Turning Nickelbased Alloys

Obtaining long tool life at high metal removal rates

August 16, 2011

Reducing BUE and Notching in Superalloys

New grade and geometry combinations allow manufacturers to run at much higher cutting speeds, which helps improve productivity.

Superalloys are high-temperature materials that generally are used above 540 degrees C because of their ability to retain strength at these high temperatures. One class of superalloys is the nickel-based group, which includes INCONEL®, Hastelloy®, and other highly alloyed compositions like Rene® and Udimet®.

The properties of these metals are developed through a combination of casting and forging, followed by heat treating. Chemical treatments, annealing, and aging are also used to maximize the corrosion resistance and material strength.

One problem with working with these materials, particularly in the heat-treated condition, is that if a lot of heat is generated in the cutting zone, work-hardening of the material will result.

This means that proper coolant usage is critical to get the chips out of the work zone as quickly as possible and also to cool the area.

"Using high-pressure coolant as part of your machining strategy means that you don't have to stop the machine to clear chips, and you are not breaking tools caused by the recutting of chips," explained David Andrews, Sandvik Coromant Canada's product and application specialist-turning products.

Use in Aerospace

Nickel-based superalloys are used extensively in both aircraft engines and gas turbines for power generation. These components can take dozens and even hundreds of hours to produce, which means that smart tooling usage is necessary.

"Tool selection is critical at each stage of production," said Andrews. "We classify it into three stages of machining: first-stage machining, intermediate-stage machining, and finish-machining."

It's critical for aerospace companies to eliminate deflection of the component caused by the cutting process. During fine-finishing operations on thin-walled components, for example, very little material is left to remove, sometimes as little as 0.005 in. on a finish pass. This means that light pressure must be used.

"Using a ground insert cutting edge during the finish-machining stage puts as little pressure on the component as possible. Also, running a small-nose radii tool helps as well," said Andrews.

New grade and geometry combinations allow manufacturers to run at much higher cutting speeds, which helps improve productivity. While costing more, advanced tooling such as ceramics will reduce the time each component spends on the machine.

"Using ceramics in the roughing stage allows you, in some cases, to run two or three times as fast, which can take an incredible amount of time out of production," said Andrews.

Insert Failure

A combination of crater wear and accelerated flank wear caused by built-up edge (BUE) is common when cutting these materials. Because of the high nickel content in these alloys, BUE is often problematic. The accumulation can very quickly tear the coating off of an insert's cutting edge.

"The nickel content makes these alloys very abrasive and very sticky. This not only causes accelerated insert wear, but also causes the built-up-edge problem," said Andrews.

Increasing the cutting speed, the typical remedy for fighting BUE, is limited in these cases because of the risk of plastic deformation of the cutting edge and increased crater wear caused by the high heat.

A sharp, physical vapor deposition- (PVD-) coated cutting edge will make it easier to remove material without sticking into the material.

Understanding BUE and Notching

"Nickel-based alloys tend to be hard, tough, and [often] work-harden," said Walter Tools Turning Product Manager Kurt Ludeking. "When cutting these alloys, the shearing action at the cutting tool rake face causes work hardening in the chip. Because of the high temperature in the cutting zone and the pressure of the chip on the rake face of the insert, a layer of the workpiece material bonds to the insert, tearing away from the bottom surface of the chip. This repeats continuously, building up layer upon layer of workpiece material on the insert."

Common cutting conditions that cause BUE in these materials are low cutting speed, high feed, high pressure, and high cutting temperature.

The most effective actions to combat the formation of BUE are:

• Increase cutting speed. This keeps the chip moving quickly so that there is less time for the bonding to occur.
• Decrease the feed rate. Reduced feed reduces the pressure on the surface, reducing the tendency of the chip to bond to the surface.
• Increase the insert rake angle. A more positive cutting edge and chipbreaker reduces the work hardening in both the chip and the workpiece.
• Use a PVD-coated insert. The smooth surface reduces the tendency for bonding.
• Ensure adequate coolant supply. Plenty of coolant reduces friction and keeps the cutting tool cool, which in turn enables higher speeds without excess wear.

Another common problem associated with cutting nickel-based alloys is notching.

As the insert cuts the material, deformation and hardening occur on the surface of the workpiece. This hardened layer creates a higher temperature as it is cut than the nondeformed material beneath it. This means that in the area of the depth of cut (DOC), higher temperatures cause accelerated wear compared to areas closer to the insert's corner radius. This accelerated wear forms the depth-of-cut notch.

"There are several ways to slow down the formation of the DOC notch and extend tool life," said Ludeking.

One of the most effective ways is to increase the lead angle of the toolholder to 15, 30, or 45 degrees, or use a round insert.

"By increasing the lead angle, the chip thickness is reduced, and the contact of the high-hardness, deformed area is spread out over a larger part of the cutting edge. Further, the main cutting edge is now cutting before the radius of the insert, significantly reducing the work-hardening effect at the maximum depth of cut. This is an effect of the changed chip formation and results in a significantly reduced notching tendency."

Other ways to reduce DOC notching are to:

• Vary the cutting depth. This reduces the concentration of the accelerated wear in one area, extending the life of the insert.
• Use a tougher cutting tool grade—preferably PVD-coated.
• Reduce the cutting speed.
• Use an insert with more open chip form geometry.
• Ensure plenty of coolant is applied to the cutting zone.

The Role of Coatings

Because of their high strength, work-hardening capability, low heat conductivity, and high shearing forces, high-temperature alloys create very demanding cutting conditions compared to other materials. The coating must play multiple roles.

"One of the most important roles for the coating is to act as a heat, abrasion, and diffusion barrier," said Ludeking. "These functions help prevent the depth-of-cut notching so prevalent when machining nickel-based alloys, as well as plastic deformation, flank wear, and crater wear."

According to Ludeking, an aluminum oxide layer in the coating is best for resisting heat and abrasion, but chemical vapor deposition (CVD) processes for depositing an Al2O3 coating do not provide a very smooth layer, adding to BUE problems.

"The ideal coating for nickel-based alloys is a PVD Al2O3 coating that provides a very smooth surface and maintains the sharp cutting edge necessary for these tough materials," said Ludeking

Although the cutting tool substrate and coating are very important, for nickel-based alloys the geometry of the cutting edge is possibly even more important.

Due to their toughness, these alloys require a sharp cutting edge to counter notching, BUE, and high cutting forces. However, the common occurrences of chip hammering, chipping, and plastic deformation require a strong cutting edge to resist these failure modes.

"The ideal cutting edge for nickel-based alloys is one that is both sharp and strong, combined with an effective chip-breaking geometry that controls the chip without creating high stresses on the cutting edge, or adding to the tendency to form built-up edge," said Ludeking.

www.coromant.sandvik.com

www.walter-tools.com