Laser assisted cold spray of ferritic alloys: oxide dispersion strengthened fe91ni8zr1 and aisi 4340 steel

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Oxide dispersion strengthened (ODS) Fe91Ni8Zr1 (at. %) and AISI 4340 steel were successfully deposited via laser assisted cold spray. The laser assisted cold spray technique includes accelerating powder particles such that they strike a substrate at supersonic speeds and metallurgically bond. A high-powered laser irradiated the surface of the deposition area making the substrate surface thermally softer promoting deposition. In situ laser heating of the substrate increased the deposition efficiency of the high strength 4340 steel from 48 % to 72 %. The increased surface temperature from 400 C to 950 C also increased the median ferrite grain size. As the ferrite grain size increased, the hardness decreased; however, at higher surface temperatures, the steel transitioned to martensite and compensated the lost hardness due to grain size with the hardness returning to the same values as the cold spray deposits. The lowest viable surface temperature achieved for multi-layered LACS deposition of ODS materials is 650 C. Increased surface temperatures led to an increase in deposition efficiency up to 32 % at 950 C and resulted in a lower hardness material. Grain sizes and particle sizes increased from the elevated temperatures as well. However, the grains did not grow the same throughout the thickness of the material. Grains near the surface of the deposit are several times larger than grains near the deposit-substrate interface. In addition to LACS processing and microstructure, this work reports compressive surface residual stresses of LACS deposits, dynamic strain aging of ODS materials, and a commentary for improving cluster analysis of nano-scale oxides measured through atom probe tomography.

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Materials science