A popular approach for the 3D printing of metals is to pass a laser spot over a layer of powder, which then melts and fuses together. The printer repeats that process for additional layers of powder to build up the desired part. The higher the density of laser power, the faster the printing process. The problem is that intense laser illumination can also introduce gas pockets into the material and degrade the mechanical properties. Just what constitutes too high a laser power density has been a matter of trial and error until now. By imaging with synchrotron x rays the formation of vapor pockets in irradiated metal, Anthony Rollett of Carnegie Mellon University, Tao Sun of Argonne National Laboratory, and their colleagues have found a simple rule to prevent laser-induced defects.

X-ray absorption images of a metal under laser illumination in the conduction (left), intermediate (center), and keyhole mode (right). The melted region is indicated by red shading, and the vapor pocket is the lighter area within the melted region.

Using lasers to print 3D metal objects resembles laser welding, which operates in one of two modes depending on the power density of the laser. In conduction mode, shown in the left panel of the figure, a laser melts a shallow, hemispherical region of the metal. In keyhole mode (right panel), a laser with higher power density penetrates deeper and boils the material until a narrow vapor pocket is formed. The same modes occur in 3D printing when the laser melts the metal powder. To avoid the introduction of pores in the final product, manufacturers aim to remain in conduction mode. But in the absence of sophisticated imaging techniques, they won’t know which mode they are in until after the printing is done.

Using ultrahigh-speed synchrotron x-ray imaging, the researchers identified the dynamics of the transition from conduction to keyhole mode for both a stationary and a moving laser. For a stationary laser, all power densities above a threshold resulted in a keyhole mode with vapor holes of the same general shape. Sun and his colleagues also observed an intermediate power density regime (center panel) that was out of conduction mode and characterized by a fluctuating, conical vapor pocket. For a moving laser, the vapor depression’s shape varied substantially with the power and velocity of the laser. But the transition out of conduction mode was still straightforward: In power–velocity space, the transition is defined by a straight line, with higher power thresholds for faster laser velocities.

What was the biggest surprise? Nearly all the commonly used combinations of laser power and velocity are not in the conduction mode regime. Manufacturers had previously blamed the resulting defects and inhomogeneities on the type of metal powder or the laser power alone. The linear dependence introduced by Rollett and his colleagues offers a simple tool for manufacturers to reevaluate, improve, and potentially speed up their 3D printing. (R. Cunningham et al., Science 363, 849, 2019.)