Treatments To Produce Stabilized Aluminum Mirrors for Cryogenic Uses

Five metallurgical treatments have been tested as means of stabilizing mirrors that are made of aluminum alloy 6061 and are intended for use in cryogenic applications. Aluminum alloy 6061 is favored as a mirror material by many scientists and engineers. Like other alloys, it shrinks upon cool-down from room temperature to cryogenic temperature. This shrinkage degrades the optical quality of the mirror surfaces. Hence, the metallurgical treatments were tested to determine which one could be most effective in minimizing the adverse optical effects of cooldown to cryogenic temperatures. Each of the five metallurgical treatments comprises a multistep process, the steps of which are interspersed with the steps of the mirror- fabrication process. The five metallurgical- treatment/fabrication–process combinations were compared with each other and with a benchmark fabrication process, in which a mirror is made from an alloy blank by (1) symmetrical rough machining, (2) finish machining to within 0.006 in. (˜ 0.15 mm) of final dimensions, and finally (3) diamond turning to a mirror finish. Two specimens ? a flat mirror and a spherical mirror ? were fabricated in each case. The blanks for all the specimens were cut from the same plate of aluminum alloy 6061-T651. (The suffix “T651” denotes a stress-relieving treatment that involves reducing residual stresses by mechanical stretching of the previously untreated alloy.) Of the five metallurgical-treatment/fabricationprocess combinations tested, the one found to exert the greatest stabilizing effect comprises the following ten steps: