You Must Unlearn Everything You've Learned

After some early success welding the inside joints, our team welder ran into trouble joining all the pieces of the front wheel supports together. Aluminum is very difficult to weld.

The surface of aluminum is made up of aluminum oxide, a tough surface that protects the interior aluminum from corrosion. It melts at around 3700 degrees, hotter than the melting point of steel. The aluminum underneath melts at around 1200 degrees. So welding aluminum requires punching through the skin at high temperature and somehow not completely melting everything underneath. The analogy I was taught is that it's like welding a banana.

At the same time, keeping extraneous gases out of molten metal is critical to the strength of the weld. Any contaminants that mix in while the aluminum is being welded will weaken the joint at best or make it impossible to weld at worst. He dealt with the latter for weeks.

His experience welding steel helped a little, but it took him a lot of thought and experimentation to figure out his process. It basically boils down to obsessively cleaning all the surfaces with acetone between each prep step.