Process-induced Defects Research Articles

Process Design for Manufacturing Fiber-Reinforced Plastic Helical Gears Using a Rapid Heating and Cooling System

In this study, a lightweight fiber-reinforced plastic (FRP) helical gear was fabricated to investigate the potential application of FRP in automobile parts that require high loads and reduced noise. High-performance aramid FRP processed using the wet-laid method was used in the tooth region, and SCR420 steel was used in the inner hub region. A hot-forming system that combines rapid induction heating and water channel cooling methods was developed to reduce the cycle time. The cooling water flow conditions were analyzed to precisely control the mold temperature. Additionally, a rotating extraction system was developed to mitigate the extraction difficulty owing to the helix angle to the extraction direction. Using the innovative hot-forming system developed in this study, a helical gear without any process-induced defects was fabricated with a significantly reduced cycle time. The performance of the gear was successfully estimated using gear durability, torsional strength, and motion noise tests. The use of FRP materials offers significant potential to realize lightweight components; however, certain challenges related to their properties that may limit their application must be addressed on a case-by-case basis.

Influence of boron implantation induced defects on solar cells: Modeling the process defects

The effect of process-induced defects on the photo-generated charge-carrier lifetime and solar cell performance is critical, which will help optimize the process recipe. In this work, we attempt to quantify the effects of process-induced defects during boron implantation on the n-type silicon wafer in different annealing ambiences. We have evaluated the role of defects that can be formed during oxygen and inert ambience annealing on n-type bifacial passivated emitter rear totally diffused solar cells using a recombination current prefactor (J0). The numerically calculated J0 is calibrated with the reported experimental J0 values using two different methods: (i) Shockley–Read–Hall lifetime and (ii) effective trap-density method. In the latter method, we used the simulated defect density profiles. Both methods capture the process-induced degradation. We observed that the process-induced defects could deteriorate by almost 1% absolute efficiency for the considered annealing conditions. We found that dislocation loops alone cause an ignorable effect on terminal characteristics, but other process-induced mechanisms could dominantly degrade the cell's performance. To further support, we show that independent defects (apart from coupled defects) other than dislocation loops could explain the experimentally reported boron-implanted diodes’ J–V curves under reverse bias conditions.

A Detailed Analysis of the Microstructural Changes in the Vicinity of a Crack-Initiating Defect in Additively Manufactured AISI 316L

The fatigue life of metals manufactured via laser-based powder bed fusion (L-PBF) highly depends on process-induced defects. In this context, not only the size and geometry of the defect, but also the properties and the microstructure of the surrounding material volume must be considered. In the presented work, the microstructural changes in the vicinity of a crack-initiating defect in a fatigue specimen produced via L-PBF and made of AISI 316L were analyzed in detail. Xenon plasma focused ion beam (Xe-FIB) technique, scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD) were used to investigate the phase distribution, local misorientations, and grain structure, including the crystallographic orientations. These analyses revealed a fine grain structure in the vicinity of the defect, which is arranged in accordance with the melt pool geometry. Besides pronounced cyclic plastic deformation, a deformation-induced transformation of the initial austenitic phase into α’-martensite was observed. The plastic deformation as well as the phase transformation were more pronounced near the border between the defect and the surrounding material volume. However, the extent of the plastic deformation and the deformation-induced phase transformation varies locally in this border region. Although a beneficial effect of certain grain orientations on the phase transformation and plastic deformability was observed, the microstructural changes found cannot solely be explained by the respective crystallographic orientation. These changes are assumed to further depend on the inhomogeneous distribution of the multiaxial stresses beneath the defect as well as the grain morphology.

Additive manufacturing of tantalum scaffolds: Processing, microstructure and process-induced defects

Extreme melting point, high density, and ease of oxidation of tantalum (Ta) make its processing rather difficult using conventional methods. Additive manufacturing (AM) serves as an alternative Ta processing technique with unique design flexibility, customization, and minimizing material wastage. However, most additively manufactured parts contain undesired microstructural features or deviations (process-induced defects). This study aimed to assess the processability of the solid and structurally porous scaffolds of Ta through laser powder-bed fusion (LPBF) AM. It will further characterize and evaluate microstructure of LPBF processed Ta, with emphasis on assessing the mechanism of the process-induced defects. The x-ray diffraction (XRD) and microstructural investigation confirmed the presence of BCC Ta phase containing columnar, equiaxed and fine grains with roughness between 3.88 nm to 10.40 nm. The presence of oxygen resulted in the formation of some oxide phases such as Ta2O5 and Ta2O3. Numerous process-induced defects, including solidification-induced micropores, pores-induced and solidification-induced microcracks were identified. The dislodge unmelted or partially fused Ta powder resulted in other form of defects including micro-concaves and microgrooves. The mechanism of pore formation during the LPBF of Ta was assessed through computational simulation and its findings rationalised the experimental results.

Fatigue life prediction considering variability for additively manufactured pure titanium clasps.

This study aims to develop a numerical prediction method for the average and standard deviation values of the largely varied fatigue life of additively manufactured commercially pure titanium (CPTi grade 2) clasps. Accordingly, the proposed method is validated by applying it to clasps of different shapes. The Smith-Watson-Topper (SWT) equation and finite element analysis (FEA) were used to predict the average fatigue life. The variability was expressed by a 95% reliability range envelope based on the experimentally determined standard deviation. When predicting the average fatigue life, the previously determined fatigue parameters implemented in the SWT equation were found to be useful after conducting fatigue tests using a displacement-controlled fatigue testing machine. The standard deviation with respect to stroke and fatigue life was determined for each clasp type to predict variability. The proposed prediction method effectively covered the experimental data. Subsequently, the prediction method was applied to clasps of different shapes and validated through fatigue tests using 22 specimens. Finally, the fracture surface was observed using scanning electron microscopy (SEM). Many manufacturing process-induced defects were observed; however, only the surface defects where the maximum tensile stress occurred were crucial. It was confirmed that the fatigue life of additively manufactured pure titanium parts is predictable before the manufacturing process considering its variability by performing only static elasto-plastic FEA. This outcome contributes to the quality assurance of patient-specific clasps without any experimental investigation, reducing total costs and response time.

“Non-equilibrium” grain boundaries in additively manufactured CoCrFeMnNi high-entropy alloy: Enhanced diffusion and strong segregation

Grain boundary diffusion in an additively manufactured equiatomic CoCrFeMnNi high-entropy alloy is systematically investigated at 500 K under the so-called C-type kinetic conditions when bulk diffusion is completely frozen. In the as-manufactured state, general (random) grain boundaries are found to be characterized by orders-of-magnitude enhanced diffusivities and a non-equilibrium segregation of (dominantly) Mn atoms. These features are explained in terms of a non-equilibrium state of grain boundaries after rapid solidification. The grain boundary diffusion rates are found to be almost independent on the scanning/building strategy used for the specimen’s manufacturing, despite pronounced microstructure differences. Grain boundary migration during diffusion annealing turned out to preserve the non-equilibrium state of the interfaces due to continuous consumption of the processing-induced defects by moving boundaries. Whereas the kinetic “non-equilibrium” state of the interfaces relaxes after annealing at 773 K, the non-equilibrium segregation is retained, being further accompanied by a nano-scale phase decomposition at the grain boundaries. The generality of the findings for additively manufactured materials is discussed.

Anisotropic high cycle fatigue property estimation for laser additive manufactured Ti6Al4V alloy dependence on tomographic imaging of defect population

Improvement of fatigue properties of additive manufactured metals is subject to the regulation of process-induced microstructure and defect population. Here, the high cycle fatigue properties of the annealed Ti6Al4V alloy manufactured by two types of laser powder bed fusion (L-PBF) processes, in orthogonal sample orientations, were studied. The fatigue performance of this alloy was also estimated dependence on the investigation of microstructure and defects using optical microscopy, X-ray diffraction, scanning electron microscopy and micro computed tomography. The results demonstrated that the fatigue properties were degraded by the negative effect of prior-β columnar grain boundaries and defects, wherein the surface keyhole governed the pronounced fatigue scatter and anisotropy due to its geometrical parameters and anisotropy severity along the building direction. The projection size √area of keyholes was well applied in the fatigue property estimation. The theorical fatigue limits were estimated individually due to the keyhole anisotropy, and provided conservatively in the improved Kitagawa-Takahashi diagram based on the modified El-Haddad model and the extreme size of keyholes. A relation derived from Paris law was proposed for fatigue life prediction, and to well establish a linear fitting correlating fatigue life with the projection size of surface keyholes and stress amplitude range with the achievement of the alleviation of fatigue anisotropy and scatter. The research provides a reference for the non-destructive estimation of fatigue properties in terms of tomographic imaging of defect population.

The Influence of Laser Powder Bed Fusion (L-PBF) Process Parameters on 3D-Printed Quality and Stress–Strain Behavior of High-Entropy Alloy (HEA) Rod-Lattices

Laser powder bed fusion (L-PBF) additive manufacturing technology is suitable for the direct 3D printing of geometrically complex periodic micro-rod-lattices. However, controlling the geometric and performance consistency remains challenging due to manufacturability limitations, non-negligible process defects, and surface roughness, which is inconvenient to measure, affecting the mechanical properties and deformation behavior of the lattice structures. To improve the forming quality of the rod lattices and the consistency of repetitive 3D printing, we theoretically analyzed the causes of the defects and the effects of the L-PBF parameters on the process defects of CoCrFeNiMn high-entropy alloy micro-rods. The forming quality of the micro-rods was evaluated and classified with control experiments, and the surface roughness was measured and analyzed. Randomly protruding metal particles on the surface were mainly caused by the diffusion of laser energy, the incomplete melting of some metal powders, and/or “balling” process-induced defects caused by laser remelting. The tensile mechanical properties of typical L-PBF-printed micro-rods with different geometric characteristics were compared and evaluated. The influence of the geometric characteristics of the defects on the mechanical properties is discussed. The mechanical properties of the L-PBF-printed rod lattices were evaluated by compression experiments. It was found that the properties of different rod lattices have a positive relationship with the relative density.

On study of process induced defects-based fatigue performance of additively manufactured Ti6Al4V alloy

Additively manufactured Ti6Al4V are high strength alloys but associated with inferior ductility and inherent defects. These process-induced defects act as a stress raiser and deteriorates the mechanical and fatigue properties of the alloy. Post-processing is generally done to enhance the ductility and fatigue properties but at the expense of strength of the additively manufactured Ti6Al4V alloy. In this study, the effect of defects on mechanical properties and fatigue performance of additively manufactured (selective laser melting) Ti6Al4V alloy has been investigated based on experimentally calibrated defects-based model. The as-built specimens were subjected to different heat treatments in order to optimize their mechanical properties and the most optimum heat treatment was then selected for investigation of changes in fatigue properties. The beneficial effect of heat treatment on mechanical properties was mainly associated with the reduction in fraction of α ’-martensitic microstructure and increase in α -lamellae thickness. Most of the fatigue failures occurred from surface and sub-surface defects, owing to the high stress concentration factor associated with them, and improved fatigue lives as a result of optimum heat treatment. A process-structure–property (PSP) linkage has been established to predict the mechanical properties. Experimentally calibrated defects’ based fatigue model has been formulated to predict the fatigue performance using the experimentally observed data i.e. the process induced defects’ size and their locations. • Different post-processing of SLM-based AM Ti6Al4V was investigated. • Heat treatment was carried out, which improved both mechanical & fatigue properties. • A Process-Structure-Property (PSP) linkage was established. • A new defects-based model was proposed to predict the fatigue properties.

Selective Laser Melting (SLM) Additively Manufactured CoCrFeNiMn High-Entropy Alloy: Process Optimization, Microscale Mechanical Mechanism, and High-Cycle Fatigue Behavior.

The equiatomic CoCrFeNiMn high-entropy alloy (HEA) possesses excellent properties including exceptional strength-ductility synergy, high corrosion resistance, and good thermal stability. Selective laser melting (SLM) additive manufacturing facilitates the convenient fabrication of the CoCrFeNiMn HEA parts with complex geometries. Here, the SLM process optimization was conducted to achieve a high relative density of as-built CoCrFeNiMn HEA bulks. The mechanisms of process-induced defects and process control were elucidated. The microscale mechanical behaviors were analyzed through in situ scanning electron microscopy observation during the compression tests on micro-pillars of the as-built HEA. The stress-strain characteristics by repeated slip and mechanism of "dislocation avalanche" during the compression of micro-pillars were discussed. The high-cycle fatigue tests of the as-built HEA were performed. It was found that a large number of nano-twins were induced by the fatigue, causing a non-negligible cycle softening phenomenon. The effects of promoted ductility due to the fatigue-induced nano-twins were illustrated. This work has some significance for the engineering application of the SLM additively manufactured CoCrFeNiMn HEA parts.

A Review on Microstructural Formations of Discontinuous Fiber-Reinforced Polymer Composites Prepared via Material Extrusion Additive Manufacturing: Fiber Orientation, Fiber Attrition, and Micro-Voids Distribution.

A discontinuous fiber-reinforced polymer composite (DFRPC) provides superior mechanical performances in material extrusion additive manufacturing (MEAM) parts, and thus promotes their implementations in engineering applications. However, the process-induced structural defects of DFRPCs increase the probability of pre-mature failures as the manufactured parts experience complicated external loads. In light of this, the meso-structures of the MEAM parts have been discussed previously, while systematic analyses reviewing the studies of the micro-structural formations of the composites are limited. This paper summarizes the current state-of-the-art in exploring the correlations between the MEAM processes and the associated micro-structures of the produced composites. Experimental studies and numerical analyses including fiber orientation, fiber attrition, and micro-voids are collected and discussed. Based on the review and parametric study results, it is considered that the theories and numerical characterizations on fiber length attrition and micro-porosities within the MEAM-produced composites are in high demand, which is a potential topic for further explorations.

Defect-based analysis of the laser powder bed fusion process using X-ray data

Due to high production costs and a limited reproducibility of quality, the high potential of laser powder bed fusion (LPBF) has not been fully exploited yet. In fact, internal defects can have a detrimental effect on the fatigue behaviour and cause final component failure. Therefore, process-induced defects must be localized and evaluated at a higher level of detail. The present study deals with the correlation amongst pores and LPBF process parameters in AlSi10Mg components. Computed tomography (CT) allows an extensive examination of internal defects. Within this work, a total number of 2,939,830 pores detected in 96 cylindrical samples were analysed using CT. The formation of pores can be adjusted by varying the modified volume energy density, for example, by using various laser scanning speeds. Furthermore, the effect of powder preparation scan strategies (pre-heating and pre-sintering) on the formation of different pore types as well as the general reproducibility was examined. For instance, the shielding gas flow, contaminated protective windows of the lasers as well as prior powder preparation influence the formation of pores. Using prior laser powder preparation reduced the total number of pores at high scanning speeds up to 45%. When the scanning speed is increased, the number of spherical pores decreases and large and irregularly shaped pores appear. Interestingly, only the pre-heating process resulted in a reduced formation of spherical pores at low scanning speeds (1000 mm/s).

Experimental investigation and numerical modeling of laser powder bed fusion process-induced angle-dependent defects in strut-based lattice structure

The emergence of additive manufacturing technology has enabled the fabrication of advanced structures with intricate geometric topologies. However, additive manufacturing induces geometric defects in as-printed structures which may result in deviation from the parent material’s properties. The defects and impaired material properties greatly influence the performance of the fabricated structure. In this paper, the effect of build angle is investigated. The angle-dependency for the geometric defect distribution characteristics is verified through X-ray tomography. The mathematical relationship between the parent material’s properties and the build angle is established. A numerical model is developed for predicting the mechanical performance of laser powder bed fusion fabricated lattice structures proposed by using the experimental results. The positioning geometric defects are introduced into this model to achieve higher predicting accuracy. This work proposes a reliable and low-cost numerical analysis method based on the angle dependence and helps in predicting the mechanical performance of additive-manufactured complex three-dimensional lattice structures.

Static assessment of flawed thin AlSi10Mg parts produced by Laser Powder Bed Fusion

Several factors must be considered within the assessment of parts produced by Additive Manufacturing (AM) such as, for example, heterogeneous microstructure, process-induced defects, surface quality, residual stresses, and dependence on the material’s properties with the building orientation. All these factors severely affect the resistance to static and fatigue loadings of AMed components. Among the possible failure mechanisms, the failure promoted by static loadings when cracks are present is one of the most important failure conditions, in particular for the as-built parts, which might have reduced fracture toughness and ductility compared to the wrought alloy counterpart. In this work, we present a comprehensive approach to the static assessment of AlSi10Mg parts manufactured by Laser Powder Bed Fusion. Two benchmark fracture geometries were designed to investigate the typical AM geometrical features: i) thin walls in tension and ii) notched components in bending. Finite Element analyses of the benchmark specimens showed that an approach based on elastic–plastic fracture mechanics parameters is needed to correctly predict the experimental failures, despite the quasi-brittle behaviour shown by the AlSi10Mg alloy. In view of these results, this paper explores the applicability of the Failure Assessment Diagram (FAD), a tool used for conventional ductile materials, to static assessment of AMed parts. The results show that the assessment approach based on the FAD makes it possible to properly predict the experimental failures of the benchmark specimens.

Influence of geometric defects on the compression behaviour of thin shell lattices fabricated by micro laser powder bed fusion

Micro laser powder bed fusion (μLPBF) is an additive manufacturing process enabling the fabrication of thin-walled metallic shell lattices at the micrometre scale. With wall thicknesses reduced down to 100 µm, this process significantly enhances the structural design freedom, enabling the design of multi-functional, lightweight, stiff, and strong components. However, from a manufacturing perspective, as the wall thicknesses approach the printing resolution, this process is likely to introduce geometric defects, and informed quality assurance of the 3D-printed structures becomes critical. From a structural perspective, the mechanical properties of thin-walled structures are known to be highly sensitive to manufacturing defects. To quantify the process-induced geometric defects and to understand the influences of the geometric defects on the mechanical properties of thin shell lattices, we propose a micro X-ray computed tomography (XCT) based finite element (FE) modelling methodology, which is validated here for Primitive-type shell lattices fabricated by μLPBF in stainless steel. Four types of geometric defects, namely thickness variations, through-thickness defects (holes) on shell surfaces, surface waviness and roughness, are incorporated into the shell element-based FE models. The detailed evaluation of numerical and experimental compression test results shows that the defect-informed simulation approach provides significantly improved prediction accuracy compared to the ideal geometry model in terms of stiffness, peak stress, plateau stress and densification behaviour. The respective influences of each type of defect and their combinations on the mechanical properties of the shell lattices are studied, highlighting that surface roughness and thickness variations lead to prominent impact on the reduction of both linear and nonlinear mechanical properties.

Effect of hot isostatic pressing on microstructural and micromechanical properties of additively manufactured 17–4PH steel

Microstructural and mechanical characterization techniques were utilized to establish the effect of the combined hot isostatic pressing (HIP) and standard H1025 heat treatment schedule on the properties of 17–4PH steel manufactured by means of laser powder bed fusion (LPBF). The as-printed (AP) sample initially consists of a dual-phase microstructure, with 78% martensite and 22% retained austenite. Then, the combined HIP and H1025 applied to the AP sample resulted in lower retained austenite of 7%. The microstructural analysis indicates that the post-processing after printing alleviated the process-induced defects reducing the overall defect content from 0.37% to 0.01%. Moreover, it is observed that the characteristic melt pools are dissolved, resulting in a homogeneous microstructure with reduced grain sizes. A room-temperature compression testing was then applied to the sample before and after post-processing treatment. It has been identified that, in both samples, the retained austenite transforms into martensite, a characteristic of the transformation-induced plasticity effect. The critical stress to initiate the dynamic transformation was then determined to be lower in the HIP sample than that of the AP sample. Based on the results, it can be concluded that the combined HIP+H1025 processing employed in this work effectively increased the mechanical properties of LPBF-fabricated 17–4PH steel. • 17–4Ph steel was fabricated via laser powder bed fusion. • Characterization of the samples was done by the means of EBSD analysis. • Using double differential method, the critical stresses for initiation of dynamic transformation of HIP sample was higher.

Prediction of the peak and steady-state ply–ply friction response for UD C/PAEK tapes

Excessive ply–ply friction can lead to process-induced defects during the hot press forming of thermoplastic composite laminates. Process simulations can be used to enable first-time-right defect-free manufacturing. However, a good understanding of the ply–ply friction is required to improve the constitutive models to allow simulation of more complex parts. We performed friction experiments on UD C/PAEK tapes, showing a typical response with a peak followed by a steady-state friction. Micrographs of tested specimens were analyzed to evaluate the matrix interlayer thickness distribution in the ply–ply interface, which we combined with the matrix viscosity to predict the peak friction. Generated fiber distributions, mimicking the ply–ply interface, were successfully used to obtain the matrix interlayer as well. The steady-state friction was accurately described by including a critical shear stress to represent wall slip, substantiating the concept of wall slip as the dominant underlying mechanism for the start-up friction response.

LASER DRILLING OF AIR-FILM COOLING HOLES IN AIR AND WITH COAXIAL WATERJET ASSISTANCE

Air-film cooling holes were drilled into a turbine blade with a 532-nm Nd:YVO4 nanosecond laser in air and with coaxial waterjet assistance. The drilling quality of the sidewalls of the holes was investigated comparatively by means of a 3D confocal laser scanning microscope, scanning electron microscopy, transmission electron microscopy, X-ray diffraction and energy-dispersive X-ray spectroscopy. Results have shown that the maximum thickness of the processing-induced defects around the air-film cooling holes drilled by the laser in air is up to 100 µm, while the holes obtained by means of the asynchronous operation of laser drilling in air and coaxial waterjet assistance indicate no spatter, oxide layer, recast layer or cracks. Compared with laser drilling in air, the minimum size of the heat-affected zone around the air-film cooling holes induced by asynchronous processing is decreased down to the sub-micrometer scale. The main phases in the oxide layer, recast layer and heat-affected zone are α-Al2O3, γ-Ni, and β-NiAl, respectively. Asynchronous processing can help us achieve high position precision and it will have wide application.
 are α-Al2O3, γ-Ni, and β-NiAl, resp

Benchmark of a probabilistic fatigue software based on machined and as-built components manufactured in AlSi10Mg by L-PBF

The possibility to obtain optimized components with a reduced weight is the main driver of space and aeronautic industries in seriously considering the metal additive manufacturing (AM) technology for production. Despite the incontrovertible advantages offered by this manufacturing technique, the material produced is usually affected by the presence of internal defects, a poor surface quality, and process-induced residual stresses. These features strongly affect the fatigue performance and reproducibility of AMed parts, limiting the adoption of deterministic criteria for fatigue assessment. A probabilistic approach is therefore needed for the analysis of critical and structural components. To this aim, a fully probabilistic finite element (FE) post-processor, ProFACE, was developed by part of the authors to assess the fatigue strength and critical locations of complex components in the presence of process-induced defects. A wide benchmark activity was supported by the European Space Agency (ESA) to test the software capabilities for the life prediction of components manufactured in AlSi10Mg by L-PBF. After tuning ProFACE parameters based on the results obtained on standard fatigue specimens, the software was used to estimate the fatigue life of the components obtaining a good description of the experimental dataset for both volumetric and surface defects. The software was then used to explore the effect of the variability of the most significant parameters affecting fatigue strength of AlSi10Mg AMed components.

Additive Manufacturing Potentials of High Performance Ferritic (HiperFer) Steels

In the present study, the first tailored steel based on HiperFer (high-performance ferrite) was developed specifically for the additive manufacturing process. This steel demonstrates its full performance potential when produced via additive manufacturing, e.g., through a high cooling rate, an in-build heat treatment, a tailored microstructure and counteracts potential process-induced defects (e.g. pores and cavities) via “active” crack-inhibiting mechanisms, such as thermomechanically induced precipitation of intermetallic (Fe,Cr,Si)2(W,Nb) Laves phase particles. Two governing mechanisms can be used to accomplish this: (I) “in-build heat treatment” by utilizing the “temper bead effect” during additive manufacturing and (II) “dynamic strengthening” under cyclic, plastic deformation at high temperature. To achieve this, the first HiperFerAM (additive manufacturing) model alloy with high precipitation kinetics was developed. Initial mechanical tests indicated great potential in terms of the tensile strength, elongation at rupture and minimum creep rate. During the thermomechanical loading, global sub-grain formation occurred in the HiperFerAM, which refined the grain structure and allowed for higher plastic deformation, and consequently, increased the elongation at rupture. The additive manufacturing process also enabled the reduction of grain size to a region, which has not been accessible by conventional processing routes (casting, rolling, heat treatment) so far.