Studies on the formability of powder metallurgical aluminum–copper composite

Formability is concerned with the extent of deformation that the materials undergo before failure; thus its investigation is critical for successful processing of materials during bulk deformation. The present investigation has been undertaken to generate the forming limit diagrams for powder metallurgical aluminium–copper composites for different initial relative densities and copper contents. Sintered aluminium–copper composite compacts of 2%, 4% and 6% copper content with different initial relative densities have been prepared by applying recommended powder compaction pressures. The material properties such as apparent strain hardening exponent and strength coefficient were determined using stage wise compression test to generate the formability limit diagram. Densification curves were plotted to investigate the effect of initial relative density and copper content on the pore closure phenomena during deformation. Theoretical and experimental investigations using standard ring compression test were carried out to determine friction factor between tool and work piece interfaces for different initial relative density and copper content. The critical transition densities vide the forming limit diagram were found to be 84%, 85.3%, 86% and 87.5% for pure sintered aluminium, Al–2%Cu, Al–4%Cu and Al–6%Cu composites respectively. The friction factor between tool and work piece interfaces has showed increasing pattern for all the cases with decrease in the initial relative density and increase in the copper content of the composite.     

The size effect on micro deformation behaviour in micro-scale plastic deformation

When the geometry of metal deformed part is scaled down to micro-scale, the understanding and prediction of micro deformation behaviour becomes difficult. This is because the conventional material deformation models are no longer valid in micro-scale due to the size effect, which affects the deformation behaviour in micro plastic deformation, and thus leveraging the traditional knowledge of plastic deformation from macro-scale to micro-scale is not meaningful. In this paper, the size effect on micro-scale plastic deformation and frictional phenomenon are investigated via micro-cylindrical compression test, micro-ring compression test and Finite Element (FE) simulation. The experimental results show the occurrence of various size-effect related deformation phenomena, including the decrease of flow stress and the increases of: (a) irrational local deformation, (b) the amount of springback, and (c) the interfacial friction stress with the decreasing specimen size. The research further verifies that the established surface layer models, with the identified surface grain, the internal grain properties and the measured friction coefficients, are able to predict micro deformation behaviour. The research thus provides an in-depth understanding of size effect on deformation and frictional behaviours in micro-scale plastic deformation.     

Microstructural evolution and flow behaviour during hot compression of twin roll cast ZK60 magnesium alloy

Abstract

Microstructural evolution and flow behaviour during hot compression of twin roll cast ZK60 magnesium alloy were characterised by employing deformation temperatures of 300, 350 and 400°C and strain rate ranging from 10^?3 to 100 s^?1. When compressed at 10^?3 s^?1, all stress–strain curves at different temperatures (300, 350 and 400°C) showed a flow softening behaviour due to active dynamic recrystallisation. When compressed at 10^?2 s^?1 and elevated temperatures (300, 350 and 400°C), all stress–strain curves showed a flow stress drop after peak stress due to twinning for 300 and 350°C deformation and recrystallisation for 400°C deformation. The balance between shear deformation and recrystallisation resulted in a steady flow behaviour after the true strain reached 0·22. When strain rate increased to 10^?1 s^?1, a small fraction of dynamic recrystallisation in shear deformation region was responsible for slight flow softening behaviour during compression. A flow hardening appeared due to basal and non-basal slips when deformed at 100 s^?1. It is suggested that the flow behaviour during hot compression of twin roll cast ZK60 alloy depends on the separating effect or combined effects of shear deformation, twinning and recrystallisation.     

Stress analysis of steel beams strengthened with a bonded hygrothermal aged composite plate

In this paper, the problem of interfacial stresses in steel beams strengthened with bonded hygrothermal aged composite laminates is analyzed using linear elastic theory. The analysis is based on the deformation compatibility approach developed by Tounsi (Int. J. Solids Struct. 43:4154–4174, 2006) where both the shear and normal stresses are assumed to be invariant across the adhesive layer thickness. The adopted model takes into account the adherend shear deformations by assuming a linear shear stress through the depth of the steel beam. This solution is intended for application to beams made of all kinds of materials bonded with a thin composite plate. For steel I-beam section, a geometrical coefficient ξ is determined to show the effect of the adherend shear deformations. This research is helpful for the understanding on mechanical behaviour of the interface and design of such structures.     

