The growing use of additive manufacturing is taking place because this technique shows promise as a more efficient and sustainable way to produce intricate metal parts. But challenges remain because some metal alloys have proven to be difficult to 3D print.

One popular method for additive manufacturing is the use of laser-based powder bed fusion (LBPF). In a previous TLT article,¹ a study involving the additive manufacturing of a new martensitic alloy, AF9628, using LBPF was discussed. Process parameters such as laser power, scan speed and layer thickness were utilized by an analytical model to predict the characteristics of the 3D printed metal. A two-dimensional process map was prepared that predicts how the combination of scan speed, laser power and laser hatch spacing can produce viable 3D printed parts. Subsequently, AF9628 was successfully produced using LBPF.

Besides minimizing porosity, another factor that must be taken into consideration in additive manufacturing is microsegregation. Raiyan Seede, doctoral student in the department of materials science and engineering at Texas A&M University in College Station, Texas, says, “Segregation is a phenomenon that is observed in traditional manufacturing practices such as casting where the cooling of a liquid alloy does not produce a homogeneous structure because a specific metal may precipitate out of the alloy. In additive manufacturing, segregation also will take place but on the micro scale. Non-equilibrium cooling may result in fine microsegregation.”

The presence of microsegregation can have a negative effect on mechanical properties and on the performance of fabricated parts. The degree that microsegregation occurs is dependent upon the alloy and on the process conditions used.

A good analogy for the effect microsegregation can have on precipitating unwanted phases in a metal is the addition of sodium chloride to water. Seede says, “In introducing sodium chloride to water, a certain solubility limit is reached that can lead to salt precipitating out and forming another phase in water.”

In an effort to develop new metal alloys specifically for additive manufacturing that can be useful commercially, a new study was initiated to better understand how material properties and additive manufacturing conditions affect alloy properties such as microsegregation and porosity. To do this, four nickel-based alloys were investigated that pose different challenges when processed using LBPF.

Single-track experiments
Seede, in collaboration with his advisor, Ibrahim Karaman, Chevron professor I and head of the department of materials science and engineering at Texas A&M University, evaluated the potential to 3D print the following nickel-based alloys: Ni-20 at% Cu, Ni-5 at% Al, Ni-5 at% Zr and Ni-8.8 at% Zr. Seede says, “We chose these four alloys because they exhibit different properties when the two metals are mixed. The nickel-copper alloy produces a uniform phase when mixed no matter the ratio of the two metals. It is known as an isomorphic alloy. The nickel-aluminum alloy should not exhibit much microsegregation because the two metals solidify at about the same temperatures. In contrast, nickel and zirconium have large differences in solidification temperature making the Ni-5 at% Zr alloy ideal to evaluate versus the nickel-aluminum alloy.

The other nickel-zirconium alloy (Ni-8.8 at% Zr) is eutectic meaning that it solidifies at a lower temperature than the two metals and two phases are expected to form simultaneously in traditional manufacturing processes. Prior to this study, it was not clear what microstructure to expect from additively manufacturing a eutectic alloy.”

In evaluating the four nickel-based alloys, the researchers first used a modeling program to predict melt pool dimensions across the parameter space. Then the researchers conducted single laser-track experiments to build the process map for each alloy. Seede says, “The single-track experiments were done to obtain a quick, yet accurate understanding of laser power, laser scan speed and other parameters that will minimize porosity formation and microsegregation.”

The researchers used optical microscopy, scanning electron microscopy and wavelength dispersive spectroscopy to evaluate the metal alloys. Figure 1 shows an example of a colorized, scanning electron microscope image of a Ni-5 at% Zr powder particle. Dendrites that formed during the solidification of the powder particle can be observed on its surface.


Figure 1. A colorized scanning electron microscope image of a Nickel-5 at% Zirconium particle is shown. Dendrite solidification, shown on the particle surface, is frequently observed when conditions are far from equilibrium, such as the solidification of a snowflake. Figure courtesy of Texas A&M University.

Primary dendritic arm spacing (PDAS) was used by the researchers to quantify some of the microstructures for different laser powers and scan speeds. Seede says, “We used PDAS to predict how results from the single-track experiments will translate into the bulk samples produced by LBPF.”

Using the PDAS data obtained from single tracks, the researchers were able to successfully develop process maps for each of the four metal alloys that showed conditions under which porosity and microsegregation are minimized. Seede says, “We also used machine learning to develop an approach for predicting process conditions that will allow for the successful 3D printing of alloys while minimizing microsegregation.”

Future work will involve developing these processing maps further for additively manufactured alloy mechanical properties. Seede says, “We intend to link the porosity and microstructure maps with mechanical property maps to find ideal regions to 3D print.”

Additional information can be found in a recent article² or by contacting Seede at rseede@tamu.edu or Karaman at ikaraman@tamu.edu.

REFERENCES
1. Canter, N. (2020), “3D printing of a high-strength martensitic steel alloy,” TLT, 76 (10), pp. 12-13. Available here.
2. Seede, R., Ye, J., Whitt, A., Trehern, W., Elwany, A., Arroyave, R. and Karaman, I. (2021), “Effect of composition and phase diagram features on printability and microstructure in laser powder bed fusion: Development and comparison of processing maps across alloy systems,” Additive Manufacturing, 47, 102258.