Comparison of the microstructure and mechanical properties of FeCrNiBSi alloy fabricated by laser metal deposition in nitrogen and air

Abstract Laser metal deposition (LMD) as a promising on-line maintenance technology can effectively reduce the maintenance cost of energy and mining machinery. However, the process of LMD for on-line maintenance is faced with two difficulties: insufficient inert gas and oxidation of materials. To overcome the difficulties, this work proposed to use nitrogen which is easily isolated from air and an FeCrNiBSi alloy powder having good oxidation resistance as the powder delivery gas and the feedstock material for repairing AISI 5140 by LMD, respectively. The microstructure and mechanical properties of the FeCrNiBSi coatings deposited on AISI 5140 substrates by LMD in nitrogen and air were compared. Both coatings had a hypoeutectic microstructure composed of a primary austenite dendrite within a eutectic structure consisting of a (Cr,Fe)2B phase and an austenite phase. The coating fabricated in air possessed lower yield stress, ultimate tensile stress and microhardness, higher total elongation than the coating fabricated in nitrogen due to the eutectic volume fraction in the former was lower. The composition and microstructure of the coating fabricated in air varied monotonously from the substrate/coating interface to 300 μm. By contrast, a continuous eutectic lamella was formed at the interface of the coating fabricated in nitrogen, which hindered the diffusion of elements, caused a sudden change in composition, and was detrimental to the mechanical properties. Therefore, for repairing AISI 5140, the FeCrNiBSi powder was deposited onto AISI 5140 by LMD in air, and the yield stress and ultimate tensile stress were slightly higher than those of the AISI 5140, indicating that AISI 5140 could be repaired with FeCrNiBSi by LMD in air. In addition, the microstructural characteristics that less eutectic in overlapping zone of the laser deposited coatings and its influence on the mechanical properties were discussed.