Quick plastic forming of aluminum alloy sheet metal

Abstract

A method is disclosed for stretching magnesium-containing aluminum alloy sheet stock into intricate shapes such as are required in automotive body panels. The sheet stock, at a temperature in the range of about 400° C. to about 510° C., is stretched under the pressure of a working gas into conformance with the surface of a forming tool. The sheet forming pressure is increased continually in a controlled manner from ambient pressure to a final forming level in the range of about 250 psi to about 500 psi or higher. A portion of the sheet can experience strain rates substantially higher than 10 −3 sec −1 and the forming of the sheet can be completed within 12 minutes.

1. A method of stretch forming a magnesium-containing, aluminum alloy sheet into a product, said alloy comprising up to about 6% by weight magnesium and having a microstructure characterized by a grain size in the range of about 5 to 30 micrometers, said method comprising
 heating said sheet to a temperature in the range of about 400° C. to about 510° C. and 
 stretching at least a portion of the heated sheet so that one side of the sheet is brought into conformance with a shaping surface by applying working gas pressure to the opposite side of the sheet, said stretching being accomplished by continually increasing said pressure from ambient pressure to a final stretching pressure in the range of about 250 psi to about 500 psi above ambient pressure and completing said stretching within a period of up to about 12 minutes. 
 
     
     
       2. A method as recited in  claim 1  comprising increasing the rate of increase of said pressure at a time after about one minute of application of said pressure to a final stretching pressure in said range of about 250 psi to about 500 psi. 
     
     
       3. A method as recited in  claim 1  comprising increasing said pressure to a level of 10 psi to 50 psi during the first minute of the application of said pressure and, thereafter, increasing said pressure at a rate faster than a linear rate of increase to a final stretching pressure in the range of about 250 psi to about 500 psi. 
     
     
       4. A method as recited in any of  claims 1 – 3  in which said magnesium-containing aluminum alloy comprises, by weight, about 3.5% to about 6% magnesium as a solid solution in said aluminum. 
     
     
       5. A method as recited in any of  claims 1 – 3  in which said aluminum alloy comprises, by weight, about 3.5% to about 6% magnesium, about 0.1% to about 1% manganese and aluminum. 
     
     
       6. A method as recited in any of  claims 1 – 3  in which said aluminum alloy comprises, by weight, about 4% to 5% magnesium, about 0.3% to 1% manganese, up to about 0.25% chromium, up to about 0.1% copper, up to about 0.3% iron, up to about 0.2% silicon and aluminum. 
     
     
       7. A method of forming an article of manufacture from superplastic magnesium-containing aluminum alloy sheet stock, comprising
 providing a sheet forming tool having a peripheral surface against which the periphery of said sheet stock can be held in sealing engagement and a sheet forming surface within said peripheral surface for forming said sheet, said tool including means for venting said cavity during the forming of said sheet, 
 heating said sheet to a temperature in said range and holding said sheet in sealing engagement with said peripheral surface of said tool, said sheet then having a first surface facing said forming surface and an opposite surface, 
 stretching said heated sheet into conformance with said forming surface by applying working gas pressure to said opposite side of the sheet, said stretching being accomplished by continually increasing said pressure from ambient pressure to a final stretching pressure in the range of about 250 psi to about 500 psi above ambient pressure and completing said stretching within a period of up to about 12 minutes. 
 
     
     
       8. A method as recited in  claim 7  in which the rate of pressure increase is greater than a linear rate of increase. 
     
     
       9. A method as recited in any of  claim 7  or  8  in which said article is an automotive vehicle body panel. 
     
     
       10. A method of stretch forming a magnesium-containing, aluminum alloy sheet into a product, said alloy comprising up to about 6% by weight magnesium and having a microstructure characterized by a grain size in the range of about 5 to 30 micrometers, said method comprising
 heating said sheet to a temperature in the range of about 400° C. to about 510° C. and 
 stretching at least a portion of the heated sheet so that one side of the sheet is brought into conformance with a shaping surface by applying working gas pressure to the opposite side of the sheet, said stretching being accomplished such that at least a portion of the sheet experiences a strain rate greater than 10 −3  sec −1 . 
 
