Malvern
When additive manufacturing (AM) was in its infancy, developing a machine that could ‘print’ a component was challenging enough, and the focus was very much on reaching a point where the hardware was commercially viable. However, those working at the forefront of AM soon realised that the hardware was only half the story; the powder was equally important.
As the knowledge base grew, it became evident that existing metal powder supplies did not serve the AM market. Today we understand far more about how to identify, optimise, manufacture and recycle metal powders for AM, and indeed the critical role that advancing this understanding will play in our realisation of the full potential of the technology.
So, why do AM powders need to be different?
As AM moves from a design and prototyping technology to a manufacturing support tool, it is being exploited more and more for metal applications. In particular, its ability to produce complex parts in a single piece, without design constraints, has attracted many industries to its promise.
In the aerospace and automotive sectors where part failure risk is everyone’s nemesis, delivering metal parts that have consistent strength throughout can be a real advantage. For example, AM-produced fuel nozzles for the GE LEAP engine are 25% lighter and five times more durable than the previous part – making them a highly attractive choice.
In these challenging applications, however, powder selection, production and quality are of critical importance. Depending on which AM process and machine is used to create a part, the powder used will be subjected to different flow, stress and processing regimes. Ensuring the raw material can stand up to the job is the difference between metal AM success and failure.
You’ve made your powder bed, now lie in it!
Powder bed AM processes involve construction of the component on a progressively retracting platform, with a fresh layer of powder spread across the bed following the selective fusing of specified areas. A roller spreads the exposed powder across the bed to create a thin, uniform layer around 20 to 50 microns in depth. A cycle of spreading, melting and fractional platform retraction is repeated, up to thousands of times, to build the finished component, layer-by-layer.
Powder perfection is essential
Current AM machines offer little opportunity for any form of responsive control, meaning that inconsistent input material properties will translate directly into inconsistent finished component properties. Reduced powder quality can produce defects in the end part including pores, cracks, inclusions, residual stresses and sub-optimal surface roughness, as well as compromising throughput.
Beyond chemistry, it is the physical characteristics of a metal powder that define AM performance. These characteristics include both bulk properties of the powder and properties of the individual metal particles. Key bulk properties are packing density and flowability. Powders that pack consistently well to give a high density are associated with the production of components with fewer flaws and consistent quality. The ability to spread evenly and smoothly across a bed, to form a uniform layer with no air voids is essential – and this, the flowability of the powder, is also critical. Both bulk density and flowability are directly, though not exclusively, influenced by particle size and shape.
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Metal powder analysis
Particle size distribution and morphology data to help determine packing density and flowability can be measured using laser diffraction and automated imaging techniques.
Laser diffraction measures particle size distributions by measuring the angular variation in intensity of light scattered as a laser beam passes through a dispersed particulate sample. Large particles scatter light at small angles relative to the laser beam and small particles scatter light at large angles. The angular scattering intensity data is then analysed to calculate the size of the particles responsible for creating the scattering pattern, using the Mie theory of light scattering. The particle size is reported as a volume equivalent sphere diameter.
Automated imaging systems capture tens of thousands of particle images in just a few minutes and, from these, generate statistically valid size and shape distributions which can be used to characterise particle morphology in a more precise, objective and robust way than is achievable with, for example, Scanning Electron Microscopy.
Those involved in metal AM, including powder suppliers, machine manufacturers and end users, are well advised to use these technologies to ensure the characteristics of the powders they select will meet their end part requirements.
Fuelling the AM revolution
Up to one-third of the production cost of an AM component is the cost of the powder used, with commercial viability resting on establishing a robust supply chain and efficient powder recycling strategies. It can be a real challenge to establish specifications for AM metal powders, especially when their application is broadening every day.
Many savvy players are turning to complementary analytical techniques, such as laser diffraction and advanced automated image analysis, to identify and specify suitable powders, optimise AM processes, monitor batch consistency, implement effective powder recycling strategies and achieve consistently high-quality parts. Together, if we can build a supply chain of consistent and appropriate quality, the feasibility, reliability and long-term viability of metal AM across countless industries may be closer than we think.
