Additive’s idiosyncrasies — producing functional parts
The effect might be insignificant for a small part built within the typical additive manufacturing machine envelope. Yet the effect becomes pronounced as build sizes grow. The facility’s electron beam additive manufacturing machine from Sciaky can build parts as long as 85 cm. The figure above shows an extreme example of the residual stress the facility has found in such large components. A 3/4-inch steel plate became bent 3/4 inch at each end because of the forces built into this part as it was formed. The second photo in this figure illustrates a solution Sciaky has applied to this problem: building mirror images of the same part on opposite sides of the plate in order to balance the forces.
On a powder bed machine, says Dickman, the metal powder is expensive enough that a user needs a strategy for reclaiming and reusing powder left over after the build. The problem with this is that the powder at the end of the cycle is slightly different from the powder that began it. Smaller particles are inclined to melt more readily and tend to agglomerate onto the larger particles, so the leftover powder is biased toward larger particle sizes. Depending on the material purity requirements of the application, it might be possible to mix this leftover powder with virgin powder. But even at that, how many cycles is the powder good for before it has changed too much to assure a quality part? In any additive manufacturing process aimed at ongoing production, he says, the answer to this question has to be part of the process specifications. For CIMP-3D and other additive manufacturing firms, the implications and best practices of powder recycling is another area of study.
One final process consideration is perhaps the most fundamental: the metal from which the part is made. The choice of metal for a machined part might not be the right choice if that same part is redesigned to be produced additively. The design freedom of additive extends to the choice of material, because some materials that are difficult to machine are actually easy to apply in additive.
The reverse is also true. Some aluminium alloys, for example, while easy to work with in machining, are more difficult to process using a laser-based additive process. Meanwhile, titanium alloys, which might be seen as challenging to machine, are the most proven and best-understood metals when it comes to additive processing. In fact, the ideal material for a topology-optimised form (point number 1) is likely to be titanium, because of its strength-to-weight properties. And if the optimisation removes enough material from the design, then the new titanium part is likely to be lighter than the previously solid part.
Though we seldom think of this, says Simpson, part materials are usually selected with an eye toward machining. Hard-to-machine materials have a strike against them in winning acceptance. Now, he says, it’s time to change that mindset, because that criterion biases us against certain metals that deserve to be more popular than they are.
Inconel, for example, has favourable properties appropriate to a range of applications. It should be used more widely than it is. It’s just that it’s difficult to machine. But in a near-net-shape additive process in which the amount of stock to be removed is much less, Inconel might be free to play a larger role. ETMM
This article first appeared in Additive Manufacturing, additivemanufacturinginsight.com, reprint courtesy of Gardner Business Media, Cincinnati, Ohio, US.