Material Qualification

Summary

The lack of repeatable process outcomes from additive manufacturing (AM) has been broadly acknowledged as an impediment to widespread implementation. While most R&D efforts have been focused on in-process parameters and metrics and how they both relate to final part properties (e.g., microstructure, mechanical properties), much less focus has been placed on the precursor material. The Additive Manufacturing Standards Collaboration (AMSC) has identified ten gaps in the standardization of characterization of precursor material (PCM), of which five are deemed a ‘medium’ or ‘high’ priority indicating that they should be addressed in the next 5 years. For reference, the 260-page AMSC V2.0 report identifies a total of 90 gaps across five topical areas. The research and development for precursor material characterization is a priority in the AM community. There is a lack of characterization techniques for metal powder that have been proven pertinent to AM processes. While existing techniques are capable of characterizing gas atomized metal powders, the research conducted, albeit limited, has revealed the inadequacies of these conventual methods. The aim of this project is to evaluate and leverage relevant, conventional methods and couple these with novel characterization techniques, either developed at NIST or externally.

Description

Objective
To develop, utilize, and analyze methods of characterizing the precursor materials in additive manufacturing in both virgin and recycled states with the goal of advancing measurement science to benefit the AM community. The components of principal interest will include the rheological, size/morphological, and thermal properties of the precursor materials. Other secondary foci include the hygroscopicity, triboelectricity, and metallurgical properties of the studied materials.  

What is the Problem?
There is a currently a lack of understanding of the role precursor material properties have in determining final part properties and intermediate process properties (e.g., quality of spread layers of powder).  While this may lead to the development of new characterization methods, because of this lack of understanding existing methods must first be evaluated for relevance and uncertainty.  There also exists in some areas a complete lack of measurement capability.  For instance, the powder fed directed energy deposition process has no commercially available means to measure the mass flow rate of powder.  

What is the Technical Idea?
Utilize the technical expertise at NIST to evaluate and develop precursor material characterization methods. The expertise and unique toolset at NIST and in particular the Intelligent Systems Division, place the PCM Project Team in an optimal position to conduct impactful research.

While there does exist various methods of evaluating metal powder rheologically, morphologically, thermally, and chemically, there is a lack of understanding how these pertain to the actual AM processes and in turn the final properties of the parts produced.  The first step in solving this problem is to understand what tools are available and to conduct in depth analysis of these measurement tools.  Not only are contributions to uncertainty like repeatability important, but also whether and if so how, the data collected is related to the AM process.  Conventional flow measurement methods such as the Hall flowmeter are still used to evaluate powder, but recent studies have shown this sort of technique inadequate in predicting a powder’s spreading performance.  

What is the Research Plan?
The research plan is broken down into 5 focuses.  They are: sizing/morphological, spreading/rheological, thermal property, and hygroscopicity characterization.  

While the methods for characterization of size and morphology of AM precursor materials seem to be sufficient, the manner in which each compares to one another and the real size and shape of the particles being measured is unknown.  First, each of techniques will be compared to one another to provide this information for industry and for future research at NIST and elsewhere.  Well controlled sampling and splitting will be used to provide each method with samples with PSD’s representative of the original powder. Later, one technique (likely SEM or XCT) will be chosen as the ground truth with some allowed uncertainty and will be used to quantify the uncertainties of the other methods for the sizing of metal AM powders.  

There currently are various conventional methods for characterizing the flow or rheology of metal powders, but also some novel methods.  The relevance to the AM processes hasn’t been proven in a robust manner.  First, conventional powder characterization methods will be evaluated for repeatability, how they compare to techniques that measure the same metric (e.g., sieve and laser diffraction sizing), and if they are relevant to AM processes. Then novel methods will begin to be evaluated in the same manner.  There have been invented unique, novel characterization methods at NIST over the past several years (FY17 and FY18) under the AM Materials Project.  These will be evaluated as well.  If a certain novel method is deemed appropriate for standardization, the Project Team will usher this proposed standard forward.  

The thermal properties of AM precursor materials will be evaluated here at NIST and via collaborations with external organizations.  The laser flash method for measuring the thermal diffusivity of materials will be the primary technique used.  This technique will be coupled with unique powder capsules designed to allow the properties of the powder to be ascertained without those of the containing material to interfere.  While capsules have been designed and fabricated, they will be further evaluated and potentially improved.  Additionally, FEA will aid in extracting only the powders contribution to the thermogram acquired using the laser flash method.   

The tendency of metal AM powders to behave differently in the presence of various moisture contents is an issue for the AM community.  Not enough is understood how the presence of moisture alters the rheology, the powder’s performance in the AM machine, and how it is chemically altered over longer exposure durations. These areas will be the focus of the hygroscopicity research on AM precursor materials.