Control of oxide inclusions in ODS FeCrAl steel by LPBF parameters
Heat input is correlated with melt pool geometry and oxide inclusion content. Our researchers have modified laser powder bed melting parameters to produce an ODS-FeCrAl steel free of Y-Al-O oxide inclusions from a Fe-22Cr-5.1Al-0.5Ti-0.26Y and TiO₂powder mixture. This research provides a feasible method for in-situ removal of oxide inclusions from alloys during PBF processes.
Oxide inclusions in alloys can degrade component fatigue life and can become preferential corrosion sites in seawater environments.
When FeCrAl ODS steel is prepared via the PBF process, Y-Al-O oxide inclusions are generated due to the high affinity of Y and Al for O. The presence of Y-containing oxide inclusions is a common problem in the additive manufacturing of oxide dispersion-strengthened steels.
This study used a prealloyed powder of Fe-22Cr-5.1Al-0.5Ti-0.26Y (wt.%) mixed with 0.5 wt% TiO₂ (50 nm, 99.99% purity) powder as an oxygen carrier. Samples measuring 70 × 10 × 15 mm³ (length × width × height) were fabricated using a laser powder bed fusion (LPBF) system. Process parameters were: laser power P = 300 W, scanning speed v = 834 mm/s to 1318 mm/s, spot spacing h = 110 μm, and layer thickness t = 30 μm. A bidirectional scanning strategy was adopted with a 90° rotation between consecutive layers. Before printing, the mild steel substrate was preheated to 150°C.
As the heat input to the melt pool increases (i.e., the scanning speed decreases), the cross-sectional area of the melt pool increases, and remelting between adjacent melt pools improves. This enhanced remelting promotes epitaxial grain growth from the previously solidified zone into the current melt pool, resulting in grain coarsening. The melt pool geometry (depth and width) was quantitatively characterized using an optical measurement system. The results showed that the depth and width of the melt pool increased with increasing heat input. When the heat input was increased to 109 J/mm³, one deposited layer underwent six remelting cycles during the printing process.
Further microstructural characterization revealed that increasing the heat input effectively removed the Y-Al-O oxide inclusions from the specimen. At a heat input of 109 J/mm³, the Y-Al-O oxide inclusions were barely visible in the matrix (less than 0.1%).
But the Increase of heat input leads to grain coarsening. The average grain size increases from 72 μm to 179 μm. Furthermore, the size of the nanoprecipitates also increases with increasing heat input, from 35 nm to 49 nm. The nanoprecipitates have a core-shell structure and have been identified as Y₂O₃/TiN.
Room-temperature tensile testing results show that despite microstructural coarsening, the yield strength of the specimens decreased by only 37 MPa after increasing heat input, and the fracture mode was ductile. The yield strength is primarily attributed to lattice friction stress, solid solution strengthening, grain boundary strengthening, precipitation strengthening, and dislocation strengthening. Among these strengthening mechanisms, dislocation strengthening dominates the yield strength contribution of the ODS-FeCrAl steel
