Stains,what is the probable cause?

[dhdz187]

We are serving an aluminum parts for our customer, it is a part that used in hard disk industry. the material is AL6061-T6 and beadblast is required at customer side after all machining process finished. We only do machining process,and beadblast is done by customer.

The problem now is that some stains was found on the surface of the part after beadblast and drying process. Below is the photo:
Stains was found on the surface near the blind holes (8-32 UNC threaded hole).
It must be strongly emphasized that stains only be found after beadblast and drying process. It looks nice before these process.

Here is something to share, it is what we are doing now :
Only water-soluble cutting fluid is allowed during machining process and ultrasonic cleaning was followed before shipment to customer, we even check all blind holes with a cotton swab.
We are making efforts to improve our cleaning process continuously, but problem still can't be solved.

What 's the probable root cause?
Ultrasound frequencies? Ultrasonic cleaning temperature? or cleaning solvent is not suitable? or other cause?

Any comment will be appreciated.

 

[CoryPad]

The staining is caused by a chemical reaction between the Al and a contaminant. It may occur only after blasting because that process is damaging the natural oxide layer and allowing the contaminant to contact the Al below. I would check for the usual contaminants, especially S and Cl, in the cleaning process fluids.

 

[ornerynorsk]

Are other material blasted in the same cabinet?

 

[Ceramicguy]

Are you sure it's your problem? The picture is a little unclear but it looks like stains from a liquid drying on the surface. Perhaps bleed-out from the tapped hole. Is your customer dry blasting or wet blasting the parts?

Bruce

http://www.accuratus.com

 

[dhdz187]

CoryPad (Materials) 5 May 10 9:20
The staining is caused by a chemical reaction between the Al and a contaminant. It may occur only after blasting because that process is damaging the natural oxide layer and allowing the contaminant to contact the Al below. I would check for the usual contaminants, especially S and Cl, in the cleaning process fluids.

As you said, staining should be found at any area that has a chemical reaction, but it only found near the threaded holes.

 

[dhdz187]

CoryPad (Materials) 5 May 10 9:20
The staining is caused by a chemical reaction between the Al and a contaminant. It may occur only after blasting because that process is damaging the natural oxide layer and allowing the contaminant to contact the Al below. I would check for the usual contaminants, especially S and Cl, in the cleaning process fluids.

As you said, staining should be found at any area that has a chemical reaction, but it only found near the threaded holes.

 

[dhdz187]

Ceramicguy (Materials) 5 May 10 15:00
Are you sure it's your problem? The picture is a little unclear but it looks like stains from a liquid drying on the surface. Perhaps bleed-out from the tapped hole. Is your customer dry blasting or wet blasting the parts?

Actually we think the problem is not our problem,but we have no evidence to prove it.

Customer should balsting the parts by dry blasting. it is a cycle use part, blasing - putting into use - acid cleaning - blasting - putting into use - acid cleaning,keep doing this loop until the parts can't be used anymore.

I attached a photo, hope it's useful.

 

[Ceramicguy]

What acid are they using? It's purity level when new? Is contamination level monitored as a process variable? Are the holes blind and the residual acid wicking out during drying. Are the threaded fasteners of the same material as the ring? Has the customer considered a final rinse in a water displacer and heated drying?

Bruce

http://www.accuratus.com

 

[FACS]

Bead blasting makes the surface wick liquids. It looks like whatever fluid is in the threaded hole is being wicked out after blasting.

Charlie

http://www.facsco.com

 

[dhdz187]

Sorry, I am on a leave during the past days? Could we back to this topic again ?

This issue has been communicated to our boss,it is very serious.My team need to solve it quickly.

Since customer insisted that the problem is at our side(machining process), not at beadblast process. So could anybody share some knowleague about beadblast ,if it can cause this problem ? in which ways?

 

[dhdz187]

Ceramicguy (Materials) 6 May 10 8:30
What acid are they using? It's purity level when new? Is contamination level monitored as a process variable? Are the holes blind and the residual acid wicking out during drying. Are the threaded fasteners of the same material as the ring? Has the customer considered a final rinse in a water displacer and heated drying?

for sure,fastener is stainless steel with silver plating, THat is only what I can confirm now.

 

[Ceramicguy]

Hi dhdz187,
I went back to the photograph in light of the fastener information you provided. I am more convinced than ever it is a liquid coming out of the blind hole and drying on the surface. Remember, that stain may be only a few atomic layers thick so it won't take much material to cause the effect. Whether the material is a compound of aluminum, silver, machining fluid or something else is not too important. The deepening coloration at smaller surface areas indicates to me that it is something dissolved in the fluid coming from the hole and depositing more thickly as the liquid evaporates (and the concentration of the culprit contaminant rises). If you must know what the stain is, perhaps a quick EDAX of the stained area compared to the "clear" area will help you track down exactly what is causing the coloration and be an interesting intellectual exercise but, it may still not resolve the situation. Getting the liquid out of the blind hole is the secret. Can your customer implement something as easy as blowing the blind holes out with compressed air between or after rinse operations?

Bruce

http://www.accuratus.com

 

[dhdz187]

Hi Bruce,

Thank you very much for your patience and help on this issue. Your analysis is convincing and easy to understand. So we will discuss with our customer as per your advice of blowing the blind holes between their operations.

Now,our sales team are doing two experiment, one is ultrasonic cleaning in Singapore,to see if it can be some helpful to the contaminant, and the other is we do the beadblast by ourself and also the acid cleaning. Hope I can get some positive result and to close this issue.

Thank you again.

 

[cpmetalurgist]

Just joined so sorry about the delayed response.

Another possibility is you may be driving your drilling operation too hard and over heating the aluminum. You don't see it but there may be chemical changes happening to your aluminum at the grain level. When the part is put in a cleaner that works like an electrolyte you may get galvanic corrosion between the two materials now on the same part. The stain looks like it could match a heat zone for the hole.

Now throw in the SS fastener and you really have a galvanic reaction with any liquid that can conduct.