Improving the Mechanical Properties of the Fusion Zone in Electron-Beam Welded Ti-5Al-5Mo-5V-3Cr Alloys

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I.

INTRODUCTION

METASTABLE BETA titanium (b-Ti) alloys have become popular in the aerospace industry due to their favorable response to heat treatments, high strengths, low forging temperatures (in comparison to a+b alloys such as Ti-6Al-4V), deep hardenability, and relatively lower flow stresses. These characteristics make them well suited for applications requiring forged components with thick cross sections and high strengths. The metastable b titanium alloys have a high enough b-stabilizer content to suppress a martensitic transformation when quenched from above the b-transus. At even higher b-stabilizer contents, 100 pct b-phase retention may be observed in some alloys.[1] Because metastable b-Ti alloys do not transform martensitically upon cooling, they respond well to solution treatments and aging in the a+b phase field after quenching to precipitate a. The size, morphology, and volume fraction of the a precipitates inherently control the mechanical properties. CHRISTOPHER J. MARVEL, MASASHI WATANABE, and WOJCIECH Z. MISIOLEK are with the Department of Materials Science and Engineering, Lehigh University, Bethlehem, PA 18018. Contact e-mail: [email protected] JOSEPH C. SABOL is with Alcoa, Pittsburgh, PA. TIMOTIUS PASANG is with the Department of Mechanical and Engineering, Auckland University of Technology, Auckland 1010, New Zealand. Manuscript submitted September 7, 2016. Article published online January 17, 2017 METALLURGICAL AND MATERIALS TRANSACTIONS A

The Ti-5Al-5V-5Mo-3Cr (Ti-5553) system is one of the metastable b-Ti alloys that has emerged in the last two decades to replace Ti-10V-2Fe-3Al (Ti-10-2-3) for landing gear applications and thick section forgings,[2] but has also found use in airframe components, flat products, and armor applications.[2–4] In comparison, the Ti-5553 alloy has better forgability and response to heat treatment, is less prone to segregation, and can be air cooled to achieve sufficient hardening in thick sections.[1,2,4] Similar to other metastable b titanium alloys, Ti-5553 is susceptible to forming continuous grain boundary a. Grain boundary a cannot be suppressed and leads to poor mechanical properties when improper thermomechanical processing is performed, such as insufficient deformation to breakup existing grain boundary a, slow cooling from solution treatment temperatures, and extended durations at high solution treatment temperatures. In addition to grain boundary a, metastable b-Ti alloys may also form x-phase precipitates during cooling and heating. The presence of x is commonly linked to poor mechanical properties and brittle behavior. Heat treatments are generally performed above its transus temperature in order to eliminate it in the final microstructure. In relation to Ti-5553, x-phase precipitation has been shown to embrittle b-Ti alloys. Sabol et al. observed that x was present in the fusion zone (FZ) of electron-beam welded (EBW) Ti-5553, and reported that it imparted brittle characteristics to the FZ because x-precipitates are obstacles for dislocation motion.[5] F

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