Laser melting modes in metal powder bed fusion additive manufacturing

In the laser powder bed fusion additive manufacturing of metals, extreme thermal conditions create many highly dynamic physical phenomena, such as vaporization and recoil, Marangoni convection, and protrusion and keyhole instability. Collectively, however, the full set of phenomena is too complicated for practical applications and, in reality, the melting modes are used as a guideline for printing. With an increasing local material temperature beyond the boiling point, the mode can change from conduction to keyhole. These mode designations ignore laser-matter interaction details but in many cases are adequate to determine the approximate microstructures, and hence the properties of the build. To date no consistent, common, and coherent definitions have been agreed upon because of historic limitations in melt pool and vapor depression morphology measurements. In this review, process-based definitions of different melting modes are distinguished from those based on postmortem evidence. The latter are derived mainly from the transverse cross sections of the fusion zone, whereas the former come directly from time-resolved x-ray imaging of melt pool and vapor depression morphologies. These process-based definitions are more strict and physically sound, and they offer new guidelines for laser additive manufacturing practices and create new research directions. The significance of the keyhole, which substantially enhances the laser energy absorption by the melt pool, is highlighted. Recent studies strongly suggest that stable-keyhole laser melting enables efficient, sustainable, and robust additive manufacturing. The realization of this scenario demands the development of multiphysics models, signal translations from morphology to other feasible signals, and in-process metrology across platforms and scales.

Figure 1

The general physical process of laser melting. With the rapid increase in temperature, the solid (a powder bed sample here) transforms into liquid, gas, and plasma through the transitions of melting, vaporization, and ionization. During this process, melt pool and vapor depression form and evolve, and their depth-to-width aspect ratios of the transverse cross sections continue to rise because of enhanced laser absorption and limited heat transfer. In the schematic in the top left corner, the approximate location of the vapor depression inside the melt pool is outlined with a yellow dashed circle, while the red arrow indicates the laser scan direction.

Figure 2

Melt pool, keyhole, and common defects in unstable-keyhole-mode melting. (a) Schematic showing the general melt pool structure. The white arrows inside the melt pool indicate the flow pattern of the melt. (b),(c) Protrusion structure on the front wall of an unstable keyhole. (b) High-speed, high-energy x-ray images. (c) Schematic illustrations. At time $t_1$, a keyhole of reverse-triangle-like shape (RTS) forms. The rear wall is directly exposed to the incident laser beam, and the generated vapor then travels upward toward the front wall, as indicated by the blue arc lines and white arrows in (c). Upon the collision with the vapor, the existing protrusions (such as Pro. #1 and Pro. #2) change from a dome shape leaning downward (denoted by ${\text{Pro}}^{-}$) to a rod shape tilting upward (denoted by ${\text{Pro}}^{+}$). Here the rapid growth and collapse of the tonguelike protrusion Pro. #1 eventually leads to the formation of extremely fast ligaments and spatters, and the collapse of protrusion Pro. #2 causes the formation of an instant keyhole pore. (b),(c) Adapted from [211].

Figure 3

Postmortem-based and process-based definitions of laser melting modes. (a) Basis for definitions. With increasing temperature the depth-to-width aspect ratio of the (fused) melt pool (and possibly also the vapor depression) increases, and the laser melting transitions from conduction to keyhole mode. (b)–(d) Historical evolution of the definitions. (b) Original and (c) updated versions of postmortem-based definitions. (d) Process-based definitions. Along the axis of aspect ratio ($AR$), the values in brackets are the characteristic aspect ratios of the vapor depression; otherwise, they are the characteristic aspect ratios of the (fused) melt pool. The $A{R}_{\mathrm{k}}$ at $T_{\mathrm{s}}$ in (c) and $A{R}_{\mathrm{sk}}$ at $T_{\mathrm{sk}}$ in (d) are aspect ratios where the vapor depression starts to deviate from the semicircular shape. The $A{R}_{\mathrm{m}}$ at $T_{\mathrm{s}}$ in (c) and $A{R}_{\mathrm{sm}}$ at $T_{\mathrm{sm}}$ in (d) are aspect ratios where the (fused) melt pool starts to deviate from the nearly semicircular shape.

Figure 4

Laser melting modes defined by postmortem transverse cross sections of the fused melt pool. (a) Original definitions of (a1) conduction mode and (a2) keyhole mode. The depth $d$ and width $w$ to calculate the aspect ratio are marked in white. From [95]. (b) Melting mode transitions via varying scan velocity in laser powder bed fusion. From [149]. (c) Aspect ratio of the fused melt pool as a function of power density. The laser is a stationary beam, and the spot size and the interaction time are constants. From [7]. (d) Depth of the fused melt pool as a function of volumetric energy density. From [160].

Figure 5

Laser melting modes defined by the vapor depression and melt pool morphologies through high-speed synchrotron x-ray imaging. (a)–(c) Conceptual definitions in stationary laser melting. From left to right, conduction, transition, and keyhole modes are displayed. The light area is the vapor depression and the red shaded area shows the melt pool. (d),(e) Strict definitions based on a quantitative morphology measurement from (a)–(c). (d) Penetration depth of vapor depression and (e) the aspect ratio of the melt pool over time at various laser powers. The transitions in (d) and the time points indicated by the vertical dashed lines in (e) define the vapor depression transition. The open circles in (e) define the melt pool transition. (f),(g) Extended definitions in the $P\text{−}V$ space for the case of a scanning laser. (f) Bare plate. (g) Powder bed. The lower blue and upper red dashed lines in (f) outline the vapor depression and melt pool transitions, respectively. The laser spot size in (f) is $95\mu \mathrm{m}$, and that in (g) is $115\mu \mathrm{m}$. Adapted from [33].

Figure 6

Integration of operando synchrotron x-ray imaging and other high-speed in situ monitoring techniques. Through multitechnique fusion and multisignal translation, the emerging knowledge obtained from synchrotron x rays can be transferred to industrial practice.

Figure 7

Schematic diagrams of a process map for the laser powder bed fusion of metals. (a) Process window. The process window is located at the center of the $P\text{−}V$ space and is surrounded by several common defect zones. The definitions of melting modes here are process based. Only part of the mixing stable-keyhole and transition region constitutes the window. At low energy density (low power and high scan velocity), balling can extend the region of lack-of-fusion porosity by causing variability in the melt pool size, and thus, the overlapping area between the “balling” and “lack-of-fusion” regions is designated as the latter. (b),(c) Boundary and origin of keyhole porosity. (b) Keyhole porosity boundary and origin. On the left side, the keyhole porosity boundary in the $P\text{−}V$ space is sharp and smooth. On the right side, around the porosity boundary the critical keyhole instability that is analogous to a double palm strike emits an acoustic wave (shock wave) and drives the pore near the keyhole tip to accelerate rapidly away from the keyhole. When the pore is captured by the solidification front, it becomes a detrimental structural defect in the build. (c) X-ray images of keyhole pore formation and motion around the keyhole porosity boundary. In the first few microseconds after a pore pinches off the keyhole, the original keyhole tip remains nearly stationary. The pore $P_1$ marked with a dashed circle is then accelerated to about $10\mathrm{m}/\mathrm{s}$ in less than $1\mu \mathrm{s}$. At the time $11.04\mu \mathrm{s}$, a microjet $P_{\text{jet}}$ (see the inset) is penetrating into the pore $P_1$ from the side facing the keyhole bottom. The x-ray images are background corrected and the contrast is then reversed to highlight the events around the keyhole. (c) Adapted from [212].

Figure 8

Synchrotron x-ray imaging of laser melting. (a) Configuration of experimental systems. (b) Experimental schematic.