Brazing alloy

Two or more metal or alloy parts (such as catalytic convertor parts) are brazed by melting a homogeneous nickel-based brazing filler alloy containing boron in an amount of not more than 0.2% by weight; converting the molten alloy to a powder of uniform composition; and using the powder as a brazing filler alloy to braze a metal or alloy part to a further metal or alloy part. Typically, the parts are of stainless steel with a relatively high aluminum content; at least one of the parts is generally in the form of a thin foil.

BACKGROUND OF THE INVENTION
 The present invention is concerned with a method of brazing together two or
 more metal or alloy parts.
 Brazing is a process in which a metal or alloy of lower melting range is
 used to join parts made from an alloy of metal of a higher melting range.
 Nickel-based brazing filler alloys are well known and are used to join
 parts of alloys from stainless steel to much more refractory alloys such
 as Nimonics, Inconels and the like. These brazing filler alloys are
 commonly supplied as powders. They melt generally in the range from
 880.degree. C. to 1200.degree. C., individual alloy compositions having
 solidus-liquidus ranges determined by melting point depressants such as
 phosphorus, boron and silicon. Combinations of boron and silicon are used
 in several alloys such as BS 1845 HTN1, HTN2 etc, the boron content being
 relatively high and in excess of 1%. These are reliably manufactured by
 charging to a melting furnace a formulation having the desired final
 composition which, when molten, is converted directly to powder by
 atomizing to provide appropriate melting characteristics without blending
 of the powder.
 GB-A-1547117 discloses nickel-based brazing alloys for use in brazing or
 diffusion welding of metal or alloy joints, of, for example, carbon steel,
 nickel-based alloys, copper alloys, and stainless steel. The brazing
 alloys disclosed contain boron in an amount of 0.6% to 1.8% by weight. If
 the brazing alloys of GB-A-1547117 were to be used for the purpose of
 brazing thin foils of stainless steel, erosion of the relevant joint can
 result.
 GB-A-2279363 discloses a metallic coating material for use in protecting
 the surface of a substrate of a copper-based alloy against high
 temperature corrosion and erosion. The document, however, does not teach
 or suggest the use of the metallic material as a brazing filler for
 brazing a metal or alloy part to another metal or alloy part. The high
 proportion (0.5% to 4.0%) of boron present, would in any case, make the
 material unsuitable for brazing together thin stainless steel parts, for
 the reasons outlined above.
 The highest melting nickel-based brazing filler alloys are those which use
 only silicon as the melting point depressant, e.g. BS1845 HTN5, mid-range
 composition of 19% chromium, 10.5% silicon, balance nickel. They offer
 particularly useful properties in the brazing of parts for very high
 temperature applications. However their inherently higher brazing
 temperatures can impair the selection of optimum brazing conditions for a
 particular choice of parent metal and design of component, or increase
 furnace maintenance costs.
 In order to overcome this problem, it is known to blend to these alloys up
 to 10% of a nickel-based brazing filler alloy of a lower melting range to
 facilitate the onset of the brazing process, especially where brazing
 alloy flow in to be on parent metal substrates having higher refractory
 surfaces. Such substrates may be, for example, iron alloys containing
 chromium and aluminum as used in the manufacture of metallic catalyst
 supports for cars, and also in honeycomb seals in gas turbines.
 Conveniently this brazing filler alloy of a lower melting range contains
 boron as at least one of the melting point depressants present. After
 assisting early braze flow, the boron then diffuses into the substrates
 leaving the final re-melt temperature of the brazing filler alloy
 unaffected by any further depression in melting temperature which might
 otherwise be caused by the presence of boron. A convenient level of boron
 in the lower melting range alloy is 1% since this then results in a level
 of 0.1% in the resultant blend. The blend route to such a low boron
 content has been formerly accepted as the normal method for several
 reasons.
 These include simple modification of a standard powder at the end of
 routine manufacture, in this case to BS1845 HTN5, to produce relatively
 small quantities of a special grade by blending. The manufacturing control
 required to produce a 1% boron alloy powder to within a given
 proportionate tolerance is also less demanding than a direct melting route
 producing 0.1%. In practice a blend constituent may be chemically analyzed
 before blending and blend ratios modified to compensate for normal
 tolerances of manufacture. For example, if the lower melting powder was
 found to contain 0.9% instead of 1% boron then the blend ratio could be
 modified from 10% to 11% to give the same final content of 0.1% boron. The
 same low melting range powder composition may be manufactured for other
 purposes, for example to blend with powders of other compositions, or for
 use as a product in its own right in the braze hardfacing process for
 which it was originally conceived.
 For these various reasons it has become standard industry practice to
 manufacture brazing filler alloys with low boron contents in the typical
 range 0.05-0.15% by blending.
