Surface Additive Manufacturing Research Articles

Metal-metal and metal-composite joints with 3D printed aluminium substrates: effect of surface treatment on the mode I fracture toughness

ABSTRACT Adhesive bonding of composite laminates to metals is an increasingly employed technique to manufacture lightweight structures. However, this type of joints is sensitive to defects and composite delamination near the bonded area. Thus, increasing their damage tolerance is fundamental to design reliable multi-material components. This work is the first step in a project aimed at improving the fracture toughness of Carbon Fibre Reinforced Polymer (CFRP)/metal joints, exploiting Additive Manufacturing (AM) to produce small features on the metal adherend. This first step is devoted to the qualification of the Mode I fracture toughness of the as-built AM surface. Thus, Double Cantilever Beam (DCB) tests are carried out on metal/metal secondary bonded and CFRP/metal co-bonded joints. The metal part is obtained by Selective Laser Melting (SLM) of AlSi10Mg aluminium alloy, without surface pre-treatment to preserve the as-built roughness. The fracture toughness of the AM un-treated joints is also compared to that of joints in which the metal adherend is sandblasted. Optical profilometry is employed to evaluate the surface morphology of both as printed and sandblasted adherends. The results show that the untreated joints have comparable fracture toughness to their treated counterparts, despite manifesting dissimilar failure modes due to the different surface morphologies.

Mass finishing of additively manufactured Ti6Al4V parts: An investigation of surface finish dependency on build orientation and processing conditions

Poor surface finish of as-built metal Additive Manufacturing (AM) remains a key challenge in fabricated parts. Metal AM parts typically require postprocessing to improve surface finish before application. Furthermore, different surface orientations and scanning strategies in Powder-Bed-Fusion (PBF) metal AM are prone to result in non-uniform surface roughness across different surfaces of a part. In recent years, Mass Finishing (MF) technologies, such as Centrifugal Disc Finishing (CDF), are gaining popularity in improving AM surfaces due to its fixture-free, batch-processing, and relatively low costs. There is a critical need to study the surface finish dependency on AM build orientation and MF processing conditions. In this study, Ti6Al4V ASTM-E8b tensile bars were fabricated in 45⁰ and 90⁰ orientation via Laser Powder Bed Fusion (L-PBF) to investigate the effects of CDF processing conditions on upward facing surface (up-skin), downward facing surface (down-skin), and side facing surface (side-skin). The surface roughness evolution on each type of surface were collected via laser scanning profilometry. It was found that both processing speed and processing time of CDF were statistically significant factors in improving AM surface roughness, however, processing time had higher impact. Furthermore, different build orientation demonstrated different surface roughness reduction rate and material removal rate. It was found that side-skin had the highest average roughness reduction, and the down-skin exhibited the least roughness reduction. In addition, this study found that the surface with the highest initial surface roughness (down-skin) had the least material removal rate, and that the highest material removal rate was observed in surface with higher periodicity (side-skin). Findings from this study will help the AM community in decision-making of CDF conditions to prevent over-finishing and under-finishing. In addition, the investigation of surface evolution dependency on AM build orientation can further improve the understanding of key mechanisms of AM + MF hybrid manufacturing system.

High temperature friction and wear of post-machined additively manufactured tool steel during sliding against AlSi-coated boron steel

In recent years, additive manufacturing (AM) of metallic materials has achieved the production of virtually fully dense parts, extending the range of potential applications. There is a growing interest in the use of AM to produce forming tools for hot stamping. The possibilities of locally tailoring the die material to tackle wear challenges and producing more complex geometries to improve die cooling are key-features driving that interest. However, there is a lack of knowledge concerning the tribological behavior of AM materials, particularly at high temperature, as well as the influence surface finishing processes after additive manufacturing. The aim of this study is to investigate the high temperature friction and wear behavior of a tool steel, produced by selective laser melting, within the context of hot forming of AlSi-coated boron steel. A high temperature strip drawing tribometer was used to perform sliding tests at 600 °C and 700 °C. Three different surface finishes were used for the AM samples: ground, milled and shot-blasted. A conventionally produced steel with the same chemical composition and a ground surface finish was used as a reference. At 600 °C, a similar stable coefficient of friction of 0.4 was observed for both materials and all surface topographies. At 700 °C, all tests resulted in a sudden increase in friction up to 0.9 due to local rupture of the AlSi-coating, severe material transfer and ploughing. The wear mechanisms observed for the ground surfaces, both AM and reference tool steel, were a combination of adhesive material transfer and abrasive material removal that promotes material pile-up, resulting in wedge formation on the tool steel surface. The characteristic wedge formation was not common in the milled surface. This is attributed to strain-hardening and topographical features from the finishing process. For the shot-blasted AM surface, deformation and flattening of the large asperities was observed, as well as material transfer. Subsurface deformation associated with high adhesion during sliding was observed, mainly for the ground surfaces. The milled surface resulted in the least amount of tool steel transfer onto the counter body, while the shot-blasted one resulted in the largest amount. AM and reference ground tool steel showed very similar friction and wear behavior in this tribosystem.

Ultra-precision surface finishing and feature generation on metal additive manufactured components with controlled tool path design

Metal additive manufacturing (AM) is a rapidly emerging technology gaining significant attention in industries. The intrinsic advantages of design freedom and mass customization establishes the dominance of AM over conventional methods. In the present scenario, nanofinished metallic surfaces with micro-features forms major requirement in industries for controlling reflectivity, fluid retention capacity and frictional performance of surfaces. However, the existing metal AM technology is incapable to produce high resolution microfeatures over the part surfaces due to restriction in laser spot diameter. Moreover, the fabricated metal AM parts are far different from nominal CAD design model due to form deviations and the presence of excessive surface irregularities. The aforementioned challenges lower the efficacy of metal AM surfaces in optical and tribological applications. Thus, a secondary surface post-treatment method is inevitable to overcome aforementioned limitations. Therefore, the present study proposes a hybrid method combining metal AM with ultra-precision surface finishing to achieve nanofinished features on customized metal AM components. The unmelted powder particles and other surface irregularities were eliminated through multiple finishing passes which ultimately yielded a surface finish of ∼ 49 nm. Based on designed tool path, micro-feature generation was successfully accomplished on metal AM surface with minimal deviations in the geometric profile of the feature.

A Study on Electrochemical Polishing of Additively Manufactured Ti6Al4V Complex Parts and Its Influence on Corrosion Behavior

Nowadays, metal additive manufacturing (AM) has become popular for creating complex geometries that are impossible to create with conventional manufacturing methods. However, the surface quality of as-built AM parts is typically not (uniformly) smooth. Among different imperfections leading to irregular surface morphology, we can mention the staircase effect due to the layer-by-layer nature of the deposition techniques, partially fused feedstock material, balling effects, spatter, and inadequate fusion, where each impact on surface finish depends on the deployed AM process method. Typically, the surface quality of an as-built part is not satisfactory for functional biomedical applications. To address this issue, many research studies are focusing on optimizing part production variables, such as feed stock material, design of the part, AM process parameters and condition of fusion deposition and binding. However, in most cases, to meet the final part requirements, post-processing after AM production is necessary. Different post-processing methods such as abrasive polishing, laser polishing, chemical etching, and electrochemical polishing are established over the years. Among others, electrochemical polishing (ECP) is a suitable post-processing method for metal additive manufactured parts, particularly those with complicated geometry and inaccessible surfaces such as lattice structures. Titanium and its alloys are commonly used in biomedical applications because of their fracture toughness, corrosion resistance, and biocompatible properties, as well as their fatigue strength. Therefore, Ti-6Al-4V is selected as AM feedstock material for lattice structure fabrication in this work.The principal objectives of this study are 1) to characterize the surface of the lattice structure’s struts to observe the influence of post-processing on the core of the lattice and ensure achieving a uniform lattice structure; and 2) to study the influence of electrochemical polishing (ECP) on corrosion behavior of AM lattice structures as well as on AM flat surfaces.In a first step (Figure 1a), the best ECP condition, based on a Taguchi method design of experiment (DOE), on a flat additively manufactured titanium surface is established. The applied potential, electrochemical polishing time, and the electrode distance (the distance between working and counter electrode) are considered as the ECP parameters in the DOE, and for each parameter two levels are considered. Therefore, four experiments are designed, and two samples are polished for each experiment. In order to observe the effect of each experiment, a Confocal characterization is performed (Figure 1b), and based on the results, the experiment with the highest improvement in roughness is selected to polish a part with lattice-like structure (Figure 1C).In a second step, it is necessary to characterize the internal struts of the lattice structures to observe the influence of post-processing on the core of the lattice and ensure achieving a uniform lattice structure after ECP. This can be accomplished by cutting the part followed by profilometry or confocal microscopy, which are destructive methods. As a non-destructive method, X-ray computed tomography (XCT) can be used to asses the internal parts of a lattice structure. However, in most literature, it is used as a qualitative tool to evaluate the uniformity of lattice structure after post-processing. In this study, the XCT output are analyzed using a novel method, which could quantitatively measure the surface roughness of internal struts and assess its improvement by ECP, in a non-destructive manner.In spite of the fact that corrosion resistance of lattice structure AM parts is important for many practical applications, few studies specifically report corrosion rates before and after post-processing of AM lattice parts. In this paper the influence of ECP on corrosion behavior of AM lattice structures and AM plane surfaces are studied. To evaluate the impact of ECP on the corrosion behavior of AM parts, linear polarization is conducted to measure both the corrosion potential and corrosion rate (Figure 1d).According to the obtained results, the highest improvement of in surface roughness is obtained in the second experiment (sample P03, P04), which contains the higher level in each parameter (20 min polishing time and 20V applied voltage). By ECP of the Ti-6Al-4V AM surface, we have proven that we can decrease the arithmetic mean surface roughness value - Sa – by 70%. Using the ECP process, we reduced the initial roughness of 12.3 μm to a polished surface of 3.5 μm.Further experiments are being conducted on the ECP of lattice structures and the evaluation of their corrosion behavior. Figure 1

