Functionally Gradient Materials Research Articles

Investigation and review of FGM functional gradient materials

With the advancement of modern industries and processes, numerous structures are now required to function in thermal environments, leading to a new class of composite materials known as functional gradient materials (FGMs). Initially developed as thermal barrier materials for structural applications in aerospace and fusion reactors, FGMs have gained global recognition as vital composite materials. The overall properties of FMG are unique and different from any of the individual material that forms it. There is a wide range of applications for FGM and it is expected to increase as the cost of material processing and fabrication processes are reduced by improving these processes. Their application is increasingly recognized as a highly effective method for achieving sustainable development across various industries. In this study, an an overview of resents a detailed examination of the development, features, and essential manufacturing processes of FGMs is presented, based on a review of existing literature.

Multi-Gradient Bone-Like Nanocomposites Induced by Strain Distribution.

The heterogeneity of bones is elegantly adapted to the local strain environment, which is critical for maintaining mechanical functions. Such an adaptation enables the strong correlation between strain distributions and multiple gradients, underlying a promising pathway for creating complex gradient structures. However, this potential remains largely unexplored for the synthesis of functional gradient materials. In this work, heterogeneous bone-like nanocomposites with complex structural and compositional gradients comparable to bones are synthesized by inducing strain distributions within the polymer matrix containing amorphous calcium phosphate (ACP). Uniaxial stretching of composite films exerts the highest strain in the center, which ceases gradually toward the sides, resulting in the gradual decrease of polymer alignment and crystallinity. Simultaneously, the center with high orientation traps most ACP during stretching due to the nanoconfinement effect, which in turn promotes the formation of aligned nanofibrous structures. The sides experiencing the least strain have the smallest amounts of ACP, characteristic of porous architectures. Further crystallization of ACP produces oriented apatite nanorods in the center with a larger crystalline/amorphous ratio than the sides because of template-induced crystallization. The combination of structural and compositional gradients leads to gradient mechanical properties, and the gradient span and magnitude correlate nicely with strain distributions. Accompanying bone-like mechanical gradients, the center is less adhesive and self-healable than the sides, which allows a better recovery after a complete cutting. Our work may represent a general strategy for the synthesis of biomimetic materials with complex gradients thanks to the ubiquitous presence of strain distributions in load-bearing structures.

Mechanical and microstructural profiling of additively manufactured cobalt–nickel functional gradient structure

In this study, a functional gradient material (FGM) structure composed of a nickel alloy (IN718), and a cobalt alloy (CoCrMo) was additively manufactured using a co-axial powder-fed laser-directed energy deposition (L-DED) system. The high manufacturing flexibility of L-DED enabled the seamless deposition of IN718-CoCrMo FGM structure through interfacial bonding driven by thermal gradient and cool-down mechanisms. Microscopic analysis confirmed the absence of cracks or delamination in the CoCrMo-IN718 interfacial bonding region. The SEM-EBSD microstructural analysis and micro-hardness testing were performed on the as-printed CoCrMo-IN718 FGM primarily to correlate the hardness properties-microstructural grain phase morphology between the parent alloys and their interfacial bonding region. It was observed that the average hardness of the CoCrMo-IN718 interface zone (322.83 HV1) fell between IN718 (268.67 HV1) and CoCrMo (359.75 HV1) regions. The EBSD analysis reports indicated that along the build direction of CoCrMo-IN718 FGM, the preferential grain orientation aligned in [100] with grain structure transitioning from equiaxed → columnar → elongated columnar dendrites → equiaxed grain, predominantly comprised of face-centered cubic (FCC)-gamma (γ) phase crystal structures. Relatively, coarser grain structures were observed in the interface bonding region, attributed to the analogous crystallography space groups, and prolonged localized thermal gradients.

Review of previous works on functionally graded materials (FGM)

This paper presents bibliographical research on previous works of FGM functional gradient materials. We notice that functional gradient materials represent a relatively new line of research. Since the appearance of the FGM concept, many researchers have been interested in it, and countless works have been published, but there is still much to be done.

