A Look into Powder Materials for Metal 3D Printing

The market for 3D printing is poised for an explosive growth. Today, a plethora of 3D printing techniques can shape objects from an ever growing list of materials – photo-polymeric resins, extruded filament, powders of plastics, pure metals and alloys, etc.

Metal additive processes such as metal powder bed fusion and directed energy deposition are potentially capable of producing high-quality, functional and load bearing parts from a variety of metallic powder materials. However, “one-size-fits-all” doesn’t apply well to industrial additive manufacturing and when it comes to high value parts and critical applications, it’s crucial to know the pros and cons of these processes and how they apply to a chosen material, or to partner with an expert who does.

Metals powders: Where do they come from?

Today, metals are the fastest-growing segment of 3D printing. The growth of additive manufacturing is tied to business opportunities and, directly, the materials available and their cost. Low-cost metal powders are the key enablers for 3D printing, to realize its potential and transform industrial production.

Available metallic powders for additive manufacturing are mostly those being used in a range of applications (medical, aerospace, jewelry, automotive, etc…):

• Tool and maraging steels,
• Stainless steels,
• Aluminum alloys,
• Cobalt-chromium and nickel-based superalloys,
• Commercially pure titanium and titanium alloys,
• Copper alloys
• Precious metals (gold, platinum, palladium, silver).

Metal powders can vary widely in size, but also in shape (spherical to irregular). As a consequence, processing characteristics in AM metal systems vary, as well. To ensure consistency and repeatability of the quality of metal powders and AM processes, the machine manufacturers are working closely with powder suppliers.

Metal AM systems manufacturers (EOS, Concept Laser, ARCAM, etc.) supply approved material powders, but many powders are likely atomized elsewhere and the cost is greater than purchasing directly from a powder manufacturer.

Companies producing powders for traditional powder processes such as hot isostatic pressing (HIP), metal injection moulding (MIM) or powder sintering (PS) have an opportunity to capture some market shares of this booming sector, if they manage to comply with the requirements of metal additive manufacturing. Supply chains for 3D printing materials are in flux and, therefore, susceptible to technological disruption by innovators.

Major third party metal powder manufacturers that one may want to consider when choosing powders are:

• ATI Powder Metals
• Carpenter
• Erasteel
• LPW Technology
• Metalysis Technology
• AP&C
• Sandvik Osprey
• TLS
• GKN Hoeganaes
• HC Starck
• Praxair
• Metco

Metals powders: How are they produced?

Water atomization is the most common and economical technique to produce metal powders. The high water pressure that impacts more energy into a molten metal stream, combined with a rapid cooling rate, give rise to powder particles that are tough and irregular in shape. An irregular shape is less desirable for powder particles devoted to additive manufacturing because it increases the flow time and possibly reduces the packing density. However, under certain conditions, it is possible to produce spherical powders with a particle size distribution optimized for additive manufacturing.

Generally, gas atomized powders are preferred over water atomization for additive manufacturing and gas atomization has become the most common technique to produce metal powders for AM. The feedstock is melted under an air or inert gas or in a vacuum atmosphere; then, the chamber is back-filled with gas to force molten alloy through a nozzle. High-velocity gas (air, nitrogen, helium or argon) gas impinges into the flowing melt and breaks it up into fine droplets.

Interfacial tensions naturally spheroidise the surface of molten metal droplets that cool down and fall at the bottom of the atomization tower, where powders are collected. Gas atomization technology provides qualified powders for various additive manufacturing processes, such as selective laser melting (SLM), electron beam melting (EBM), direct energy deposition (DED) and infiltration.

