You can't weld steel and aluminum together. But you can weld steel and titanium together. And you can also weld aluminum and titanium together. So, in order to weld steel and aluminum, all you need to do is use a layer of titanium.
Proposal (more of a gimmick than a proper innovation) is to build unnaturally light, unnaturally expensive, high-end custom knife blades using a small piece of high quality steel for several millimeters of edge, a thin titanium layer, and rest of the blade as an aluminum alloy.
A composite sword could also be built using a high-quality edge steel, a titanium buffer, and an aluminum alloy blade. The aluminum could be reinforced with small steel insets. It would all be welded together using electromagnetic induction. Such a sword might be lighter, faster, and even sharper and tougher than an all-steel blade.
It would even be possible to build a fiberglass, kevlar, or carbon composite blade with a steel edge. Removable, replaceable steel plates could be used to protect the composite core. This should allow for a flexible, lightweight, edge-retaining sword. It could also be reconfigured as an un-edged weapon.
Why does this matter? For absolutely no reason other than the fact that people would buy it.
