Building the Part
The start sequence is critical to establish and verify proper process performance before transitioning to fully automated operation. Once it is ensured that all subsystems are functioning together properly and initiated any process monitoring, then process can be initiated. Having an AM operator on call to attend or to a stop the operation is needed to assure proper termination, remove the part, and prepare the system for the next build. Certain AM system designs reduce the downtime between building one component and the next by utilizing dual or modular powder bed configurations, allowing part or powder module removal of a completed build simultaneous with starting the next.
The successful completion of a build cycle often includes a cooling interval particularly for EB, DED. and arc-based systems. PBF-EB, PBF-L, and DED-L are more efficient in melting and experience less heat buildup due to the high-energy heat source. However, PBF-EB uses a powder bed preheat of up to ~700°C and is performed in a vacuum chamber that slows heat transfer and cooling after the build cycle due to the preheating of the powder bed, combined with the additional heat input of the electron beam. An inert gas purge of the chamber may be required to assist in heat transfer and cooling until the part is below a reactive temperature and allowed to cool in air to a temperature that allows effective handling, powder removal, and post-processing.
In the case of PBF-L systems, a shorter cooldown interval is required as powder bed heating is less than with PBF-EB. After cooldown, removal of the powder will include special powder handling procedures to allow removal of the build platform, the part, and support structures.
Powders optimized for AM use are often expensive; therefore, reclaiming and reusing them is critical to the process. The claims that used powder can be re-sieved and reused indefinitely are disputed. Changes to powders may include losses due to re-melting, vaporization, oxidation, and moisture pick resulting from normal processing, or improper handling or storage. These changes may in many cases be insignificant with slight charges to particle size distribution but will also be dependent upon the alloys used and types of powder. This is an active area of research, but now it is best to follow the vendor’s recommended procedures. Powder vendors are beginning to fill the need by offering powder characterization services and tracking software to assist customers in maintaining their AM powder inventory.
Removal of the support structures and part from the build plate may require conventional machining, sawing, or EDM (electrode discharge machining) operations as well as finishing the build plate and measurement to assume a minimum build plate thickness and flatness specs are met. Post-build procedures may include cleaning of the chamber or post-processing the powder by sieving to allow reuse and recycle. In some cases, a part may be left connected to the build platform as a support fixture for subsequent finishing operations such as machining or inspection. In other cases, the build plate may become integral to the final part following through all post-process heat treatment and machining operations.