Microstructural development in laser-based powder bed fusion - from ferritic stainless steel to medium entropy alloys
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.
Laser-based powder bed fusion
interstitial solid solution strengthening
medium entropy alloys
ferritic stainless steels
Sri Bala Aditya Malladi
Chalmers, Industri- och materialvetenskap, Material och tillverkning
Grain refinement in additively manufactured ferritic stainless steel by in situ inoculation using pre-alloyed powder
Scripta Materialia,; Vol. 194(2021)
Artikel i vetenskaplig tidskrift
Sri Bala Aditya Malladi, Zhuoer Chen, A. Durga, Greta Lindwall, Sheng Guo, Lars Nyborg, Influence of in-situ inoculation on microstructure of ferritic stainless steel manufactured by laser-based powder bed fusion
Sri Bala Aditya Malladi, Zhuoer Chen, Sheng Guo, Lars Nyborg, Laser-based powder bed fusion of interstitial nitrogen strengthened CoCrNi medium entropy alloy.
Design av nya material och processer för nästa generations additiv tillverkning
VINNOVA (2018-00803), 2018-05-16 -- 2021-05-15.
Bearbetnings-, yt- och fogningsteknik
Metallurgi och metalliska material
Chalmers tekniska högskola
VDL, Chalmers Tvärgata 4C
Opponent: Ulf Jansson, Department of Chemistry - Ångström, Inorganic Chemistry, Uppsala University, Sweden