Publications


Multi-scale mesh modeling software products and controllers
Pal, D., N. Patil, and B. Stucker, USPTO Publication number US20160321384 A1, 2014.

Simulation systems, manufacturing systems, software products and controllers are provided with multi-scale modeling in which a coarse mesh and a fme mesh that models a stimulus are decoupled. The fine mesh can be moved within the coarse mesh with a cut and paste operation. The coarse mesh is updated by sparsely propagated effects through the coarse mesh. Simulations of the invention can be conducted in real-time, and be used as controllers in manufacturing systems, such as additive manufacturing systems. A number of efficient methods are provided for solving meshing determinations that arise from movement of a stimulus modeled within a fine mesh.

Selective laser melting of HY100 steel: Process parameters, microstructure and mechanical properties
JJS Dilip, GDJ Ram, TL Starr, B Stucker, Additive Manufacturing, 13:49-60. 2017

Selective Laser Melting (SLM) of a high strength low alloy steel HY100 is considered in the present investigation. The current work describes (i) optimization of SLM process parameters for producing fully dense parts in HY100 steel and (ii) the effects of post-processing heat treatment on the microstructure and mechanical properties. Samples have been fabricated by SLM using different combinations of laser power, laser scan speed, and hatch spacing. Fully dense samples were achieved at an energy density of 65 J/mm3. Microstructures of the as-built and heat treated samples were investigated using optical and scanning electron microscopes, X-ray diffraction, and electron backscattered diffraction techniques. The as-built sample showed fully martensitic microstructure with alternate bands of untempered (hard) and auto-tempered (soft) regions. The as-built parts are unsuitable for direct application due to untempered, hard and brittle martensite microstructure. The as-built parts were subjected to post-processing heat treatments (“direct temper” and “quench and temper”). The direct tempered samples exhibited higher yield strength and ultimate strength than the quench and temper ones. Noticeable amounts of anisotropy with respect to the build orientation, especially in tensile elongation, were observed in the direct tempered samples due to in-homogenous microstructure. Quench and temper treatment of the parts resulted in recrystallized grains with uniform microstructure. The current investigation shows that quench and temper at 650 °C is an optimum post processing treatment for HY100 SLM parts as it manifests desired strength with good tensile elongation.

A review of defect modeling in laser material processing
C Teng, D Pal, H Gong, K Zeng, K Briggs, N Patil… -Additive Manufacturing, 2017

Thermomechanical modeling of laser material processing in general, and defect modeling in particular, has raised attention in both academia and industry for the last twenty years. Additive manufacturing (aka, 3D printing) is increasingly studied and utilized by researchers and engineers. Defects created during a part building process are costly to identify and could cause premature part failure, and thus numerous studies and research projects have been conducted in order to predict and analyze defects in laser material processing. The available information for defect modeling is scattered widely in the literature and mostly dedicated to very small and specific areas of focus, making it difficult for others to follow, even though the quantity of information is not small. In this work, a review of defect modeling which focuses specifically on the defect types existing in additive manufacturing industry has been carried out, including over 140 referenced articles.

Simulating melt pool shape and lack of fusion porosity for selective laser melting of cobalt chromium components
C Teng, H Gong, A Szabo, JJS Dilip, K Ashby, S Zhang… – Journal of Manufacturing Science and Engineering, 2017

Cobalt chromium is widely used to make medical implants and wind turbine, engine and aircraft components because of its high wear and corrosion resistance. The ability to process geometrically complex components is an area of intense interest to enable shifting from traditional manufacturing techniques to additive manufacturing (AM). The major reason for using AM is to ease design modification and optimization since AM machines can directly apply the changes from an updated STL file to print a geometrically complex object. Quality assurance for AM fabricated parts is recognized as a critical limitation of AM processes. In selective laser melting (SLM), layer by layer melting and remelting can lead to porosity defects caused by lack of fusion, balling, and keyhole collapse. Machine process parameter optimization becomes a very important task and is usually accomplished by producing a large amount of experimental coupons with different combinations of process parameters such as laser power, speed, hatch spacing, and powder layer thickness. In order to save the cost and time of these experimental trial and error methods, many researchers have attempted to simulate defect formation in SLM. Many physics-based assumptions must be made to model these processes, and thus, all the models are limited in some aspects. In the present work, we investigated single bead melt pool shapes for SLM of CoCr to tune the physics assumptions and then, applied to the model to predict bulk lack of fusion porosity within the finished parts. The simulation results were compared and validated against experimental results and show a high degree of correlation.

