Material design method for the functionally graded cemented carbide tool

The aim of this study is to apply the concept of functionally graded materials (FGMs) to tool materials and to develop high-performance cutting tools. The requirement of the graded structure is that the surface is highly wear resistant cermet, and the inside is tough cemented carbide. Compressive residual stress was introduced to the material surface by grading the composition. To develop the new material, the cutting condition of broken cermet was investigated and their cutting temperature distribution was measured by a newly developed measuring method. Then Computer Aided Engineering (CAE) analysis was performed to calculate the generated thermal stress. The new material was developed with the aim to introduce the compressive residual stress over the calculated thermal stress. As a result developed tools demonstrated higher wear resistance, breakage resistance, thermal crack resistance and peeling resistance over those of conventional tools in the market.     

Thermal fracture resistance of a functionally graded coating with periodic edge cracks

This work investigates the thermal fracture resistance of a functionally graded coating with an array of periodic edge cracks. The integral equation method is used to analyze the thermal stress intensity factors (TSIFs) at the crack tips and the critical thermal shocks that cause crack initiation. The effects of crack density (crack spacing) and thermal property gradients on the critical thermal shocks and TSIFs are examined using an Al₂O₃/Si₃N₄ graded coating on a Si₃N₄ substrate. Numerical results show that for a given crack density, the graded Al₂O₃/Si₃N₄ coating exhibits higher critical thermal shocks than the homogeneous Al₂O₃ coating, and hence higher thermal fracture resistance. For a given material gradation profile, a higher crack density (smaller crack spacing) enhances the critical thermal shock significantly.     

Fundamentals of liquid phase sintering for modern cermets and functionally graded cemented carbonitrides (FGCC)

Metallurgical reactions and microstructure developments during sintering of modern cermets and functionally graded cemented carbonitrides (FGCC) were investigated by modern analytical methods such as mass spectrometer (MS), differential thermal analysis (DTA), differential scanning calorimeter (DSC), dilatometer (DIL), microscopy and analytical electronic microscopy with energy dispersive spectrometer (EDS). The complex phase reactions and phase equilibria in the multi-component system Ti/Mo/W/Ta/Nb/C,N-Co/Ni were studied. The melting behavior models in the systems of TiC–WC/MoC–Ni/Co, TiC–TiN–WC–Co and TiCN–TaC–WC–Co have been established. By an in-depth understanding of the mechanisms that govern the sintering processing and metallurgical reactions, new cermets and different types of FGCC with desired microstructures and properties were developed.     

功能梯度材料板安定区域优化

采用遗传算法优化功能梯度材料陶瓷相体积分布并考虑了金属相材料的强化,得到了循环力/热耦合作用下的功能梯度材料板的最佳安定区域,其中功能梯度材料热物参数空间分布规律采用一个指数函数描述,并给出了Al/SiC功能梯度材料板优化后的安定区域图.     

双相结构功能梯度WC-Co合金的微观组织结构与小负荷维氏硬度

用光学显微镜观察了采用两步法工艺制备的双相结构功能梯度WC-Co硬质合金的微观组织结构，并测量了其小负荷维氏硬度。结果表明：合金芯部的η相呈点状弥散分布于硬质相WC颗粒之间和硬质相WC颗粒与Co粘结相之间；在3相区，由于超细η相的存在，WC晶粒大部分已失去其多角特征，出现晶粒圆化现象；DP合金前驱体在后续渗碳处理过程中，合金表层的部分Co在碳势差的作用下向合金内部发生迁移，使合金中间过渡层及其附近区域中Co粘结相呈现梯度变化；合金表层与合金中间过渡层在由原来的3相组织变为两相组织的同时，合金中细小的WC晶粒明显减少。WC晶粒的均匀度明显增加；合金截面的小负荷维氏硬度出现由高变低再由低变高的规律性变化，与合金内部微观组织结构的变化相对应。     

