Design Optimisation for 3D printed SLM objects

Selective laser melting of a high strength Al[sbnd]Mn[sbnd]Sc alloy: Alloy design and strengthening mechanisms

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 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" class="first" dir="auto" id="d13128e120">Laser powder bed fusion (LPBF), a typical metal additive manufacturing (MAM) process, generates large thermal gradients and rapid cooling rates, making it challenging to fully understand the relationship between nano-scaled solidification microstructure and process parameters. This study aims to uncover the nano-scaled solidification microstructure characteristics of 316L stainless steel (SS) produced through LPBF. A deeper understanding of microstructural development is crucial for fabricating high-performance materials by effectively controlling the AM process. <p xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" dir="auto" id="d13128e122"> <b>1. Introduction</b> <p xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" dir="auto" id="d13128e127">Laser powder bed fusion (LPBF) is currently the most versatile metal additive manufacturing (MAM) process, enabling the production of complex parts without the design limitations of traditional manufacturing methods. The rapid and repeated thermal cycles create intense thermal gradients, leading to metastable chemical, structural, and mechanical states, resulting in metallurgical defects that compromise material properties. Extensive research of LPBF-processed 316L SS has been conducted to improve the mechanical properties and highlight the need to correlate different microstructural features with the manufacturing process. Most studies have focused on characterizing microstructures from macro to sub-micrometer scales, with few investigating features at the nano or atomic levels. Therefore, this study aims to explore these nano/atomic scale microstructural features to better understand the relationship between solidification microstructure and process parameters. <p xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" dir="auto" id="d13128e129"> <b>2. Experiment</b> <p xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" dir="auto" id="d13128e134">The nominal composition of the 316L SS powder was 18Cr-14Ni-2.5Mo-0.03C (wt.%), and the powder size was under 53µm. LPBF fabrication was conducted using a 3D printer (EOSM290, EOS GmbH, Germany) equipped with a Yb-fiber laser by the “X-scan strategy,” i.e., the laser beam was scanned bidirectionally along the X-axis without rotation.

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.

Additive manufacturing (AM) technology can potentially disrupt offshore and marine industry by effectively reducing manufacturing processes and leadtime, and enabling new product designs. A review of existing literature has revealed limited knowledge in using AM to process shipbuilding materials, which can otherwise enhance existing shipbuilding manufacturing workflow. This has led to the research motivation to establish the technical feasibility of using selective laser melting (SLM) to process ASTM A131 EH36 shipbuilding steel. SLM process is a widely understood AM technology that has been used extensively to process other type of steels, for example stainless steel 316L. EH36 belongs to a class of high tensile low alloy steel which has traditionally been processed through casting. However, knowledge on using lasers to process EH36 is largely unknown, especially in AM applications. Using SLM to process EH36 is novel and will solve the problem of lack of knowledge in this field. The study thus aims to further the understanding of the mechanical properties and microstructure of SLM processed EH36. A preliminary investigation was first carried out to establish the technical feasibility of using SLM to process EH36. The process parameters obtained were then used to further the investigations. Heat treatment process was applied as a possible post processing technique. Mechanical testing was carried out to characterise its mechanical properties. The fracture surfaces and microstructures were then studied to characterise the material. The results showed that EH36 can be processed using SLM without any visible cracks. The mechanical properties of as built SLM processed EH36 exhibit very high tensile strength, but low ductility. The ductility can be improved through tempering heat treatment process, but at the partial sacrifice of tensile strength. Fractography analysis also affirmed the lack of ductility in the SLM processed EH36 samples. Finally, the microstructure showed that fine grain size and martensitic microstructure were the primary drivers behind its high tensile strength. Tempered samples experience grain coarsening and phase transformation to a mainly ferritic structure, which led to recovery in its ductility but a decrease in the tensile strength. A scaled model was fabricated to validate the results from the microstructure studies, and the findings indicate similar microstructures formed. This demonstrates the repeatability of the developed process parameters on fabrication of complex joints. The study contributes to the scientific knowledge with regard to material and mechanical property characterisation of SLM processed EH36. The findings from the fractography and microstructural analysis will contribute towards building the knowledge and facilitate future work on AM of EH36.

In the field of metal additive manufacturing (AM), one of the most used methods is selective laser melting (SLM)—building components layer by layer in a powder bed via laser. The process of SLM is defined by several parameters like laser power, laser scanning speed, hatch spacing, or layer thickness. The manufacturing of small components via AM is very difficult as it sets high demands on the powder to be used and on the SLM process in general. Hence, SLM with subsequent micromilling is a suitable method for the production of microstructured, additively manufactured components. One application for this kind of components is microstructured implants which are typically unique and therefore well suited for additive manufacturing. In order to enable the micromachining of additively manufactured materials, the influence of the special properties of the additive manufactured material on micromilling processes needs to be investigated. In this research, a detailed characterization of additive manufactured workpieces made of AISI 316L is shown. Further, the impact of the process parameters and the build-up direction defined during SLM on the workpiece properties is investigated. The resulting impact of the workpiece properties on micromilling is analyzed and rated on the basis of process forces, burr formation, surface roughness, and tool wear. Significant differences in the results of micromilling were found depending on the geometry of the melt paths generated during SLM.

Fatigue Assessment of Laser Additive Manufactured AlSi12 Eutectic Alloy in the Very High Cycle Fatigue (VHCF) Range up to 1E9 cycles

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.

