There are as many different welding robots as there are different welding applications. Actually, robots are more than applications because different industrial robot manufacturers compete in this field. This is not without a reason and this reason is quite strong. Robotic welding comprises around 25 percent of the industrial robots market.
Copper is a good conductor. Better than many other metals. If you place a steel plate between two copper plates and connect those copper plates to electricity there's a good chance that the steel plate will start melting. This principle is used in spot welding.
Steel (or some other metal) parts that need to be connected are squeezed between two electrodes. Then those electrodes are switched to electricity. In this process the squeezed parts melt together.
Typical thickness for parts to be welded with this method ranges from 0.5 to 3.0 mm. As you can imagine - the thickest are the parts the more electricity needs to be conducted through copper electrodes. Also, the squeezing force that needs to be applied is rather large. Therefore, the spot welding tool, usually, is rather heavy. You can see a manual spot welder in a picture above.
So, robot arms used for spot welding have to be able to lift heavy weights. According to FANUC their robots that can be used for spot welding applications are F-200iB, M-900iA Series, R-2000iB Series, R-2000iB/200T, R-2000iA/EW Series. If you are looking for a used one then you may want to look at older R series robots, they should be sturdy enough.
KUKA, on the other hand, offer to use their KR 240-2, KR 210-2 K, KR 210-2 K, KR 200 comp, as well as their heavy duty models for spot welding. There are also robots with smaller payloads that can be used for spot welding applications, however, I would do some serious thinking about those. A spot welding robot should be powerful enough.
Last but not least - there are MOTOMAN and ABB spot welding robots. MOTOMAN models: HP20-6, HP165-100, ES200RN-120, ES165N, ES165RN, HP165, HP165R, ES200N, ES200RN, HP200, HP200R, HP200T, HP350-200, HP350, UP400RN, HP500, HP600. A number in a name of a robot indicates its payload. So again - low payload models should be double-considered.
ABB models: IRB 6620, IRB 6640, IRB 6650S, IRB 6660, IRB 7600. Again, if you are going for a used robot then older ABB 6000 series' robots should do the job.
Arc welding is an umbrella term which includes different popular methods of welding like MIG, TIG, MMA also called stick welding, and others. However, all these methods have something in common, namely, an electric arc. An electric arc forms between an electrode and the base metal thus melting it. The electrode can be non-consumable and consumable. In case if an electrode is a consumable one it melts down too and forms a weld.
The most popular non-consumable electrode arc welding method is TIG. TIG stands for "tungsten inert gas welding". In this case an electrode is made from tungsten. Because the electrode does not melt down filler material is needed to form a weld. Also, an inert or semi-inert gas mixture is used in the process.
Consumable electrode methods include MIG (metal inert gas) welding, MMA (manual metal arc) welding also called stick welding, SAW (submerged arc welding) and others. All these methods have at least one thing in common - the electrode melts down and forms a weld. In case of stick welding, the electrode is a metal rod covered with flux. In case of MIG welding, continuous electrode wire and inert gas are fed through a welding pistol.
As you can imagine this process creates a lot of heat, fumes and other hazardous things. So a robot usage is perfectly justified in this case. Also, the percentage of defective articles is reduced to a minimum when welding robots are used.
What about robots? Well, a tool is rather light in this case so most robot arms can be used for this task. Except for those that have been designed for a specific task, for example - painting or picking. So, there are many new and used robotic arms from FANUC, ABB, KUKA, MOTOMAN and others that can be used as arc welding robots.
There's only one last thing. You should check out if the controller software provided with the robot arm is able to do arc welding operations effectively. If there's a special piece of software designed for arc welding it should be enough to do the task. Anyway, I've heard that older versions of Japanese robot controllers don’t do well in this field.
If you are serious about arc welding robots, there are specifically tailored solutions designed for arc welding like IRB 1410 ArcPack from ABB and “Arc welding solution arm” from FANUC. I'm pretty sure that KUKA and others have something similar too.