Design Guidelines and Porosity Evolution in Metal Binder Jetting Components

Abstract

In Metal Binder Jetting (MBJ), a binder is selectively jetted onto a powder bed to form a green part, which is then debound and sintered. Green parts reach only about 53% relative density, so sintering causes large shrinkage, gravity-driven distortion and residual porosity. This thesis studies design guidelines, porosity evolution during sintering and densification by Hot Isostatic Pressing (HIP) for 316L stainless steel printed on a Markforged DM P2500 system. Bridge specimens sintered at 1310 ◦C were used to vary one design parameter at a time. Unsupported span had by far the strongest effect: sag rose from 0.4mm at a 20mm span to 9.1mm at 60 mm. Increasing the top thickness from 3 to 7mm reduced sag from 3.8 to 1.7 mm, a base reduced it from 3.6 to 2.3 mm, and a 20mm inner fillet reduced it by 55 %. Z-oriented bridges sagged about 15% more than flat ones. In pin arrays, pins down to 0.5mm diameter were printed, but at half scale five of six slots retained powder; depowdering rather than printing resolution limited the minimum feature size. Dense and shell cubes were sintered in vacuum at 1250, 1270, 1290 and 1310 ◦C and characterised by Archimedes density measurements and light optical microscopy with image analysis. The relative density of dense specimens rose from 89.0 to 99.0 %, and shell specimens were denser at every temperature. Open porosity fell from 8.7% to below 0.5% already at 1270 ◦C, where closed porosity peaked at 4.7 %. Bulk density alone therefore does not show the open-to-closed porosity transition. The pores became fewer, smaller and rounder, and the share of quasi-circular pores rose from 40 to 70 %. HIP was applied in six conditions between 1070 and 1170 ◦C and 50 and 150MPa. The response depended mainly on the sintered state, not on the HIP temperature or pressure. Dense specimens sintered at 1270 ◦C, with the most closed porosity, went from 5.1% to 0.7–0.9% total porosity. Dense specimens sintered at 1250 ◦C kept 8.0–9.8% porosity because their pores were still open. After HIP, all groups from 1270 ◦C upwards reached about 99% relative density, a level that depends on the assumed theoretical density. Design, sintering temperature and HIP must therefore be chosen together: sintering must first close the pore network before HIP can remove it.