Liquid phase sintering of functionally graded WC–Co composites

Functionally graded cemented tungsten carbide (WC–Co) is an example of functionally graded materials (FGM) in which mechanical properties are optimized by the presence of microstructural gradients such as cobalt gradient and grain size differences within the microstructure. In particular, a cobalt gradient is preferred. However, the manufacture of FGM WC–Co with a cobalt gradient is difficult because the flow of the liquid phase during liquid phase sintering (LPS) would eliminate any initial cobalt gradient built into the powder compacts. In this paper, different factors, which can be used to influence the migration of liquid during sintering, are investigated. These factors include gradients in grain size, carbon and cobalt content, and sintering time. It is shown that a difference in particle size may induce a step-wise profile of cobalt concentration. Initial carbon content differences, however, can be used to obtain a gradient of cobalt during sintering. The effects of these factors are explained based on the roles of capillary force and phase reactions.     

Fracture toughness of coated TiCN–WC–Co cermets with graded composition

Mixed TiCN–WC–Co cermets are developed to improve at the same time toughness and resistance to deformation of materials for cutting tool applications. Moreover, graded materials joining optimum properties according to the functional part of the tool are elaborated. To this end, TiCN–WC–Co cermets are interesting because they develop a WC–Co layer at the surface during the sintering. This tough layer at the surface limits the crack propagation that can lead to the rupture of the tool. Such materials show a good resistance to the deformation in the bulk and a good toughness at the surface, where the cracks are initiated upon machining. Cutting tools are often coated by CVD to improve the wear resistance. This paper proposes a method to measure the toughness K_IC at high temperature by using this CVD coating for initial crack formation. The coating thickness is the precrack length of traditional K_IC measurements. Samples are fractured by three point bend tests. The rupture stress is measured by Weibull statistics. This method is particularly interesting for graded structure materials where the influence of surface layers on toughness must be estimated. The comparison between cermets with and without WC–Co layer shows an improvement of 28% of the toughness when the layer is present. The possible bias of internal stresses on the results is discussed.     

Microstructural, mechanical and tribological properties of tungsten-gradually doped diamond-like carbon films with functionally graded interlayers

A series of tungsten-gradually doped diamond-like carbon (DLC) films with functionally graded interlayer were prepared using a hybrid technique of vacuum cathodic arc/magnetron sputtering/ion beam deposition. With ‘compositionally graded coating’ concept, the deposition of wear-resistant carbon-based films with excellent adhesion to metallic substrate was realized. In the films, a functionally graded interlayer with layer sequence of Cr/CrN/CrNC/CrC/WC was first deposited onto the substrate, and then, a DLC layer doped with gradually decreasing content of W was coated on. The W concentration gradient along depth of the film was tailored by adjusting the W target current and deposition time. The characterized results indicate that the microstructural, mechanical and tribological properties of these films show a significant dependence on the W concentration gradient. A high fraction of W atom in carbon matrix can promote the formation of sp² sites and WC_1 − x nanoparticles. Applying this coating concept, strongly adherent carbon films with critical load exceeding 100 N in scratch test were obtained, and no fractures or delaminations were observed at the end of the scratched trace. The hardness was found to vary from 13.28 to 32.13 GPa with increasing W concentration. These films also presented excellent tribological properties, especially significantly low wear rate under dry sliding condition against Si₃N₄ ball. The optimum wear performance with friction coefficient of 0.19 and wear rate of 8.36 × 10⁻⁷ mm³/Nm was achieved for the tungsten-gradually doped DLC film with a graded W concentration ranging from 52.5% to 17.8%. This compositionally graded coating system might be a potentially promising candidate for wear-resistant carbon-based films in the demanding tribological applications.     

Temperature profile optimization for microwave sintering of bulk Ni–Al2O3 functionally graded materials

Bulk multilayer graded Ni–Al₂O₃ samples have been sintered under heating by millimeter-wave radiation using a gyrotron system for high-temperature materials processing. By using a purposely designed thermal insulation arrangement, the temperature profile has been adjusted along the concentration gradient to accommodate for different sintering temperatures of the components. The sintered samples have flat boundaries between layers, and their microstructure is free from cracks and delamination. In addition to metal-ceramic graded transitions, metal-ceramic-metal graded insulator structures have also been fabricated.     