复合材料管接头与钢管间摩擦力及其对管接头强度的影响——数值分析

通过定义接触单元, 建立了复合材料套管接头与钢管连接的有限元分析模型。分析了在钢管端部受到弯曲、压缩和扭转载荷条件下, 复合材料管接头的应力状态, 并采用Tsai-Wu 强度准则对复合材料管接头进行了强度分析。重点研究了随着摩擦系数的变化复合材料管接头与钢管间摩擦力的变化规律及其对复合材料接头强度的影响。结果表明, 随着摩擦系数的增大, 复合材料管接头与钢管间最大正应力减小, 最大摩擦力增大; 在以弯曲载荷为主的组合载荷作用下, 复合材料管接头的安全裕度增大。      

Micromechanical interaction in two-phase iron–copper polycrystals

The plastic deformation of two-phase iron–copper polycrystals was studied experimentally and modelled in a semi-analytical approach, taking into account work-hardening behaviour, initial texture, slip processes and volume fractions of the phases. Iron–copper polycrystals including the single-phase materials were produced by powder metallurgy in various compositions of iron and copper. The two-phase materials had microstructures ranging between interpenetrating networks and matrix/inclusion type. Samples were deformed by rolling and compression at room temperature. Besides stress vs. strain during compression the texture and the microhardness distribution were measured before and after the deformation. The determined quantities (stress, strain, texture) were compared with model calculations performed with a viscoplastic self-consistent (VPSC) model from R. Lebensohn and C. Tomé. The best predictions of this model were found in the case of copper inclusions in an iron matrix whereas for interpenetrating networks a viscoplastic Taylor model was in better accordance with experiment.     

A100超高强度钢的流变应力曲线修正与唯象本构关系

在Gleeble-3500型热模拟试验机上对A100超高强度钢进行热压缩实验,获得了在变形温度为850~1200℃,应变速率为0.001~10s -1 以及变形程度为60%条件下的流变应力曲线,分析热压缩过程中摩擦和温升效应对流变应力的影响,修正了流变应力曲线;并在Arrhenius双曲正弦函数方程的基础上引入应变量参数构建了基于应变量耦合的唯象本构模型。结果表明:随着变形温度的降低或应变速率的增加,摩擦和温升效应对流变应力的影响逐渐显著;所建立的本构模型预测值与实验值的绝对平均相对误差为4.902%,相关系数为0.99,能够用于准确预测不同应变下的流变应力。     

Experimental investigation on constitutive behaviour of superconducting powder BSCCO

As a superconducting material, Bi 2233/Ag tape needs high-critical transport current density J _c, which is influenced by the uniform deformation and density of BSCCO powder in filaments during the forming process. The aim of this paper is to investigate the constitutive behaviour of BSCCO powder. The modified Drucker–Prager/Cap model is introduced to describe the constitutive behaviour of BSCCO powder. A series of cyclic loading experiments for BSCCO powder in a cylinder die were carried out. Based on the experiments, the relationships between the radial stress and the axial stress were obtained, and the parameters in the constitutive model were calculated. By pushing the compact powder from the die, the coefficient of friction between the BSCCO powder and the cylinder die was determined. Finally, the modified Drucker–Prager/Cap model is proposed and used to simulate the confined compression test of BSCCO powder.     

An experimental and numerical investigation on micro rolling for ultra-thin strip

The demand for miniaturized parts and miniaturized semi-finished products is increasing nowadays, because microforming processes can improve production rate and minimize material waste due to less forming passes. However, traditional macro metal forming processes and modelling cannot be simply scaled down to produce miniaturized micro parts. In this study, a 2-Hi micro rolling mill has been successfully built. Experimental and numerical investigations on the micro rolling process for ultra-thin SUS 304 stainless steel strip have been conducted. The experimental results show that the micro rolling deformation of ultra-thin strip is influenced by size effect which results from the specimen size difference and this size effect is embodied in the flow stress and the friction coefficient. Analytical and finite element (FE) models in describing size effect related phenomena, such as flow stress, friction, rolling force and deformation behaviour, are proposed. The material surface constraint and the material deformation mode are critical in determination of material flow stress curve. The analysis of surface roughness evolution with rolling conditions has also been performed. The identified analysis on deformation mechanics provides a basis for further exploration of the material behaviour in plastic deformation of micro scale and the development of micro scale products via micro rolling.     

Optimizations of wear resistance and toughness of hydroxyapatite nickel free stainless steel new bio-composites for using in total joint replacement

The aim of this work was to produce novel bio-composites made of hydroxyapatite and nickel free stainless steel (prepared by heat treating bone ash) and studying their mechanical properties including their tribology under various loads, toughness, and compressive and bending strengths. Different amounts of nickel free stainless steel powder (30, 40, 50 and 60 wt.%) was added to this hydroxyapatite powder to get bio-composites. Their hardness, wear resistance and friction coefficient, as a function of the metal (nickel free stainless steel) content were investigated. Hardness and wear resistance were decreased by increasing of the weight percentage of stainless steel, while friction coefficient was increased. Strength and toughness of composites increases considerably by increasing of NFSS content. The toughness enhancement is contributed mainly by crack bridging and plastic deformation of the nickel free stainless steel. The strengthening effect is contributed by both the matrix grain refinement and the toughness enhancement. According to results of all mechanical tests done on composites, composite with 50 wt.% nickel free stainless steel has the most appropriate mechanical properties among other composites for using in orthopaedic applications.     

