sheath in proportions to produce a composite having the overall 85 wt % Au 15 wt. % Sn composition. This composite may then be drawn using any conventional method and apparatus to produce 85 wt % Au 15 wt. % Sn wires. The Au will be the predominant phase and hence maintain sufficient ductility for the forming process.

EXAMPLE 2

(41) Consider a Au92-Sn8 wt. % alloy.

(42) Under equilibrium conditions of slow solidification encountered in conventional gravity casting, the solidified as-cast alloy will consist of a solid solution of Sn in Au () containing about 2 wt. % Sn and an intermetallic compound Au.sub.5Sn ().

(43) Even though Sn distribution is expected to be rather uniform within the alloy material, the relative volume fractions of the two components (see 2.sup.nd line of Table 1) are such that the alloy will be inherently brittle.

(44) A possible solution for obtaining a ductile material would be designing a cored structure in which a Sn powder is cored into an Au sheath for subsequent wire drawing operations. It is anticipated that such structures will produce a workable product, but perhaps one with a non-uniform distribution of Sn given the low volume fraction of Sn (see 3.sup.rd line of Table 1).

(45) Alternatively, one can design a two component alloy such that both good Sn distribution and improved ductility can be accomplished. For this particular case, a cored structure consisting of a soft Au sheath and an equiatomic compound of Au and Sn such as the intermetallic AuSn () is anticipated to produce a relatively ductile feedstock for subsequent drawing operation with a substantially improved distribution of Sn based on the relative volume fraction of the two compounds (see 4.sup.th line of Table 1)

(46) TABLE-US-00001 TABLE 1 Volume fractions of material structure constituents Structure f.sub.v.sup. f.sub.v.sup. f.sub.v.sup.Sn f.sub.v.sup.Au f.sub.v.sup. As-cast (/) 29% 71% / / / Structured (Au/Sn) / / 12% 88% / Structured (Au/) / / / 69% 31%

(47) As can be seen, in both cases shown as the 3.sup.rd and 4.sup.th lines of Table 1, the ductile component Au predominates. Composite products of such composition should be sufficiently ductile to be worked into a wire.

EXAMPLE 3

(48) For applications such as nozzles for industrial gas turbines, typically Au70/Ni22/Pd 8 alloys are used as having suitable ductility.

(49) The present invention would allow low gold, or even no gold, low ductility alloys to be used, for example Ni57.1/Pd30/Cr10.5/82.4. Such an alloy could, for example, be produced as a composite of a Pd/Ni rich phase for ductility with the Cr and B in a low ductility phase. For a given composition, different mixtures of low ductility and ductile phases may be used, as has been demonstrated above for AuSn alloys.

(50) For similar compositions, with lower Cr content, a Pd/Cr rich phase could form the ductile phase, with a NiB brittle intermetallic phase forming the low ductility phase.

(51) The present invention is not restricted to the above alloys, which are merely representative of the sort of low ductility composition to which this invention may be applied.

(52) Manufacturing Methods

(53) The following disclose specific exemplary methods of making cored wires, but is not restricted thereto.

(54) A cored wire of up to 0.063 diameter can be produced by the means of roll forming equipment or the same. This equipment folds at room temperature a strip of ductile alloy component to form a sheath that encapsulates the brittle powder component or a ductile powder component that has a high propensity to react with the other component (such as in the case of AuSn) to form a brittle alloy.

(55) Further wire size reductions can be accomplished with previously cited wire drawing equipment.

(56) Alternatively, the core powder constituent can be encapsulated in a container (which may be cylindrical) consisting of the drawable ductile material, evacuated at room temperature and sealed to provide a larger feedstock workpiece for swaging or drawing mill operations and further wire drawing processing.

(57) For example, as shown in FIG. 3, a starting preform 6 comprising a ductile sheath 7 may consist of a tube open at one end and filled with a core powder material 8 constituent of the brazing alloy. Complete encapsulation of the powder is accomplished with a plug 9 comprising an exit stem tube 10 joined with the sheath by means of a weld or a brazed joint to form a vacuum tight seal 11.

(58) The alloy composition and wall thickness of the sheath component are determined by the composition and the packing density of the core powder component as exemplified below. The sheath can be made by any suitable method. For example, a wrought cast bar may be prepared by vacuum induction melting of a charge of alloy elements weighed to meet the nominal composition of the sheath. For instance, a NiPdCr melt charge may comprise electrolytic nickel pieces, palladium shots and chromium flake of 99.95% purity or higher. Melting may be performed in a zirconia or alumina crucible, although zirconia is a preferred (but not essential) refractory material for melting chromium containing alloys.