表面无立方相层功能梯度硬质合金的研究进展

综述了目前应用于涂层基体的无立方相层含氮功能梯度硬质合金的研究进展;详细介绍无立方相层的形成热力学基础、梯度结构特征、机理和动力学研究进展以及力学性能和切削性能;重点评述C、N含量以及组分对无立方相层的影响规律;提出获取合金系统真实的热力学相图和动力学数据是今后研究工作的重点。     

Thermal analysis of functionally graded coating AlSi alloy and steel pistons

Functionally graded coatings are coating systems used to increase performances of high temperature components in diesel engines. These coatings consist of a transition from the metallic bond layer to cermet and from cermet to the ceramic layer. In this study, thermal behavior of functional graded coatings on AlSi and steel piston materials was investigated by means of using a commercial code, namely ANSYS. Thermal analyses were employed to deposit metallic, cermet and ceramic powders such as NiCrAl, NiCrAl + MgZrO₃ and MgZrO₃ on the substrate. The numerical results of AlSi and steel pistons are compared with each other. It was shown that the maximum surface temperature of the functional graded coating AlSi alloy and steel pistons was increased by 28% and 17%, respectively.     

A novel single-step method for fabrication of dense surface porous aluminum

A new manufacturing process was developed for functionally graded porous Al foams. Morphological variations were monitored by scanning electron microscopy. Mechanical properties were measured by compressive tests. Results indicated that the maximum temperature of 773 K and minimum interior porosity of 60% are the best conditions. Scanning electron microscope (SEM) images show a strong connection between Al particles with high necking radius. Micro-hot molding of Al particles in the space between salt particles is probably responsible for porosity formation.     

功能梯度 WC-Co/WC-Fe-Ni双层硬质合金的研究

使用新的制备方法成功制备了功能梯度WC-Co/WC-Fe-Ni双层结构硬质合金。冷压成型所需的压制压力需要保持在15 MPa,在这个压力下WC-Co和WC-Fe-Ni层的烧结收缩率相同,制备的双层合金没有分层和裂纹等不利现象出现。采用X射线衍射（XRD）、光学显微镜（OM）和扫描电子显微镜（SEM）等实验手段研究了双层合金的相组成与微观结构,发现合金中没有η或者石墨相的存在,而且,WC-Co和WC-Fe-Ni层间的界面处结合良好。同时,在WC-Co/WC-Fe-Ni双层结构硬质合金的界面处有明显的连续变化的 Fe, Ni 和 Co 成分梯度,两层间的成分梯度导致界面附近的硬度梯度的形成。制备的功能梯度WC-Co/WC-Fe-Ni双层结构硬质合金同时高的硬度、耐磨性和韧性。     

Fabrication of SS316L-IN625 functionally graded materials by powder-fed directed energy deposition

ABSTRACT

In this work, functionally graded materials of Stainless Steel 316L and Inconel 625 were fabricated using Directed Energy Deposition. Intermediate layers were built in between of SS316L and IN625. Pure SS316L and IN625 samples were also prepared. The results show microstructure varied sharply at the interface on pure SS316L and IN625 while changed gradually on gradient samples. Quantitative analysis reveals the consistence of designed and tested material composition at the gradient layers. Tensile testing shows the yield strength of graded samples is similar to that of pure IN625 while the ultimate tensile strength is approaching to that of pure SS316L due to the fracture at SS316L side. Microhardness measurements reveal the gradual change of hardness values over the gradient zone.     

