Characterization of Defects and Designed Flaws in Metal Additive Manufacturing (AM) Parts with X-ray Computed Tomography (XCT)

Metal Additive Manufacturing (AM) has great potential to revolutionize manufacturing industries, but a reliable method to detect defects in AM-produced parts with complex internal structures must be developed prior to its widespread adoption. In this research, defects occurring in additively manufactured metal parts are characterized with X-ray Computed Tomography (XCT). In addition, the concept of metrological XCT is introduced, with the goal of providing more accurate dimensional measurements of the internal features. Preliminary experiments have been conducted toward the larger goal of evaluating XCT as a viable option for nondestructive evaluation of AM-produced components. Critical to this evaluation is the principle of the Probability of Detection (PoD), which has not been previously determined for typical AM defects using XCT. The first step in determining the PoD for XCT is to develop a suitable artifact with embedded features that are representative of the defects occurring in AM-produced parts. Two sets of samples were built by AM and their embedded defects measured by XCT. In the first set of samples, the chosen AM processing parameters were suboptimal, ensuring that defects would be present in the parts. These parameters were also varied to understand their effect on the resulting microstructure and defect formation. Measurements of porosity and the pore size distribution were determined from the XCT images. In the second set of samples, synthetic internal features were added, some approximating typical AM defects. Dimensional XCT was used to evaluate the quality of these features. Based on the results, a future experiment with the goal of estimating the PoD of critical AM defects with an XCT system is suggested

Using Design of Experiments in Finite Element Modeling to Identify Critical Variables for Laser Powder Bed Fusion

Input of accurate material and simulation parameters is critical for accurate predictions in Laser Powder Bed Fusion (L-PBF) Finite Element Analysis (FEA). It is challenging and resource consuming to run experiments that measure and control all possible material properties and process parameters. In this research, we developed a 3-dimensional thermal L-PBF FEA model for a single track laser scan on one layer of metal powder above a solid metal substrate. We applied a design of experiments (DOE) approach which varies simulation parameters to identify critical variables in L-PBF. DOE is an exploratory tool for examining a large number of factors and alternative modeling approaches. It also determines which approaches can best predict L-PBF process performance.Mechanical Engineerin

Data related to the effect of specimen geometry and orientation on tensile properties of Ti-6Al-4V manufactured by electron beam powder bed fusion

Additive manufacturing quality assessment often relies on tensile testing as the preferred methodology to qualify builds and materials. The data included in this article provides additional supporting information on our manuscript [1] on the effect of specimen geometry and orientation on tensile properties of Ti-6Al-4V manufactured by electron beam powder bed fusion. As such, the data in brief provides in-depth details on the tensile specimen specifications, the tensile specimen build layout and replicate notations, and the tensile testing datasets. The information presented herein complements the manuscript.Natural Sciences and Engineering Research Counci

High-speed micro-electro-discharge machining

If the speed and precision of micro-electro-discharge machining (micro-EDM) processes can be significantly enhanced, then they have the potential to be used for a wide variety of micro- and meso-machining applications. Toward this end, a better understanding of the impacts the various machining parameters have on material removal has been established through a single discharge study of micro-EDM and a parametric study of small hole making by micro-EDM. The single discharge study revealed craters resembling spherical caps surrounded by a rim of re-solidified metal. The volume of an individual crater varies linearly with applied energy while its diameter is related to the cube-root of applied energy. Modeling the micro-EDM discharge as an instantaneous spherical heat source allows accurate prediction of material removal by a single discharge. The main avenues for improving the speed and efficiency of micro-EDM are in the areas of more controlled pulse generation by the power supply and more controlled positioning of the tool electrode during the machining process. Controlling the gap between the tool and workpiece is critical to the microEDM process and operating at a larger gap spacing by using increased applied voltage proved beneficial. Further investigation of the micro-EDM process in two dimensions leads to important design rules, specifically the smallest feature size attainable by the process. While the smallest feature realized in the present study was 12μm, the lower bound on feature size appears limited only by existing power supply technology. An estimation of minimum feature size attainable is obtained by measuring Rz of a characteristic surface