     
     
       11. A method as recited in  claim 10  comprising stretching said sheet such that at least a portion of the sheet experiences a strain rate greater than 5×10 −3  sec −1 . 
     
     
       12. A method as recited in  claim 10  comprising continuously increasing said gas pressure from ambient pressure to a final stretching pressure and completing said stretching within a period of up to about 12 minutes. 
     
     
       13. A method as recited in  claim 12  in which said stretching is completed within a period of up to about six minutes. 
     
     
       14. A method as recited in  claim 12  in which said stretching is completed within a period of up to about three minutes. 
     
     
       15. A method of forming an article of manufacture from a metal alloy sheet comprising:
 providing a sheet forming tool having engagement surfaces for engaging the sheet near peripheral edges thereof so that the sheet can be held in sealing engagement with the sheet forming tool, the sheet forming tool having a sheet forming surface for forming the sheet;   heating the sheet and holding the sheet in sealing engagement with the engagement surfaces of the sheet forming tool, the sheet having a first surface facing the sheet forming surface and an opposite surface;   stretching the heated sheet into conformance with the forming surface by applying working gas pressure to the opposite surface of the sheet, the stretching being accomplished by applying the pressure in a series of stages comprising a first stage, and a second stage; and wherein the first stage comprises increasing the pressure at a first rate from ambient, and the second stage comprising increasing the pressure at a second rate, and wherein the second stage occurs after the first stage and the second rate is greater than the first rate and further comprising applying a final stretching pressure during the second stage, and wherein the final stretching pressure ranges from about 250 psi to about 500 psi above ambient pressure.   
     
     
       16. A method as set forth in claim 15 wherein the stretching further comprising a third stage comprising applying a final forming pressure to the sheet after the second stage, and wherein the final forming pressure is applied at a substantially constant pressure. 
     
     
       17. A method as set forth in claim 15 wherein the sheet comprises an aluminum alloy. 
     
     
       18. A method as set forth in claim 17 wherein the aluminum alloy comprises magnesium. 
     
     
       19. A method as set forth in claim 18 wherein the aluminum alloy comprises magnesium present in an amount, by weight, up to 6 percent of the aluminum alloy. 
     
     
       20. A method as set forth in claim 15 wherein the alloy sheet has a microstructure grain size ranging from about 5-30 micrometers. 
     
     
       21. A method as set forth in claim 15 wherein the heating of the sheet is conducted so that the temperature of the sheet ranges from about 400° C. to about 510° C. 
     
     
       22. A method as set forth in claim 15 wherein the first rate of increasing the pressure is non-linear. 
     
     
       23. A method as set forth in claim 15 wherein the second rate of increasing the pressure is non-linear. 
     
     
       24. A method as set forth in claim 23 wherein the first rate of increasing the pressure is non-linear. 
     
     
       25. A method as set forth in claim 15 wherein the stretching further comprising applying a final stretching pressure after the series of stages, and wherein the final stretching pressure is substantially constant. 
     
     
       26. A method as set forth in claim 15 wherein the stretching further comprising applying a final stretching pressure stage after the second stage, and wherein pressure is increased during the final stretching pressure stage at a linear rate. 
     
     
       27. A method as set forth in claim 15 wherein the metal sheet comprises an aluminum alloy comprising magnesium present in an amount, by weight, of about 1.0 percent to about 6 percent magnesium. 
     
     
       28. A method as set forth in claim 15 wherein the metal sheet comprises an aluminum alloy comprising magnesium present in an amount, by weight, of up to 6 percent, and further comprising additional materials. 
     
     
       29. A method as set forth in claim 28 wherein the additional materials consist of manganese, chromium, copper, iron, silicon, and mixtures thereof. 
     
     
       30. A method as set forth in claim 15 wherein the stretching of the sheet into conformance with the forming surface is completed in less than 12 minutes. 
     
     
       31. A method of forming an article of manufacture from a metal alloy sheet comprising:
 providing a sheet forming tool having engagement surfaces for engaging the sheet near peripheral edges thereof so that the sheet can be held in sealing engagement with the sheet forming tool, the sheet forming tool having a sheet forming surface for forming the sheet;   heating the sheet and holding the sheet in sealing engagement with the engagement surfaces of the sheet forming tool, the sheet having a first surface facing the sheet forming surface and an opposite surface;   stretching the heated sheet into conformance with the forming surface by applying a working gas pressure, in a range of about 250 psi to about 500 psi above ambient pressure, to the opposite surface of the sheet, the stretching being accomplished by applying the pressure in a series of stages comprising a first stage, and a second stage; and wherein the first stage comprises increasing the pressure from ambient at a first rate to a first stage ending pressure, and the second stage comprising increasing the pressure at a second rate from the first stage ending pressure, and wherein the second rate is greater than the first rate, and wherein the second rate is non-linear.   
     
     
       32. A method of forming an article of manufacture from a metal alloy sheet comprising:
 providing a sheet forming tool having engagement surfaces for engaging the sheet near peripheral edges thereof so that the sheet can be held in sealing engagement with the sheet forming tool, the sheet forming tool having a sheet forming surface for forming the sheet;   heating the sheet and holding the sheet in sealing engagement with the engagement surfaces of the sheet forming tool, the sheet having a first surface facing the sheet forming surface and an opposite surface;   stretching the heated sheet into conformance with the forming surface by applying working gas pressure, in a range of about 250 psi to about 500 psi above ambient pressure, to the opposite surface of the sheet, the stretching being accomplished by applying the pressure in a series of nonlinear pressure increasing stages comprising at least a first stage and a second stage, and wherein the average rate of pressure increase in the first stage is greater than the average rate of pressure increase in the second stage.   
     
     
       33. A method of forming an article of manufacture from an aluminum alloy sheet comprising:
 providing a sheet forming tool having engagement surfaces for engaging the sheet near peripheral edges thereof so that the sheet can be held in sealing engagement with the sheet forming tool, the sheet forming tool having a sheet forming surface for forming the sheet;   heating the sheet to a temperature ranging from 400° C. to 510° C. and holding the sheet in sealing engagement with the engagement surfaces of the sheet forming tool, the sheet having a first surface facing the sheet forming surface and an opposite surface;   stretching the heated sheet into conformance with the forming surface by applying working gas pressure, in a range of about 250 psi to about 500 psi above ambient pressure, to the opposite surface of the sheet, the stretching being accomplished by applying the pressure so that at least a portion of the sheet experiences a strain rate greater than 5×10 −3  sec −1 .   
     
     
       34. A method of forming an article of manufacture from a metal alloy sheet stock comprising:
 providing a sheet forming tool having engagement surfaces for engaging the sheet near peripheral edges thereof so that the sheet can be held in sealing engagement with the sheet forming tool, the sheet forming tool having a sheet forming surface for forming the sheet;   heating the sheet and holding the sheet in sealing engagement with the engagement surfaces of the sheet forming tool, the sheet having a first surface facing the sheet forming surface and an opposite surface;   applying pressure, in a range of about 250 psi to about 500 psi above ambient pressure, on the heated sheet in a series of stages to bring the sheet into conformance with the forming surface, the series of stages comprising a first stage, a second stage, and a final forming stage; and wherein of the pressure applied in the first stage is increased at a first average rate, and wherein the pressure applied in the second stage is at a second average rate that is greater than a first average rate, and wherein the pressure applied in the final forming stage is substantially constant.   
     
     
       35. A method of forming an article of manufacture from a metal alloy sheet comprising:
 providing a metal alloying sheet stock having a microstructure grain size ranging from about 5-30 micrometers, and providing a sheet forming tool having engagement surfaces for engaging the sheet near peripheral edges thereof so that the sheet can be held in sealing engagement with the sheet forming tool, the sheet forming tool having a sheet forming surface for forming the sheet;   heating the sheet and holding the sheet in sealing engagement with the engagement surfaces of the sheet forming tool, the sheet having a first surface facing the sheet forming surface and an opposite surface;   applying pressure, in a range of about 250 psi to about 500 psi above ambient pressure, on the heated sheet in a series of stages to bring the sheet into conformance with the forming surface, the series of stages comprising a first stage, a second stage, and a final forming stage; and wherein the pressure applied in the first stage is increased at a first rate, and wherein the pressure applied in the second stage is at a second rate that is greater than a first rate, and wherein the pressure applied in the final forming stage is substantially constant.   
     
     
       36. A method as set forth in claim 35 wherein the heating of the sheet is conducted so that the sheet has a temperature less than 510° C. 
     
     
       37. A method as set forth in claim 35 wherein the heating of the sheet is conducted so that the sheet has a temperature ranging from 400° C. to 510° C. 
     
     
       38. A method as set forth in claim 35 wherein the metal alloy is an aluminum alloy. 
     
     
       39. A method as set forth in claim 35 wherein the rate of pressure increase in the first stage is non-linear. 
     
     
       40. A method as set forth in claim 35 wherein the rate of pressure increase in the second stage is non-linear. 
     
     
       41. A method as set forth in claim 40 wherein the rate of pressure increase in the second stage is non-linear. 
     
     
       42. A method of forming an article of manufacture from an aluminum alloy sheet comprising:
 providing a sheet forming tool having engagement surfaces for engaging the sheet near peripheral edges thereof so that the sheet can be held in sealing engagement with the sheet forming tool, the sheet forming tool having a sheet forming surface for forming the sheet;   heating the sheet and holding the sheet in sealing engagement with the engagement surfaces of the sheet forming tool, the sheet having a first surface facing the sheet forming surface and an opposite surface;   stretching the heated sheet into conformance with the forming surface by applying working gas pressure, in a range of about 250 psi to about 500 psi above ambient pressure, to the opposite surface of the sheet, the stretching being accomplished by applying the pressure in a series of stages comprising a first stage, and a second stage; and wherein the first stage comprises increasing the pressure at a first average rate from ambient, and the second stage comprising increasing the pressure at a second average rate, and wherein the second stage occurs after the first stage, and the second average rate is greater than the first average rate.   
     
     
       43. A method as set forth in claim 42 further comprising applying a final forming pressure to the sheet after the second stage, and wherein the final forming pressure is applied at a substantially constant pressure. 
     
     
       44. A method as set forth in claim 42 wherein the aluminum alloy comprises magnesium. 
     
     
       45. A method as set forth in claim 44 wherein the magnesium is present in an amount, by weight, up to 6 percent of the aluminum alloy. 
     
     
       46. A method as set forth in claim 42 wherein the alloy sheet has a microstructure grain size ranging from about 5-30 micrometers. 
     
     
       47. A method as set forth in claim 42 wherein the heating of the sheet is conducted so that the temperature of the sheet ranges from about 400° C. to about 510° C. 
     
     
       48. A method as set forth in claim 42 wherein the rate of increasing the pressure in the first stage is non-linear. 
     
     
       49. A method as set forth in claim 42 wherein the rate of increasing the pressure in the second stage is non-linear. 
     
     
       50. A method as set forth in claim 49 wherein the rate of increasing the pressure in the first stage is non-linear. 
     
     
       51. A method as set forth in claim 42 wherein the stretching further comprising applying a final stretching pressure stage after the second stage, and wherein pressure is increased during the final stretching pressure stage at a rate that is linear. 
     
     
       52. A method as set forth in claim 42 wherein the aluminum alloy comprising magnesium present in an amount, by weight, of about 1.0 percent to about 6 percent magnesium. 
     
