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Up Topic Welding Industry / Technical Discussions / Abrasives for X-ray quality welds??
- - By ADELUCA (*) Date 01-22-2004 14:18
I am required to do a GTAW x-ray quality weld on 6242 Titanium the part is large and not easily machined. Is there an abrasive out there that will not contaminate the edge??? The parts will be acid etched before welding. Is there a robotic machine with a carbide cutter that would be sutable? Maybe some kind of high power rotory file? Any suggestion would be appreciated. Thanks in advance

Al
Parent - By Niekie3 (***) Date 01-22-2004 18:49
Hi Al

Are you asking about how to abrasively prepare the weld edge? If so, any grinding disc suitable for S/Steel should be OK for Ti. If I have misunderstood the question, please ask again.

Regards
Niekie Jooste
Parent - - By Lawrence (*****) Date 01-22-2004 21:30

Al,

Tell us a little more about the project.

If money is no object than I would suggest contacting TWI http://www.twi.co.uk/j32k/index.xtp as these folks are preeminent in the field. And they do have a bit of free info on their website you might find helpful.

If your doing a large production run than a mechinized system might make sense. If however your just needing a little surface oxide removal right in the weld area than Neikie is right.

All grit type abrasives will leave some residue, if you use them there is no escape. This does not however, mean that you can't use them and still get acceptable results.

While some folks (and some codes) requre silicon carbide sanding discs for edge prep. Most mettalurgists agree that it really has no practical advantage over aluminum oxide discs.

Tell us more, we love talking about titanium around here.

Parent - - By G.S.Crisi (****) Date 01-23-2004 14:46
Lawrence,
let me clear up one thing and don't get angry with me.
Silicon carbide and aluminum oxyde grinding disks are two different things, each one having its own applications.
Silicon carbide, of which Carborundum is the most known brand, is used for carbon and low alloy steel. It's no good for stainless, because tiny quantities of the carbon contained in the silicon carbide remain trapped in the steel. This, in turn, originates the well known problem of "intergranular corrosion". For stainless steels, aluminum oxyde grinding disks should be used.
Recently, another type of material has appeared for stainless steel: zirconium oxyde. As titanium belongs to the same family of zirconium, I'd suggest Al to use this type of disk.
Giovanni S. Crisi
Sao Paulo - Brazil
Parent - By Lawrence (*****) Date 01-23-2004 17:55
Prof. Crisi

You said it perfectly.

Some of the Major Aircraft engine makers (GE, CFM, and Pratt come to mind) produce and repair a variety of titanium componants and they have a standard practice manual that most vendors and airlines follow, these docs have had a call out for silicon carbide 120 grit discs for pre-weld oxide removal. My personal inquiries with EWI back when we had membership, into why they were superior to aluminum oxide resulted in a rather hazy explaination that there was no perceptable difference (in our welding environment) when it came down to it but that we should still stick to our specs for the sake of consistency.

You rightly point out that silicon carbide discs (we used Norton) are not recommended for Stainless/inco/super alloys.

Good point.
Parent - By brande (***) Date 01-25-2004 06:33
If you want to be absolutely sure there is no contamination on weld prep, then the only real alternative you have is a carbide burr or similar mechanical means.

All abrasives can leave residue (grains) in the weld area. Some abrasives are better than others in the amount of residue left, but they will all leave residue.

Is it a big thing-depends on your code and requirements.

If some contamination is acceptable-go abrasives-they are much quicker.

If not-mechanical means, including carbide burrs, milling, and the like should be employed.

