Good advice
Good advice
Thanks Denis. I have wondered about the issue of anodizing "flight critical" parts recently. Most tempers for 6061 and 7075 require heating at temperatures above 375 degrees for a considerable period of time. When I looked into powder coating, it appeared that the oven time and temperature was probably less than that needed to affect the temper, but I'm not willing to take that chance. Also powder coating has the potential for covering tiny cracks. I did powder coat a 4130 engine mount recently, white, so that any cracks would be more visible. As far as anodizing parts, it is likely true that anodizing might have a small effect on strength as it does compromise a tiny bit of the surface, a few thousandths perhaps. My rule of thumb in the past for life safety products has been to use material capable of holding at least 5 times the working load, in some applications I have seen designs calling for 10 times the working load. Aviation design seems to cut these numbers a bit, and fatigue life, given the vibratory loads seems to be a more important issue. But the simple fact is that if the compromise induced by anodizing is sufficient to cause failure, then I think that the part was greatly under sized for the load. I recently replaced some mast brackets of 2024 with 7075-T7 aluminum of greater thickness. 7075 has greater tensile strength but is known to be more brittle, but the T7 temper has less propensity for crack spread. A trade-off, like most things.
It is a rule of thumb that most aluminum tempers are unaffected by temperatures less than 200 degrees Fahrenheit. Above that some effect can be seen, dependent on the temperature and time. 212 degrees, boiling water, would have minimal effect unless maintained for a long period of time. 600 degrees would have a much quicker effect. The heat treatment for 6061-T6 is 990 degrees, followed by 320 degrees for 10 hours. It is unlikely that most things that we might do would reach these temperatures, with the exception of exhaust exposure. Otherwise we couldn't make engine cylinders from aluminum.