GB2325430A - Brazing method - Google Patents

Abstract

Two or more metal or alloy parts (such as catalytic convertor parts) are brazed by melting a nickel-based brazing filler alloy containing boron in an amount of 0.05 to 0.5% by weight; converting the molten alloy to a powder; and using the powder as a brazing filler 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

Brazing Method 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 8800C to 12000C, 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.

The highest melting nickel-based brazing filler alloys are those which use only silicon as the melting point depressent, 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 is to be on parent metal substrates having highly refractory surfaces. (For example, iron alloys containing chromium and aluminium 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 analysed 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 metal 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 metal 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 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 these compositional differences. This in turn results in differences in brazing performance leading to differences in brazing conditions and results.

It is therefore an object of the present invention to provide an improved method of brazing two or more parts with less compositional differences (which are harmful to brazing).

According to the present invention, there is provided a method of brazing two or more metal or alloy parts, said method comprising: (a) melting in a furnace ingredients such as to provide a molten nickel based brazing filler alloy containing boron in an amount of 0.05 to 0.58 by weight; (b) converting the molten alloy to a powder; and (c) using said powder as a brazing filler for brazing a metal or alloy part to a further metal or alloy part.

Typically, the brazing filler alloy is used to join 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. It is therefore desirable when brazing such metals or alloys, for boron to be present in low amounts. Boron assists flow in the presence of such oxides, but because of interaction at high temperatures between the alloys which are being brazed and excessive amounts of boron, it is for this reason that the amount present is not more than 0.5% and preferably should not exceed approx. 0.2%. However, it is more preferred that the amount of boron should not exceed 0.1%.

Preferably, the brazing filler alloy contains 12% to 20% chromium, 8% to 12% silicon, up to 0.1% boron, the balance being nickel, incidental ingredients and impurities. A particularly preferred brazing filler alloy contains about 17% chromium and about 9.78 silicon. 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.

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 used in a method according to the invention.

Referring to Figure 1, elements la 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 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).

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% 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, 9.78 silicon, 0.18 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 premelting 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 preapplied 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 104 mbar Heat to 10000C at a rate of 15 C/min Hold at 10000C for 10 min Heat to 11700C at a rate of 10 C/min Hold for 5 min Cool in vacuum to 9000C Inert gas cool to 1000C 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 premelted - 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 O.lmgram/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 (13)

CLAIMS:

1. A method of brazing two or more metal or alloy parts, said method comprising: (a) melting in a furnace ingredients such as to provide a molten nickel-based brazing filler alloy containing boron in an amount of 0.05 to 0.5% by weight; (b) converting the molten alloy to a powder; and (c) using said powder as a brazing filler for brazing a metal or alloy part to a further metal or alloy part.

2. A method according to claim 1, wherein said brazing filler alloy contains boron in an amount of 0.05 to 0.28 by weight.

3. A method according to claim 1, wherein said brazing filler alloy contains boron in an amount of 0.5 to 0.1% by weight.

4. A method according to claim 3, wherein said brazing filler alloy contains chromium in an amount of 12 to 20% by weight.

5. A method according to claim 3 or 4, wherein said brazing filler alloy contains silicon in an amount of 8% to 12% by weight.

6. A method according to claim 4, wherein said brazing filler alloy contains about 17% by weight of chromium.

7. A method according to claim 5, wherein said brazing filler alloy contains about 9.7% by weight of silicon.

8. A method according to any preceding claim, wherein said alloy is converted to said powder by gas atomisation.

9. A method according to any preceding claim, wherein the parts are of stainless steel containing aluminium.

10. A method according to claim 9, wherein said stainless steel parts contain up to 10% by weight of aluminium.

11. A method according to claim 9 or 10, wherein said stainless steel parts contain up to 20% by weight of chromium.

12. A method according to any preceding claim, wherein said parts comprise components of a catalytic convertor.

13. A method of brazing, substantially as described herein with reference to the Example.