Metal-binder-jet (MBJ) layering technology is a fast-emerging approach in additive manufacturing (AM). Like laser-powder-bed fusion (LPBF), the industry-leading technology, MBJ promises greater performance and functionality and higher productivity within its target markets—and promises to provide the same manufacturing agility as LPBF, as well as complement traditional shape-forming and finishing processes.
At present, MBJ typically utilizes 316L and 17-4 PH stainless steel to address performance goals for industries such as automotive, aerospace, and hydraulic or pump components.
Porosity in printed products is a common challenge used as a “pass-fail” measurement for all AM systems. In most cases, AM systems provide better part density than castings. And MBJ, at least in some quarters, is on a rigorous path to address porosity and related stresses through the use of computed-tomography (CT) scanning and software analysis.
CT analysis provides insights at every phase of product development and manufacture. Because of its ability to view interior regions of a part non destructively, CT provides valuable quantitative data on both specific, critical and overall regions of a part and assembly. High-performance AM components usually exhibit fresh and quite novel geometries. There are unique stresses and vulnerabilities inherent in creating purely function-driven passageways and features, be it with LPBF or MBJ systems.
Unlike the laser-induced heat approach of LPBF, MBJ is driven by the interaction of powder particles, the compaction by the roller and the application of the adhesive binder. Parts are later heat cured in a two-step process to burn off the binder additives, reduce porosity, and achieve target strengths and desired quality.
In the first, “colder,” phase the green (non-sintered) part is debindered. Next, a higher temperature is applied—usually in the same oven—to sinter the part to its final strength. Achieving a strong initial part is crucial in the first stage to remove excess powder and allow for handling of the green part within the sintering stage.
With all evolving (and even mature) AM technologies, ongoing process knowledge, inspection, and final control are paramount to success. In order to reach commercial reliability, every approach has had to move away from black-art practices and recipes that can’t be replicated machine to machine, or even batch to batch on the same systems.
Here is where CT analysis can help developers understand process causality and interrelated factors that influence outcomes. Pinpointing causes and effects can aid in the use and development of sensors, feed rollers, print heads, feeders, binders, and overall operating parameters. Analysis helps greatly in effectively addressing shrinkage, distortions, brittleness, and porosity.
Early insight into print-process behaviors and conditions that lead to observable, measurable trial outcomes is crucial in shortening development cycles and establishing long-term operating parameters that reliably deliver the highest productivity and commercial success.
Reaching results faster in early development helps compress all the following stages of product creation. It also establishes a sound methodology for continuous improvement and future expansion of system features and capabilities.
CT analysis enables a deeper understanding in the development of MBJ process parameters, e.g., when the user is evaluating different combinations of powder, layer thickness, and binder saturation. Other testing methods are very limited and don’t give a deep volumetric insight into the green part.
One interesting effect identified through CT analysis of MBJ parts has been the tendency for segregation of fine powder particles in the bottom layers of a print, which leads to higher porosity. This can happen when finer particles agglomerate. Understanding this behavior allows engineers to evaluate original powder sizes in relation to layering methods and binder mixes.
For the development of early, critical system parameters, low-resolution CT-slice images have proven to be excellent for overview and initial analysis. In one study, high-resolution slice images were comparable to microsection images and were good for targeted analyses. Layer thicknesses of 90 µm revealed larger inhomogeneities that later led to poorer quality parts in the post-sintering stage.
Production of MBJ parts entails several stages. These include the initial print (sensitive to layering heights, binder amounts and powder quality); high-temperature debindering (still a highly porous state for the part); and longer-cycle, lower-heat sintering to further bond the metal, reduce porosity, and eliminate binder additives.
As with early CT analysis, each phase of MBJ part development can be measured via one-off scans, batch, or inline-automated inspections, according to circumstances such as production controls for aerospace-spec parts. Together, early and mid-process tracking reveals these main findings:
Density variations can be observed in MBJ in sintered parts. These variations can be traced back to the powder application by roller, the binder application itself, and the powder-binder interactions.
Binder application can lead to segregation of powder particles in one layer.
The binder application process can lead to porosity in green and sintered parts due to droplet impact. Higher binder saturation levels can compact the particle structure in green parts due to capillary forces and mechanical impact effects.
With final part density, strength, and geometric accuracy so important to commercialization—not to mention that shrinkage rates and other process impacts affect quality and yield—in process inspection and knowledge collection is fundamental to success.
Two compatible CT software analysis (measurement- and simulation-based) approaches can help in-process analysis and geometry compensation.
Measurement-based analysis serves to benchmark stages and conditions in the process leading to quantification of approaches and goals, improvements, and establishment of required, final part geometries—including comparing nominal-actual CAD tolerances and geometries against the as-manufactured part to show deviations.
The simulation-based approach relies on prior process simulation of the shrinkage, e.g., in Simufact Additive. In such software solutions, the part geometry and the occurring shrinkage during the sintering process can be simulated and the part geometry can be adjusted to the different shrinkage behavior in the X, Y, and Z axes. For more complex part geometries, the simulation-based approach can be used to make detailed adoptions for complex part designs.
Overall, many challenges can be addressed using simulation, including shrinkage compensation and process adjustments going back to powder sizes and anomalies in air spaces to aspects of curing and post processing.
Advantages of the measurement-based compensation software approach, include: compensation for distortions that are beyond the accuracy of simulation; and the ability to tackle non-uniform distortions within the build space and sintering furnace, providing machine repeatability.
Advantages of simulation-based compensation include: predicatively solving manufacturing problems; identifying an orientation/support concept that is fit for distortion compensation; and the ability to iterate designs free of the cost of physical tests and numerous trial runs.
In summary, CT software analysis of scanned data for MBJ and other additive processes provides the insights and actionable information needed to reduce development time and help ensure that production examples reach the series quality demanded by customers. In the race to use AM for creation of breakthrough designs and much-needed, quickly produced, direct-part replacement markets, CT analysis software offers the depth, breath and level of automation required for advancing new and evolving AM technologies.
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