for cars or automotive welding, some of the job is finished by submerged arc welding process. SAW process needs welding flux and wires. basic welding flux can provide excellent mechanical properties, while acid welding flux can provide excellent welding performance. see this http://www.ylflux.com/saw_flux.html to know more about the knowledge about fluxes.
aluminum plate welding is not an easy job. Al under high temperature would react with atmospheric oxygen, and turns into alumina, which is an obstacle for good welding result. So, gas shield welding is necessary, SAW welding for steels is not suitable for aluminum. Argon arc welding method is recommended.