I don't know what you mean by "normal welding", but processes which use argon include TIG and MIG. The argon is used to create an atmosphere around the heated metal to prevent atmospheric gasses from making the weld brittle. Other processes use fluxes to produce shielding, such as SMAW and SAW welding.
The strength of the weld will depend among other variables on joint design, filler metal yield strength, compatibility of base metal with filler metal, and execution of the weld. Different mixes of Argon are used in welding. Some are 100% argon, some 75%/25% argon/C02, and others are mixed with helium. Which mixes to use depend on the welding process, material, finish desired, welding speed and personal preference.