Unprocessed machining chips as a practical feedstock in directed energy deposition

Abstract

Directed energy deposition (DED) is an additive manufacturing process capable of creating parts or coatings from powder or wire feedstocks, and may be adapted to use machining chips. Re-use of steel chips as opposed to landfilling or even recycling, has implications for CO2 emission savings. This is particularly the case of like-for-like replacement of powder and other feedstocks. This work addresses two key concerns which currently impede the implementation of chip-based DED – the impact of residual coolant present on chips on part properties, and whether chip type feedstocks can be delivered in a consistent, practical manner similar to powders. The study demonstrates that DED can generate dense, repeatable deposits from 304L stainless steel milling chips, using both pre-placed and vibrationally fed strategies. Milling chips created with and without the use of machining coolant can both be successfully deposited, without compromising the density of the part. EPMA/XRD/EBSD analysis showed no evidence of coolant in deposits, explained by rapid evaporation, and micro-hardness analysis showed it had no effect on hardness. Practical implementation of chip-based feedstocks into DED systems is validated via use of a bespoke vibration feeding system, which is calibrated in this work, and capable of controlled delivery beneath the laser. Melting behaviour under low power is also investigated, revealing chips are readily melted into droplets and incorporate surrounding chips, ultimately yielding efficient material incorporation into tracks.