A new algorithm for optimal process parameters based on minimum building time in additive manufacturing

BackgroundAdditive manufacturing method is used for manufacturing of solid three-dimensional parts. It requires less human efforts and manufacturing time for parts is less. Different process parameters such as layer thickness, building orientation, infill type, and infill percentage affect the building time, model cost, mechanical properties, and surface roughness. The presented paper develops an algorithm for adapting layers and generating tool-paths. This algorithm can improve the fabrication efficiency and geometrical accuracy in the additive manufacturing (AM) of complex models. The proposed algorithm consists of three modules that identify the optimal process parameters, named as part building orientation, layer thickness, strategy type for internal filling, and slope of the tool-path.ResultsThe input is the PTS file that contains the points of the layers contour of the computer-aided design (CAD) model. All the modules for the proposed algorithm were implemented using the MATLAB R2019a programming language software. The main finding results showed that the fabrication with an adaptive layer thickness was more time-efficient. The build time was reduced up to 47.3%. The developed tool-path generation strategies (contour offset and zigzag line tool-path) can effectively balance the AM surface quality and fabrication efficiency requirements.ConclusionIn this research, the AM users can benefit by saving the cost and time. The parts were fabricated with a high degree of accuracy, and the surface finish was suitable for determining the optimal process parameter.

Fast and Accurate Defects Detection for Additive Manufactured Parts by Multispectrum and Machine Learning.

Traditional defect detection methods for metal additive manufacturing (AM) have the problems of low detection efficiency and accuracy, while the existing machine learning detection algorithms are of poor adaptability and complex structure. To address the above problems, this article proposed an improved You Only Look Once version 3 (YOLOv3) algorithm to detect the surface defects of metal AM based on multispectrum. The weighted k-means algorithm is used to cluster the target samples to improve the matching degree between the prior frame and the feature layer. The network structure of YOLOv3 is modified by using the lightweight MobileNetv3 to replace the Darknet-53 in the original YOLOv3 algorithm. Dilated convolution and Inceptionv3 are added to improve the detection capability for surface defects. A multispectrum measuring system was also developed to obtain the AM surface data with defects for experimental verification. The results show that the detection accuracy in the test set by YOLOv3-MobileNetv3 network is 11% higher than that by the original YOLOv3 network on average. The detection accuracy for cracking defects of the three types of defects is significantly increased by 23.8%, and the detection speed is also increased by 18.2%. The experimental results show that the improved YOLOv3 algorithm realizes the end-to-end surface defect detection for metal AM with high accuracy and fast speed, which can be further applied for online defect detection.

