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!!!