Machinability of Super Alloys – General Machining
Machinability of Materials Group “S” (HRSA – Heat Resistant Super Alloys) is considered difficult because : HRSA materials are metallurgically composed to have high strength at high temperatures, the stresses that are generated when machining also are high. The unique capability of these Titanium Alloys, Nickel, Iron and Cobalt-based super alloys to perform close to the melting point of their basic metal gives them varied but generally poor machinability.
Annual Global Consumption Super Alloys
There is a growing number of workpieces produced using Super Alloys Over USD 1.35 Bi / year
Source: Dedalus consulting
Main Application Areas for Super Alloys
Super Alloys have a growing demand of application. The main 3 are :
Super Alloys – Group S Materials of Group S can be initially split in 2 main groups : 1) Fe, Ni and Co Based ( Lamina Group 9 ); 2) Ti Based (Lamina Group 10 ).
Super Alloys – Group 9
The Lamina Material Group 9 is divided in 3 main sub-groups:
1) Fe-Based : Incoloy 800, Incoloy 901, Discalloy ( Hardness : 180-300 HB )
2) Ni-Based : Inconel 718, Nimonic 75, Waspalloy ( Hardness : 180-380 HB )
3) Co-Based : Stellite 21, Steelite 32, Haynes 25 ( Hasdness : 300-400 HB )
Fe-Alloys + Ni + Alloys Features
The Fe and Ni based alloys have austenitic structure which brings features as high ductility and work hardening, producing a gummy machining behavior similar to that austenitic stainless steel. In addition, these alloys designed for high temperature applications remain strong at the temperatures of chip formation during machining, creating a difficult chip control, the thermal conduction is much less than steel and other common materials and the temperature stays always much high on the cutting edge.
Co - Alloys Features
The Co-Alloys are the most difficult material on the “S” Group. They combine excellent mechanical wear resistance, especially at high temperatures, with very good corrosion resistance. Its composition is based on Cobalt in addition with Chromium, Tungsten, Carbon and Silicon in most cases. This material composition has an abrasive behavior and increases significantly the wear rate (Flank and Crater Wear).
Work Hardening Working hardening occurs when the metal ahead of the cutting tool, especially one that is cutting poorly, is plastically deformed. This hardened layer is more difficult to penetrate in subsequent passes or following operations. It increases the risks of notch wear as the first failure mechanism during the machining process.
Plastic Deformation When machining HRSA, the heating created on the cutting zone is not quickly dissipated as when machining steels and stainless steels for example. It increases the risks of Plastic Deformation as a possible failure mechanism.
Minimizing Work Hardening In order to minimize the work hardening effect, it is recommended :
1. Tools with sharp cutting edge
2. Tools with positive rake angle
Chip Breaker Recomendation Negative inserts with Radius 0.8mm (e.g. CNMG 120408)
Titanium Corrosion Resistance
The combination between Titanium and Oxigen brings a non-corrosive property to this material due to the inert layer (approx. 0.01mm) on the surface containing TiO2. If there is some damage on the surface and there is oxygen available, this surface is immediately rebuilt. It makes this material very suitable for jet engine parts, landing gears, heat exchangers and many other parts in aerospace industry (eg.: structural parts)
Pieces made of Titanium can be used on the must aggressive environments.
Cutting Parameters Remarks
Characteristics: Super Alloys are materials which succeed in applications that Stainless Steels are not able to solve it, for example. Usually they are very difficult to machine because of its very narrow range of cutting speeds : ▪ If the cutting speed is too low, the material sticks to the cutting edge. ▪ If it’s too high, the high quantity of chemical components produces abrasive wears in the cutting edge. The risk of plastic deformation also increases substantially
Keep the Focus on…
In order to maximize the success rate, it is strongly recommended : 1. Avoid vibration: Reduce risk of insert Breakage 2. Abundant Coolant: Remove the heating from the cutting zone 3. Respect the Speed Limit: On this Material Group it is very important do not exceed the speed limit, only 5% more can “Kill” the insert (Cutting Speed)
Trouble Shooting The main failure mechanisms in Super Alloys are:
• Notch Wear: Vary depth of cut when possible / reduce the feed rate / apply a more positive geometry.
• Crater Wear: Decrease cutting speed / Check coolant direction / Apply a more positive geometry
• Decrease cutting speed / Decrease feed rate
In summary, for successful machining of Super Alloys, it is very important to gather all the information and pay special attention to the cutting data, mainly cutting speed and feed rate, as they have a very narrow recommended application range for good results. It is important to keep in mind that, only optimized cutting speeds are able to achieve expected performance and a small variation on it can change the tool life enormously.