I want to weld or braze the pedal to the crank arm, but what metals are they? I ruined the threading in my crank arm, and would like to weld or braze it back on, instead of buying a new crank set. It is a cheap old bike, so the result does not have to be perfect, but it should be cheap and durable. 

I have welded on iron (an exhaust pipe and the like) in the past, and if this was just iron, I could fix it with no problems, but in this case it is not that easy. 

Pedal arm: I think it is of aluminium. It seems light. Not magnetic. Looks like aluminium. I have no other guesses. 

Pedal shaft: Steel, maybe even cast iron? I don't know how to tell the difference. It is definitely not aluminium. The material is stronger than the alu in the pedal arm, since the thread is intact. It looks sort of dirtyish / dark gray. Dark gray like some heavy parts in an engine which I believe are made from cast iron. Only slight rust despite a long life. Very far from the look of a stainless bolt. 



Can you help me figure out what metals they are made from, so I can move on to finding the appropriate welding or brazing? Thanks.
Welding aluminum and steel is not a DIY skill - read This. 

If you must repair rather than replace, a helicoil is the correct way to address the problem. A crank would be cheaper than the coil alone, let alone the time to fix it. Chemical bonding (AKA. Glue) is probably the only DIY solution. 

The issue I see is that when a pedal comes off while riding, it usually ends in tears (at best) up to a stay in hospital or worse. Why take the risk? 

Second hand cranks are readily available - I have a box of bit with at least 2 sets and an old bike with another if I needed it.  Your LBS will probably have something for a few dollars. Visit you local rubbish tip/recycle center - I can almost guarantee they will have an old, unride-able bike with usable cranks.