Method for forging titanium alloy forging and forged titanium alloy material

Abstract

The invention provides a titanium alloy having a narrow distribution of material properties in the thickness direction, allowing easy to finish the surface of the forged material after forging, during working the product to the final shape. The forged titanium alloy has a low sensitivity for cracking, excellent workability, and favorable ductility and fatigue properties, and provides a method for forging the titanium alloy. In order to attain the forged titanium alloy and the method for forging thereof, forging the titanium alloy, which has Tβ ° C. of the β-transus, is conducted, while keeping the relation of 400° C.≦Td and (Tβ−400)° C.≦Tm≦Tβ at a strain rate of a range from 2×10 −4 s −1 to 1 s −1 , further limiting the chemical composition of the titanium alloy, keeping the relation of [(Tm−Td)≦250° C.] during forging, where Tβ (° C.) is the β-transus of the titanium alloy, Tm(° C.) is the temperature of the work material for being forged, and Td(° C.) is the temperature of a die. The titanium alloy, which is produced by the above-described method, has a fine microstructure to be controlled for forming a specified microstructure, has a uniform and homogeneous material properties in the thickness direction, and has an excellent ductility and a fatigue properties.

What is claim is:  
     
         1 . A method for forging a titanium alloy comprising the steps of: 
 preparing a titanium alloy as a forging stock, wherein the titanium alloy has a thickness center portion and near the surface area;    forging the titanium alloy as the forging stock to have a work hardening factor, whose value is 1.2 or less, for obtaining a forged titanium alloy having a uniform and homogeneous material properties, wherein the work hardening factor is defined as,   work hardening factor= Hv ( def )/ Hv ( ini )   where, Hv(ini) is the hardness of the titanium alloy as the forging stock before forging, and, 
 Hv (def) is the hardness of the forged titanium alloy under the reduction of 20%.  
   
     
     
         2 . The method according to  claim 1 , wherein a difference of hardness between the thickness centers portion of the forged titanium alloy and near the surface area of the forged titanium alloy is 60 or less of Vickers hardness.  
     
     
         3 . A method for forging a titanium alloy comprising the steps of: 
 preparing a titanium alloy as a forging stock;    forging the titanium alloy as the forging stock, at a strain rate within a range from 2×10 −4  s −1  or more to 1 s −1  or less, while keeping a relation of (Tβ−400)° C.≦Tm≦900° C. and 400° C.≦Td≦700° C., to obtain a forged titanium alloy having a uniform and homogeneous material properties;    where, Tβ (° C.) is a β-transus of the titanium alloy, 
 Tm(° C.) is a temperature of the work material for being forged, and  
 Td(° C.) is a temperature of a die.  
   
     
     
         4 . The method according to  claim 3 , wherein the temperature of the die, Td(° C.), and the temperature of forging stock, Tm (° C.), are controlled to satisfy the relation of (Tm−Td)≦250° C.  
     
     
         5 . The method according to  claim 3 , wherein the titanium alloy as the work material for being forged contains Al: 4 to 5%, V:2.5 to 3.5%, Fe:1.5 to 2.5%, and Mo:1.5 to 2.5%, by mass percentage.  
     
     
         6 . The method according to  claim 3 , wherein, 
 the titanium alloy as the forging stock has an α+β microstructure, an aspect ratio of primary α-phase is 5 or less, an average grain size of primary α-phase is 10 μm or less, and    a volume fraction of primary α-phase is within a range from 20% or more to 80% or less,    wherein the aspect ratio is defined as the following,    the aspect ratio=longitudinal length of the grain/width of the grain, which is perpendicular to the longitudinal direction.    
     
     
         7 . A forged titanium alloy comprising: 
 a thickness center portion and near the surface area;    a work hardness factor of 1.2 or less, wherein the work hardening factor is defined by Hv(def)/Hv(ini), where, 
 Hv (ini) is the hardness of the titanium alloy as the forging stock before forging, and,  
 Hv (def) is the hardness of the forged titanium alloy under the reduction of 20%,  
 wherein the forging stock within a temperatures range from (Tβ−400)° C. or more to less than 900° C., and Tβ (° C.) is a β-transus (° C.) of the titanium alloy.  
   
     
     
         8 . The forged titanium alloy according to  claim 7 , wherein difference of the hardness between the thickness center portion of the forged titanium alloy and near the surface area of the forged titanium alloy is 60 or less of Vickers hardness.  
     
     
         9 . The forged titanium alloy according to  claim 7  consisting essentially of 4 to 5% Al, 2.5 to 3.5% V, 1.5 to 2.5% Fe, 1.5 to 2.5% Mo, by mass, and balance of substantially Ti.  
     
     
         10 . The forged titanium alloy according to  claim 7 , wherein, 
 the titanium alloy as a before forging has an α+β microstructure,    an aspect ratio of primary α-phase is 5 or less,    an average grain size of primary α-phase is 10 μm or less, and    a fraction of primary α-phase is within a range from 20% or more to 80% or less,    wherein the aspect ratio is defined as the following;    the aspect ratio=longitudinal length of the grain/length of the grain, which is perpendicular to the longitudinal direction.