Interface characteristics and performance of additively manufactured steel-titanium bimetals prepared through composition gradient layers

In this study, laser-directed energy deposition (LDED), a powder-feeding laser additive manufacturing technique, was utilized to fabricate bimetals comprising high-strength steel (HSS) and Ti6Al4V (TC4). Through meticulous compositional control, four distinct regions (labeled Areas 1 through 4) comprising Fe-Ti intermetallics with varying sizes and morphologies were obtained at the LDEDed HSS/TC4 interface. These zones exhibited thicknesses of approximately 190 μm, 210 μm, 525 μm, and 205 μm, respectively. The effect of these intermetallics on the performance of LDEDed HSS/TC4 bimetals was investigated. The results indicated that most Fe-Ti intermetallics, characterized by reticular/granular morphologies or fine precipitates, were dispersed at the bonded interfaces (Areas 1, 3, 4). Nevertheless, irregular and long-striped C14_Laves phases were observed in Area 2, indicating the presence of a pure Fe2Ti region. Performance testing revealed that fractures predominantly occurred within a hard and brittle Fe2Ti region (Hardness: 891.5HV) during mechanical grinding or tensile clamping, highlighting the formidable challenge of achieving high-quality LDEDed HSS/TC4 bimetals through a component-regulated building strategy. The difficulty arises from the high Fe content in HSS and high Ti content in TC4, resulting in the formation of a pure Fe2Ti region at the interface. Thermodynamic calculations suggest that the composition-gradient-layer strategy may not be optimal for the integrated laser additive manufacturing of HSS and TC4. These findings provide valuable insights into the design of steel-titanium functional gradient materials and other multi-material components.

Research on the microstructure and mechanical properties of functional gradient materials of TC4/TC11 titanium alloys for wire arc additive manufacturing with different transitional forms

Functional gradient materials (FGMs) have garnered increasing interest for their potential in material and structural design. Additive manufacturing, as a kind of free manufacturing and near-net shaping technology with in-situ controllability of materials, is the dominant technology for fabricating FGMs. While numerous studies have been conducted on titanium alloy FGMs, there is still a lack of research on the structure and properties of the material with the number of transitional layers. This study employs double-wire arc additive manufacturing to fabricate FGMs composed of TC4 and TC11 alloys. The study examines the utilization of direct and continuous transition forms in conjunction with stress relieving and solution aging heat treatment processes. The composition, grain morphology, microstructure, and mechanical properties of FGMs are analyzed. Results indicate that in samples employing direct transition, higher heat treatment temperatures and durations lead to a more uniform composition and microstructure. Additionally, the interface morphology becomes increasingly indistinct, and transversal elongation at the interface is enhanced by the strain compensation of the two materials. In continuous transitional samples, an increase in the number of transitional layers results in a more uniform microstructure near the interface, leading to a reduction in interface morphology and an enhancement of mechanical properties. The fracture position tends to shift closer to the TC4 side, with both process forms exhibiting ductile fractures. The plasticity of the TC4 part is superior, and the ductile fracture morphology is more pronounced. This study offers a valuable experimental foundation for investigating the direct and continuous transition of titanium alloy FGMs.

A Survey on Fused Filament Fabrication to Produce Functionally Gradient Materials.

Fused filament fabrication (FFF) is a key extrusion-based additive manufacturing (AM) process for fabricating components from polymers and their composites. Functionally gradient materials (FGMs) exhibit spatially varying properties by modulating chemical compositions, microstructures, and design attributes, offering enhanced performance over homogeneous materials and conventional composites. These materials are pivotal in aerospace, automotive, and medical applications, where the optimization of weight, cost, and functional properties is critical. Conventional FGM manufacturing techniques are hindered by complexity, high costs, and limited precision. AM, particularly FFF, presents a promising alternative for FGM production, though its application is predominantly confined to research settings. This paper conducts an in-depth review of current FFF techniques for FGMs, evaluates the limitations of traditional methods, and discusses the challenges, opportunities, and future research trajectories in this emerging field.

