Influence of the Processing Atmosphere on Binder Jetting of Stainless Steel: From Printing to Sintering

Licentiate thesis, 2024

Binder Jetting (BJT) is considered a promising Additive Manufacutring (AM) technology for the production of complex metal components due to its high productivity and cost efficiency compared to other AM technologies. The BJT technology is a multi-step process that consists of printing, curing, depowdering, debinding and sintering. Each process step includes individual process parameters. One crucial variable across each step is the processing atmosphere.

A robust process is a prerequisite for manufacturing consistent green parts. Printing in BJT is conducted at ambient temperature. The reusability of the metal powder is considered unproblematic since no oxidation is expected during printing. However, the impact of powder reuse and specifically curing on printing behavior has not been extensively studied in BJT. Therefore, the first part of this study investigated the influence of powder reuse on powder characteristics and printing results for 17‑4 PH stainless steel. Furthermore, different curing atmospheres were compared. The green densities obtained after printing decreased from 4.71 g/cm³ to 4.47 g/cm³ after 20 build jobs, which correlated with an oxygen pickup of the powder during curing. In addition, the median particle size increased by ~1 µm after 20 build jobs. Curing in inert environments such as nitrogen (N2) is shown as advantageous since oxidation of the powder is limited.

The debinding process aims to remove the binder efficiently without contaminating the material. Debinding can be conducted in air to utilize oxygen (O2) for binder decomposition. The powder is, however, prone to oxidation at elevated debinding temperatures. Consequently, the decrease of the oxygen content in the debinding atmosphere at 300 °C for 2 h was studied. The results demonstrated that the carbon introduced by the binder can enhance oxide removal by carbothermal reduction. Debinding in inert argon (Ar) resulted in low binder removal, but led to nearly complete oxygen removal after sintering. The formation of δ-ferrite was prohibited due to the high carbon content resulting in low sintered densities of ~88 %. Debinding in 1 vol.% O2 + Ar reduced oxygen content by 46 % relative to the virgin powder. At the same time, high densities of ~98 % were obtained with no carbon pick-up after sintering. Debinding in 3 vol.% O2 to 20 vol.% O2 removed the binder almost completely. While oxygen reduction was measured from brown to sintered components, the oxidation caused during debinding was not sufficiently reversed by carbothermal reduction during sintering in an inert Ar atmosphere.