Fabrication and properties of heavy section extrusions of Ti-6Al-6V-2Sn and Ti-8Mo-8V-2Fe-3AI alloys
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THE
alloy, Ti-8Mo-8V-2Fe-3A1, henceforth t e r m e d Ti-8823, is a metastable beta titanium alloy that when heat treated can through harden in sections up t o 6 in. (152 mm) in thickness.1 While high strength levels have been achieved at excellent fracture toughness levels, the ductility as measured by tensile elongation or reduction-of-area is on the low side.2 The cause for the low ductility values has been a t tributed to the l a r g e beta g r a i n size2 which remains a f t e r the heavy sections have been hot worked, usually by forging. With this in mind, Bohanek~ attempted to m a x i m i z e the amount of hot working by p r e s s forging, thereby realizing a significant reduction in the g r a i n size. Specifically, Bohanek utilizing a forging temperature of 1600°F (1144 K) p r e s s forged Ti-8823 from a 25 in. (635 mm) square cross-section to 8 1//2 in. (216 mm) round billet using the following sequence: 25 in. (635 mm) square ~ 18 in. (457 mm) square ~ 12 in.~305 mm) square ~ 8 1//2 in. (216 ram) square 8 1/2 in. (216 mm) round. This represents a total of 90 pct reduction during hot working. However, even with this processing history, a relatively c o a r s e g r a i n size persisted leading t o less than d e s i r e d ductility values. As a result, Bohanek concluded3 that only by continuous reduction such as exists in an extrusion process can the g r a i n size be effectively r e duced. The present paper reports the details and results of attempts to achieve a l a r g e amount of continuous reduction by extruding a heavy section cylindrical hollow of Ti-8823. Also discussed are the results of utilizing the same extrusion procedure heavy sections of the alpha beta Ti-6A1-6V-2Sn alloy, henceforth t e r m e d Ti-662. In contrast t o Ti-8823 w h e r e the processing aim is to achieve a reduced beta g r a i n size, Ti-662 should be processed within temperature limits to reduce the likelihood that regions of chemical segregation known as beta flecks will appear in the microstructure.4 R. CHAIT is Supervisory Materials Engineer, Army Materials and Mechanics Research Center, Watertown, MA 02172, and C. STEAD is Chief Metallurgist, Forged Products, Cameron Iron Works, Houston, T X 77001. Manuscript submitted August 1 6 , 1976. METALLURGICAL
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MATERIALS AND HOT WORKING PROCEDURE Ti-8823 Alloy The Ti-8823 of concern in this study was melted by Titanium Metals Corporation of A m e r i c a (TMCA) at their Henderson, Nevada facility. A consumable vacuum arc r e m e l t process was used to produce a heat (No. K9278) with the following chemical composition (wt pct): 7.9 Mo, 8.1 V, 2.1 Fe, 3.1 A1, 0.035 C, 0.133 O, 0.006 H and 0.010 N. The extremely l a r g e beta g r a i n size that exists within the ingot is shown in Fig. 1. Hot working of the ingot was conducted by TMCA on t h e i r 2500 ton (22 • 106 N) forging p r e s s at the Toronto, Ohio facility. Here the ingot was reduced from 28 in. (711 mm) t o approximately 20 in. (508 mm) in diam billet through a s e r i e s of forging and upsetting operatio
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