Porosity Problems Spraying Metco 72F-NS / Wc-114

[J_rock]

Hello All,


I am experiencing difficulty creating a Tungsten carbide coating to meet GE B50TF27 class A/B

My setup is as follows

Metco 9MCE Control
Metco 9MRB Power supply
Praxair / Taffa SG100 torch
Praxair / Taffa 1264 Dispenser
ABB IRB 2400 Manipulator

3in standoff
100mm/s traverse
3mm advance

Primary Ar @ 104 scfh
Secondary He @ 40 scfh

Carrier 4 l/m

Volts 40
Amps 650

Powder federate 25g/m

I have tried a number of parameter sets and have managed to create a coating which meets the spec in all areas apart from porosity.

The application is spraying on a titanium alloy substrate.


Any help would be appreciated

[Alexangel1226]

J_rock,

Few queries before I proceed further:

1. You said u managed to come to this so far and meet all requirements except porosity, do you mean you do not start from the OEM recommended starting parameter?

2. Have your parameter been deviating too far from what is recommended?

3. What is the porosity level evaluated? I supposed the req. is @ V3, what is your current coating outcome worse than V4? Marginal? I wont elaborate further but you understand what I mean?

4. Do you also manage to achieve other property requirement e.g Tensile & the Rockwell hardness per the requirement confidently?

5. What is the thickness you build upto? Is the porosity equally well spread or only concentrate to certain layers, (top, middle or bottom).

My comment on your parameter will be:
1. The ratio of 1st & 2nd seems abit not right. Primary gas shd be ok, maybe u can try increase the 2nd by 50% or more. - When we increase gas flow(2nd gas), generally, the heating capacity increases, giving better melting effect and a higher velocity to the spray stream, particle will be traveling in a higher Velocity and result in producing a denser coating with lower porosity level.

2. The Voltage seems a bit too low with the Amperage u set which I guess it is due to your low 2nd gas, try to have it reached and increased by another 25 to 35%. By doing the 1st step of increasing the 2nd gas, the voltage shall increase.

3. Feedrate is rather low, as u increase 2nd gas, the process would be able to take on more powder, load up more, 5.0 to 6.0 Lbs shd be alrite. If you were to do this, check you carrier gas, should compensate with the feedrate increase too.

4. Stand-off u can try to increase a little to 3.5 or 4.0" if your oxide level is ok, assume you have no unmelt prob. This may allow your powder to have a higher dwell time, better melting and absorb more energy, result in more impactful splats as it deposit to the substrate, which then leads to a denser coating~ lower porosity

Well, this is so far I could advice, but not suggesting u to do all but one at a time, more important is to adjust one parameter at a time, and spot the difference of the outcome, and understand the mechanism how porosity is formed and how it would be gone.

Hope this help.

Regards,
Alexangel1226

[Gordon]

Hi J_rock

to the Surface Engineering Forum.

In addition to Alexangel1226's helpful post, I would first carefully evaluate your metallographic preparation procedures. I have seen this many times where people spend months on coating parameter optimisation, only to find they are not going anywhere fast. Establishing the true porosity for these coatings is far more difficult than you might think

[Gordon]

Hi J_rock and Alex

Copied this across from the similar thread porosity-problem-with-metco-71-fns

Quote:Hi Alex

Quote:We are spraying the Metco-72F-NS(Tungsten carbide 12% cobalt aggregate) Not sure if these 2 powder are the same?

They are pretty much the same chemically (88/12 WC/Co), but produce quite different coatings. Metco 71 variants are cast and crushed powders while Metco 72 variants are sintered and crushed powders. The cast version tends to have many intermediate phases between WC and Co, while the sintered is pretty much just WC in a relatively pure Co matrix. The sintered 72F forms coatings (when sprayed right ) that produce much more abrasion resistant and ductile coatings than the cast 71 type forms.

Just want to make the point that to reduce porosity (and sometimes increase hardness) in these types of coating can be relatively all too easy by increasing thermal input. Beware, too much heat will breakdown the primary WC producing unwanted intermediate metastable phases that will be detrimental to ductility and wear resistance.

[Alexangel1226]

Gordon- Thanks for the great info.. I have learnt something new today!! =P

[J_rock]

Alexangel1226

Answers to your queries below,

1 - My starting point was the Taffa technical bulletin parameters converted to run on the Metco 9mc panel which is pretty much what I am running with at the moment. I have tried another high power parameter set (900amps) which again produced a coating with a high porosity.

2 - No. My parameters have not deviated too far from the recommended parameters.

3 - The porosity requirement is less than or equal to V-3. I would say I am currently half way between V-3 and V-4.

