Although some engineers have had success in spot welding steel and aluminum together, it has largely been considered impossible to achieve reliable, continuous welds directly between the two dissimilar metals. That changed last Thursday, however, when Honda Motor Company announced that it has devised a technique for doing that very thing. The results, which include lower vehicle weight and better performance, can be seen in the 2013 Accord.
The Honda team developed a variation on Friction Stir Welding, in which metals are joined via mechanical pressure – it's the same technique that has been used for experimental steel/aluminum spot welds in the past. As Honda explains it, “This technology generates a new and stable metallic bonding between steel and aluminum by moving a rotating tool on the top of the aluminum which is lapped over the steel with high pressure.” The welds that result are reportedly as strong or stronger than those made using regular Metal Inert Gas welding.
Steel/aluminum subframes built with the new technique are said to be 25 percent lighter than those made entirely out of steel, which should translate into improved fuel economy. The process also made it possible to alter the structure of the subframe, so that the mounting point for the suspension could be relocated – this change has reportedly increased the rigidity of the mounting point by 20 percent, and thereby improved the car’s dynamic performance.
Additionally, the new process uses about half the amount of electricity as Metal Inert Gas welding, and the machinery it requires isn’t as large as that traditionally used for Friction Stir Welding – in fact, it can be attached to an industrial robot. The technique can also be used for aluminum-to-aluminum welding, without any hardware changes.
A new non-destructive inspection system, incorporating an infra-red camera and a laser, is used to check all of the steel/aluminum welds.
Source: Honda
http://en.wikipedia.org/wiki/File:Anand-FSW-Figure1-B.jpg
It heats the materials with friction and the rotating action of the tool mushes the two metals together
Use of Galvanised steel reduces corrosion... (zinc, so it may in fact be the zinc bonding to the aluminium, not the steel...)
Zinc alloy melting point ~420C Zinc-Aluminium Alloy Melting point 400-600C Aluminium Melting point 660C
Melting point Steel... 1370C
Phase change to alpha + austenite 738 C (to hot, for any of the other metals..)
Sure Stir welding does not melt the steel, it just heats it until it becomes plastic... (in the red range) which is, around the above chase change temperature....
Interesting that these materials are compatible.... have to be very careful not to overheat the aluminium....
However, hasn't stir welding been used for a while for welding dissimilar metals... (Often used to weld high strength steel shafts to cast iron, or to join castings.. or sifferent al/ Ti alloys... )
It appears that friction stir welding reduces the formation of the bad inter metallic compounds, which makes mig etc. welding of dissimilar metals a bad idea... This is most likely because it doesn't actually melt either of the parent metals (and doesn't transition the steel back to Gamma.), but plasticises them...resulting in a mixed phase, without the complete transition through the phases from solid to molten, as experienced by conventional welding... in the HAZ.
As for the galvanic corrosion issue, there is no worry of corrosion in the weld itself because there is nowhere for the electrolyte to go. The surface of the weld (indeed, the whole piece) would need some sort of barrier coating, sacrificial anode, or other protection from the elements. No amount of galvanization or surface sealant would prevent any corrosion in the weld anyways, as mixing in the weld zone occurs up to two or three times the depth of the pin...welding a chunk of galvanized steel would simply drive grains of zinc into the center of the weld.
With the different coefficients of thermal expansion issue, I believe the exceptionally fine grain size (and resultant flexibility) of the weld itself would prevent the thermally-induced fatigue from causing problems. The friction stir process isn't just used for solid-state welding, it is also used to increase flexibility in stamped sheet metal - one pass with the friction stir tool along the areas of the greatest deformation reduces or eliminates tearing during the forming process.