Machining of thermal sprayed coatings

[ServiceTech]

Dear All;

Regarding the single point turning of thermal sprayed coatings, I am seeking the latest information available on methods, techniques and equipment.

At my shop, we predominately plasma spray Amdry 960, Metco 443NS, Metco 480NS, Amdry 718 and arc spray Tafa 73MXC and 75B. There are a few others but these are the heavy production materials.

While our success rate is good, I still get alot of calls to the floor to examine poor finishes, porosity in the thermal spray coating after turning, some chipping, small edge chips etc.

As usual, the coating process is blamed and not much accountability is held to the machine shop.

As I continue to develop our thermal spray processes, I believe the machine parameters should be examined in depth as well.

This leads me here to ask my fellow thermal sprayers what they would be willing to share in regards to finishing these coatings. Any information you can provide or point me to is greatly appreciated.

Thanks in advance.

[sreenuvundela]

Dear All;

Regarding the single point turning of thermal sprayed coatings, I am seeking the latest information available on methods, techniques and equipment.

At my shop, we predominately plasma spray Amdry 960, Metco 443NS, Metco 480NS, Amdry 718 and arc spray Tafa 73MXC and 75B. There are a few others but these are the heavy production materials.

While our success rate is good, I still get alot of calls to the floor to examine poor finishes, porosity in the thermal spray coating after turning, some chipping, small edge chips etc.

As usual, the coating process is blamed and not much accountability is held to the machine shop.

As I continue to develop our thermal spray processes, I believe the machine parameters should be examined in depth as well.

This leads me here to ask my fellow thermal sprayers what they would be willing to share in regards to finishing these coatings. Any information you can provide or point me to is greatly appreciated.

Thanks in advance.

---

Dear All;

Regarding the single point turning of thermal sprayed coatings, I am seeking the latest information available on methods, techniques and equipment.

At my shop, we predominately plasma spray Amdry 960, Metco 443NS, Metco 480NS, Amdry 718 and arc spray Tafa 73MXC and 75B. There are a few others but these are the heavy production materials.

While our success rate is good, I still get alot of calls to the floor to examine poor finishes, porosity in the thermal spray coating after turning, some chipping, small edge chips etc.

As usual, the coating process is blamed and not much accountability is held to the machine shop.

As I continue to develop our thermal spray processes, I believe the machine parameters should be examined in depth as well.

This leads me here to ask my fellow thermal sprayers what they would be willing to share in regards to finishing these coatings. Any information you can provide or point me to is greatly appreciated.

Thanks in advance.

---

Hi

As you know that all thermal spray coatings will bond to substrate mechanically. So, it is always better to do grinding operation rather than carry turning on coating to get better surface finish and dimensional accuracy.

However, if you want to do machining only, then you have to concentrate much on metodology rather than on tooling.

During turning, you have to carry such that turner has to start from the middle of coating to one end so that cutting force will not act in opposite direction of mechanical bonding which will lead to peel off of coating at edges.

You have to take care about feed and depth of cut will be minimum.


Regards,

SREENIVAS

[Lars.Patrick]

I can speak to 443ns...

We had to reduce to 0.005" per pass to avoid chipping, particularly with magnesium and aluminum substrates.

For whatever reason, this coating just does not like to bond very well to our parts.

[ServiceTech]

Thank you both for your reply. It does seem that the best machining techniques are to start at the center of the coating and machine outwards and then inwards using depth of cut and infeed of .003" - .005" per revolution. Sometimes a deeper cut can provide a good finish at the risk of chipping the coating. I don't think it's worth the risk. We are still experimenting with various insert types and have mostly settled on traditional carbides due to most surfaces having interruptions in the form of bolt holes which really negates the use of diamond type inserts.

[J!m]

You need to keep the force on the coating low. This means sharp (small radius inserts) tools and low infeed and low surface speed.

The best results I've had, when dealing with machine shops that do not have coating experience, is to "treat it like cast iron". Like cast iron, coatings (unless completely dense HVOF) behave like cast iron- interrupted cut that dulls the tool quickly if you get too aggressive. This is exacerbated by higher oxide in the coating, which acts like embedded grit.

Unless it's an abradable, machine with oil as coolant on metals. Cermets and ceramics should be ground, even alumina.