Influence of surface temperature in the laser assisted cold spray deposition of sequential oxide dispersion strengthened layers: Microstructure and hardness

摘要

Six-sequential layer deposits of an oxide dispersion strengthened (ODS) Fe_(91)Ni_8Zr_1 alloy were laser assisted cold sprayed (LACS) at 650 °C and 950 °C. The nanoscale zirconia phase in this alloy precipitated from residual oxygen within the high-energy ball milled constituent elemental powders used for the spray deposition. The laser provided a local heating of the substrate (or the surface of the previous deposited layer), which thermally softens it, increasing the mass deposition efficiency (DE). At a surface temperature of 650 °C during deposition, the ODS alloy was ferrite with a DE of 7.3%. This low DE resulted in a peening/tamping effect of ricocheted powders off the surface resulting in the grains at the substrate/deposit interface being notably refined. At a surface temperature of 950 °C, the ODS alloy was in the austenitic phase field during deposition resulting in a DE increase to 32.4%. With this increase in deposition temperature, the deposited grains grew and the oxide particles coarsened. This resulted in a reduction in Vickers hardness from 598 ± 56 (650 °C) to 293 ± 38 (950 °C). Hall-Petch grain size effects, Orowan strengthening between oxide particles, and Taylor hardening contributions estimated the hierarchy in the strengthening mechanisms with these microstructural changes, with grain size being determined as the dominant mechanism. These deposits demonstrate the advantages of using a laser for relatively difficult-to-spray (high hardness and strength) materials, albeit with associated changes in microstructure with their corresponding changes in hardness.