Edward W. Reutzel

A survey of sensing and control systems for machine and process monitoring of directed-energy, metal-based additive manufacturing

Purpose – The purpose of this paper is to surveys classic and recently developed strategies for quality monitoring and real-time control of laser-based, directed-energy deposition.Additive manufacturing of metal parts is a complex undertaking. During deposition, many of the process variables that contribute to overall build quality – such as travel speed, feedstock flow pattern, energy distribution, gas pressure, etc. – are subject to perturbations from systematic fluctuations and random external disturbances. Design/methodology/approach – Sensing and control of laser-based, directed-energy metal deposition is presented as an evolution of methods developed for welding and cladding processes. Methods are categorized as sensing and control of machine variables and sensing and control of build attributes. Within both categories, classic methods are presented and followed by a survey of novel developments. Findings – Additive manufacturing would not be possible without highly automated, computer-based control...

Additive Manufacturing of Ti-6Al-4V Using a Pulsed Laser Beam

Microstructural development in directed-energy additive manufacturing of metal components is a complex process that produces parts with materials whose microstructure and properties are influenced by multiple heating and cooling cycles. Much work has been undertaken to correlate microstructural development with processing conditions, such as laser power and processing speed. Here, the microstructure and indentation hardness of a Ti-6Al-4V component processed with a pulsing laser beam and a continuous wave (CW) laser beam are investigated. It is found that the pulsed-beam build showed no statistically significant variation in lath width or indentation hardness with build height while the build deposited with the CW beam showed a statistically significant decrease in hardness and an increase in lath width near the middle of the build. The reduction in variability with beam pulsing is attributed to rapid cooling rates within the melt pool, a greater degree of melt pool stirring, and reduced aging during part build-up.

Laser-silicon interaction for selective emitter formation in photovoltaics. II. Model applications

Laser doping is an attractive way to manufacture a selective emitter in high efficiency solar cells, but the underlying phenomena, which determine performance, are not well understood. The mathematical model developed in Part I solves the equations of conservation of mass, momentum, and energy and is used here to investigate the effects of processing parameters on molten zone geometry, average phosphorus dopant concentration, dopant profile shape, and sheet resistance. The empirically calculated sheet resistance values are in good agreement with independently measured sheet resistance values reported in the literature. Process maps for output power and travel speed show that molten zone geometry and sheet resistance are more sensitive to output power than travel speed. The highest molten zone depth-to-width aspect ratios and lowest sheet resistances for 532 nm laser beams are obtained at higher laser powers (>13 W) and lower travel speeds (<2 m/s). Once the power level is set, the travel speed can be vari...

A brief survey of sensing for metal-based powder bed fusion additive manufacturing

Purpose – Powder bed fusion additive manufacturing (PBFAM) of metal components has attracted much attention, but the inability to quickly and easily ensure quality has limited its industrial use. Since the technology is currently being investigated for critical engineered components and is largely considered unsuitable for high volume production, traditional statistical quality control methods cannot be readily applied. An alternative strategy for quality control is to monitor the build in real time with a variety of sensing methods and, when possible, to correct any defects as they occur. This article reviews the cause of common defects in powder bed additive manufacturing, briefly surveys process monitoring strategies in the literature, and summarizes recently-developed strategies to monitor part quality during the build process. Design/methodology/approach – Factors that affect part quality in powder bed additive manufacturing are categorized as those influenced by machine variables and those affected by other build attributes. Within each category, multiple process monitoring methods are presented. Findings – A multitude of factors contribute to the overall quality of a part built using PBFAM. Rather than limiting processing to a pre-defined build recipe and assuming complete repeatability, part quality will be ensured by monitoring the process as it occurs and, when possible, altering the process conditions or build plan in real-time. Recent work shows promise in this area and brings us closer to the goal of wide-spread adoption of additive manufacturing technology. Originality/value - This work serves to introduce and define the possible sources of defects and errors in metal-based PBFAM, and surveys sensing and control methods which have recently been investigated to increase overall part quality. Emphasis has been placed on novel developments in the field and their contribution to the understanding of the additive manufacturing process.

Medicaid Reform: Programming Solutions to the Equity Problem

This paper demonstrates the application of a mathematical programming model to a longstanding policy issue in the Medicaid reform debate: the redistribution of program funds necessary to achieve equity in eligibility and benefit coverage across states. The model is used to estimate the potential degree of equity achievable in the current Medicaid system given various budgetary and political constraints. Two model simulations, based on a 1979 data set for aged recipients of Supplementary Security Income, are presented. The results indicate that half or more of the interstate differences in spending for this population group are due to actuarial and efficiency factors rather than deviations from equity potential. The implications of eliminating the remaining differences are discussed.

