Patent Publication Number: US-2007102073-A1

Title: Near-beta titanium alloy heat treated casting

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
      This application claims priority and benefits of U.S. provisional application Ser. No. 60/578,737 filed Jun. 10, 2004, the disclosure of which is fully incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION  
      The present invention relates to a near-beta titanium alloy and, more particularly, to a heat treatment and heat treated near-beta titanium alloy casting.  
     BACKGROUND OF THE INVENTION  
      Near-beta titanium alloys are known in the art and are described in published European application 2003/0164212 A1 and published Japanese abstract JP 7011406 A2.  
      There is a need to improve the mechanical properties, such as strength and ductility, of near-beta titanium alloys that are cast and optionally hot isostatically pressed to provide a desirable combination of mechanical properties.  
     SUMMARY OF THE INVENTION  
      The present invention provides in an illustrative embodiment a heat treatment for a near-beta titanium alloy as well as a heat treated near-beta titanium alloy casting having a Widmanstätten microstructure comprising primary alpha phase precipitates and secondary alpha phase precipitates in a beta phase matrix. The heat treatment produces a hardness that correlates to a desirable combination of tensile strength and ductility of the heat treated near beta-titanium alloy casting for load-bearing structural applications.  
      Other advantages, features, and embodiments of the present invention will become apparent from the following description taken with the following drawings. 
    
    
     DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a graph of Vickers hardness at different cooling rates versus aging temperature for a near-beta Ti-5Al5Mo-5V-3Cr (Ti-5553) alloy. Ultimate tensile strength, yield strength, and % elongation are also set forth for certain data points.  
       FIG. 2  is a graph correlating Vickers hardness versus ultimate tensile strength (UTS), yield strength (YS), and ductility (El) for the heat treated near-beta Ti-5553 alloy.  
       FIGS. 3   a ,  3   b ;  4   a ,  4   b ; and  5   a , 5   b  are photomicrographs at 1000× of the heat treated near-beta Ti-5553 alloy showing a Widmanstätten microstructure having primary and secondary alpha phase needle-shaped precipitates in cross-sections of the photomicrographs (i.e. cross-sections through primary and secondary alpha phase platelet-shaped precipitates in the actual alloy body).  
       FIGS. 6   a ,  6   b  are photomicrographs at 2500× and 10000×, respectively, of the heat treated near-beta Ti-5553 alloy having a Vickers hardness of about 380 showing a Widmanstätten microstructure having primary and secondary alpha phase needle-shaped precipitates in cross-sections of the photomicrographs (i.e. cross-sections through primary and secondary alpha phase platelet-shaped precipitates in the actual alloy body).  
       FIG. 7  is a graph of room and elevated temperature strength and ductility of the Ti-5553 alloy casting.  
       FIG. 8  is a table comparing room temperature mechanical properties of the heat treated Ti-5553 casting versus those of a Ti-6Al-4V (designated Ti-64) casting.  
       FIG. 9  is a graph showing room temperature (T=70 degrees F.) high cycle fatigue (HCF) strength over cycles for the Ti-5553 alloy casting and the Ti-64 casting. The HCF testing was conducted at a stress ratio R=0.1. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
      The present invention provides a heat treatment for near-beta titanium alloys and especially for a cast and optionally hot isostatically pressed near-beta titanium alloy as well as a near-beta titanium alloy casting having a heat treated, refined Widmanstätten microstructure. A near-beta titanium alloy is one which is quenchable from a solution temperature at or above the alpha/beta transformation temperature and which retains some or all of the beta phase upon quenching to room temperature. For purposes of illustration, a near-beta titanium alloy (designated Ti-5553) that can be heat treated pursuant to the invention comprises, in weight percent, about 4.4 to about 5.7% Al, about 4.0 to about 5.5% Mo, about 4.0 to about 5.5% V, about 2.5 to about 3.5% Cr, about 0.3 to about 0.5% Fe, and balance essentially titanium (designated Ti-5553 alloy). Table 1 sets forth an illustrative alloy composition (Specification) as well as actual (Target) tested alloy composition.  
