A Couple of Questions
Is welding only done with a welding torch?
There are many other forms of welding which do not use a welding torch to connect materials. Ultrasonic welding uses high frequency vibration to connect thin metal sheets or plastics. There is also roll welding where two pieces are run through two high pressure rollers to clamp them together. Electricity can be used to weld metals but applications are a bit limited because the cost can be high.
Can all metals be welded together?
Many types of metals out there can be welded to create a strong bond but there are exceptions to this rule. For example, aluminum and steel cannot be welded together because they have different melting points. As steel takes roughly twice the temperature of aluminum to weld, the aluminum becomes a liquid and will not stay near the steel. But beyond this, when aluminum and steel fuse together they form a brittle bond that is not very effective.