A new and efficient multi-scale simulation architecture for prediction of performance metrics for parts fabricated using additive manufacturing
D Pal, B Stucker – TMS 2015 144th Annual Meeting & Exhibition, Annual Meeting Supplemental, 2016

Performance metrics for parts made using Additive Manufacturing (AM), such as inter-and intralayer strength, are a function of the energy source, scan pattern(s) and material(s). Similarly, residual stress, surface finish and part distortion are a function of the state change of the material(s), scan pattern(s), overall geometry and post-fabrication steps.

A brief overview of newly developed computational tools for metal based AM to simulate performance metrics along with their experimental validations are discussed in this paper, including five major areas of AM simulation. These five areas are: (1) thermo-mechanical (2) the use of Euler angles for guaging static and dynamic strengths, (3) the intelligent use of matrix algebra and homogenization to extract the spatiotemporal nature of AM processes, (4) a fast GPU architecture and (5) algorithms targeted towards attaining an accurate faster than real-time simulation efficiency.

A novel method to fabricate TiAl intermetallic alloy 3D parts using additive manufacturing
JJS Dilip, H Miyanaji, A Lassell, TL Starr, B Stucker – Defence Technology, 2016

The present work explores the feasibility of fabricating porous 3D parts in TiAl intermetallic alloy directly from Ti–6Al–4V and Al powders. This approach uses a binder jetting additive manufacturing process followed by reactive sintering. The results demonstrate that the present approach is successful for realizing parts in TiAl intermetallic alloy.

The effects of material property assumptions on predicted meltpool shape for laser powder bed fusion based additive manufacturing
C Teng, K Ashby, N Phan, D Pal, B Stucker – Measurement Science and Technology, 2016

The objective of this study was to provide guidance on material specifications for powders used in laser powder bed fusion based additive manufacturing (AM) processes. The methodology was to investigate how different material property assumptions in a simulation affect meltpool prediction and by corrolary how different material properties affect meltpool formation in AM processes. The sensitvity of meltpool variations to each material property can be used as a guide to help drive future research and to help prioritize material specifications in requirements documents. By identifying which material properties have the greatest affect on outcomes, metrology can be tailored to focus on those properties which matter most; thus reducing costs by eliminating unnecessary testing and property charaterizations. Futhermore, this sensitivity study provides insight into which properties require more accurate measurements, thus motivating development of new metrology methods to measure those properties accurately.

Microstructures of Friction Surfaced Coatings – a TEM Study
J Akram, JJS Dilip, D Pal, B Stucker, PR Kalvala… – Practical Metallography, 2016

The microstructures of dissimilar metal welds between 9Cr-1Mo (Modified) (P91) and austenitic stainless steel (AISI 304) with Ni-based alloy interlayers (Inconel 625, Inconel 600 and Inconel 800H) are reported. These interlayers were deposited by the friction surfacing method one over the other on P91 alloy, which was finally friction welded to AISI 304. In this paper, the results of microstructural evolution in the friction surfaced coated interlayers (Inconel 625, 600, 800H) are reported. For comparative purposes, the microstructures of consumable rods (Inconel 625, 600, 800H) and dissimilar metal base metals (P91 and AISI 304) were also reported. Friction surfaced coatings exhibited dynamic recrystallization. In friction surfaced coatings, the carbide particles were found to be finer and distributed uniformly throughout the matrix, compared to their rod counterparts.

Selective Laser Melting of the Eutectic Silver-Copper Alloy Ag 28 wt % Cu
H Rieper, A Gebhardt, B Stucker – Navigation, 2016

The aim of this work was to perform a detailed investigation of the use of Selective Laser Melting (SLM) technology to process eutectic silver-copper alloy Ag 28 wt. % Cu (also called AgCu28). The processing occurred with a Realizer SLM 50 desktop machine. The powder analysis (SEM-topography, EDX, particle distribution) was reported as well as the absorption rates for the near-infrared (NIR) spectrum. Microscope imaging showed the surface topography of the manufactured parts. Furthermore, microsections were conducted for the analysis of porosity. The Design of Experiments approach used the response surface method in order to model the statistical relationship between laser power, spot distance and pulse time.