Bonding between aluminium and copper in cold spraying: story of asymmetry

Abstract

The bonding mechanism in cold spraying is still a matter of some debate, which requires further investigation. In the present work, aluminium powder was cold sprayed onto a copper substrate and copper powder was cold sprayed onto an aluminium substrate using the same process gas and spray parameters. Separate experiments were performed to produce thick (～400 μm) coatings and isolated particle impacts. Deposits were characterised using scanning electron microscopy and image analysis. The coating–substrate interfacial bonding was assessed via a method in which, following a short heat treatment at 400°C, intermetallics grow at the interface where metal to metal contact has been established. In addition, the bond strength values of deposits were determined using a standard pull-off test. It was found that the copper particles deposited onto an aluminium substrate resulted in significant substrate deformation, whereas aluminium particles caused minimal deformation of the copper substrate. Furthermore, the former displayed a higher degree of metallurgical bonding at the coating/substrate interface in comparison with the latter. These results suggest that the removal of oxide films from the surfaces was greater when copper was the material being sprayed rather than aluminium. The impact behaviour of the two materials and the removal of oxide due to deformation at high strain rate are discussed with the aid of the Johnson–Cook plasticity model.     

木粉/PE复合材料与高速钢刀具摩擦特性的研究

目的 切削加工时木工刀具的摩擦磨损形式较为复杂,旨在研究高速钢与木粉/PE复合材料之间的摩擦特性.方法 通过木粉/PE复合材料与高速钢的摩擦试验,围绕木粉质量分数、往复频率、轴向载荷等因素,分析木粉/PE复合材料与高速钢摩擦因数的变化规律,以及试验后木粉/PE复合材料的表面形貌.结果 木粉/PE复合材料与高速钢刀具的摩擦因数随着木粉质量分数的增加呈现出先升高再降低的趋势,在木粉质量分数为50％时达到最大,摩擦特性为磨粒摩擦及粘着摩擦.结论 随着频率、载荷的升高,木粉质量分数较高的木粉/PE复合材料摩擦因数明显降低,高速钢刀具更适用于切削加工木粉质量分数较高的木粉/PE复合材料;摩擦试验后木粉/PE复合材料粗糙度显著降低,使用高速钢刀具切削加工木粉/PE复合材料,木粉质量分数为50％的材料试验后形貌较为平整,这为获得较好的表面切削质量提供了有力参考.     

铜/钢双金属复合制备工艺技术研究现状

介绍了目前国内外生产铜/钢双金属复合材料的主要制备工艺以及它们的原理和特点,在此基础上分析了铜/钢复合界面的冶金结合行为和铜/钢双金属复合材料制备的研究动态,并对铜/钢复合材料在各方面的应用做了简单介绍,最后对各制备工艺的未来发展趋势进行总结。     

Werkstoffverhalten metallischer Faserverbunde unter Druckschwellbeanspruchung

Behaviour of metallic fiber composites under pulsating compression The behaviour of metallic fiber composite materials is investigated in stress controlled pulsating compression tests in the low cycle fatigue range. Two fiber composites are chosen as the test material which are used in electric industry as contact materials: copper matrix reinforced by continuous austenitic steel fibers as well as silver matrix reinforced by nickel fibers. Especially the successive buckling of the slim fibers arising during cyclic compression loading is studied by means of metallographic methods. Buckled fibers form a kinkband with localized fiber bending. The kinkband formation is found to be independent on fiber volume and stress amplitude.     

Adiabatic flow curves of metallic materials at high strain rates

Dynamic compression tests are carried out on Armco iron, Cr-V-steel, Ni-Cr-Mo-V-steel, an austenitic Ni-Cr-Mo-steel, tantalum, nickel and Ni₃Al and magnesium. The flow curves are analysed to determine the influence of the deformation energy which is tranformed into heat on the flow behaviour and mechancial stability. Not only the material properties but also the conditions of friction between the specimen and the compression tool are found to have a greate influence on the flow stress reduction and stability. High frictional forces promote mechanical instability of materials with low strain hardening and low strain rate sensitivity.     

Deformation twinning in zircaloy 2

Abstract

Fully recrystallised zircaloy 2 samples were subjected to different degrees of uniaxial compression. Grains of high Taylor factors showed {1012}〈1011〉 deformation twins, noticeable up to 13–16% compression. Twinning strongly affected the crystallographic texture and also brought in clear differences in stored energy and residual stress between the suspected parent and product grains/orientations of twinning. At later stages of deformation, where presence of twinning was insignificant, aforementioned heterogeneity was further supplemented by heterogeneity in microstructure – clear presence of fragmenting and non-fragmenting grains. Direct observations on twin fraction, twin deviation and twin continuity had shown an apparent peak in twinning by ～7·5% compression, an observation explainable through a simple model of twin decay by in grain misorientation development.     

