Brazing in aerospace application 2

[gfbotha]

We are considering attaching a large number of relative small (steel?) lugs onto a highly stressed thin walled marage steel tube using a brazing process (Ag, Cu, …. brazing fillers). It is a critical aerospace application and we are concerned about possible corrosion/embrittlement problems.

We consider doing the brazing all at once heating the assembly in an oven (lugs held in place by a jig). The assembly will be painted and exposed to the open environment. One supplier of brazing fillers claims they can supply filler material formulated to minimize corrosion at the interface and also that we might get away without using any flux. Strength-wise the concept seems OK. The assembly will be exposed to vibration and temperatures of say 400°C for a few seconds during once-off operation.

Would highly appreciate any thoughts or experience to share.

Regards

 

[tomwalz]

It’s possible.

I would need to see drawings and specifications to have any idea as to how good an idea it is.

Braze alloys are specified by AWS and generally allow tolerances of 1% plus or minus. A few ppm in an alloy might not mean anything in normal use but could be critical in your application.

We get ppm levels of zero or very close to it for tool brazing so it would be a reasonable spec for you.

The steel will be effected by brazing temps but post braze tempering can readjust that somewhat.

Fixturing for brazing can be tricky. Everything grows and moves between ambient and 600C.

What sort of corrosive environment?

Ag, Cu braze filler metals (AWS Bag class) typically contain zinc which makes them unsuitable for use in vacuum furnaces which limits your options for fluxless brazing. We do vacuum and flux brazing with various methods but don’t do atmosphere brazing so don’t want to comment on it.

Flux is an oxygen interceptor and can be a cleaner as well. Common fluxes for Bag alloys are potassium salts of boron and fluorine. There are other fluxes that are less corrosive.

Overall I would be really leery about advice from an alloy supplier unless they also did brazing. Remember that everything is easy if someone else has to do it.

Best to put it out for bid.

Tom


Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com

 

[gfbotha]

Tom, excellent, and most appreciated – just started to think I’m not going to get any response!

Sorry for asking it explicitly - if you say it’s possible, are you indicating that you actually know about or have experience about brazing specifically maraging steel? I am wondering whether one will have to prepare a flow formed maraging steel surface by chemical or mechanical means (apart from degreasing of course). As you probably know, maraging steel has an 18% Ni content, etc. but ultimately it can/will corrode (however, in-between production steps it will be protected against corrosion). Our metallurgical guy is worrying about possible diffusion of elements into the quite thin marage wall and the possible effect thereof…

The maraging steel will be aged at 480°C for 3h after a laser weld step and this worked fine in the past to restore properties in that HAZ.

Regarding the jig or fixture: positional tolerances for the lugs will not be that tight along the length & circumference of the tube and we thought of holding it spring loaded onto the tube (in the radial direction). I guess steel would be a reasonable material to make the fixture of?

The painted assembly will have to pass a salt spray test (wet & dry for 4 x 24h).

Wonder whether controlled atmospheres brazing has any consequences apart from cost.

For reasons mentioned in my OP, we need to avoid/limit contact with acids/hydrogen, sulfates, phosphates, nitrates and H2S. I am not sure: when washing off flux remains afterwards by means of hot water, does it come off easily? - rinse, soak or strong spray required?

We so far had a look at what Handy & Harman of Canada can offer; but will now also explore your website.

Thank you for your valued assistance.

Regards

 

[israelkk]

You need control atmospher3 to prevent corrosion or coloring of the maraging steel. What is the brazing material melting temperature? It has to be equal or lower than the 480C of the aging temperature. Otherwise it may affect the heat treatment. I will avoid any flux, it is very difficult to remove or neutralize and it can continue to "work" under the paint. If you do an oven brazing I think you can go without the flux if the parts are appropriately cleaned and degreased.

 

[unclesyd]

I would give these people a call. It would ideal for joining your thin wall tubing.

All I can say about the product is that it works.

http://www.rntfoil.com/applications/joining/

 

[TVP]

What does everyone think of TIG brazing for this type of work? Obviously it depends on the number of pieces to be joined, but I have read a number of threads in the last several months that refer to the problems with brazing and fluxes, and I always think of TIG brazing as being the solution to this problem.

