Image Credit: NASA/JPL-Caltech
3D-printing in metals is made either by having a flat powder bed and locally melt the material with a laser or electron beam, or by depositing powder- or wire material and locally melt the added material with a laser or electric arc. In this project, powder bed Selective laser melting (SLM, a powder bed fusion technique) is used to produce components for companies. It is a mature technique (”plug and play”) and already used in production in many places. In the figure below, a schematic of the SLM process can be seen, as well as high speed photographs of processing (note that the laser light is invisible here).
The process is not always called SLM, since all manufacturers currently has their own name for the process. One good video explaining how the process works is this video.
The SLM system used in this project is located in Nivala and is run by Future Manufacturing Technologies (FMT) at Oulu University.
This system has exchangeable platform sizes, depending on application requirements:
- 280x280x365 mm
- 50x50x150 mm
- High temperature platform (heating up to 600 degrees)
Direct Metal Deposition (DMD)
Or Direct Energy Deposition (DED) is an alternative for SLM when printing larger components. Either wire or powder can be deposited into the laser beam and it is not constrained to work in a cabinet. These techniques have higher deposition rates and can also build on already existing parts to add features.
There are a few materials certified materials (powders) for the SLM process, where some are used more than other and with varying prices. Examples are:
- Tool steel
- Stainless steel
The material properties are often very good for the 3D-printed components. However, the properties also depend upon a number of parameters, among a few are here illustrated:
The surface roughness is highly dependent upon the powder particle size (or wire width), but also orientation of the work piece and produced layer thickness. In most cases, surfaces can be polished (or sand/glass ball-blasted) but this is not always possible or wanted. The surface quality is smoother on the upper part of a surface (due to melting), compared to overhangs where semi-molten powder particles are fused. Detail resolution is best at on vertical surfaces (e.g. for producing text or structured graphics on the component).
FMT has investigated use of 20 µm layer thickness in aluminium (AlSi10Mg) in order to improve surface quality (particle size 20-65 µm). For small component features, good surface quality is important. Printing time is increased, but surface roughness is improved, having possible Ra of 3.2. On the upper part ”skin” were best when having 37% laser power compared to sub-layers, which can also be improved by adding small chamfers. More information can be found in this report.
For DMD process the surface is generally more uneven than SLM produced parts. Therefore post treatment (milling) is even more needed if tolerances of produced components are high. For this purpose, there are also hybrid approaches, where DMD and milling is combined in one machine to be able to mill at certain regions of the components during the build (that otherwise would be difficult to reach).
When 3D-printing, support structures are needed during component processing. These structures needs to be removed after processing and also consumes material and therefore part of the cost of manufacturing and adds to post-processing of the 3D-printed component. Some knowledge of the process is needed when designing these. While keeping the amount of support structure low and making it easy to remove during post-processing, still have it positioned well enough to prevent failure of the support or deformation of the part. Common strategies involve rotation of the component and design to keep overhangs low and work piece angles at certain lateral angles. In the left figure below, the component is tilted in order to reduce amount of support needed, while process settings for the support structure was first incorrect, leading to deformation of the component. In the right figure, angles are considered throughout the work piece so that only a minimal amount is needed to the build plate (always needed).
Besides the required sawing components of the build plate (also Grinding, polishing and sand paper roughening of the base plate before next build), there are a number of post treatment tools available. Depending on components, some are required while some remain optional:
- Removal of support structures
- Heating of the component to relieve stress due to the thermal cycles the component has experienced during manufacturing
- Optional surface finishing, e.g.
- Sand or glass ball blasting
Project related results
In addition to above mentioned processing reports, fundamental process knowledge has been generated and presented in these reports:
- CYCLAM processing: Comparative analysis of variants of the CYCLAM-technique; presented at LANE 2018: Industrial paper, Presentation
- DMPD processing: Powder particle attachment mechanisms onto liquid material; presented at LANE 2018: Journal paper, Presentation
- DMPD processing: Powder Catchment in Laser Metal Deposition; presented at ICALEO 2018: journal paper, Presentation
- DMWD processing: Laser Enhancement of wire Arc Additive Manufacturing; presented at ICALEO 2018: journal paper, Presentation
This includes improved knowledge about powder catchment in DMD processing with powders (DMPD), improved process stability in DMD processing with wire (DMWD) by adding a laser and strategies of using the CYCLAM process. Selected images for these are here shown: