AS ISO/ASTM 52911.1:2021 pdf free.Additive manufacturing – Design Part 1: Laser-based powder bed fusion of metals.
5 Characteristics of powder bed fusion (PBF) processes
5.1 General
Consideration shall be given to the specific characteristics of the manufacturing process used in order to optimize the design of a part. Examples of the features of AM processes which need to be taken into consideration during the design and process planning stages are listed in 5.2 to 5.8. With regards to metal processing, a distinction can be made between, for example, laser-based PBF (applied for metals and polymers) and electron beam-based PBF (applied for metals only).
Polymers PBF uses, in almost every case, low-power lasers to sinter polymer powders together. As with polymer powders PBF, metals PBF includes varying processing techniques. Unlike polymers, metals PBF often requires the addition of support structures (see 6.4.3). Metals PBF processes may use low-power lasers to bind powder particles by only melting the surface of the powder particles or high-power
(approximately 200 W to 1 kW) beams to fully melt and fuse the powder particles together.
Electron beam-based melting and laser-based melting have similar capabilities, although the beam energy transferred from the electron beam to the metal is of a higher intensity and the process most commonly operates at higher temperatures than the laser counterpart, therefore typically also supporting faster build rates at lower resolutions. In general, since the powder bed is preheated and kept close to the melting temperature during the building operation, electron beam processes subject parts to less thermal induced stresses and have faster build rates, but the trade-off often comes with much longer times needed for the build chamber to cool down after the build cycle has been completed, and in general larger minimum feature sizes and greater surface roughness than laser melting.
5.2 Size of the parts
The size of the parts is not only limited by the working area/working volume of the PBF-machine. Also, the occurrence of cracks and deformation due to residual stresses can limit the maximum part size. Another important practical factor that can limit the maximum part size is the cost of production having a direct relation to the size and volume of the part. Cost of production can be minimized by choosing part location and build orientation in a way that allows nesting of as many parts as possible. The cost of the volume of powder required to fill the bed should be considered. Powder reuse rules impact this cost significantly. If no reuse is allowed then all powder is scrapped regardless of volume solidified.
5.3 Benefits to be considered in regard to the PBF process
PBF processes can be advantageous for manufacturing parts where the following points are relevant.
— Integration of multiple functions in the same part.
— Parts can be manufactured to near-net shape (i.e. close to the finished shape and size).
— Degrees of design freedom for parts are typically high. Limitations of conventional manufacturing processes do not usually exist, e.g. for:
— tool accessibility, and
— undercuts.
— A wide range of complex geometries can be produced, such as:
— free-form geometries, e.g. organic structures,
— topologically optimized structures, in order to reduce mass and optimize mechanical properties, and
— mull structures, e.g. honeycomb.
— The degree of part complexity is largely unrelated to production costs, unlike most conventional manufacturing.
— Assembly and joining processes can be reduced through part consolidation, potentially achieving en bloc construction.
— Overall part characteristics can be selectively configured by adjusting process parameters locally.
— Reduction in lead times from design to part production.
5.4 Limitations to be considered in regard to the PBF process
Certain disadvantages typically associated with AM processes shall be taken into consideration during product design.
— Shrinkage, residual stress and deformation can occur due to local temperature differences.
— The surface quality of AM parts is typically influenced by the layer-wise build-up technique (stairstep effect). Post-processing can be required, depending on the application.AS ISO/ASTM 52911.1 pdf download.