This paper some technical issues with Ti6Al4V airframe investment castings during the engineering and manufacturing development phase of the F-22 Raptor program. The fracture-critical nature of the F-22 wing-to-body and aileron castings demands a good understanding of defect sources. Focus is brought upon oxygen- and nitrogen-containing inclusions. These defects include melt stock-source low-density inclusions, investment shell ceramic inclusions, and foundry-source 'hard alpha' inclusions. All occur as hard, brittle regions that are effective crack nuclei. As such, knowledge of their size and frequency is important for accurate life prediction models for fatigue-critical components. This paper considers the origins, microstructure, size distribution and frequency, and property effects of these defects within titanium investment castings. Methods for assessing the presence of these inclusions, and for their mitigation, are discussed. Success in these endeavors is important to maintaining the cost reduction and design efficiency possible with casting when applied to durability- and fracture-critical airframe structure.


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