Modeling and Simulation of Metal AM

Additive manufacturing with metal components is a complex, and currently cyclic, process due to the physical phenomena that are occurring. These phenomena can be mathematically modeled in order to predict the outcome of a specific aspect of the build. Coupling the mathematical models can then be used to develop a complete simulation, which can produce estimates for a range of characteristics for a part built using additive manufacturing techniques. This chapter will investigate the main models used in the simulation of metal AM. These models will include the modeling of thermal behavior, fluid dynamics, stress, and a selection of other auxiliary models, which are necessary to complete the simulations. For each of the models investigated, the various modeling techniques that have been developed will be presented along with their limitations, validation techniques, and parameters necessary to model the process correctly

Review of Metal AM Simulation Validation Techniques

Due to the complexity of metal AM (additive manufacturing), it can require many trial runs to obtain processing parameters which produce a quality build. Because of this trial and error process, the drive for simulations of AM has grown significantly. A simulation only becomes useful to researchers if it can be shown that it is a true representation of the physical process being simulated. Each process being simulated has a different method of validation to show it is an accurate representation of the process. This paper explores the various methodologies for validation of laser-based metal AM simulations, focusing mainly on the modeling of the thermal processes and other characteristics derived from the thermal history. It will identify and explain the various validation techniques used, specifically looking at the frequency of reported use of each technique

Review of AM Simulation Validation Techniques

Due to the complexity of Additive Manufacturing (AM), it can require many trial runs to obtain processing parameters which produce a quality build. Because of this trial and error process, the drive for simulations of AM has grown significantly. Simulations only become useful to researchers if it can be shown that they are true representations of the physical process being simulated. All simulations have different methods of validation to show that they are an accurate representations of the process. This paper explores the various methodologies for validation of laser based metal AM simulations, focusing mainly on the modeling of the thermal processes and other characteristics derived from thermal history. It will identify and explain the various validation techniques, specifically looking at the frequency of reported use of each technique

Anisotropy in Impact Toughness of Powder Bed Fused AISI 304L Stainless Steel

The current effort involved investigation into the anisotropy of AISI 304L fabricated through laser powder bed fusion. Charpy V‐notch specimens made from material fabricated at three different build orientations were tested and analyzed. A statistically significant difference among the toughness values indicates the presence of anisotropy within the additively manufactured material. While the lowest toughness was found in vertically built specimens, the horizontal specimens were found to exhibit the highest toughness. From the fracture surfaces, an atypical mode of failure was observed. Exclusive crack propagation along the interlayer track boundaries was observed. The toughness variation correlated with the ease of access for crack propagation along the interlayer track boundaries. From Weibull distribution fits of toughness data, the toughness of 3D printed 304L was more variant and lower in comparison with wrought 304L

Characterization of Impact Toughness of 304L Stainless Steel Fabricated through Laser Powder Bed Fusion Process

In this research, the impact toughness of powder bed based additively manufactured 304L stainless steel was investigated. Charpy specimens were built in vertical, horizontal and inclined (45⁰) orientations to investigate the variation in toughness with build direction. These specimens were tested in as-built and machined conditions. A significant difference in toughness was observed with varying build directions. The lowest toughness values were recorded when the notch was oriented in line with the interlayer boundary. The highest toughness was recorded when the notch was perpendicular to the interlayer boundary. A significant scatter in toughness values was also observed. The variation and distribution among the toughness values were modeled by performing 3-parameter Weibull fits. The performance and variation of the additively manufactured 304L were also compared with the toughness values of wrought 304 stainless. The additively manufactured material was observed to be significantly less tough and more variant in comparison to wrought material

CSAC Flight Experiment to Characterize On-Orbit Performance

Precise positioning, navigation, and timing requirements are driving a need for increasingly accurate spacecraft timing systems. This paper describes an experiment being developed at the University of Colorado Boulder to quantify the stability and behavior of a chip-scale atomic clock (CSAC) onboard an Air Force Research Laboratory (AFRL) University Nanosatellite Program (UNP-9) MAXWELL CubeSat mission. The CSAC experiment will run onboard MAXWELL, enabling the GPS receiver measurements to occur using the unsteered CSAC as an external clock. The experiment will record and downlink the position, clock bias, pseudorange, phase, and temperature. These data will allow us to characterize the on-orbit performance of the CSAC

Escape from an optoelectronic tweezer trap: experimental results and simulations

Optoelectronic tweezers (OET) are a microsystem actuation technology capable of moving microparticles at mm s−1 velocities with nN forces. In this work, we analyze the behavior of particles manipulated by negative dielectrophoresis (DEP) forces in an OET trap. A user-friendly computer interface was developed to generate a circular rotating light pattern to control the movement of the particles, allowing their force profiles to be conveniently measured. Three-dimensional simulations were carried out to clarify the experimental results, and the DEP forces acting on the particles were simulated by integrating the Maxwell stress tensor. The simulations matched the experimental results and enabled the determination of a new “hopping” mechanism for particle-escape from the trap. As indicated by the simulations, there exists a vertical DEP force at the edge of the light pattern that pushes up particles to a region with a smaller horizontal DEP force. We propose that this phenomenon will be important to consider for the design of OET micromanipulation experiments for a wide range of applications

Predictive Model for Thermal and Stress Field in Selective Laser Melting Process -- Part II

Finite Element Analysis (FEA) is used to predict the transient thermal cycle and optimize process parameters to analyze these effects on deformation and residual stresses. However, the process of predicting the thermal history in this process with the FEA method is usually time-consuming, especially for large-scale parts. In this paper, an effective predictive model of part deformation and residual stress was developed for accurately predicting deformation and residual stresses in large-scale parts. An equivalent body heat flux proposed from the single layer laser scan model was imported as the thermal load to the layer by layer model. The hatched layer is then heated up by the equivalent body heat flux and used as a basic unit element to build up the macroscale part. The thermal history and residual stress fields of two solid parts with different support structures during the SLM process were simulated. Layer heat source method has the capability for fast temperature prediction in the SLM process, while sacrificing modeling details for the computational time-saving purpose. Thus numerical modeling in this work can be a very useful tool for the parametric study of process parameters, residual stresses and deformations

Large expert-curated database for benchmarking document similarity detection in biomedical literature search

Document recommendation systems for locating relevant literature have mostly relied on methods developed a decade ago. This is largely due to the lack of a large offline gold-standard benchmark of relevant documents that cover a variety of research fields such that newly developed literature search techniques can be compared, improved and translated into practice. To overcome this bottleneck, we have established the RElevant LIterature SearcH consortium consisting of more than 1500 scientists from 84 countries, who have collectively annotated the relevance of over 180 000 PubMed-listed articles with regard to their respective seed (input) article/s. The majority of annotations were contributed by highly experienced, original authors of the seed articles. The collected data cover 76% of all unique PubMed Medical Subject Headings descriptors. No systematic biases were observed across different experience levels, research fields or time spent on annotations. More importantly, annotations of the same document pairs contributed by different scientists were highly concordant. We further show that the three representative baseline methods used to generate recommended articles for evaluation (Okapi Best Matching 25, Term Frequency-Inverse Document Frequency and PubMed Related Articles) had similar overall performances. Additionally, we found that these methods each tend to produce distinct collections of recommended articles, suggesting that a hybrid method may be required to completely capture all relevant articles. The established database server located at https://relishdb.ict.griffith.edu.au is freely available for the downloading of annotation data and the blind testing of new methods. We expect that this benchmark will be useful for stimulating the development of new powerful techniques for title and title/abstract-based search engines for relevant articles in biomedical research.Peer reviewe