Manufacturing and characterization of functionally gradient material from cemented carbide to diamond via filament-based material extrusion 3D printing

Functionally gradient material (FGM) fabricating via additive manufacturing draws great attention on alleviating the abrupt material discontinuity at the interface of different materials. Introducing gradient structure into polycrystalline diamond composites to produce diamond/cemented carbide FGM is an effective method to reduce the residual stress and enhance the bonding strength of diamond layer and cemented carbide substrate. However, achieving a smooth transition of material properties from cemented carbide to diamond is challenging. In this research, diamond/cemented carbide FGM with a well-controlled gradient was manufactured via filament-based material extrusion. Initially, green bodies of diamond/cemented carbide FGM were obtained by printing with customized composite filaments. Subsequently, thermal decomposition characteristic of the printing filament was analyzed by TGA, and the influence of different heating rates during the binder decomposition stage on the morphology and structure of the green bodies was investigated. After debinding, diamond/cemented carbide FGM without flaw was synthesized at 6.5 GPa and 1550℃. Subsequently, microstructure characterization was performed to investigate the distribution characteristics of the materials inside the diamond/cemented carbide FGM. A continuous transition from tungsten carbide to diamond was achieved inside the gradient layer. Furthermore, the stress state of diamond in different positions of the diamond/cemented carbide FGM was analyzed by Raman spectroscopy. It was found that all the residual stresses in the axial interface of the gradient layer were compressive stress, which can prevent occurrence of microcracks resulted from the tensile stresses formed in the interface. Finally, the impact tests reveal that gradient structure is conductive to improving the bond strength between PCD layer and cemented carbide substrate as the impact times before fracture of diamond/cemented carbide FGM has been increased by 15 %. The gradient structure design strategy, generated via the filament-based material extrusion technology, pioneers new ideas to the development of high-performance FGMs in mass production.

Directed energy deposition of functional gradient material to enhance mechanical properties of heterogeneous stainless steels

In this study, 316L/18Ni300 functional gradient material (FGM) were designed and prepared via directed energy deposition, aiming to achieve synergistic enhancement of strength and toughness between two different lattice-structured steels. Macroscopic analyses showed that the interlayer metallurgical bonding of the 316L/18Ni300 FGM was achieved, and no obvious defects such as unfused and inclusions were observed. Spatially heterogeneous structures cellular dendrite, epitaxially grown columnar dendrites, columnar dendrites were observed along the building direction. The microhardness of the 316L/18Ni300 FGM surface is 329 HV, which is an improvement of 185 % as compared to 316L. The static tensile and three-point bending results show that 316L/18Ni300 FGM combines the toughness of 316L with the strength of 18Ni300. In the reciprocating sliding wear test, 316L/18Ni300 FGM showed an increase in wear resistance along the building direction, with a 35 % increase on the surface. This study not only provides innovative ideas and theoretical guidance for 316L/18Ni300 FGM in terms of mechanical property enhancement, but also opens up a new path for broadening 316L for a wide range of applications.

Free vibration Analysis of Beams with Functional Gradient Materials with Cracks

The article presents the development of a dynamic model for a functionally graded material (FGM) beam incorporating cracks. Initially, it assumes an exponential distribution of material properties along the thickness direction of the beam and simulates the opening crack using a zero-mass rotational spring model. This approach enables the calculation of bending stiffness and local flexibility at the cracked section. Subsequently, drawing upon Timoshenko beam theory and von Kármán geometric nonlinear theory, the study formulates the energy equation of the beam and establishes the partial differential control equations for the cracked FGM using Hamilton's principle. The method of separation of variables is employed to discretize the partial differential motion equations into ordinary differential motion equations. Beam functions serve as mode functions, whose unknown coefficients are determined according to the boundary and continuity conditions, thereby yielding the natural frequencies and mode shapes of the cracked FGM beam. Numerical analysis is conducted to evaluate the impact of boundary conditions, the relative position of cracks, and the length-to-thickness ratio on the natural frequencies of the cracked FGM beam.

