Welded armor is VERY common with tanks, as almost every tank from WW2, the Cold War, and some modern tanks have welded armor. When welding is done well, it can be one of, if not the best way of putting armor together. When done poorly, it is horrible. When welds are done poorly, they can break in combat, or just break when the tank is trying to get to combat. Most German tanks and Russian tanks used welding.
This way of putting steel together was used in WW1. A lot of early WW2 British tanks used riveted armor, which are just better nails or screws, as they keep the two things together until they break. but it got phased out in favor of better methods like welded and cast. While light tanks like the M3 Stuart would use them, and the Little Willie used riveted armor, and so did the later adaptation, Mother.
Cast iron had one major flaw. With cast iron, the armor usually wasn't as effective as welded. The upside was that it was REALLY easy to make tanks with this. American Shermans used this in WW2, as it took little time to throw together a tank with it. Instead of using precision tanks like the Germans, they went with numbers, which is why they used cast iron. You can tell if a Sherman is cast iron if the front hull is rounded, not sharp, which gives it a huge weak point, which was easily exploited by German tankers.
While technically a countermeasure, this armor is designed to counter HEAT warheads such as RPG-7 rounds. They work by letting the rocket hit the bar, making it trigger and fire the molten copper insides at the tank, not into it. It barely helps against normal shells, but they get stopped in their tracks against TOW missiles and other types of shells that detonate on impact.
While composite armor can provide the same effective armor as steel, it takes up more space. This is used on modern tanks as while it takes more space, it uses plastics, ceramics, and other materials that are lighter than steel to make the tank weigh less. Some tanks, like the TAM 2IP, use outdated hulls and put composite screens on them to give them more armor while keeping them light.
Spall lining, or spall liner, is a small steel/cloth sheet that catches spall, which is shrapnel that is created when a shell enters a tank. When the spall enters a tank, it can hit crewmembers and destroy components, which can disable tanks. A spall liner is a liner that catches the spall and keeps the crew safe. This is nice because it keeps the tank alive. It doesn't help against shells though.