In recent years, additive manufacturing (AM) has evolved from a prototyping tool to a still new, but established and economically viable choice for component production. Annual sales of metal AM machines have grown from fewer than 200 in 2012 to almost 2,300 in 2018 as the aerospace, energy, automotive, medical, and tooling industries have embraced the technology . The use of AM in manufacturing is driving growth in the market segment dedicated to metal materials – which is expected to account for a quarter of the market by 2023 .
AM oﬀers several advantages compared with alternative powder metallurgy methods, ranging from design ﬂexibility to the potential for high material use eﬃciency, and is particularly suitable for producing small to medium volumes of relatively small components, as well as enabling the creation of complex parts that were previously unachievable. The development of AM machines is an important focus area as the technology is adapted to produce larger components and deliver higher throughputs. However, there is now an equal emphasis on the properties of the powders used.
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 eﬀective powder recycling strategies. Identifying analytical tools that can reliably set speciﬁcations for AM metal powders to validate quality and manage their use is vital. In this white paper, we review the key processes used in AM, how they determine the requirements of metal powders for this application, and how they can be measured.
2. A process like no other
AM is ‘the process of joining materials to make objects from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing methodologies such as machining’. As a tool-less manufacturing technique, it oﬀers superior design freedom to any other and, uniquely, similar scalability for making one part or many. Other beneﬁts include the possibility to create lightweight structures and build multicomponent parts in one step, reduced material consumption compared with machining, and short production cycle times. To fully utilize these potential beneﬁts, manufacturers need to understand the process just as they would any other, the properties of material inputs, and interactions between the two, to exert eﬀective control.
There are several alternative technologies used within AM machines, each subjecting a metal powder to diﬀerent ﬂow, stress, and processing regimes. Therefore, matching powder characteristics to any speciﬁc application/machine is crucial. The most common commercial technologies can be classiﬁed as powder bed or blown powder. A brief overview of how these processes work is useful in setting powder requirements in context.
2.1 Powder bed AM
Powder bed AM processes involve constructing the component on a progressively retracting platform, with a fresh layer of powder spread across the bed following the selective fusing of speciﬁed areas. With laser powder bed fusion (PBF), a laser beam is used to locally melt the upper layer of the spread powder. PBF machines vary in terms of, for example, build volume and the number of lasers used, and are suitable for a wide range of materials including titanium, nickel and aluminum alloys, stainless and tool steels, and cobalt chrome. That said, build times are slow – in the order of 25g per hour – so a primary aim is to reduce processing times.
Figure 1. Powder bed AM processes such as PBF call for rapid, even powder spreading and effective recycling of the excess powder