1 6 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 | J A N U A R Y / F E B R U A R Y 2 0 2 3 given tool design tomaximize the deposition rate for each material. SUCCESS FROM VARIOUS WASTE STREAMS To date, research has identified key barriers for deposition of recycled material. By learning from new feedstock deposition strategies, successful depositions for both recycled machine chips and strips of damaged material feedstock were achieved as depicted in Fig. 4 and detailed microstructure and resulting mechanical behavior in the open literature[15-16,24]. To achieve adequate compaction and deposition of machine chip feedstocks shown in Fig. 4, the chips required an additional processing step of ball milling to sizes less than 30 mm regardless of whether the chips were compacted into a rod fed AFSD machine or directly fed through an AFSD machine outfitted with an auger feeder. From a forward operating base (FOB) standpoint, the auger system is incapable of depositing hard alloys such as steel or titanium alloys due to wear and fracture toughness issues and would require time intensive machine modifications to switch between processing auger feedstock or rod feedstock. Therefore, the use of a continuous chip recycling machine as discussed later would be a more viable option to mitigate machine downtime from swapping out AFSD components. Alternatively, strips of material instead of monolithic rods can be loosely deposited by the AFSD process by creating stacks of thin strips of a material together to fill the approximate geometry of traditional AFSD feedstock. With these stacked strip recycling depositions, exterior contaminants such as sand or other abundance of high friction particles will result in jamming of material in the tool and require extensive cleaning with machine downtime. Interestingly, the current SERDP research project was able to demonstrate the viability of depositing the strips of material with the non-skid coating or even internal sand embedded between layers to create a successful MMC deposition. Here, these depositions highlight the robustness of the process to fabricate parts using contaminated feedstock material. POINT-OFNEED DAR IN ACTION C u r r e n t l y , AFSD provides a complementary process that can overcome barriers created by liquid-solid phase transformations of fusion-based AM processes, but there are still limits to the nascent technology. Specifically, additional research and development are required for items such as tooling advancement s , process monitoring and feedback controls, and automatic feeding of solid rod feedstock. For example, in the AA6061 AFSD build shown in Fig. 5 that was a 64 mm tall build with a length of 152.4 mm, the actual deposition rate was 0.25 kg/ hr compared to the theoretical deposition rate of 1.03 kg/hr. The difference between the two values is that only two layers were able to be deposited per rod of feedstock. Therefore, the machine and tool had to cool down for a certain amount of time before the operator could load new feedstock rods. Additionally, since the machine was only able to deposit in position control there was no spatial-temporal information to provide feedback control from subsequent heat generation as the build height increased to the maximum height. The successful completion of the SERDP project has demonstrated the ability for DAR of aluminum alloy secondary feedstocks. As AFSD is still in the early maturation stages there is still additional development required to transition this technology to the field. Particularly, automatic feeding is currently in development, which will create significantly improved microstructures and mechanical behavior by eliminating transient regions of starting/stopping the process to reload new feedstock. The AFSD process is at a stage where there are still more scientific discoveries to make as the process matures, but the process does offer key immediate benefits for production repair, specifically by fabricating and repairing long lead time large aluminum alloy components at production facilities such as military depots. However, since fabricating the square feedstock is a significant limitation from a time and material standpoint, the ability to further investigate round feedstock depositions is highly beneficial to the user community. Limited investigations to date for round feedstock demonstrate the ability to produce similar AFSD builds and properties to builds fabricated with square feedstock depositions (Fig. 6). The preliminary work in Fig. 6 comparing round and square feedstock Fig. 4 —Successful AFSD depositions from recycled feedstock including compactedmachine chips, loose machine chips, and strips of damagedmaterial.
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