Metco 58NS unmelt particles

[thezombiesprayer]

[attachment=263][attachment=263][attachment=264]
Hi

Recently for the past few months, I have noticed in the increasing number of globular or unmelt particles in our coupons sprayed for the Metco 58NS coatings. Pls refer to pictures attached. They are quite big in size and sometimes the number exceeds the stated limits.
I have checked the feedrate of the powder as well as the spray distance and etc. But even when we spray according to our established parameters, we still experience quite a number of these globular particles in our microstructural tests.
Gun nozzles and spares are changed quite regularly according to our schedules.

Can advise what are the possible root cause to the problem and what changes can we do to eliminate these unmelts in the Metco 58NS coatings?
We have some spraying restrictions regarding the spraying distance due to the part configuration though.

Thanks

thezombiesprayer

[Joris Kraak]

Hi Zombiesprayer,

What gun are you using?!
- is your grain size distribution of the powder the same ?
- check injection in the plasma, the powder port might be mis-aligned. (sometimes happens with 3MBT or 9MB)

[thezombiesprayer]

(02-10-2012 04:01 AM)Joris Kraak Wrote:  Hi Zombiesprayer,

What gun are you using?!
- is your grain size distribution of the powder the same ?
- check injection in the plasma, the powder port might be mis-aligned. (sometimes happens with 3MBT or 9MB)

Hi there

I'm using the F4MB 90 degrees gun.
For the powder grain size distribution, do you mean that we need to sieve the powder?
We did try sieving in the past, but ever since we have some experiments before sieving and w/o sieving seemed to be of similar results and we stopped sieving.
I shall check out the alignment of the gun.

P.S. I have read in another post that Metco 58NS is common to have some unmelts but as long as it falls within the stated limits of size and amount, we will be safe in the range?

Thanks

thezombiesprayer

[vivian]

Hi thezombiesprayer,

I have similar issue. Can you advise have the the unmelt particles resolved? If so, can you shear with me what to look for?

Thanks.

[Spraytube]

Hi,

on the first hand, generally speaking,, having unmelt particle in a coating could be a lack of power transmitted to the particle.

There could be several reasons for that :

Carrier gas is not correct and the powder don't get in the plasma, the first things to do is leak checking the complete powder line.

Viscosity of the plasma is not correct and the powder can't get in the plasma or pass through : check your plasma gas parameters and calibrate your system.

"Power" given to particle is not correct, even if the gun maintenance is correct, most of the time we forget to check electrical hoses: a simple Voltage recording with fixed test parameters after a gun maintenance will give you good overview of the situation of your system.

On the other end, water conductivity could be an issue in your case, the unmelt particle in your view could come from the nozzle of the 9MB gun.

If you have a high water conductivity on a 9MB gun, small round "bubbles" of copper will appear on the nozzle and you will found them in the coating.
It will be really hard to detect it on a micrograph view especially in Cu/Ni/In coating.

You can visually see those copper "bubbles" on 9MB nozzle but the easiest thing to do is to measure the water conductivity.

Regards

[vivian]

Thanks for the reply. I will work on these areas.

The OEM recommedation for water conductivity is <10 uS. The reading i have now is 2 uS. Not sure whether to have tighter control or not?

I have the folowing observations:
When the powder port is placed at +45 deg, the coating having high porosity % and unmelt % . However, the powder port is placed at -45 deg, the coating quality much more better and accetable to the spec.

I should get similar result regardless + or - 45. I not really understand this.

To get acceptable result, powder port is always set at -45 deg, but just recently observe higher unmelt particles in coating.

Also I am facing coating with low Vickers hardness result generally below 150 to 160 Hv. So far, I am trying to find out what are the factors affect this.

Regards
[/quote]

[Spraytube]

(10-18-2012 11:24 AM)vivian Wrote:  Thanks for the reply. I will work on these areas.

The OEM recommedation for water conductivity is <10 uS. The reading i have now is 2 uS. Not sure whether to have tighter control or not?

That sound good for me.

(10-18-2012 11:24 AM)vivian Wrote:  I have the folowing observations:
When the powder port is placed at +45 deg, the coating having high porosity % and unmelt % . However, the powder port is placed at -45 deg, the coating quality much more better and accetable to the spec.

I should get similar result regardless + or - 45. I not really understand this.

To get acceptable result, powder port is always set at -45 deg, but just recently observe higher unmelt particles in coating.

This might be related to the relative position of the gun regarding the part or to the exhaust system. Check for eventual turbulence source.

(10-18-2012 11:24 AM)vivian Wrote:  Also I am facing coating with low Vickers hardness result generally below 150 to 160 Hv. So far, I am trying to find out what are the factors affect this.

Hardness is normally between 140-200 HV 0.3, you should increase you surface/traverse speed.

What are your parameters/system/powder feeder?
Are you using airjet?

Regards

[vivian]

Thanks again for the advices.

Parameters as follow:
powder flow rate: 45 g/min
H2: 6.6 NLPM
Ar: 37 NLPM

Equipment: APS Multicoat 9MB gun, with powder feed unit 9MPE-CL

[Spraytube]

Hi,

I give you mine :

Spray distance : 90mm
Surface speed : 50 m/mn step 5 mm

Powder rate : 45 g/mn at 6l/mn (you should adjust this value according of how you see the powder get into the plasma), injector #2

Airjets : 4.5 bar crossing at 118 mm

Plasma : 550-570 amps if you are using thyristor power source, else remove 40-50 amps.
Gaz : ar:37 nlpm H² 6.8 nlpm

Nozzle GH

anyway you should be around 64-67volts.

I got better porosity result with an F4 gun.

Regards