Variable Polarity GTAW of SMP14 Single Crystal Superalloy: Part One — Effects on Microstructure and Tensile Properties

Gas tungsten arc welding (GTAW) using AC or DC current types has been utilised for the manufacture and repair of turbine engine components for many years. The weld repair of single crystal components such as turbine blade tips is also important. However, welding of a single crystal alloy has its associated problems (which will be discussed later). SMP14 is an advanced single crystal, Ni-base superalloy used for turbine blading. This alloy has enhanced mechanical properties at elevated temperatures when compared to equiaxed, directionally solidified and first generation single crystal superalloys. The objective of this task is to investigate the weldability of SMP14 using the new variable polarity GTAW process. Metallurgical investigations were undertaken to evaluate the microstructure of the welded region. Microporosity was found in the weld and there was no evidence of recrystallization, sub-grains beneath the weld or microcracking/microfissuring. Tensile test evaluations revealed that a high strength weld equivalent to equiaxed MAR-M247, Ni-based superalloy could be achieved. This appears to be the highest weld mechanical strength achieved to date when joining any Ni-base single crystal alloy together. Fractography was also utilised to analyse the fracture surfaces of the tensile test samples. Oxidation tests also revealed that the oxidation resistance of the weld was good and will be suitable for weld tip restoration where the weld needs to have good oxidation resistance.