A fastener or fastening is a hardware device that mechanically joins or affixes two or more objects together. In general, fasteners are used to create non-permanent joints; that is, joints that can be removed or dismantled without damaging the joining components. Steel fasteners are usually made of stainless steel, carbon steel, or alloy steel.
Typical fasteners (US quarter shown for scale)
Structural bolt DIN 6914 with DIN 6916 washer and UNI 5587 nut
Welding is a fabrication process that joins materials, usually metals or thermoplastics, primarily by using high temperature to melt the parts together and allow them to cool, causing fusion. Common alternative methods include solvent welding using chemicals to melt materials being bonded without heat, and solid-state welding processes which bond without melting, such as pressure, cold welding, and diffusion bonding.
Overhead stick welding
The iron pillar of Delhi, India
Bridge of Maurzyce
Acetylene welding on cylinder water jacket, US Army 1918