Cobalt Chrome Alloy in 3D Printing

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  What the alloy is before it becomes a part
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  How the melt pool builds a dense part
• 3
  The four defects that actually show up
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  From printed blank to finished part
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  When printing wins and when it loses
• 6
  Printed CoCr versus machined CoCr

Cobalt chrome is not one material. It is a family of cobalt-base alloys where chromium sits between roughly 26 and 30 percent by weight, and the balance decides the behavior. Medical grades such as CoCrMo (ASTM F75, F1537) add 5 to 7 percent molybdenum for corrosion resistance and castability. Dental and wear grades add tungsten instead, which raises hardness at the cost of ductility. Some aerospace grades carry nickel; many do not.

For additive manufacturing the alloy arrives as gas-atomized powder, typically 15 to 45 μm for laser powder bed fusion and 45 to 105 μm for electron beam melting. Particle shape matters more than the datasheet suggests. A spherical powder with few satellites flows evenly across the recoater blade; an irregular batch bridges in the hopper and leaves streaks in the layer.

The physical numbers explain most of the process difficulty. Melting range sits around 1200 to 1400 °C, thermal conductivity is low at roughly 13 to 15 W/m·K, and the alloy holds hardness at temperature far better than aluminium or stainless. Low conductivity means heat does not leave the melt pool quickly, so the pool stays large and the surrounding powder stays hot.

That combination is the whole story of cobalt chrome alloy in 3D printing. Good melt wetting, high hardness, slow cooling. Every parameter and every defect follows from those three facts.

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  CoCrMoMolybdenum-bearing. Medical and dental standard, corrosion resistant, moderate hardness.
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  CoCrWTungsten-bearing. Higher hot hardness, used for wear surfaces and some tooling.
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  Powder cut15–45 μm for L-PBF, 45–105 μm for EBM. Match the cut to the machine.
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  ReuseSieved powder can be reused, but oxygen and satellite content drift upward with each cycle.

In laser powder bed fusion a 200 to 400 W fiber laser traces the cross-section at 600 to 1200 mm/s, with hatch spacing around 0.08 to 0.12 mm and layer thickness of 20 to 40 μm. The laser does not melt a clean hole. It creates a keyhole or conduction-mode pool that wets into the previous layer, and the track width must overlap its neighbor enough to avoid porosity between passes.

Because cobalt chrome conducts heat poorly, the pool is wider and stays liquid longer than the equivalent stainless steel parameter set. That is useful for bonding and dangerous for geometry. Overhangs down to about 45° from vertical usually print without support. Below that, the unsupported edge curls as it cools and the recoater blade hits it.

Cooling rates in the range of 10⁵ to 10⁶ K/s freeze the microstructure into a fine cellular or dendritic pattern. That is not the equilibrium structure the alloy would reach in a casting. Residual stress builds layer by layer, and the part can bow once it is cut from the plate. Stress relief before removal is standard practice, not an optional step.

Electron beam melting works differently. A preheated bed at 700 to 1000 °C, a faster scan, and a vacuum chamber mean lower residual stress and less distortion. The trade is surface finish. EBM parts come out rougher and need more material removed afterwards.

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  Laser power200–400 W typical for CoCr on a 100 μm spot machine.
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  Layer thickness20–40 μm. Thinner layers cost time, not quality, past a point.
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  Overhang limitRoughly 45° from vertical before supports become mandatory.
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  AtmosphereArgon for L-PBF, vacuum for EBM. Oxygen pickup hardens and embrittles the part.

Gas porosity appears as round voids a few tens of microns across, trapped when the keyhole collapses or when the powder carries internal voids from atomization. It is usually a powder quality problem, not a parameter problem. Switching to a different powder lot often clears it faster than re-tuning the laser.

Lack-of-fusion defects are flatter and sharper-edged. They sit at the boundary between two hatch lines or two layers where the pool did not wet in. These are the dangerous ones, because a sharp internal notch under cyclic load becomes a crack origin. X-ray or CT inspection is the only reliable way to find them without destroying the part.

Distortion from residual stress shows up as a bowed base, a curled overhang, or a crack that opens during cutting from the build plate. Annealing in the 800 to 1150 °C range before wire EDM release removes most of the driving stress. Skip it and the part moves after the last operation, which is the worst possible time.

Ballooning on downfacing surfaces is a thermal problem. The pool sags into loose powder beneath it and solidifies as a rough, partially sintered skin. Support structures limit the sag. They also add the next problem, which is removing them from a hardened alloy without gouging the part.

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  Gas porosityRound voids. Check powder lot and atomization quality first.
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  Lack of fusionSharp voids between tracks. Raise energy density or reduce hatch spacing.
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  Residual stressAnnealing in the 800–1150 °C range before cutting from the plate.
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  BallooningRough downfacing skin. Add supports or reorient the part.

A printed cobalt chrome part is a near-net blank. As-built surfaces sit around Ra 8 to 15 μm, far from a bearing surface or a mating face. The first operation after stress relief is support removal, usually by hand with a cut-off wheel, then by CNC on any critical face. Cutting supports off a 40 HRC alloy with a hand tool is slow and easy to get wrong.

Hot isostatic pressing closes internal porosity and homogenizes the microstructure. Typical cycles run 1000 to 1200 °C at 100 to 150 MPa for two to four hours, followed by a controlled cool. For fatigue-critical or implant-grade parts, HIP is normal. For a bracket that sees static load, it may be unnecessary cost.

Heat treatment then sets the final hardness. Solution treatment and aging can push CoCrMo toward 35 to 45 HRC depending on grade and cycle. The exact recipe belongs to the alloy supplier, not to a generic table. Ask for the certificate and follow it.

Machining and finishing come last. Printed CoCr is abrasive, work-hardens under a dull tool, and holds heat at the cutting edge. Carbide with a hard coating, modest depth of cut, and plenty of coolant is the baseline. A printed face that needs ±0.02 mm will almost always be milled rather than printed to size.

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  Stress reliefBefore cutting the part off the build plate, not after.
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  HIP1000–1200 °C at 100–150 MPa. Closes internal porosity.
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  Age hardeningGrade-specific. Target 35–45 HRC for wear-facing CoCrMo.
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  Finish machiningCarbide, coated, low depth of cut, heavy coolant.

Printing wins on complexity that cannot be reached by a tool. Internal channels, lattice structures, conformal cooling passages, and organic brackets with no straight reference face all favor additive. Cobalt chrome adds the option of a hard, corrosion-resistant, biocompatible surface in the same operation, which is why dental frameworks and orthopedic implants were early adopters.

Printing loses on simple geometry at volume. A cylindrical bushing, a shaft, a flanged fitting: these are faster and cheaper on a lathe or a mill, and the material properties are more predictable. Printed CoCr has direction-dependent properties. Horizontal and vertical specimens from the same build do not test the same, and a design that ignores this will fail in a way the FEA did not predict.

Size is another boundary. Most metal powder bed machines build within a 250 to 400 mm envelope. Parts beyond that need segmentation and joining, which adds a joint that must be qualified. At GreatLight we machine cobalt chrome and other hard alloys up to 4,000 mm on 5-axis centers, so large CoCr parts usually start as bar or plate rather than powder.

The practical rule: print the shape, machine the interface. A printed CoCr part with machined mating faces, bores, and threads gets the geometric freedom of additive plus the tolerance and finish of subtractive. That hybrid route is the one most production programs settle on.

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  Choose printingInternal channels, lattices, organic geometry, low-to-mid volume.
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  Choose machiningSimple geometry, tight tolerance, high volume, large envelopes.
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  HybridPrint the complex body, machine the critical interfaces.