HVOF Flame affected by cooling air jet

[Rickydraconis]

Hi All,

I am using a GTV K2 to spray WC -12% Cobalt on aerospace components. The area to be coated is a inner diameter of 10" and depth of 1.5".

As this is a kerosene based system, the heat generated is much hotter as compared to a hydrogen based system. Therefore, I introduced a few stream of cooling air jet (30 C) to cool the component during the process.

However, it was observed the these air jet tend to create a 'vortex' within the inner diameter. The flame of the HVOF will 'dance' or flickle, which is highly unusual given its high velocity. The stand-off distance is 28cm (approx 11"). The 'dance' of the flame had casued the deposition to drop to 0.0001" (or nil) even after numberous passes.

Hope to heard your views on this situation. Removal of the cooling jet is not possible as the coating will be become burnt.

many thanks

[LEN WOOD]

Ricky,

The air jet is causing turbulence (my favourite phenomona again!) and disrupting the spray stream/deposition zone and the coating would be really compromised, cooled, but compromised.
Is the component 'closed end' or the spray location deep inside the bore? You need to allow spent powder, fumes, etc to vacate at both ends. You may need to look at fixturing or raising so that the component has spacing or gaps where it is held in the rotator, chuck, turntable, etc. I would also suggest you cool the part from the outside and maybe run cooling jet(s) on the gun. Angle, direction, positioning and pressure of which are critical to good results. You may also try running a really fast traverse speed down inside to beyond the lower limit of the sprayed area and then traversing back at regular speed. This would minimise re-entrapment of 'spent' powder and improve your coating.

Let me know more if I'm missing something.

Cheers

GlenB

[Gordon]

Hi Ricky

to the Surface Engineering Forum.

Pretty much go along with GlenB

[Rickydraconis]

Hi Glen & All,

It as described. I had tried closed end and open end trials, both exhibit similar coating build up (0.0001"/ pass).

Here additional info,

1) Spray angle - 35 degrees
2) Combustion pressure - 7.5 Bar
3) Powder flow - 100g/min
4) Vertical tranverse speed - 1 mm/s

I had obtained few advises which I am going to try:

1) reduce spray angle
2) Increase transverse speed to 2mm/s or 3mm/s
3) extend spray beyond coating area

Lastly, can Gordon kindly advise if it possible to attach pictures? It would be better if you advise on my set-up. I am unable to put any photos directly.

thanks a lot.

[Gordon]

Hi Ricky

You should now be able to upload images via the attachment facility. Members need to make 2 or more posts before the forum allows this function. Just a measure to help protect us from spam and unwanted images.

0.0001"/ pass in itself is not a bad thing, for this type of coating I would recommend keeping deposit per pass below 0.0005" anyway.

At 1mm/s traverse and 100g/min feed rate on 10" diameter your deposit efficiency sounds painfully low. I would expect 10X + that. I can not see your cooling set-up, spray angle effecting it this much.

I would conduct a deposit efficiency test and first establish whether you are getting reasonable efficiency in more ideal conditions.

I had obtained few advises which I am going to try:

1) reduce spray angle
2) Increase transverse speed to 2mm/s or 3mm/s
3) extend spray beyond coating area

Generally good advice
"2) Increase transverse speed to 2mm/s or 3mm/s" well really the traverse speed should be set so that the coating layers just overlap without "barber pole spirals". Set part rotation and traverse speed so that on each rotation the gun moves relative to part a bit less than the spray footprint width. Set the speeds so that you achieve around 0.0002"/per pass if possible, if not go as low as possible. Doing this will go some way to help you control temperature.