problems with T800 HVOF

[stokee]

We are currently experiancing problems with spraying T800 HVOF using sultzer metcos DJ system. At the moment we have a set of spray parameters which we use, but when sectioning the test samples we will see a different oxide and porosity content with each application. We can generally meet any macro hardness figures and micro hardness figures. Our tensile strength properties can be a bit hit and miss ranging from 7500psi up to 1100 psi. Our main problem is the porosity content as when we machine the parts we tend to see the porosity in the machined surface.
Any help or tips would be much appreciated.

[Gordon]

Hi stokes

to the Surface Engineering Forum.

It seems you have problems with gaining consistent coating results. This would suggest something is changing or producing inconsistent conditions in your process. Try checking that your powder feed rate is consistent and is not pulsing. Check that your fuel flow is consistent, propane and propylene fuels flows can be problematic due to temperature changes during draw off.

Which Diamond Jet system, parameters and T800 powder are you using?

[stokee]

Hi Gordon, thank you very much for your reply. We use the DJ2600 with a METCO 9MP DJ closed loop powder feed unit. We use an oxygen/hydrogen mix for the fuel and nitrogen for the carrier gas. The units with which we control our gas flows are Metco units (not sure how these convert to to normal flow rates) but we run the oxygen at 26 units the hydrogen at 70 units and the carrier gas at 35 units. We use a powder sray rate of 6lb/hour which is very consisitent and a spray distance of 9.25 inches from exit at the nozzel to part.

[stokee]

Hi again, the powder we use is diamalloy 3001

[Gordon]

Hi stokes

Are your spray parameters the standard ones recommended by Sulzer Metco?

Have you been able to get any help on your problem from the equipment and powder supplier Sulzer Metco?

The information I have on parameters is different to yours, but mine are old and may have been revised. The fact that your coating results are inconsistent tends to indicate a problem with your equipment. I would advise that you carefully check out your equipment and procedures before considering process parameter changes. Even a poorly developed parameter set should give fairly consistent coating results.

[jeffsays163]

Hi, Stokee, may I ask a few questions? Are you spraying T-800 (HVOF) for GE? You finally proved by them? Do you know any people got proved by GE on this T-800 coating by using DJ gun? You are using DJ H2 (DJ2600) or DJ2700 (Propane or natrual gas)? jeffsays163@hotmail.com. Thank you and Happy New Year! Jeff

[Alexangel1226]

To all,

not sure if I could help.. T800 is never an easy powder to deal with..be it APS or HVOF. Per the aviation metarllugical evaluation, it is almost of the highest and most stringent requirment you will see. Thus it is setting a very high standard to produce an acceptable coating esp for GE aviation.

First, even the lab test passed all test confidently, the machined surface will still see some light "porous" looks profile. And if it is justified by the lab result, this should be accpetable. Depend also on the machining technique, be it single point cutting or others.. is there surface roughness/profile requirement? Also as I know, a reasonable chipped off at edges are allowed. other than that, Thickness build up and max thickness control should also be a concern. It is always limited coating thickness u could go, though it is a HVOF operation. Check that.

Flow meter indicated reading is a conversion, can easily get the converted Pressure range from the Manual Appendix Flowmeter Reading vs SLPM.

Powder feedrate which caused by equipment problem could be an area to look into. even thouh it is Closed Loop, but if the fluctuation is out of wrack, pulsation of the flame will occur. that leads to all kinds of problem..

Check if you see the "diamonds" in the ignition..u shd observe 4~5 diamonds ..that indicates process are optimum that give u great velocity that translate into high coating density..which will help in reducing porosity..

If u experience oxide.. and porosity.. probably your stand-off is not optimum(too far).. try on that..If stand-off and all process parameter and equipment is ok, maybe then try looking into the feedrate... you may increase slightly to reduce oxide..as I suspect if your oxide is overmax.. the porosity sometimes are due to pull-out particles on the oxide plane..As tiny pull-out and porosity soimetimes looks alike and need very expereince lab tech to differentiate it..

All said, what gordon said is very true.. dun get all your effort in vain when most of the time you may be played out by the faulty equipment. But then it should give you some clue if all this while you are spraying ok, only happen recently and u did not change anything..

