Funny I see this thread pop up as I’m currently looking up some numbers for elevated temperature processing on 7075-T6 parts at work. Warm forming/working of 7075-T6 aluminum IS perfectly acceptable and a standard industry practice… I have a spec from NASA open right now regarding that very subject.
I aim for about a 0.001”-0.0015” interference fit between the upper and barrel when doing a thermal fit if I’m doing it with shims. I verify the level of interference with a micrometer on the barrel extension and a very accurate dial bore gauge in the upper. Personally, I won't attempt a thermal fit before verifying the numbers as if things are too tight you're probably going to overheat the upper trying to get it assembled-- you're tempted to just keep heating until it fits! I’m not sure what level of interference fit manufacturers who make thermal fit uppers are boring their uppers to.
For 7075-T6, this spec I’m looking at has a table of maximum acceptable soak times at various temperatures before forming, the table showing the maximum temp and soak times which result in no more than a 5% strength loss. The acceptable soak time at 204C / 400F is 5-10 minutes, 191C / 375F is 30-60 minutes, 177C / 350F is 1-2 hours, 163C / 325F is 2-4 hours, 149C / 300F is 10-12 hours. Heating to 218C / 425F is listed as "time to temperature" only with *zero* permitted dwell time, and at 232C / 450F you will have reduced the strength beyond the 5% allowable and now the part must be re heat treated and aged to restore it to T6 conditions. As the temp goes down the acceptable soak time without losing more than 5% of the strength increases dramatically, which makes perfect sense. Going above those temps and/or beyond those soak times at temp starts to degrade the heat treat and mechanical properties beyond the allowable 5% reduction, and it will require heat treating and aging the material again to get the mechanical properties back to T6 levels.
For example, if you’re wanting a 0.0015” diametric interference fit between the upper and barrel extension you’ll need to make some clearance with a temperature delta so it’s a slip fit.
Putting the steel barrel and extension in a 0F freezer in your kitchen or garage fridge will shrink it 0.0005” from a 75F room.
Heating your aluminum upper to 300F from a 75F room will expand the upper by 0.0028”. That means you’ll now have a 0.0018” slip fit of the extension into the upper and that’s workable as long as you move quickly. According to the spec I’m reading a soak time of up to 10-12 hours is acceptable at 300F. If you want to control that, put the upper in an oven with a thermocouple attached to monitor it and don’t heat it with a torch or heat gun which can create local hot spots on the outer contact surfaces (even though the excellent thermal conductivity of aluminum will help prevent overheating the torch / heat gun contact area.) At 300F you have 10-12 hours of acceptable soaking time before you degrade the mechanical properties of the 7075-T6 upper beyond 5%; that is a very safe working temp without worrying about messing up the heat treat on your upper unless you forget it in the oven overnight.
Heating to 325F will get the upper to expand another 0.0003” (0.003” upper expansion, now you have 0.0021” slip fit between the barrel and upper) and per the spec that temperature is acceptable for 2-4 hours soak. Taking the upper to 350F will get you another 0.0003” expansion for 0.0024” slip fit between the barrel and upper and 350F is acceptable for 1-2 hours.
Heating the upper to 400F for no more than 5-10 minutes dwell time is also acceptable per the spec I’m looking at and taking it to 375-400F will give you even more clearance to make slipping the barrel extension in easier without it locking up, but at 375-400F the permissible dwell times are much shorter; IMO you’re much safer keeping the upper around 300-325F max as you have a lot more safety margin regarding strength decrease from the very long permissible dwell times at the lower temps, and doing it in a temperature controlled oven gives you much better control over the temp of the part than say “about 10 seconds rotating in the flame of a propane torch or 30 seconds in front of a 1500W heat gun.”
If you are really worried about altering the mechanical properties of your upper, bonding is certainly the safest solution… but disassembly without destruction of the upper or barrel can be a bit difficult depending on what adhesive you used, and if you need to use heat to weaken the adhesive to remove the barrel, all the above mentioned temperature and time guidelines apply.
Note that the spec I'm referencing says heating 7075-T6 above 450F for any amount of time will reduce strength beyond 5% and requires re heat treating and aging to restore the mechanical properties, and now recall the disassembly instructions for loctite 609 and 620 suggests heating it to 500F... Now what's easier on the upper, a 0.001-0.0015" thermal interference fit done at 300-325F, or reusing an upper that was heated hot enough to release a barrel bedded in with loctite 609 or 620?
IMO thermal fitting a 7075-T6 upper is perfectly safe without degrading the mechanical properties of the upper provided you don’t overheat or overdwell the upper at the elevated temp.
