A lightweight and support-free design method for selective laser melting

The manufacture of lightweight components is one of the most important requirements in the automotive and aerospace industries. Gears, on the other hand, are among the heaviest parts in terms of their total weight. Accordingly, a spur gear was considered, the body of which was configured as a lattice structure to make it lightweight. In addition, the structure was optimized by topology optimization using ProTOP software. Subsequently, the gear was manufactured by a selective laser melting process by using a strong and lightweight material, namely Ti-6Al-4V. This study defeated the problems of manufacturing orientation, surface roughness, support structure, and bending due to the high thermal gradient in the selective laser melting process. To experimentally investigate the benefits of such a lightweight gear body structure, a new test rig with a closed loop was developed. This rig enabled measurements of strains in the gear ring, hub, and tooth root. The experimental results confirmed that a specifically designed and selectively laser-melted, lightweight cellular lattice structure in the gear body can significantly influence strain. This is especially significant with respect to strain levels and their time-dependent variations in the hub section of the gear body.

This study aims to explore lightweight and selective laser melting (SLM) process design for automobile wheels to improve the fuel economy and environmental sustainability of automobiles, which are studied using Altair Inspire and Simufact Additive software, respectively. First, a lightweight design is achieved while ensuring the safety factor through topology optimization and stress analysis. Then, the effects of the part placement, laser power, scanning speed, post-annealing temperature and time on the residual stress and distortion of the wheel are studied. The optimal printing method and process parameters that can minimize stress and distortion are obtained. The research results show that the weight of the automobile wheel after topology optimization is reduced by 7%, which reduces the material cost while ensuring the safety factor under the specified load. Through the simulation of the SLM process, it is found that adopting a horizontal placement method, reducing the laser power to 300 W, increasing the scanning speed to 3 m s−1, and increasing the annealing temperature to 500 °C can minimize the residual stress and strain of the wheel. Since the residual stress is basically stable when the annealing holding time is more than 0.3 h, 0.3 h should be the optimal holding time in consideration of saving post-processing time and shortening the production cycle.

Due to recent trends in the growing demand for customized and better product quality, inappropriate selection of linear energy density values during the selective laser melting (SLM) process will not only result in a decrease in productivity but will also have negative implications on environmental performance. The SLM process is complex and involves multiple inputs; therefore, the physical-based models are difficult to be formulated. In this context, the present work proposes two variants of the evolutionary approach using genetic programming (GP) in the formulation of a functional expression between the density of fabricated parts and seven inputs of the SLM process. These variants are proposed by introducing two model selection criteria from statistical learning theory to be used as fitness functions in the GP framework. The performance of the proposed energy-based density models is evaluated against the actual experimental data based on five statistical metrics and hypothesis testing. The relationships between the system parameters are unveiled and can be used for effectively monitoring the economic and environmental performance of SLM processes. It was found that the hatching space and linear energy density have the highest impact on the SLM density component. A major contribution of the study is that the optimum values of the inputs can be selected to curtail the energy usage from the SLM process.

Selective laser melting (SLM) is an important additive manufacturing process. It applies focused laser energy to quickly melt and solidify material powders, and a controlled layered operation can result in a free form build that is often out of reach for machining processes. As such, it has attracted much attention in recent years. However, metal components produced by this process often have inferior mechanical properties, as compared with the counterparts by the traditional manufacturing processes. To strengthen the metal components by SLM, adding reinforcement particles and applying post treatment are regarded as the two effective ways. Although adding reinforcement particles to create metal matrix composites has been studied by researchers in literature, much fewer has been done to use post treatment processes to further improve the properties and performance of the metal matrix composites from SLM. In this study, a nano-TiC reinforced Inconel 718 composite is prepared using SLM technique. The material has 0.5 wt.% nano-TiC addition. Solid solution treatments at three levels of temperature (940, 980, 1020 °C) are carried out to evaluate the effect of the heat treatment methods on the microstructure and resulted mechanical properties of the composite material. The results of samples with and without heat treatment are also compared. SEM observations are carried out to analyze the microstructure of the composite and understand the reinforcing mechanism. Tensile tests are conducted to evaluate the mechanical properties of the formed composites. It is discovered that compared with the pure Inconel 718 by SLM, the Inconel 718-TiC composite exhibits improved ultimate tensile strength. Microscopy observation of as-built samples indicates that the dendritic structures of Inconel 718 is remarkably refined by the TiC particles. Suspected laves phase particles are observed in as-built Inconel-TiC composite, and they partially transform to large amount of needle-like δ phase during the solid solution treatment.

