Filler wire for Aluminium to CS TIG welding

Dear experts, we are having a project of aluminum heat exchangers with tube side MOC as aluminum SB 209. Here I want to make confirm the suitable filler wires for  TIG welding of Aluminium to SA 516 GR 60/70.

I saw your query on LinkedIn also.  Aluminum could be welded to steel a couple of ways, e.g friction stir welding, explosion bonding, not certainly by Arc Welding Process. Some of the problems are as stated below.

• With respect to fusion, welding says, GTAW iron and aluminum are not compatible metals. Their melting temperatures differ greatly: 1220°F for aluminum vs. 2800°F for iron.
• Both metals have almost no solubility for the other in the solid-state, especially iron in aluminum, and several brittle intermetallic phases can form (FeAl, Fe2A1, or FeAI3)'
• Consequently, resultant fusion welds joining steel and aluminum would be extremely brittle. 

For the sake of fabrication, your only option would braze welding or brazing. However, for a heat exchanger, I have doubt if Sec-IX welding qualification criteria could be met for such a joint.

Aluminum to steel combination is popular in ships, as ship superstructure contains extensive  Aluminum alloys for the sake of weight reductions. Friction or explosion bonded inserts are used as intermediatory to facilitate GTAW  or GMAW. This is not the choice for process equipments e.g heat exchangers.

If the MOC aluminum on the tube side, then the preferred tube-sheet would be either Aluminum or C.S Tube sheet clad with Aluminum.

Wherever C.S# Aluminum welding may occur,  facilitate the design change for Aluminum to aluminum welding.

Thanks.