Sand Blasting on Titanium Substrate

[ykang]

Hi can anyone help to solve my problem with the interface contamination on titanium substrate by sand blasting process?

Blast Parameter:
Media Size: #60
Angle: 55 Deg
Distance: 12"
Pressure: 25 psi

I am facing problem with those very fine particles that appeared at the interface that failed to meet the acceptance limit (35%). May I know how could I resolve this problem?

Thank You

[Alexangel1226]

Ykang,

Hi, a few question:
1. what is the surface roughness you intend or control at?
2. What is the grit balsting material you use? corundum?
3. Is this manual or CNC grit blasting operation?
4. Your surface speed or nozzle traverse speed?

Your parameter seems ok. You may try to use finer grit size # 80 or 100
Use compressed air to blow-off all the surface after grit blasting.
Check if you have over blasted the surface which cause fatigue or excessive embeded grit on substract. Otherwise, you may look your traverse speed to ensure the optimum result.

Last but not least, you can try to use M.E.K or other solvent to wipe clean the substract after air blow by a lint free cloth.

Hope this helps.

Regards,
Alexangel1226

[Gordon]

Hi Ykang

to the Surface Engineering Forum.

I would repeat what Alexangel1226 said and also that your blast parameters without knowing blasting equipment and nozzle design/bore size makes it difficult to comment on specific parameters.

[ykang]

Hi:

Please see my parameter:

Roughness: Min 100 uin (Ra)
Media: Aluminium Oxide (#60)
Console Type: Automatic Blast with Pressure Type
Rotation Speed: ~1RPM
Diameter of Part: 300mm
Nozzle Size: 8mm ID Boride

My equipment is very limited to adjust the transverse speed, so that I would need to compensate it by adjusting other parameters. Compressed Air-blowing is always done to remove loose particles, however it seems not possible to remove those tiny particles, which can be observed clearly under microscope.

Thanks

Regards
Ykang

[Gordon]

Hi Ykang

I know titanium is prone to this problem. I'm hoping some of our members with more day to day production experience of coating titanium will comment.

Factors to consider in reducing grit entrapment:

Use minimum of blasting to achieve the desired etch. Over blasting will increase grit retention, apart from being wasteful.

Blast pressure is critical. Too high will make the problem worse, too low will be inefficient. Blast nozzle type/bore size is also important aspect here.

Blast angle is generally reported to produce higher grit retention at 90 degrees than at lower angles like 55, but tensile bond strength tests indicate higher values at 90 degrees. Also the angle of thermal spray relative to grit blast angle can effect bond strength. Generally using the same direction for both produces best results down to around 55 degrees. Using opposing angles say 55 degree blast and spraying 55 degree from opposite direction though would not be good

Blast media - try to use only fresh/new, avoid spent and broken down media containing fines and contaminants. I know some like to use the white higher purity alumina grit, but I think this tends to break down quicker than the tougher brown versions containing titanium dioxide/silica. I've seen silicon carbide grit used, though I don't really know how effective it is, but may be worth a try. Grit size and particle distribution size range will be other factors.

I've heard some go to the extremes of ultra-sonic cleaning to remove some retained grit. I don't know if this is effective and there is always an increased risk of contamination and deactivation of the surface.

I can only advise that you experiment with your grit blasting procedure, bearing in mind the above comments.

Good luck

[J_rock]

Hi Ykang

Brush your gritted surface with a nylon nylon brush (a fingernail cleaning brush with white bristles works well) and blow off with compressed air. We use this procedure on Ti Blades and there is a noticable difference under the microscope with and without the additional brushing.

good Luck

[ykang]

Hello... Thanks for your prompt reply. I hope these will help me to solve the problem.

[Alexangel1226]

J_rock Wrote:Hi Ykang

Brush your gritted surface with a nylon nylon brush (a fingernail cleaning brush with white bristles works well) and blow off with compressed air. We use this procedure on Ti Blades and there is a noticable difference under the microscope with and without the additional brushing.

good Luck

This approach sounds great, creative and effective if it is already tested and proven at your end. Will leverage this best practice!! Thanks.

Another way is what mentioned by Gordon, the ultrasonic treatment will normally romove this embedded grit quite effectively, but depends on if you really need to go to that extend, perhaps it is worth trying.

Gordon,
Can you enlighten me more on the Titanum substrate genral problem with its property esp on the typical defects when thermal coated? I do not spray much Ti parts, but it is within our shop repair capability, just to get myself ready. :ashamed0002:Thanks.

Regards,
alexangel1226

[ykang]

Something that pop-up in my mind. I am not sure if somebody uses Grit-blasting to 'strip' the coating? Do you have any problem with the coating adhesion after re-sprayed, when the part is 'thoroughly blasted' ?

Since Ti is prone to those fine tiny particles, do you think such contamination will cause the delamination of coating?

Thanks
Ykang