Multi-material Parts Research Articles

On the effect of material density in dimensional evaluations by X-ray computed tomography of metal-polymer multi-material parts

An important aspect to consider in the evaluation of parts and assemblies by X-ray computed tomography (XCT) is the attenuation coefficient of the different materials involved, which are directly related to their density; depending on this coefficient, the X-ray penetration varies and, therefore, varies the contrast between different materials and with the background. This becomes more critical in those assemblies in which materials are characterized by a high difference in density, where the lighter material could be difficult to be characterised. In this paper, the effect of the presence of metals in the dimensional evaluation of polymeric geometries (having lower density than the metal parts) is studied, to evaluate the errors caused in dimensional measurements of different geometries and surface texture characterization. Based on a common geometry, four scenarios have been experimentally tested with variations of metal amount, in which macro geometries (precision spheres made by different polymers) and micro geometries (inclined ramps manufactured by fused deposition modelling (FDM)) have been characterised. Results show errors in the surface determination of the polymeric features directly related to the presence of metal: a high amount of steel makes significantly difficult to accurately determine the interface between background and material due to the noise and artifacts created, while aluminium has less influence on the irregularities of the features extracted. This effect is more evident for polymers with lower density due to the higher difference. Numerically, most affected parameters are those sensible to variations in surface determination, such as spheres’ form error and ramps’ maximum surface texture (Sz), while more solid features as spheres’ diameters, distances and ramps’ average surface texture (Sa and Sq) remain more stable. In conclusion and to sum up, it has been found that the quantity of metal present in assemblies made of polymeric and metallic materials is correlated with distortions in the dimensional evaluation of polymeric features by XCT.

Shape Memory Polymers in 4D Printing: Investigating Multi-Material Lattice Structures

This paper evaluates the design and fabrication of a thermoplastic polyurethane (TPU) shape memory polymer (SMP) using fused deposition modeling (FDM). The commercially available SMP filament was used to create parts capable of changing their shape following the application of an external heat stimulus. The characterization of thermal and viscoelastic properties of the SMP TPU revealed a proportional change in shape fixity and recovery with respect to heating and cooling rates, as well as a decreasing softening temperature with increasing shape memory history due to changes in the polymer microstructure. Inspired by the advancements in 3D and 4D printing, we investigated the feasibility of creating multi-material lattice structures using SMP and another thermoplastic with poor adhesion to TPU. A variety of interlocking lattice structures were evaluated by combining SMP with another thermoplastic that have poor adhesion with TPU. The tensile strength and failure modes of the fabricated multi-material parts were compared against homogenous SMP TPU specimens. It was found that the lattice interface failed first at approximately 41% of the ultimate strength of the homogenous part on average. The maximum recorded ultimate strength of the multi-material specimens reached 62% of SMP TPU’s ultimate strength. These characterizations can make 4D printing technology more accessible to common users and make it available for new markets.

Additive manufacturing and mechanical analysis of multi-material polymer parts combining thermosets and thermoplastics

Multi-material additive manufacturing (AM) represents one of the most promising solutions to target the contemporary demand for complex products with high individuality and inherent functionalities. Besides continuous advances in machinery and the available material spectrum, a substantial aspect of multi-material AM is still underrepresented: the simultaneous combination of thermosets and thermoplastic within a single AM process. A promising technology in this regard is the newly developed Fusion Jetting (FJ) process. This investigation focuses on the combination of acrylate-based photopolymers (thermoset) and thermoplastic polyurethane (TPU) with FJ. Tensile specimens are built with strategic variations of the process parameters and experimentally analyzed to derive beneficial processing conditions. A proof of concept is delivered by demonstrating a significant increase in Young’s modulus of TPU specimens from approximately 65 to 160 MPa through integration of photopolymer reinforcements. Further experiments regarding variable layer heights and laser powers identify an optimum layer height of 100 µm along with a tolerable laser power of 15 W for maximum mechanical properties. An overall challenging aspect of the FJ process is the presence of unwanted delamination between reinforced and non-reinforced layers. The failure mode is observed during tensile testing on multiple multi-material specimens of this investigation. The origin of delamination can be correlated to the deviation of integrated reinforcements from their originally intended dimensions as well as the unwanted crystallization within reinforced layers. First, countermeasures to minimize delamination are identified, such as decreasing the rotational increment of the laser hatch orientation from 90 to 10° per layer.

The influence of building parameters on residual stress, interfacial structure, and cracking control of laser powder bed fusion processed Ti6Al4V/AlMgScZr multi-material parts

With the growing demand for lightweight and high-performance components applied in extreme aerospace environments, there is a prospective interest in producing the integrated multi-material components consisting of titanium alloy and aluminum alloy. Laser powder bed fusion (LPBF), as a laser powder bed based additive manufacturing process with a high production flexibility, shows a promising potential in creating multi-material components. However, due to the possibility of the generation of various intermetallic compounds (IMCs) at the interface, it has been a significant challenge to fabricate reliable Ti/Al alloy multi-material components without any crack formation by LPBF. In this study, the Ti6Al4V/AlMgScZr multi-material parts were fabricated by LPBF and the effects of building parameters of AlMgScZr on the residual stress and crack control of Ti6Al4V/AlMgScZr multi-material parts were investigated. The evolution mechanisms of IMC phases at the interface under laser interaction were further revealed. Due to the change of elemental compositions and difference in Gibbs free energies of IMCs, a gradient ordered transition zone with a width of about 450 μm consisting of Al3Ti, TiAl, and Ti3Al was formed from AlMgScZr to Ti6Al4V. A finite element model was established to explain the effects of process parameters on residual stress evolution, cracking formation and its control. As the scan speed increased from 500 to 1100 mm/s and the attendant building height of AlMgScZr increased to 60 layers, the residual stress of laser processed Ti6Al4V decreased from 439 to 364 MPa, but the residual stress of AlMgScZr increased from 195 to 303 MPa. As the applied scan speed was enhanced to process AlMgScZr, the element diffusion was mitigated, thereby reducing the formation of IMCs and stress concentration. This study shows the possibility of in-situ control of IMCs and residual stress by process optimization during LPBF preparation of titanium alloy/aluminum multi-material parts without crack formation.

Transition zone parameter development in multi-material powder bed fusion: a general approach

Power bed fusion of metals using a laser beam (PBF-LB/M) offers unique possibilities to manufacture functionally graded materials (FGM) consisting of different alloys. These so-called multi-material parts enable their material properties to be tailored to local material requirements. In this paper, a new methodical approach for the production of metal FGM with transition zones oriented in different directions and manufacturing sequences of the different materials is investigated. Existing approaches for the manufacturing of these transition zones were enhanced with graded parameter variations, spatial laser movement modulation techniques (wobbling), and geometric approximations using a step structure. For the validation of the approach and the characterization of the transition zones, the manufactured samples were investigated and characterized using optical microscopy and hardness profile measurements. Furthermore, the density of the transition zones was analyzed by image data processing. The feasibility of the presented methods is shown and the production of defect-free transition zones with controlled expansions for functionally graded materials via PBF-LB/M achieved

Potential and challenges for Powder Bed Fusion – Laser Beam (PBF-LB) in industrial ceramic additive manufacturing

In recent years, ceramic 3D printing has proven its potential for increasing the cost and time efficiency of ceramic manufacturing, especially for customizable products, small series production and for (multi-material) parts with complex design. This work gives an overview of the methods for ceramic additive manufacturing that are currently available for industrial production with their advantages and disadvantages. Ceramic Powder Bed Fusion- Laser Beam (PBF-LB) or also known as selective laser sintering or melting (SLS/SLM) is introduced, using a ceramic powder as starting material for shaping by selective laser irradiation. Amongst others, the technique offers a superior productivity, which might be the central argument in the future for evaluating the applicability of PBF-LB for industrial manufacturing. Additionally, a novel approach to ceramic PBF-LB is presented, using a thermoset binder for incorporating ceramic particles in the starting material that, due to the non-meltable nature of the binder, improve the dimension stability of the green parts during thermal debinding and sintering. The production of large, complex structures opens up a wide range of applications. One promising application for porous structures made of photocatalytic titanium oxide is water treatment. Such printed, debinded and sintered filter modules enable the degradation of organic residues in water, thus contributing to safer and higher water quality. Furthermore, high-resolution printing can be realized via micro-PBF-LB (μ-PBF-LB, also known as μ-SLS).

The role of graded layers in interfacial characteristics and mechanical properties of Ti6Al4V/AlMgScZr-graded multi-material parts fabricated using laser powder bed fusion

Graded multi-material parts achieve a compositionally graded transition between two different materials, mitigating undesirable consequences such as cracking and delamination due to property mismatch and significantly improving the comprehensive performance of parts. In this study, the Ti6Al4V/AlMgScZr-graded multi-material parts were fabricated using laser powder bed fusion technology, introducing a composition-graded layer with 25 wt.% Ti6Al4V and 75 wt.% AlMgScZr at the interface to reduce the mismatch between the two materials. The effect of the graded layer’s laser scanning speed on the densification behavior, microstructure evolution, and mechanical properties of the Ti6Al4V/AlMgScZr-graded multi-material parts was investigated. It was revealed that the crack area at the interface reduced from 0.325 to 0.067 mm2 as the scanning speed increased from 2400 to 2800 mm/s and then increased to 0.161 mm2 at 3000 mm/s. A smooth, continuous-graded layer with good metallurgical bonding was fabricated at 2800 mm/s. The TiAl3 intermetallic compound was formed at the interface and underwent a transition from rod-like to coarse dendritic and finally to finer dendritic structure along the building direction. The Ti6Al4V/AlMgScZr-graded multi-material parts exhibited a graded decrease in microhardness from 374 HV0.2 on the Ti6Al4V side to 122 HV0.2 on the AlMgScZr side, and an excellent compressive strength of 1531 MPa was obtained at the optimal parameter of 2800 mm/s.

Factors affecting interface bonding in multi-material additive manufacturing

Additive manufacturing or 3D printing is the process of depositing material layer-by-layer to create 3-dimensional products. When creating 3D-printed products from two or more materials, multi-material additive manufacturing processes are used which eliminate the need for assembly operations. Fused filament fabrication multi-material additive manufacturing permits the production of a single printed item employing multiple materials in fused filament fabrication. This work studied the factors affecting fused filament fabrication multi-material additive manufacturing, by reviewing existing works, designing part(s), conducting design of experiments, and carrying out parts’ performance test. An E3D multi-material filament 3D printer was utilised throughout this study. The chosen polymer combination was polycarbonate (PC) and poly(methyl methacrylate) (PMMA), whilst the design and testing of the multi-material parts was limited to lap-shear testing. Results showed that the interface bonding of the PMMA/PC (PMMA printed first and followed by PC) specimens was stronger than the one of the PC/PMMA specimens. Furthermore, to investigate the effect of the contact or overlapping area on the interface bonding strength, PMMA/PC specimens with varying dimensions were designed, printed, and tested. When the contact area was reduced, a strong interface bonding between the PMMA and PC layers was still maintained.

Short carbon fiber-reinforced PLA composites: influence of 3D-printing parameters on the mechanical and structural properties

3D printing, particularly “fused filament fabrication” (FFF), plays a crucial role in Industry 4. FFF is widely used for creating complex structures and multi-material parts across various industries such as food industry, fashion industry, and manufacturing sectors. The properties of FFF-produced objects are remarkably affected by printing parameters. This study explores the impact of printing parameters and the addition of short carbon fibers on the strength of polylactic acid (PLA) printed samples. The lowering layer height, increasing feed rate and extrusion temperature boost impact strength, while a smaller raster angle enhances it. Meanwhile, an improved flexural strength is achieved by adjusting layer height, extrusion temperature, and raster angle. Higher extrusion temperatures enhance tensile strength, microstructure, and reduce porosity. Lower layer height improves flexural and impact strength (28.05% increase in 0.1 mm layer height), higher feed rate boosts strengths (12.56% improvement in 7 mm3/s feed rate), and elevated extrusion temperatures enhance impact strength (14.49% increase in 230 °C extrusion temperature) but reduce flexural strength (14.44% decrease). Incorporating carbon fibers in PLA negatively affects the microstructure but increases crystallinity, raising the melting temperature and lowering cold-crystallization temperature. The introduction of carbon fibers into PLA results in a complex interplay of mechanical and thermal properties.Graphical abstract

Interface behaviors and mechanical properties of diffusion bonded 45 steel/additive manufactured 316L steel joints

Diffusion bonding of additive manufactured (AM) metal parts to traditionally manufactured metal parts could simultaneously achieve large-scale, complex-structure, and multi-material parts. However, there is a lack of study on diffusion bonding as well as its interface formation mechanism between AM metal parts and traditionally manufactured metal parts. In this study, diffusion bonding was performed to join AM 316L stainless steel and casting 45 steel at different temperatures. The element distribution, interface characteristics and shear strength of the joints were investigated, and the interface formation mechanism at different bonding temperature was analyzed in detail. The results reveal that the interface transition zone from the AM 316L steel side to the 45 steel side is composed of the metallic elements diffusion layer and the carbide layer. Moreover, an increase in bonding temperature can effectively promote interface voids closure and increase the thickness of the metallic elements diffusion layer, while the thickness of the carbide layer firstly increases and then decreases as the temperature increases. The component of the carbide layer was identified as brittle compounds (CrFe)23C6 and (CrFe)7C3. The shear strength of the joints exhibited an increasing trend with the increasing bonding temperature. Interestingly, a sudden decrease in strength was found at 950 °C due to the significant increase of the brittle carbide layer, while high-quality joints could be achieved as the brittle carbides dissolve at 1000 °C. This study could lay the foundation for understanding diffusion bonding interface behaviors between AM metal parts and traditional casting metal parts.

A preliminary study on multi-material fused filament fabrication of an embedded strain gauge for low-cost real-time monitoring of part strain

Unlike other manufacturing techniques, additive manufacturing enables part consolidation through the production of multi-material parts with enhanced functionality. With reference to the functionality of monitoring the structural integrity of a product during its use, conductive filaments can be used in additive manufacturing. This work aims to investigate the applications of multi-material fused filament fabrication to produce embedded strain gauges for real-time monitoring of part deformations. In layer-by-layer fabrication, conductive filaments can be used to produce strain-sensitive elements inside products at a low cost. This preliminary study demonstrated the feasibility of the proposed approach using tensile samples fabricated through additive manufacturing. The samples were produced using a polyethylene terephthalate glycol filament and an acrylonitrile styrene acrylate filament, while electrically conductive polylactic acid was used for the strain gauge. The characterization and testing activities were conducted by comparing the results of the tensile testing with data acquired through an experimental system set up with an Arduino board, aligning with the resistance-based strain gauge theory. The findings show that the co-fabricated strain gauge successfully traces part deformation, enabling real-time monitoring of strain in the elastic field. Nevertheless, further optimization of the proposed approach is imperative to enhance the reliability and accuracy of the methodology.

The Effect of Epoxy Resin on the Infiltration of Porous Metal Parts Formed through Laser Powder Bed Fusion

Laser powder bed fusion (L-PBF) additive manufacturing technology can print multi-material parts with multiple functions/properties, and has great potential for working in harsh application environments. However, the metal blank formed by sintering metal powder material with binder added through L-PBF has an obvious porous structure and insufficient mechanical properties, and few studies have been conducted studying this. In this paper, epoxy resin was used to impregnate the blank of porous metal parts formed by L-PBF with iron-based powder material at a certain temperature, and a cross-linked curing reaction was carried out with three kinds of phenolic resin in different proportions under the action of a curing agent, so as to fill the pores and achieve the desired mechanical properties. The characteristic peaks of each group of epoxy resin were characterized using Fourier transform infrared spectroscopy (FT-IR) and H-nuclear magnetic resonance (1H-NMR) spectrums. The microstructure, decomposition temperature, and residue of four epoxy resin dispersion systems were analyzed with a scanning electron microscope (SEM), a thermal gravimetric analyzer (TGA), and derivative thermogravimetry (DTG). The results show that the density of the porous metal parts was obviously improved, the heat resistance temperature of the parts could reach 350 °C, and the tensile strength of the sample after EP2-1 impregnation was increased by 4–6 times after curing at 160 °C for 6 h. Therefore, the use of an epoxy resin dispersion system can increase the porosity of L-PBF porous metal parts, but can also significantly improve their mechanical properties, which can help them to meet the requirements of applications as model materials, biological materials, and functional materials to provide a feasible solution.

Effect of Core Material Thickness on the Shore Hardness of the Sandwich-Structured Multi-Material 3D-Printed Parts

Fused Filament Fabrication (FFF) continues to experience improvements in terms of its flexibility and functionality, therefore it attracts public attention to use this technology. Multi-Material Additive Manufacturing (MMAM) is an approach in the FFF technology that allows the manufacturing of 3D-printed products composed of two or more materials in a single printing process. MMAM enables the user to apply various configurations to obtain a 3D-printed material with adjustable properties. This study aims to determine the effect of core material on the Shore hardness of the FFF printed parts with the MMAM approach. There were two types of materials combined with the MMAM approach in this work, namely polylactic-acid (PLA) and thermoplastic elastomer (TPE). The Shore hardness test was conducted according to the ASTM D2240-15 standard. The results showed that the thickness of the core material inserted into the printed material had a significant effect on the hardness value of printed multi-material parts. In addition, the hardness value was highly dependent on the modulus of elasticity of the material. Therefore, the hardness value changed following the proportion of the printed material.

Interfacial analysis by optical monitoring of layer level multi-material parts fabricated via Powder Bed Fusion–Laser Beam

This experimental work aims to contribute to the development of Powder Bed Fusion–Laser Beam (PBF-LB) processes for the fabrication of multi-material structures by studying the interfacial characteristics using an off-axis optical monitoring system and relating in-situ results with metallurgical and mechanical properties of built parts. The study was developed employing an innovative approach for intralayer multi-material fabrication by PBF-LB, based on the use of a partitioning system in the powder chamber.Image analysis was used to extrapolate information needed to describe possible contamination effects between powders, the size of the interface and the presence of metallurgical defects.Chemical, metallographic and statistical analyses were also carried out to study the interface between the two materials of the samples and to support the results from optical monitoring.The results showed that the optical monitoring system enables real-time verification of the required quality parameters during the joining of different materials with PBF-LB processes.

Evaluation of durability of 3D-printed multi-material parts for potential applications in structures exposed to marine environments

Traditional methods of manufacture require the assembly of individual components to create multi-material structures, leading to increased costs and weight, as well as potentially reduced usable lifetimes. However, there are numerous applications that cannot be realized using conventional manufacturing techniques, as they need a multi-material approach. Additive manufacturing presents a ground-breaking opportunity to fabricate components that combine diverse materials in a single part or incorporate three-dimensional gradient areas throughout the entire cross-section of the component. The utilization of additive manufacturing has transcended its initial prototype phase and has garnered attention for its potential in facilitating low-scale production of products employed across various areas, including underwater field, such as components of the underwater acoustic recorder used by the Brazilian Navy. In this study, the durability of 3D-printed multi-material parts for potential applications in structures exposed to marine environments was investigated. Multi-material sandwich structures with outer layers made of ABS and a PLA core were fabricated using a Fused Deposition Modeling (FDM) 3D printer. Three different configurations were studied, varying the number of ABS layers (one, three and five layers of ABS were used). The ageing process was carried out in seawater for 60 days. Tensile and flexural tests were used to measure the mechanical properties of both unaged (control) and aged specimens. As expected, the highest water absorption was observed in the PLA samples, while the lowest in ABS samples. For the multi-material samples the lower absorption was found for the one layer ABS samples. For the unaged samples, the mechanical properties (tensile and flexural) varied as a function of varying the number of ABS layers, while for the aged samples, a plateau tendency was observed.

Powder sheets additive manufacturing: Principles and capabilities for multi-material printing

In this work, a novel Metal Additive Manufacturing using Powder sheets (MAPS) method for printing multi-material composites in one process is proposed. MAPS employs powder sheets (i.e. metal powder-polymer matrix flexible films) as the feedstock material. Its key advantages include a relatively rapid change from one material to another and a minimum wastage of materials due to the elimination of the powder bed. The powder sheets were fabricated using a ‘solvent casting’ method. They were then employed in a commercialised metal printer for printing metal multi-material composites. To prove the disruptive concept of MAPS, a 60-layer trimetallic multi-material composite (304 L stainless steel, In718 and CoCrFeMnNi high entropy alloy) was additively manufactured using three different types of powder sheet material in the same manufacturing system for the first time. Experimental results indicate a high density (99.80 %) multi-material composites was printed by MAPS. EDX and SEM observations of the multi-material composites revealed variations of chemical composition and microstructure along the build direction. The newly proposed MAPS manufacturing method and results of this study provide insights into a new avenue for multi-material metallic parts.

Inverted laser sintering of metal powders

We demonstrate the ability of the inverted laser sintering process to manufacture parts composed of metal powder. We fabricate a 10-layer part by depositing a layer of copper powder onto a sapphire plate, then pressing the plate against the part being built and sintering the powder onto the part by shining a 14W 445 nm laser through the glass. The process was then repeated multiple times, each time adding a new layer to the component being printed until completion. We discuss the potential applications and impacts of this process, including the ability to directly fabricate multi-material metallic parts without the use of a powder bed.

Practical application of circularity micro-indicators to automotive plastic parts in an industrial context

The increasing amount of plastics in new vehicles, and the consequent growth of plastics in the automotive shredder residue, makes it imperative and urgent to improve reuse and/or recovery of plastics at the vehicle's end-of-life. Circularity micro-indicators can identify improvement guidelines and monitor the transition to a more circular economic model; however, the complexity, variety of scope and approach, and scarcity of information on most of the circularity micro-indicators hamper their practical adoption by companies.In this paper, we identify relevant micro-indicators for the automotive sector, focusing on key aspects for automotive plastic parts. Further, to showcase the application of these micro-indicators in a real industrial context, a case-study plastic automotive component was identified in collaboration with a tier II supplier and five complementary circularity micro-indicators were evaluated.We found 23 circularity micro-indicators (at the product level) that are highly relevant for automotive parts and we discuss individually how each can provide useful information. Results indicate an enormous improvement opportunity to improve circularity of plastic automotive parts, namely in the use of more sustainable material sources (recycled, bio-based, or compostable materials) and promotion of end-of-life recovery approaches (reuse or refurbish, and take-back systems).We concluded that practices adopted by industry to optimize production, in terms of design for manufacturing and design for assembly, as well as in the composition of plastic automotive parts, namely integral design, multi-material parts, or welding components made from different materials, severely limit the implementation of circular economy approaches, a situation that needs to be reverted by legislators and companies.

Treatment of AlSi12 feedstock with water to manufacture multi-material parts by laser powder bed fusion process: A potential route for the fabrication of inorganic 3D metamaterials

A new L-PBF process is proposed to manufacture, from a single powder chemically activated by pure hot water, parts comprising in selected areas of space, a ceramic or a metal alloy. The chemical activation of the starting alloy powder, the melting and densification of the metallic zones as well as the oxidation of metal alloy powder and sintering of the ceramic zones, have been studied and optimized. The results obtained allowed the realization of multi-material functional structures. Thus, small coaxial conductors have been fabricated. Beyond these relatively simple parts, 2D and 3D periodic structures made of ceramics and metal have been successfully realized. The achievement of these periodic structures gives hope for the further development of 3D inorganic metamaterials for microwave applications.

Development of a Test Bench for the Experimentation of the Electrical Performance of 3D Printed Multi-Material Parts

Obtaining multi-material parts by material extrusion processes is becoming more interesting as the available materials permit achieving superior properties in the 3D printed products. Combining conductive filament with other with elastomeric properties makes it possible to materialise electrical circuits for introducing active elements in specific parts, such as sensors, triggers or antennas. In this context, a test bench has been designed, manufactured and set-up, to evaluate the electrical behaviour of multi-material 3D printed test samples composed of two or more materials, being one a conductor of electricity (at least) and the other(s) non-conductive but flexible. The functionalities of the test bench include the possibility to apply tensile, compressive, shear, or flexural loads to the test samples. The electrical performance of the samples can be assessed in terms of resistivity and capacitance, in real time, when the test bench stands still and when it conducts the series of movements that produce the elastic deformation of the samples. To achieve this, three electronic circuits have been designed with their own corresponding control with Arduino: a circuit to measure the variation of the resistance of the test samples, a circuit to measure the variation of the capacitance of the test samples, and a circuit controlling the movements of the mechanical set (motor and terminals) that generates the deformation of the test samples. The test bench is connected to a desktop computer to ease the data export, treatment, and visualisation. As a set-up of the test bench, several preliminary experimentation measurements have been done to assess factors of interest such as sensitivity and a correlation index. The present work also frames the requirements of the parts to be tested in the bench and outlines the work procedure to carry out the series of experiments.