Browsing by Author "Ranjan, Rajit"

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Classification and analysis of common simplifications in part-scale thermal modelling of metal additive manufacturing processes
(Springer, 2023-11) Ranjan, Rajit
Computational process modelling of metal additive manufacturing has gained significant research attention in recent past. The cornerstone of many process models is the transient thermal response during the AM process. Since deposition-scale modelling of the thermal conditions in AM is computationally expensive, spatial and temporal simplifications, such as simulating deposition of an entire layer or multiple layers, and extending the laser exposure times, are commonly employed in the literature. Although beneficial in reducing computational costs, the influence of these simplifications on the accuracy of temperature history is reported on a case-by-case basis. In this paper, the simplifications from the existing literature are first classified in a normalised simplification space based on assumptions made in spatial and temporal domains. Subsequently, all types of simplifications are investigated with numerical examples and compared with a high-fidelity reference model. The required numerical discretisation for each simplification is established, leading to a fair comparison of computational times. The holistic approach to the suitability of different modelling simplifications for capturing thermal history provides guidelines for the suitability of simplifications while setting up a thermal AM model.
Controlling local overheating in topology optimization for additive manufacturing
(Springer, 2022-05) Ranjan, Rajit
A novel constraint to prevent local overheating is presented for use in topology optimization (TO). The very basis for the constraint is the Additive Manufacturing (AM) process physics. AM enables fabrication of highly complex topologically optimized designs. However, local overheating is a major concern especially in metal AM processes leading to part failure, poor surface finish, lack of dimensional precision, and inferior mechanical properties. It should therefore be taken into account at the design optimization stage. However, including a detailed process simulation in the optimization would make the optimization intractable. Hence, a computationally inexpensive thermal process model, recently presented in the literature, is used to detect zones prone to local overheating in a given part geometry. The process model is integrated into density-based TO in combination with a robust formulation, and applied in various numerical test examples. It is found that existing AM-oriented TO methods which rely purely on overhang control do not ensure overheating avoidance. Instead, the proposed physics-based constraint is able to suppress geometric features causing local overheating and delivers optimized results in a computationally efficient manner
Design for manufacturability in additive manufacturing using a graph based approach
(ASME, 2015-09) Ranjan, Rajit
Additive Manufacturing (AM) processes are used to fabricate complex parts using a layer by layer approach. This enables designers to be more creative with their designs and build parts which may be difficult to manufacture using conventional processes. However, as AM is in its infancy, relevant literature with respect to design guidelines for AM is not readily available. This research proposes a novel approach to implement design guidelines in AM using a systematic graph based approach. These design rules will assist designers to come up with efficient part designs that can be manufactured with minimum part errors. The design rules are formulated by studying the relationship between input part geometry and AM process parameters. A feature graph based design analysis method is proposed along with a Producibility Index (PI) which is used to compare the designs. Modifications in part design based on these rules and their comparison is presented in the form of three case studies.
Fast detection of heat accumulation in powder bed fusion using computationally efficient thermal models
(MDPI, 2020) Ranjan, Rajit
The powder bed fusion (PBF) process is a type of Additive Manufacturing (AM) technique which enables fabrication of highly complex geometries with unprecedented design freedom. However, PBF still suffers from manufacturing constraints which, if overlooked, can cause various types of defects in the final part. One such constraint is the local accumulation of heat which leads to surface defects such as melt ball and dross formation. Moreover, slow cooling rates due to local heat accumulation can adversely affect resulting microstructures. In this paper, first a layer-by-layer PBF thermal process model, well established in the literature, is used to predict zones of local heat accumulation in a given part geometry. However, due to the transient nature of the analysis and the continuously growing domain size, the associated computational cost is high which prohibits part-scale applications. Therefore, to reduce the overall computational burden, various simplifications and their associated effects on the accuracy of detecting overheating are analyzed. In this context, three novel physics-based simplifications are introduced motivated by the analytical solution of the one-dimensional heat equation. It is shown that these novel simplifications provide unprecedented computational benefits while still allowing correct prediction of the zones of heat accumulation. The most far-reaching simplification uses the steady-state thermal response of the part for predicting its heat accumulation behavior with a speedup of 600 times as compared to a conventional analysis. The proposed simplified thermal models are capable of fast detection of problematic part features. This allows for quick design evaluations and opens up the possibility of integrating simplified models with design optimization algorithms.