Comparison of AlSi7Mg0.6 alloy obtained by selective laser melting and investment casting processes: Microstructure and mechanical properties in as-built/as-cast and heat-treated conditions

Among the many additive manufacturing (AM) processes for metallic materials, selective laser melting (SLM) is arguably the most versatile in terms of its potential to realize complex geometries along with tailored microstructure. However, the complexity of the SLM process, and the need for predictive relation of powder and process parameters to the part properties, demands further development of computational and experimental methods. This review addresses the fundamental physical phenomena of SLM, with a special emphasis on the associated thermal behavior. Simulation and experimental methods are discussed according to three primary categories. First, macroscopic approaches aim to answer questions at the component level and consider for example the determination of residual stresses or dimensional distortion effects prevalent in SLM. Second, mesoscopic approaches focus on the detection of defects such as excessive surface roughness, residual porosity or inclusions that occur at the mesoscopic length scale of individual powder particles. Third, microscopic approaches investigate the metallurgical microstructure evolution resulting from the high temperature gradients and extreme heating and cooling rates induced by the SLM process. Consideration of physical phenomena on all of these three length scales is mandatory to establish the understanding needed to realize high part quality in many applications, and to fully exploit the potential of SLM and related metal AM processes.

<p>A common misconception about additive manufacturing (3D printing) is that any shape can be made in any material at the press of a button. The reality is that each process and material requires distinct Computer Aided Design (CAD) files that need to be optimised to the physical limitations of the manufacturing process. This optimisation process can have significant effects on the designer’s aesthetic intentions. Selective Laser Melting (SLM) is the new benchmark for functional 3D printed titanium designs where the optimisation process plays an important role in the outcome of the end product. The limitations imposed by the manufacturing process include build support material, heat transfer and post processing and designs are required to be optimised before the manufacturing process can commence. To date, case studies written on the SLM process have focused largely on engineering and functional applications in particular within the medical industry. However; this process has not been extensively studied from a visual and aesthetic industrial design perspective. This research will gather specific knowledge about the technical limitations involved in the Selective Laser Melting process and explore through a case study approach how a designer s intentions can be maintained or even enhanced when using this technology. With greater understanding of the SLM technology, the optimisation process may further provide positive outcomes to the designer by saving time, money and waste. This case study is built on an existing product design file as a base model. Refinements to the model were made based on findings from existing design research as well as digital and physical models. The existing design research was focused on challenges designers encounter using 3D printing technologies including SLM as well as the optimisation process. Models and design iterations were developed using Nigel Cross’s four step model of exploration, generation, evaluation and communication. By iteratively redesigning aspects of the model to conform to the SLM limitations, this study reviews opportunities for areas to reduce material without compromising the design intent.</p>

In selective laser melting (SLM) process, the build part quality is determined by process parameters such as laser scanning speed and power. The presence of porosity, a major printing defect that significantly affects part performance, may arise in laser melting process due to insufficient or excess energy input. The improvement of build quality heavily depends on fundamental understanding of porosity formation in the SLM process. In this study, the discrete element method (DEM) has been utilized to simulate the creation of a newly deposited powder layer. A computational fluid dynamics (CFD) model was developed to simulate the melting and solidification process of Ti-6Al-4V powders in the SLM process. The thermo-fluid model includes effect of surface tension and recoil pressure as well as laser ray multi-reflection in keyhole. The predictability of the developed CFD model has been validated against literature experimental data. It is found that the collapse of an unstable deep keyhole was responsible for the formation of pores. In addition, higher laser scanning speeds tend to form unstable melt pools, e.g., melt pool break-up.

WC–Co cemented carbides, well-known as the conventional tooling materials, have not been successfully produced by one step additive manufacturing processes such as selective laser melting (SLM) yet. The microstructure evolution as well as WC grain growth behavior has rarely been investigated in detail during SLM process. In this study, the WC–Co cemented carbides with different Co contents (12–32 wt%) were prepared by optimized SLM processes for comparative investigation of densification behavior, microstructure characterization and mechanical property. The increase in Co content in feedstock carbide granules can improve the densification behavior during SLM process. The SLM processed WC-12Co shows larger average WC grain size and higher percentage of coarser WC grains as compared with both WC-20Co and WC-32Co. The microstructure characterization, combined with finite element simulation, shows the WC grain growth mechanisms include agglomeration and dissolution-deposition of WC during SLM process and agglomeration of WC is an important mechanism especially for WC–Co cemented carbides with Co content as low as 12 wt%. The comparison between horizontal (perpendicular to the SLM laser beam) and vertical (parallel to the SLM laser beam) cross sections of carbides shows that SLM process introduces a certain degree of microstructure and mechanical behavior anisotropy for WC-12Co, WC-20Co, and WC-32Co.

Repeatability and reproducibility of parts produced by selective laser melting is a standing issue, and coupled with a lack of standardized quality control presents a major hindrance towards maturing of selective laser melting as an industrial scale process. Consequently, numerical process modelling has been adopted towards improving the predictability of the outputs from the selective laser melting process. Establishing the reliability of the process, however, is still a challenge, especially in components having overhanging structures.In this paper, a systematic approach towards establishing reliability of overhanging structure production by selective laser melting has been adopted. A calibrated, fast, multiscale thermal model is used to simulate the single track formation on a thick powder bed. Single tracks are manufactured on a thick powder bed using same processing parameters, but at different locations in a powder bed and in different laser scanning directions. The difference in melt track widths and depths captures the effect of changes in incident beam power distribution due to location and processing direction. The experimental results are used in combination with numerical model, and subjected to uncertainty and reliability analysis. Cumulative probability distribution functions obtained for melt track widths and depths are found to be coherent with observed experimental values. The technique is subsequently extended for reliability characterization of single layers produced on a thick powder bed without support structures, by determining cumulative probability distribution functions for average layer thickness, sample density and thermal homogeneity.