Gas Atomisation is mostly used for Fe, Ni and Co alloys, but is also available for Al and Ti alloys. Other variations of this technique exist, such as:

• Water atomization: for unreactive materials, produces irregular shaped particles
• Plasma atomization: high-quality and extremely spherical powder, limited to alloys that can be formed into a wire feedstock
• Electrode induction melting gas atomisation: suitable for all alloys, but most economic with reactive alloys like Ti. A bar feedstock is rotated and melted by an induction coil before it flows downwards into a gas stream for atomisation. A cheap, clean, and good process for small batches and to produce small diameter powder particles.
• Centrifugal atomisation: Good trade-off between Gas atomisation and Plasma atomisation, best suited to larger batch sizes of less reactive, low-melting temperature alloys; however, it can also produce Ni-base superalloy powders

After production, powders can be characterized according to various standard techniques used for granular materials:

• Hall flow: Flow rate and apparent density.
• Powder flow and rheological properties analysis.
• Angle of repose: Steepest angle of descent to which powders are piled without slumping.
• Tapped density: Bulk density of the powder after consolidation/compression.
• Morphology by scanning electron microscopy.
• Entrapped porosity by scanning electron or optical microscopy.
• Laser diffraction: Analysis of the particle size.
• Sieve analysis: Assess the particle size distribution.
• Moisture determination: water mechanically held water on the surface or between the particles of the material.
• Chemical composition analysis: amount of metallic, non-metallic impurities (elemental form, or in dissolved form as solid solution or as compounds).

Finally, powders are packaged in robust and moisture resistant containers in HDPE. Pure titanium and alloys are packed under argon while other materials are usually packed under normal air atmosphere.

Metal additive manufacturing: What makes a powder suitable?

Metal powders used in additive manufacturing should have:

• spherical shape to ensure good flow/coating ability and a high packing density,
• particle size usually below 50 μm or 150 μm depending on machine type and surface finish or productivity required,
• particle size distribution tailored to the application and properties,
• controlled chemical composition and gas content.

The particle size distribution of metallic powder particles impacts the density of AM parts. Although it’s possible to attain high densities with different powder types, the parameters of the process must be adjusted accordingly. Ultimately, the productivity varies. Moreover, the particle size distribution does not only affect the density but also the mechanical properties and surface quality of the parts.

A significant issue in metal ALM is the progressive degradation of metal powders during processing as a result of the powder bed being exposed to oxygen and other contaminants. There is large potential demand for cost-effective methods of reconditioning metal powders.

In search of a low porosity, fine microstructure and uniform properties!

Repeatedly obtaining additively manufactured materials with 100% of the reference density is doubtlessly challenging. Metal additive manufacturing techniques can yield densities in excess of 99%. Some materials are reported with a full density while some others present a spread of densities.

Density is influenced by the development of pores or entrapment of unmelted powders during the layering. Occasionally, hot isostatic pressing is used to increase as-fabricated densities.

From a mechanical standpoint, porosity (especially “open” porosity) jeopardizes AM parts’ fracture toughness and fatigue properties. Indeed, under cyclic stress conditions, porosity or partial de-lamination can initiate cracks and lead to part failure. For load-bearing applications of the aerospace industry, toughness and fatigue resistance are critical requirements.

When casting metal alloys, the element with the highest melting point starts to solidify first. As the casting cools from the surface towards the center, grains will present a significantly different alloying elements’ concentration. Concentration will vary throughout the part and grains will form in specific orientations. The material properties won’t be uniform or isotropic.

In metal additive manufacturing processes tiny amounts of material are melted at a time. For alloys, some segregation of alloying elements occurs but on a much smaller scale. The rapid solidification leads to a more uniform chemical composition and microstructure throughout the part.

Metal additive manufacturing gives rise to unique microstructure and mechanical behavior

Additively manufactured metal parts can experience very high cooling rates, giving rise to several unusual effects depending on the material, as:

• Suppression of diffusion-controlled solid-state phase transformations
• Formation of supersaturated solutions and non-equilibrium phases
• Formation of extremely fine, refined microstructures with little elemental segregation
• Formation of very fine second-phase particles such as inclusions and carbides

Sometimes, these effects are desirable but they must be considered on a case-by-case basis.

Generally, because of the refined microstructure of metals produced with additive manufacturing, an increase in strength and decrease in ductility is expected compared with conventional casted or wrought alloys.

The layering building of the part is responsible for directional solidification and anisotropy, while the scanning pattern of the energy beam naturally generates unique microstructures. It is perceived as a disadvantage of the technique. However, it offers the possibility to customize the microstructure by controlling the scanning pattern and take advantage of a judicious orientation of the part in the platform.