Improving the manufacturability of metal AM parts
(Mikroniek, 2019) Ranjan, Rajit
Numerous challenges of additive manufacturing (AM) are tackled in the European Horizon 2020 project PAM^2 by studying and linking every step of the AM process cycle. For example, PAM^2 researchers from the design, processing and application side have collaborated in this work to optimise the manufacturability of metal AM parts using an improved Topology Optimisation (TO) approach, including a thermal constraint. Additionally, the project is focusing on modelling, post-processing, in- and post-process quality control and industrial assessment of AM parts, with the aim of moving beyond the state-of-the-art of precision metal AM.
Integration of design for manufacturing methods with topology optimization in additive manufacturing
(ASME, 2017) Ranjan, Rajit
Additive manufacturing (AM) processes are used to fabricate complex geometries using a layer-by-layer material deposition technique. These processes are recognized for creating complex shapes which are difficult to manufacture otherwise and enable designers to be more creative with their designs. However, as AM is still in its developing stages, relevant literature with respect to design guidelines for AM is not readily available. This paper proposes a novel design methodology which can assist designers in creating parts that are friendly to additive manufacturing. The research includes formulation of design guidelines by studying the relationship between input part geometry and AM process parameters. Two cases are considered for application of the developed design guidelines. The first case presents a feature graph-based design improvement method in which a producibility index (PI) concept is introduced to compare AM friendly designs. This method is useful for performing manufacturing validation of pre-existing designs and modifying it for better manufacturability through AM processes. The second approach presents a topology optimization-based design methodology which can help designers in creating entirely new lightweight designs which can be manufactured using AM processes with ease. Application of both these methods is presented in the form of case studies depicting design evolution for increasing manufac
A mold insert case study on topology optimized design for additive manufacturing
(2019) Ranjan, Rajit
The Additive Manufacturing (AM) of injection molding inserts has gained popularity during recent years primarily due to the reduced design-to-production time and form freedom offered by AM. In this paper, Topology Optimization (TO) is performed on a metallic mold insert which is to be produced by the Laser Powder Bed Fusion (LPBF) technique. First, a commercially available TO software is used, to minimize the mass of the component while ensuring adequate mechanical response under a prescribed loading condition. The commercial TO tool adopts geometry-based AM constraints and achieves a mass reduction of ~50 %. Furthermore, an in-house TO method has been developed which integrates a simplified AM process model within the standard TO algorithm for addressing the issue of local overheating during manufacturing. The two topology optimized designs are briefly compared, and the advantages of impleme
New support structures for reduced overheating on downfacing regions of direct metal printed parts
(2019) Ranjan, Rajit
In Laser Powder Bed Fusion (LPBF), the downfacing surfaces usually have increased surface roughness and reduced dimensional accuracy due to local overheating and warpage. To partially overcome this a new supporting structure is developed in this study, namely the contactless support. This is a thin blade parallel to the critical area which transfer the heat away from the melt pool via conduction through the powder bed instead of direct contact. The support is tested in different geometries and printing conditions to define the optimal distance from the part and its effectiveness is evaluated by measuring the surface roughness of the samples. Numerical modelling of heat transfer phenomenon is also employed to determine the thermal history of the printing process and understand which parameters define the optimal distance for the thermal supports. Finally topology optimization is used to create a support structure which minimize the wasted material while keeping the heat flow optimal.
