Bulk-Explosion-Induced Metal Spattering During Laser Processing

Spattering has been a problem in metal processing involving high-power lasers, like laser welding, machining, and recently, additive manufacturing. Limited by the capabilities of in situ diagnostic techniques, typically imaging with visible light or laboratory x-ray sources, a comprehensive understanding of the laser-spattering phenomenon, particularly the extremely fast spatters, has not been achieved yet. Here, using MHz single-pulse synchrotron-x-ray imaging, we probe the spattering behavior of Ti-6Al-4V with micrometer spatial resolution and subnanosecond temporal resolution. Combining direct experimental observations, quantitative image analysis, as well as numerical simulations, our study unravels a novel mechanism of laser spattering: The bulk explosion of a tonguelike protrusion forming on the front keyhole wall leads to the ligamentation of molten metal at the keyhole rims and the subsequent spattering. Our study confirms the critical role of melt and vapor flow in the laser-spattering process and opens a door to manufacturing spatter- and defect-free metal parts via precise control of keyhole dynamics.

Popular Summary

In laser welding, machining, and 3D printing of metals, incandescent droplets are frequently ejected from the laser’s path. This phenomenon is commonly known as spattering. Although these fireworkslike sparks are strikingly beautiful, they can cause structural defects—rough surfaces, craters, and porosity—resulting in the degradation of material properties. For years, researchers have been trying to understand the origins of spattering by doing fast optical imaging experiments. But, limited by their penetration power, visible light and infrared imaging could not capture the subsurface structural dynamics. Therefore, many key processes that lead to the initial formation of spatters have never been directly observed, and the occurrence of those extremely fast spatters still lacks affirmatory explanation. Here, we use extremely bright x rays to probe the spattering process.

At the U.S. Department of Energy’s Advanced Photon Source, high-flux, high-energy x rays allow us to see into the metal sample and record the dynamics of the laser-induced keyhole with micrometer spatial resolution, subnanosecond temporal resolution, and megahertz imaging rate. We elucidate the major forces involved in different stages of spattering and identify the substantial role of melt and vapor flow in this process. More importantly, we discover that the explosion of some unusual protrusions arising from the front keyhole wall drives the initial escape of molten metal around the keyhole rims and the subsequent ejection of fast spatters.

Our study advances the fundamental understanding of the laser-induced spattering phenomenon and paves the way for manufacturing defect-free metal parts via the control of keyhole dynamics.

Figure 1

MHz x-ray images of metal spattering of Ti-6Al-4V during laser processing. Four events can be extracted. Event no. 01 (sky blue dashed rectangles): A protrusion forms at the top surface and runs down along the front keyhole wall, accompanied by the keyhole morphology changing from a $\mathsf{J}$-like shape to a reverse-triangle-like shape. Event no. 02 (purple dashed rectangles): A following protrusion appears, grows, and collapses around the horizontal center of the keyhole. A minikeyhole on top of the protrusion is outlined by a light yellow dashed curve. Event no. 03 (dark blue arrows): The local curvature on the rear keyhole wall changes. Event no. 04 (light green dashed and solid rectangles): Melt ligaments form, elongate, and break up into spatters (light green dashed circles numbered SP01–SP05). A separate event KP (sky blue solid rectangles) describes the formation and vanishing of a keyhole pore. The laser beam scans from left to right, with spot size of approximately $80\mu \mathrm{m}$ ($1/e^2$), power of 210 W, and scanning speed of $500\mathrm{mm}/\mathrm{s}$. The imaging frame rate is $1.087\times {10}^6$ frames per second, synchronized with the x-ray pulses. Each individual image is generated by a single x-ray pulse (pulse width approximately 100 ps). All images shown here are background corrected using the images collected before the laser melting. The contrast is then reversed to highlight the events around the keyhole. Frame-by-frame images as well as schematic illustrations are documented in the Supplemental Material Figs. S3 and S4 and Video S2 [6].

Figure 2

Bulk explosion inside a keyhole. (a) Schematic illustrations of the sequential events that lead to melt ligamentation. At $t_1$, a keyhole of reverse-triangle-like shape (RTS, marked using sky blue dashed rectangle) forms. The rear wall of this unique keyhole is directly exposed to the incident laser beam, and the generated vapor plume (olive green arrows) travels upwards towards the front keyhole wall. Upon impact, the existing protrusions (e.g., Pro. no. 01 and Pro. no. 02) transform from a dome shape slightly leaning downward (denoted by ${\text{Pro}}_{.}^{-}$) to a rod shape tilting upward (denoted by ${\text{Pro}}_{.}^{+}$). At $t_2$, a tonguelike protrusion ($P$) forms around the horizontal center of the keyhole with a minikeyhole on its top surface. The bulk explosion of the tonguelike protrusion (purple dashed circle and arrows) releases a mixture of gas and fine droplets. At $t_3$, the high-momentum mixture reaches the rims of the keyhole. Simultaneously, the local curvatures (dark blue dashed circles) of the keyhole walls change, and thin melt ligaments (Lig, olive green dashed and solid rectangles) appear. After the explosion, the remains of $P$ protrusion (${\text{Pro}}_{.}^r$) continue to flow down and quickly vanish. The last panel shows an instant keyhole pore. (b) Simulation of laser absorption intensity with and without the tonguelike protrusion via ray tracing. (c) Keyhole depth as a function of time. Most local minima below a depth threshold correlate with the formation of reverse-triangle-like keyholes. The point marked by the sky blue dashed rectangle is the case shown here. The inset in (c) shows the distribution of the downflow speeds of the protrusions on the front keyhole wall, with the sampling volume of 182. (d) Relationship between the action time of the tonguelike protrusion $t_3\text{−}{t}_2$ and its formation time $t_2\text{−}{t}_1$. Data points in this plot are collected from x-ray videos (frame rate varies from 1.087 to 5 MHz) of multiple spattering processes.

Figure 3

Spatter behaviors and pressure field. (a) Trajectories of five ejected particles in one spattering process. SP01–SP03 are forward-flying spatters, and SP04 and SP05 are backward-flying spatters. The reference black dashed lines highlight the curved trajectories of the spatters SP04 and SP05. The diffused sky blue background on the left side of the laser indicates the vapor plume zone arising from the front wall of the keyhole. (b),(c) Vertical and horizontal travel speeds (upper panels) and the calculated pressures on melt ligaments and spatters (lower panels). The half-solid symbols in each plot indicate the times when the thin melt ligaments appear. The solid symbols mark the times when spatters separate from the melt pool. The negative pressure represents a net force that slows down the motion of the liquid structure, while the positive pressure accelerates the objects. The inset of (b) is a closer look at the vertical pressure field the spatters experience. (d) X-ray images of the keyhole morphology. The top region of the front keyhole wall is highlighted. The protrusion appearing between 149.96 and $155.48\mu \mathrm{s}$ increases the local laser absorption, creating a stronger vapor plume that is responsible for the increased pressures on spatters SP04 and SP05. All images shown here are background corrected using the images collected before the laser melting. The contrast is then reversed to highlight the keyhole dynamics.