THE CHALLENGE: Additive manufacturing has progressed from being mainly a tool for low-cost prototyping to a manufacturing activity that can produce large volumes of high-value metallic components. The industry is now focused on establishing repeatable and standardized processes to ensure consistent performance across various printed parts and new materials. Industry experts have identified quality control as one of the biggest obstacles to widespread additive manufacturing adoption. Myriad factors are in play when lasers fuse thin layers of powdered alloys, and localized heterogeneity may cause pores or micro faults that result in part failures. Researchers are working to determine the effects that link printing parameters to fatigue and fracture behavior. Chemical characterization is a key component to this link, and the industry needs new analytical methods to keep pace with the increasing complexity of feedstock material and printed parts.
THE SOLUTION: A new mass spectrometry technique can quantify the metallic, trace, and low-mass constituents in additive manufacturing powders or builds. In addition to bulk characterization, it can also perform elemental mapping. With spatial resolution ranging from 5-200 microns, it provides chemical data at both the particle level and at the same scale as the melt pool in many additive manufacturing techniques. The instrument’s mapping abilities can identify both spatial distributions and depth profiles of any element of interest. Such results are useful across the additive manufacturing ecosystem from verifying homogeneity of a feedstock to diagnosing a build’s failure.
A recent study demonstrated chemical mapping results from an additively manufactured commercially pure aluminum sample. The sample contained several large pores that the laser failed to fuse during the build process. Another analytical technique had identified differences in grain structure above and below the pore and this study verified the change in grain structure was associated with localized chemical heterogeneity. Within the pore, chemical maps revealed lower aluminum concentrations and higher concentrations of impurity elements like carbon, oxygen, and iron. Above the pore in the build direction, the maps showed higher concentrations of elemental impurities.
THE TECHNOLOGY: Laser Ablation Laser Ionization Time of Flight Mass Spectrometry (LALI-TOF-MS) addresses many challenges associated with other analytical techniques to offer rapid, high-sensitivity quantification of nearly the entire periodic table. As illustrated in Figure 1, the ionization source, LALI, uses two lasers to first ablate, or release, material from a solid sample’s surface and then ionize neutrals present in the ablated material. The laser ablation process allows direct analysis of solid materials, like additive manufacturing powders or printed parts, without the complicated sample preparation procedures of other techniques that require liquid sample introduction. The ionization laser targets the neutral particles created by ablation, which are more representative of the sample’s constituents than plasma-generated ions.
Overall, LALI results in more reliable elemental verification and reduces sample matrix effects. After ionization, the TOF mass analyzer creates a full mass spectrum at each laser spot. This system does not require an inert gas, and it is fully contained within a compact package that fits on a desktop. LALI-TOF-MS provides versatility, high-throughput, broad elemental coverage, and low detection limits and has potential to meet the growing characterization needs of the additive manufacturing industry.
CHEMICAL MAPPING CAPABILITIES: An additively manufactured commercially pure aluminum build was analyzed to investigate lack-of-fusion pores for local chemistry variations. The process involved rastering a small area of 1.12 mm by 0.3 mm with a 20-micron laser spot size. Figure 2 shows images captured by the instrument’s microcamera before and during the analytical process. The left image displays one of the investigated lack-of-fusion pores while the right image shows the live microcamera image during the analysis.
Figure 3 shows six example chemical maps. The maps display concentrations for individual elements of interest: aluminum, carbon, oxygen, titanium, iron, and molybdenum. For each, lighter blues represent higher concentrations. Each pixel is a laser spot and includes a full mass spectrum of data, and results are not limited to the elements displayed here. In this example, detection ranged from light elements (boron and carbon) to metallic elements (gallium and zirconium). On the aluminum map, the orange circle highlights the lack-of-fusion pore.
Figure 3: Mapping results from a 1.12-mm-by-0.3-mm scan area on a commercially pure aluminum sample. Each pixel is a 20-micron laser spot. The color scale shows the concentration of each element of interest.
As shown in Figure 3, the concentration of the matrix element, aluminum, decreases within the pore while impurity concentrations increase. Additionally, the maps identify differences in titanium and molybdenum above the pore, which correspond with the observed grain size variations. Such information is useful for understanding how the lack-of-fusion pores were formed, and the impact the pores will have on part performance, including fatigue and fracture behavior.
SUMMARY: This study highlights LALI-TOF-MS as a promising new and alternative technique for quality control and failure analysis in the additive manufacturing industry. Examining the lack-of-fusion pores of an aluminum build revealed chemical heterogeneity both within and nearby the pores. Because LALI-TOF-MS collects a full mass spectrum at each laser spot, the user can examine low-mass elements (e.g., carbon and oxygen) as well as metallic and trace elements in a single analytical session. Traditional mass spectrometry or spectroscopy methods struggle to quantify carbon and oxygen. For these elements, users rely on inert gas fusion and combustion, which are destructive methods that preclude the types of localized analyses shown in Figure 3.
This innovative technology was designed to simplify elemental analysis and allow a user with no experience in mass spectrometry to obtain accurate bulk quantification and high-resolution chemical maps with minimal sample preparation and analysis time. For quality control applications, spatial distributions of elements can identify impurities, contaminants, or poorly-distributed particles in the feedstock which would impact the build. After the build, elemental mapping can determine the chemical composition of part weak points or failures. LALI-TOF-MS provides chemistry data at the scale of 3D printing with spatial resolution comparable to both the feedstock particle size and the printer’s melt pool.
Note: This article was prepared using Federal funds under award 70NANB22H128 from NIST, US Department of Commerce. The statements, findings, conclusions, and recommendations are those of the authors and do not necessarily reflect the views of NIST or the US Department of Commerce.