Forged titanium alloy material and method for manufacturing same

Abstract

Provided is a titanium-alloy forging material in which fatigue-strength characteristics are improved without worsening ultrasonic flaw detection. A β-forged titanium-alloy forging material ( 1 ) is characterized in that the area ratio of non-flat grains, which are prior β-grains ( 2 ) having an aspect ratio of 3 or less and a diameter in the forging direction of at least 20 μm, and an α-phase ratio at the crystal grain boundary ( 3 ) of at least 80%, is less than 10%, and the area ratio of flat grains, which are prior β-grains having an aspect ratio greater than 3 and a diameter in the forging direction of 20-700 μm, and an α-phase ratio at the crystal grain boundary ( 3 ) of at least 80%, is 85% or greater, and the average orientation difference of the α-phase crystal orientation deposited at the crystal grain boundary ( 3 ) of the flat grains is at least 6°.

1 . A β forged titanium alloy material,
 wherein an area fraction of non-flattered grains of prior β grains having 80% or more its grain boundaries decorated alpha phase, which aspect ratio is 3 or less, and which diameter in a forging direction is 20 μm or more, is less than 10%; 
 an area fraction of flattered grains of prior β grains having 80% or more its grain boundaries decorated alpha phase, which aspect ratio exceeding 3, and which diameter in the forging direction is 20 μm or more but not more than 700 μm, is 85% or more; and 
 an average misorientation of crystal orientation of an α-phase precipitated along the grain boundary of the flattered grains is 6° or more. 
 
     
     
         2 . The β forged titanium alloy material according to  claim 1 , comprising a titanium alloy having a Mo equivalent [Mo]eq, which are represented by the following formula (1), more than 2.7 but less than 15:
   [Mo]eq=[Mo]+[Ta]5+[Nb]/3.6+[W]/2.5+[V]/1.5+1.25[Cr]+1.25[Ni]+1.7[Mn]+1.7[C o]+2.5[Fe]  (1),
 
 
       wherein, each element symbol in brackets on the right side of the formula (1) represents a mass % content of each element in the titanium alloy. 
     
     
         3 . The β forged titanium alloy material according to  claim 1 , having a thickness of 30 mm or more at the thinnest portion thereof and 70 mm on average. 
     
     
         4 . A method of manufacturing the β forged titanium alloy material as claimed in  claim 1 ,
 wherein the β 0  forging comprises: 
 heating a titanium alloy material is heated to (T β +10)° C. or higher, wherein T β  represents β-transus temperature, until a β crystal grain diameter of the titanium allow material falls within a range of 300 μm or more but not more than 1000 μm, 
 forging the heated titanium alloy material at forging temperature, T F  [° C.] which satisfies the following formula (2) and under conditions in which the forging temperature T F  [° C.] satisfy the formulas (3) and (4), respectively, to produce a forged titanium alloy material; and 
 cooling the forged titanium alloy material obtained by the above-mentioned forging is cooled to temperature lower than (T β −150)° C.;
     T   β −150≦ T   F   ≦T   β +100  (2)
 
   Ln( S   R )+22800/( T   F +273)−18.6≦0  (3)
 
   Ln( S   R )+22800/( T   F +273)−13.2≧0  (4),
 
 
 
       wherein in the formulas (2) to (4), T β  represents the β-transus temperature [° C.], T F  represents the forging temperature [° C.], and S R  represents a strain rate [s −1 ] upon forging. 
     
     
         5 . The method of manufacturing the forged titanium alloy material according to  claim 4 , further comprising:
 billet forging an ingot composed of a titanium alloy into a corresponding billet; and   α+β forging the billet prior to the heating step, in which the billet obtained from the titanium alloy is forged in an α+β two phase region.   
     
     
         6 . The method of manufacturing the forged titanium alloy material according to  claim 5 , wherein the billet obtained from the titanium alloy has an acicular microstructure. 
     
     
         7 . The method of manufacturing the β forged titanium alloy material according to  claim 4 , further comprising:
 an ultrasonic inspection step after the cooling step, comprising irradiating ultrasonic waves in a direction parallel to a direction in which an amount of forging in the β forging is the largest to inspect a flaw of the forged titanium-alloy material. 
 
     
     
         8 . The method of manufacturing the forged titanium alloy material according to  claim 5 , further comprising:
 an ultrasonic inspection step after the cooling step, comprising irradiating ultrasonic waves in a direction parallel to a direction in which an amount of forging in the β forging is the largest to inspect a flaw of the forged titanium-alloy material.   
     
     
         9 . The method of manufacturing the forged titanium alloy material according to  claim 6 , further comprising:
 an ultrasonic inspection step after the cooling step, irradiating ultrasonic waves in a direction parallel to a direction in which an amount of forging in the β forging is the largest to inspect a flaw of the forged titanium alloy material.   
     
     
         10 . The method of manufacturing the forged titanium alloy material according to  claim 4 ,
 wherein the forged titanium alloy material is suitable for manufacturing an aircraft engine component.   
     
     
         11 . An aircraft engine component comprising the β forged titanium alloy material according to  claim 1 . 
     
     
         12 . The method of manufacturing according to  claim 7 , further comprising selecting a β forged titanium alloy material based on a number of flaws determined by the inspection step.