Automakers are working hard to find cost-effective ways to join dissimilar metals, in particular, aluminum and steel.
As car makers strive to lower the weight of vehicles, physics and chemistry have become one of the major challenges. Automakers want to use the best available material for each component of a vehicle’s construction. They have learned that all aluminum or all steel is not the best way to build a vehicle and that in order to produce a cost-effective vehicle with light weight, joining aluminum to steel becomes inevitable.
However, there are problems in doing so. Dissimilar metals are difficult to join. One problem is that as metals are heated and then cooled during welding, the different materials expand and then contract at different rates. This builds in weaknesses and stress in the material that can later become cracks and fractures. A second problem is corrosion. When two dissimilar metals are joined together, or even touch one another, they begin an electrochemical process that results in corrosion. This was discovered by Luigi Galvani in the late 1700s. Engineers have been working with ways to limit and prohibit this type of corrosion for centuries. When the French built the Statue of Liberty, designers used a shellac between the iron inner and copper outer layers. It lasted about 100 years but was recently discovered to have failed.
In automobiles, designers want to use sheets with few folds and curves made of aluminum, for example, as a door panel. They also want to use complex forms made of steel such as the inner structure of a door. Joining these together is tricky. Honda found a novel way of joining the two metals for the Acura MDX. The idea is to fold, or hem, the aluminum and steel over one another twice. That creates a “lock seam.” To beat the inevitable corrosion Honda patented a way to use an adhesive to separate the materials and used a coated steel to prevent the materials from conducting electricity and thus corroding.
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While helpful for that specific application, automakers want to be able to use existing processes and tooling in their factory as much as possible. GM and other companies first began to design and patent ways for the same spot-welders that were used to weld steel to steel to also weld aluminum to aluminum. GM designed a copper electrode that had small rings that could break through the aluminum oxide layer on sheet aluminum to get to the pure aluminum underneath. That solved the problem of using one tool to weld both metals, but another problem crept in. Going back and forth was causing small bits of the material to be transferred which could cause corrosion, so GM developed and patented an abrasive cleaning step in between to wipe away the bit of leftover metal when the tool was switching to a new metal.
Automakers worked hard to find a way to simply weld aluminum and steel directly to one another. Honda and GM took different approaches and have different results, but both have recent important breakthroughs. Honda’s method is called friction stir welding. This method actually creates a compound (or alloy) of aluminum and the iron-based steel between the two metals. Honda’s technical description of how corrosion is prevented is complex, but in effect, the Honda process requires that coatings be applied to both metals and that a sacrificial section is introduced. In other words, it won’t last forever, but it will last longer than the car does. Honda has used this method to join the Accord’s aluminum and steel subframe sections for five model years now.
In 2015 General Motors began spot-welding aluminum and steel together. It was tried first on the Cadillac CT6 premium sedan in a section of the seatback.
During a recent New England Motor Press Association (NEMPA) meeting, former NEMPA president and current BestRide managing editor, Craig Fitzgerald, asked a representative from GM how it manages the Galvanic corrosion problem.
GM’s Charlie Klein artfully dodged the direct question by summarizing that it is manageable. Perhaps the question of how steel and aluminum in cars can be joined can be summed up by a lesson once explained by a seasoned handyman. The handyman was using steel screws to secure two sections of an aluminum gutter. The homeowner, an engineer, said: “Won’t that cause Galvanic corrosion?” The handyman answered, “Yes, but not in our lifetimes.”