4 - The bond strength is 8000 psi (5500requirement) and the Rockwell is 88 (83 Requirement)

5 - Typically 0.012" - the porosity is equally spread.

Thank you for your comments - I plan to look a bit further into the sample preparation method with our lab before changing any spray parameters.

Gordon,

Can you recommend any reading material or courses regarding metallographic preparation? My experience in this area is limited?


Thank you both,



JRock

[Gordon]

Hi J_rock

Quote:Gordon,

Can you recommend any reading material or courses regarding metallographic preparation? My experience in this area is limited?


Thank you both,

To be honest I haven't read anything on this subject for years. One old publication sticks in my mind a Metco technical report (1971) "Preparation of Specimens for Process Control of Flame Sprayed Coatings by Metallography" which if you are very lucky you might be able to get from Sulzer Metco.

Thermal spray coating metallography is still a bit of a "black art". Metallographers really do need to accept that not everything they see is "true" particularly in the case of porosity determination. You may find many different preparation "recipes" procedures, but really there is nothing like a metallographer, that puts in the time and effort experimenting with all the different variables and gaining that experience needed to interpret the results and understand the limitations. Sorry that probably is not what you wanted to hear , but I don't think it is something you can do just purely from a book or procedure.

[Gordon]

Hi J_rock

Quote:Can you recommend any reading material or courses regarding metallographic preparation? My experience in this area is limited?

Some ASM courses in Maastricht just prior to the ITSC 2008 conference event in Maastricht, The Netherlands.

[ycwbycwb]

There is one chapter in "Handbook of thermal spray technology" described the metallography of coatings. I think it is worthy to read it.
https://www.amazon.co.uk/Handbook-Therma...0871707950

BR,
Ycwbycwb

Quote:To be honest I haven't read anything on this subject for years. One old publication sticks in my mind a Metco technical report (1971) "Preparation of Specimens for Process Control of Flame Sprayed Coatings by Metallography" which if you are very lucky you might be able to get from Sulzer Metco.

Thermal spray coating metallography is still a bit of a "black art". Metallographers really do need to accept that not everything they see is "true" particularly in the case of porosity determination. You may find many different preparation "recipes" procedures, but really there is nothing like a metallographer, that puts in the time and effort experimenting with all the different variables and gaining that experience needed to interpret the results and understand the limitations. Sorry that probably is not what you wanted to hear , but I don't think it is something you can do just purely from a book or procedure.

[CHI3]

J_rock what was the outcome of the porosity of your 72F.

[KevinS]

(03-12-2008 04:27 PM)Gordon Wrote:  Hi J_rock

Quote:Can you recommend any reading material or courses regarding metallographic preparation? My experience in this area is limited?

Some ASM courses in Maastricht just prior to the ITSC 2008 conference event in Maastricht, The Netherlands.

The "Handbook of Thermal Spray Technology" has a pretty good section in it about polishing. For WC FNS strongly suggest the final step be 1 micron diamond on a DAC type cloth for 15 minutes.

[KevinS]

Sorry gave advice for NS not FNS coatings. FNS coatings are one of the hardest to polish for an accurate porosity level. What normally appears to be porosity are areas that were full of fine metallic powders and oxides. The coating is torn apart easily. Silicon carbide papers tend to drag and smear the coating. The silicon carbide does not cut the tungsten carbide well and tends to bludgeon the carbides. For some reason 3 micron diamond also tends to create problems with this coating. Suggest you try a prewet (Metadi) 6 micron diamond lapping film (placed on top of a hard flat polishing cloth) and then a 1 micron diamond polish on a DAC cloth. That should give you good results if you get the times right.

(09-30-2013 08:25 PM)KevinS Wrote:  

(03-12-2008 04:27 PM)Gordon Wrote:  Hi J_rock

Quote:Can you recommend any reading material or courses regarding metallographic preparation? My experience in this area is limited?

Some ASM courses in Maastricht just prior to the ITSC 2008 conference event in Maastricht, The Netherlands.

The "Handbook of Thermal Spray Technology" has a pretty good section in it about polishing. For WC FNS strongly suggest the final step be 1 micron diamond on a DAC type cloth for 15 minutes.

[JeffVigor]

You may want to investigate a finer powder. Powder Alloys Corp,

https://www.powderalloy.com/

makes a fine cut of the GE Tungsten Carbide material that will significantly reduce the porosity. I would get a sample of their material and try it. Make sure you specify the fine cut as they produce several different cuts.

Also, when polishing the samples make sure you are using a no-nap cloth. The longer nap polishing cloths will induce porosity in your sample.