Last but not least, dun forget to certify your material prior to usage..as this is often an area we missed out. Try a different lot of powder and perform a Gauge R&R. It could also due to that particular lot of powder which is defective. this boils down to how tight u control your material, machines, and your man, your QA system in equipment calibration, recal, powder sprayability testing, and maybe your technician certification...etc.

All the best.

Regards,
Alexangel1226

[Gordon]

Thanks Alex, some sound advice.

Its been said before, but to stress, check equipment and calibration, check powder and feeding characteristics, question metallographic interpretation before you start embarking on parameter optmisation.

[bhellman]

Thanks Alex, some sound advice.

Its been said before, but to stress, check equipment and calibration, check powder and feeding characteristics, question metallographic interpretation before you start embarking on parameter optmisation.

Or buy a better (not so known) equipment, like AC-HVAF, which in my opinion gives much better coating qualities than any "HVOF". This is not sales talk, but actual experience (of course from some 3 yrs ago).
bhellman

[Gordon]

Or buy a better (not so known) equipment, like AC-HVAF, which in my opinion gives much better coating qualities than any "HVOF". This is not sales talk, but actual experience (of course from some 3 yrs ago).
bhellman

Better for some coatings, but certainly not all. Debatable with T800 coatings particularly using the typical commercially available powders like Diamalloy 3001, 68F, Praxair Co-111 etc. with tight aerospace specs.

[erick212]

I will be starting the qual process with the exact equipment in the next week or two. The only differance is that I will be using the Praxair powder. This is if the priorites don't get get changed again.

[Gordon]

Hi Erick

Good luck with your qualification process. I'd be interested to know how you get on.

[jeffsays163]

Yes, Erick, keep us posted...

[erick212]

The priorities have just changed... This will put my testing out another week or two. I will however keep you posted.

[erick212]

I am currently working on the T800. I have an acceptable micro but, cannot pass the bond testing.. I have tried a few things( I will go into later) but, no luck. It is breaking in the coating so, I have ruled out the epoxy(glue test is over 12,000) and interface issues. It is definately a fail in the coating(8300 is my best). I just don't understand... I can make over 8500 with my Mettech system(plasma). If you guys have any ideas please feel free to share.

Thanks

Erick

[Gordon]

Hi Erick

I take it that you are mainly getting a coating cohesion failure rather than a coating/substrate interface bond failure. Which does sound strange, you don't normally expect to get bond test failures when the metallography is good.

I would do some metallography on the actual failed bond strength test pieces to see if that provides any clues to the cause of your problem.

[erick212]

Well, after further review it seems that the coating was failing towards the interface... then into the coating. I chatted with a few friends @ Sulzer and they thought that pre-heating the bond buttons was a bad idea(on Inconel ?). I understand there logic and reasons but I am new to HVOF so I learn as I go.. I sprayed another set with no pre-heat and putted them this morning...... My average bond went up almost 1000psi... not quite there yet but making headway. I just figured I would share this little tidbit.


Erick

[Gordon]

Hi Erick

No pre-heating I would have thought a pre-heat would benefit bond strength, unless you are having trouble controlling coating/substrate temperature during spraying.

Are your bond strength test pieces sprayed under simulated conditions of that of the actual component parts or is it just general proving of process/powder?

[jeffsays163]

Erick, but I heard from Sulzer different saying, someone in US said pre-heating is almost a must no matter you use DJ2600 or JetKote. preheat will be also good for macro.

[erick212]

Hi Erick

No pre-heating I would have thought a pre-heat would benefit bond strength, unless you are having trouble controlling coating/substrate temperature during spraying.

Are your bond strength test pieces sprayed under simulated conditions of that of the actual component parts or is it just general proving of process/powder?

Well here it is.. I have a good micro and good bond pulls. The parameter that was recommended was making the coating way to brittle.I did micro hardness and was fracturing the coating with the diamond @ a 300g load(I had to use a 100g to get anything ) I lowered the H2 and O2 and that made a huge differance. The coating structure is great, the hardness is a little towards the low side(i'm workning on that), the oxides are great..... now for the best part.... my tensile pulls are now in the 9 to 10k range. As for preheat I found that it made a slight increase in tensile readings with no preheat. This was over 20 runs of data review. This testing is just to prove powder/process. I will be doing this again in a few days... I just setup another HVOF booth and it should be running today or tomorrow.

Erick