具有连续组分的Ti-Mo系梯度材料的制备

从异相颗粒共沉降的角度出发,通过理论分析影响颗粒堆积体内组分分布的沉降各要素之间的关系,确立了粉末的粒度分布同沉积层组分之间的定量关系式,在此基础上,建立了用共沉降法制备梯度材料的数学模型.通过沉降实验和热压烧结技术,制备出了组分连续变化的Ti-Mo系梯度材料.将实验测试结果同模型的计算结果相比较,表明建立起的数学模型能很好地预测材料中的组分分布.     

Characterization of aluminum based functionally graded composites developed via friction stir processing

Al/SiC functionally graded material (FGM) was developed through a novel multi-step friction stir processing (FSP) method. SiC particles with a mean size of 27.5 μm were embedded in the groove on the 6082-Al plate. To create a graded structure over a predefined value, FSP was carried out with three tools with different pin lengths and with varying volume fractions of SiC particles. The structure was formed by passing tools with 1−3 passes with a constant rotational and traveling speeds of 900 r/min and 20 mm/min, respectively. The experiments were conducted at room temperature. Microstructural features of functionally graded (FG) samples were examined by using scanning electron microscopy (SEM) and 3D light microscopy. Mechanical properties in terms of wear resistance and microhardness were thoroughly assessed. The results indicate that the increase in FSP pass number causes more uniform SiC particle dispersion. The microhardness values were impacted by the number of passes and improved by 51.54% for Pass 3 when compared to as-received 6082-Al. Wear resistance of Al/SiC FG samples was found to increase as a result of the addition of SiC particles.     

Strength and residual stresses of functionally graded Al2O3/ZrO2 discs prepared by electrophoretic deposition

Biaxial strength testing of functionally graded Al₂O₃/ZrO₂ discs revealed that the strength of such discs, prepared by electrophoretic deposition, was almost doubled from 288 MPa for pure Al₂O₃ to 513 MPa for the graded discs; this was due to the compressive surface residual thermal stresses in the Al₂O₃ surface layer caused by the graded compositional profile. The surface compressive stress measured by means of X-ray diffraction was compared with the analytically calculated stress distribution in the graded component.     

Mechanical properties of CNT reinforced hybrid functionally graded materials for bioimplants

The hybrid functionally graded materials (FGM) of hydroxyapatite (HA), stainless steel 316L (SS316L) and carbon nanotubes (CNT) were synthesized for biomedical implants. Three different types of FGM were produced by the combination of SS316L and CNT to reinforce HA in discrete layers of FGM. In the first type of FGM, concentration of SS316L was varied from 10% to 40% (mass fraction) with an increment of 10% to reinforce micro HA. In the second type of FGM, 0.5% (mass fraction) functionalized CNT was added by maintaining the rest of composition as that of the first type of FGM. In the third type of FGM, mixture of micro and nano HA (mass ratio1:1) was used, keeping rest of composition similar to the second type of FGM. All types of FGM were subjected to uniaxial compaction and sintered by pressureless sintering technique at similar compaction and sintering parameters. The results show that the densification is enhanced with the addition of CNT and nanocrystalline HA in the FGM. Hardness and fracture toughness increase in both FGM reinforced with CNT, but the increase of the hardness and fracture toughness are more pronounced in FGM with micro and nanocrystalline HA.     

Characteristics of the functionally graded coating fabricated by plasma transferred arc centrifugal cladding

Iron-based powders were deposited on the internal wall of a cylinder by means of plasma transferred arc centrifugal cladding. The as-fabricated coating was a functionally graded triple layer coating with microstructures varying from hypereutectic firstly to near eutectic, and then to hypoeutectic structures along the radial direction. Significant enrichment of carbides [M₇C₃ and M₂₃(C, B)₆] was observed in the inner layer. The centrifugal force was responsible for the chemical composition gradient which resulted in the microstructures gradient. Theoretical analysis revealed that Cr, C and B atoms moved toward the inner layer whereas Fe, W and Mo atoms moved toward the outer layer under the effect of centrifugal force, which was confirmed by the quantitative analysis and line-scan profile. Wear resistance of the inner layer of the coating was fairly higher than that of the substrate.