Because of expansion of aluminium with temperature should I glue an aluminum load taking fixture after post curing or I can do that safely before? I am afraid of delamination of the fixture due to contraction when temperature will go down. I am using 3M DP420 as a structural adhesive.
Thank you for sharing your knowledge.

Ultimately it depends on the laminate used as well as the adhesive properties.   If the part needs to be high temperature use, you might need 2 post cures, one for the part, one for the glue. Not being familiar with that specific adhesive, I do not know what its CTE or flexural strength is like so hard to say what is best. Many structural adhesives with a high flexural strength would be fine.   However safer option is bond after initial post cure, then re-post cure for the adhesive if it needs it. 

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant