Lubrication of magnesium workpieces for hot forming

Abstract

The elevated temperature forming of magnesium based alloy workpieces by stretching, drawing, bending (or the like) a surface of the heated workpiece over the forming surface of a forming tool is improved by forming an integral adherent layer of magnesium hydroxide on the tool-contacted surface(s) of the magnesium alloy workpiece. The magnesium hydroxide layer may be formed by treating the surface(s) of the sheet with an aqueous salt solution (e.g., sodium chloride) at a temperature and for a time to form a protective layer of desired thickness (e.g., up to about thirty micrometers). If desired, an additional layer of forming lubricant, such as a film comprising boron nitride, may be applied to the magnesium hydroxide layer.

1 . A method of preparing a magnesium alloy workpiece for a forming process in which the workpiece is heated to a temperature above room temperature and a surface of the workpiece is caused to slide against a surface of a forming tool, the method comprising:
 forming an adherent layer of magnesium hydroxide on the surface of the magnesium alloy workpiece before the sliding contact of the forming process, the thickness of the magnesium hydroxide layer being predetermined for the sliding contact in the forming process.   
     
     
         2 . A method as recited in  claim 1  in which the adherent layer of magnesium hydroxide is formed by contacting the surface of the magnesium alloy workpiece with an aqueous solution of a salt, the temperature of the aqueous solution and the duration of the contact providing the predetermined thickness of the magnesium hydroxide layer. 
     
     
         3 . A method as recited in  claim 1  in which the adherent layer of magnesium hydroxide is formed by contacting the surface of the magnesium alloy workpiece with an aqueous solution comprising sodium chloride, the temperature of the aqueous solution being above room temperature and the duration of the contact being up to about thirty minutes. 
     
     
         4 . A method as recited in  claim 1  further comprising applying a film of a forming lubricant over the adherent layer of magnesium hydroxide before the sliding contact of the forming process. 
     
     
         5 . A method as recited in  claim 1  in which the magnesium hydroxide layer has a thickness in the range from about one micrometer to about thirty micrometers. 
     
     
         6 . A method as recited in  claim 1  in which the magnesium alloy workpiece is a sheet. 
     
     
         7 . A method of forming a magnesium alloy workpiece into a predetermined three-dimensional shape, the method comprising:
 forming an adherent layer of magnesium hydroxide on at least one surface of the magnesium alloy workpiece; and   bringing the surface of the workpiece with the layer of magnesium hydroxide into loaded and sliding contact with the surface of the forming tool to form the predetermined shape, the workpiece and forming tool being at a forming temperature above ambient temperature.   
     
     
         8 . A method of forming a magnesium alloy workpiece as recited in  claim 7  in which the thickness of the magnesium hydroxide layer is in the range of about one micrometer to about thirty micrometers. 
     
     
         9 . A method of forming a magnesium alloy workpiece as recited in  claim 7  in which the forming temperature is above 200° C. 
     
     
         10 . A method of forming a magnesium alloy workpiece as recited in  claim 7  in which an adherent layer of magnesium hydroxide is formed on both sides of a magnesium alloy sheet, and the sheet is formed by stamping between complementary stamping tools at a forming temperature above 200° C. 
     
     
         11 . A method of forming a magnesium alloy workpiece as recited in  claim 7  in which the workpiece is a sheet that is formed under loaded sliding contact with the forming surface of a forming tool at a forming temperature above 200° C., the surface of the sheet workpiece with the layer of magnesium hydroxide being in sliding contact with the forming surface of the forming tool. 
     
     
         12 . A method as recited in  claim 7  in which the adherent layer of magnesium hydroxide is formed by contacting the surface of the magnesium alloy workpiece with an aqueous solution of a salt, the temperature of the aqueous solution and the duration of the contact providing a predetermined thickness of the magnesium hydroxide layer. 
     
     
         13 . A method as recited in  claim 7  in which the adherent layer of magnesium hydroxide is formed by contacting the surface of the magnesium alloy workpiece with an aqueous solution comprising sodium chloride, the temperature of the aqueous solution being above room temperature and the duration of the contact being up to about thirty minutes. 
     
     
         14 . A method as recited in  claim 7  further comprising applying a film of a forming lubricant over the adherent layer of magnesium hydroxide before the sliding contact of the forming process. 
     
     
         15 . A method as recited in  claim 14  in which the forming lubricant comprises boron nitride, the boron nitride comprising film having a thickness ranging from about 2 micrometers to about 35 micrometers. 
     
     
         16 . A method as recited in  claim 7  in which the magnesium alloy workpiece is a sheet. 
     
     
         17 . A method of forming a magnesium alloy sheet workpiece into a predetermined three-dimensional shape, the method comprising:
 forming an adherent layer of magnesium hydroxide on at least one surface of the magnesium alloy sheet, the thickness of the magnesium hydroxide layer being up to about thirty micrometers; and   bringing the surface of the sheet workpiece with the layer of magnesium hydroxide into loaded and sliding contact with the surface of a forming tool to form the predetermined shape, the magnesium alloy sheet and forming tool being at a forming temperature above about 200° C., the magnesium alloy sheet having an opposite side.   
     
     
         18 . A method as recited in  claim 17  in which gas pressure is applied to the opposite side of the magnesium alloy sheet to load and urge the sheet workpiece into sliding contact with the forming tool. 
     
     
         19 . A method as recited in  claim 17  in which a complementary forming tool applied to and engages the opposite side of the magnesium alloy sheet to load and urge the sheet workpiece into sliding contact with the forming tool. 
     
     
         20 . A method as recited in  claim 17  in which the adherent layer of magnesium hydroxide is formed by contacting the surface(s) of the magnesium alloy sheet workpiece with an aqueous solution of a salt, the temperature of the aqueous solution and the duration of the contact providing a predetermined thickness of the magnesium hydroxide layer.