The Effect of Yttrium on Ti-5111 Gas Tungsten Arc Welds
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TITANIUM alloys are technologically important and, because of their high strength-to-weight ratio, biocompatibility, and excellent corrosion resistance, they are used in many aerospace, defense, and commercial applications. Ti-5Al-1Sn-1Zr-1V-0.8Mo (Ti-5111), which is a near-a Ti alloy, was designed by Timet Corp. (Pittsburgh, PA) in conjunction with the United States Navy for use in naval applications. This alloy combines the properties of high toughness, stress corrosion cracking resistance, and room temperature creep resistance.[1] One of the drawbacks of this alloy, however, is that welding processes may have a deleterious effect on the material ductility. Titanium is a strong getter for oxygen, and during welding, oxygen will likely be picked up from impurities in the shield gas. Some oxygen also may diffuse toward the weld fusion zone (FZ) because of the thermal gradient.[2] Oxygen is an a-stabilizer, and oxygen atoms can reside in octahedral interstitial sites in the hexagonally close-packed lattice of a-titanium. Increasing oxygen content is known to decrease the ductility of a-titanium alloys.[3] Furthermore, oxygen diffusion toward the FZ may lead to softened, oxygen-depleted regions away from the FZ.[4,5] Weld mechanical properties also may be affected by the grain structure in the weld region. Gas tungsten arc welding (GTAW), which is considered in this investigation, is associated with high heat input and low cooling rate. For near-a titanium alloys, grain growth in the FZ typically occurs via epitaxial solidification,[6] with the grain size determined in part by the grain size in the B.W. NEUBERGER, Researcher, formerly with the Department of Materials Science and Engineering, University of Maryland, College Park, College Park, MD 20742, is now with Bettis Atomic Power Laboratory, West Mifflin, PA 15122. P.G. OBERSON, Research Assistant, and S. ANKEM, Professor, are with the Department of Materials Science and Engineering, University of Maryland. Contact e-mail: [email protected] Manuscript submitted February 17, 2009. Article published online November 18, 2010 1296—VOLUME 42A, MAY 2011
heat-affected zone (HAZ).[7] The grain size in the HAZ may be large as a result of relatively long exposure to temperatures above the b-transus,[8] which contributes to a large prior-b grain size in the FZ. The coarse, columnar grain structure in the FZ is associated with low ductility.[9] Therefore, one approach to improve the weld ductility is to refine the prior- grain size and promote heterogeneous nucleation in the FZ.[10] In previous investigations,[11–14] yttrium was added to the filler metal for titanium alloy welds to improve the weld ductility. Yttrium was chosen because it has a higher affinity for oxygen than titanium.[11] Furthermore, yttrium has low solubility in titanium and a high melting temperature of approximately 1796 K (1523 C). Simpson[11] studied the addition of yttrium to Ti-6Al-6V2Sn welds. He believed that yttria (Y2O3) particles could control the grain size by acting as heterogeneous nucleation site
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