Porous Machined Surface on Part Lower Inner Diameter on Inco 718 Coating Surface

[Alexangel1226]

Hi Gordon,

I am having some prob on a work piece, coating on part lower ID on the dimension which forms a "L" shape from cross section where coating on the whole L surface (vertical and parallel base of the "L"). Porous surface finishing after final machining to limit.

Some info on the process:
-Powder : Inco 718
-Gun spraying at ~60deg angle to assess to the ID
-ID @ 6.8"
-RPM set @ 140
-Gun traverse speed @ 10mm/min
-Parameters is optimum when check on the test piece.
-Thk build up @ ~ 0.012".
-After machined coating thk on surface ~0.006"-0.008"



Click image for larger picture

It should show very smooth and shiny surface after machine and not the "porous" like that. It is worse after the step on the vertical side.

Pls advice.

Thanks.

[Gordon]

Hi Alex

First, I think I would make up an L shape test piece to duplicate part surface, fit on component or simulate same conditions and spray. Do metallography on whole L shape cross-section to establish what's happening to the coating. If coating looks good then you may need to concentrate on improving the machining/grinding procedure.

Chances are that you are suffering from:

ID spraying problems with ventilation of dust and debris away from coating area, but much worse than that is the L shape surface.



Excuse my scribble, but I think you will get the picture When spraying one surface, spray material bouncing off will form a very poor quality secondary deposit on the adjacent perpendicular face.

The only way I can suggest for curing this problem is to mask either the vertical or horizontal face and spray the other at the best angle you can get. Remove masking and spray again with optimal spray angle for uncoated area.

[Aerospace Mfg]

Alex,
One other thing to consider is your spray angle. Is it possible with your geometry to spray at 45°? The bottom of the 'L' looks to be more sound (what Gordon is calling the primary coating), and the back of the 'L' is hwere the porosity is forming. On this surface you are essentially spraying at a 30° angle and possibly causing peaks with the coating, which in turn creates pores behind these peaks relative to the spray direction.

[Alexangel1226]

Thanks Aerospace & Gordon for your advice,

I have gotten a scrap work piece and will carry out a test by putting test pieces at both vertical & horizontal dimension and see the outcome. I agree with you on the rebounce and expected the angled spraying with rebounce will deteriorate the coating integrity.

Masking this area and build up one surface follow by another seems right move. I will do both trial simultaneously. And shall get back to you on the outcome and improvement.

Thanks again.

[Gordon]

Hi Aerospace Mfg & Alex

Good point from Aerospace Mfg

This image shows the effect: click to enlarge


I think spray reflection or bounce will be a big factor here and by spraying the faces in a two part process instead of all at once will enable you to optimise the angle for each face. Two birds with one stone