Spectral representation of higher-order localization relationships for elastic behavior of polycrystalline cubic materials

A computationally efficient higher-order spectral framework has been formulated for the calculation of the elastic localization tensors for polycrystalline material systems using the generalized spherical harmonics as the Fourier basis. This new approach offers tremendous potential for rapid analysis of the elastic performance of a very large set of microstructures in any selected polycrystalline material system. The spectral framework transforms the complex integral relations for local stress and strain fields (derived from established generalized composite theories) into relatively simple algebraic expressions involving polynomials of structure parameters and morphology-independent influence coefficients. These coefficients need to be established only once for a given material system. In this paper, we formulate and demonstrate a viable approach to establishing the values of the second-order influence coefficients for cubic polycrystals by calibration to the results of micromechanical finite element models.     

Bending-fatigue behavior of bulk metallic glasses and their composites

Amorphous alloys with high glass-forming ability and thermal stability were discovered in the 1990s. In the following years, the alloy design increased the critical casting thickness to several centimeters, and a homogeneous dispersion of nanoscale particles was found to improve the ductility. Therefore, bulk metallic glasses (BMGs) are being studied widely because of their potential as structural materials. The fatigue behavior is an important characteristic of structural materials. The current review documents the bending-fatigue studies of BMGs and their composites. The fatigue characteristics of these alloys in different loading conditions and environments are summarized and compared in this paper. The factors affecting the fatigue behavior of BMGs and their composites are discussed. The mechanisms of fatigue-crack initiation and propagation in BMGs are addressed in this review. In order to broaden the scope of applications of BMGs, a fundamental understanding of the fatigue behavior is critical for the design of new alloy systems and the development of the processing techniques.     

Electro-deposition of oxide-dispersed nickel composites and the behavior of their mechanical properties

Oxide-dispersed nickel composites were produced by electro-deposition of nickel with various types of oxide nano-powders in a sulfamate bath. The effect of the oxide concentration in the bath on the mechanical properties of the electro-deposited composites was studied. The current efficiency of the electro-deposition process was analyzed for different types of oxide nano-powders and for in terms of the amounts embedded in the nickel (Ni) matrix. To evaluate the mechanical strength and the thermal stability, micro-hardness, tensile strength and elongation were measured at room temperature for composites that were heat-treated in a temperature range of 298 K to 1123 K. The results of the experiments showed that a higher oxide powder concentration in the bath led to a greater amount of the oxide particle co-deposited in the nickel matrix. This resulted in higher micro-hardness and improved high-temperature thermal stability. However, an inhomogeneous distribution of the co-deposited oxide powders in the composites was observed to weaken the thermal stability of their tensile strength and elongation.     

Progress in studying the fatigue behavior of Zr-based bulk-metallic glasses and their composites

Zr-based amorphous alloys with a high glass-forming ability and thermal stability were discovered in 1990. In the following years, the alloy design increased the critical casting thickness to several centimeters, and a homogeneous dispersion of nanoscale particles was found to improve the ductility. Therefore, Zr-based bulk-metallic glasses (BMGs) are being studied widely because of their potential as structural materials. The fatigue behavior is an important characteristic of structural materials. The current review documents the fatigue studies of Zr-based BMGs and their composites. The fatigue characteristics of these alloys in different loading conditions and environments are summarized and compared in this paper. The factors affecting the fatigue behavior of the Zr-based BMGs and their composites are discussed. The mechanisms of fatigue-crack initiation and propagation in BMGs are addressed in this review. In order to broaden the scope of applications of BMGs, a fundamental understanding of the fatigue behavior is important for the design of new alloy systems and the development of the processing techniques.     

一种建立板料成形极限应力图的新方法

基于塑性理论建立了比例加载条件下双向拉伸应力应变关系,结合Swift分散性失稳准则,提出了一种建立板料成形极限应力图的方法。分别应用Hill 48和Hosford屈服准则以及单向拉伸性能参数,建立了铝合金板(r<1)和薄钢板(r>1)两种材料的成形极限应力图(FLSD),分析表明,不同的屈服准则的选取对于成形极限应力曲线有不同的影响,对于不同类型的材料屈服准则的影响程度也不同。与由通常的成形极限图(FLD)转换所得到的成形极限应力图(FLSD)进行了对比分析,结果表明,所提出的方法计算过程更为简便,并能较为准确地建立成形极限应力图,可以作为复杂加载路径下的成形极限破裂判据。     

SiC颗粒氧化行为及SiCp/铝基复合材料界面特征

界面反应和界面产物对SiCp/Al基复合材料的性能具有重要影响。对SiCp 的高温氧化行为进行了试验研究。结果表明 :SiCp 氧化起始温度为 80 0～ 85 0℃ ,其氧化增量和氧化产物SiO2 的体积分数及厚度与高温氧化处理的保温时间呈抛物线关系。以氧化处理的SiCp 为增强体 ,含Mg铝合金为基体 ,通过挤压铸造工艺制备复合材料。利用TEM和FE TEM对所得的复合材料界面进行观察 ,结果表明 ,在SiCp 表面形成了一定数量的尖晶石(MgAl2 O4 ) ,其数量和尺寸与Mg含量有关。由此 ,通过控制SiCp 的氧化处理工艺参数和基体合金成分 ,可以实现对SiCp/Al基复合材料界面反应及产物的控制     

