Machining Super Alloys

Imagine that you gathered a group of manufacturers, engineers, and machine operators in a room and asked, "What do you think of heat-resistant superalloys?" Depending on who answered, you would likely receive one of two contradictory responses.

Those whose industries rely on parts made of durable iron- or nickel-based superalloys would most likely express their appreciation for the materials due to their stability and hardness. On the other hand, those who must create the pieces would likely express feelings of frustration.

The cutting-edge materials have revolutionized the medical field and aerospace industry. Even so, the features that make heat-resistant superalloys (HRSAs) so valuable also make them difficult to create by machine.

For example, aircraft engines rely on HRSAs, which can withstand the high temperatures generated during flight. Unfortunately, HRSAs require considerable time and effort to cut in the factory. In addition, the lengthy machining process once made them less efficient to produce compared to conventional materials like iron or steel.

Luckily, recent advances in milling processes have simplified HRSA machining. Iron and steel still require less time, energy, and effort to produce. However, the use of ceramic tools, improvements in tool pressure, enhanced cutting parameters, and high-speed systems have resulted in significant changes in the HRSA milling process.

TYPES OF HRSAs

Upon their introduction, metallurgically crafted heat-resistant superalloys represented a revolution in several industries. The resilient, highly refined metals withstand extreme corrosion and pressure without losing their integrity. Further, HRSAs such as nickel-based superalloys, iron, cobalt, and titanium alloys display the extraordinary characteristic of being heat resistant.

Most experts categorize HRSAs into two groups: Lamina Group 9 and Lamina Group 10. Lamina Group 9 includes alloys comprised of iron (Fe), nickel (Ni), or cobalt (Co), such as:

• Incoloy 800

• Inconel 700

• Inconel 718

• Stellite 21

Lamina Group 10 features titanium (Ti)-based superalloys such as Ti-6Al-4V and T40. With hardness ranges from 180 to 400 HBs on the Brinell Scale, the superalloys can withstand up to twice the pressure as stainless steel, which rings in at about 200 HBs.

HRSA FEATURES

Also known as high-performance alloys, superalloys combine key features that have resulted in their use in almost every industry. Defining characteristics of superalloys include:

• Low thermal conductivity

• Extreme heat resistance

• Superior ductility

• Wear resistance

• Corrosion resistance

• Oxidation resistance

• Surface integrity

• High strength-to-weight ratio

However, the features that make superalloys desirable also make them difficult to machine. Because of the strength and toughness of the materials, standard cutting tools experience a significantly shorter tool life, as the heat-resistant superalloys require high temperatures to fabricate, which in turn stresses the cutting tools.

Further, machine operators frequently need to switch tools or blades when machining superalloys. The regular changes can result in inconsistent operations and slower cycle times.

Even so, the benefits of HRSAs far outweigh the additional effort required to machine them. Below, we'll look at the many uses of HRSAs across various industries, including the aerospace, chemical processing, and medical fields.

HOW HRSAS ARE USED

Currently, HRSAs play significant roles in pivotal industries, including :.

THE MEDICAL INDUSTRY

The medical industry has relied on superalloys – namely titanium-based alloys – for years. The alloy's toughness, low elasticity, and wear resistance make it indispensable for equipment such as:

• Dental implants

• Heart pumps

• Bone clips

• Artificial hearts

• Heart valves

THE AEROSPACE AND AIRCRAFT INDUSTRY

As mentioned above, the aircraft and aerospace industries rely on nickel-based superalloys to withstand the high-temperature environments within engines. Both aircraft and rocket engines employ nickel-based superalloys for numerous components, including:

• Combustors

• Afterburners

• Disks, vanes, blades, bolts, and shafts

• Cases

• Aerodynamic skins

THE CHEMICAL PROCESSING INDUSTRY

From the chemical industry to the coal and petrochemical fields, countless manufacturers rely on superalloys to create products used every day. The temperature- and oxidation-resistant nature of superalloys makes them an invaluable solution for equipment such as:

• Pipes

• Valves

• Bolts

• Reaction containers

• Springs

• Ducts

THE ENERGY INDUSTRY

Finally, the energy industry has put HRSAs to use for decades. Unlike other metal materials, HRSAs can withstand the intense temperature of nuclear power production and oil and gas mining. Because of their unparalleled hardness and toughness, HRSAs easily lend themselves to applications such as:

• Nuclear reactors

• Bolts

• Blades

• Springs

• Control-rod mechanisms

• Ducting

• Tubulars

• Wellheads

• Valve stems

THE MACHINING PROCESS FOR MANUFACTURING HEAT-RESISTANT SUPERALLOYS

Despite the wide variety of uses for HRSAs, some may argue that outdated and inefficient milling procedures have slowed their growth. However, thanks to ongoing research, manufacturers can now rely on newly recommended cutting parameters to improve the machining process.

Properly machining HRSAs relies on several factors, including:

• Cutting speeds

• Tool materials

• Cutting tool pressure

The machining process may vary based on different types of alloys. That said, you may improve your roughing operations and finishing machining processes by:

CONSIDERING YOUR MACHINES

Many machine shops that work with HRSAs face a dilemma due to the age and size of their machines. Some shops will insert oversized tools to cut conventional materials like aluminum or brass. Instead, HRSAs demand a higher-powered machine with significant torque to reach high speeds.

Alongside the increased horsepower, the machines tend to be more rigid compared to lower-powered models. Rigid contact with the superalloy materials will result in a secure hold. A tighter connection reduces the risk of vibration, which can shatter the ceramic cutting tools often used when milling HRSAs.

KEEPING YOUR CUTTING TOOL COOL WHILE MACHINING HRSAS

Due to the intense heat required to machine HRSA material, consider applying high-pressure coolant to the cutting tool. While monitoring the heat of the tool may slow the overall process, the cutting speed will likely increase, along with the insert's tool life.

Using a high-pressure or flooding coolant can also minimize the risk of plastic deformation. Plastic deformation often occurs when machining an HRSA because the hot section of the HRSA's surface doesn't cool as quickly as other materials.

Using a coolant on the surface of the materials during the milling process can reduce failures like notch wear and chip buildup.

INCREASING YOUR SPEEDS WITH CERAMIC TOOLS

As we mentioned above, many shops choose ceramics when working with heat-resistant superalloys. Ceramic milling cutters provide enhanced machining compared to the carbide tools used to cut other metals. In addition, ceramic tools boast unbeatable metal removal rates compared to carbide.

Most importantly, ceramics won't become brittle at high temperatures, unlike carbide. The durable material results in improved cutting speed compared to carbide tools. For example, a shop may need hours to cut HRSAs using a carbide tool. A ceramic tool will reduce the same job to mere minutes.

USING THE CORRECT CHIP GEOMETRY

Using a ceramic insert instead of a metal material like carbide can improve your cutting speeds and reduce the overall cost of machining. However, to significantly enhance your milling process, consider the insert geometry.

Ensure that you direct the chip material away from your cutting edge. If you allow them to come in contact with the cutting edge, chips can create failure mechanisms, including:

• Increased friction

• Machining insert breakdown

• Uneven cut depth

Instead, adjust the angle of your ceramic chip breaker tool and send the chips flying away from your titanium-, iron-, or nickel-based alloy. By using this strategy, you'll extend the lifespan of your ceramic tools and streamline your roughing and finishing methods.