Have a ball-Good Luck

brande
Parent - - By G.S.Crisi (****) Date 02-18-2004 22:07
At this time you must have solved your problem, so this opinion is posted just for reference of future readers.
I am Thais Maldonado, student of Material Engineering at Mackenzie Presbyterian University in Sao Paulo, Brazil.
In order to get my engineering degree, I had to prepare a monograph on how to select the right grinding wheel for engineering applications, so I feel I am qualified to give an opinion on Al's question. By the way, my monograph was approved with very good qualification notes.
The suggested type of grinding wheel for Al's application is silicon carbide. Silicon carbide is recommended for relatively soft materials like titanium (it is softer than steel) and will leave no residues which can show up in the radiography.
Any comment to this posting will be appreciated.
Thais Maldonado
Parent - By ssbn727 (*****) Date 02-19-2004 06:06
Hi Giavonni, Thais!
Thais, if you're saying that all Ti alloys are "softer" than all steels then I beg to differ if you mean what I interpreted so if you could elaborate further, I'd appreciate it.
Remember Thais, the original post mentioned that they were using a 6242 Ti alloy and not a CP (Commercially Pure) grade.
Ti is successfully ground by selecting the proper combination of grinding fluid, abrasive wheel, wheels speeds and feed rates. Both aluminum oxide and silicon carbide wheels can be used. Considerably lower wheels speeds than in conventional grinding of steels are recommended.
Feeds should be light, positive, constant and particular attention paid to the coolant.
A water-sodium nitrite coolant mixture gives good results with aluminum oxide wheels. Silicon carbide wheels operate best with sulfo-chlorinated oils but these can present both a fire hazard, not to mention the increased potential for corrosion if left on the surface without a subsequent acid pickle and it is important to flood the work when using these oil based coolants. (use the water based coolant instead) After grinding, it is highly recommended to acid pickle the worked surface to remove any residue left from grinding. Bottom line with Ti is cleanliness on the surfaces to be welded.
Now if you want to use a carbide burring tool bit then be aware that the tool life of whichever type you chose will be considerably less than when using these bits on let's say - mild steel because the Ti metal chips being removed tend to fuse to the cutting surfaces or "teeth" on the burring tool bit which eventually breaks off so a cutting fluid such as the above mentioned water based type is recommended to be used in conjunction with these bits also.
Any type of machining (turning, tapping, drilling, grinding, milling, etc.) used on CP Ti is similar in the characteristics when one machines 18-8 type stainless steels, with the Ti alloy grades being somewhat harder to machine.
In milling Ti, when the cutting edge fails it's usually because of chipping.
Thus, the results with carbide tools is often less satisfactory than with high speed steels. The increase in cutting speeds of 20-30% which is possible with carbide tools compared with high speed steel tools does not always compensate for the additional tool grinding costs.
Consequently, it is advisable to try both to determine which works better for a specific Ti grade. Always use a water based cutting fluid when milling Ti and it's alloys.
When I had to use a burring tool (die grinder) on Ti, I also used a stainless steel wire brush to clean the teeth on the bit in order to prevent chips from fusing on to the teeth. This required constant attention to the condition of the burring tool bit so, I grinded for no more than 30 seconds at a time with a water based cutting fluid like the type mentioned above and very carefully!!! The best burring tool bits were the metal carbide ones like a C-91 or similar types. Of the high speed steels, Colbalt types seem to be the best.
In summary; Good tool life and successful machining of Ti alloys can be assured if the following guidlines are observed:

1) Maintain sharp tools to minimize heat buildup and galling.

2) Use rigid setups (Clamping) between tool and workpiece to counter workpiece flexure.

3) Use a generous quantity of cutting fluid to maximze heat removal.

4) Utilize lower cutting speeds. 5) Maintain high feed rates.

6) Avoid interruption in feed (positive feed)

7) Regularly remove turnings or chips from the tools and the machines.

Finally, after any type of machining is completed on Ti and it's alloys -You MUST clean the metal with an acid pickle to ensure that no residual contaminants are left on the surfaces that are to be welded together.
The same applies whether or not the surfaces that are worked or machined are to be fused/welded together or not!

Respectfully,
SSBN727 Run Silent... Run Deep!!!

Up Topic Welding Industry / Technical Discussions / Abrasives for X-ray quality welds??

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