Scanning patterns in SLM
In selective laser melting, scanning patterns influence critical features such as porosity, microstructure, surface roughness and heat build-up in the finished the metal components. Different scanning strategies documented in literature are now commercially accessible in production SLM machines.
During the overall SLM process workflow, components are digitalised and sliced in a stack of layers. In turn, each layer is split into various regions based on algorithms that determine whether a specific location has material above or below. In short, these algorithms are the logical versions of these 2 questions: is there material above this in-layer region? Is there material below it? The answer defines each area as core, skin, upskin, downskin.
Why is it important to define regions in each layer? Defining these in-layer regions gives the flexibility to assign them different parameters. And why does it matter? Because the melting process and heat transfer is intrinsically different depending on the material and its location (is it powder? Is it resolidified material? Is it a thin wall? Is it part of a larger solid volume?). And so are the final properties of the finished product (microstructure, surface roughness, etc). More on that later…
For now, let’s see what kind of typical patterns are used to scan large areas such as core regions.
The three main scanning configurations are commonly referred to as stripe, checkerboard and islands patterns.
ln-layer patterns: Stripes, islands, chessboard
The stripe pattern is a band defined by the scan vector width (ie stripe width), the hatching space between adjacent tracks and the scan direction as well as the overlap with the neighbouring stripes
Chess board pattern
The chessboard pattern – or checkerboard pattern - is defined by its squares, similar to, as the description suggests, the squares of a chessboard. This pattern is defined by the side length of the square, the hatching distance between adjacent tracks and the overlap between squares. The equivalent of the white squares of the checkerboard are printed first before the black squares.
This is a random version of the chessboard pattern. Here each square is printed randomly across the layer, in no particular order. The main variables are square side width, overlap and hatching distance to ensure no powder is left unmelted.
Scanning directions – unidirectional, bidirectional, spiral, double scan – can also be changed.
Patterns orientation in successive layers
These in-layer patterns can be shifted (or not) in x, y directions and/or at various angles across successive layers . This give rise to a large number of permutations available!
Laser beam motion is typically driven by mechanical mechanisms of mirror through theta scan heads. Practically, this involves ‘jumping’ time (ie beam switched off) while the mirrors reposition to process successive stripes or squares. Worth keeping in mind is this have a non negligible impact on the building time of large components.
To sum up...
Scanning patterns across large regions are a combination of in-layer features and scan vector directions coupled with translational (x and/or y direction) or angular shifts across successive layers in the building direction. These patterns influence building time and properties of the SLM components and supports.
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