DRY or WET Blasting

[ir_minja]

Dear all,

For before we proceed with HVOF process, the parts that we want to coat need to clean the point we want to coat first. My situation is, nowaday i want to buy a blasting machine which is dont know what type that i want to buy. It is wet blasting machine or dry blasting machine. Wy we need to used wet blasting or dry blasting before proceeding HVOF process. Need your explaination regarding the above matter.

Warmest Regards,
Ir_minja

[William]

dry blasting before proceeding HVOF process

[Gordon]

Hi Ir_minja

I'm with William, DRY grit blasting.

Wet blasting may have it uses in surface preparation, but I would not recommend using it as final preparation method before coating. Surface needs to be dry and free from contamination.

[ir_minja]

Thank for your prompt reply,

I have a few more question:

1) What is the effect (Metallurgy and mechanical properties) on surface when we do surface prepation by using dry blasting?
2) What is the best surface roughness to do HVOF process

I need the answer ASAP. I have to explain the details to my bos by today. Please help me..

Looking forward to hear your prompt reply.

Best Rgrds,
Ir_minja

[karunanidhi]

Dear,
\
Before reporting your boss you have to understand the following,

Types of blasting procedures
types of blasting media
blasting parameters
Types of HVOF coating materials
Thickness requirement of each coating material
Surface roughness requirement with respect to coating thickness

When you found answers for the above , then only, you ll be able to answer your boss.

regards
karunanidhi-

[ir_minja]

Thank for your reply karunanidhi,

For surface roughness parameter with respect to thickness coating, where can i get the correct for this parameter. FYI, my company agreed to buy a new dry blasting machine brand Pan Blast Equipment. What is the effect on shaft surface when we use dry blasting process??

[William]

Hi Ir_minja
My recommend as follow:
1. After blasting shall be to visual inspect grit blast surface for complete coverage, even matte finish and no shiny or reflective surfaces.
2. The blasted Surface Roughness: 100 - 140 Ra.

Best regards,

[William]

Dear all,
I have question with the Interface condition.
How do your procedure prevent the embeded foreign particles fo contamination between the base metal and the coating?
Thanks,

[Gordon]

Hi Ir_minja

(04-08-2009 02:00 AM)ir_minja Wrote:  Thank for your prompt reply,

I have a few more question:

1) What is the effect (Metallurgy and mechanical properties) on surface when we do surface prepation by using dry blasting?
2) What is the best surface roughness to do HVOF process

I need the answer ASAP. I have to explain the details to my bos by today. Please help me..

Looking forward to hear your prompt reply.

Best Rgrds,
Ir_minja

(04-08-2009 07:23 AM)ir_minja Wrote:  Thank for your reply karunanidhi,

For surface roughness parameter with respect to thickness coating, where can i get the correct for this parameter. FYI, my company agreed to buy a new dry blasting machine brand Pan Blast Equipment. What is the effect on shaft surface when we use dry blasting process??

The effect on metallurgy and mechanical properties on part due to surface preparation by grit blasting blasting in most cases is not significant. Although, with some materials grit blasting and coating can influence fatigue strength properties. Thin section parts may be prone to distortion.

Quote:What is the best surface roughness to do HVOF process

I will make the point that surface roughness as in a Ra reading is not always the best or only guide. More important is the quality of texture, grit should be cutting as a posed to peening surface to produce a "sharp" etch. Extreme example; surface blasted with sharp fine alumina grit producing a very low Ra reading will be far far superior to say a very rough high Ra surface prepared by shot blasting. Generally, fine texture for thin coatings and coarser for thicker coatings. This should be controlled by correct selection of grit media and grit size more so than changing other blasting parameters. Also be aware that the hardness of the substrate will influence the grit blasting effect.

[Gordon]

Hi William

(04-09-2009 03:12 AM)William Wrote:  Dear all,
I have question with the Interface condition.
How do your procedure prevent the embeded foreign particles fo contamination between the base metal and the coating?
Thanks,

A few previous posts on this subject, although aimed at titanium substrates should be applicable generally.

(05-27-2008 03:11 PM)Gordon Wrote:  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

(05-27-2008 04:36 PM)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

[ir_minja]

Dear Gordon and william,

Thank for your great explaination and valuable information. I'm really appreciated your fast response.

Warmest Regards,
ir_minja