Hydrogen-Affected Tensile and Fatigue Properties of Ti-6A1-4V Sheets by Superplastic Forming

Abstract

Mill-annealed Ti-6A1-4V (Ti64) sheets were formed into rectangular shaped pans with two square recesses by the superplastic forming (SPF) technique under high pressure using commercial grade argon gas. The SPF process induced a significant decrease in the tensile ductility and high cycle fatigue (HCF) resistance of the Ti64 alloy. The fractographic analysis of as-fabricated Ti64 sheets showed a hydrogen-affected layer on the surface, possibly incurred by a small amount of water vapor in the commercial grade argon gas. This hydrogen-affected layer with a depth of approximately 60 pm tended to decrease tensile ductility and the resistance to HCF, by encouraging easy crack initiation on the surface. The effect of hydrogen on the mechanical properties of the SPF processed Ti64 sheets was discussed based on fractographic and micrographic observations.