Dynamic Analysis of an FGM Beam Undergoing Large Overall Motion in Temperature Field

The dynamic analysis of a functionally gradient material (FGM) beam undergoing large overall motion based on meshless methods is studied in the variable temperature field environment. Considering the high-order terms of nonlinear coupling deformation which were often ignored in previous studies, the high-order rigid–flexible coupled (HOC) dynamic model of a rotating FGM beam with a lumped mass in a temperature field is established by employing Lagrange’s equations of the second kind. Compared with the first-order approximate coupled (FOAC) dynamic model, the HOC dynamic model is more accurate and has a wider scope of application. The validity of the point interpolation (PIM) method and radial point interpolation method (RPIM) in this paper is verified by comparison with simulation results of the assumed mode method (AMM) and the finite element method (FEM). On this basis, the factors affecting the dynamic characteristics of the FGM beam such as the form of temperature field, the functionally gradient index, and the location of added lumped mass are discussed. In this paper, the meshless method with higher computational efficiency and the HOC dynamic model with more accurate results are adopted. This study will provide guidance and reference for dealing with problems in engineering, machinery, aerospace, and other fields in complex environment.

Vibration, Buckling and Stability Analyses of Spinning Bi-Directional Functionally Graded Conical Shells

This paper investigates the free vibration, buckling and dynamic stability of spinning bi-directional functional gradient materials (BDFGMs) conical shells. The material properties vary along the thickness and axial direction. The dynamics model is established based on the first-order shear deformation theory and the governing equations and boundary conditions of the conical shell are derived employing Hamilton’s principle. Subsequently, the differential quadrature (DQ) method is employed to discretize the governing equations into an algebraic system of equations for solving and analyzing the free vibration characteristics of the conical shell. The theoretical model’s accuracy and the solution method’s reliability are rigorously verified. The effects of temperature, functional gradient index, and rotation on the vibration characteristics, traveling wave vibration and critical speed of the conical shell in a thermal environment are systematically explored through numerical analysis. The results indicate that both the material gradient index and temperature increase lead to a decrease in the shell’s natural frequency. For the spinning BDFGMs shell, elevated temperature causes the occurrence of trailing wave vibration to advance to the critical speed. Centrifugal force emerges as the primary factor influencing the critical buckling load and unstable region variation of the spinning shell.

LCA and LCC of wire arc additively manufactured and repaired parts compared to conventional fabrication techniques

Multi-material wire arc additive manufacturing (MM-WAAM) exhibits considerable promise for producing functional gradient materials, components with complex design, and enable a more local supply chain. However, it is still to be determined for which industrial applications MM-WAAM present more benefits than conventional production routes both in terms of environmental and economic impacts. This study undertook a comprehensive cradle-to-grave assessment encompassing two distinct metallic components, manufactured via either additive manufacturing (AM) or conventional fabrication procedures. One of those is a repair case of a forging die, and the other is the production of an industrial tool. Analysis outcomes manifestly revealed pronounced benefits associated with MM-WAAM for these components. For instance, the utilization of MM-WAAM in crafting an optical fiber case injection mould tool yielded energy consumption reduction, attributed to enhanced conformal cooling channels. Additionally, the repair of a forging die via MM-WAAM showcased the potential to curtail new scrap generation during post-processing, prolonging tool lifespan. This study conclusively shows the application-specific nature of MM-WAAM's favourability over traditional processes, thereby offering valuable insights for strategic implementation.