Thermographic Measurements of the Commercial Laser Powder Bed Fusion Process at NIST

Measurement of the high-temperature melt pool region in the laser powder bed fusion (LPBF) process is a primary focus of researchers to further understand the dynamic physics of the heating, melting, adhesion, and cooling which define this commercially popular additive manufacturing process. This paper will detail the design, execution, and results of high speed, high magnification in-situ thermographic measurements conducted at the National Institute of Standards and Technology (NIST) focusing on the melt pool region of a commercial L-PBF process. Multiple phenomena are observed including plasma plume and hot particle ejection from the melt region. The thermographic measurement process will be detailed with emphasis on the ‘measurability’ of observed phenomena and the sources of measurement uncertainty. Further discussion will relate these thermographic results to other efforts at NIST towards L-PBF process finite element simulation and development of in-situ sensing and control methodologies.Mechanical Engineerin

Additive Manufacturing Round Robin Protocols: A Pilot Study

As the number of users of additive manufacturing (AM) steadily increases, and considering their demand for material and process specifications, the need for standard protocols for round robin studies is increasing accordingly. Researchers at the National Institute of Standards and Technology (NIST) have conducted and participated in several AM round robin studies with the aim not only to characterize the AM process, and material but also to improve the understanding of AM round robin studies themselves. One simple study, a pilot round robin study investigating geometric performance of NIST-owned consumer-grade 3D printers, provides excellent examples of typical results and lessons learned. While individual printers produced relatively consistent results, there was significant variability between the printers. This variability existed despite best efforts to ensure participants followed consistent procedures in building the test parts. Further, the variability made it apparent that collecting pedigree data from each build was required to draw any conclusions about potential causes of the variability.Mechanical Engineerin

Repeatability and Sensitivity of a Rotating Drum Method for Rheological Characterization of Stainless Steel Powders Used for Additive Manufacturing

There remains a need for rheological characterization of metal powder used for powder- spreading-based additive manufacturing (AM). Novel powder rheometers introduced to the commercial market for this purpose must be rigorously evaluated for repeatability and sensitivity before widespread adoption for predicting AM powder performance. The work presented here focuses on the quantification of the repeatability and sensitivity of a commercially available rotating drum powder rheometer for testing metal AM powder. This assessment is accomplished via a set of tests that include the following independent variables: cleaning method, the mass of the sample, particle size distribution, material, and hysteresis.Mechanical Engineerin

Characterization of Defects and Designed Flaws in Metal Additive Manufacturing (AM) Parts with X-ray Computed Tomography (XCT)

Metal Additive Manufacturing (AM) has great potential to revolutionize manufacturing industries, but a reliable method to detect defects in AM-produced parts with complex internal structures must be developed prior to its widespread adoption. In this research, defects occurring in additively manufactured metal parts are characterized with X-ray Computed Tomography (XCT). In addition, the concept of metrological XCT is introduced, with the goal of providing more accurate dimensional measurements of the internal features. Preliminary experiments have been conducted toward the larger goal of evaluating XCT as a viable option for nondestructive evaluation of AM-produced components. Critical to this evaluation is the principle of the Probability of Detection (PoD), which has not been previously determined for typical AM defects using XCT. The first step in determining the PoD for XCT is to develop a suitable artifact with embedded features that are representative of the defects occurring in AM-produced parts. Two sets of samples were built by AM and their embedded defects measured by XCT. In the first set of samples, the chosen AM processing parameters were suboptimal, ensuring that defects would be present in the parts. These parameters were also varied to understand their effect on the resulting microstructure and defect formation. Measurements of porosity and the pore size distribution were determined from the XCT images. In the second set of samples, synthetic internal features were added, some approximating typical AM defects. Dimensional XCT was used to evaluate the quality of these features. Based on the results, a future experiment with the goal of estimating the PoD of critical AM defects with an XCT system is suggested.</p