SLM workflow and build preparation steps for metal components

Taking adequate steps during build preparation ensures high production yield. In contrast, overlooking one or few of these could make life difficult: use the wrong type of supports or forget to repair the .stl files and you could see the build fail half way through, use the wrong substrate temperature and you could overage the material and ruin the fatigue properties of the components, use the wrong build orientation and you could waste expansive powder in the form of useless supports… 

Production yield and build succes is decided partly at the planning and preparation stage and is equally defined by suitable process parameters development. Here we mention the critical aspects you should be careful to address to ensure high yield production at the planning stage.

1. Part design and 3D CAD file generation

Regardless of the CAD software used (Solidworks, CATIA, …) to create components 3D representtions, none of these are specifically tailored for the desgn of parts made using SLM. SLM-specific design, rendered all the more complicated by the fact it is sill a trial-and-error, experience-based activity (science or art?), is nevertheless critical to ensuring reliable and repeatable build success, improving production yield and minimising post processing time or waste.

2. *.stl file generation and repair

The 3D model is converted into an *.stl file or digitalised into a mesh structure. At this stage some meshing conversion errors can occur and may be repaired automatically or manually using STL manipulation software (Magics, …). It is also important to ensure fine meshing for precision and keep shape accuracy. The finer the mesh, the larger the *.stl file and this can eat up time during the slicing step but it's a small price to pay for realistic digitalisation.

3. Supports generation

The first step involves finding the most suitable part orientation to maximise build success and minimise the quantity of supports (hence limit wasted material and time consuming supports removal). It also means choosing the right type of supports with adequate part/support boundaries (or “teeth”). This ensure 1) appropriate support and heat transfer and 2) this makes sure the first part layer in contact with the support structure is not peeling off (insufficient heat heat transfer) and subsequently swept by recoater (part failure!). Tips include minimising overhangs to 45deg angle, ensuring the sturdiness of the supports so they won't bounce or bend against the recoating blade.

On some occasions, some judicious sacrificial structures are a good compromised or alternative to automatically or manually software-generated supports that can seriously miniise the amount of powder required for supports while maximising build success.

Given that the component will be chopped off the substrate it is usually useful to add a few 0.1mm at the base of the part to ensure dimension accuracy after cutting off the plate.

4. Slicing and repair of sliced file

Slicing is the act of dividing the component and its support along its heigth in a finite set of layers along the z-axis (build axis) with user defined thickness, usually in the range of 20um to 100um. More often than not, the supports need to be easy to remove and require only to be strong enough to support the build and withstand the recoating motion with its potential “grating” effect. Being more feeble (buut not so much they would collapse or crumble) the supports are usually sliced with twice the thickness of the components and lower laser density processing parameters are used.

5. Assigning parameters to the build

Once the components+supports *.stl files have been sliced , they can be loaded on the SLM machine and respectively assigned suitable processing and “machine” parameters, previously determined during the critical proces and parameters development stage.

On the EOS M280, there are:

• “machine” parameters, such as layer thickness, O2 levels, substrate temperature, recoating speed, and number of skipped layers, HWI offset ,...
  
  
• “processing” parameters, such as laser power, hatching distance, scanning pattern, scanning speed, etc…
  
  

Building supports and components usually requires different respective parameters set, as the components require stringent mechanical properties, whereas the supports need optimised only for strength and easy removal.

The substrate temperature, as well as machine and parameters for supports and components, are optimised during process development.

Conclusions?

As important as determining the processing parameters, each step of the SLM production workflow is important to ensure hassle-free successful builds and high production rate. Judicious design, part orientation and appropriate supports types can make the difference between repeatable high production yield or a costly time consuming mistakes.