Influence of defects on mechanical properties of Ti–6Al–4V components produced by selective laser melting and electron beam melting
H Gong, K Rafi, H Gu, GDJ Ram, T Starr, B Stucker – Materials & Design, 2015

This study evaluates the mechanical properties of Ti–6Al–4 V samples produced by selective laser melting (SLM) and electron beam melting (EBM). Different combinations of process parameters with varying energy density levels were utilized to produce samples, which were analyzed for defects and subjected to hardness, tensile, and fatigue tests. In SLM samples, small pores in amounts up to 1 vol.% resulting from an increase in energy density beyond the optimum level were found to have no major detrimental effect on the mechanical properties. However, further increase in the energy density increased the amount of porosity to 5 vol.%, leading to considerable drop in tensile properties. Samples produced using lower-than-optimum energy density exhibited unmelted powder defects, which, even at 1 vol.% level, strongly affected both tensile and fatigue properties. In EBM, insufficient energy input was found to result in large, macroscopic voids, causing serious degradation in all mechanical properties. These findings are helpful in process optimization and standardization of SLM and EBM processes.

Simulation of Powder-Based Additive Manufacturing Processes
D Pal, C Teng, B Stucker – Additive Manufacturing: Innovations, Advances, and Applications, 2015

With the advent of new manufacturing technologies such as additive manufacturing (AM) processes, it has become increasingly possible to test design indeas and decrease the time between design, concept improvement iterations, and market-ready finished products, along with reducing costs, to produce and deliver these products to the desired customers. In particular, metals-based AM products are of significant interest because they find applications in the aircraft, automotive, medical implant, power tool, and traditional manufacturing tooling fields. Read more …

An efficient multi-scale simulation architecture for the prediction of performance metrics of parts fabricated using additive manufacturing
D Pal, N Patil, K Zeng, C Teng, B Stucker – Metallurgical and Materials Transactions A, 2015

In this study, an overview of the computational tools developed in the area of metal-based additively manufactured (AM) to simulate the performance metrics along with their experimental validations will be presented. The performance metrics of the AM fabricated parts such as the inter- and intra-layer strengths could be characterized in terms of the melt pool dimensions, solidification times, cooling rates, granular microstructure, and phase morphologies along with defect distributions which are a function of the energy source, scan pattern(s), and the material(s). The four major areas of AM simulation included in this study are thermo-mechanical constitutive relationships during fabrication and in-service, the use of Euler angles for gaging static and dynamic strengths, the use of algorithms involving intelligent use of matrix algebra and homogenization extracting the spatiotemporal nature of these processes, a fast GPU architecture, and specific challenges targeted toward attaining a faster than real-time simulation efficiency and accuracy.

A generalized feed forward dynamic adaptive mesh refinement and derefinement finite element framework for metal laser sintering — Part I: Formulation and algorithm development
D Pal, HK Rafi, K Zeng, A Moreland, A Hicks, D Beeler… – Journal of Manufacturing Science and Engineering, 2015

A novel multiscale thermal analysis framework has been formulated to extract the physical interactions involved in localized spatiotemporal additive manufacturing processes such as the metal laser sintering. The method can be extrapolated to any other physical phenomenon involving localized spatiotemporal boundary conditions. The formulated framework, named feed forward dynamic adaptive mesh refinement and derefinement (FFD-AMRD), reduces the computational burden and temporal complexity needed to solve the many classes of problems. The current study is based on application of this framework to metals with temperature independent thermal properties processed using a moving laser heat source. The melt pool diameters computed in the present study were compared with melt pool dimensions measured using optical micrographs. The strategy developed in this study provides motivation for the extension of this simulation framework for future work on simulations of metals with temperature-dependent material properties during metal laser sintering.

A generalized feed-forward dynamic adaptive mesh refinement and derefinement finite-element framework for metal laser sintering — Part II: Nonlinear thermal simulations and validations
D Pal, N Patil, KH Kutty, K Zeng, A Moreland, A Hicks… – Journal of Manufacturing Science and Engineering, 2016

A novel multiscale thermal analysis numerical tool has been developed to address the micro–macro interactions involved in localized melting and sintering processes, such as laser sintering of metals exhibiting nonlinear thermal response. The method involves extension of a feed-forward dynamic adaptive mesh refinement and derefinement finite-element framework to incorporate nonlinear thermal phenomenon in the vicinity of the energy source and further reduce computational time and complexity when simulating spatiotemporally periodic problems posed by metal laser sintering. The thermal and microstructural predictions computed using this framework are in good agreement with the thermal contours measured using a forward-looking infrared (FLIR) imaging system and microstructures observed using an optical microscope.

