I'm going to add the machining info as a separate post, since most won't be interested, but I'll reference this post in the OP. OK. Here goes:
Traditional Machining of Titanium
The technology supporting the machining of titanium alloys is basically very similar to that for other alloy systems. Efficient metal machining requires access to data relating the machining parameters of a cutting tool to the work material for the given operation. The important parameters include:
Tool life
Forces
Power requirements
Cutting tools and fluids
Guidelines. The following guidelines, based in large part on the inherent factors affecting the machinability of titanium described above, contribute to the efficient machining of titanium:
Use low cutting speeds: A low cutting speed helps to minimize tool edge temperature and maximize tool life. Tool tip temperature is strongly affected by cutting speed. Lower speeds are required for alloys such as Ti-6Al-4V than are necessary for unalloyed titanium.
Maintain high feed rates: The highest rate of feed consistent with good practice should be used. Tool temperature is affected less by feed rate than by speed. The depth of each succeeding cut should be greater than the work-hardened layer resulting from the previous cut.
Use a generous quantity of cutting fluid: A coolant provides more effective heat transfer. It also washes away chips and reduces cutting forces, thus improving tool life.
Maintain sharp tools: Tool wear results in a buildup of metal on the cutting edges and causes poor surface finish, tearing, and deflection of the workpiece.
Never stop feeding while tool and titanium are in moving contact: Allowing a tool to dwell when it is in moving contact with titanium causes work hardening and promotes smearing, galling, and seizing. This can lead to a total tool breakdown.
Use rigid setups: Rigidity of the machine tool and workpiece ensures a controlled depth of cut.
Although the basic machining properties of titanium metal cannot be altered significantly, their effects can be greatly minimized by decreasing temperatures generated at the tool face and cutting edge. Economical production techniques have been developed through application of the previously mentioned basic rules in machining titanium.
Tool Life. Tool life data have been developed experimentally for a wide variety of titanium alloys. A common way of representing such data is shown in Fig. 10.2, where tool life (as time) is plotted against cutting speed for a given cutting tool material at a constant feed and depth in relation to Ti-6Al-4V. It can be seen that tools for machining titanium alloys are very sensitive to changes in feed. At a high cutting speed, tool life is extremely short; as the cutting speed decreases, tool life dramatically increases. Industry generally operates at cutting speeds promoting long tool life.
Figure 10.2: Effect of cutting speed and feed on tool life during the turning of Ti-6Al-4V alpha-beta alloy:
Forces and Power Requirements. Cutting force is important because, when multiplied by the cutting velocity, it determines the power requirements in machining. For general approximations, the power requirements in turning and milling can be obtained by measuring the power input to the drive motor of the machine tool during a cutting operation and by subtracting from it the tare, or idle power. A good approximation of the horsepower required in most machining operations can be predicted from unit power requirements. Table 10.2 shows the power requirements for titanium in comparison with other alloys.
Table 10.2: Average unit power requirements for turning, drilling, or milling of titanium alloys compared with other alloys systems:
Tool Materials. Cutting tools used to machine titanium require abrasion resistance and adequate hot hardness. Despite the use of new tool materials—such as special ceramics, coated carbides, polycrystalline diamonds, and cubic boron nitride—in metal removal of steels, cast irons, and heat-resistant alloys, none of these newer developments have found application in increasing the productivity of titanium machined parts.
Generally, only straight carbide and general-purpose high-speed or highly alloyed tool steels can be used. Carbide tools (such as grades C-2 and C-3), if feasible, optimize production rates. General-purpose high-speed tool steels (such as grades Ml, M2, M7, and M10) also are used. However, better results are generally obtained with more highly alloyed tool steel grades, such as T5, T15, M33, and the M40 series. Cutting tool performance is influenced by many factors. Setup, processing methods, grinding techniques, material quality, and the condition of the machine tool and fixturing all influence cutter performance.
In early studies, the straight tungsten carbide cutting tools, typically C-2 grades, performed best in operations such as turning and face milling, while the high-cobalt, high-speed steels were most applicable in drilling, tapping, and end milling. The situation remains much the same today. C-2 carbides are used extensively in engine and airframe manufacturing for turning and face milling operations. Solid C-2 end mills and end mills with replaceable C-2 carbides find application, particularly in aerospace plants. M7 and the M42 and M33 high-speed steels are recommended for end milling, drilling, and tapping of titanium alloys.
Cutting Fluids. The correct use of coolants during machining operations greatly extends cutting tool life, and this is particularly true for titanium alloys. Chemically active cutting fluids transfer heat efficiently and reduce cutting forces between tool and workpiece. Of course, cutting fluids should not cause any degradation of the properties of the workpiece. Chlorine at one time was considered a suspect element in cutting fluids, regardless of the concentration and specific conditions used in titanium alloy manufacturing operations. The aversion to cutting fluids containing chlorine was based on the early discovery of hot-salt stress-corrosion damage in titanium alloys through mechanical property studies (see Chapter 13) and on the unexpected cracking of titanium alloys in cleaning and heat-treatment operations.
