Exploring the role and measurement of particle size and shape in metal additive manufacturing

Introduction

In recent years, metal additive manufacturing (AM) has transitioned successfully from a prototyping tool to a still new, but established and economically viable, choice for component production. Indeed, as the aerospace, energy, automotive, medical, and tooling industries have embraced this technology [1], annual sales of metal AM machines have risen from less than 200 in 2012 to almost 2,300 in 2019. Alongside this trend, the use of AM in manufacturing is also driving an increase in the proportion of the market dedicated to metal materials. This segment is expected to account for a quarter of the whole manufacturing market by 2023 [2].

Powder bed AM processes such as selective laser melting (SLM), electron beam melting (EBM), and binder jetting offer certain advantages over alternative powder metallurgy methods, including design flexibility and potential for high material-use efficiency. These processes are particularly suitable for producing small to medium volumes of relatively small components, as well as enabling the creation of new, complex parts that were previously unachievable. As these technologies are adapted to enable larger components and higher throughputs, the development of AM machines is an increasingly important focus area. However, there is now also an equal emphasis on the properties of the powders used [3].
SLMProcessIllustration.jpg
Figure 1: Powder bed AM processes such as SLM require rapid, even powder spreading and effective recycling of the excess powder.

What makes a good metal AM powder?

AM processes typically operate with ‘fixed’ parameters for a specific application; most currently available machines offer little opportunity for responsive control. As such, inconsistent properties in the input material will directly translate to inconsistent properties in the finished component. Poor 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. To avoid these issues, manufacturers must understand the relationship between material properties, processing performance, and end-component properties [3].

Chemistry is paramount in metal AM powders. A powder needs to be compatible with the alloy composition of the material specified. The grade must also be carefully selected to control the interstitial elements present such as oxygen or nitrogen, as well as particulate contaminants which can impact the properties of the finished part. Beyond chemistry, it is the physical characteristics of a metal powder, such as packing density and flowability, that determine AM performance. Powders that pack consistently with high density are associated with the production of consistent-quality components with fewer flaws, while good flowability enables the powder to spread evenly and smoothly across a bed to form a uniform layer with no air voids [3].

Bulk density and flowability are directly, though not exclusively, influenced by morphological characteristics such as particle size and shape, as illustrated in Figure 2. For instance, smooth, regular-shaped particles typically flow more easily than those with a rough surface or an irregular shape [4]. This is because rougher surfaces result in increased interparticle friction, while irregularly shaped particles are more prone to mechanical interlocking; both effects decrease flowability. Similarly, spherical particles tend to pack more efficiently than those that are irregular, giving rise to higher bulk densities [5].
Fig 3 particles.jpg
Figure 2: Smooth, regularly shaped particles tend to flow more easily than those that are irregular and/or rougher, because of reduced interparticle friction and a lower risk of mechanical interlocking.
 
When it comes to particle size, AM metal powders must be fine – for example, so that they can form a powder bed just tens of microns thick. However, these ‘fines’ can be problematic for health and safety, as well as flowability. Because the forces of attraction between particles increase with decreasing particle size, finer powders usually flow less freely than coarser analogs. Optimizing particle shape can help to mitigate this effect [4, 6]. Particle size and particle size distribution also influence packing, as illustrated in Figure 3. Specifically, distributions including both coarse and fine particles, with finer particles filling the interstices left by larger ones [5], enable maximum packing density.
Figure-2-AN210408-Polymer-Powder-Particle-Size-Shape.jpg
Figure 3: Packing density reaches a maximum when the particle size distribution includes both fine and coarse particles.

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