 

[tomwalz]

Flux may or may not be easy to remove depending on what flux and how it is used. It is common to use White flux or Black flux for Ag, Cu based alloys if you mean AWS Bag alloys. Flux traps oxygen and can get used up. If it gets used up it is very difficult to remove. If you use enough and don’t use it all up it removes easily with warm water. Several years ago our US Handy & Harman rep showed us some flux remover. We tried it and it was just o.k. but we didn’t use it because we like our technology better.

I better confess that I have a prejudice against Lucas Milhaupt, the US Handy & Harman; because of the customer service rep we are assigned. My staff thinks he is rude and it typically takes him several days to reply to a RFQ. We use Prince & Izant to purchase alloy that comes from Handy & Harman because the prices and the service are much better from Prince & Izant.

Try Lucas Milhaupt for tech support. I really like and respect the tech support guys.

I am a surface prep guy and our specialty is hard to braze materials for tools. Our standard is to develop technologies for advanced material that can be used by a guy in a sawmill with an oxy-acetylene torch. We use oven, torch and induction. We have another division that does vacuum brazing. That is where we do electronics and exotic materials.

If I understand what you are talking about the type of steel and even the individual batch could make a difference. The materials will grow and shrink during brazing. They will grow and shrink uniformly and evenly because of the inherent properties. They will also twist and bend and warp depending on how they have been treating previously.

If you are worried about corrosion then the atmosphere used in atmosphere brazing can have a huge effect. Hydrogen atmosphere is common. A hydrogen furnace can seriously weaken tungsten carbide because the H reacts with the C and creates the equivalent of an Eta phase. (In WC the C is packed in the lattice of the W) This can weaken the carbide by as much as 40%.

There are a great number of finishing technologies to deal with corrosion. If you look at my patents you will see that we use a chemical or electro-chemical approach to surface preparation. Were I use I would worry more about getting a good assembly than corrosion. There are many simple passivation techniques that might apply.

What you are asking for here is theory but what you want is finished parts. I have given you the answers I think most likely to be correct however I could reverse my position and argue the other side given specific circumstances.

I would recommend that you specify materials and do a drawing then send it out for quotes. Don’t worry about getting it perfect now. Good suppliers will be happy to help you with the design rather than try to work with something only theoretically possible. Try a couple reputable brazing houses. Also try finishing.com for corrosion issues. They are plating but also general metal preparation. Activation of metal is a big deal in plating thus so is its opposite.

Tom


Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com

 

[gfbotha]

Seems that in this application flux may well be a problem child.

israelkk, most brazing materials seem to have melting points of 600-900°C. We do have the option of solution annealing at 820°C but only as an much earlier process step in my mind (not suitable when the tube is in its final thin state). So, brazing temperature seems to be a problem...?

unclesyd, I had a look at the interesting Nano processes offered by RNT. Will have to investigate; gut feel is it may well be a bit expensive and exotic…?

TVP, I am not familiar with TIG brazing but a quick web search revealed issues like very localised heating and difficult to avoid melting base metal. Each assembly involves about 48 lugs, so as you said that is likely to be an issue with a manual or labour intensive process.

Tom, thanks again for your valuable input! Just to put all my questions in perspective; I firmly belief in a thorough conceptual phase to avoid bigger expenses and schedule problems later on. And these exotic materials like maraging have the habit of surprising one with some nasty phenomenon if not careful.

Best regards

 

[TVP]

gfbotha,

I don't consider either localized heating or base metal melting to be valid issues when TIG brazing of steel parts using suitable fillers (bronze alloys). 48 lugs per assembly is probably too many for any type of manual process, so fixture brazing probably is the best way to go.

 

[tomwalz]

Very courteous response, thank you.

Which reminds me that I probably owe unclesyd at least a few thank you’s for help he has given me.

I looked at the brazing source he recommends a while ago. As I recall you had to pretin (tin) both sides them you mated them with a bit of the nano-foil in the middle. You ignited the foil and the heat remelted the braze alloy. Didn’t make sense for me then but the extremely localized heat may work for you.

Tom


Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com