This blog is translated from German with DeepL.
Metal parts produced additively. Is this the future for obtaining a series part as quickly as possible?
The process briefly explained
This article deals with the SLM (Selective Laser Melting) process, i.e. the melting of metal powder with lasers. The process belongs to the powder bedding processes. As with the SLS process for plastics, the finished part is created from an existing powder supply and then released from the powder.
The design procedure
- Analyze the part to be manufactured and its functions
The entire assembly and its functions are taken into account. As a rule, a cost-optimal solution is only achieved if as many functions as possible are integrated into the part to be manufactured additively. Additively manufactured parts can become arbitrarily complex without significant additional costs (complexity for free).
Expose functional surfaces
Expose elements such as mounting holes, friction surfaces or precise seats from the original geometry. For manufacturing reasons, “classically” manufactured parts usually have too much material between the individual functional surfaces.
Integrate functionsAs far as possible, integrate adjacent functions into the part. Additional supports or fixtures that become necessary for reworking should also be integrated immediately into the part to be printed.
Redesign and optimize the partThe part is then redesigned, taking into account design principles and taking advantage of design freedoms and calculation space. If part weight is an important consideration, topology optimization is applied. This is an FEM calculation method which iteratively calculates the optimum geometry for the workpiece after the boundary conditions (forces, restraints, etc.) have been entered.
Topology optimization can be performed using CAD-internal plug-ins (usually included in high-end variants of simulation software) or tools such as “Hyperworks”. However, the FEM-based approaches have the disadvantage that the data cannot be fed back parametrically.
Quotation and cost analysisCompared to SLS plastic parts, longer lead times must be expected. The layer thickness is 20-75µm compared to approx. 100µm for SLS. The height of the part to be machined is the major driving cost and time factor. The extensive rework (separation from the base plate and removal of support structures) is especially significant in terms of lead time.
Compared to SLS, the material costs are higher, but do not have a significant impact. Even with different metals (steel, aluminum or titanium), there are no major differences between different materials for the same SLM part size.
Production – the production processFrom the powder supply, which is located next to the actual build area, individual layers are pushed into the build area with a squeegee. After application, each layer is melted or fused with the underlying layers by a laser (see Fig. 1). By lowering the workpiece fixture, layer after layer is thus applied one by one. Overhanging structures, large holes or horizontal structures require support geometry. This primarily serves as a thermal bridge so that the energy of the melt can be dissipated and the parts deform less (see Fig. 2).
This process is repeated until the part is finished. The remaining metal powder (which is 100% recyclable and can be mixed with new powder in any ratio) is then removed, leaving the finished part. This must then be freed from the base plate and support structures. In contrast to plastic parts, this process is much more time-consuming and must be taken into account when considering deadlines.
The support structures are removed by eroding, milling or sawing, depending on the requirements of the parts to be manufactured.
SLM execution of 7 parts. The job was not completed here, so the support structures end “in the air”. The parts were placed “crooked” in the build space to get by without support structures in the interior as far as possible.
- Very short delivery times
- Complex parts can be produced, even with undercut geometries
- Parts can or must be further machined
- Threads, ground surfaces, fits, etc. are downstream processes that are often manufactured by third-party suppliers
- Reduction of individual parts in an assembly
- Weight savings
- Powder can be 100% reused
Support geometries are required, which must be cut off
- Parts must be reworked (threads, ground surfaces, fits, etc.)
- Support structures are waste, disposed of with normal scrap metal
- Single pieces (special solutions, medical implants)
- Prototypes (medical technology, automotive, mechanical engineering, etc.)
- Small series production 50-100 pieces/year
Spare parts demand
- Design tips
In order to obtain good and usable parts, certain boundary conditions must be observed.
- Minimum wall thickness of 1mm should not be fallen short of, since they cannot be manufactured with all materials and are additionally strongly geometry-dependent. As a rough rule of thumb, the ratio of height to wall thickness is 40:1.
- Minimum size of raised details: 0.5mm high and 0.8mm wide
- Minimum size of recessed details: 0.5mm deep and 1.0 mm wide
- Drill holes larger than 8mm in Z-direction if possible
- Overhangs from 45° do not require a support structure
- Roughness of surfaces is best on vertical walls
- Channels and holes often do not need a support structure (depending on size), but all holes collapse somewhat during the manufacturing process. For this reason, holes larger than 8mm should be designed round at the bottom and teardrop-shaped at the top.
- Changes in cross-sections in the same component can lead to distortion in the component.
As the name “additive manufacturing” implies, many processes have changed from rapid prototyping applications to manufacturing processes for small and medium series. Due to the extensive post-processing, SLM is usually not worthwhile for single pieces. However, as soon as complex geometries, high part integration or short throughput times are required, production of 5-10 pieces or more can pay off very well.