Twin-Wire-Arc process

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The twin-wire-arc process of thermal spray is actually a fairly well-established process, wherein two wires are fed through and an electric arc struck between them. This melts the wire and a carrier gas "shoots" the melted particles on-to the substrate material to be coated. It has been used for several decades to re-build worn out surfaces on pump components, etc. Nickel-aluminum wire is the most common. However, there have been several applications where cored wires that comprise of harder core materials such as tungsten carbide particles encapsulated in a wire have been used. The TAFA MX-C series of wires fall under this category. Years ago, there was a wonderful application, wherein the wire arc process was used as a low-cost replacement to developing a hard wear resistant surface on to barrel bars used in the corn oil extraction industry where the previous method was PTA ( Plasma Tungsten Arc ) welding. Again, for sensitive applications, proper control of the wire feed rates, stand-off distance, and current/voltage parameters are needed for superior coating quality.

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9 comments:

Renergy said...

The article is very interesting.We are using TWAS and want to undertake wear resistant coatings of Oil Expeller Worms and Cones.
Nickel aluminide is very expensive
bond coat.Can we use SS 310 or some
other cheaper bond coat material for this purpose to contain costs.
Your expert advise will be of great
help.Can worn out Crankshafts be
reclaimed by TWAS

Renergy said...

In TWAS what should be the distance of the spray gun and
the substrate to be coated.Is
it possible to undertake machining
process like turning operations
of the coated parts or only grinding is recommended.What
is the best test for checking the
bonding of coating

Raj K said...

Renergy:

Regarding bond coat material, a high nickel content material is preferred and hence nickel aluminide is very common. SS 310 might not work as well. With regards to testing, the correct method IS still to coat bond slugs and perform the standard tensile bond test procedure.

Regarding crank shaft repair, of course they can be reclaimed by TWAS. Make sure that you adhere to the best developed parameters.

I have seen many of the TWAS built up coatings machined with single point tooling.

Good luck and thanks for stopping by!

Renergy said...

Dear Raj
Thanks for your expert advise.We
will be using Nichrome 80/20 wire as bond coat as it is available
readily in small quantities.Could
you please suggest the appropriate voltage for getting good bond spray
quality using nichrome 80/20 wire.
In oil expeller worms and cones we
intend using high carbon high chromium wires for spraying after
bond coat.Is it the right material
in your opinion for improving life
of worms.and cones.

Thanks with regards

Renergy said...

Sir
We are awaiting your reply with
regards to twin wire spray coating
of oil expeller worms and cones for
increasing life.Your expert advise
will be of great help to us.

Thanks with regards

for Renergy
Gurmit Singh

Renergy said...

We tried TWAS on Oil Expeller
Flights and encountered problem
of peeling off of the coating.
The job was grit blasted at 80PSI
using cast iton grit of 25 mesh.
Bond coat of Nichrome was given prior to the coating of Flights
with High Carbon High Chromium
Steel wire.The finish was very good
but the coating started peeling off.How can we improve the quality
of coating.Kindly give your valuable
suggestions.

Thanks
for Renergy
Gurmit Singh

Renergy said...

We had tried coating of oil expeller worm flights using the
twin wire spray process.We encountered problems with regard to peeling of coating even though
the finish was good.Kindly suggest
improvements for better bonding of
coating.After grit blasting bond
coat of Nichrome 80/20 was given and
then High Carbon High Chromium
metal sprayed.

Thanks

Yours truly
for Renergy

Gurmit Singh

Raj K said...

Renergy:

I am troubled that you are still having problems. Let us go back to the basics. Ensure that the surface is thoroughly and properly cleaned. After cleaning, handle ONLY with clean white gloves -- no bare hands allowed. I see you are using cast iron grit. Can you switch to aluminum oxide grit blasting material please? Next is your coating thickness too high per pass? If so, increase the traverse speed and deposit smaller thickness per pass to build up. Next, you WANT to have a coating with some porosity for strain relief. Is your coating devoid of all porosity -- that will induce too much of stress. Try using a pair of air jets that intersect about half an inch PAST the stand off distance. For example if your stand off distance is 4 inches, intersect the air jets at 4.5 inches. This will blow away the junk and presolidified material from hitting the part.

Sorry could not get back to you sooner, with the million things I am involved in! Good luck and thanks for stopping by.

Raj K said...

Renergy:

And by the way try also reducing wire feed rate.