Characterizing surface finish and fatigue behavior in binder-jet 3D-printed nickel-based superalloy 625

Abstract

In this study, the fatigue properties of binder-jet 3D-printed nickel-base superalloy 625 were evaluated. Standard fatigue specimens were printed and sintered, then half of the samples were mechanically ground, while the other half were left in their as-sintered state. They were then characterized using micro-computed x-ray tomography, metallographic sample examination, and optical and stylus profilometry for surface topography. The micro-computed tomography observations showed that density of the as-printed sample was ~50%, while the sintered sample neared full densification (98.9 ± 0.3%) upon sintering at 1285 °C for 4 h in a vacuum atmosphere. The metallographic examination showed equiaxed grains. The roughness of the as-sintered samples was significant with an RMS roughness of Rq = 1.39 ± 0.20 μm as measured over a linescan of 5 mm, but this was reduced to Rq = 0.47 ± 0.02 μm after mechanical grinding. All samples were tested to failure in fatigue, under fully-reversed tension-compression conditions. While the as-sintered samples showed poor fatigue properties compared to prior reports on cast and milled parts, the ground samples showed superior performance. Scanning electron microscopy observation was conducted on the fractured surfaces and showed that the samples underwent transgranular crack initiation, followed by intergranular crack growth and final failure. In the mechanically ground sample, hardness increased nearly twofold up to 75 µm beneath the sample's surface and X-ray diffraction indicated an in-plane compressive stress, grain refinement, and micro-strain on the mechanically ground sample. The reduced roughness, surface hardening, and compressive stress resulted in increased fatigue life of the binder-jetted alloy 625.

A Review on Binder Jet Additive Manufacturing of 316L Stainless Steel

“…Unlike PBF processes which apply laser or electron beam to melt metal powders, no power heat source involved during the BJ building process, thus BJ can work with a wide range of metals and is regarded as one of the most cost-effective AM techniques [5,12,14,26,31,32]. Furthermore, the surrounding powder sufficiently supports any overhanging geometry in the powder bed and eliminates the need for support structure design [5,27].…”

Powder-bed additive manufacturing for aerospace application: Techniques, metallic and metal/ceramic composite materials and trends

“…[17,55]. As is reported in the recent literature, use of BJP technology permitted to many researchers successful printing of different types of alloys, including superalloys and ceramic matrix composites (CMCs) using BJP process [18,19,[56][57][58][59].…”

Beamless Metal Additive Manufacturing

“…Basically, the metal BJ process generates a relatively fine equiaxed grain microstructure, while the EBM and DMLS processes are conducive to comparable columnar grain microstructures as illustrated in Figure 3. Fatigue performance of the printed nickel-based superalloy 625 part could be improved by mechanical grinding of its surface, surpassing that of the cast alloy [66]. Steel Parts with tailored material gradients have been 3D printed using nanoparticle carbon black ink [67].…”