Advanced Tip Repair of Single Crystal HPT Blades With LW3 and LW4280 Welding Materials

Abstract

High pressure turbine (HPT) blades manufactured from single crystal (SX) materials exhibit tip degradation during service resulting in loss of coatings and parent metal, primarily from abrasion, thermal-mechanical fatigue cracking (TMF), creep, and oxidation. Currently, Gas Tungsten Arc Welding (GTAW) and Laser Beam Welding (LBW) with Merl 72 and Rene 142 (R142) welding materials are used for repairing the tips of SX HPT blades. Tips repaired with Merl 72, despite the superior oxidation resistance of the cobalt welding material, are prone to cracking due to the low mechanical properties of the Merl 72 welds at temperatures exceeding 1800°F (982°C). Additionally, despite the high strength of R142 in its cast condition, R142 welds are prone to weld stress-strain cracking and thus require preheating of the blades above 1700°F (926°C) to repair the part with a predetermined level of micro cracking present. Preheating can adversely affect the inert atmospheric conditions of the argon protection. This inadequate shielding of the welding area may result in contamination of welds with non-metallic inclusions which reduce creep and TMF properties. The current study focuses on substantiating the replacement of Merl 72 with alternative LW3 and LW4280 nickel based welding materials for minor dimensional restoration and full tip replacement on SX HPT blades with a solid tip cap. LW3 and LW4280 contain 28 vol.% and 49 vol.% gamma prime phase respectively, after post weld aging heat treatment. A time-transient thermal mechanical Finite Element Analysis (FEA) of the SX HPT blade was completed for takeoff, cruise, and landing conditions. The resultant temperature and stresses from the FEA study were used as the basis for qualification of the tip repair. Tensile and stress rupture properties of dissimilar SX-LW3 and SX-LW4280 welds produced at ambient temperature using manual GTAW and Laser Direct Energy Deposition (L-DED) on a LAWS1000 welding system utilizing a 3D additive manufacturing (AM) concept were studied. It was demonstrated that LW4280 welds had superior stress rupture, and fatigue properties when compared to M 72. Cyclic oxidation resistance of LW4280 at 2048°F (1120°C) was found to be sufficient to ensure required durability of repaired blades for 6,000 cycles in cases of damage to protective coatings. Some examples of repairs of HPT blades developed using these materials and technologies are provided.