Post-Processing and Finishing

What sorts of post-processing operations (Figure 10.1) are required? Can you perform these operations or will you send the parts to a service provider to complete the post-build finishing operation?

Full functionality of a deposited metal part will most often require post-processing and finishing to achieve the desired dimensions and properties. Post-processing operations for metals may require heat treatments, machining operations, and access to resources that require specialized precision equipment, expert knowledge, and operations beyond simple media blasting, sanding, or coating. Knowledge of these post-processing operations, where and how to access these resources, will help you better choose the process and materials to make the best up-front design decisions. Knowing how and when to apply these operations is critical to efficiently achieve the desired full performance of your metal part.

FIGURE 10.1 Post-processing and finishing operation.

One of the first operations after completion of a build cycle and cooling is to remove the part from the powder bed, build platen, or fixtures. As introduced above, this will include powder removal, recovery, and physically removing support structures or fixtures, emptying powder volumes internal to the part through drain holes, and clearing internal passages. Figure 10.2a and b show a support structure and its removal for a nozzle component. Recovering the powder and recycling it back into the process for subsequent builds may require different procedures depending upon the powder type and build conditions. Recommended practices for tracking and mixing new virgin powder with sieved and reused powders will vary depending on applications, standards, and certifications still in development.

Finishing may include media blasting, peening, sanding, abrasive slurry honing, or grinding to smooth surface features and allow visual inspection. Washing may be used to help remove powders from internal features. In cases such as medical applications, sterilization may be specified. Coating and painting, as used in plastic prototype finishing, may improve surface finish or appearance. Alternatively, smoothing or finishing may employ slurry polishing, electro-etch, electro-polishing, or plating operations. These operations may require specialized equipment and processes provided by a dedicated service provider.

Partially fused powder particles may become dislodged and affect in-service part performance. Machining, grinding, polishing, or coating are all candidates to modify as-deposited powder, but the more post-processing required by your design, the more you diverge from having a straight-out-of-the-machine functional object and the more you reduce the benefits of AM fabrication, when compared to conventional processing. Hybrid systems combining improvements in laser optics and beam delivery may extend capabilities to include automated finishing or inspection into the build space. External supports or base features may need to be removed as part of the post-build finishing operations. The design of support structures and optimization to

FIGURE 10.2 (a) As-built nozzle structure with support structure; (b) Finished nozzle with supports removed.

allow ease of removal is incorporated into the build cycle using software, while the removal of supports is typically performed by sawing, cutting, machining, grinding, EDM, or other mechanical means. Knowledge of what post-processing methods are best for a specific material and design is critical to optimizing the AM process for a specific part.

Heat treatment is often required to achieve the engineering properties of the AM metal. Heat treatments may require 2 -4 hours in an inert or vacuum furnace at temperatures ranging from 650°C to 1150°C. These treatments may be required to improve or meet the desired strength, hardness and ductility, fatigue, or bulk properties. As stated earlier in the book, heat treatments, such as annealing, homogenization, solutionizing, or recrystallization, may be needed to achieve uniform bulk properties or achieve the desired microstructure. The layer-by-layer deposition can result in directionally dependent properties and can vary with respect to the orientation of the part within the build chamber. HT furnaces used to treat metal parts may need to operate at high temperatures and use inert atmospheres, such as argon or vacuum, when processing certain materials. Relief of residual stresses present in an AM part may also require HT to assure dimensional stability. All these operations require specialized HT equipment and may take hours to complete. Research is ongoing to understand and define heat treatment conditions for AM deposited materials. In one example research indicated differing heat treatments of the same material were required to optimize either hardness or wear resistance.

As mentioned earlier, hot isostatic pressing (HIP) is a process that uses high temperatures and high gas over pressures to heat a part to a temperature below melting and at pressures of 100s of MPa, and temperatures in the range of 900-1000°C for 2 to 4 hours to help close and fuse internal pores, voids, and defects. HIP can also provide heat treatment benefits by optimizing the temperature and pressure cycles to improve mechanical properties such as strength, elongation, ductility, and to improve the structural integrity of the component. The equipment is large and costly and may require a specialty service provider.

HIP pressure chambers typically are limited in size ranging from ~75 mm to 2 meters, limiting the size of an AM part to be consolidated, although custom HIP system designs and services are available in industry.

High precision parts will often require subtractive post-processing operations, such as machining, grinding, or drilling, to achieve the dimensional tolerances and surface finishes of the final functional shape. Specialty operations such as plunge EDM or polishing may be needed to achieve the final surface contours of molds, punches, and dies. The tradeoffs between the accuracy, surface finish, or microstructure of the as-deposited part and the desired final finish of the part entail a complex set of decisions. Regardless of the process used to fabricate the part, either by PBF or DED, laser, electron beam, arc, powder, or wire, a careful evaluation of the final requirements will contribute to the decisions made during process, material, and procedure selection. Creating machining blanks may relax the dimensional requirements of the deposit. In cases such as in DED, deposition rates may be more important than accuracy.

If the as-deposited near net shape requires machining to achieve the final dimensions, surface finish or distortion may be less important than optimizing the build rate. As an example discussed earlier, DED-EB may be used to create very large objects to be subsequently machined to final dimension. The cost savings result from not having to machine very large billets resulting in the creation of a significant amount of wasted material. In cases such as these, the degree of stair stepping, distortion, and relatively crude deposition to the near net shape is less important because the final shape and tolerances will be achieved by machining. The hybrid combination of DED and CNC machining holds promise to incorporate DED using lasers and arc-based systems into multi-axis machining centers. Again, speed may take precedence over accuracy if all the AM deposit will be machined to final dimension.