Posted by Alex Chan (188.8.131.52) on 15:44:36 09/01/06
In Reply to: Re: Cluster oxide in Inco718 posted by Hong Wang
First and foremost, I would like to express my heart felt
appreciation to Gordon in expressing this theory to me. I knew that
these oxide clusters are formed by fine particles that are over
heated and oxidized. But with a good explaination from you has
indeed enhanced my knowledge understanding on its occurrence.
I have not sent those pictures from you. But i will definitely do so. Fyi, I have worked out the coating which has clusters oxide reduced to less than 5 and eventually to less than2 to none. (I have quantified those ratable clusters in my evaluation). I have work on the parameter using my existing system. But still working on the repeatability of the result. Currently still not stable, about 70% coupons sprayed using dif. machines (all sulzer metco unicoat system) and dif. operators could get coating which is free from ratable cluster oxide. Ratable in aerospace standard is 0.003" and above.
Here I would like to share my evaluation result and would appreciate if Gordon or any expert could comment. Generally, I have changed my nozzle from 731 to 730, which is of a larger dia which i use to spray other metallic powder, and bond coating. Then I have increased the primary gas flowrate to extremely high about 2.5 times from my original parameter. And the stand-off distance from initial 4.5" to 6". Generally, this is the best refinement tentatively I have got after more than 50 coupons evaluated. Sound terrible, but it's real tough!
Now I have almost eliminated clusters oxide, but I m seeing increasing number of unmelted particles and linear oxides which if going greater would be considered as delamination or micro crack as ther will be pull-out from these linear oxides. Theoritically, I think it is due to high primary gas result in higher velocity, and the particle has not melted properly. Yet, it is still within limit.
My present challenge is to produce repeatable result and on the other hand work on another set of parameter using N2/Argon, which suggested by my counter parts.
Another thing to take njote is that i did perform a thorough check on my powder feeding system to ensure the powder feeding is constant without pausing and is injected towards the center of the flame. Air jet has to be parrallel without interfere the flame zone, and turn off external air jet which could possibly induced turbulence.
I will try to get some standard sieve with is withijn the powder spec eg:45 micron to 90 micron to sieve thro the powder and heated up before use. Though it has work out, but I still do not understand what I have done to this far. Pls give your comment on this. Txs.
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