Effect of directed energy deposition processing parameters on laser deposited Inconel® 718: External morphology

Through laser-based, directed energy deposition, single-track bead-on-plate clads of Inconel® 718 were deposited onto substrates of the same composition. Postprocessing analyses of the geometry of the single beads were made to assess the effects of changes in processing parameters. Laser power, travel speed, working distance, and initial substrate temperature were varied to alter the shape of the laser deposited material. The resulting geometries were analyzed through metallography and optical profilometry. This study concludes that laser power has the largest effect on bead width, and that working distance has the largest effect on bead height and angle of repose. Additionally, substrate preheating was found to amplify the effects of varying power on bead height and width. Empirical models were developed to describe the geometry of single beads based on chosen processing parameters. These models were compared to optical profilometry measurements for accuracy.

Effect of directed energy deposition processing parameters on laser deposited Inconel® 718: Microstructure, fusion zone morphology, and hardness

Single-bead, laser-deposited Inconel® 718 tracks atop substrates of the same composition were studied to ascertain the influence of laser power, processing speed, working distance, and substrate preheat on the fusion zone geometry, microstructure, and hardness. Modifying working distance encompassed both a change in powder flow distribution and beam diameter. Laser power and processing speed linearly affected fusion zone width and area, though laser power was found to have the most significant effect of all processing parameters. Preheating the substrates increased the width of the fusion zone by an average of 16% and led to a more uniform hardness throughout. The fusion zone cross-section was found to morph from semicircular to double-parabolic (wavy) with increasing laser power. This was attributed to surface tension induced Marangoni flow and the influence of surface-activated species on surface tension. The applicability of coupled parameters, including linear heat input and normalized enthalpy were i...

Sensing for directed energy deposition and powder bed fusion additive manufacturing at Penn State University

Additive manufacturing of metal components through directed energy deposition or powder bed fusion is a complex undertaking, often involving hundreds or thousands of individual laser deposits. During processing, conditions may fluctuate, e.g. material feed rate, beam power, surrounding gas composition, local and global temperature, build geometry, etc., leading to unintended variations in final part geometry, microstructure and properties. To assess or control as-deposited quality, researchers have used a variety of methods, including those based on sensing of melt pool and plume emission characteristics, characteristics of powder application, and layer-wise imaging. Here, a summary of ongoing process monitoring activities at Penn State is provided, along with a discussion of recent advancements in the area of layer-wise image acquisition and analysis during powder bed fusion processing. Specifically, methods that enable direct comparisons of CAD model, build images, and 3D micro-tomographic scan data will be covered, along with thoughts on how such analyses can be related to overall process quality.

Modeling of contact geometry and dopant profile during laser-silicon interaction

Laser fired contacts (LFCs) and laser doped selective emitters can be used to improve manufacturing throughput of photovoltaic devices without sacrificing device conversion efficiency. However, the laser parameters used to form these features can vary significantly. LFCs can be formed with short pulses (hundreds of nanoseconds) while selective emitters can be formed using either a pulsed or CW mode. Here, mathematical models for a pulsed laser and CW laser are used to evaluate how variations in processing parameters affects alloy formation, molten pool geometry and dopant concentration profiles. The models solve the conservation equations for mass, energy, and momentum to study the effects of heat and mass transfer and fluid flow on the formation of LFCs and emitters. Comparisons between experimental data and theoretical calculations for molten pool geometry and concentration profiles demonstrate good agreement. For LFCs, when assuming complete melting and mixing of the Al contact layer, the Al concentration varies significantly with power level, which drastically impacts the calculated pool shape. The dimensionless Peclet number is used to understand dominant heat and mass transfer mechanisms. Conduction is the dominant heat transfer mechanism at power levels around 20W for both LFCs and emitters. As the power level is increased to 50W, however, the dominant heat transfer mechanism changes to convection. Changes in laser parameters also impact fluid flow velocities and dopant concentration profile for emitters processed in CW mode, which suggests that convection-based models should be used to accurately predict concentration profiles within emitters.

fs laser surface processing of platinum

The production of nanometer-scale features on platinum samples was investigated with the use of a femtosecond (fs) laser system operational at IR, visible and UV wavelengths. Laser induced surface structures were produced having globular nanoprotrusions with apparent length scales ranging from sub-nanometer diameters to 100’s of nanometers, dependent on the laser parameters employed. Analysis of Field Emission Scanning Electron Microscope (FE-SEM) images revealed that each surface contained a distribution of structure sizes for which the frequency of occurrence for a given size increased appreciably as the particle/globule diameter decreased. In the field of biomedical implants, there is increasing interest in surface texturing to promote tissue in-growth as well as porous structures which promote delayed drug release. The nanometer-scaled features investigated may be applicable to such implants.The production of nanometer-scale features on platinum samples was investigated with the use of a femtosecond (fs) laser system operational at IR, visible and UV wavelengths. Laser induced surface structures were produced having globular nanoprotrusions with apparent length scales ranging from sub-nanometer diameters to 100’s of nanometers, dependent on the laser parameters employed. Analysis of Field Emission Scanning Electron Microscope (FE-SEM) images revealed that each surface contained a distribution of structure sizes for which the frequency of occurrence for a given size increased appreciably as the particle/globule diameter decreased. In the field of biomedical implants, there is increasing interest in surface texturing to promote tissue in-growth as well as porous structures which promote delayed drug release. The nanometer-scaled features investigated may be applicable to such implants.