     
       53. A method as set forth in claim 42 wherein the aluminum alloy comprising magnesium present in an amount, by weight, of up to 6 percent, and further comprising additional materials. 
     
     
       54. A method as set forth in claim 53 wherein the additional materials comprise an alloying component based on a metal selected from the group consisting of manganese, chromium, copper, iron, silicon, and mixtures thereof. 
     
     
       55. A method as set forth in claim 42 wherein the stretching of the sheet into conformance with the forming surface is completed in less than 12 minutes. 
     
     
       56. A method of forming an article of manufacture from an aluminum alloy sheet comprising:
 providing a sheet forming tool having engagement surfaces for engaging the sheet near peripheral edges thereof so that the sheet can be held in sealing engagement with the sheet forming tool, the sheet forming tool having a sheet forming surface for forming the sheet;   heating the sheet to a temperature ranging from about 400° C. to about 510° C. and holding the sheet in sealing engagement with the engagement surfaces of the sheet forming tool, the sheet having a first surface facing the sheet forming surface and an opposite surface;   stretching the heated sheet into conformance with the forming surface by applying working gas pressure, in a range of about 250 psi to about 500 psi above ambient pressure, to the opposite surface of the sheet, the stretching being accomplished by applying the pressure in a series of stages comprising a first stage, and a second stage; and wherein the first stage comprises increasing the pressure at a first average rate from ambient, and the second stage comprising increasing the pressure at a second average rate, and wherein the second stage occurs after the first stage and the second average rate is greater than the first average rate.   
     
     
       57. A method of forming an article of manufacture from an aluminum alloy sheet comprising:
 providing a sheet forming tool having engagement surfaces for engaging the sheet near peripheral edges thereof so that the sheet can be held in sealing engagement with the sheet forming tool, the sheet forming tool having a sheet forming surface for forming the sheet;   heating the sheet and holding the sheet in sealing engagement with the engagement surfaces of the sheet forming tool, the sheet having a first surface facing the sheet forming surface and an opposite surface;   stretching the heated sheet into conformance with the forming surface by applying working gas pressure, in a range of about 250 psi to about 500 psi above ambient pressure, to the opposite surface of the sheet, the stretching being accomplished by applying the pressure in a series of stages comprising a first stage, and a second stage; and wherein the first stage comprises increasing the pressure at a first rate from ambient, and the second stage comprising increasing the pressure at a second rate, and wherein the second stage occurs after the first stage and the second rate is greater than the first rate, and wherein the aluminum alloy has a microstructure grain size of 5-30 micrometers.   
     
     
       58. A method of forming an article of manufacture from a metal alloy sheet comprising:
 providing a sheet forming tool having engagement surfaces for engaging the sheet near peripheral edges thereof so that the sheet can be held in sealing engagement with the sheet forming tool, the sheet forming tool having a sheet forming surface for forming the sheet;   heating the sheet and holding the sheet in sealing engagement with the engagement surfaces of the sheet forming tool, the sheet having a first surface facing the sheet forming surface and an opposite surface;   stretching the heated sheet into conformance with the forming surface by applying working gas pressure to the opposite surface of the sheet, the stretching being accomplished by applying the pressure in a series of stages comprising a first stage, and a second stage; and wherein the first stage comprises increasing the pressure from ambient at a first rate to a first stage ending pressure, and the second stage comprising increasing the pressure at a second rate from the first stage ending pressure, and wherein the second rate is greater than the first rate, and wherein the second rate is non-linear, the stretching including a final stretching pressure in a range of about 250 psi to about 500 psi above ambient pressure.   
     
     
       59. A method of forming an article of manufacture from a metal alloy sheet comprising:
 providing a sheet forming tool having engagement surfaces for engaging the sheet near peripheral edges thereof so that the sheet can be held in sealing engagement with the sheet forming tool, the sheet forming tool having a sheet forming surface for forming the sheet;   heating the sheet and holding the sheet in sealing engagement with the engagement surfaces of the sheet forming tool, the sheet having a first surface facing the sheet forming surface and an opposite surface;   stretching the heated sheet into conformance with the forming surface by applying working gas pressure, in a range of about 250 psi to about 500 psi above ambient pressure, to the opposite surface of the sheet, the stretching being accomplished by applying the pressure in a series of stages comprising a first stage, and a second stage; and wherein the first stage comprises increasing the pressure from ambient at a first rate to a first stage ending pressure, and the second stage comprising increasing the pressure at a second rate from the first stage ending pressure, and wherein the second rate is greater than the first rate, and wherein the second rate is non-linear, said stretching being accomplished such that at least a portion of the sheet experiences a strain rate greater than 10 −3  sec −1 .   
     
     
       60. A method of forming an article of manufacture from a metal alloy sheet comprising:
 providing a sheet forming tool having engagement surfaces for engaging the sheet near peripheral edges thereof so that the sheet can be held in sealing engagement with the sheet forming tool, the sheet forming tool having a sheet forming surface for forming the sheet;   heating the sheet and holding the sheet in sealing engagement with the engagement surfaces of the sheet forming tool, the sheet having a first surface facing the sheet forming surface and an opposite surface;   stretching the heated sheet into conformance with the forming surface by applying working gas pressure to the opposite surface of the sheet, the stretching being accomplished by applying the pressure in a series of nonlinear pressure increasing stages comprising at least a first stage and a second stage, and wherein the average rate of pressure increase in the first stage is greater than the average rate of pressure increase in the second stage, the stretching including a final stretching pressure in a range of about 250 psi to about 500 psi above ambient pressure.   
     
     
       61. A method of forming an article of manufacture from a metal alloy sheet comprising:
 providing a sheet forming tool having engagement surfaces for engaging the sheet near peripheral edges thereof so that the sheet can be held in sealing engagement with the sheet forming tool, the sheet forming tool having a sheet forming surface for forming the sheet;   heating the sheet and holding the sheet in sealing engagement with the engagement surfaces of the sheet forming tool, the sheet having a first surface facing the sheet forming surface and an opposite surface;   stretching the heated sheet into conformance with the forming surface by applying working gas pressure to the opposite surface of the sheet, the stretching being accomplished by applying the pressure in a series of nonlinear pressure increasing stages comprising at least a first stage and a second stage, and wherein the average rate of pressure increase in the first stage is greater than the average rate of pressure increase in the second stage, the stretching including a final stretching pressure in a range of about 250 psi to about 500 psi above ambient pressure.   
     
     
       62. A method of forming an article of manufacture from an aluminum alloy sheet comprising:
 providing a sheet forming tool having engagement surfaces for engaging the sheet near peripheral edges thereof so that the sheet can be held in sealing engagement with the sheet forming tool, the sheet forming tool having a sheet forming surface for forming the sheet;   heating the sheet and holding the sheet in sealing engagement with the engagement surfaces of the sheet forming tool, the sheet having a first surface facing the sheet forming surface and an opposite surface;   stretching the heated sheet into conformance with the forming surface by applying working gas pressure, in a range of about 250 psi to about 500 psi above ambient pressure, to the opposite surface of the sheet, the stretching being accomplished by applying the pressure in a series of stages comprising a first stage, and a second stage; and wherein the first stage comprises increasing the pressure at a first average rate from ambient, and the second stage comprising increasing the pressure at a second average rate, and wherein the second stage occurs after the first stage, and the second average rate is greater than the first average rate, the stretching being accomplished such that at least a portion of the sheet experiences a strain rate greater than 10 −3  sec −1 .   
     