 However, a disadvantage of blending two nickel based brazing filler alloy
 powders of different compositions is that these compositional differences
 from point to point will persist after melting in certain applications. In
 particular, when the brazing filler alloy is spread extremely thinly,
 which can for example involve essentially monolayers of powder particles,
 depending on the application. It can be shown in these circumstances that
 small scale statistical variations in homogeneity will lead to
 considerable localized variations in composition--in this case of boron
 concentration. Thus, the resultant film of motion filler metal immediately
 after melting is so thin that mixing does not occur on a scale sufficient
 to remove these compositional differences. This in turn leads to
 differences in brazing performances and therefore differences in brazing
 conditions and results. Sometimes the results can be very deleterious,
 with erosion of thin foils in the region of the brazed joint.
 OBJECTS OF THE INVENTION
 It is therefore an object of the present invention to provide an improved
 method of brazing two or more metal or alloy parts, with less deleterious
 compositional differences in the brazed joint.
 It is a further object of the invention to provide an improved method of
 brazing together metal or alloy parts, at least one of which being thin
 and/or of stainless steel.
 It is also an object of the present invention to provide a homogeneous
 brazing filler alloy having a relatively low boron concentration which may
 be used to effectively braze together thin stainless steel parts.
 It is a still further object of the present invention to provide a
 homogeneous powdered brazing filler alloy having a uniform composition
 which produces a substantially homogeneous brazed joint between metal or
 alloy parts.
 SUMMARY OF THE INVENTION
 These and other objects are solved according to the invention, in a first
 aspect, which comprises:
 (a) melting ingredients such as to provide a homogeneous molten
 nickel-based brazing filler alloy containing boron in an amount of not
 more than 0.2% by weight of the molten alloy;
 (b) converting the molten alloy to a powder of uniform composition;
 (c) interposing the resulting powder between a first metal or alloy part
 and a second metal or alloy part; and
 (d) melting the powder between the first part and the second part so as to
 braze the first part to the second part.
 DESCRIPTION OF PREFERRED EMBODIMENTS
 Typically, the brazing filler alloy is used to join one or more parts of
 stainless steel with a relatively high aluminum content (such as an alloy
 containing up to about 10% aluminum, up to 20% chromium, with the balance
 being iron, incidental ingredients and impurities). Such alloys are
 conventionally used for purposes such as metallic catalyst supports for
 vehicles.
 A high aluminum content in one or both of the metal or alloy parts to be
 brazed can restrict the brazing filler alloy flow, because stable
 superficial oxides form during heating in vacuum, which inhibit brazing. A
 boron concentration of not more than 0.2% by weight is sufficient to allow
 the boron to interact with and substantially break-up the superficial
 oxides which form. This amount of boron also assists flow in the presence
 of such oxides.
 It is disadvantageous to employ excess boron, because the excess can
 interact with thin foils of metals or alloys being brazed. This can lead
 to a non-uniform and weakened bond between the brazed parts. For this
 reason the amount present is not more than approximately 0.2% by weight.
 However, it is more preferred that the amount of boron should not exceed
 0.15% by weight.
 Preferably, the brazing filler alloy contains 12 to 20% chromium, and/or 8
 to 12% silicon, the balance consisting essentially of boron, nickel,
 incidental ingredients and impurities. A particularly preferred brazing
 filler alloy contains about 17% chromium and about 9.7% silicon.
 Preferably, the brazing filler alloy is substantially free of copper and/or
 manganese. That is, it is preferred that the amount of copper is less than
 0.2% and the amount of manganese is preferably less than 0.4% by weight.
 The brazing filler alloy is preferably solidified as a powder (in step (b)
 of the method according to the invention) by a process comprising
 conventional gas atomization.
 The macro melting and solidification behaviour of such brazing filler alloy
 powders is not significantly different from that of blended powders, but
 their micro compositional uniformity and hence micro melting and
 solidification behaviour are significantly improved. When, for example,
 the brazing filler alloy is used for a substantially linear joint between
 a thin foil of metal or alloy and a further part of metal or alloy
 (typically at a rate of about 0.1 milligram per millimeter length of one
 or more of the parts being brazed) the resulting brazed joint is
 significantly more homogeneous than the joints achieved by the prior art
 "blending" process. By "thin foil", I mean herein a thickness of less than
 100 microns (typically about 50 microns or even less).

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT
 Referring to FIG. 1, elements 1a and 1b forming part of a catalytic
 convertor are brazed to a curved metal surface 2 using as brazing filler
 alloy a thin layer of powder 3, made according to steps (a) and (b)
 outlined above. This brazing filler alloy will be referred to subsequently
 as "pre-melted", in order to distinguish from the prior art process in
 which the ingredients are mixed together during brazing.
 FIG. 1 shows the desirability of uniformity of the boron content in a thin
 layer of brazing filler alloy.
 The present invention will now be illustrated by description of the
 following exemplary embodiment (in which FIGS. 2 and 3 are described in
 more detail).
 EXAMPLE 1
 A parent metal was selected on which brazing filler alloy flow is sensitive
 to such filler alloy compositional variations; for example, 50 micron
 thick foil of a composition of approx. 12% chromium, 6% aluminum, the
 balance being iron. The relatively high aluminum content greatly restricts
 brazing filler alloy flow because stable superficial oxides form during
 vacuum brazing which inhibit wetting. Boron was therefore present in an
 amount to assist the flow.
