HVOF WCCoCr Coating defect

[Patrick K]

Hey guys,

This is my first time posting, but I've done quite a bit of reading on this forum.

I have an issue that I would like some other opinions on:

We were superfinishing a shaft that was coated with WCCoCr via HVOF and there were quite a few pores in the coating (maybe 2-3 per sq. in.). They were varying diameters, but they were hard to see without a flashlight. Also, there was a trailing line behind some of the bigger ones as if it was a pullout of some sort.

We're thinking either grit contamination or unmelts. Any other thoughts?

Also, we are considering baking the power before spraying to remove any moisture and maybe investing in a tumbler. How many of you use these methods?

I'm sure I left something out, but let me know what your questions are and I can be more specific.

Any help I can get would be greatly appreciated!

Patrick

[derek]

We always heat and mix the powder before using it. You don't ?!!!!
Have you clean the shaft before spray it? There maybe some oil on it will cause the problem.

[Patrick K]

We always heat and mix the powder before using it. You don't ?!!!!
Have you clean the shaft before spray it? There maybe some oil on it will cause the problem.

To my knowledge, we've never heated the powder in all our years of doing it, but they have hand shaken the powder before using.

And yes, the shaft is clean and freshly grit blasted.

[k09]

Hi Patrick,

Heating the powder would surely help in eliminating any moisture in the powder. Especially in humid conditions pre-heating powder helps in reducing lumps in powder which could restrict free flow of powder. Did you observe lumps in the powder before transferring the powder in the powder hopper?

Regarding the coating defect it would be difficult to comment unless you post a picture of the defect.. Which spray equipment are you using?

K09

[Patrick K]

Hi Patrick,

Heating the powder would surely help in eliminating any moisture in the powder. Especially in humid conditions pre-heating powder helps in reducing lumps in powder which could restrict free flow of powder. Did you observe lumps in the powder before transferring the powder in the powder hopper?

Regarding the coating defect it would be difficult to comment unless you post a picture of the defect.. Which spray equipment are you using?

K09

I wasn't present to see any lumps, but none were spoken of, and the operators are normally aware of any inconsistencies.

I wish I could post a picture of the defect, but by the time I got around to it, it was already gone. We are running the DJ2700 Watercooled HVOF gun.

It literally looks like a small divot (no bigger than 0.5 mm in diameter), with a line trailing behind it like a comet.

[pakalano]

Hi Patrick,

Heating the powder would surely help in eliminating any moisture in the powder. Especially in humid conditions pre-heating powder helps in reducing lumps in powder which could restrict free flow of powder. Did you observe lumps in the powder before transferring the powder in the powder hopper?

Regarding the coating defect it would be difficult to comment unless you post a picture of the defect.. Which spray equipment are you using?

K09

I wasn't present to see any lumps, but none were spoken of, and the operators are normally aware of any inconsistencies.

I wish I could post a picture of the defect, but by the time I got around to it, it was already gone. We are running the DJ2700 Watercooled HVOF gun.

It literally looks like a small divot (no bigger than 0.5 mm in diameter), with a line trailing behind it like a comet.

Sounds like you either had contamination(from grit blasting) on the substrate OR there was a bit of buildup in the nozzle wall that "spit" onto the part from either moisture and/or too cold of water inlet temp. We run same equipment and have experienced the same kind or problems, finding one of these two things being the issue.

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One other note; I have been told heating your powder to 400deg before spraying is the best method of drying. Some may find that excessive, but it does ensure the moisture has been taken from the powder.