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Black "baked in" staining on aluminum extrusions after brazing process

Black "baked in" staining on aluminum extrusions after brazing process

(OP)
I work with aluminum castings and aluminum extrusions. The extrusions go through a brazing oven as part of a "core assembly"...the braze oven gets up to around 1100 degrees F, though does not stay at the temp long before starts to "cool down".

I ma seeing parts with a black stain on the machined surface of the aluminum extrusion. It can't be simply wiped off, like whatever was in the material was baked on during the process.

The parts are machined at a sub-supplier that uses alcohol as a cutting lube (sounds crazy but is what we require). The parts are also put through a mechanical "soap and clean water rinse" bath post-machining. Parts are typically shipped in a cardboard box. I know what FRETTING is and know the black/brown staining isn't that (as you can usually easily wipe that off).

We also "prep" the aluminum surface with stainless steel scrub pad and alcohol to break the oxide layer before work piece is assembled and core put through brazing process.

Could the "baked on" stain be from all these handling processes, and whatever has "soaked" into the surface of the aluminum is simply being "baked out"? Can aluminum "absorb" atmosphere/moisture and the stain is the results of that being "cooked out"?

Note: the issue all this creates is just a visual concern on the finished part. (Will plunk a picture in here next time I am here.)

RE: Black "baked in" staining on aluminum extrusions after brazing process

Your process sounds pretty clean, it doesn't sound like there are baked-in organic contaminants. You mention castings, which are certainly porous enough that all kinds of nasty stuff could be held up in them, but it sounds like these stains are happening on extrusions, which should be pretty sound. Is it possible that tramp iron is left behind by the s.s. scrubber? Try a ceramic abrasive (silicon carbide, or corundum) instead? Is the furnace a combustion atmosphere, and what is the fuel, and what is the extrusion alloy - if there is enough sulfur content in the fuel, and zinc content in the alloy, the stain may be zinc sulfide.

RE: Black "baked in" staining on aluminum extrusions after brazing process

(OP)
Thanks for responding, btrue.

The "furnace" is a vacuum braze oven. We build radiator cores out of all aluminum material. There is cladded aluminum sheet stock (cladding is 4004 material, I think the base metal is 3003)...but the cladding isn't the issue.

I did a DOE last year when this occurred before (or actually when people were concerned with its sudden appearance). My first thought was something was in the extruded aluminum piece that exhibited the stain as I know Al CASTINGS are porous and "suck in" everything...thus that is why we mechanically wash them now. I know extrusions are more dense, yet still aluminum. Anyway, my DOE, I heated up a "clean" extrusion (prior to going into high temp vaccum braze oven) and the work piece started to sweat water. Within the water was very fine particulates, and once the water was burned off, the particulates (upon further cooking from the propane torch I was using) started to darken, ie, burn/bake/cook. Thus I assumed that, under heat, I was drawing stuff out of the extruded aluminum material...heck, maybe I was simply burning off whatever was in the oxide layer.

We reviewed the parts yesterday. One of the guys mentioned possibly there was residual machining lube/coolant trapped within the oxide layer from when the extruded aluminum piece is machined at supplier...and once the piece goes through the hi temp vacuum braze process, it cooks/bakes/burns that stuff off...thus the black "stain".

We are thinking perhaps if we send the parts through a mechanical wash process (basic soap and clean water rinse), it will wash off whatever residual STUFF is on the extrusions.

Anyway, my bigger question is if perhaps extruded aluminum (denser than a plain ole aluminum casting) can "soak in" stuff, retain it in the oxide layer, and then possibly be "burning off" during the vacuum braze process...thus leaving that black stain. (Like boiling off the water in a aluminum pan and not taking it off the burner in time.)

Note: the stain can be removed but usually takes a wire wheel or heavy wire brushing.

RE: Black "baked in" staining on aluminum extrusions after brazing process

I would not have thought an extrusion would be that porous, but it sounds like yours are. You might think about a warm vacuum oven prior to washing - to draw whatever is in the material "pores" out to the surface where the detergent & water can clean it off.

RE: Black "baked in" staining on aluminum extrusions after brazing process

Are you sure your aluminum "sweats"? It is a common misconception when people see water forming on the surface of metal when a torch flame is applied. It is actually water vapor in the flame condensing on the cold metal surface.

As btrueblood said, any sulfur in the the fuel or in lubricant residues will stain aluminum. Small amounts of sulfur in fuels is very common and it is an ingredient in some lubricants.

RE: Black "baked in" staining on aluminum extrusions after brazing process

(OP)
Most of our larger extrusions, that are basically "big" slabs of aluminum, between 8mm to 10mm thick, with some thicker flanges to them, are machined. Though we require machine sources to cut with denatured alcohol, the ones who don't wash their parts through a "soap and clean water" bath. I am thinking now perhaps lubricant residue that has absorbed into the natural oxide layer of the material might the culprit???

Note, not every work piece exhibits this staining.

As I think I've mentioned, our facility is thinking about sending the parts out for a final wash so we can bring them back and use them JIT more or less. We have done this with aluminum castings (more for weldability purposes) and it has worked very well.

RE: Black "baked in" staining on aluminum extrusions after brazing process

You may have remnants of flux left by manipulation which causes the stains.

RE: Black "baked in" staining on aluminum extrusions after brazing process

We sometimes rub soap on aluminium before annealing items, the soap turns black when the correct temperature is reached, could this be the problem ?

Bob
www.metals4u.co.uk

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