Rheological behavior of semi-solid TiB_2/7075 composites and simulation of their rheocasting processes

In the present study, in-situ Ti B2 particle-reinforced 7075 aluminum alloy was produced by adding a mixture of K2 Ti F6 and KBF4 to the molten base alloy. The effects of the addition of 4.5wt.% and 9wt.%Ti B2 on the apparent viscosity and microstructure were investigated. The results showed that adding Ti B2 is effective for optimizing primary α-Al, but compared with the 4.5wt.%Ti B2/7075 composite, the addition of 9wt.%Ti B2 had no further significant refinement role in the 9wt.%Ti B2/7075 composite due to particle aggregation. The viscosities of semi-solid 7075 alloy and Ti B2/7075 composite slurries increased with an increase in solid fraction, but decreased with an increase in shear rate. The viscosity of 4.5wt.% Ti B2/7075 was the lowest among the three samples, and that of 7075 alloy was the highest under the same conditions. The primary α-Al grain size was decreased, and the dendritic grains grew into spherical shapes after shearing. Based on the experimental results, viscosity models of the semi-solid 7075 alloy and 4.5wt.% and 9wt.%Ti B2/7075 composites were formulated. According to the simulation results, the shrinkage porosity of the 4.5wt.%Ti B2/7075 wheel was lower than those of the 7075 alloy and 9wt.%Ti B2/7075 wheels.     

Equation for the density of particle-reinforced metal matrix composites: A new approach

This paper presents a novel equation for the density of ceramic particle reinforced metal matrix composites. An overall density change occurs in composites due to the thermal mismatch between the metal matrix and the reinforcement. The thermal mismatch occurs because the coefficient of thermal expansion and the elastic properties are different for the matrix and the reinforcement. The values obtained using the proposed equation for density were compared with both the rule of mixtures for density and the experimental values obtained for aluminium and zinc alloy composites. The composite specimens were fabricated using compocasting technique (one of the types of liquid metallurgy route). The proposed mathematical model is found have better agreement with the experimental results at lower volume fractions of the reinforcement; however, some deviations were observed at higher volume fractions of the reinforcement. The proposed equation yields agreeable results for aluminium composites and fairly agreeable results for zinc alloy composites.     

A new model to describe effect of plastic deformation on elastic modulus of aluminum alloy

In order to improve the prediction capability of spring-back in aluminum sheet metal forming, the influence of the plastic deformation on elastic modulus is considered when the material undergoes a large plastic deformation. The present work focused on establishing a new model to accurately describe the relation of elastic modulus and plastic deformation. The tensile tests were performed to investigate the influence of plastic deformation on elastic modulus at low strain rate. Two different aluminum sheets were used, AA2024-T3 and LY12-CZ, and the thickness of sheet metals was 1.3 mm and 2.0 mm, respectively. In order to overcome the drawback, which is directly measuring the slope of tension curve to obtain elastic modulus, an extrapolation method was adopted. The proposal macroscopic piecewise sinusoidal function can accurately model the elastic modulus variation.     

A new technology for the production of aluminum matrix composites by the plasma synthesis method

Aluminum matrix composites were produced via the plasma injection of reinforcing particulates into aluminum melts stirred electromagnetically. In this process, a metallic wire supplied to the active zone of plasma torch disintegrated into fine metallic particles. In the plasma arc, the particles are heated above the melting point and accelerated to close to sonic speed, which helps with the incorporation of the particles into the melt. In this study, the possibility of producing aluminum matrix composites reinforced by intermetallic compound particles via plasma synthesis is demonstrated.     

Reheating process of metal matrix composites for thixoforming and their inductive coil design

In this work, the fabrication processes of particulate metal matrix composites (PMMCs) with a homogeneous distribution of reinforcement and their reheating for thixoforming were studied. Electromagnetic stirring was used to fabricate PMMCs to vary particle size. PMMCs were tested before and after the reheating process using a tensile test with and without heat treatment. The combined process conditions for fabricating the PMMCs are also suggested for a variety of particle sizes. For the thixoforming of PMMCs, fabricated billets are reheated by using an induction heating system with a maximum capacity of 20 kW. The effects of the dispersion state of the reinforcements on the reheating temperature and microstructural morphology were investigated.     

Improvements in finite-element simulation for stamping and application to the forming of laser-welded blanks

During stamping-die design, the formability in sheet-metal forming process has been evaluated by the geometrical functions in ‘Die-Face CAD’, which has been developed and improved by Toyota Motor Corporation. When evaluation by these functions is difficult, formability has been estimated by performing experiments using test dies in which the forming defects are similar to those in the actual process.

