KEY CONCEPTS
• Pore formation during laser powder bed fusion (LPBF), a common 3D printing technique used on metals, can cause shortened fatigue life during cyclic loading.
• Operando high-speed X-ray imaging was conducted to better understand how pore formation occurs during LPBF.
• Acoustic waves generated due to keyholes suspended in an unstable environment can push pores into the body of the metal, trapping them after solidification and leading to the initiation of fatigue cracks. 

Additive manufacturing (also known as 3D printing) is slowly becoming a significant technology that can be used in metalworking. One of the advantages in using additive manufacturing is the flexibility the technique has to produce complex metal parts, which is why it is used widely in prototyping.

In a previous TLT article,¹ 3D printing of commonly used wrought aluminum alloys was discussed. The researchers indicated that alloys such as 7075 aluminum were not able to be 3D printed due to the formation of cracks in the microstructure during the process. This problem was overcome through the addition of nanoparticle grain refiners (such as hydrogen stabilized zirconium particles) to aluminum alloy metal powder prior to 3D printing.

The most commonly used 3D printing technique used with metals is laser powder bed fusion (LPBF). Anthony Rollett, professor of materials science and engineering at Carnegie Mellon University in Pittsburgh, Pa., says, “In LPBF, a cycle starts with a machine spreading a layer of a specific metal powder that is 20 micrometers thick over the metal undergoing 3D printing. A laser is then applied where the user wants the part made to melt the metal in a specific area. This process is repeated for a few thousand times to manufacture the finished part. Once completed, residual powder is then removed.”

One problem associated with LPBF is the formation of pores in the 3D printed metal. Rollett says, “Metals manufactured through the use of LPBF exhibit good static mechanical properties such as tensile strength. But LPBF-produced metals can display shortened fatigue life during cyclic loading that can be traced to the formation of cracks. Pores are a significant part of this process because they have been found to initiate fatigue cracks.”

Past work to determine the cause for the formation of pores has focused on the keyhole mode of melting. Rollett explains, “A laser acts as a high-intensity drill by focusing light into a metal. The result can be drilling slightly more than 100 microns into the metal powder, which creates a deep and narrow cavity known as a keyhole.”

Rollett points out that keyholes do not contribute to the formation of metal fatigue cracks.

Further work has now been done to better understand how the generation of keyholes can lead to the production of pores in the metal.

Operando high-speed X-ray imaging
Rollett and his colleagues applied a laser beam along a straight line to a sample of the metal Ti-6Al-4V that has a layer of powder on top and is sandwiched between two glassy carbon plates. The researchers developed a power-velocity (PV) map to determine those regions where pore formation was occurring by varying the power of the laser and the scan speed across the metal surface.

Rollett says, “The two most important parameters that need to be evaluated in LPBF are power and scan speed. These two characteristics provide a good view as to how the 3D printing process is progressing.”

The broad applications for Ti-6Al-4V in aerospace and medical applications was the reason this alloy is used in this study, according to Rollett.

The researchers used a technique known as operando high-speed X-ray imaging to better understand how pore formation occurs based on instabilities found in the keyholes. Rollett says, “Operando high-speed X-ray imaging generates extremely bright high-energy X-rays that can be used to produce movies at high speed showing pore formation. This technique enabled us to follow the process under the microscope at a microsecond time scale.”

The PV map produced a very defined boundary between an area of stable melting and a region where pores are formed from keyholes. Rollett says, “Scanning at slower speeds can produce pores no matter the power generated by the laser.”

A megahertz X-ray image showing the formation of a keyhole is shown in Figure 1. As shown in the first image in the upper left, as the keyhole fluctuates, the tip forms a “J” shape that pinches off (as shown in progressing to the fourth image to the right) to form a pore. Further formation of pores is shown in the bottom set of four images.


Figure 1. A series of megahertz X-ray images illustrates how pore formation takes place during LPBF. Figure courtesy of Carnegie Mellon University.

Rollett says, “The keyhole is suspended by the liquid metal in an unstable environment that leads to oscillations of a magnitude of 100 kilohertz and the formation of acoustic waves. This results in the pores being pushed away from the keyhole into the body of the metal where they are trapped after solidification and can initiate the fatigue cracks, causing premature failure.”

This research provides a foundation for establishing specific laser power and scan speed conditions to minimize the formation of pores from the keyhole mode of melting. Rollett says, “We have developed a qualification procedure that can be used with specific metals to minimize defect formation through the use of specific qualification protocols.”

Additive manufacturing of metals can now be done more effectively, which should enhance the wider use of LPBF. Further information can be found in a recent article² or by contacting Rollett at rollett@andrew.cmu.edu.

REFERENCES
1. Canter, N. (2018), “Manufacturing of aluminum alloys by 3D printing,” TLT, 74 (1), pp. 12-13.
2. Zhao, C., Parab, N., Li, X., Fezzaa, K., Tan, W., Rollett, A. and Sun, T. (2020), “Critical instability at moving keyhole tip generates porosity in laser melting,” Science, 370 (6520), pp. 1080-1086.