Plasma descaling of titanium and titanium alloys

Abstract

The invention provides a method of removing surface scale from a titanium or titanium alloy substrate. The method includes the steps of heating the substrate to a temperature in the range from about 100° C. to about 600° C., and thereafter subjecting the heated surface to a plasma formed from a gas selected from the group of consisting of CF 4 and SF 6 . The plasma reacts with the surface scale, removing the scale, without attacking the underlying crystalline titanium or titanium alloy. Properly controlled, the plasma reaction terminates when the plasma has penetrated the scale, and encounters the underlying crystalline metal. As a result, the method of the invention is capable of uniform removal of the entire surface scale of a crystalline titanium-containing substrate, without intergranular attack of the substrate.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows: 
     
       1. A method of removing a heat-treatment induced scale from surfaces of an underlying crystalline titanium or titanium alloy body of an aircraft component, the method comprising: (a) heating at least the surfaces of the aircraft component having the heat-treatment induced scale to a temperature in the range from about 100° C. to about 600° C.;   (b) removing the scale from the surfaces by reacting the heated surfaces of the aircraft component with a plasma formed from a gas selected from the group consisting of CF 4  and SF 6  to remove the scale without intergranular attack of the underlying crystalline titanium or titanium alloy body beneath the scale; and   (c) auto-terminating the reacting when the plasma has reacted through the scale and encounters the underlying crystalline titanium or titanium alloy body.   
     
     
       2. The method of claim 1, wherein the scale comprises an oxide scale from about 0.001 to about 0.005 of an inch in thickness. 
     
     
       3. The method of claim 1, wherein heating of step (a) comprises heating to a temperature is in the range from about 220° C. to about 520° C. 
     
     
       4. The method of claim 1, wherein the titanium alloy is Ti-6A1-4V. 
     
     
       5. The method of claim 1, wherein the step of heating comprises heating by heating in an enclosed vacuum chamber. 
     
     
       6. The method of claim 5, wherein the reacting with plasma is in the enclosed chamber. 
     
     
       7. The method of claim 1, wherein the scale comprises alpha case. 
     
     
       8. The method of claim 1, wherein the reacting is at a rate sufficient to remove at least about 0.0005 to about 0.002 of an inch per hour. 
     
     
       9. A method of removing a heat-treatment induced scale from surfaces of a titanium or titanium alloy substrate, the method comprising: (a) heating at least the surfaces of the substrate having the heat-treatment induced scale to a temperature in the range from about 220° C. to about 520° C.,   (b) subjecting the heated substrate to a reactive plasma, containing fluoride ions to remove the scale, without intergranular attack of the titanium or titanium alloy substrate; and   (c) terminating the subjecting step when the plasma has reacted through the scale and encounters underlying crystalline titanium or titanium alloy of the substrate.   
     
     
       10. The method of claim 9, wherein the subjecting comprises subjecting to a plasma of a gas selected from the group consisting of fluorocarbon compounds, sulfur fluorides and phosphorous fluorides. 
     
     
       11. The method of claim 9, wherein the subjecting to a plasma to remove scale comprises subjecting to a plasma at a concentration and under temperature conditions to cause removal of the scale at a rate of from about 0.0005 to about 0.002 inches per hour. 
     
     
       12. A method of removing a scale from surfaces of titanium or titanium alloy substrates, the method comprising: (a) heating the substrate to a sufficient temperature to allow chemical components of the scale to react with a plasma generated from a gas selected from the group consisting of fluorocarbons, phosphorous fluorides and sulfur fluorides at a rate that removes at least about 0.0001 inch per hour from the scale; and   (b) reacting the scale with a plasma generated from the gas, the step of reacting carried out without intergranular attack of underlying substrate metal.   
     
     
       13. The method of claim 12, wherein the heating is to a temperature in the range from about 220° to about 520° C. 
     
     
       14. The method of claim 12, wherein the heating comprises heating to react at a rate of about 0.0005 to about 0.002 inches/hr. 
     
     
       15. The method of claim 12, wherein the heating is to a temperature in the range about 100° C. to about 600° C. .Iadd. 
     
     
       16.  The method of claim 1, wherein removing step (b) comprises reacting the heated surfaces of the aircraft component with the plasma within an enclosed vacuum chamber at a pressure below atmospheric..Iaddend..Iadd.17. The method of claim 16, wherein the pressure is below 10 Pascal..Iaddend..Iadd.18. The method of claim 17, wherein the pressure is below 8 Pascal..Iaddend..Iadd.19. A heat treated titanium aircraft component produced in accordance with the method of claim 1, wherein the titanium aircraft component is free of scale and intergranular attack..Iaddend..Iadd.20. The titanium aircraft component of claim 19, wherein the titanium aircraft component is free of hydrogen inclusion so that hydrogen content of the titanium aircraft component is substantially the same as the content of the titanium aircraft component before the heat treatment that produced the scale..Iaddend..Iadd.21. The method of claim 9, wherein subjecting step (b) comprises subjecting the heated substrate to the reactive plasma within an enclosed vacuum chamber at a pressure below atmospheric..Iaddend..Iadd.22. The method of claim 21, wherein the pressure is below 10 Pascal..Iaddend..Iadd.23. The method of claim 22, wherein the pressure is below 8 Pascal..Iaddend..Iadd.24. A heat treated titanium part produced in accordance with the method of claim 1, wherein the part is free of scale and intergranular attack..Iaddend..Iadd.25. The substrate of claim 24, wherein the titanium aircraft component is free of hydrogen inclusion so that hydrogen content of the titanium aircraft component is substantially the same as the content of the titanium aircraft component before the heat treatment that produced the scale..Iaddend..Iadd.26. A heat treated titanium part that is free of 
     
     
        scale and intergranular attack..Iaddend..Iadd.27.  The heat treated titanium part of claim 26, wherein the titanium aircraft component is free of hydrogen inclusion so that hydrogen content of the titanium aircraft component is substantially the same as the content of the titanium aircraft component before heat treatment..Iaddend.