Some metals cannot be welded to each other
Dissimilar metals, and the brittle compounds that form between them.
Some metals cannot be welded to each other because melting two different metals together can produce entirely new brittle chemical compounds at the boundary between them, where two similar alloys would simply mix. Those compounds have none of the useful strength or ductility of either original metal, so the act of joining the two becomes the reason the joint fails.
Levy and Salvadori's Why Buildings Fall Down works through real collapses, and many begin at a joint somebody assumed was fine. The assumption that any two metals can be melted and joined given enough heat and skill is exactly the kind their book keeps returning to.
When the weld metal is a new substance
Welding two pieces of the same alloy works because the molten metal, once it cools, resolidifies into essentially the material it started as, reconnected across what used to be a gap. Welding two different metals asks the two molten metals to mix at the boundary and solidify into whatever compounds their combination of elements forms. For some pairings, aluminium and steel being a familiar case, the result includes intermetallic compounds: chemically distinct substances with a crystal structure and mechanical behaviour unlike either parent, often hard, brittle and prone to cracking under stress either original metal would have absorbed.
A bad same-metal weld is still, chemically, the metal everybody expected, only poorly processed. A bad dissimilar-metal weld can be a substance nobody wanted, formed by the chemistry of the combination with no mistake in technique at all.
Oil and vinegar, and the opposite problem
Shaking a bottle of vinaigrette mixes oil and vinegar into a cloudy blend that separates again as soon as it stands, because the two liquids never combine chemically however hard they are shaken. They sit as droplets of one inside the other until gravity sorts them out. Some metal pairings behave in the opposite and more dangerous way when melted together, combining too readily into a new, unwanted compound.
That is arguably the worse outcome. A separated vinaigrette announces the problem plainly, the oil floating visibly above the vinegar the moment the bottle is set down. A joint that has combined into a brittle compound gives no such warning, presenting a smooth, fully bonded surface that can look sound right up until it is loaded and fails.
Chemistry problems and skill problems
Certain pairings are recognised across the welding trade as essentially unweldable to each other by conventional fusion methods, because the compounds they form are reliably brittle however carefully the weld is executed. The failure belongs to the chemistry, so a more experienced welder or a refined technique changes nothing. The distinction matters because the two kinds of problem call for different responses. A skill problem is solved by better welding. A chemistry problem is solved only by changing what is being melted together: the filler, an intermediate material, or the joining method itself.
Joining such pairs therefore often routes around fusion welding altogether, using a mechanical fastener, an adhesive bond, brazing with a filler chosen to avoid the problem compound, or a transition piece of an intermediate material that welds acceptably to the metals on either side. Identifying the pairing early is the difference between choosing the right method from the outset and discovering the wrong one after the joint has been built into a larger assembly and put under load.
A transition piece works on the same logic as the vinaigrette in reverse. Each of its faces meets a metal it combines with well, so no single boundary in the finished joint has to bring together the two metals that were never going to cooperate. The awkward pairing is simply never asked to meet directly.
Pairings that can be joined after all
Not every dissimilar pairing is doomed. Decades of engineering effort have gone into ways of joining different metals where a naive fusion weld would fail: solid-state processes such as friction welding, which join the parts with heat and pressure below their melting points and so give the two metals little chance to mix into a compound, specialised fillers designed to avoid the worst compounds, and careful control of how much of each metal mixes at the joint. The blanket rule that dissimilar metals cannot be welded is shorthand for a more specific and checkable claim, that a given pairing joined by a given process forms a given compound with known properties. Design decisions should rest on that specific claim, checked for the pairing actually in front of the engineer. Left unchecked, the blanket version tends to be corrected first by the joint's own failure.