Milling

To provide dimensional accuracy and a smooth surface finish during milling operations, it is important to use sharp tools along with a rigid machine and setup. Since most milling operations involve interrupted cutting, high speed steel cutting tools are often used.

Low cutting speeds (10-20 sfm) and light chip loads are required. In some applications, it is necessary to mill nickel and cobalt base alloys at speeds as low as 5 sfm for acceptable tool life. Typical milling parameters for roughing operations are 10-20 sfm with feed rates of 0.001-0.004 in. per tooth, while finishing is conducted in the range of 5-15 sfm at feeds of 0.001-0.004 in. per tooth. Too light a feed, approximating rubbing, will cause an excessive work hardened layer. Because this rubbing action at the beginning of the cut is avoided in climb milling, it is preferred to conventional or up-milling. In addition, the downward motion of the cut assists rigidity and diminishes chatter. However, climb milling requires a very rigid setup and a machine equipped with a backlash

Direction of Cut

Direction of Cut

Nose Radius

Fig. 6.24. Effect of Turning Tool Nose Radius on Workpiece43

Work

Nose Radius

Sharp Point

Fig. 6.24. Effect of Turning Tool Nose Radius on Workpiece43

eliminator. Cuts deeper than 0.060 in. are seldom attempted with climb milling, because it is virtually impossible to obtain the required rigidity. Face milling is preferable to slab milling, because it reduces work hardening and chatter.

Two cutter design principles require special consideration for milling cutters: (1) tooth strength must be greater than that required for milling steel or cast iron, and (2) relief angles must be large enough to prevent rubbing and subsequent work hardening. Milling cutters should be designed so that the teeth have positive rake and helix angles. Inserted blades are used on nearly all but the smallest cutters, because even under the most favorable machining conditions, the life of the cutting edges is short. Mechanical methods of securing the blades in the cutter body are preferred because replacement of chipped or broken blades is easier. Because of the interrupted cutting action, high speed steel is used for cutters in most applications. However, carbide is frequently more economical than high speed steel when milling the more difficult to machine alloys, such as the highly alloyed precipitation hardening grades.

Sulfochlorinated oil introduced in copious amounts at the exhaust side of the cutter is the preferred condition for milling superalloys. Soluble oil emulsions are often used, and they provide better cooling for the tools and workpieces than straight oils. However, some sacrifice in surface finish and tool life occurs with the use of soluble oil emulsions compared to sulfochlorinated oils. The latter are often diluted with mineral oil (up to 50%) to obtain fluidity with no large sacrifice in the ability to promote cutting and achieve a good surface finish. Workpieces milled with sulfochlorinated or other chemically active oils must be thoroughly cleaned before being heated to elevated temperature.

6.9.3 Grinding44

Grinding is often used to machine highly alloyed heat treated cast turbine blades. When the machining operation calls for a significant amount of material removal, a rough grind followed by a finish grind can be used to produce a defect-free surface. Because high temperature superalloys are sensitive to the level of energy used during grinding, metallurgical alterations and microcracking may occur at the surface. If an extremely accurate surface is required, the work should be allowed to cool to room temperature after the final roughing grind. This allows a redistribution of internal stresses, and the resulting distortion, if any, can be corrected in the final grinding operation.

Aluminum oxide wheels work best for superalloys; however, CBN is also used for some precision grinding applications. Grinding pressures should be great enough to cause slight wheel breakdown. Coarse wheels (46-60 grit) produce the best finishes during surface grinding. Low pressures help prevent distortion during grinding, especially with annealed material. Using moderate wheel speeds (e.g., 4000sfm) reduces grinding heat and the probability of the workpiece cracking. Reciprocating tables are preferred to rotary tables, because they have reduced wheel contact, generate less heat, and cause less distortion of the workpiece.

Flood cooling is used during grinding to prevent heat checking (cracking) of the workpiece surface. Due to the low thermal conductivity of superalloys, copious amounts of grinding fluid are important. Highly sufurized water based soluble oils provide the best heat removal. Chlorinated oils and chlorinated water soluble oils with about 1% chlorine are often used for wet dressing of form grinding wheels to a tolerance of 0.002 in. or less. However, residual or entrapped fluid will react with the alloy during high temperature service and are often avoided for this reason.

