Metco 444 Unmelts

[Aerospace Mfg]

Great forum! First time poster but have used it as a reference for a while now.

I am having a problem with passing a lab for Metco 444 (a NiCrAl derivative). We have a nagging problem with unmelts, we have been increasing the volts in an attempt to heat up the coating and now we are going over hardness. Any ideas?

[Gordon]

Hi Aerospace Mfg

to the Surface Engineering Forum.

Can you let us know equipment and parameters (original and modified). Also some coating photomicrographs (email to me or see here) may help.

I assume when you increase the plasma voltage you achieve this by increasing the hydrogen secondary gas, which is certainly one way to reduce unmelts. It will also have a slight tendancy to increase hardness due to increasing oxides. Alternately, you could try one or more of the following:

• reducing powder feed rate
• reducing primary plasma gas flow slightly
• make sure spray distance is not too short
• make sure powder feed is smooth and not pulsing or fouling port and nozzle face.
  

Hope that is of some help.

[Aerospace Mfg]

Thank you for your reply. We have tried varying the feed and flow rates without success. The spec may be a bit stringent (3 unmelts max per view) and we are always at the max or o/max limit. Attached is a typical photo of the unmelts we are experiencing. In an effort to remove the oxide clusters and reduce the hardness, to allow for more heat in the system and rid the unmelts, air was 'swept' through the flame at the substrate surface. This picture shows the oxides were removed, but the unmelts were greatly increased, and the hardness actually went up!? Any insights?

Click images to enlarge.

Typical


Typical



With Air Sweeping


With Air Sweeping

[Gordon]

Hi Aerospace Mfg

Placed images in your post above. Hope they are ok.

Give me a little time to think about this one

[Gordon]

Hi Aerospace Mfg

You have certainly made some progress.

Not an easy one. The reduction in oxide clusters probably has little effect on hardness overall, but the increase in the finer (normal) oxides will increase hardness slightly. To reduce unmelts further, you need to get more heat into the particles. This unfortunately may result in an upward trend in hardness.

My thinking now is directed to powder injection, obviously if this is not set to optimum conditions, then some of the powder may not be positioned properly in the plasma stream. The more massive fraction of the powder will tend to penetrate the plasma further than the lighter fraction (classification effect). I would try playing with different carrier gas flows to see if you get any improvement. Also check powder port condition and alignment. I know 444 can have a nasty tendency to foul powder ports and nozzle faces particularly on long runs. If you notice any spits or pimples on the coatings, then this is where they come from.

A little puzzled why a maximum hardness is specified. Also, we need to be careful how we define what is an umelted particle. May worth discussing with your lab, just to make sure you are not rejecting coatings unnecessarily.

What were the original and modified spray parameters? Have you tried the standard Metco parameters for the 3MB/7MB (not keen on high spray rate 35lb/hr? 9MB parameters as far as I can remember). That's assuming you are using these guns

[Aero_Guy]

I am also running into issues with Metco 444 unmelts and hardness. I feel your pain. Our samples are soft (80) and we need to get the hardness up to around 84. We tried increasing volts and reducing powder feedrate, but still unable to achieve hardness.

Aerospace Mfg: Did you ever get it to work? I was wondering if I could send someone my parameters for review. We are up against the wall in terms of deadlines. Any info. would be appreciated. Thanks in advanced.

Aero_Guy

[kschewe]

spray distance effects the hardness. 5.5" plasma 8" flamespray.

[wilfred1963]

you could try to feed at around 3-5 lbs an hour at a stand off of 4- 4.5 inches with frontal intersect of 4.5-5.0 inches this has always worked for myself never really had an unmelt issue with 444 also we used a gh nozzle in a metco 3m torch with argon hydrogen. and number 2 port and block with the powder flow set to center of nozzle which we adjusted with the gasses flowing pror to lighting the torch.

[derek]

Our Spec called diameter bigger than 0.003" unmelt, you only have one bigger then that, are you sure the lab report is right?

[Aero_Guy]

Thank you Derek and wilfred1963 for your response. We were able to achieve the hardness and unmelts requirement with wilfred's listed parameters. Really appreciate your help.

Aero_Guy

[wilfred1963]

anytime