Features for production of dissimilar graded materials using additive manufacturing methods

The article considers the method of obtaining heterogeneous compounds using gradient transitions from intermediate materials. The results of studies of the structure of gradient transitions have shown the possibility of using additive manufacturing methods for obtaining products from heterogeneous metals and for the manufacture of functional gradient materials.

Impact Responses and Wave Dissipation Investigation of a Composite Sandwich Shell Reinforced by Multilayer Negative Poisson's Ratio Viscoelastic Polymer Material Honeycomb.

This analysis investigated the impact wave response and propagation on a composite sandwich shell when subjected to a low-velocity external shock, considering hygrothermal effects. The sandwich shell was crafted using face layers composed of functional gradient metal-ceramic matrix material and a core layer reinforced with negative Poisson's honeycomb. The honeycomb layer consisted of a combination of viscoelastic polymer material and elastic material. The equivalent parameters for the functional gradient material in the face layers were determined using the Mori-Tanaka and Voigt models, and the parameters for the negative Poisson's ratio honeycomb reinforcement core layer were obtained through Gibson's unit cell model. Parameters relevant to a low-velocity impact were derived using a modified Hertz contact law. The internal deformations, strains, and stress of the composite sandwich shell were described based on the higher-order shear deformation theory. The dynamic equilibrium equations were established using Hamilton's principle, and the Galerkin method along with the Newmark direct integration scheme was employed to calculate the shell's response to impact. The validity of the analysis was confirmed through a comparison with published literature. This investigation showed that a multilayer negative Poisson's ratio viscoelastic polymer material honeycomb-cored structure can dissipate impact wave energy swiftly and suppress shock effectively.

Experimental investigation of compressive behaviors of functionally graded composites material based on engineering cementitious composites and normal concrete

A new kind of functional gradient material (FGM) based on engineering cementitious composites (ECC) and normal concrete (NC) was proposed in this study. This material offers enhanced crack control while minimizing ECC consumption. Foremost, uniaxial compression tests were carried out on single-layer NC-ECC composites featuring varying ECC volume fractions (0%, 20%, 40%, 60%, 80%, and 100%). These tests yield an elastic modulus formula, E(y), for the NC-ECC composite. Subsequently, based on the E(y) formula, a 5-layer NC-ECC FGM is designed, with each sub-layer having distinct ECC volume fractions. This is contrasted with pure ECC and 2-layer NC-ECC specimens for reference. The study concludes with an analysis of the failure patterns, load-displacement curves, and elastic moduli of the NC-ECC FGM. The results demonstrate consistent performance trends of elastic modulus in single-layer NC-ECC composite specimens, confirming the feasibility of employing a functional gradient between NC and ECC. Utilizing the formula E(y) derived from single-layer NC-ECC, the ECC volume fractions in the 5-layer NC-ECC FGM are computed as 100%, 86.89%, 68.13%, 41.55%, and 0% for each layer. Importantly, the NC-ECC FGM exhibits a failure mode resembling ECC specimens, with primary cracks not extending along interfacial layers. In contrast, 2-layer NC-ECC specimens experience cracking along interfacial boundaries. The compressive strength of NC-ECC FGM surpasses that of ECC specimens by 1.42 times while retaining ECC-like ductility, showcasing a 25.85% improvement over 2-layer NC-ECC. The findings from this study provide valuable insights for the design and subsequent engineering applications of functional gradient materials.

Progress of ceramic materials in the application of armor protection

Bulletproof ceramics have emerged as crucial elements within contemporary ballistic protection systems, aiding in safeguarding individuals and assets against various threats. As the demand for enhanced protection continues to rise, understanding the evolution and advancements in ceramic armor materials becomes imperative. This paper presents a comprehensive analysis of the evolution, properties, and advancements in ceramic armor materials. The nuanced comparative study of these ceramics includes highlighting the distinct ballistic properties of each, with a particular emphasis on the balance between hardness and potential fragility, as well as Silicon Carbide’s promising composite ceramics. The paper spotlights the transformative potential of graphene-modified ceramics, functional gradient materials, and micro-laminates. Alumina ceramics underscore the significance of microstructural optimization and the role of grain size adjustments. Conclusively, this paper offers a panoramic view of the past, present, and future trajectories of ceramic armor materials, advocating for continued research and innovation in this critical domain.

