Many of the same characteristics that make superalloys good high temperature materials also make them difficult to machine, namely:
• Retention of high strength levels at elevated temperature
• Rapid work hardening during machining
• Presence of hard abrasive carbide particles
• Generally low thermal conductivities and
• Tendency of chips to weld to cutting edges and form built-up edges.
However, it should also be noted that superalloys cover a wide range of alloys that have somewhat different machining characteristics. For example, the iron-nickel based alloys, which resemble stainless steels, are easier to machine than the nickel and cobalt based alloys. In general, as the amount of alloying elements increases for higher temperature service, the alloy becomes more difficult to machine.
These high temperature characteristics place cutting tools under tremendous heat, pressure, and abrasion, leading to rapid flank wear, crater wear, and tool notching at the tool nose and/or depth of cut region, as illustrated in Fig. 6.22.
End Clear, Face
End Clear, Face
Fig. 6.22. Typical Tool Wear Features40
Due to their high temperature strengths, superalloys remain hard and stiff at the cutting temperature, resulting in high cutting forces that promote chipping or deformation of the tool cutting edge. In addition, since superalloys retain a large percentage of their strength at elevated temperatures, more heat is generated in the shear zone resulting in greater tool wear than with most metals. Since the forces required to cut superalloys are about twice those required for alloy steels, tool geometry, tool strength, and rigidity are all important variables.
Their low thermal conductivities cause high temperatures during machining. The combination of high strength, toughness, and ductility impairs chip segmentation, while the presence of abrasive carbide particles accelerates tool wear. Superalloys also have a tendency to rapidly work harden which can create a hardened surface layer that degrades the surface integrity and can lead to lower fatigue life.
General guidelines for machining superalloys are very similar to those for titanium alloys:41
• Conduct the majority of machining in the softest state possible,
• Use positive rake angles,
• Use sharp cutting tools,
• Use strong geometries,
• Prevent part deflection,
• When more than one pass is required, vary the depth of cut.
To make machining somewhat easier, precipitation hardened alloys are often solution heat treated, rough machined, aged, and then finish machined. This approach results in superior surface finishes with minimal distortion. Positive rake angles minimize work hardening and built-up edges, by shearing the chip away from the workpiece. Sharp cutting edges also help to minimize built-up edges and provide better surface finishes. Dull or improperly ground cutting tools cause higher machining forces leading to built-up edges, metal tearing, and excessive part deflection. However, since sharp edges are more fragile and susceptible to chipping, honed edges are frequently used for roughing operations and sharp edges for finishing.
A large nose radius, where possible, also results in a stronger, less chip prone tool. Rigid set-ups with minimal part deflection reduce vibration and chatter that can cause tool failures and deteriorate tolerances and surface finishes. Large leading edge angles, which engage more of the cutting tool in the workpiece and spread the wear over a larger distance, help to prevent localized notching. In turning operations using NC equipment, the depth of cut can be varied from one pass to another to spread the wear along the entire insert and thus prolong tool life.
Standard grades of cemented carbide are usually used for uninterrupted turning operations with C-2 grade used for roughing and C-3 grade used for finishing. Although high speed steel cutting tools wear much faster than carbides, they are sometimes required if heavy interrupted cuts are being made due to their greater resistance to chipping and breakage. The cobalt grades, such as M33 and M42, are recommended because of their greater heat resistance than the standard high speed grades. A comparison of the cutting speeds in surface feet per minute (sfm) illustrates the productivity advantages when carbides can be used. High speed steel:
- Roughing: 0.250in. depth of cut, 12-18 sfm, 0.010ipr
- Finishing: 0.050 in. depth of cut, 15-20 sfm, 0.008 ipr
- Roughing: 0.250in. depth of cut, 30-40 sfm, 0.010 ipr
- Finishing: 0.050 in. depth of cut, 40-50 sfm, 0.008 ipr
Even further productivity gains, through higher cutting speeds, can often be achieved with cubic boron nitride (CBN) and ceramic tools, but the setup needs to be extremely rigid to preclude tool chipping.42 CBN tools are often used when turning the harder nickel base (wrought and cast) and cobalt base cast alloys. Ceramic cutting tools allow speeds of 500-700 sfm with feeds of about 80% those of carbide. Typical ceramic tools include SiAlON and SiC whisker reinforced Al2O3.
The recommended geometries for single point turning tools are shown in Fig. 6.23. As mentioned earlier, it is important to use positive rake angles to insure that the metal is cut, rather than pushed. Positive rake angles also help to guide the chip away from the workpiece surface. The side and end relief angles, which provide clearance between the tool flank and the workpiece, are usually a compromise: too small of an angle will cause rubbing, while too large of an angle will provide insufficient cutting edge strength.
The side cutting angle affects the load on the cutting edge and provides thickness and directional control to the chips. The nose radius provides strength to the tool nose and helps to dissipate heat generated by the cut. A scallop produced by a tool with a nose radius (Fig. 6.24) gives a better surface finish, shallower scratches, and a stronger workpiece, with less tendency to crack at sharp corners than the notch effect produced by a sharp tool.
Water soluble oils in mixtures (1 part oil with 20-40 parts water) are frequently used in turning. Flood cooling is used to minimize excessive heat build-up. If sulfurized or chlorinated oil is used as a cutting fluid, the workpieces must be thoroughly cleaned before heat treatment or high temperature service. Serious damage to workpieces during heating cycles can result if any residue remains.
End Cutting Edge Angle Roughing 6° Finishing 60°
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