A numerical method has been developed in order to substitute numerical analysis for experiments using test dies for the accurate prediction of defects in sheet-metal forming. The elastic-plastic FEM with the commercial code ‘JNIKE3D’ has been improved in the areas of: (1) the material constitutive equation; (2) the consideration of the pressure distribution on the blank-holder; and (3) the evaluation of breakage initiation. Using the improved method, the square-cup drawing process and the hemming process have been analyzed. Numerical results for strain, breakage initiation, and hemming deflection were in good agreement with experimental results. The formability of laser-welded blanks and the most efficient process to form them were evaluated also using the improved method.     

The solidification characteristics of 6061 and A356 aluminum alloys and their ceramic particle-reinforced composites

The solidification curve and viscosity are the two most important solidification characteristics. The solidification curves of the 6061 and A356 alloys and their ceramic particle-reinforced composites are determined by using differential thermal analysis (DTA) coupled with mathematical modeling. In applying this method, the cooling curves of the alloys and composites are first determined by using DTA, then mathematical modeling is used to simulate the cooling curves, and their solidification curves can thus be calculated. It has been found that the principal characteristics of the solidification curves of the composites and their matrix alloy are similar. The viscosities of these alloys and composites are determined by using a Searle-type viscometer. The viscosity of the semisolid slurry increases with increasing solid content. A sharp increase in the viscosity is observed during solidification, and the value of the critical solidified fraction is a function of the shear rate. Both the aluminum alloys and their respective composites displayed similar critical solidified fractions, and these values are hardly affected by the addition of the ceramic particles.     

Mechanical analysis of woven composites at high strain rates and its application to predicting impact behavior

The deformation behavior of woven composites at high strain rates was analyzed using a constitutive equation developed to describe the nonlinear, anisotropic/asymmetric and rate-dependent mechanical behavior of woven composites. The rate-dependent nonlinear behavior of woven composites was characterized at high strain rates (1 s⁻¹ to 100 s⁻¹) using a tensile testing method first proposed in this research. The material properties for the developed constitutive equation were determined and subsequently used in a finite element analysis of the deformation behavior of woven composites at high strain rates. Finally, the impact behavior of woven composites was predicted using the constitutive equation and the results were compared with experiments, showing that the current constitutive equation including the characterization method is adequate to describe the deformation behavior of woven composites at high strain rates up to impact level.     

弥散铜-WC复合材料的热变形行为及动态再结晶临界条件

利用Gleeble-1500D热力模拟试验机,在变形温度为350～750℃、应变速率为0.01～5 s-1、总应变量约为0.5的条件下,对复合材料的高温热变形行为及动态再结晶临界条件进行研究。结果表明:弥散铜-WC复合材料高温流动应力-应变曲线主要以动态回复和动态再结晶软化机制为特征,峰值应力随变形温度的降低或应变速率的升高而增加;在真应力-应变曲线基础上,建立的Al2O3/Cu-WC复合材料高温变形本构模型较好地表征了其高温流变特性,其热激活能为208.35 kJ/mol;同时,利用其θ-σ曲线出现拐点及-dθ/dσ曲线上出现最小值研究了动态再结晶的临界条件。     

A new approach for the geometrical calibration of parallel kinematics machines tools based on the machining of a dedicated part

A main limitation of parallel kinematics machine tools (PKM) in high-speed machining tasks is their low level of accuracy compared with serial kinematics machine tools, which is largely due to geometrical transformation errors. These errors can be reduced by identifying the geometrical parameters of the inverse kinematics model integrated in the controller by exteroceptive calibration. The aim of this paper is to propose a new external measurement method in order to perform the geometrical calibration of PKM, taking into account machining requirements. This method is implemented in three steps: machining of a dedicated part, measurement, and identification of the geometrical parameter values. In this paper, the method is described with a particular emphasis on the machined surface profiles of the dedicated part and on the numerical calibration approach. Measurement errors on the machined surface enable the identification of the PKM geometrical parameters. Thus, calibration is performed with respect to machined surface defects without taking into account the entire tool pose defect, as is the case in usual calibration methods. The study is illustrated using the Verne PKM, which is located at IRCCyN (Nantes, France).     

A new approach to identifying the dynamic behavior of CNC machine tools with respect to different worktable feed speeds

The dynamics of the machine tool structure are important in high precision machining. Some researchers have studied that the dynamics are expected to change under different machining conditions. However, the dynamic behaviors of the machine tool at different worktable feed speeds are rarely studied. In this paper, an output-only modal identification available to predict the dynamics of the machine tool at different feed speeds is proposed. The excitation of this method uses the inertia force sequence caused by random idle running of the worktable. The first six modes of the entire machine tool structure are estimated using the proposed method. The results indicate that the running state of the worktable can influence the modes in which the worktable vibrates. The estimated natural frequencies and damping ratios decrease obviously as the feed speed increases. Furthermore, because this method enable to determine modal parameters by measuring the response of machine tool structure without using any artificial excitation, it can be used to predict the dynamic behaviors of the machine tool in entire working space effectively.