Fusion zone fracture behavior of weldments in alpha-beta titanium alloys
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Ill AND
D. W. BECKER
Weldments in alpha-beta titanium alloys often exhibit considerable quantities of martensite in the fusion and near heat affected zones. 1,2 Tensile fracture of these typically large grained regions under monotonic loading conditions reportedly occurs at low levels of m a c r o scopic deformation by cleavage, 3,4 a fracture mode not commonly observed in alpha-beta titanium alloys.5 Recent studies by Chesnutt and Williams 6 have demonstrated the difficulties encountered in distinguishing true cleavage from ductile tearing in an alpha-titanium alloy, and have suggested that particular caution must be exercised when analyzing titanium alloy fracture surfaces. In view of these observations, the authors felt it n e c e s s a r y to evaluate the fracture c h a r a c t e r i s tics of weldments in alpha-beta titanium alloys in order to determine whether cleavage does indeed occur in the partially martensitic m i c r o s t r u c t u r e s . The present study essentially reinvestigated the fusion zone fracture behavior of two high strength alpha-beta titanium alloys, Ti-6A1-2Sn-4Zr-6Mo (Ti-6246) and Ti-6A1-6V-2Sn (Ti-662), monotonically loaded in tension and bending. Autogenous, full penetration weldments were produced on mill annealed sheet using the gas tungsten arc welding p r o c e s s . Weldments exhibiting m a c r o s t r u c t u r e s , m i c r o s t r u c tures, and chemical compositions nearly identical to those which had reportedly experienced tensile f r a c ture by cleavage 3,4 were produced through proper control of welding p a r a m e t e r s and conditions. Tables I and II provide base and weld metal chemical compositions and welding p a r a m e t e r s , respectively. Subsequent to welding, weld reinforcement was removed by grinding and coupons were machined into dogbone type tensile and bend specimens. The weld fusion zone in both specimen types was oriented along the longitudinal specimen axis, thereby providing an all fusion zone tensile specimen gage section. Fusion and near heat affected zone regions in Ti-662 weldments were characterized by a large grained m a c r o s t r u c t u r e (Fig. l(a)) and a fine, acicular, p a r tially martensitic m i c r o s t r u c t u r e which was virtually identical morphologically to that which reportedly fractured by cleavage. 3 F r a c t u r e of the weldment fusion zone in both tensile and bend specimens occurred predominantly transgranularly at low levels of m a c r o scopic deformation (Table III). When viewed at low and intermediate magnification, transgranular regions on the fracture surfaces often exhibited a step-like, faceted appearance similar to what might be anticipated for a cleavage fracture (Fig. 2(a) to (c)). Observation of these facets at increased magnification, however,
W. A. BAESLACK III and D. W. BECKER are Materials Joining Engineer/Metallurgist and Group Leader, respectively, Joining Technology Group, Air Force Materials Laboratory, Wright-Patterson AFB, OH 45433. Manuscript submitted April 30, 1979. METALLURGICAL TRANSACT
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