Laser welding of thin Al layers offers a silver-free and highly flexible option for the interconnection of Al-metallized solar cells. Welding requires the melting of the Al layers in order to form a reliable electrical and mechanical contact. Here, we investigate the process driving the melt front of the aluminum, which is attached to a transparent substrate, toward the interface between the two Al layers. In experiments, we observe two different mechanisms depending on the thickness of the irradiated layer. In the case of Al layers thinner than 5 μm, a melt-through of the Al-layer is observed, whereas for thicker layers, thermal expansion causes a breakage of the surface and ejection of molten Al, which enables the contact formation. Using simulations that are based on the finite-element method, we instigate both mechanisms. The simulation results match the experimental observations within the measurement uncertainty. In case of thin layers, the simulation shows that the process is limited by thermal diffusion. For thicker Al layers, the onset of melting on the irradiated side initiates the breakage of the surface and the ejection of the aluminum.