EFFECTS OF MICROSTRUCTURE ON RESISTANCE TO SHEAR LOCALIZATION FOR A CLASS OF METAL MATRIX COMPOSITES

The development of shear bands in a class of materials with inhomogeneous and composite microstructures is analysed using dynamic finite element calculations. The analysis is based on experimental studies of the deformation localization and constitutive behaviour of tungsten composites having two-phase microstructures consisting of hard tungsten grains and a soft alloy metal matrix. The calculations concern the effects of variations in microstructures on the resistance of the materials to shear banding. Specifically, three variations from the currently used microstructure are considered: (1) the alteration of the matrix phase to one that has a lower thermal conductivity; (2) the introduction of an interfacial layer between the grains and the matrix which limits microscopic heat exchange between the phases by having a low thermal conductivity; and (3) the modification of the matrix to one having a lower strength at high temperatures, but otherwise the same thermal and mechanical properties as the nickel–iron–tungsten matrix currently used in many tungsten composites. Numerical results show that the introduction of the low-conductivity interfacial layer significantly enhances the formation of shear bands and reduces the overall flow stress of the composite throughout the deformation. On the other hand, accelerated thermal softening of the matrix phase at high temperatures expedites localization in later stages of deformation while allowing the material to maintain a strong stress–strain response in the early part of the deformation. The results point out possible directions for the improvement of advanced composites in applications where the localization of deformation plays an important role.     

Microstructure and enhanced atomic diffusion of friction stir welding aluminium/steel joints

Lap joints of friction stir welding between aluminium and stainless steel sheets were conducted using a welding tool with a cutting pin. The atomic diffusion of Fe–Al system during the severe plastic deformation was investigated. The interfacial microstructure and metallurgical reaction was analysed. The metallurgical reaction layers were identified as a compounds containing a phase of Al₃Fe, partial solid solution of Fe and Al, and amorphous with a thickness of 0.9–3.3?μm which depending on the process parameters. The interdiffusion coefficient between Al and Fe atoms is about 4 orders of magnitude compared with that under thermal equilibrium state. The nanocrystalline and partial amorphous were formed near the interface which may caused by the enhanced atomic diffusion.     

Determination of interfacial mechanical properties of ceramic composites by the compression of micro-pillar test specimens

A novel method to determine the fiber-matrix interfacial properties of ceramic matrix composites is proposed and evaluated; where micro-pillar samples containing inclined fiber/matrix interfaces were prepared from a SiC fiber-reinforced SiC matrix composites and then compression-tested using the nano-indentation technique. This new test method employs a simple geometry and mitigates the uncertainties associated with complex stress state in the conventional single-filament push-out method or tensile unloading–reloading hysteresis loop analysis method for the determination of interfacial properties. Based on the test results using samples with different interface orientations, the interfacial debond shear strength and the internal friction coefficient are explicitly determined and compared with values obtained by other test methods. SEM observation showed that micro compression caused an adhesive type of debonding between the fiber and the pyrolytic carbon interface. The results suggest that the debonding/failure behavior of the micro-pillars followed the Coulomb fracture criterion. The determined interfacial debond shear strength is ~100 MPa, which appears to be smaller than that determined from fiber push-out test for similar composite systems. The difference can be explained by the effect of normal stress (clamping stress) on the apparent interfacial debond shear strength.     

增材制造铜/钢双金属材料研究进展

铜/钢双金属材料具有力学强度高、物理化学性能优良等优势,在交通运输、电力能源和建筑工业等领域应用前景广阔。然而,传统熔铸工艺在制造铜/钢双金属材料时,容易在铜/钢界面处产生偏析现象,在一定程度上限制了铜/钢双金属材料的发展。与传统工艺相比,增材制造技术不仅能实现复杂加工零件的快速制造,而且在成形过程中较短的保温时间能缓和或消除异种金属材料界面产生的冶金缺陷,进而增强铜/钢双金属材料的力学性能。由于双金属材料是近年来的研究热点,有关增材制造铜/钢双金属材料的综述性文章较少,故综述了近年来激光、电子束及电弧增材制造技术制造铜/钢双金属材料的研究发展现状,分析了各技术的优缺点,并从制备方法、工艺参数及界面合金元素等角度,分析了影响材料界面组织性能变化的关键因素。发现在增材制造铜/钢双金属材料方面,目前激光增材制造技术主要应用于精度要求较高的小尺寸零部件,电子束增材制造技术适用于某些具有特殊性能的合金,如钛合金,而电弧增材制造技术适用于精度要求较低的大型复杂零部件。在铜/钢双金属材料增材制造过程中,界面处易形成显微组织分布不均匀、界面晶粒尺寸差异较大等现象,导致界面处产生应力集中,从而造成材料...