A comparison of post-processing techniques for Additive Manufacturing components

Additive Manufacturing (AM) processes can now produce components with homogeneous mechanical properties, but the physical phenomena involved during the build make AM surfaces not always suitable to be used as-is.This paper first focuses on the properties of as build AM surfaces and highlights the need to employ post-processing techniques to improve these properties and meet the function requirements. Besides High Speed Machining, which is the usually chosen process, others can be interesting candidates. Waterjet finishing and laser polishing can be employed to smooth peaks and valleys that characterize AM surface and to remove unmelted particles. In addition, these processes are also used for surface texturing. Friction Stir Processing promises a high productivity. The results of several experimental campaigns carried out in the laboratory are provided in this paper. The major interests and drawbacks of the studied processes are identified with the view to define performance windows.

Confined Electrochemical Finishing of Additive-Manufactured Internal Holes with Coaxial Electrolyte Flushing

Adhered powders and undesirable surface quality represent significant challenges for additive manufacturing (AM) technology. AM internal surface finishing is more difficult owing to the poor visibility of internal geometries and the inconvenient manipulation to machine tools. This paper proposed a confined electrochemical finishing process with a coaxial electrolyte flushing manner to remove the AM internal surface asperity. The insulation was conducted in the non-machining area of cathode to cease its electrochemical reaction which consumed the electric charge to generate gas bubbles, and the electrochemical dissolution could be confined within the AM internal hole. More intensive electrochemical dissolution was performed to the AM surface asperity and thus higher material removal and better surface finishing enhancement could be achieved. The coaxial electrolyte flushing manner created a uniform and efficient flow field to facilitate the discharge of electrolytic products. This technique could eliminate the partially melted powders adhered to the AM internal surface and reduce their Ra from 15.620 μm to 3.494 μm and Rz from 78.402 μm to 19.272 μm. The AM internal hole with a high aspect ratio of 10 could be well handled by this process to eliminate the adhered powders from its internal surface.

The effect of machining parameters on the surface quality of 3D printed and cast polyamide

Fused deposition modeling (FDM) is an additive manufacturing (AM) technique that has emerged as a suitable application in different areas, including machine design and manufacturing. The main advantages of this method over conventional methods include that it is faster and produces less material waste. Besides, AM offers computer-aided design and manufacturing but does not include any limitations on the product's geometry and does not require any extra tools. End milling is a conventional manufacturing process used for profiling, slotting, and facing. In this study, at the point of overcoming the weaknesses of AM surface quality, it was investigated whether the cast polymer's surface quality could be reached with hybrid manufacturing (AM + milling). For this reason, the parts produced by FDM were subjected to end milling, and the effect of cutting depth, feed rate, and rotation speed on surface quality and chip type were investigated. The results obtained are compared with the results of the milling operation of cast polyamide. For all results, surface quality increases with a rising feed rate. In general, the surface quality obtained by milling parts produced using FDM is low, but each manufacturing technique is affected differently by the end milling conditions. Low rotation speed and high feed rates should be preferred to obtain the desired surface quality from FDM printed polyamide parts.

A hybrid framework to estimate the surface state and fatigue performance of laser powder bed fusion materials after shot peening

Despite the rapid growth of additive manufacturing (AM) technologies, AM parts usually present relatively poor surface quality in as-built condition. This necessitates the application of customized post surface treatments to efficiently enhance the surface quality and alleviate the adverse effects of high surface roughness and extremely irregular surface morphology on fatigue strength of AM metallic materials. In this paper, starting from an as-built AM surface, a detailed numerical model is developed to effectively estimate the surface morphology of laser powder bed fusion (LPBF) AlSi10Mg specimens after shot peening. The obtained results are then incorporated into two analytical approaches to evaluate the fatigue strength of different series. The accuracy of the model is validated by comparisons made with experimental data obtained considering as-built and shot peened specimens both before and after heat treatment. The results demonstrate that the proposed hybrid approach can serve as a powerful package not only to determine the beneficial contribution of shot peening on the surface morphology of AM material but also to provide an accurate and rather reliable approximation of their fatigue strength.

Prediction of Extreme Value Areal Parameters in Laser Powder Bed Fusion of Nickel Superalloy 625

Important to the success of additive manufacturing (AM) is the ability to inspect and qualify parts. The research community is pushing to identify correlations between part function and surface topography, yet little guidance specific to AM surface measurement exists. Thus, development of inspection methods for surface finish are Required. In laser powder bed fusion (LPBF) AM, parts are built through a complex process with many variables, and the length scales of interest on the surface cover a wide range. Full characterization of the surface is time consuming and costly as high resolution in surface measurements decreases field-of-view (FoV), requiring stitching multiple FoVs to cover large areas. Statistical methods exist to estimate the maximum value based on a sample of FoVs, but are not yet commonplace in AM surface measurement. The goal of this work is to understand the use of these statistical methods in the estimation of maximum area valley depth (Sv) of a surface, an extreme value parameter, for which researchers have already found relationship to fatigue life. This work also investigates the effect of microscope objective, measurement region size, and nesting index of areal filters on Sv. A large (i.e., greater than 40 mm × 40 mm) planar LPBF surface is fabricated in nickel superalloy 625 and measured using a focus variation microscope with a 10x objective and again with a 20x objective. Results show that there is little difference in the maximum value of Sv between the two objectives, but the nesting index does have some effect. Results also show that a Type 1 Generalized Extreme Value, or Gumbel, distribution can be used to accurately estimate the maximum value of Sv for a surface from a small set of measurements, providing a framework for users to develop inspection routines that balance measurement time and accuracy of estimation.

Effect of Cut-Off, Evaluation Length, and Measurement Area in Profile and Areal Surface Texture Characterization of As-Built Metal Additive Manufactured Components

Surface texture characterization of components built using additive manufacturing (AM) remains a challenge. The presence of various asperities and random roughness distributions across a surface poses several challenges to users in selecting an appropriate cut-off wavelength (λc), evaluation length (ln), and measurement area. This paper investigates a modified framework for surface texture characterization of AM components. First, the surface asperities in an AM component were identified through scanning electron microscope (SEM) analyses. The maximum diameter (φm) of the surface asperities were determined through image processing and were used as cut-off for surface texture evaluation. Second, another set of surface texture results were extracted using standard measurement procedures per ISO 4287, 4288, 25178-1, -2, and -3. Third, the investigative measurement framework’s effectiveness and suitability were explored by comparing the results with ISO standard results. Last, the effects of using non-standard cut-off wavelength, evaluation length, and measurement area during surface texture characterization were studied, and their percentage deviations from the standard values were discussed. The key findings prove that (a) the evaluation length could be compromised instead of cut-off, (b) measurement area must be 2.5 times the maximum asperity size present in the surface, and (c) it is possible to identify, distinguish, and evaluate specific features from the AM surface by selecting appropriate filters, thereby characterizing them specifically. The investigations and the obtained results serve as valuable data for users to select appropriate measurement settings for surface texture evaluation of AM components.

Potential and challenges of additive manufacturing for topology optimized spacecraft structures

This study focused on the potential of topology optimization (TO) for metallic tertiary structures of spacecrafts produced by the additive manufacturing (AM) technique laser powder bed fusion. First, a screening of existing conventionally manufactured products was carried out to evaluate the benefits of a redesign concerning product performance and the associated economic impact. As a result of the study, the most suitable demonstrator was selected. This reference structure was redesigned by TO taking into consideration the AM process constraints. Another major aim of this work was to evaluate the possibilities and challenges of AM (accuracies, surface quality, process parameters, postmachining, and mechanical properties) in addition to the redesign process. A comprehensive approach was implemented including detailed analysis of the powder, mechanical properties, in-process parameters, and nondestructive inspection (NDI). All measured values were used for a back loop to the design process, thereby providing a final robust redesign. Finally, the fine-tuned demonstrator was built up in an iterative process. The parts were tested under representative conditions for the application to verify the performance. The demonstrator qualification test campaign contained thermal cycling, vibration testing, static load testing, and NDI. Thus, an improvement in technology readiness level up to “near flight qualified” was reached.

Surface integrity of additive manufacturing parts: a comparison between optical topography measuring techniques

Abstract Additive Manufacturing (AM) presents significant industry-specific advantages allowing the creation of complex geometries and internal features that cannot be produced using conventional manufacturing processes. However, a current limitation of AM is the degraded dimensional control and surface integrity of specific surfaces. The parts are constructed through layer-by-layer approach, each layer presenting a characteristic ‘fingerprint’. The functional performance of the final part is influenced by the morphology of the outer surface as well as by the surface quality introduced at intermediate layers. Surface texture metrology therefore can play an enabling role in AM-related manufacture and research. The use of optical topography measurement instrumentation allows for a high level of detail in the acquisition of topographic information. Some of the most commonly used optical measuring instruments are Vertical Scanning Interferometry (CSI), Imaging Confocal Microscopy (CONF), and Focus Variation (FV), each one has benefits and drawbacks in terms of acquisition time and measurement resolution. AM surfaces overall present complex topographical features, requiring the acquisition of large surface areas and large z-scans which considerably increases the acquisition time. Speed is a key factor in industrial practice, and time optimization is required for quality control and surface analysis before down-stream processes. This paper reports on the measurement and characterisation of the surface texture of metal powder bed fusion AM parts. All measurements were performed in the same SENSOFAR S-NEOX instrument using the commonly used optical technologies (CSI, CONF, and FV) and the latest step in confocal measurement technology called Continuous Confocal (C-CONF). The resolution and acquisition time of each technique is analysed in order to check the suitability of each method to characterize and describe the AM surface microstructures in a time-efficient way.

Optimal space filling for additive manufacturing

Purpose In additive manufacturing (AM) process, the physical properties of the products made by fractal toolpaths are better as compared to those made by conventional toolpaths. Also, it is desirable to minimize the number of tool retractions. The purpose of this study is to describe three different methods to generate fractal-based computer numerical control (CNC) toolpath for area filling of a closed curve with minimum or zero tool retractions. Design/methodology/approach This work describes three different methods to generate fractal-based CNC toolpath for area filling of a closed curve with minimum or zero tool retractions. In the first method, a large fractal square is placed over the outer boundary and then rest of the unwanted curve is trimmed out. To reduce the number of retractions, ends of the trimmed toolpath are connected in such a way that overlapping within the existing toolpath is avoided. In the second method, the trimming of the fractal is similar to the first method but the ends of trimmed toolpath are connected such that the overlapping is found at the boundaries only. The toolpath in the third method is a combination of fractal and zigzag curves. This toolpath is capable of filling a given connected area in a single pass without any tool retraction and toolpath overlap within a tolerance value equal to stepover of the toolpath. Findings The generated toolpath has several applications in AM and constant Z-height surface finishing. Experiments have been performed to verify the toolpath by depositing material by hybrid layered manufacturing process. Research limitations/implications Third toolpath method is suitable for the hybrid layered manufacturing process only because the toolpath overlapping tolerance may not be enough for other AM processes. Originality/value Development of a CNC toolpath for AM specifically hybrid layered manufacturing which can completely fill any arbitrary connected area in single pass while maintaining a constant stepover.

Surface Roughness and Morphology Customization of Additive Manufactured Open Porous Ti6Al4V Structures

Additive manufacturing (AM) is a production method that enables the building of porous structures with a controlled geometry. However, there is a limited control over the final surface of the product. Hence, complementary surface engineering strategies are needed. In this work, design of experiments (DoE) was used to customize post AM surface treatment for 3D selective laser melted Ti6Al4V open porous structures for bone tissue engineering. A two-level three-factor full factorial design was employed to assess the individual and interactive effects of the surface treatment duration and the concentration of the chemical etching solution on the final surface roughness and beam thickness of the treated porous structures. It was observed that the concentration of the surface treatment solution was the most important factor influencing roughness reduction. The designed beam thickness decreased the effectiveness of the surface treatment. In this case study, the optimized processing conditions for AM production and the post-AM surface treatment were defined based on the DoE output and were validated experimentally. This allowed the production of customized 3D porous structures with controlled surface roughness and overall morphological properties, which can assist in more controlled evaluation of the effect of surface roughness on various functional properties.