Working with Titanium Alloys
Titanium and stainless steel are generally comparable in terms of machinability, except that when there is a higher alloy content, the machinability decreases. There should not be any problems in the machining of titanium and its alloys, if the special characteristics of titanium are taken into account.
Titanium can be cut very easily, but only if the tools you use are kept sharp. It is always easier to sharpen a tool than to have a wearland develop. Proper tool angles, adequate coolants and the use of slow speeds and heavy feeds are also advised.
Due to the fact that titanium has low thermal conductivity, when cutting, the chips have a tendency to gall and weld to the cutting edges on the tool. This always speeds up the wear on the tool itself. Rather than lose production, it is best advised to work the tool to its maximum capacity and then replace it when productivity decreases.
To lengthen tool life, use of the proper coolant is necessary to reduce cutting temperature and inhibit galling. Cutting fluids containing chlorine, fluorine, bromine and iodine should not be used, in order to avoid corrosion problems.
Titanium, whether commercially pure or alloyed, can be turned easily. Carbide tools are highly recommended for turning. Best results are acquired from metal carbides such as C-91 and similar types. Of the high speed steels, cobalt-types seem to be the best. If carbide is not available, Stellite, Tantung, Rexalloy, or other types of Castalloy tools may be used.
Drilling may be accomplished successfully with ordinary high speed steel drills. When drilling titanium, the most important factor is the length of the unsupported section of the drill. This section of the drill should not be any longer than is required to drill the depth of the hole and allow the chips to flow through the flutes and out the hole. Following this process permits maximum cutting pressure and quick removal to clear chips without breaking the drill. Using a "Spiro-Point" drill for grinding is recommended.
Tapping titanium is one of the more difficult machining operations, due to the problem of chip removal. The use of a gun-type tap where the chips are pushed ahead of the tap can make this process less difficult. When titanium smears on the land of the tap, it may cause another problem, tap freezing or binding in the hole. Sulfurized and chlorinated cutting oils are recommended for this. Best performance in tapping titanium is when using a 65% thread.
For grinding titanium, the ideal combination of grinding fluid, abrasive wheel and wheel speeds can quicken this form of shaping titanium. Alundum and silicon carbide wheels are used. The ideal way to perform this procedure is to use lower wheel speeds than in general grinding of steels. An excellent coolant is a water-sodium nitrite mixture, although, unless proper precautions are taken, this mixture may be very corrosive to equipment.
Sawing titanium requires slow speeds - in the 50 fpm [15.25 mps] range - and heavy, constant pressure from the blade.
Parts that have been machined and will be exposed to higher temperatures should have all cutting oils thoroughly removed. The solvent basically used for this is methyl-ethyl-ketone.
Low flash point cutting oils are not recommended, due to the fact that during machining, high heat is generated, which may cause the oil to ignite. High flash point cutting fluids or water-soluble oils are recommended.
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