Ultra-high strength steels are widely used in fatigue critical load carrying aerospace components, such as the landing gear. These steels are able to operate at very high service stresses, but their drawback is that they are fracture sensitive and have limited toughness. Therefore, crack propagation from common damage scenarios, such as stress corrosion and fatigue, is a problem to the structural integrity. These components are very difficult to repair and are usually replaced when the grind-out of the damage exceeds dimensional limits, leading to an expensive and time consuming replacement procedure. This project explores the possibility of implementing laser cladding to repair ultra-high strength steels. Laser cladding uses a laser beam to melt an alloy powder creating a clad layer that fuses with the substrate material. The aim is to re-build the geometry in a grind-out which has exceeded the permissible limit to restore the mechanical properties, especially fatigue properties, back to a minimum safety level. Results indicate that the repair of AISI 4340 and AerMet 100 ultra-high strength steels, using powders of the same composition, can produce a clad layer with a good fusion bond and very little defects. However, the thermal cycling during the cladding process results in very rapid heating and cooling rates. As a result, the microstructure in and around the repaired regions leads to very brittle properties, significantly reducing the fatigue life. Changing the powder composition demonstrated flexibility to improving the fatigue properties. Also, post clad heat treatment offers control of the microstructure, therefore opportunities for accurate tailoring of the fatigue properties. The overall results show that laser cladding offers a very promising path to restore and possibly increase the structural integrity of damaged ultra-high strength steel components.
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