Tag Archive for: nickel alloys

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.

Machining Precious Metals

Precious metals can be particularly difficult to machine due to their wide range of material properties and high cost if a part has to be scrapped. The following article will introduce these elements and their alloys as well as provide a guide on how to machine them effectively and efficiently.

About the Elements

Sometimes called “noble” metals, precious metals consist of eight elements that lie in the middle of the periodic table (seen below in Figure 1). The eight metals are:

  1. Ruthenium (Ru)
  2. Rhodium (Rh)
  3. Palladium (Pd)
  4. Silver (Ag)
  5. Osmium (Os)
  6. Iridium (Ir)
  7. Platinum (Pt)
  8. Gold (Au)

These elements are some of the rarest materials on earth, and can therefore be enormously expensive. Gold and silver can be found in pure nugget form, making them more easily available. However, the other six elements are typically found mixed in the raw ore of the four metals they sit below on the periodic table: Iron (Fe), Cobalt (Co), Nickel (Ni), and Copper (Cu). These elements are a subset of precious metals and are generally called Platinum Group Metals (PGM). Because they are found together in raw ore, this makes mining and extraction difficult, dramatically increasing their cost. Because of their high price tag, machining these materials right the first time is incredibly important to a shop’s efficiency.

machining metals

Figure 1: Periodic table with the 8 precious metals boxed in blue. Image source: clearscience.tumblr.com

Basic Properties and Compositions of Precious Metals

Precious metals have notable material properties as they are characteristically soft, ductile, and oxidation resistant. They are called “noble” metals because of their resistance to most types of chemical and environmental attack. Table 1 lists a few telling material properties of precious metals in their elemental form. For comparison purposes, they are side-by-side with 6061 Al and 4140 Steel. Generally, only gold and silver are used in their purest form as the platinum group metals are alloys that consist mainly of platinum (with a smaller composition of Ru, Rh, Pa, Os, Ir). Precious metals are notable for being extremely dense and having a high melting point, which make them suitable for a variety of applications.

Table 1: Cold-worked Material Properties of Precious Metals, 4140 Steel and 6061 Aluminum 

precious metals

Common Machining Applications of Precious Metals

Silver and gold have particularly favorable thermal conductivity and electrical resistivity. These values are listed in Table 2, along with CC1000 (annealed copper) and annealed 6061 aluminum, for comparison purposes. Copper is generally used in electrical wiring because of its relatively low electrical resistivity, even though silver would make a better substitute. The obvious reason this isn’t the general convention is the cost of silver vs. copper. That being said, copper is generally plated with gold at electrical contact areas because it tends to oxide after extended use, which lowers its resistivity. As stated before, gold and the other precious metals are known to be resistant to oxidation. This corrosion resistance is the main reason that they are used in cathodic protection systems of the electronics industry.

Table 2: Thermal Conductivity and Electrical Resistivity of Ag, Au, Cu, and Al 

machining metals

Platinum and its respective alloys offer the most amount of applications as it can achieve a number of different mechanical properties while still maintaining the benefits of a precious metal (high melting point, ductility, and oxidation resistance). Table 3 lists platinum and a number of other PGMs each with their own mechanical properties. The variance of these properties depends on the alloying element(s) being added to the platinum, the percentage of alloying metal, and whether or not the material has been cold-worked or annealed. Alloying can significantly increase the tensile strength and hardness of a material while decreasing its ductility at the same time. The ratio of this tensile strength/hardness increase to ductility decrease depends on the metal added as well as how much is added, as seen in Table 3. Generally this depends on the particle size of the element added as well as its natural crystalline structure. Ruthenium and Osmium have a specific crystal structure that has a significant hardening effect when added to platinum. Pt-Os alloys in particular are extremely hard and practically unworkable, which doesn’t yield many real-world applications. However, the addition of the other 4 PGMs to platinum allow for a range of mechanical properties with various usages.

Table 3: PGM material properties (Note: the hardness and tensile strength are cold worked values) 

machining metals

Platinum and its alloys are biocompatible, giving them the ability to be placed in the human body for long periods of time without causing adverse reactions or poisoning. Therefore, medical devices including heart muscle screw fixations, stents, and marker bands for angioplasty devices are made from platinum and its alloys. Gold and palladium are also commonly used in dental applications.

Pt-Ir alloys are noticeably harder and stronger than any of the other alloys and make excellent heads for spark plugs in the automobile industry. Rhodium is sometimes added to Pt-Ir alloys to make the material less springy (as they are used as medical spring wire) while also increasing its workability. Pt and Pt-Rh wire pairs are extremely effective at measuring temperatures and are therefore used in thermocouples.

Machining Precious Metals

The two parameters that have the most effect when machining are hardness and percent elongation. Hardness is well-known by machinists and engineers across the manufacturing industry as it indicates a material’s resistance to deformation or cutting. Percent elongation is a measurement used to quantify material ductility. It indicates to a designer the degree to which a structure will deform plastically (permanently) before fracture. For example, a ductile plastic such as ultrahigh molecular weight polyethylene (UHMWPE) has a percent elongation of 350-525%, while a more brittle material such as oil-quenched and tempered cast iron (grade 120-90-02) has a percent elongation of about 2%. Therefore, the greater the percent elongation, the greater the material’s “gumminess.” Gummy materials are prone to built-up edge and have a tendency to produce long stringy chips.

Tools for Precious Metals

Material ductility makes a sharp cutting tool essential for cutting precious metals. Variable Helix for Aluminum Alloy tools can be used for the softer materials such as pure gold, silver, and platinum.

machining metals

Figure 2: Variable Helix Square End Mill for Aluminum Alloys

Higher hardness materials still require a sharp cutting edge. Therefore, one’s best option is to invest in a PCD Diamond tool. The PCD wafer has the ability to cut extremely hard materials while maintaining a sharp cutting edge for a relatively long period of time, compared to standard HSS and carbide cutting edges.

machining metals

Figure 3: PCD Diamond Square End Mill

Speeds and Feeds charts:

machining metals

Figure 4: Speeds and Feeds for precious metals when using a Square Non-ferrous, 3x LOC

machining metals

Figure 5: Speeds and Feeds for precious metals when using a 2-Flute Square PCD end mill