Metallurgical Investigation of Air Intake Casing of Gas Turbine Starter Unit (GTSU) of a Military Aircraft

摘要

One of the struts of the part Air Intake Casing was found cracked during the qualification test of the GTSU. The part was newly fitted into the engine before the vibration test and the crack in the part had developed just within 1 hour and 30 minutes of the test. The casing was made of Al-Cu-Ni cast alloy of specification MSRR 8009, was in solutionized and aged condition.The chemical composition and heat treatment condition of the casing met the specified requirements. However, micro examination revealed presence of numerous microshrinkage porosities in the casing.The crack in the part was opened-up for fractographic studies and Scanning Electron Microscopy revealed presence of fatigue striations on the fracture surface of the casing. The orientations of the striations indicated that fatigue failure had originated at multiple locations from the surface of the strut of the casing. SEM analysis also revealed presence of microshrinkage porosities at the fatigue failure origins.It was concluded that the Air Intake Casing has failed by fatigue that had originated from microshrinkage porosities present on the strut surface of the casing. Al-Cu cast alloys have poor castability due to long freezing range that results in casting defects like microshrinkage porosities and hot tears. Therefore, laboratory had recommended changing the material used for the casing to Al-Si premium grade cast alloys or Al-Gu wrought alloys subject to fulfilling the design requirements.