September_AMP_Digital

FEATURE A D V A N C E D M A T E R I A L S & P R O C E S S E S | S E P T E M B E R 2 0 2 0 4 6 A dditive manufacturing (AM) processes vary in build method, and the path to a finished product affects the resultant microstructure and mechanical properties. In the case of fusion/melt AM processes, such as laser pow- der bed fusion, also called selective laser melting (SLM) and direct metal laser sintering (DMLS); and electron beam addi- tivemanufacturing (EBAM) processes, such as electron beam powder bed fusion and directed energy deposition (DED), re- current heating and cooling of layers has a significant effect on the final microstructure and can result in anisotropy with- in the build itself. Additionally, each build is dependent on the starting feedstock material (powder or wire). The rate of layer deposition and cooling kinetics for each layer also has a distinct effect on the internal stress in the build and build plate. Therefore, as-built products typically must undergo a post build heat treatment to remove stresses, improve the final microstructure, and improve mechanical properties. Because many of the metal alloys used for aerospace and medical parts are highly sensitive to reactions with air at ele- vated temperatures, vacuum heat treatment is necessary to achieve optimummechanical properties. The selection of heat treatment processes depends on part geometry, properties required for the end use of the material, and the build method. The resulting micro- structure has a very small grain size, which results in loss of ductility and the risk of brittle fracture. Thus, the selection of the proper thermal treatment determines the final me- chanical properties of the build. Inkjet, or binder jet (BJ) deposition, a non-melt AM bonding process, uses either liquid polymer deposition or metal-polymer matrix deposition and does not entail the strain of melt AM processes. Parts made using this process require final densification via sintering in a vacuum fur- nace. Binder removal becomes a major concern, especially when carried out in a cold-wall vacuum furnace. This article discusses several practical concerns in- volved in using vacuum heat treatment [1] , including tem- perature measurement, unvented cavities, loose pow- der, and direct contact of metals in the high-temperature vacuum [2] . PRACTICAL CONSIDERATIONS FOR VACUUM HEAT TREATMENT OF AM METALS Temperature measurement, unvented cavities, loose powder, and direct contact of certain metals must be considered during process development of vacuum heat treatment of additively manufactured parts. Virginia Osterman, FASM,* Solar Atmospheres Inc., Souderton, Pa. Fig. 1 — Placement of a thermocouple in a hole in an additively manufactured part provides the most accurate temperature profile during heat treatment. Fig. 2 — Placing an external dummy block containing a thermocouple near the additively manufactured part provides a good comparative temperature profile for the part. 8