Abdalla R. Nassar

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...

Investigations of laser-sustained plasma and its role in laser nitriding of titanium

Laser-sustained plasma (LSP) and CCD imaging of reactant species were employed to investigate the role of near-surface plasma in CO2 laser nitriding of titanium in open atmosphere. Insights were gained regarding the role of plasma processes and the role of reactive nitriding species in the nitriding process. Studies of single nitrided trails have identified the following regimes, as a function of LSP off-focal distance and beam translation speed, characterized by (1) the formation of heavily oxidized surfaces, (2) the formation of titanium nitride (TiN) nanoparticulate, (3) nitride formation in the absence of a surface-struck or LSP and (4) the formation of near-stoichiometric, oxide-free TiN surfaces with a LSP. For the first time it will be shown that the LSP can access nitriding conditions beyond those achieved with surface-struck plasma (or in the absence of plasma) to produce uniform, near-stoichiometric, titanium nitride coatings.

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.

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.

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.

Assessment of optical emission analysis for in-process monitoring of powder bed fusion additive manufacturing

ABSTRACT Developing methods which allow real-time monitoring of powder bed fusion (PBF) additive manufacturing (AM) processes is key to enabling in situ assessments of build quality (e.g. lack of fusion and porosity). Here, we investigate the use of optical emission spectroscopy and high-speed (100 kHz) measurement of select wavelength emissions, based on a line-to-continuum approach, to determine if a correlation between PBF AM process inputs, sensor outputs, and build quality exists. Using an open protocol system interfaced with a 3D Systems ProX 200 machine, sensor data were synchronised with the scanner position and the laser state during the buildup of Inconel-718 components under varying powers, scan speeds, and hatch spacing parameters. Sensor measurements were then compared against post-build computed tomography scans. We show evidence that sensor data, when combined with appropriate analyses, are related to both processing conditions and build quality.

Gutter design and selection for roof rainwater catchment systems

Water, while being critical for the survival of all of humanity, is not readily available to everyone. Within much of the developing world, water delivery systems are sub-par, leaving many homes without accessible and sustainable water resources. To address this issue, the use of rainwater harvesting systems is common. However, many extant systems are flawed in design, efficiency or sustainability. This paper investigates the impact of gutter cross-section on the performance and efficiency of rain water harvesting from roof catchments. Multiple gutter systems, with varying cross-sectional profiles, including a novel wrap-gutter design, were built and tested experimentally using a rainwater simulator. Experimental data, together with theoretical analyses, were used to rate gutter performance in terms of water lost though overflow, rate of water drainage, amount of standing water remaining in the gutter, amount of water loss via overshoot and the total amount of rain caught by the gutters. It was found that a wrap design, not normally highlighted in the literature, had the most consistent performance, regardless of rainfall intensity. Analyses regarding context-appropriate designs along with broader economic impacts of RWH systems are discussed.

The feasibility of rice bags as a low-cost and locally available alternative to greenhouse glazing

Greenhouses can help farmers increase their yields and improve their livelihoods while reducing spoilage and furthering food security. As farms are getting smaller and access to water is getting more difficult, greenhouses are gradually gaining popularity in the agrarian economies of sub-Saharan Africa. Most greenhouses sold in the market are designed for commercial farmers and are beyond the reach of smallholders. The Humanitarian Engineering and Social Entrepreneurship (HESE) program at Penn State has developed and commercialized affordable greenhouses that utilize locally-sourced materials. The only exception is the glazing - the plastic covering on the greenhouse structure - which is imported from abroad. The cost of this glazing is too high, and is subject to foreign exchange fluctuations and supply chain anomalies. In an effort to further decrease the cost of the greenhouse, and thereby increase its accessibility in the market, this article investigates the feasibility of locally-available, inexpensive materials that can be used as substitutes for typical glazing materials. The primary emphasis of this paper is on rice bags, which are an abundant, inexpensive material found commonly in developing countries. Three properties of rice bag glazing were tested: light transmission, UV resistance, and water conservation. Results indicated that while rice bags are not an ideal substitute for standard glazing, they may be appropriate as low-cost shade nets. It was also found that common bubble wrap, coated with a UV-absorbent coating, may adequately replace typical glazing.