Brazing method

Abstract: 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 aluminium content.

Description: 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 or 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°C to 1200°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 atomising 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. These alloys can offer particularly useful properties in the brazing of parts for very high temperature service. 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 is to be on parent metal substrates having highly refractory surfaces. Such substrates may be, for example, iron alloys containing chromium and aluminium as used in the manufacture of automobile metallic catalyst supports, 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 further depression in melting temperature which might otherwise be caused by the presence of boron. A convenient proportion of boron in the lower melting range alloy is 1% since this can result in a proportion 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 alloy 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 analysed before blending, and blend ratios may be modified, in order 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 typically in the range 0.05-0.15% by blending. However, a disadvantage of blending two nickel based brazing filler metal powders of different compositions is that these compositional differences from point to point will, in certain applications, persist after melting. In particular, this effect can happen when the brazing filler metal is spread extremely thinly, which can, for example, involve essentially monolayers of powder particles. It can be shown that in these circumstances, small scale statistical variations in homogeneity will lead to considerable localised variations in composition - in this case of boron concentration. Thus, the resultant film of molten filler metal immediately after melting is so thin that mixing does not occur on a scale sufficient to remove the compositional differences mentioned previously. This in turn results in differences in brazing performance leading to 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. It is therefore the purpose 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 also a purpose 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 a purpose of the present invention to provide an homogeneous brazing filler alloy having a relatively low boron concentration which may effectively braze together thin stainless steel parts. It is also a purpose of the present invention to provide a homogenous powdered brazing filler alloy having a uniform composition which produces a substantially homogenous brazed joint between metal or alloy parts. A known method of brazing two or more metal or alloy parts (such as that described in the abovementioned GB-A-1547117) broadly comprises the following steps: (a) melting ingredients such as to provide a homogeneous molten nickel-based brazing filler alloy containing boron; (b) converting the molten alloy to a powder; and (c) using the powder for brazing a metal or alloy part to a further metal or alloy part. According to the invention, the boron is present in the molten brazing filler alloy and therefore in the powder used as a brazing filler alloy in an amount of not more than 0.2% by weight, and the powdered alloy is of uniform composition. Typically, the brazing filler alloy is used to join one or more parts of stainless steel with a relatively high aluminium content (such as an alloy containing up to about 10% aluminium, 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 aluminium content in the metals or alloys 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% (typically about 17%) chromium, and/or 8% to 12% (typically about 9.7%) silicon, the balance being the boron, the nickel, incidental ingredients and impurities. 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 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 atomisation. 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 substantially linear joints between a thin foil and a further metal or alloy part (typically at a rate of about 0.1 milligram per millimetre 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 "blending" process. By "thin foil", we mean herein a thickness of less than 100 micrometres (typically about 50 micrometres or even less). An illustrative embodiment of the method according to the invention will now be described, with reference to the accompanying drawings, in which: Figure 1 is a schematic illustration showing joining of metal parts according to the invention; Figure 2 shows the spread of a (prior art) blended powder braze deposit; and Figure 3 shows the spread of a pre-melted braze deposit obtained in a method according to the invention. Referring to Figure 1, elements 1a and 1b forming part of a catalytic convertor are brazed to a curved metal surface 2 using as brazing filler a thin layer of powder 3, made according to steps (a) and (b) outlined above. This brazing filler 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. Figure 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 Figures 2 and 3 are described in more detail, and in which all percentages are by weight). Example A parent metal was selected on which brazing filler alloy flow is sensitive to filler alloy compositional variations; for example, 50 micron thick foil of a composition of approx. 12% chromium, 6% aluminium, the balance being iron. The relatively high aluminium 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 30mm 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, with 10% of a powdered nickel-based alloy containing 2.5% silicon and 1% boron. A 20g 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 simulate the very small brazing filler alloy deposition 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 draughtsman'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 800cps 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 volatilises, 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 exceeds 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 x 10 -4 mbar Heat to 1000°C at a rate of 15°C/min Hold at 1000°C for 10 min Heat to 1170°C at a rate of 10°C/min Hold for 5 min Cool in vacuum to 900°C Inert gas cool to 100°C and discharge. The vacuum brazing furnace used was of a type maintained for brazing difficult to wet parent metals and in particular was prepared for this trial by high temperature bake out to meet stringent conditions of internal cleanliness and inleakage, as will be understood by experts in this field. After brazing the samples, macro photographs were taken at 75 times magnification. Figure 2 shows that the spread of the filler metal is irregular corresponding with the scale of the statistical variations predicted for the random variation in boron content. Approximately 10% of the total length has abnormally restricted flow and a less easily calculable proportion shows greater flow than average. Figure 3 shows no such variation and because the only difference between the specimen of Figure 2 and that of Figure 3, was the state of aggregation of boron - i.e. blended or pre-melted - it was concluded that this was responsible for the differences in uniformity of flow. Although the pepperpotting technique used for sample preparation did not feasibly permit deposition rates below 0.1mgram/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.

Claims: A method of brazing two or more metal or alloy parts, said method comprising: (a) melting ingredients such as to provide a homogeneous molten nickel-based brazing filler alloy containing boron; (b) converting the molten alloy to a powder; and (c) using said powder for brazing a metal or alloy part to a further metal or alloy part; characterized in that said boron is present in said molten brazing filler alloy in an amount of not more than 0.2% by weight such that said powder is of uniform composition and contains boron in said amount. A method according to claim 1, wherein said molten brazing filler alloy contains boron in an amount of 0.05% to 0.15% by weight. A method according to claim 1 or 2, wherein said brazing filler alloy contains chromium in an amount of 12% to 20%, for example 17% by weight. A method according to any preceding claim, wherein said brazing filler alloy contains silicon in an amount of 8% to 12% for example 9.7% by weight. A method according to any preceding claim, wherein said brazing filler alloy contains substantially no copper and/or substantially no manganese. A method according to any of claims 1 to 5, wherein at least one of said metal or alloy parts has a thickness of less than 100 micrometres. A method according to any of claims 1 to 6, wherein said brazing is performed by applying said brazing filler alloy to a zone on a surface of at least one of said parts at a rate of not more than 0.1 milligrams of said alloy per millimetre length of said zone. A method according to any preceding claim, wherein at least one of the parts is of stainless steel containing aluminium, for instance up to 10% by weight of aluminium. A method according to claim 8, wherein said stainless steel parts contains up to 20% by weight of chromium and/or wherein said parts comprise components of a catalytic convertor. A powdered nickel-based brazing filler alloy of substantially uniform composition and consisting of: (a) boron in an amount of not more than 0.05 to 0.2% by weight; (b) chromium in an amount of 12% to 20% by weight; (c) silicon in an amount of 8% to 12% by weight; (d) substantially no copper; (e) substantially no manganese; and (f) the balance being nickel, incidental ingredients and impurities.

Inventors: ALLNATT NORMAN, ALLNATT, NORMAN
Applicants: WALL COLMONOY LTD, WALL COLMONOY LIMITED
Application year: 1998
Main classification: B23K35
IPC codes: B23K1, B23K35, C22C19