AS ISO ASTM 52911.1:2021 – Additive manufacturing -Design Part 1: Laser-based powder bed fusion of metals
AS ISO ASTM 52911.1:2021 – Additive manufacturing -Design Part 1: Laser-based powder bed fusion of metals.
— Consideration shall be given to deviations from form, dimensional and positional tolerances of parts. A machining allowance shall therefore be provided for post-production finishing. Specified geometric tolerances can be achieved by precision post-processing.
— Anisotropic characteristics typically arise due to the layer-wise build-up and shall be taken into account during process planning.
— Not all materials available for conventional processes are currently suitable for PEW processes.
— Material properties can differ from expected values known from other technologies like forging and casting. Material properties can be influenced significantly due to process settings and control.
— Excessive use and/or over-reliance on support structures can lead to both high material waste and increased risk of build failure.
— Powder removal post processing is necessary.
5.5 Economic and time efficiency
Provided that the geometry permits a part to be placed in the build space in such a way that it can be manufactured as cost-effectively as possible, various different criteria for optimization are available depending on the number of units planned.
In the case of a one-off production. height is the [actor that has the greatest impact on building time and build costs. Parts should be orientated in such a way that the build height is kept to a minimum.
II the intention is to manufacture a larger number of units, then the build space should be used as efficiently as possihle Parts should he orientated so as to minimize the number of build runs required. Strategies for nesting can also be included to maximize the available build space. If the same parts are oriented differently for best packing. i.e. results in building at different angles, then the mechanical properties can vary from part to part.
The use of powder that remains in the system depends on the application, material and specific requirements. Powder changes can be inefficient and time consuming. Though they are necessary when changing material type, powders from same-material builds can be reused if permitted In the governing specification. It is important to note, however, that recycling of powder can affect the powder size distribution, surface characteristics and alloy composition, and this in turn affects final part characteristics. In addition, the reusable powder characteristics and therefore recyclability can be different for electron beam-based and laser beam-based powder bed fusion. The number of times a powder can be recycled is dependent on the machine manufacturer and the part specification.
Many poorly designed parts (particularly those designed for conventional processes with little or no idaptation) necessitate a specific orientation either to minimize the use of supports or to increase the likelihood of build success. Indeed, parts designed for additive manufacture should be devised such that build orientation is obvious and/or specified.
5.6 Feature constraints (islands, overhang, stair-step effect)
Since AM parts are built up In successive layers, separation of features can occur at some stage of the build. This depends on the part geometry. The situations described in 52 to can be regarded as critical (the level of criticality depends on the P1W technology in focus) in this respect.
Areas with an overhang angle of O produce an overhang with length a (see Figure 2, right). Small overhangs do not need any additional geometry in the form of support structures. In such cases, the projectrng area Is self-supporting during manufacturing. The permissible values for a depend on the specific PBF process, the material and the process parameters used. Significant overhangs can induce a collapse or deformation of the length a of Figure 2, which can lead to the machine standstill.
5.6.4 Stair-step effect
Due to the layer-wise build-up, the 3D geometry of the part is converted into a 2,5D image before production, with discrete steps in the build direction. The resulting error caused by deviation of this 2,5D image from the original geometry Is described as the stair-step effect. The extent of this is largely dependent on the layer thickness (see Figure 3).