Different metals and alloys require different voltage and current charcteristics, the big thing I see is switching from negative ground to positive ground for aluminum and using flux core wire (most common I see) as far as voltage and current settings,, the aforementioned miller website is good, and a few I also would add ,,
http://www.weldreality.com/(explanations ,,weld processes,, some settings haha a good guy I know)
also the AWS site (American Welding Society) is our defacto,,but they are heavy on commercial charge money for their books but they set the standard in the USA,, they are one of 2 main standards as a American Welder I have had to meet (the other is US Military Spec.)
http://www.aws.org/w/a/Another I googled (I don't like the site but a couple videos and explanations.)
http://www.weldingtipsandtricks.com/ My humble experience is that a good weld should be stronger then the material your welding.
Different alloys and metals,, its nice to know if its mild steel or stainless or aluminum or cast iron, the hardness, if it has chrome or other alloys or if its a high tensile ,,or uhm most large companies have a metallurgist ,,who can do scratch or etch tests to determine what,,a piece is,, if not known then what process is best (cost, time, technique,,) then proper settings usually the welding engineers determine what process,, if the company is big otherwise the welders usually individualy determine settings,, gas, wire or electrode sizes, and types,, then voltage and current,, or frequency ,trim,, etc
The majority of welding is now GMAW (Gas Metal Arc Welding) genericaly called mig (Metal Inert Gas, which it is not always,,so this is a misnomer) -think the guy holding a welding gun mostly mild steel, high tensile steel and stainless, you can do aluminum,,but this is often the cheap high volume production process most used in my experience. (main transfer types are Spray -very good quality if done right but literally sprays material deposition on and requires most skill but very fast and high quality but due to high travel speed most tough to do repetitively, I used to make Helicopter parts using this then we had them x-rayed,
Pulsed spray is similar to Spray but less travel speed, and is gaining popularity here also called Rapid-Arc,, tough to set the welder unless the welder is a Pulse type, uhm great for high quality production,,
Most common used is Globular Transfer (imagine small balls of molten metal) Globular is okay for most steel and common,, Then Short Circuit,, I have used this for welding odd out of position type robot welds,, on thin materials,, very sharp,,quick,, not often used by me anyway,,
(the videos and pics on welding reality will explain much better then I)
Tig is on tv alot, the shows where people build motorcycles,, tig can do most things GMAW can, with better quality but takes more practice more time and better skill, (Tungsten Inert Gas)
Arc is used most often for heavy thick (2 inches or more thick steel) Construction, or pipe and where a emergency exists that a alternative is impractical,,(maybe the second most primitive but a art unto itself,,)
Think a guy with 2 cables,,one grounded and a rod on the other ,,this is capable and can be good quality but takes the most skill, in my opinion,,,knowing which rod for the job,, (there are books and charts,, but no shielding gas,,can make things tough and the older welders had touchy heat controls,, uhm a powered unit I was taught to go gas engine powered, that diesel powered ones would idle up and down and the welder would suffer with heat,,deviations,, I am sure newer regulators may have solved some of this but,,
I like big solid state transformer style,,, the inverters are lighter, but a employee spilling a can of pop into one means big money,, and they last on average 3-6 years,,whereas the transformer style we had some from the 1950's open them up,, after unplugging them ,,spray soap and water,, oil the slide bicycle chain arm,, and they last ,,, cheap solid and heavy so hard to steal,,
good luck, I remember people building small mig machines with flux core using transformors from,,(microwaves)? but usually like 90-120 amp machines,, so they could weld thin metal,,or medium if they layered,,