 The samples of foil used in these tests were marked as to rolling
 direction, degreased and abraded in the length direction with 400 mesh
 silicon carbide paper to give a clean and reproducible surface condition.
 The foil was then cut into 30 mm square coupons.
 The brazing filler alloy selected had a composition of 17% chromium, 9.7%
 silicon, 0.1% boron, the balance being nickel.
 A first brazing filler alloy was manufactured by mixing 90% of a powdered
 nickel-based alloy containing 19% chromium, 10.5% silicon and 10% of a
 powdered nickel-based alloy containing 2.5% silicon and 1% boron. A 20 g
 sample was used in this experiment; the required tumbling time to produce
 a uniform blend of the two constituents was 3 minutes.
 A second brazing filler alloy was manufactured by pre-melting all
 constituents in a furnace to directly achieve a composition of 17%
 chromium and 9.7% silicon and 0.1% boron.
 In both cases the balance of the composition was nickel and small
 quantities of normally occurring impurities.
 All powders were produced from liquid melts by conventional gas
 atomization, the powder then being screened to give a size fraction for
 these tests of largely 45 microns to 106 microns.
 In order to stimulate the very small brazing filler alloy description rates
 typical of an industrial process, it was necessary to develop an
 appropriate application method for applying the brazing filler alloy
 powder. It was found that narrow lines of a suitable adhesive could be
 drawn on the parent metal foil using a draftsman's pen of the type which
 has an adjustable claw tip. Brazing filler alloy powder was then gently
 sifted on to this line of adhesive and the excess blown away, leaving a
 line of brazing filler alloy particles adhering to the foil. When the
 adhesive was dry, the brazing filler alloy powder deposit was ready for
 brazing. The direction of application of the line of brazing filler alloy
 was along the rolling direction, that is parallel with the subsequent
 direction of abrasion when the foil was prepared.
 This technique of two stage application in which the uptake of powder is
 limited by the location and amount of pre-applied adhesive is known among
 brazing experts as "pepperpotting".
 In order to achieve an essential monolayer of powder particles it was
 necessary to select binder characteristics with some care. A solution of a
 proprietary resin in a slow drying solvent (ethyl lactate) was adjusted to
 a viscosity of 800 cps to suit the particular type of pen in use. The
 resin was a type carefully selected to be fully fugitive--that is, on
 heating in vacuum during the early part of the subsequent vacuum brazing
 cycle, it completely volatilizes, leaving powder particles cohering in
 place by the weak forces known to exist between small particles and a
 substrate.
 A long drying solvent was necessary in order that the particles were drawn
 into intimate contact with the substrate as drying proceeded.
 It is known from prior investigations that this intimate contact is
 necessary in order that the filler alloy may ultimately wet the type of
 substrate when the temperature during heating in vacuum reaches and
 extends the melting range of the brazing filler alloy.
 It was found by tare weighing, reweighing when the samples were dry, and
 measurement of the length of the lines of brazing filler alloy deposited
 that the deposition rate was approximately 0.1 mg per mm length. In order
 to obtain two comparable samples of the blended powder and of the
 pre-melted powder, the deposition technique was repeated until two samples
 were obtained, one each of the blended and pre-melted powders, having
 deposition weights/mm within 10% of each other.
 The samples were then brazed according to the following conventional vacuum
 brazing cycle:
 Evacuate to less than 1.times.10.sup.-1 mbar
 Heat to 1000.degree. C. at a rate of 15.degree. C./min
 Hold at 1000.degree. C. for 10 min
 Heat to 1170.degree. C. at a rate of 10.degree. C./min
 Hold for 5 min
 Cool in vacuum to 900.degree. C.
 Inert gas cool to 100.degree. C. and discharge.
 The vacuum brazing furnace used was of a type used for brazing difficult to
 wet parent metals; the furnace was prepared for this trial by high
 temperature bake out to meet stringent conditions of internal cleanliness
 and in-leakage, as will be understood by experts in this field. After
 brazing the samples, microphotographs were taken at 75 times
 magnification.
 FIG. 2 shows that the spread of the brazing filler alloy is irregular,
 corresponding to the scale of the statistical variations predicted for the
 random variation in boron content. Approximately 10% of the total length
 exhibits abnormally restricted flow, while a less easily calculable
 proportion shows greater flow than average.
 FIG. 3 shows no such variation, and because the only difference between the
 specimen of FIG. 2 and that of FIG. 3 was the state of aggregation of
 boron (i.e. blended or pre-melted), it was concluded that this difference
 was responsible for the differences in uniformity of flow.
 Although the pepperpotting technique used for sample preparation did not
 feasibly permit deposition rates below 0.1 mgram/mm, it is possible that
 the associated industrial process may be improved to achieve yet lower
 deposition rates with benefits including but not limited to, economy in
 the use of brazing filler alloys. At such lower deposition rates a
 statistical analysis shows that the random variations between boron
 contents in such deposits of blended brazing filler alloy powder will
 become greater. The use of pre-melted brazing filler alloy powder
 eliminates such variations and permits full advantage to be taken of such
 industrial process improvements.