People ask this standing over all kinds of things: a cracked steel bracket, an aluminum boat trailer, a stainless railing, an old cast iron part. The honest answer is that most common metals can be welded, but the metal you are working with decides everything else, which process, which filler rod, and how fussy the job is.
This is the metal most things are made of: trailer frames, railings, brackets, structural beams, gates. It melts at around 2,500 degrees F, it is forgiving, and it welds easily with MIG, stick, or flux-core. If you have a broken steel something, it is almost always fixable, and usually ends up as strong as new or stronger.
Stainless resists rust, which is why it shows up on railings, food and kitchen equipment, marine hardware, and anything outdoors that has to stay clean. It melts a bit hotter than mild steel, around 2,750 degrees F, and it needs clean technique and the right matching filler so it keeps its rust resistance. TIG is the usual choice when the weld shows.
Aluminum is light and great for boats, truck beds, trailers, and racks. Here is the catch: it melts at only about 1,220 degrees F, less than half the temperature of steel, and it pulls heat away fast. That combination makes it trickier to weld without warping or blowing through. It takes TIG or a MIG setup built for aluminum (a spool gun), plus clean metal. Doable, just not a beginner's metal.
Cast iron (engine blocks, old vises, wood stoves, antique machinery) is loaded with carbon, which makes it hard, brittle, and fussy about sudden heat. It can be welded, but it is one of the trickiest repairs in the trade and where a lot of cheap fixes fall apart. We wrote a whole honest rundown here: Can You Weld Cast Iron?
The real first step on any job is identifying the metal correctly, because that decides the process and filler. Guessing is how welds fail. That part is on us.
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