     
       63. A method of forming an article of manufacture from an aluminum alloy sheet comprising:
 providing a sheet forming tool having engagement surfaces for engaging the sheet near peripheral edges thereof so that the sheet can be held in sealing engagement with the sheet forming tool, the sheet forming tool having a sheet forming surface for forming the sheet;   heating the sheet and holding the sheet in sealing engagement with the engagement surfaces of the sheet forming tool, the sheet having a first surface facing the sheet forming surface and an opposite surface;   stretching the heated sheet into conformance with the forming surface by applying working gas pressure to the opposite surface of the sheet, the stretching being accomplished by applying the pressure in a series of stages comprising a first stage, and a second stage; and wherein the first stage comprises increasing the pressure at a first average rate from ambient, and the second stage comprising increasing the pressure at a second average rate, and wherein the second stage occurs after the first stage, and the second average rate is greater than the first average rate, the stretching including a final stretching pressure in a range of about 250 psi to about 500 psi above ambient pressure.   
     
     
       64. A method of forming an article of manufacture from an aluminum alloy sheet comprising:
 providing a sheet forming tool having engagement surfaces for engaging the sheet near peripheral edges thereof so that the sheet can be held in sealing engagement with the sheet forming tool, the sheet forming tool having a sheet forming surface for forming the sheet;   heating the sheet to a temperature ranging from about 400° C. to about 510° C. and holding the sheet in sealing engagement with the engagement surfaces of the sheet forming tool, the sheet having a first surface facing the sheet forming surface and an opposite surface;   stretching the heated sheet into conformance with the forming surface by applying working gas pressure, in a range of about 250 psi to about 500 psi above ambient pressure, to the opposite surface of the sheet, the stretching being accomplished by applying the pressure in a series of stages comprising a first stage, and a second stage; and wherein the first stage comprises increasing the pressure at a first average rate from ambient, and the second stage comprising increasing the pressure at a second average rate, and wherein the second stage occurs after the first stage and the second average rate is greater than the first average rate, the stretching being accomplished such that at least a portion of the sheet experiences a strain rate greater than 10 −3  sec −1 .   
     
     
       65. A method of forming an article of manufacture from an aluminum alloy sheet comprising:
 providing a sheet forming tool having engagement surfaces for engaging the sheet near peripheral edges thereof so that the sheet can be held in sealing engagement with the sheet forming tool, the sheet forming tool having a sheet forming surface for forming the sheet;   heating the sheet and holding the sheet in sealing engagement with the engagement surfaces of the sheet forming tool, the sheet having a first surface facing the sheet forming surface and an opposite surface;   stretching the heated sheet into conformance with the forming surface by applying working gas pressure to the opposite surface of the sheet, the stretching being accomplished by applying the pressure in a series of stages comprising a first stage, and a second stage; and wherein the first stage comprises increasing the pressure at a first rate from ambient, and the second stage comprising increasing the pressure at a second rate, and wherein the second stage occurs after the first stage and the second rate is greater than the first rate, and wherein the aluminum alloy has a microstructure grain size of 5-30 micrometers, the stretching including a final stretching pressure in a range of about 250 psi to about 500 psi above ambient pressure.   
     
     
       66. A method of forming an article of manufacture from an aluminum alloy sheet comprising:
 providing a sheet forming tool having engagement surfaces for engaging the sheet near peripheral edges thereof so that the sheet can be held in sealing engagement with the sheet forming tool, the sheet forming tool having a sheet forming surface for forming the sheet;   heating the sheet and holding the sheet in sealing engagement with the engagement surfaces of the sheet forming tool, the sheet having a first surface facing the sheet forming surface and an opposite surface;   stretching the heated sheet into conformance with the forming surface by applying working gas pressure, in a range of about 250 psi to about 500 psi above ambient pressure, to the opposite surface of the sheet, the stretching being accomplished by applying the pressure in a series of stages comprising a first stage, and a second stage; and wherein the first stage comprises increasing the pressure at a first rate from ambient, and the second stage comprising increasing the pressure at a second rate, and wherein the second stage occurs after the first stage and the second rate is greater than the first rate, and wherein the aluminum alloy has a microstructure grain size of 5-30 micrometers, the stretching being accomplished such that at least a portion of the sheet experiences a strain rate greater than 10 −3  sec −1 .