Cutting fluids used in machining titanium alloys require special consideration because chlorine ions have, under certain circumstances, caused stress-corrosion cracking in laboratory testing of these alloys for mechanical properties. Consequently, chlorine at one time was considered a suspect element regardless of the concentration and specific conditions used in manufacturing operations, such as machining.
When specifying cutting fluids for machining titanium, some companies have practically no restrictions other than using controlled-washing procedures on parts after machining. Other manufacturers do likewise, except that they do not use cutting fluids containing chlorine on parts which are subjected to higher temperatures in welding processes or in service. Also when assemblies are machined, the same restrictions apply because of the difficulty in doing a good cleaning job after machining. Still other organizations in aerospace manufacturing permit no active chlorine in any cutting fluid used for machining titanium alloys.
A program to define the effect of experimental chlorinated and sulfurized cutting fluids on the mechanical properties of the Ti-6AL-4V alloy (annealed, 34 Rc) was performed. Mechanical property evaluations included:
✦ High-cycle fatigue at both room and elevated temperatures
✦ Fatigue crack propagation at two cyclic frequencies
✦ Fracture toughness
✦ Stress-corrosion/surface-embrittlement exposures
Within the scope of the program, and within the range of variables investigated, the results indicated generally that no degradation of mechanical properties relative to those obtained from neutral cutting fluids occurred. Similar results were obtained by using chlorinated and sulfurized fluids in machining, or by having those cutting fluids present as an environment during testing. The use of chlorine-containing (or halogen-containing) cutting fluids generally is not a recommended practice, despite the above-noted results which pertain to only a single titanium alloy.
There are excellent cutting fluids available which do not contain any halogen compounds. In fact, from extensive test data collected by the Air Force Materials Laboratory, it can be concluded that chlorine-containing cutting fluids do not always provide better tool life. For certain alloys and operations, dry machining is preferred. Usually the heavy chlorine-bearing fluids excel in operations such as drilling, tapping, and broaching.