Creep feed grinding is frequently used for turbine components, such as cast single crystal blades. A comparison of creep feed grinding with conventional grinding is shown in Fig. 6.25. In conventional reciprocating surface grinding, the grinding wheel makes rapid traversals removing only a small amount of material for each pass. In creep feed grinding, the workpiece is traversed at a very low table speed with a large depth of material removed per pass. In addition, the entire width of the wheel is used in creep feed grinding. Since creep feed grinding uses a low workpiece speed and a large depth of cut, it requires a larger total force, and therefore more power, than conventional surface grinding. However, creep feed grinding allows much higher metal removal rates. A typical grinding cycle would be to take the majority of the stock in one or two roughing passes, dress the wheel, and then take a finishing pass.

Area of Contact

Reciprocating Motion Short Arc of Contact

Area of Contact

Reciprocating Motion Short Arc of Contact

Depth of Cut

Cross Feed

Depth of Cut i

Cross Feed

Conventional Grinding

Conventional Grinding

Creep Feed Grinding

Fig. 6.25. Conventional and Creep Feed Grinding44

Creep Feed Grinding

Fig. 6.25. Conventional and Creep Feed Grinding44

An extension of creep feed grinding is the Very Impressive Performance Extreme Removal (VIPER) process developed by Rolls Royce in the UK. In the VIPER process, small diameter (<8in.) aluminum oxide wheels are combined with high pressure coolant (1000 psi) that is precisely injected into the grinding wheel ahead of the grind. Centrifugal force moves the coolant out of the wheel during the grind, both cleaning the wheel and cooling the workpiece. Productivity improvements in the range of 5-10 times have been reported, depending on whether or not the wheel is intermittently dressed or continuously dressed during the grind operation.

6.10 Joining

Superalloys can be joined by welding, diffusion bonding, brazing/soldering, adhesive bonding, and with mechanical fasteners. Although adhesive bonding is rarely used for superalloys, adhesive bonding is covered in Chapter 8 on Adhesive Bonding and Integrally Cocured Structure. Mechanical fastening is covered in Chapter 11 on Structural Assembly.

6.10.1 Welding4546

The solid solution strengthened iron-nickel, nickel, and cobalt based superalloys are readily welded by arc welding processes such as GTAW and GMAW. However, the y' strengthened iron-nickel and nickel based alloys are susceptible to hot cracking during welding, or may crack after welding (delayed cracking). The susceptibility to hot cracking is a function of the aluminum and titanium contents, which forms y', as shown in Fig. 6.26. Cracking usually occurs in the HAZ and welding is usually restricted to wrought alloys with about 0.35 or less volume fraction of y'. Casting alloys with high aluminum and titanium contents, like Inconel 713C and Inconel 100, are considered unweldable because they will

Fig. 6.26. Influence of Titanium and Aluminum on Superalloy Weldability1

usually hot crack during the welding operation. Borderline alloys, such as René 41 and Waspaloy, can usually survive the welding process but may crack later.

Alloys that are hardened with y'' (Ni3Nb), such as Inconel 718, are not subject to strain age cracking, because y" precipitates at a much slower rate than y'. This allows them to be heated into the solution temperature range without suffering aging and resultant cracking. The delayed precipitation reaction enables the alloys to be welded and directly aged with less possibility of cracking. Inconel 718 is readily weldable in the solution treated condition, followed by a combined stress relieving and aging treatment. The weldability of Inconel 718 is yet another reason for its high usage.

If the surface of the material to be welded is not free of oxide, superalloys, due to the same oxides that give them good oxidation resistance, can experience trapped oxides in the weld metal and lack-of-fusion defects at the weld metal/parent metal interface. The oxide cannot be removed by simple wire brushing; an abrasive grinding operation is needed to positively remove the oxide. Ineffective inert gas protection can allow the reformation of an oxide film on the surface of the deposited weld metal. Care needs to be taken to remove this oxide before multipass welding to avoid the problems of entrapped oxide and lack-of-fusion defects.

Typical joint designs are shown in Fig. 6.27. Beveling is not required for material that is 0.1 in. or thinner. Thicker material should be beveled using either

V-Groove

Double V-Groove

U-Groove

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