Influence of shielding environment on solidification cracking in the laser deposition of Aermet®100 on 300M steel Available to Purchase

Inert gas shielding is a vital component of laser deposition as this prevents adverse interactions between the deposit and atmospheric gasses. Nozzle and chamber based shielding are the most common forms of protection. While easier to implement, nozzle based shielding provides protection at a local level only, leaving surfaces more prone to oxidation compared to chamber shielding. In this study, the role of shielding on large solidification cracks in Aermet100 deposited onto 300M was investigated. Large cracks present in nozzle shielded samples were eliminated when chamber shielding was implemented, with oxide films found inside the large cracks confirming the role of environmental oxygen on crack formation. The formation of these cracks is suggested to occur through a combination of three factors: (1) macrosegregation of the 300M substrate into the Aermet100 rich melt pool, (2) the unique solidification sequence between 300M and Aermet100, and (3) the formation of oxygen bubbles due to the remelting of surface oxides. Oxygen bubbles entrapped by rapidly solidified macrosegregation trials are thought to allow oxide films to form between dendrites as the bubble escapes to the surface. The more complete solidification of Aermet100 rich liquid ahead of the bubble then seals in the gas, leading to a large crack like defect. The elimination of environmental oxygen via chamber shielding therefore prevents these cracks occurring.