Procedure, materials and tips for surface finishing Ti-6Al-4V 2

[DuncanGibbons]

Hi all,

I am wanting to improve the surface finish on some Ti-6Al-4V ELI test specimens and a demonstrator component. These parts are in two surface conditions currently, some in the machined conditions +/- 1 um Ra and +/- 6.5 um Rz, and some in the electropolished condition, +/- 3.3 um Ra and +/- 19.5 um Rz. What is the best approach for hand polishing these parts down to Ra < 0.2 um and Rz < 1 um?

Also what types absrasives and materials are compatible with Ti-6Al-4V ELI and are recommended that can help me achieve such a finish (ideally that won't cost an arm and a leg as it's for a student project, I have a dremel, the usual power tools and lots of elbow grease)?

Many thanks,
Duncan

 

[mfgenggear]

Look into super finish. Hand polish or by commercial methods.

 

[TugboatEng]

Electro-polishing can be surprisingly inexpensive if you don't mind a short lead time. My understanding is that TiAlV will not make a mirror finish if appearance is a concern.

This is on Reddit, for what it's worth:

Ti is kind of a bitch, it's the only material we polish anywhere near like the method we use… 220 grit to 9µm diamond to 0.04µm OP-S colloidal silica mirror polish combined with 10% by volume 30% H2O2. The final polish with the H2O2 is the trick… you need the chemical activity to get mirror.

For any other material, we'd NEVER skip from 220 to 9µm, nor from 9µm to final polish. But it's what works. Check the guides people have linked to.

I see here that silicon carbide was used which is common for softer materials. The polish contaminates the surface some and requires removal by ultrasonics cleaning.

https://www.researchgate.net/post/How-to-polish-titanium-disc-by-using-silicon-carbide

 

[EdStainless]

We used cushioned abrasive pads and wet polished (DI water with a small amount of surfactant, don't use soap).
We never went from 220 straight to 9um, but we did go in larger steps than with other materials.
I want to recall 180 or 220 (depending on starting condition) then 400, 800, 9um, and 0.05um for finish.
Each pass should be 90 deg to the previous pass so that you can see that you removed all prior scratches.
EP will give you a surface that is microscopically smooth.
Ra or anything measured with a profilometer is wave and lay.
And reading below 25um are basically worthless, the stylus is too large.
For those surfaces you have to measure optically.

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[DuncanGibbons]

Thanks for the tips,

I will try SiC sandpaper/abrasive pads of grit 220, 400 then 800, I suppose using an angle grinder with the pads will give better results than sanding by hand? I see online that SiC is preferred to aluminium oxide for titanium and hard metals.

For the finishing polish is it recommended to use polishing compounds (seems popular for jewellery) or is it preferred to use the diamond suspension of 9 and 0.05 microns? Also, should these polishing media be applied to a buff wheel or is there a special cloth that should be used (such as those use for metallographic specimens)?

Thanks

 

[EdStainless]

If these samples are flat then you should be doing this on a very flat platen.
Otherwise you will end up with a wavey surface.
I don't know why you would need an angle grinder unless these are a few feet long.
If you can hold the piece in your hand then doing them by hand makes sense.
You will only be removing a couple of thousandths of an inch of material.
I used SiC for rough passes, diamond for 9um, and a ceramic (alumina or ceria) suspension for 0.05um

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P.E. Metallurgy, consulting work welcomed

 

[mfgenggear]

https://roslerblog.com/2016/11/18/superfinishing/

 

[DuncanGibbons]

Last question, what media (isopropanol, water etc.) should be used to clean the surfaces after each grit and before starting the next?

 

[IRstuff]

I suppose using an angle grinder with the pads will give better results than sanding by hand?

An angle grinder is typically pretty heavy and you do not want to leave deep ruts, so an orbital sander with a light touch would be more appropriate. I've polished marble tiles, which I think are more challenging, to a near mirror finish with a series of water-wetted grits starting at 1000 and I think I ended with 6000.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[EdStainless]

between grits you wash with water and rinse with isop or acetone.
Make sure to wash your hands also, you have to eliminate all grit carryover.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed