Hello,
I am working on an experimental project that would require slightly modifying the outside of an AR receiver, adding material to the rear area where the receiver extension threads in, specifically to the top where it is flat.  I cannot weld, especially not aluminum.  Would it be reasonable to use a high temp silver solder to do this?  Would I just need to strip off the anodization?  Would I necessarily need to use 7075 or could I use another type?  Would applying this kind of heat to this area cause a problem w/ the temper?

What do you think about potentially removing the solder and block later?

Please let me know if you have any tips, I have some high temp solder I got from brown ells and I suspect I will give it a go on an SBR'd lower.

Thank you.

This stuff works well, just follow the instructions.

https://www.alumiweld.com/

You could alumiweld (solder) it on. You would need special aluminum solder, not high temp silver solder.

Would have to have the area stripped of anodized and cleaned.
the soldered area would be weaker than the parent 7075.

.

Will your project not have a charging handle?

I agree, they do work well, but they have a huge learning curve.  I prefer map gas or oxy-acetylene for them.  My host object tends to melt prior to the stick properly flashing out with most techniques I tried.  

I have not tried it on a gun, I fixed a pretty thin aluminum hull boat with it using the yellow map gas bottle and made sure to use the stainless brush and it is still holding after several years.

Side charger, non reciprocating

Yikes!

Sounds like you need to find a welder that can do aluminum

7075 apparently doesn't weld particularly well, Braceman being the exception to the rule.  6061 is weldable, and 80% lowers are available in 6061. OP could go that route, and make whatever he's welding on out of 6061, weld it to a 6061 80% lower, then if the weld is good, finish out the lower.

Alternatively, since he's welding, make what he wants out of steel, and weld it to a Flat Spot steel lower.  Probably easier, much heavier.

I found a local welder recommended for aluminum by a local gun shop.

Who is this "Braceman?"

Yeah, googled it a bit I had no idea 7075 was so risky to weld, something about grain structure and cracks...

I guess I should look for a cheap 6061 lower, if such a thing exists...

Maybe I should go back to that alumiweld stuff, but you have to heat the parts up to 730f!!  Ive no idea if that will ruin the temper of a lower, but from what I know about steel metallurgy I'd be surprised.

Braceman is a pretty reputable smith and well known on the retro forums here. The guy can weld the crushed remains of a lower to an 80% to make a lower with legit M16 markings.

I see, so that is his user name here.

So he will weld the mag well sidewall of a broke lower into a new lower.  Seems somewhat less structurally important than welding at the rear area of the upper, its weakest point to begin with.  But perhaps Ill see what he says about it.

Thanks

Zinc is not known for outstanding strength.
Find a welder.

A load on a block like this creates both a shear and a tensile load.

The shear is sideways and attempts to 'slide' the block over the area of the joint.
It is usually NOT the failure node that occurs if the area is large compared to the protrusion of the block from the surface.

The tensile load occurs as the block attempt's to rotate around the bottom edge of the attachment.
If the block 'sticks out' from the surface a significant amount compared to its attachment area this mode will fail.

The block starts to lift opposite the lower surface and the extra protrusion increases the leverage of the force.

It is actually common for even things held together with screws.
Shearing off multiple screws does not usually occur.
The screws on one side fail in tension often from threads stripping out.

Their cross section area to resist tensile failure is the smallest section loaded.