Comparison of 3DSIM thermal modelling of selective laser melting using new dynamic meshing method to ANSYS
K Zeng, D Pal, HJ Gong, N Patil, B Stucker – Materials Science and Technology, 2015

Selective laser melting (SLM) is an additive manufacturing (AM) process in which parts are fabricated by selectively melting regions of the surface of a metallic powder bed in a layer-by-layer fashion. Various thermal phenomena such as heat conduction, convection, radiation, melting and solidification, dynamic phase changes, and evaporation occur during the SLM process. In addition, laser intensity and powder bed scan speeds during processing complicate understanding of the process due to complex dynamic interactions between the powder bed and laser. In order to study these dynamic interactions, a finite element model has been developed which uses a dynamic mesh with spatial non-linear thermal properties to track the point of laser exposure on the powder bed to study thermal evolution during SLM. The model is able to achieve a refined, localised mesh in the melt zone and heat affected zone (HAZ), surrounded by a relatively coarse mesh outside of the HAZ regions. The dynamic meshing for this implementation is achieved using both the sub-modelling functionality in ANSYS and a new set of algorithms being commercialised by 3DSIM, LLC. It was found that dynamic meshing reduces the model size and computational burden. In this paper, the use of the sub-modelling approach for dynamic meshing was verified by comparing it against a uniform fine mesh model. The results of the two models match within an acceptable tolerance. Also, a mesh sensitivity analysis was carried out in order to show solution convergence as a function of increasing mesh density. The results of this analysis were also validated using experiments to show a match between experimental and simulated melt pools. Finally, the ANSYS solution was compared with a new set of dynamic meshing finite element analysis algorithms running in Matlab. It was found that these new algorithms were significantly faster than their ANSYS counterparts for solving problems using a dynamic mesh.

Evaluations of effective thermal conductivity of support structures in selective laser melting
K Zeng, D Pal, C Teng, BE Stucker – Additive Manufacturing, 2015

Simulations capable of predicting the complex thermal behavior which occurs in a selective laser melting (SLM) process would help design and manufacturing engineers build more optimum designs in a reliable manner. A multiscale feed forward adaptive refinement and de-refinement (FFD-AMRD) finite element framework has been developed in response to this need. Support structures fabricated during SLM to overcome residual stress induced part distortion are a key part of the process, and a representation of these support structures in a finite element framework must be considered. If support structures could be designed with minimal material usage while still maintaining an ability to withstand the residual stresses generated during the part fabrication, this would significantly impact industrial use of SLM. In this work, the effective thermal properties of support structures are represented using thermal homogenization. The effective thermal properties of the support structures have been found to be a function of their geometry, anisotropy and constituent independent thermal properties. The results from this study have been compared against standard models and a good match has been found. The objective of this work is to derive effective thermal property functions which could be directly incorporated in the FFD-AMRD framework mentioned above to enhance computational speed.

A study of transverse laser modes using a novel multi-scale simulation architecture for laser-based manufacturing technologies
N Patil, D Pal, C Teng, K Zeng, T Sublette, B Stucker – SPIE LASE, 2015

The present work presents an investigation of transverse laser modes in Selective Laser Melting (SLM). It includes detailed descriptions of process physics and various simulation tools that were developed at 3DSIM for SLM simulation. The SLM process depends on a focused laser directed towards a powder bed to selectively melt and solidify layers of powder to create a complex three dimensional geometry. The thermo-mechanical interaction of laser, powder bed and partially solidified part involves various nonlinear phenomena leading to final part microstructure, mechanical properties and geometrical accuracy. One important aspect of these interactions is the laser beam profile. Traditionally, Gaussian laser profiles with 00 transverse modes are used for SLM, since these are the only modes readily available for commercial purposes. The present work utilizes the SLM simulation tools at 3DSIM to study the potential for the use of transverse mode lasers for SLM. The interaction of transverse laser modes with characteristic thermal Eigenmodes of a typical powder bed has been modeled to further understand the effects of higher order laser modes on SLM performance. © (2015) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Additive manufacturing of metal cellular structures: design and fabrication
L Yang, O Harrysson, D Cormier, H West, H Gong… – JOM, 2015

With the rapid development of additive manufacturing (AM), high-quality fabrication of lightweight design-efficient structures no longer poses an insurmountable challenge. On the other hand, much of the current research and development with AM technologies still focuses on material and process development. With the design for additive manufacturing in mind, this article explores the design issue for lightweight cellular structures that could be efficiently realized via AM processes. A unit-cell-based modeling approach that combines experimentation and limited-scale simulation was demonstrated, and it was suggested that this approach could potentially lead to computationally efficient design optimizations with the lightweight structures in future applications.


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