Soft Functionally Gradient Materials and Structures - Natural and Manmade: A Review.

Functionally gradient materials (FGM) have gradual variations in their properties along one or more dimensions due to local compositional or structural distinctions by design. Traditionally, hard materials (e.g., metals, ceramics) are used to design and fabricate FGMs; however, there is increasing interest in polymer-based soft and compliant FGMs mainly because of their potential application in the human environment. Soft FGMs are ideally suitable to manage interfacial problems in dissimilar materials used in many emerging devices and systems for human interaction, such as soft robotics and electronic textiles and beyond. Soft systems are ubiquitous in everyday lives; they are resilient and can easily deform, absorb energy, and adapt to changing environments. Here, the basic design and functional principles of biological FGMs and their manmade counterparts are discussed using representative examples. The remarkable multifunctional properties of natural FGMs resulting from their sophisticated hierarchical structures, built from a relatively limited choice of materials, offer a rich source of new design paradigms and manufacturing strategies for manmade materials and systems for emerging technological needs. Finally, the challenges and potential pathways are highlighted to leverage soft materials' facile processability and unique properties toward functional FGMs.

Effects of TiB2 content and gradient structure on microstructural characterization and mechanical properties of TiB2/Al7050 composites

TiB2 particle reinforced Al7050 matrix homogeneous composites (HC), TiB2/Al7050 three-layer conventional linear gradient functionally gradient materials (TFGM), and TiB2/Al7050 five-layer functionally gradient materials with a bionic structure (FFGM) were successfully prepared using vacuum hot-pressed sintering (VHS) method. The effects of gradient structure and TiB2 particle content on the microstructural characterization and mechanical properties of these composites were investigated. The results demonstrate that the bending strength of all composites improves with the increase in TiB2 content. Comparatively, the bending strength of functionally gradient materials (FGM) are generally superior due to a combination of the interlayer strength of each layer, heterogeneous deformation-induced strengthening, strength-plasticity synergy of the whole material, and synergistic interlayer strengthening under strong bonding. Notably, the enhancements in bending strength and strain exhibited by FFGM are more pronounced, highlighting its "soft-hard interleaved" and "strong-tough complementary" structural pattern. The research provides valuable insights for the application and further development of FGM.

Microstructure and mechanical properties of functional gradient materials of high entropy alloys prepared by direct energy deposition

In this study, FeCrCoNiMo0.5W0.75 HEAs functional gradient materials were prepared by the direct energy deposition (DED) technique, and the phase transition, microstructure and mechanical properties of the materials along the building direction were investigated. The material realizes a transition from a single FCC phase to an FCC + TCP phase from the bottom to the top. The bottom and middle of the material are typical dendritic (DR)-interdendritic (ID) structures, and at the top, due to the precipitation of the eutectic structure σ phase and μ phase, the material transforms into a hypoeutectic structure of FCC + eutectic. According to the EBSD results, the interior of the material is mainly columnar crystals that grow vertically to the molten pool. This unidirectional columnar growth makes the material have a strong texture in the <100> direction. In addition, a large degree of grain refinement is exhibited from the bottom to the top, with the grain size reduced from 19 μm to 6.5 μm. According to the TEM results, the density of dislocations is much higher near the phase and grain boundaries than in other regions, and dislocation entanglement is formed by the continuous accumulation of loose dislocations. The micro-hardness of the material increased along the construction direction up to 892.5HV0.5, and the material has good compressive strength up to 2041 ± 45 MPa. The material has excellent wear resistance and high-temperature wear resistance, forming a continuous oxide layer at 800 °C, which greatly reduces the friction coefficient and wear rate.