Brazing Silver Plated Molybdenum

[engrQA]

One part is a .0016" thick disc which is about .4" dia. It is made of moly type 100. It is nickel plated to .00008-.00012 thick per side, then silver plated to .00008-.00012. It is then sintered @ 800 degrees C. The other part is a pin .46" long, and .040" dia., and Nickel plated to .00040-.00050" thick, then sintered @835C. My problem is that when we braze these two pieces together(the pin goes through a hole in the center of the disc), the solder blisters in spots and flakes off. Parts are bubbly, and other parts have come clean off as if the solder didn't "stick" to the surface of the part. We use a solder ring that is 68%Ag/27%Cu/5%Pd and the parts are brazed at about 835 degrees C for 15 minutes. Thanks for any help you can offer.

 

[tomwalz]

Surface preparation?

Do the Ni and Ag oxidize? How hot are they when pulled from the sintering oven? When the solder comes off what is left on the part? Is any of the Ni or Ag stuck on the bottom on the solder that comes off?

That's a really thin plating. What happens to it during
sintering? When we braze ceramics we plate after sintering and just before brazing.

I have a pic of a similar failure that may help if you can figure out how to get it to you.

Send me pics if you like.

Tom

Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessor.com

 

[btrueblood]

Are both parts powder-metal Moly, or wrought Moly? Agree with Tom, you should sinter, and possibly in H2 (to remove organics) before plating, then do a low-temp. (400 F max) bake-out after plating, then go to braze furnace. Is the silver/braze alloy discolored? What is the brazing atmosphere?

 

[engrQA]

Thanks for your reply.
The plating doesn't oxidize. I'm going to have to research how hot the parts are after they're sintered. they're sintered at 800 and 835C for 2-6 min.
I believe tht when the solder comes off, the plating is intact.
After sintering, the parts are checked for blisters or peeling before they are brazed.

Another point of interest is that on some of the parts, the solder just bubbles. It puffs up in various places. Could this be an overheating issue?
I would be interested to see the pictures you have, unfortunately, our microscope is down right now. you can send pictures to jeff_lehman@hotmail.com

 

[engrQA]

The material is wrought moly. The braze alloy is not discolored.

The furnace is conveyor style belt driven, with 3 temp zones, open to atmosphere.
thanks

 

[engrQA]

I should note that the parts are chem cleaned before they are plated, would it be better to sinter them before plating?

 

[btrueblood]

Does the braze alloy de-bond more from the nickel plated surface, or the nickel+silver plated surface? Or neither...my theory here is that the silver plating ought to bond better, nickel can get a tough oxide film on it.

Beyond that, you might consider a vacuum or hydrogen atmosphere braze, I've never used moly in atmoshperic furnace, it just oxidizes too rapidly. Wondering if some oxide is diffusing through the plating, too. Or, are there un-plated areas of the moly -- its oxide is highly volatile at those temperature, and it will float around and coat adjacent surfaces quite readily. Tungsten acts that way too, and both of them can have some really wild oxidation reactions if there is even a few ppm of water vapor in the furnace.

 

[btrueblood]

In re-reading your first post - it sounds like some parts come through okay? If so, maybe there are variations in where the parts are located on the belt vs. temperature or atmosphere in the furnace, could you run trials with varying the locations?

 

[engrQA]

The braze alloy debonds from the nickel plated surface. I agree that it would better to braze in a hydrogen or vacuum furnace, but due to the volume of these parts, this is quite difficult.

Some parts come out ok which leads me to suspect placement on the belt, etc. I'm testing this theory today.

Part of the moly pin is unplated. Are you saying that the moly's oxide could possibly cause areas of the solder to blister?

 

[tomwalz]

If the furnace is open to atmosphere then, are you using a flux?

At 0.0016" thick by 4" wide you may be getting some strange thermal movement.

I have just sent you a pic (50x natural ligth)of a tungsten carbide saw tip that came off the saw. The saw body had been chrome plated. The saw tip had a cobalt enhanced surface to aid wetting. The gold is the baze alloy. The top of the pic shows a gray film over the braze alloy that came off the saw body. The bottom of the pics show where the surface treatment came off the carbide leaving bare carbide exposed.

Tom Walz
President@carbideprocessors.com







Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessor.com

 

[engrQA]

Thanks for all of your input. I just finished inspecting a batch that is finished with brazing. There is no correlation to the rejects and their placement on the trays that ride on the conveyor. On some of the rejects, the plating and solder bubbled. On others, just the solder bubbled and flaked in places. This seems like an over heating problem. I'm going to adjust the temp. a little on the next batch. Any ideas? Thomas, thanks for the pics.

 

[btrueblood]

"Part of the moly pin is unplated. Are you saying that the moly's oxide could possibly cause areas of the solder to blister?"

Yes. Moly oxide may prevent wetting of the braze alloy on whatever surface it is on, and will vaporize at temperatures near 800 C, causing bubbles to form if the alloy is covering it. Moly oxide can be transferred to your plated surfaces by handling, or even by sublimation and re-plating. The trioxide, MoO₃, melts at 795 C, and sublimes (tranforms from solid to vapor) at temperatures around that. It has a vapor pressure of 10mmHg at 814 C, and 1mmHg at 734 C.

The oxide forms fairly slowly at room temperature, but quite quickly at hot (400 F) temperatures; your sintering process may be creating a bunch of the oxide, that then transfers to the plated areas.

Try cleaning the parts just before they enter the furnace, using a process that avoids re-contaminating the plated areas. You can usually wipe away the oxides with a clean dry cloth. They are also soluble in acid solutions, so you could dry dipping & wiping.

 

[tomwalz]

As btrueblood said, start with cleanliness. I would add that you will want to make sure there are no incidental oils and greases on the surface. I would recommend a strong caustic cleaner such as Easy Off oven cleaner followed by a clean water rinse. Check cleanliness by how the water breaks across the surface. If you are not familiar with it see a plating web site for water break testing.

Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessor.com