(59) The sheath inner channel may be formed by gun drilling or alternatively by sink electric discharge machining (EDM); however the first method is preferable for higher recovery of precious metal containing chips.

(60) FIG. 4 shows an apparatus for degassing the core powder component in the sheath. Without degassing, adsorbed gases on powder particles surface may evolve upon processing into blisters that could adversely affect the integrity of the consolidated powder body. Degassing while the powder is in the sheath reduces the risk of re-adsorption of gases onto the powder. Degassing comprises heating the powder while under an appropriate vacuum.

(61) As shown in FIG. 4, the starting preform 6 is connected via the exit stem tube 10 to a vacuum apparatus 12. The vacuum apparatus 12 may comprise a mechanical pump 13, diffusion pump 14, and with appropriate valves [roughing valve 15, backing valve 16, and high vacuum valve 17] to permit control of the vacuum. Both a mechanical pump and diffusion pump are shown, but for some materials a mechanical pump alone may provide sufficient vacuum.

(62) The preform 6 is housed in a heater 19, which may be of any type appropriate to provide the desired degassing temperature.

(63) The application of heat to the capsule is aimed at accelerating the degassing operation and forcing desorption of gas species. Typically, heating to 0300-600 C. range is desirable to thermally activate the desorption process. Typically evacuation of the encapsulated powder is carried out until a suitable vacuum is reached [e.g. 1.3 Pa (about 0.01 torr) at a leak rate of no more than 13 Pa (about 0.1 torr) per minute]. At this point the degassing process is typically complete and the stem tube may be crimped to keep the encapsulated powder under vacuum. [A like degassing procedure can be applied to manufacture of composite wires as described above].

(64) FIG. 5 shows schematically the steps of a wire forming process that may be used to reduce the sealed preform 6 into a wire of less than 1 mm diameter size. First reductions of the preform by 22% of area size are accomplished by swaging 20. The radial compression associated with swaging process helps in preventing damage to the vacuum seal 11 and promotes optimal packing and homogeneous distribution of the powder component within the sheath 7. Swaging dies of incrementally decreasing sizes are used for each reduction at a given die size.

(65) Annealing 21 can be either performed after completion of the forming process step or at any given stage during forming depending on the rate of strain hardening of the sheath material. Excessive strain hardening reduces formability of the material and may cause premature rupture of the sheath 7. Annealing is preferably conducted for sufficient time at a temperature at which full recovery from imparted cold work in the sheath material is achieved.

(66) Further reduction steps may be accomplished by roll drawing 22. In this process, the wire is drawn between two rolls within a groove of the desired size. The forming stress is predominantly compressive with a minor tensile component. This stress condition prevents tearing of wall of the sheath 7 and promotes homogeneous flow of the powder which in turn prevents necking of the wire. This step may account for a 40% area reduction.

(67) A further area reduction [e.g. 38%] may be carried out on a deep drawing bench 23. As the deformation mechanism in deep drawing proceeds essentially by axial tension, care needs to be exercised not to stretch the sheath 7 beyond its rupture strength. In general it is recommended to proceed with small deformations and die size reduction increments while performing stress relieving anneals more often. Due to the higher risk of wire rupture encountered in deep drawing, this process is usually performed with sufficient lubrication with commercially available lubricants, for example those consisting of water soluble sodium sulfonates mixed with fatty oils.

(68) The invention is not limited to the specific steps and specific order shown schematically in FIG. 5 but encompasses any method of reducing a preform down to the brazing alloy wire of the present invention. Like steps may be performed on homogeneous composite wires as on cored wires.

PROSPECTIVE EXAMPLE 4

(69) The applicants believe the alloy of Example 3 (Ni57.1-Pd30.0-Cr10.5-B2.4) wt. % can be produced as a 1 mm (0.04) or less cored wire by the above described methods.

(70) The powder component used would be CrB.sub.2, chromium diboride, which is readily available and can be found in purities of 99.9% or higher. However, the particles of commercially available CrB.sub.2, tend to be rough and jagged in form, resulting in particle agglomeration during the drawing process leading to rupture of the sheath. To reduce the risk of this happening, the CrB.sub.2 may be spheroidized, for example spheroidized by plasma spraying either as particles or agglomerates of particles, to form more rounded particles. The particles should be selected to have a size distribution in which 25% by weight or less comprise particles less than 25 m in size. Preferably less than 5% by weight comprises particles less than 20 m in size.