Although the presence of chlorine ions (e.g., those found in fingerprints on a part) can cause stress corrosion in some alloys during processing, it is not thought to always damage titanium alloys during machining. Nevertheless, cutting fluids used in machining titanium alloys require special consideration. If chlorinated cutting fluids are used on alloys that may be subject to stress-corrosion cracking, carefully controlled postmachining cleaning operations must be followed. The general prohibition against the use of cutting fluids containing chlorine is not universally observed.
When specifying cutting fluids for machining titanium, some companies have practically no restrictions other than the use of con-trolled-washing procedures on parts after machining. Other manufacturers do likewise, except that they do not use cutting fluids containing chlorine on parts that are subjected to higher temperatures in welding processes or in service. Also, when assemblies are machined, the same restrictions apply due to the difficulty of 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.
Mechanical property evaluations to define the effect of experimental chlorinated and sulfurized cutting fluids on Ti-6Al-4V alloy indicated 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. These results and others suggest that under certain conditions, chlorine-containing cutting fluids are not detrimental to titanium alloys.
Usually the heavy chlorine-bearing fluids excel in operations such as drilling, tapping, and broaching. The use of chlorine-containing (or halogen-containing) cutting fluids generally is not a recommended practice, however. There are excellent cutting fluids available that do not contain any halogen compounds. Actually, for certain alloys and operations, dry machining is preferred. Figure 10.3 shows the effect of various cutting fluids on tool life in drilling Ti-6Al-4V.
Machining Speeds and Feeds. Cutting speed and feed are two of the most important parameters for all types of machining operations. Table 10.3 gives some speed and feed data on turning of selected titanium alloys. Because speed and feed rates have a direct influence on tool life, it is desirable to have charts or graphs for all possible tool and titanium alloy combinations, as well as machining techniques. Considering the range of alloys, tool compositions, and machining techniques possible, such charts are not likely to be available for all situations. However, charts such as Table 10.3 have been compiled for some other machining techniques.
Machining recommendations, such as noted above in Table 10.3 and similar sources, can require modification to fit particular circumstances in a given shop. For example, cost, storage, or other requirements can make it impractical to accommodate a very large number of different cutting fluids. Savings achieved by making a change in cutting fluid can be offset by the cost of changing fluids. Likewise, it might not be economical to inventory cutting tools that have only infrequent use. Furthermore, the design of parts can limit the rate of metal removal in order to minimize distortion (e.g., of thin flanges) and to corner without excessive inertia effects.
An illustration of typical machining parameters used to machine Ti-6Al-4V bulkheads containing deep pockets, thin flanges, and floors at an airframe manufacturer is given in Table 10.4. A bulkhead frequently contains numerous pockets and some flanges as thin as 0.76 mm (0.030 in.). Typical bulkhead rough forgings can weigh in excess of 450 kg (1000 lb), but the finished part is less than 67.5 kg (150 lb) after machining. Extensive machining is done on gas turbine engine components, just as is done on the larger airframe components. Table 10.5 lists typical parameters for machining Ti-6Al-4V jet engine components, such as fan disks, spacers, shafts, and rotating seals.
Table 10.4: Some typical machining parameters used to machine airframe bulkheads from an alpha-beta (Ti-6Al-4V) alloy:
Table 10.5: Example of typical parameters for machining gas turbine components from an alpha-beta (Ti-6Al-4V) alloy:
Increased Productivity with Special Techniques. The inability to improve cutting tool performance for titanium alloys by developing new cutting tool materials—coatings in particular—has been very frustrating. Likewise, very little improvement in productivity has been experienced by exploring new combinations of speeds, feeds, and depths. Some developments of interest include specially designed turning tools and milling cutters, along with the use of a special end-mill pocketing technique.
One of the practical techniques for increasing productivity is to determine the optimum cost in machining a given titanium part for a specific machining operation. If specific data are available relating tool life to speed, feed, and depth for a given operation and cutter, it is possible to calculate the overall cost and time of machining as a function of the cutting parameters. Some companies are using computers to perform such cost analyses and to arrive at minimum costs and optimum production rates for specific machining operations.
Fire Prevention. Fine particles of titanium can ignite and burn. Use of water-base coolants or large volumes of oil-base coolants generally eliminates the danger of ignition during machining operations. However, an accumulation of titanium fines can pose a fire hazard. Chips, turnings, and other titanium fines should be collected regularly to prevent undue accumulation and should always be removed from machines at the end of the day.
Salvageable material should be placed in covered, labeled, clean, dry, steel containers and stored, preferably in an outside yard area. Unsalvageable fines should be properly disposed. Titanium sludge should not be permitted to dry out before being removed to an isolated, outside location.
Dry powders developed for extinguishing combustible metal fines are recommended for the control of titanium fires. For maximum safety, such extinguishers should be readily available to each machinist working with titanium. Dry sand retards, but does not extinguish, titanium fires. Carbon dioxide and chlorinated hydrocarbons are not recommended.
Water should never be applied directly to a titanium fire.