A comparison of experimental compressive axial loading testing with a numerical simulation of topologically optimized cervical implants made by selective laser melting

Selective laser melting (SLM) is one of the Additive manufacturing (AM) processes that can build physical part in an added material method from digital data. In such a process, computer designed part model will be decomposed into hundreds of thousands of layers. The layered information is then transferred to SLM equipment and the part is built in a layer by layer fashion. Each powder layer will be scanned and melted in the required region by a high energy laser beam in a given scanning pattern so as to form a desired geometry. Finally, fully functional parts can be produced by repeatedly powder deposition, melting and solidification process. This process offers numerous advantages such as tooling-free productions and design freedom in geometry. In addition, SLM process is quite suitable for complicated parts such as customer designed medical implants and internal channels which are difficult to manufacture by conventional methods such as casting and machining. However, the localized heating and cooling process can lead to defects such as high residual stress, part distortion or delamination failure in SLM fabricated parts. These potential defects may impede the wide application of this technology. It is known that the laser beam scanning path will affect the thermomechanical behaviors of the build part, and thus, altering the scanning pattern may be a feasible strategy to reduce residual stresses and deformations by influencing the heat intensity input distribution. In this study, a 3D sequentially coupled finite element method (FEM) model, incorporating a volumetric moving Gaussian heat source, powder as well as solid material temperature dependent properties and layer addition features, was developed to study the complex thermomechanical process of SLM. The model was applied to evaluate six different scanning strategies effect on part temperature, stress and deformation. The major results have been summarized as follows. (1) Among all cases tested, the out-in scanning pattern has the maximum stresses along the X and Y directions; while the 45 degree inclined scanning may reduce residual stresses in both directions. (2) Large directional stress difference can be caused by back and forth line scanning strategy while minor directional stress difference is observed for other tested cases. (3) X and Y directional stress concentration is shown around the edge of deposited layers and the interface between deposited layers and substrate for all cases. (4) The 45 degree inclined scanning case has the smallest build direction deformation while the in-out scanning case has the largest deformation among the tested cases.

PurposeAdditive manufacturing raw material cost has been recently confirmed as a significant obstacle to widespread deployment of these technologies in industry. Aiming at reducing the cost of the selective laser melting (SLM) process, the purpose of this paper is to evaluate the different properties of products fabricated by SLM using low‐cost ($10/Kg) feedstock 304L stainless steel powders. The entire process cost was also evaluated.Design/methodology/approachUsing an experimental approach, 24 samples with different shapes and sizes were fabricated with layer thickness of 30, 50 and 70 μm and laser scanning speed set at 70 and 90 mm/s. Part geometry, dimensional tolerance, surface quality, density, mechanical properties and microstructure were evaluated.FindingsResults confirmed that the SLM of low‐cost 304L powder was successful and could produce functional parts with fine details and small wall thickness. Using small layer thickness and low scanning speed improved the properties by more than 20 per cent. At a layer thickness of 30 μm and speed of 70 mm/s, density was 92 per cent and hardness was 190 HV. At layer thickness of 70 μm porosity increases and cracks started to form which decreased strength and ductility. The steel remained austenitic with no carbide films at grain boundaries due to the high melting and cooling cycles.Research limitations/implicationsThis research was limited to 304L powders. Future work should be done on different materials and should include the effect of post processing heat treatment on improving the mechanical properties and microstructure.Practical implicationsThe cost of the SLM process using feedstock powders was less than 10per cent of the cost of using the special powders from a machine manufacturer with almost no effect on product quality.Originality/valueThe paper describes how cost reduction in the SLM process was achieved by using 304L powder.

The unstable forming quality of parts formed by selective laser melting (SLM) process has been one of the obstacles of its development and application, and the thermal process directly influences the forming quality in SLM process, such as the melt pool geometry. For the sake of studying the influence of different process parameters on the melt pool size in SLM forming process, a finite element model by ANSYS was established and single-layer single-channel temperature field imitation of the SLM 316 L stainless steel part under the combination of different laser power, scanning speed, focusing spot diameter and layer thickness was conducted in this paper. Since the neural network (NN) can fully approximate the complex nonlinear relationship, the melt pool size obtained by simulation is used as the training samples, and the NN model is trained to establish the mapping relation model between the SLM process parameters and the melt pool size, which provides the reference for the SLM process parameter optimization. The experimental results indicate that the deviation between the predicted results and the measured results is less, which indicates that the model has high prediction accuracy. A good mapping relation between the studied process parameters and the melt pool size is established.

Structural optimization of re-entrant negative Poisson's ratio structure fabricated by selective laser melting