Microstructural development in laser-based powder bed fusion - from ferritic stainless steel to medium entropy alloys

Licentiate thesis, 2021

Ever since the advent of additive manufacturing (AM), the interest in AM technologies has skyrocketed due to its intrinsic ability to produce near net shaped components. Laser-based powder bed fusion (LB-PBF), being one of the most widely adapted AM technologies, has been especially a game changer due to its ability to produce components of complex geometries with improved designs and to reduce not only the final weight of the products but also the amount of the waste produced. However, the rapid melting and solidification in the LB-PBF process usually result in anisotropy in properties with respect to different printing directions, due to the epitaxial growth of columnar grains. One way to address this issue is by developing the alloy systems tailored to the LB-PBF process conditions, thereby achieving more control over the solidification dynamics to produce parts with more isotropic properties.

One possible solution to inhibit the columnar growth of grains is by inducing in-situ inoculation via alloy design, thereby reducing the undercooling required for the growth of columnar grains. The first part of this work focuses on in-situ inoculation for ferritic stainless steels via alloy design to achieve the columnar to equiaxed transition. Three different ferritic steel grades based on SS441 were studied, with the aim of exploring the effectiveness of TiN as an inoculant in the LB-PBF process. The results showed that a substantial reduction in grain size with concomitant changes in the solidification behaviour occurred in the alloys pre-alloyed with inoculant forming elements, compared to the alloy without those inoculant forming elements.

The second part of this thesis explores the possibility of improving the strength of the equiatomic CoCrNi medium entropy alloys by the addition of nitrogen. The motivation is to take advantage of the rapid melting and solidification that are intrinsic to the LB-PBF process and to stabilize nitrogen as an interstitial solid solution strengthening element. Two different grades of CoCrNi, one without nitrogen and one pre-alloyed with nitrogen, have been studied. Printing with optimized parameters resulted in parts with densities greater than 99.9% with cellular solidification structure. Heat treatments of printed CoCrNi specimens resulted in the nucleation of chromium rich oxides, while no nitrogen rich phases were observed. The expected interstitial solid solution strengthening resulted in improved yield and ultimate tensile strength values from about ~730 and ~970 MPa to ~890 and ~1110 MPa, respectively.