You should consider the temperatures the Al will see. If 6061 T6 does get over 350 F then it will reduce it strength. Look at the capabilities of the new 7068 and it seems to state a time (100 hrs is not much) over 250 F will reduce its strength.
http://www.kaiseraluminum.com/wp-con...8-brochure.pdf
I work with 6061 and its cast equal C355 and we limit the temps to 350 F and even then it will only be for short time, i.e. 10 % of total life of 20 years.
Also the thermal expansion differences make a difference too and can impart HUGE stresses if not considered.
Steel is about .06 and alum is about .1 for a relative comparison.
Lee