(71) Spheroidization by plasma spraying is a process in which the particles are injected into a plasma and undergo in-flight heating and melting [in whole or in part] followed by cooling and solidification. The particles produced by this process tend to be spherical or rounded in form.

(72) Other methods of producing spherical or rounded particles can be used [e.g. gas atomization]. An appropriate alloy composition for the sheath was calculated as Ni62.18-Pd32.67-Cr5.15 wt. %.

(73) A measured packing density for CrB.sub.2 powder was 2.76 g/cm.sup.3, which yielded a wall thickness of 7.34 mm (0.289) for a sheath external diameter of 28.58 mm (1.125).

(74) The calculation for this example was as follows. Similar calculations apply to other examples and compositions.

(75) Calculation of Required Sheath Alloy Composition

(76) TABLE-US-00002 Ni Pd Cr B Desired composition 57.1 30 10.5 2.4 Amount contributed by CrB.sub.2 5.77 2.4 Balance required in sheath 57.1 30 4.73 0 Alloy composition of sheath 62.18 32.67 5.15 0 (normalised to 100%)

(77) Calculation of Sheath Thickness

(78) FIG. 6 shows a section of a cylindrical tubular preform comprising a sheath having an inside radius r.sub.ins and outside radius r.sub.out.

(79) The weight of sheath per unit length w.sub.s is given by:
w.sub.s=(r.sub.out.sup.2r.sub.ins.sup.2).sub.s

(80) where .sub.s is the theoretical density of the sheath material.

(81) The weight of powder constituent per unit length w.sub.p is given by:
w.sub.p=r.sub.ins.sup.2.sub.p

(82) where .sub.p is the packing density of the sheath material

(83) The ratio of the weight of sheath per unit length to the weight of powder constituent per unit length is the same as the ratio of the weight fraction of sheath constituent f.sup.s.sub.w to the weight fraction of powder constituent f.sup.p.sub.w

(84) $\frac{w_s}{w_p}=\frac{\left(r_{\mathrm{out}}^2-r_{\mathrm{ins}}^2\right){}_s}{r_{\mathrm{ins}}^2{}_p}=\frac{f_w^s}{f_w^p}$

(85) Which can be solved to determine the inside radius r.sub.ins required to obtain desired weight fractions of the core and the sheath

(86) $r_{\mathrm{ins}}=\sqrt{\frac{f_w^p{}_s}{f_w^s{}_p+f_w^p{}_s}}{r}_{\mathrm{out}}.$

EXAMPLE 4a

(87) It was found that when making such a composition with a 26.52 mm (1.044) outer diameter sheath of wall thickness 6.096 mm (0.24) comprising 63.5 wt % Ni and 34.5 wt % Pd and with a filling comprising plasma spherodized Cr-18.9B wt. % powder, problems arose with cracking of the sheath is despite the thick sheath wall permitted by this formulation. This was attributed to suspected formation of an ordered phase in the sheath rather than a continuous (Ni,Pd) solid solution. For this reason, for such compositions, and where the sheath comprises predominantly Ni and Pd, the amount of Pd should preferably be at least 30 at % (for pure NiPd equivalent to 43.7 wt %).

PROSPECTIVE EXAMPLE 5

(88) The applicants believe an alloy comprising (Ni50.0-Pd36.0-Cr10.5-83.0-Si0.5) wt. % may be produced as a 1 mm (0.04) or less cored wire by the above described methods.

(89) The powder component used in this example would be a blend of 91.4 wt % CrB.sub.2 chromium diboride and 8.6 wt % CrSi.sub.2 chromium disilicide, both readily available and which can be found in purities of 99.9% or higher. As with Example 4, the particles of the powder should be selected to have a size distribution in which 25% by weight or less comprise particles less than 25 m in size, preferably comprising less than 5% by weight of particles less than 20 m in size, and may be spheroidized if too jagged in form.

(90) The calculated alloy composition for the sheath was determined to be Ni56.29-Pd40.53-Cr3.18 wt. %.

(91) The measured packing density for CrB.sub.2 and CrSi.sub.2 powder blend was 2.75 g/cm.sup.3, which yields a wall thickness of 6.32 mm (0.249) at for a sheath external diameter of 28.58 mm (1.125).

EXAMPLE 6

(92) An alternative alloy comprising (Ni50.0-Pd36.0-Cr10.5-83.0-Si0.5) wt. % (same composition as Example 5) was produced as a 0.889 mm (0.037) cored wire by the above described methods.

(93) The powder component used in this example is a gas atomized Ni46.26-Cr40.30-1311.52-Si1.92 wt. % powder prepared with electrolytic nickel pieces, and chromium flake of 99.95% purity or higher.

(94) Boron melt stock is supplied in pellets 95% pure in which most impurities are volatile elements that were readily eliminated during vacuum induction melting prior to atomization. Silicon was supplied in high purity lump form. The resulting purity of the gas atomized powder was about 99.97%. The powder produced was a 100 mesh powder [90% by weight having a particle size less than 149 m] which was highly spherical with a small fraction of fine particles (particles less than 25 m constituting less than 25% by weight). It has a liquidus temperature of about 1788 C. (3250 F.).

(95) The calculated alloy composition for the sheath was determined to be Ni51.32-Pd48.68 wt. %.

(96) Measured packing density for the gas atomized Ni46.26-Cr40.30-1311.52-Si1.92 wt. % powder was 6.21 g/cm.sup.3, which yields a wall thickness of 4.13 mm (0.1625) at for a sheath external diameter of 28.58 mm (1.125).

(97) Stress relief anneals were conducted at about 788 C. (1450 F.) to soften the sheath between the reduction steps in the wire drawing process. No evidence was found of sintering of the core.

(98) Based on the stress relief temperature that was found to be effective for softening the NiPd(Cr) sheath compositions of Examples 4 and 5, all NiCrB powder compositions having a liquidus temperature equal to or higher than about 1593 C. (2900 F.) should not sinter. These compositions include, for example: a. NiB (0Ni30 at. %) b. CrB (0Cr82 at. %) c. NiCrB (45B70 at. %; 0Cr10 at. %; 30Ni45 at. %) d. NiCrB (18B45 at. %; 10Cr82 at. %; 0Ni45 at. %)

EXAMPLE 6a

(99) An alloy of the same composition as Examples 5 and 6 was produced as a 0.762 mm (0.030) cored wire by the above described methods using a 28.58 mm (1.125) outer diameter sheath having a 4.06 mm (0.16) thick wall comprising 51.5 wt % Pd and 48.5% nickel filled with 100 mesh gas atomized powder of composition in wt % Ni 46.2%,Cr 40.4%, B 11.5%, Si 1.9%.

PROSPECTIVE EXAMPLE 7

(100) Many other cored wire alloy designs are possible and not limited to the examples listed in this disclosure. In particular, one preferred alternative would be to use a nickel bar of the required purity (99.9% or higher) and formulate a single pre-alloyed powder or a blend of alloy powders of suitable composition to obtain the desired alloy for the cored wire. This approach minimizes the amount of loss of other elements in the drilling process, which is particularly important when the overall alloy composition comprises precious metals.

EXAMPLE 8

(101) A 6.35 mm (0.25) outer diameter 0.89 mm(0.035) thick wall nickel tube filled with a NiCrBSi high melting temperature alloy gas atomized powder was drawn through a single swagger pass and a sequence of deep drawing steps with alternating anneals at 1350 C. down to 0.76 mm (0.030) wire diameter size. This can enable the manufacture of brittle alloys wires such as: Icronibsi-7 [nominal composition by weight: 7.0% Cr, 4.5% Si, 3.2% B, 3.0% Fe, balance Ni]; Nibsi-4 [nominal composition by weight: 4.5% Si, 3.2% B, balance Ni], Nibco-4, [nominal composition by weight: 20.0% Co, 4.0% Si, 2.7% B, balance Ni] Nibsi-M, [nominal composition by weight: 5.6% Si, 1.6% B, balance Ni] Icronibsi-14 [nominal composition by weight: Cr 14.0%, Si 4.5%, B 3.2%, Fe 4.5%, balance Ni]; and Nicro-B [nominal composition by weight: Cr 15.0%, 4% B, balance Ni].

(102) As mentioned above, the present invention requires that the particles have a size distribution in which 25% by weight or less comprise particles less than 25 m in size, preferably comprising less than 5% by weight of particles less than 20um in size. It is of course required that the particles are not so large as to disrupt the formation of the wire. Accordingly it is required that the core powder does not comprise particles greater than 75% the diameter of the wire, preferably less than 50% the diameter of the wire. Typically an upper bound for the particles is 300 m [e.g. 50 mesh (about 297 m)] with smaller upper limits [e.g <250 m, <200 m, <150 m] being preferred.

(103) Forming Shapes

(104) The wires can be bent to shape [e.g. rings] using conventional methods and if necessary annealed before use.

(105) By the methods disclosed resilient wires and shapes can be formed of brazing alloys that conventionally would lack required resilience. The person skilled in the art will readily see variants of the invention described, and all such variants are intended to be covered by the invention.