Method of Manufacturing Semi-Finished Sheet Products From Titanium Alloy

Abstract

The invention relates to plastic working of metals, more specifically to rolling sheets, and is concerned with a method of manufacturing a semi-finished sheet product from a titanium alloy having a submicrocrystalline structure suitable for low-temperature superplastic deformation. The invention can be most efficiently used to manufacture semi-finished thin sheets, including foil, from a low-plastic two-phase titanium alloy. The object of the invention is to improve quality of semi-finished sheet products made from a titanium alloy adapted for further low-temperature superplastic deformation. A method of manufacturing a semi-finished sheet product from a titanium alloy adapted for low-temperature superplastic deformation, including rolling a billet with a prepared structure at a temperature below the polymorphous transformation temperature in isothermal or quasi-isothermal conditions provided by heating the rolls, the method characterized in that said rolling is carried out in conditions of low-temperature superplastic deformation, the deformation being performed, predominantly in a first pass, to a strain amount of ε≧ε min , where ε min is the minimum amount at which a structural state required to provide cooperative grain boundary sliding in the deformation process is formed in the alloy in selected rolling temperature/rate conditions; after each subsequent rolling pass the billet is cooled immediately on exiting the deformation region to maintain the structural state obtained in the deformation; a time period of heating the billet in a furnace for a subsequent rolling pass is restricted to prevent disturbance of the alloy structural state obtained in the previous rolling pass.

1 . A method of manufacturing a semi-finished sheet product from a titanium alloy adapted for low-temperature superplastic deformation, including rolling a billet with a prepared structure at a temperature below the polymorphous transformation temperature in isothermal or quasi-isothermal conditions provided by heating the rolls,
 the method characterized in that   said rolling is carried out in conditions of low-temperature superplastic deformation, the deformation being performed, preferably in a first pass, to a strain amount of ε≧ε min , where ε min  is the minimum amount at which a structural state required to provide cooperative grain boundary sliding in the deformation is formed in the alloy in selected rolling temperature/rate conditions;   after each subsequent rolling pass the billet is cooled immediately on exiting the deformation region to maintain the structural state obtained in the deformation process;   a time period of heating the billet in a furnace for a subsequent rolling pass is restricted to prevent disturbance of the alloy structural state obtained in the previous rolling pass.   
   
   
       2 . The method according to  claim 1 , wherein said rolling is carried out at a temperature in the range from T pt -450° C. to T pt -350° C. 
   
   
       3 . The method according to  claim 1 , wherein said rolling is carried out with a strain rate in the range from 10 −3  to 10 −1  s −1 . 
   
   
       4 . The method according to  claim 1 , wherein in said rolling, prior to achieving a strain amount of 30-60%, the billet is rotated through 90 degrees after every three to five longitudinal passes and a transverse rolling pass is performed, the remaining strain amount being gained by rolling in single direction. 
   
   
       5 . The method according to  claim 1 , wherein when manufacturing a semi-finished sheet product having a thickness not exceeding 1 mm the billet is heated through contact with working rolls. 
   
   
       6 . The method according to  claim 1 , wherein a billet with a prepared globular structure having a grain size less than 1 μm is used in said rolling. 
   
   
       7 . The method according to  claim 1 , wherein a billet with a prepared lamellar structure having a cross-sectional grain size about 1 μm is used in said rolling. 
   
   
       8 . The method according to  claim 7 , wherein the billet structure is prepared for rolling by preliminarily rolling an original billet having a grain size not exceeding 10 μm at least in one section to a strain amount of at least 80%, said rolling being started at a temperature in the range from T pt -300° C. to T pt -200° C. and finished at a temperature not lower than the basic rolling temperature, wherein the strain rate is in the range from 10 −2  to 10° s −1 . 
   
   
       9 . The method according to  claim 7 , wherein the billet structure is prepared for rolling by preliminary two-stage rolling of an original billet having a grain size of from 10 to 80 μm, the first stage comprising rolling the original billet to a strain amount not exceeding 60%, the rolling being started at a temperature in the range from T pt -200° to T pt -50° and finished at a temperature not lower than the basic rolling temperature, wherein the strain rate is in the range from 10 −2  to 10 s −1 ; the second stage comprising rolling the billet in isothermal conditions at the basic rolling temperature and strain rate to a strain amount of 20-30%. 
   
   
       10 . The method according to  claim 1 , wherein said rolling is carried out at a rolling mill comprising two working rolls and at least four backup rolls. 
   
   
       11 . The method according to  claim 10 , wherein deflection of the backup rolls directly contacting the working rolls is modified by changing the intensity of cooling bearings units of the backup rolls. 
   
   
       12 . The method according to  claim 1 , wherein said working rolls are heated by electric resistance heating units mounted inside the rolls.