Additive Manufacturing Simulation - Melt-pool Formation Modes Simply Explained.

Metal Additive Manufacturing (AM) is an innovative technology that allows for the direct production of intricate metal parts via depositing metal powder layer by layer and selectively melting and fusing it to fabricate the final component.

By subjecting the deposited material surface to a heat source, such as a laser beam, an intense heating effect takes place, rapidly elevating the temperature of the material. As the temperature exceeds the metal's melting point, the material undergoes a phase transition, transforming into a liquid state of molten metal, forming a distinct pool referred to as the melt-pool.

Melt-pool formation illustration

In industry and academia, mathematical models and computational algorithms, are utilized to accurately predict the heat transfer, fluid flow, and solidification behaviors within the melt pool. By inputting parameters such as laser power, scanning speed, material properties, and applying thermal boundary conditions into the Finite Element simulation models, researchers and engineers can utilize continuum elements and heat source models to simulate a time-dependent heat transfer of laser path trajectories. Such simulations provide valuable insights into the melt pool formation process, including the size, shape, and temperature distribution within the melt-pool. This knowledge enables the optimization of process parameters and facilitates the production of high-quality parts with desirable properties.

Finite Element model with applied Gaussian heat source model

The melt-pool undergoes its creation and evolution through the interaction of various material and production process parameters, which ultimately determine how the melt-pool is formed; via conductive or keyhole formation.

Conductive vs. keyhole melt-pool formation

Conductive Melt-pool Formation

During the conductive melt-pool formation, the laser beam is focused onto the surface of the material with low to moderate laser energy. As the laser beam moves across the surface, the surface temperature exceeds the melting point, and the metal undergoes a phase transition and turns into a molten state, forming a wide and shallow molten pool.

Schematics of conductive melt-pool formation

The dimensions of the conductive mode melt-pool are predominately shallow and ball-shaped and vary in size based on the material and laser beam settings. As the laser moves along its path, the molten metal solidifies behind it, forming a solidified track.

Keyhole Melt-pool Formation

During the keyhole melt-pool formation, an intense laser beam is focused onto the surface of the material with high laser energy. As the laser beam moves across the surface, it rapidly heats the material starts evaporating, and forms a "keyhole" or a deep, narrow cavity in the molten pool. The keyhole acts as a localized, high-temperature reservoir, surrounded by a solidifying shell of metal.

Schematics of keyhole melt-pool formation

The laser then penetrates the evaporated space, bouncing off the internal surfaces of the melt-pool. This exposure of laser to the internal melt-pool surfaces causes heat conduction to the surrounding material of the melt-pool. This results in a distinctly deep and narrow melt-pool formation called the keyhole melt-pool.

Conductive vs. Keyhole Melt-pool Formation

It is important for every AM user to develop a thorough understanding of melt-pool formation allowing them to choose the appropriate mode for the desired part production. Both modes have distinguished hallmarks, some of which are listed in the chart below:

​Conductive mode Occurs at relatively low to moderate energy density The melt-pool's width is bigger than its depth (depth to width ratio < 1.0) Shallow, ball shaped Low(er) energy absorption Relatively stable melt pool geometry Often produces smoother surface that requires less post processing

Keyhole mode Occurs at higher volume energy density The melt-pool's depth is bigger than its width (depth to width ratio > 1.0) Deep penetration High energy absorption Very efficient energy delivery Keyhole collapses can lead to increased porosity

Selected conductive- vs. keyhole-formation hallmarks

Conclusion

For users, it is important to keep in mind that the size and shape (conductive vs. keyhole) of the melt-pool can vary with the scanning speed, laser power, and the interaction between the heat source and the material. Therefore, careful control and optimization of process parameters are necessary to achieve desired melt-pool characteristics and ensure high-quality printed parts.

Both, the conductive and the keyhole melt-pool formation come with their set of pros and cons regarding topics including:

• Energy efficiency and high power requirements.

• Risk of introducing defects (porosity, warping, surface imperfections, etc.)

• Penetration depth

• Build Speed

• Material Density

• Design Flexibility

It is important to note that melt-pool formation is a dynamic process that evolves over time and can vary throughout the entire process. Furthermore, and for the sake of completeness, it needs to be stated that melt-pool formation can as well occur as a combination of conductive and keyhole formation.

Understanding melt-pool formation through modeling and simulation techniques allows researchers and engineers to gain insights into the potential outcome and risks of the metal AM process and allows them to optimize process parameters, enhance part quality, and design new materials and structures for a wide range of applications.

To learn more on how AdditiveLab process simulation can help you predict melt-pool formations and guide you during the optimization of your process parameters, reach out to the AdditiveLab team via our Contact page.