Octree data structure for support accessibility and removal analysis in additive manufacturing
(Elsevier, 2018-08) Ranjan, Rajit
Metal Additive Manufacturing (AM) processes have made it possible to build parts with complex geometric features by adopting a layer-by-layer approach. However, additional support structures are needed to support overhanging surfaces and reduce distortion that may occur in these parts. This increases the overall build time of the part and leads to additional post processing efforts for removal of support structures. Often, removal of these supports becomes difficult due to complex part features that may interfere with support removal. Further, support structures have a detrimental effect on the surface finish on the areas of the part that come in contact with the supports. Thus, minimizing the need for support structures and ensuring its maximum removal is essential for an efficient part build in AM. Part build orientation is the main parameter that influences the need for support structures to build a part. This paper presents an approach to identify the best build orientation for a part such that the overall part build time is minimized while ensuring maximum removal of supports and minimizing the contact area between the part surface and supports. A hierarchical octree data structure has been used to analyze support accessibility and the area of support in contact with part. In addition, this paper also focuses on identification of optimal number/direction of part set-ups required to remove the maximum possible support structures from a part. A 2D setup map highlighting the feasible directions of setups for support removal has also been presented. The results of these analyses have been presented with the help of four sample parts.
Overheating control in additive manufacturing using a 3D topology optimization method and experimental validation
(Elsevier, 2023-01) Ranjan, Rajit
Overheating is a major issue especially in metal Additive Manufacturing (AM) processes, leading to poor surface quality, lack of dimensional precision, inferior performance and/or build failures. A 3D density-based topology optimization (TO) method is presented which addresses the issue of local overheating during metal AM. This is achieved by integrating a simplified AM thermal model and a thermal constraint within the optimization loop. The simplified model, recently presented in literature, offers significant computational gains while preserving the ability of overheating detection. The novel thermal constraint ensures that the overheating risk of optimized designs is reduced. This is fundamentally different from commonly used geometry-based TO methods which impose a geometric constraint on overhangs. Instead, the proposed approach takes the process physics into account. The proposed method is validated via an experimental comparative study. Optical tomography (OT) is used for in-situ monitoring of process conditions during fabrication and obtained data is used for evaluation of overheating tendencies. The novel TO method is compared with two other methods: standard TO and TO with geometric overhang control. The experimental data reveals that the novel physics-based TO design experienced less overheating during the build as compared to the two classical designs. A study further investigated the correlation between overheating observed by high OT values and the defect of porosity. It shows that overheated regions indeed show higher defect of porosity. This suggests that geometry-based guidelines, although enhance printability, may not be sufficient for eliminating overheating issues and related defects. Instead, the proposed physics-based method is able to deliver efficient designs with reduced risk of overheating.
A physics-based topology optimization method for enhancing precision in metal am parts
(TU Delft, 2022) Ranjan, Rajit
A physics-based topology optimization method for enhancing precision in metal am parts | TU Delft Repository TU Delft Library search Press enter to search in title/abstract in title/abstract in authors local_library Repository Title Metadata Abstract Files A physics-based topology optimization method for enhancing precision in metal am parts Conference paper (2022) Authors R. Ranjan Computational Design and Mechanics - Mechanical, Maritime and Materials Engineering Zhuoer Chen Chalmers University of Technology C. Ayas Computational Design and Mechanics - Mechanical, Maritime and Materials Engineering M. Langelaar Computational Design and Mechanics - Mechanical, Maritime and Materials Engineering Matthijs Langelaar Computational Design and Mechanics - Mechanical, Maritime and Materials Engineering A. van Keulen Mechanical Engineering Research Group Computational Design
Topology optimisation techniques
(CRC Press, 2020) Ranjan, Rajit
This chapter presents topology optimisation techniques suitable for designing parts that will be produced by additive manufacturing, with special attention to precision aspects. First, challenges associated with ‘design for additive manufacturing’ are briefly discussed. Next, an introduction to density-based topology optimisation is given. Specific additive manufacturing limitations relevant for high-technology precision parts are summarised and topology optimisation methods which address these limitations are described. Specifically, critical overhang elimination, overheating prevention and distortion reduction are considered. The industrial relevance of these additive manufacturing–friendly topology optimisation methods are demonstrated through case studies. Finally, an outlook on the current research trends is given and challenges ahead are highlighted.