Dimension accuracy of metal components built by selective laser melting using EOS M280 (2/2)

As with any craft, it is important to understand the advantages and limitations of tools. In the case of the EOS M280, which offers one of the most powerful and flexible control software  among SLM machines, it is vital to pick out the most suitable parameters amongt the 84 parameters available (and combinations!) for dimension accuracy. 

Assuming a reasonably good as-built surface roughness (Ra<10um) and minimal distortion due to internal stresses, let’s identify the parameters and settings responsible for

1.       in-plane accuracy (= in-layer = x/y plane parallel to building substrate),
2.       accuracy in the z/building direction.

Small features (thin walls, holes, sharp angles/corners, …) accuracy or volume/dimensions variations after heat treatment will be discussed separately.

In-layer dimension accuracy - x/y plane

In each layer, the dimension accuracy depends on machine settings and processing parameters. We define ‘machine settings’ as fixed material-dependent settings:

·         HWI offset: found in the PSW 3.6 software in the HWI window, this parameters can be positive or negative in value. It reflects an offset between original CAD file and the one being machined.

·         Presence or absence of contour parameters: optional, the presence of contour parameters can improve surface roughness. These parameters are scanned along the CAD outer contour of the component in each layer.

·         In-layer scaling factors x,y in the HWI window: here to take into account volume variation after heat treatment – not usually needed (values = 0) for as-built dimension accuracy. Also the offset may be used to compensate for phase change volume variation if the component is subsequently heat treated.  These scaling parameters may also compensate for distortion generated by internal stresses.

We define as ‘processing parameters’ factors that are laser-dependent such as P, s, h and skywriting option:

·         Pre- or Post-Contour parameters: Power, scanning speed used to scan the outer CAD contour (offset optional) of the component.

·         Contour offset: given that the contour laser track has its own track width, an additional contour offset (in addition to HWI offset) may be used to ensure the overall dimensions of the components are within tolerances.

·         Skywriting: recommended, this option ensures the laser is Off as the scan head redirect the beam path at the end of the path and starts processing the adjacent track. If the laser is on, material is melted during positioning and superfluous material is processed that decrease dimension accuracy.

Dimension accuracy in the vertical/build direction

In the build direction, dimension accuracy relies on the precision positioning of the motion system, very reliable. It is also function of layer thickness: the accuracy is usually given by the CAD/theory height +/-1layer. Also, it is common practice to add sacrificial layers at the boundary between part and substrate to allow safe removal of the component of the build platform.

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