3D Printing Cobalt Chrome (CoCr)

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December 22, 2022

Cobalt Chrome is an alloy that was developed initially for the aerospace industry thanks to its excellent corrosion resistance and mechanical properties. It then became highly utilized in the medical industry mainly due to its biocompatibility. With high toughness, hardness, and strength-to-weight ratio, cobalt-chromium alloys became some of the most common materials in specific applications such as jet engines, gas turbines, and knee implants.

Metal 3D printing provided a stronger case for Cobalt Chrome. While maintaining the integrity and properties of Cobalt Chrome, additive manufacturing (AM) has facilitated and boosted its production and use. Localized manufacturing, rapid prototyping, complex geometries, minimized material waste, and on-demand spare part production have all become possible for Cobalt Chrome with the advent of 3D printing.

In this article, we will look into the details of Cobalt Chrome, how it fares against other alloys, how it can be used in 3D printing, and where it is commonly used.

What is Cobalt Chrome (Cobalt Chromium alloy)?

Cobalt Chrome, also known as Cobalt-Chromium or CoCr, is an alloy made mainly of Cobalt (53 – 67 %) and Chromium (25 – 32 %) with small added amounts of Molybdenum, Nickel, Tungsten, Silicon, or Aluminum for application-specific strengthening. Cobalt Chrome alloys are generally classified into two categories:

  1. CoCrMo alloys
  2. CoNiCrMo alloys

The most commonly used alloys are cast/wrought Co28Cr6Mo and wrought Co35Ni20Cr10Mo.

Just like stainless steel, Cobalt Chrome develops its high corrosion resistance by forming a dense and passive oxide layer on its surface (i.e., Chromium oxide). This oxide layer protects the alloy from environmental effects, helping it maintain its mechanical properties. Those properties include a high strength-to-weight ratio, high tensile and yield points, high hardness, and great toughness.

Such mechanical properties come from Cobalt Chrome’s multiphase structure and carbide precipitation, reinforcing its hardness. Cobalt Chrome also has better hot corrosion resistance, higher melting point, and higher thermal fatigue resistance compared to nickel-based alloys.

The table below shows the main properties of the two most common Cobalt Chrome alloys, Co28Cr6Mo (ASTM F75) and Co35Ni20Cr10Mo (ASTM F562).

Co28Cr6Mo

 

Co35Ni20Cr10Mo

 

Chemical
composition 

(main elements)

Co

Balance

Balance

Cr

28%

19 – 21%

Mo

6%

9 – 10.5%

Ni

0.1%

33 – 37%

Other

3.14% max

2.37% max

Density (23 °C)

7.90 g/cm³

8.34 g/cm³

Tensile strength (23 °C)

1080 MPa

800 – 1000 MPa

Yield strength (23 °C)

630 – 841 MPa

965 –
1000 MPa

Hardness, Rockwell C (23
°C)

27 [-] (may go up to 60
HRC in certain hard spots)

Thermal expansion
coefficient (23 °C)

1.05 *10-5 1/K

1.28 *10-5 1/K

Melting temperature

1360 °C

1315 – 1440 °C

3D printing process

Direct Metal Laser
Sintering (DMLS), Selective Laser Melting (SLM)

Mean particle size (D50) of
metal powder

~ 25 –
33 µm
or 30.22
µm

How is Cobalt Chrome used in 3D printing?

Cobalt Chrome’s high hardness, strength, and toughness make it difficult to machine. Its hardness may also vary to reach high levels in particular hard spots. Those hard spots can cause inconsistencies in machining.

Cobalt Chrome is also quite abrasive, which can result in wear and shortened lifespan of cutting tools. In addition, its low thermal conductivity can make the cutting process challenging for the cutting tool, as heat is not dissipated through the tool as it usually is with standard steels that are relatively less heat resistant.

These challenges have always been around in traditional manufacturing of cobalt chrome. This is where 3D printing can surpass traditional manufacturing methods as it not only helps bypass those challenges but also enables the production of lightweight, complex geometries that would be impossible otherwise.

It allows for predetermined surface roughness and is a relatively easy, controllable, and fast process, as the material does not need to be melted and cooled numerous times. It also results in minimal material waste and enables new manufacturing possibilities that can streamline and shorten the supply process, including rapid prototyping, localized manufacturing, and on-demand production.

Cobalt Chrome is mainly 3D printed using Direct Metal Laser Sintering (DMLS) and Selective Laser Melting (SLM) methods. It is prepared in powder form using the process of gas atomization, which creates ready-to-print spheroidal particles of CoCr. The 3D-printing processes of DMLS and SLM are quite similar but differ in particular details.

DMLS, also referred to as Selective Laser Sintering (SLS), directs a high-energy laser onto a bed of CoCr powder and fuses the target particles at a temperature just below the melting point. This sintering process enables particle diffusion beyond the particle boundaries. As a result, a highly dense layer of fused CoCr powder is generated, which then solidifies upon cooling. Then, more powder is spread evenly over the bed, and further layers are generated based on the preset CAD design.

SLM, on the other hand, applies the laser onto the CoCr powder bed and heats the target particles beyond their melting point, causing them to melt and fuse. This results in high-density layers of CoCr produced on top of one another until a near-net-shape part is created.

You may learn more about SLS and SLM in our previous article on the differences between additive manufacturing methods.

What are the application areas of Cobalt Chrome?

In addition to their high strength, hardness, and biocompatibility, Cobalt Chromium alloys are non-magnetic, heat-treatable, acid- and corrosion-resistant materials. All these properties made CoCr alloys suitable for a variety of applications, mainly in the aerospace, medical, energy, and chemical industries.

This range of applications has expanded with 3D printing as it allows the use of pure alloys without adding further alloying elements, as opposed to traditional manufacturing, which requires alloying additives.

Cobalt Chrome excels in applications with high-temperature conditions and corrosive environments, the most common of which are:

  • Injectors, swirlers and other parts of turbine engines
  • Fuel nozzles
  • Industrial equipment
  • Medical implants (mainly orthopedic implants)
  • Frameworks of removable partial dentures and other dental implants
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