                           TABLE 1                                      Ti-5553 Target   Specification                                 Element   Alloy Composition, w %   min, w %   max, w %   mid, w %                                         Al   5.12   4.4   5.7   5.05       Mo   4.71   4.0   5.5   4.75       V   4.72   4.0   5.5   4.75       Cr   2.77   2.5   3.5   3.0       Fe   0.31   0.3   0.5   0.4       C   0.009       0.1       Zr   NA       0.3       Si   0.05       0.15       02   0.13       0.18       N2   0.0055       0.05       H2   0.0038       0.015       Other           0.3       Ti   Balance       Balance                  
 
      The Ti-5553 alloy has potential use as a cast load-bearing structural component including but not limited to an airframe structural component, such as a bulkhead casting, landing gear component, and other components. In use as an airframe structural component, the alloy typically is investment cast to the desired airframe shape using the well known “lost wax” technique followed by hot isostatic pressing (HIP&#39;ing) of the casting (e.g. HIP&#39;ing at 1650 degrees F. at 15 ksi for 2 hours). The HIP&#39;ed airframe structural casting then is heat treated pursuant to the invention to develop a desirable combination of mechanical properties, such as tensile strength and ductility. The invention of course envisions heat treating components cast using other casting methods.  
      An illustrative vacuum heat treatment of the invention comprises a solution heat treatment for a time above the alpha/beta transformation temperature (1580 degrees F. for Ti5553) of the alloy followed by cooling to a low aging temperature relative to the alpha/beta transformation temperature (e.g. at least 400 degrees F. below the transformation temperature) to provide a relatively large amount of undercooling and then aging at an aging temperature to form a duplex refined Widmanstätten microstructure comprising primary alpha phase needles when viewed in sectioned metallographic samples and secondary alpha phase needles precipitated when viewed in sectioned metallographic samples in a beta phase matrix. Although not wishing to be bound by any theory, it is thought that coarser alpha platelets initially nucleate and grow during the controlled cooling to the aging temperature and then secondary alpha platelets nucleate and grow from the remaining retained beta phase during the subsequent aging treatment. The vacuum heat treatment produces a hardness that corresponds with a desirable combination of tensile strength and ductility of the heat treated near beta-titanium alloy casting. The invention is not limited to a vacuum heat treatment since the heat treatment can be conducted in an inert gas or other gas atmosphere that is not adversely reactive to the alloy.  
      For the above Ti-5553 alloy, a preferred vacuum heat treatment (e.g. conducted at 1×10 −4  to 1×10 −5  torr) includes a solution treatment of the optionally HIP&#39;ed casting at 1620 degrees F. for 2 hours followed by cooling in vacuum at a rate of 300 degrees F./hour to a lower temperature of about 1000-1200 degrees F. and aging at an aging temperature, such as for example 1000-1200 degrees F., for 8 hours in vacuum to produce a Vickers hardness of about 380, more generally 375 to 385, as measured using a 300 gram load, and the above-described microstructure. Cooling at 300 degrees F./hour can be achieved by computer controlled power-down of the vacuum heat treatment furnace. After the heat treatment, the heat treated casting can be gas fan cooled (GFC) in the heat treatment furnace to room temperature. Alternately, the casting can be cooled to the lower temperature and then gas fan cooled (GFC) in the heat treatment furnace to room temperature. The casting then can be reheated to and aged at an aging temperature such as 1000-1200 degrees F. for a period of time such as 8 hours.  
