Selective laser melting (SLM) is an emerging production technology in the field
of Rapid Prototyping (RP) and Rapid Manufacturing (RM). More specific,
this technique is catalogued as an additive layered manufacturing technology,
since a part is built by adding material layer by layer.
Figure 1.1 is helpful to understand the process. The part is generated
in the build cylinder, on top of a base plate or substrate. Next to the build
cylinder, there is a feed container (also called powder cylinder; sometimes two,
at both sides). A layer of powder material is deposited on the build cylinder,
by lowering the build cylinder and raising the feed container. A coating system
adds powder from the powder cylinder to the build cylinder. After a layer is
deposited, a cross section of the component to be built is scanned with the
laser. These cross-sections are calculated from a CAD model. By scanning a
surface, heat is added to material, causing it to melt after which is solidifies
as a bulk mass. This process is repeated layer by layer until a component
is created. Afterwards the component is removed from the base plate and is
cleaned. Often a post-processing step is applied.
Many RP and RM technologies work very similar [116]. The differences
are mainly the nature of the material, which can be liquid (Stereolithography),
paste (Optoform) or powder (Selective Laser Sintering and SLM). The
fusion of the material can be done by different chemical or physical reactions.
For stereolithography and optoform, this is a photopolymeric reaction. For
selective laser sintering and SLM, this is a partial or full melting of the particles
[121]. As a result, selective laser sintering and SLM are very closely
related to each other. At a certain level, it is very hard to distinguish between
both. When speaking about SLM in this work, the aim is to obtain
a density reaching nearly 100 % for the final product. Since this involves a
complete melting of the material, this is called SLM. However, for some experiments,
the resulting density is so low, that the difference between selective
laser sintering and SLM vanishes, although it is still called SLM.
The strength of this process is that it is theoretically possible to build any
possible geometry. This is a direct result of the layered manufacturing: every
single part of the component can be reached with the laser at a certain step
in the process. Unfortunately, the physics behind the process make reality
somewhat more difficult, as will be seen throughout this dissertation. The
drawback of the process is that it is not competitive for large batches or
less complex parts, because the process is rather slow and thus expensive.
Some special sectors can benefit largely from these technologies, like medical,
aerospace and automotive: often complex, tailored components are needed