Metal parts production is the fastest growing segment of the 3D printing market, indicating the value placed on additive manufacturing (AM) as a critical sector in the future of manufacturing. A vital aspect of this is the metal powders used in the process.
Two key laser-generated processes of powder metallurgy are currently in use: selective laser melting (SLM), and laser metal deposition (LMD).Both have the potential to produce high-quality, high-functionality load-bearing parts from metal powder, but critical to the production process is an understanding of metal powders. While metal in powdered form is certainly not a newcomer to the manufacturing industry, AM requires its own set of understandings associated with the technique of building layer-by-layer using fine powders in a laser-generated environment.
Important to this process is identifying the most suitable metal powder, as powders vary widely in particle size and shape, and this affects the processing characteristics. For AM, metal powders should meet the following requirements:
- Spherical shape. This is important to ensure good flow and ‘coating’ capability. The ‘shape’ of particles is influenced by the methods used in processing the powder. The water atomisation process occurs where high-pressure water is forced into a molten metal stream. Rapid cooling follows, resulting in tough but irregular particle shapes. Plasma atomisation uses multiple non-transferred direct-current arc plasmas to accelerate the atomisation gas – metal wire is fed into the apex of the plasmas, melts and is atomised in a single step, producing spherical powders. However, the preferred method for producing AM powders is gas atomisation. During gas atomisation, inert gas jets atomise molten steel into fine metal droplets that cool as they fall in the atomising tower to be collected at the bottom. The result is perfectly spherical shapes, highly suitable for AM processes with an additional benefit of high cleanliness. Whichever process is used, the powder can be characterised according to various standard techniques for granular materials. These standards usually accompany the metal powders on delivery to the manufacturing site.
- Particle size. This is usually produced at 50 or even 150 microns. Selection of size will depend on powder type, machine type and required surface finish. In AM, tiny amounts of material are melted at a time during fusion, creating unique microstructures with low porosity and uniform properties. Researching the appropriate powder is essential to attain key characteristics of a finished product.
- Particle size distribution. This must be tailored to the application and part properties as it impacts the density of the finished item. Different powder types offer different densities. While processing parameters can adjust for varying densities, particle-size distribution also effects mechanical properties and the surface quality of the finished part.
Metal powder is available for AM across a range of industry groups such as medical, aerospace, jewellery and automotive, with each area in turn presenting development opportunities for parts production. Demand from these sectors has led to the availability of metal powders on the open market such as titanium, tool steel, stainless steel, aluminium alloys, cobalt-chromium, nickel-based alloys, copper alloys and precious metals.
Fundamental to the use of metal powders are putting secure safety procedures in place at all times during powder handling and manufacturing. When shipped to AM sites, powders are packed in robust, moisture-resistant high-density polyethylene containers. Pure titanium and alloys are packed under argon, while other powders are packed under normal air pressure. Transporting and handling powders demand specific safety measures, generally detailed with the supplier information. While this new area of manufacturing offers exciting opportunities, companies have an obligation to meet safety standards required with their AM processes.
As indicated above, metal powders can vary in size and shape. Subsequently processing characteristics will also vary. To overcome these related issues and to ensure manufacturers find consistency and repeatability in quality of both powder and process, machine manufacturers are working closely with powder suppliers. Indeed many laser system manufacturers now supply powders suited to the specifications of their laser systems. If growth in this industry sector is to be successful, consideration of the features and benefits of metal powders, along with safety procedures, will be critical considerations to successful additive manufacturing.