      For certain airframe structural castings (e.g. bulkhead castings), a Vickers hardness (measured using a 300 gram load) of about 380 provides a desirable combination of tensile strength and ductility of the heat treated near beta-titanium alloy casting. For example, a Ti-5553 casting having such a Vickers hardness provides a desirable combination of tensile strength and ductility; namely, room temperature ultimate tensile strength (UTS) of 164 Ksi, room temperature tensile yield strength (YS) of 150 Ksi, and elongation (El) expressed as ductility of 7-9%. The Ti5553 titanium alloy is heat treatable pursuant to the invention to produce uniform, high strength microstructures over a broad thickness range up to, for example, 1.5 to 2 inches thickness of a casting.  
       FIGS. 3   a ,  3   b ;  4   a ,  4   b ;  5   a ,  5   b ; and  6   a ,  6   b  illustrate Widmanstätten microstructures produced using different cooling rates from the 1620 degrees F. solution temperature and different aging temperatures as well as the corresponding Vickers hardness achieved.  FIGS. 6   a ,  6   b  more clearly show that the heat treated microstructure pursuant to the invention comprises a combination of primary (coarse) alpha phase appearing in the photomicrographs as coarse needles and secondary (fine) alpha phase appearing as secondary needles in the photomicrographs precipitated during cooling and aging in a beta phase matrix.  FIG. 1  shows the measured Vickers hardness at different aging temperatures and at different cooling rates from the solution temperature.  FIG. 2  correlates the Vickers hardness to room temperature strength and ductility.  
      From  FIG. 1 , it is apparent that a faster cooling rate (e.g. 500 degrees F./hour) and a slower cooling rate (e.g. 100 degrees/hour) from the solution temperature also were evaluated in the vacuum heat treatment studies for comparison to the above preferred heat treatment (cooling rate of 300 degrees/hour and aging at 1000 degrees F). In addition, aging temperatures of 1050 degrees F. and 1100 degrees F. also were evaluated. These alternative cooling rates and aging temperatures produced the Vickers hardness and corresponding combination of mechanical properties shown in  FIG. 1  compared to the optimum combination of properties produced by the preferred vacuum heat treatment described above.  
      The invention envisions using alternative cooling rates and aging temperatures to achieve the optimum combination of mechanical properties produced by the preferred vacuum heat treatment described above. For example, a cooling rate of 500 degrees F./hour from the solution temperature and an aging temperature of 1060 degrees F. for 8 hours may produce such an optimum combination of properties. In practicing the invention, obtainment of the optimum combination of mechanical properties for a given service application involves controlling the heat treated microstructural refinement and concomitant Vickers hardness through a combination of controlled undercooling (and thus nucleation density of the alpha phase) and aging.  
       FIG. 7  shows the room temperature and elevated temperature (up to 800 degrees F.) mechanical properties of the heat treated Ti-5553 alloy (Vickers hardness of about 380).  FIG. 8  is a table comparing certain room temperature mechanical properties of the heat treated Ti-5553 casting pursuant to the invention versus those of a Ti-6Al-4V casting (designated Ti-64). The heat treated Ti-5553 alloy exhibits a substantial improvement in ultimate tensile strength (UTS) and tensile yield strength (TYS) compared to the Ti-6Al-4V casting with the same elongation. Also the compressive strength and bearing ultimate tensile (UTS) and bearing yield strength (YS) of the heat treated Ti-5553 alloy were improved over the Ti-6Al-4V casting.  
       FIG. 9  is a graph showing room temperature high cycle fatigue (HCF) strength for the heat treated Ti-5553 casting and a Ti-6Al-4V casting (designated Ti-64). The room temperature high cycle fatigue (HCF) strength of the heat treated Ti-5553 casting is much better than that of the Ti-6Al-4V casting. The room temperature high cycle fatigue (HCF) strength of the heat treated Ti-5553 casting is generally equal to that of wrought titanium alloys, which for example exhibit HCF strengths of 110 ksi at 10 7  cycles. Moreover, the heat treated Ti-5553 casting exhibited good fracture toughness.  
      Although the invention has been shown and described with respect to detailed embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail thereof may be made without departing from the spirit and scope of the claimed invention.