Surprisingly, yours is a very common question that I feel should be addressed. While solid-state joining processes like friction stud welding and flash butt welding can join aluminum and steel together, you will not be able to do this using GTAW or GMAW. In this case, it would be better to actually bolt the parts together. However, before you bolt these parts together, it pays to review a lesson from your high school chemistry class. Try to recall the electromagnetic field (EMF) chart, which showed the electrochemical potential of each of the metallic elements. When you put two dissimilar metals in electrical contact in the presence of a conductor such as salt water, a voltage is produced and the metal that is higher in the EMF chart is preferentially corroded. This is called galvanic corrosion. When you join aluminum and steel together, the aluminum will be preferentially corroded. Surprising, isn’t it, since we think of aluminum as being more corrosion-resistant than steel? In fact, aluminum is more resistant to general corrosion than steel. What I am describing here is a preferential corrosion that occurs locally at an aluminum/steel joint. So how do you avoid galvanic corrosion? You electrically insulate the aluminum from the steel. The best way to do this is to use bolts and nuts to make the joint. Use an isolating coating or paint on the aluminum and the steel to isolate them electrically. However, all of your effort will go for nothing if you just bolt the joint together. You must isolate the bolt and nut from the aluminum and steel. You do this by putting a nonconducting plastic washer between the bolt head and one side of the joint and another nonconducting washer between the nut and the other side of the joint. If the pieces being bolted are thick enough, you should also buy a nonconducting sleeve to fit on the shaft of the bolt to keep it from making contact with the steel or aluminum. So what sort of bolts and nuts should you use—steel, aluminum, stainless steel? If you have done a good job insulating the nut and bolt from the aluminum and steel, it doesn’t really matter.