Method and apparatus for creep forming of and relieving stress in an elongated metal bar

Abstract

A hot creep stretch wrap forming method includes heating a metal bar to a forming temperature within a temperature range suitable for creep deformation thereof, applying a stretching force to the metal bar at a strain rate no greater than 0.05 inch/inch/second, and wrapping the metal bar around a die, preferably having a thermally and/or electrically insulative work surface. The stretching force is typically applied to a strain ranging from 0.5% to 15.0%. The metal bar most preferably is a titanium alloy with a forming temperature ranging from 0.45 to 0.60 of its melting temperature. The wrapped metal bar is held in position and its temperature maintained within the temperature range typically for 5 to 120 minutes for stress relief. Preferably, the metal bar is held substantially at the forming temperature throughout the process. Thermal insulation around the die and metal bar reduce heat loss from the metal bar.

1 . A method comprising the steps of: 
 heating an elongated metal bar to a forming temperature within a temperature range suitable for creep deformation of the metal bar;    applying a stretching force to the heated metal bar at a strain rate no greater than 0.05 inch/inch/second; and    wrapping the heated metal bar around a die to form a wrapped metal bar.    
   
   
       2 . The method of  claim 1  wherein the step of applying includes the step of applying a stretching force to the heated metal bar at a strain rate no greater than 0.05 inch/inch/second to a strain ranging from 0.5% to 15.0%.  
   
   
       3 . The method of  claim 2  wherein the step of heating includes the step of heating a metal bar formed of a titanium alloy to a forming temperature ranging from 0.45 to 0.60 Tm wherein Tm is the melting point of the titanium alloy.  
   
   
       4 . The method of  claim 2  wherein the step of applying includes, prior to the step of wrapping, the step of applying a stretching force to the heated metal bar at a strain rate no greater than 0.05 inch/inch/second to a strain ranging from 0.5% to 3.0%; and wherein the step of applying includes, during the step of wrapping, the step of applying a stretching force to the heated metal bar at a strain rate no greater than 0.05 inch/inch/second to a strain ranging from 0.5% to 15.0%.  
   
   
       5 . The method of  claim 1  wherein the step of heating includes the step of heating a metal bar formed of a titanium alloy to a forming temperature ranging from 0.45 to 0.60 Tm wherein Tm is the melting point of the titanium alloy.  
   
   
       6 . The method of  claim 1  wherein the step of heating includes the step of heating the metal bar to a temperature ranging from 650° C.-925° C. (1202° F.-1690° F.).  
   
   
       7 . The method of  claim 1  wherein the step of heating includes the step of heating a metal bar formed of a titanium alloy to a temperature ranging from 1250° F.-1450° F.  
   
   
       8 . The method of  claim 1  wherein the step of applying includes the step of applying a stretching force to the heated metal bar at a strain rate ranging from 0.00005 inch/inch/second to 0.005 inch/inch/second.  
   
   
       9 . The method of  claim 1  wherein the step of heating includes the step of passing an electrical current through the metal bar to heat the metal bar resistively.  
   
   
       10 . The method of  claim 9  further including the step of maintaining the metal bar at temperature which is substantially uniform throughout the metal bar and which is substantially constant throughout the steps of applying and wrapping.  
   
   
       11 . The method of  claim 9  wherein the step of wrapping includes the step of wrapping the heated metal bar around a die face of a metal die with a layer of electrically insulative material separating the die face from the heated metal bar to prevent electrical communication between the metal bar and the metal die.  
   
   
       12 . The method of  claim 1  further including the steps of holding the wrapped metal bar against the die after the step of wrapping and simultaneously maintaining the temperature of the wrapped metal bar within the temperature range for at least 5 minutes.  
   
   
       13 . The method of  claim 12  further including the step of maintaining the temperature of the wrapped metal bar within the temperature range for at least 10 minutes.  
   
   
       14 . The method of  claim 13  further including the step of maintaining the temperature of the wrapped metal bar within the temperature range for at least 20 minutes.  
   
   
       15 . The method of  claim 11  further including the step of maintaining the metal bar at a temperature which is within 30.0° C. of the forming temperature throughout the steps of applying, wrapping, holding and maintaining.  
   
   
       16 . The method of  claim 1  further including the step of positioning thermal insulation around the die and metal bar to reduce heat loss from the metal bar.  
   
   
       17 . The method of  claim 1  wherein the step of heating includes the step of heating the metal bar to a temperature which is substantially uniform throughout the metal bar.  
   
   
       18 . The method of  claim 17  further including the step of maintaining the substantially uniform temperature throughout the steps of applying and wrapping.  
   
   
       19 . The method of  claim 18  wherein the step of wrapping includes the step of wrapping the heated metal bar around a die face of the die and a layer of thermally insulative material disposed between the die face and the heated metal bar without independently heating the die face.  
   
   
       20 . The method of  claim 1  wherein the step of wrapping includes the step of wrapping the heated metal bar around a working surface which is formed of at least one of a thermally insulative material and an electrically insulative material.