Topology optimisation is an effective approach to design extreme lightweight components. However, most of the resulted optimum lightweight components usually have complex structures which cannot be produced successfully by traditional manufacturing processes. Selective laser melting is one of the additive manufacturing processes. It shows powerful capacity in the manufacturing of metal components with complex structures. Therefore, the combination of topology optimisation and selective laser melting shows a promising prospect for metal components. However, support structures were usually introduced during the selective laser melting manufacturing process, which resulted to some disadvantages, for example, the support structures are generally difficult to remove from the original components because it is difficult to clamping and machining. Therefore, a design method was proposed in this study, named lightweight and support-free design method, the detailed design process and advantages were presented. In the design process, topology optimisation was applied to realise lightweight design, and support-free design process was developed to meet the support-free requirement. Finally, as a case, a cross-beam component was designed using the proposed method, and the final model was produced successfully using selective laser melting process. The case study result verified that the proposed design method is effective to design lightweight and support-free industrial metal components for SLM process.

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.

In the Laser Powder Bed Fusion (L-PBF) process, 3D components with complex geometries are fabricated in a layer-by-layer fashion by using a controlled laser beam to selectively melt particular regions of the metal powder bed. However, due to the stochastic nature of the L-PBF process, the top surface roughness of each solidified layer tends to be different even when the optimal processing conditions for the different positions on the build plate are employed. As a result, the mechanical properties of the built components frequently vary from one component to the next. Accordingly, this study proposes an Intelligent Additive Manufacturing Architecture (IAMA) for controlling the surface roughness of each build layer through an appropriate adjustment of the laser re-melting parameters. The IAMA architecture comprises five modules, namely In-Situ Metrology (ISM), Ex-Situ Metrology (ESM), Automatic Virtual Metrology (AVM), Additive Manufacturing Simulation (AMS) and Intelligent Compensator (IC). The feasibility of the proposed architecture is demonstrated by comparing the top surface roughness of cubic and mechanical strengths of tensile test samples built using the proposed method with those built using a traditional L-PBF approach without surface roughness control. It is found that the samples fabricated using the IAMA approach have an average top surface roughness of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.6~\mu \text{m}$ </tex-math></inline-formula> and a standard deviation is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.7~\mu \text{m}$ </tex-math></inline-formula> . By contrast, the samples produced using the traditional L-PBF approach have an average surface roughness of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$13.45~\mu \text{m}$ </tex-math></inline-formula> and a standard deviation of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2.5~\mu \text{m}$ </tex-math></inline-formula> . In addition, the specimens produced with the assistance of IAMA architecture have an average tensile strength of 1013 MPa with a standard deviation of 69.5 MPa, while those printed without surface roughness control have an average tensile strength of 903 MPa with a standard deviation of 101.4 MPa Note to Practitioners—As L-PBF produce part in a layer-by-layer manner, therefore, the roughness on the top surface of previous layer have a strong influence on the printing quality of current layer. The variations of surface roughness will lead to the fluctuation of mechanical properties of the fabricated components. Additionally, to the best of author’s knowledge, current commercial L-PBF machines can not actively control the surface roughness of samples during the process. The proposed IAMA in this work can predict and control the roughness on the top surface of each layer during L-PBF process. Therefore, the constructed architecture has strong potential for integrating into the commercial L-PBF machine for controlling the surface roughness on the top of the parts during the manufacturing process. As a result, the quality of the fabricated components is expected to be in consistent. Accordingly, L-PBF machine equipped with IAMA will have a strong potential in applying for mass production of the components in aerospace and automobile industries.

&lt;p&gt;Additive Manufacturing (AM), more popularly known as 3D Printing, is a process for producing functional artifacts by adding layers of materials from data generated directly from 3D solid CAD models. Additive Manufacturing (AM) is the formalized term for what used to be called Rapid Prototyping and what is commonly referred to as 3D Printing. The key to how AM works is that parts are made by adding layers of material; each layer corresponding to a thin cross-section of the part derived from the original CAD data. Although most AM machines produce parts using polymers, there are an increasing number of machines that can directly fabricate in metals. The majority of these machines fabricate from raw material in powder form using a directed energy beam to create a local melt zone. Total hip replacement is recommended for people who have medical issues related to excessive wear of the acetabular, osteoarthritis, accident or age. Researches have shown that large numbers of hip arthroplasties (where the articular surface of a musculoskeletal joint is replaced), hip remodelling, or realignment are carried out annually and will increase in the next few decades. Manufacturing of acetabular shells by using AM is a promising and emerging method that has a great potential to improve public health. Lost wax casting or investment casting is currently used to produce acetabular shells followed by lengthy and complex secondary processes such as machining and polishing. Living organs and medical models have intricate 3D shapes that are challenging to identity in X-ray CT images. These images are used for preparing treatment plans to improve the quality of the surgeries regarding waiting and surgery time per procedure and care regime. For instance, a limited number of hip replacement procedures can be carried out on each acetabulum due to a decrease of bone thickness. Rapid prototyping is a suitable treatment planning tool in complex cases to enhance the quality of surgical procedure and provide long-term stability that can be used to customize the shape and size of the acetabular shell. In this paper, to analyse the manufacturing of a prosthetic acetabular shell, built-up lines resulting from a thermal stress flow and process stopping during the selective laser melting (SLM) AM process, with regarding Gibbs free energy, interfacial energy, and equilibrium temperature will be discussed. Geometrical measurements showed 1.59% and 0.27% differences between the designed and manufactured prototype for inside and outside diameter respectively. Experimental results showed that thermal stress flow in outer surfaces are compressive, but for inner surfaces are tensile, so built-up lines in inner and outer surfaces appear as a groove and dent respectively. The results also indicate that SLM is an accurate and promising method for fabrication of acetabular cup.&lt;/p&gt;

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.