I hate to bring problems up for you, but you can't weld aluminum to steel or vice versa. The two metals will not mix. Also to weld aluminum your tig welder must have high freqincy on it also. Mosy welders don't. If you are going to use sheet steel for the shelves and want to use the aluminum bars as the post, there is a way you can do so, however it will require buying some steel pipe with the inside diameter a hair larger than the aluminum bars. drill a series of small holes 1" apart down one side of the pipe and then cut into small nipples between the holes. leave about 3/4" on one side of the hole and 1/4" on the other. Tig the pipe to the plate metal with the 3/4" toward the plate where you want the post to go on top and bottom of the bottom shelves,( if you want some short legs on the bottom of the rack) on bottom of the top shelves and on top and bottom of all the shelves in between. Cut the bars to fit and then use self tapping screws to attach them as you put them together. All it will only take is a small tack weld on both sides of the pipe. Maybe a 10% to 15% weld. There isn't any sense using up a lot of argone gas, welding rods and time welding them 100%. It is also a question of how heavy the sheet steel is, how far between the uprights, and what will be put on them as to whether you will need bracing such as angle iron under the front and rear lip of the shelves. The heaver the load, the bigger the angle needs to be but 3/4" x 3/4" should be enough even if you have something as thin as 20 gauge sheet for the shelves. If you are useing something like 1/4" plate for the shelves, you shouldn't even nee the angle at all unless you want the post spread a part a long ways. Good luck and God Bless Dennis. P.S. If you have any more questions e-mail me and I will be happy to help. The extra that you added didn't help me and I doubt anyone else at fifureing out what you are trying to do. If you will e-mail me, I will give you my home number and you can call me and then maybe we can figure out what you need to know. I mean I have some questions about what you are talking about and can't give you a correct answer with out my questions being answered and it will be a series of question, answer, question, answer, etc. I'll be happy to help you if you will e-mail me. If I can that is. lol. I did find out from someone that read your question that you don't have to have Hi Fre. if you use helium gas instead if argon gas. First time I ever heard of it after 35 years of welding but the guy seemed like he was correct. Learn some new trying to answer someone elses question. Oh well. Lol