AbstractThe phenomena occurring in a weld seam during advancement of a laser beam over a metallic component are still under dispute. The occurrence and evolution of porosity and the occasional blowout of melt need to be understood. Here, a recently developed X‐ray tomoscopy setup is applied, providing one hundred 3D images per second to capture the temporal evolution of the melt pool in an AlSi9Cu3(Fe) die‐casting while a laser beam advances. The number of pores, their size, shape and distribution are quantified with 10 ms time resolution and reflect a complex dynamic pattern. Apart from conventional welding, a variant involving a dynamic beam modulation superimposed onto the linear motion is studied. Reductions of porosity and surface roughness are observed and explained by increased pore mobility and stepwise degassing as the beam repeatedly cuts through pores. The keyhole formed in the melt pool integrated over 10 ms is represented in 3D.