Multiphase Flow Simulations For Additive Manufacturing

Multiphase Flows 

Multiphase flows are flows that include more than one fluid or more than one thermodynamic phase of the same fluid. Most of the real-life applications of fluid dynamics include multiphase flows.

Boiling water can be regarded as a standard example of a multiphase fluid flow problem where water serves as one fluid and vapor is another phase; it will also include a phase for air.

In CFD, Multiphase fluid flow has many industry applications, from chemical plants to aerospace design multiphase flows are studied to optimize the processes and the designs.

Additive Manufacturing

Everything we humans are doing today is the extension of existing phenomena of mother nature.

Everyone has seen the wasps creating a nest. They bring the clay or wood pulp in their mouth and deposit it to create a shape, by repeating this they build their intricate and complex nests.

Another similar example is of spiders. Whoever first thought of additive manufacturing must have been influenced by the creativity of such species.  

Anyone who is aware of additive manufacturing technology knows that it is a modern manufacturing technology but for centuries our ancestors have been using these techniques for pottery.  Large earthen pots were usually made in layers by adding clay and shaping it layer by layer.

In modern manufacturing technology, we iteratively add the material using an automated machine setup and thus create a desired shape. There are metal-based and polymer-based additive manufacturing processes that are widely used.

Physics Behind Modern Additive Manufacturing Processes.

The key phenomenon that is in the essence of any additive manufacturing process is the rapid curing of liquids to form a solid.

Let’s discuss one of the widely used metal additive manufacturing processes, known as Laser Powder Bed Fusion.

Laser Powder Bed Fusion (LPBF): In LPBF, metal powder is used as a raw material. A thin powder bed is formed over a metal substrate and a laser is used to melt the powder in the desired shape and it solidifies rapidly, this will form a thin cross-section of the desired whole part we intend to manufacture.

Repeating this process forming multiple layers thus yields a complete part.

Though it looks simple, it is a very intricate process where we are actually melting the metal powder which has a melting point of thousands of degrees Celsius. A high-power laser is used to perform this task rapidly, the laser not only melts the metal powder but also metal vapor fumes are also formed during this process.

We require the right set of laser power and other other involved process parameters to attain a precise amount of melting. The process involves complex phenomena of physics which include laser reflection, fluid flows, heat transfer, and phase change all this is happening at a few micrometres in length.  

The physics involved at such a small length is very difficult to impossible to capture using measurement tools. The only way we can attain the right set of process parameters experimentally is through performing iterative experiments and learning from them.

Multiphase Flow Simulation In Additive Manufacturing

Over the centuries we have developed the science behind fluid flows, heat transfer phenomenon, optics, and most of the other dominating phenomena involved in these processes. What if we could create a simulation of such complex processes?  It will be a game changer in the additive manufacturing industry. 

This is exactly where we point at when we talk about multiphase flow simulations in additive manufacturing.

Simulating The LPBF Process Using Multiphase CFD

In LPBF, a laser hits on a powder bed made of powder alloy and melts the powder to form a melt pool which then solidifies to form a continuous bead.  To simulate this process we must use interface tracking methods such as VOF which keeps track of any two phases in the cell using their volume fraction. 

 In LPBF since we have multiple phases which include solid alloy, molten alloy, alloy vapor, and shielding gas, we may use these methods to simulate multiple phases. Simulating phase tracking with multiple phases gives us an accurate prediction of fluid flow with multiple phases. We will be able to predict how two or more fluids will be interacting in the melt pool hence this simulation can be useful to predict pore formation due to vapor entrapment or shielding gas entrapment which is one of the major defects in LPBF manufactured parts.

 The animation shown is generated using AM PravaH which is an all-inclusive additive manufacturing simulation software. You can see the interaction between inert gas and molten alloy and how the gas is being trapped inside the melt pool leaving behind a small pore.

This video showcases how we can create a virtual process governed by mathematical equations to predict real-life phenomena.  The multiphase simulation used here not only tracks fluid interaction through volumetric forces acting on fluid but also accounts for surface forces such as surface tension, interfacial tension, and the Marangoni effect.

The phase change phenomena in multiphase simulation which includes melting, evaporation, and solidification during the heating and cooling cycles of the process can accurately predict the effect of laser power on the alloy powder and substrate. The amount of melting when the laser hits the powder bed which in turn creates the melt pool which is of a certain shape and size and hence can be predicted. 

The melt pool size and shape has an effect on the homogeneity of the built part, its microstructural properties, and surface finish. Using melt pool simulation thus can give us information about the quality of the built part before we print the part in real. 

The most important process involved in the additive manufacturing cycle when working with new materials is process parameter optimization. This process includes multiple iterations with altered process parameters till we produce defect defect-free build. This process can be tedious without any understanding of the effect of process parameters on the new alloy and may consume lots of time and other resources when trying to reach optimized process parameters.

The simulation can help you not only to understand how different properties of material and process parameters affect the final product but also you will be able to optimize the process parameters for a defect-free build.

Let’s witness some of the stunning animation videos from AM Pravah LPBF simulations.

The video shows how a keyhole is formed as an effect of marangoni convection and vapor pressure.  Multiple laser reflections have also been captured and the effect is visible at the inner walls of the keyhole.

The video shows high fidelity simulation of the LPBF single-track process. You can see the realistic melt pool dance as the laser melts the powder and various forces acting inducing motion inside the melt pool.

The video shows the gas entrapment due to keyhole formation and permanent pore formation after solidification. This type of defect reduces the density and hence overall strength of the built part.

Author: Pruthvi Uttarwar (Reaserch Engineer/Developer-Paanduv Applications)