Tag Archive for: machining nickel alloys

Machining Nickel Alloys: Avoiding All-Too-Common Mishaps

Nickel-based alloys are growing in popularity across many industries such as aerospace, automotive, and energy generation due to their unique and valuable mechanical and chemical properties. Nickel alloys exhibit high yield and tensile strengths at low weights and have high corrosion resistance in acidic and high temperature environments. Because of these advantageous properties, nickel alloys have increasingly become popular in machine shops.

Unfortunately, nickel alloys have a reputation for creating issues at the spindle. These metals present themselves to be problematic as they easily work harden. Further, nickel alloys generate high temperatures during machining, and have gummy chips that can weld onto cutting tools, creating built-up edge (BUE). Fortunately, with the correct approach, one can be successful in cutting nickel alloys.

Work Hardening

Across machine shops, nickel alloys are notorious for being difficult to machine. This reputation stems, largely, from work hardening, or a metal’s microhardness increasing due to the addition of heat. According to the Nickel Development Institute, this heat is generated through friction and plastic deformation of the metal. As the metal is cut, the friction between the cutting tool and workpiece generates heat which is concentrated around the cutting area.

Simultaneously, the metal is being physically worked. This means that as it is being machined, it is experiencing plastic deformation, which is a physical property that measures how much a material can be deformed to the point that it cannot return to its original shape.

This physical working of a nickel alloy increases its hardness faster than it does most other metals. The combination of high heat generation and physical work quickly increases the alloy’s hardness, causing tools to dull quickly and fail. This may result in scrapped parts and broken tools.

HVNI End MIll for Machining Nickel Alloys
Shown above is Helical Solutions’ End Mill for Nickel Alloys. This tool, engineered to excel in Inconel 718 and other nickel-based superalloys, is fully stocked in 6 and 8 flute styles.

Tool Adhesion

As nickel alloys are being machined and heat is generated, chips tend to become stringy and weld themselves to a tool’s cutting edge. This phenomenon, built up edge or BUE, rounds the cutting edge of the tool, resulting in poor cuts and increased friction, thus further contributing to work hardening. An example of BUE is seen in the image below.

BUE On End Mill Flutes
On the above tool, chips from the workpiece (Inconel 718) have welded onto the cutting edge, severely decreasing the tool’s effectiveness. Image Source: International Journal of Extreme Manufacturing

Built-up edge also speeds up tool wear, as the rotational forces involved in the cut increase. Now that the cutting edge is rounded from welded chips, a blunt tool is being forced into the workpiece.

With a blunt edge, the cutting motion changes from a shearing action to plowing. In other words, instead of cutting through the metal, the tool is pushing the material, resulting in poor cuts and increased friction.

Excessive Heat Generation

With poor cutting, internal heat of the tool rises, which can cause thermal cracking, defined by cracks that form perpendicular to the cutting edge. The fractures within the tool are created by extreme internal tool temperature fluctuations.

As a cutting tool rapidly overheats while cutting nickel alloys, cracks may form which can lead to catastrophic tool failure. With high temperatures, galling may also occur, which is characterized by pieces of the tool flaking off due to the same adhesion that causes BUE. As the tool is being welded to the workpiece and the machine continues to rotate it, pieces of the tool may start to break off resulting in tool failure.

Overcoming Nickel Alloy Difficulties

Temperature Control and Coolant Usage

The first step to effectively machine nickel alloys is to keep temperatures manageable as the workpiece is cut. Using high pressure coolant is mandatory. Coolant pressure should be 1000 psi or greater. This high pressure concentrating on the cutting zone of the workpiece will dissipate heat within both the cutting tool and workpiece. By doing so, the chances of work hardening lessen.

High pressure coolant will also aid in clearing out chips from the cutting area. Those hot gummy chips are responsible for BUE. Removing them as quickly as possible reduces the risk of BUE forming on the cutting edge. Additionally, chip removal is important to avoid chip recutting.

Chips absorb much of the heat and often work harden themselves. Recutting these hardened chips will dull the cutting edge resulting in poor cuts and decrease tool life. In general, water-based cutting fluids are preferable as they have higher heat removal rates and have a lower viscosity, which is needed for high metal removal operations.

Using the Proper Techniques

To also assist with heat removal, utilize climb milling techniques, where possible. When climb milling, the chip thickness is at its maximum at the beginning of the cut and tapers off until the cut is complete. Due to this, less heat is generated, as the cutting tool does not rub on the workpiece. Most of the heat from the cut is transferred into the chip.

