Hi, Gordon:

We have sprayed AlSi-polyester coating with acceptable porosity and HR15Y. However, the polyester was not considered to be evenly distributed.
The spray parameters as follows:
450 & 500A, 55 lpm Ar, 5.0 lpm H2, carrier gas Ar 3.0 lpm, stand-off 110-120 mm, surface speed 1000 & 1500 mm/s.

Regards,

fhyuan

Hi Fhyuan

Is the uneven distribution ordered in any way (layering effect) or is it totally random?

Two possible causes for this effect:

1. Powder is a blend, so uneven distribution could start here, with settling or poor blending. This will tend to give a random variation in distribution in the coating. Thoroughly tumble powder before use.

2. During powder injection into the plasma, some powder classification effects occur. So you end up with concentration gradient across the plasma stream. This can show up in the coating as a layering effect, alternating layers of polyester rich and aluminum/silicon rich. To reduce this effect on the coating, changes to the way you handle gun and part motion and possible changing powder port orientation with respect to gun traverse direction (If powder port is pointing down, try rotating 45 or 90 degrees, which will change plane of powder classification) should help.

Hi Fhyuan

I suspect you did not mean to open a new thread, so I've put your post below. Possibly, a little out of order as I was replying to the above when you posted this.

Quote:Uneven distribution-Al polyester coating

Hi, Gordon:

We have sprayed AlSi-polyester coating with acceptable porosity and HR15Y. However, the polyester was not considered to be evenly distributed.
The spray parameters as follows:
450 & 500A, 55 lpm Ar, 5.0 lpm H2, carrier gas Ar 3.0 lpm, stand-off 110-120 mm, surface speed 1000 & 1500 mm/s.

Is the primary Ar flow too high or the surface speed too large? By the way, the powder was internally fed behind the nozzle.

Regards,

fhyuan

Hi Fhyuan

What equipment are you using?

Surface speed by itself is limited information. Rotating part - rotation frequency (RPM), part diameter and traverse speed or Static object - ladder scan, step distance, traverse/surface speed would be more useful.

Hi Gordon:
You are right, layering effect was often found in the coating, this is especially true in the case of a rotating part.
These days we sprayed the coating in the ladder scan way, the polyester was found interconnected in the coating, while the ideal microstructure shall be such that the polyester is evenly distributed in the AlSi matrix.

The coating is sprayed using PT A2000 (powder port outside) and Unicoat (spray gun of Internal feeding).

Hi Fhyuan

Where you can, rotate powder port 45 or 90 degrees and see if this improves things.

Try spraying the coating in much thinner layers per pass of the gun.

Check that your carrier gas flow is set correctly. You could try a few runs plus and minus a few points on the carrier gas flow to see if this has any effect.

Good luck and let us know how you get on.

Hi Gordon:

The coating is sprayed using Unicoat(powder port outside) and the coating has been greatly improved concerning microstructure, esp. higher volume fraction of Al/Si.
We are now trying to achieve further improvement in the distribution of the polyester though the coating was sometimes satisfactory. This is a hard working process!
Yesterday, we sprayed two test pieces simultaneously. However, a puzzling result is that the microstructue of the coating are much different. More Al/Si phase is present in the coating on one specimen than that on the other one.

Hi Fhyuan

Quote:Yesterday, we sprayed two test pieces simultaneously. However, a puzzling result is that the microstructue of the coating are much different. More Al/Si phase is present in the coating on one specimen than that on the other one.

I find this a bit worrying, as it suggests that there are inconsistencies in your process or testing. I would try to find the cause of these differences, as it surely must put doubt in your mind as to the validity of previous work.

A careful comparison found that the microstructure is a little (not much) different. I think the distribution might be not even "macroscopically" to a certain extent.

Hi Fhyuan

I suppose you are seeing the natural variance within your sample. Sometimes you can take two cross-sections from the same test piece and find they look slightly different. A 2 dimensional view on a cross-section really is a very small view of the bulk structure. Try taking a few more sections from your previous samples and see if you are just seeing this normal variance.

It sounds like you are making good progress anyway, good luck

The spray parameters as follows:
450 A, 70 lpm Ar, 8.0 lpm H2, carrier gas Ar 4.5 lpm, stand-off 110-120 mm,

zhoudesign Wrote:The spray parameters as follows:
450 A, 70 lpm Ar, 8.0 lpm H2, carrier gas Ar 4.5 lpm, stand-off 110-120 mm,

Thank you Zhou.
We always have more polyester near the Al polyester/bond coat interface, even unacceptable poyester phase interconnection on the bondcoat, which is believed to cause the Al polyester coating susceptible to delamination in service. To some extent more Al(Si) phase is distributed away from the interface.