Selecting the Proper Tooling and Coating

The next step is selecting the right end mill. Your end mill of choice should have a proper tool coating, such as Helical’s Tplus coating. Tool coatings are specifically engineered to improve tool performance by reducing friction, increasing tool microhardness, and extending tool life.

Next is selecting flute count. Tools used for nickel alloys need to be rigid to withstand the cutting forces present when machining high hardness alloys. Therefore, higher flute counts are necessary. If using traditional roughing toolpaths, your end mill should have at least 6 flutes. With 6 flutes, there is sufficient flute valley depth to allow for chip evacuation, while having a larger core diameter keeps the tool strong and rigid.

For finishing operations and instances of implementing high efficiency milling, higher flute counts should be considered. A tool used this way should have 8 flutes to provide excellent surface finish.

Helical’s End Mills for Nickel Alloys

CNC tooling manufacturer Helical Solutions’ End Mills for Nickel Alloys product offering, its HVNI tool family, specializes in machining nickel alloys as it exhibits these key tool features.

The tools shown above are Helical Solutions’ End Mills for Nickel Alloys. These tools are coated in Tplus for high hardness, resulting in improved tool life and increased strength,

With its Tplus coating and variable pitch to minimize chatter, these solid carbide end mills are engineered to perform in all grades of nickel alloys. Coupled with their geometry to maximize cutting performance, Helical’s End Mills for Nickel Alloys utilize faster speeds and feeds, which are readily available on the Helical Solutions website and Machining Advisor Pro.

For more information about the chemical make-up, uses, and categorization of nickel alloys, read “In the Loupe’s” post “Understanding Nickel Alloys: Popularity, Chemical Composition, & Classification”.

Understanding Nickel Alloys: Popularity, Chemical Composition, & Classification

Nickel-based alloys have been a cornerstone of manufacturing for decades, desirable for their broad range of varying resistances to heat, oxidation, and corrosion. Nickel alloys also have a high strength-to-weight ratio and superior electrical conductor abilities. Because of these mechanical and chemical properties, they are primarily used in aerospace, oil, electrical, and chemical industries.

Understanding this valuable metal and how to properly machine it is imperative to delivering an optimal final part.

Nickel Alloy Chemical Composition and Classification

Nickel is commonly found in the form of an alloy, as its crystalline structure allows the element to be paired well with other metals. These atoms are arranged in a face centered cubic lattice; this structure is shown in figure 1 below.

Lattice structure of Nickel. Image source: PriyamStudyCentre.com

According to Priyam Study Centre’s Learning Chemistry, an open face lattice has the highest atomic packing number (the number of atoms per unit volume) of any metallic lattice configuration, with an atom present at each of the 6 faces and 8 corners of the cube. This structure is largely responsible for nickel’s strength and ability to create strong metallic bonds to chromium, cobalt, iron, and molybdenum, the most common metals found in these alloys.

According to City Special Metals’ article on Machining Nickel and Nickel Alloys, nickel alloys are organized into five main categories: Groups A through E. These groups are determined through the percentage of nickel present, as well as the most prominent metal that the nickel is chemically bonded with.

Table 1 displays the breakdown of these groups, showing each group’s chemical composition and a few examples of common types of nickel alloys found in that category.

GroupPercentage of NickelPaired MetalsExamples
Group A95% and greaterAlmost pure nickelNickel 200, 201, 205, and 212
Group B29% to 42%CopperMonel 400, Invar 36
Group C70% to 75%Chromium and ironInconel 600, Monel K-500, and Nickel 270
Group D50% to 56%Chromium and ironInconel 718, Inconel 625, and Hastelloy C-22
Group E63%Copper and ironMonel R405 is the only Nickel alloy in this category
Table 1: Categories of nickel alloys and their chemical compositions. Table data source: Machining Nickel and Nickel Alloys: A Guide from CSM; Nickel Based Alloys: Everything You Need to Know.

Understanding your workpiece material is just as important as understanding your machinery and tools. According to Global Market Insights (GMI), the nickel alloy market has been growing over 4% each year since 2017, and this growth is seeing an upward trend. As these alloys increase in popularity and demand, knowing the chemical compositions and classification of your specific workpiece will play a key role in successfully machining it.

Fabricating products made of nickel alloys present common struggles in every machine shop. Learn how to select proper tooling and implement machining techniques to overcome these challenges by reading CNC Machining Nickel Alloys: Avoiding All-Too-Common Mishaps.