Pitch-matrix composites for electrical,electromagnetic and strain-sensing applications

Pitch-matrix composites for deicing, electromagnetic shielding and strain sensing have been developed by using carbon fiber (discontinuous) and carbon black as electrically conductive fillers. A composite with carbon fiber (5 vol%) as the sole filler is effective for strain sensing, which functions by the electrical resistivity increasing reversibly with tensile strain. A composite with carbon fiber (3.4 vol%) and carbon black (1.5 vol%) is less effective for strain sensing and is lower in tensile strength, modulus and ductility, but it is lower in the electrical resistivity. A composite with carbon black (7 vol%) as the sole filler is very high in resistivity, but exhibits high storage modulus. Either carbon fiber or carbon black as filler increases the storage modulus, decreases the resistivity, renders the ability to provide EMI shielding and increases the softening temperature.     

Carbon fibre composites with improved fatigue resistance due to the addition of tin-lead alloy particles

The addition of tin-lead (60 wt% Sn) alloy particles (about 21–25 μm in diameter, in an amount up to 37 wt% or 7.2 vol%) between continuous unidirectional carbon fibre layers in an epoxy-matrix composite was found to improve the fatigue life by over 100 times. The alloy addition had little effect on the tensile strength, tensile modulus, compressive strength, compressive modulus (with the compressive force parallel and perpendicular to the fibres), but increased the electrical resistivity. The composites were fabricated by compression moulding at 185–200°C and 1 MPa for 30 min. The heating allowed the alloy to melt while the epoxy cured. The fatigue life enhancement is probably due to the hindering of the fatigue crack growth by the alloy particles.     

A study of the effect of filler metal thickness on tensile strength for a stainless steel plate-fin structure by experiment and finite element method

This paper presents a study of the effect of filler metal thickness on tensile strength for a stainless steel plate-fin structure by finite element method and experiment. The results show that the filler metal thickness has a great effect on tensile strength. The tensile strength is increased with the filler metal thickness increase, then it keeps stable when the filler metal thickness is 105–140 μm. But it decreases rapidly when the filler metal thickness is larger than 140 μm. The fracture location is shown at the end of vertical fin when the filler metal thickness is 105–140 μm. Specimens with filler metal thickness smaller or larger than 105–140 μm rupture in the brazed filler metal. The optimal filler metal thickness is 105 μm, using which can get higher strength for 304 stainless steel plate-fin structures.     

Studies on nickel coated carbon fibres and their composites

Uniform and continuous coating of nickel was given to the carbon fibres by cementation, electroless or electroplating techniques. The coating thickness was ranged between 0.2 and 0.6 m for all the three methods used. Coating thickness less than 0.2 m showed discontinuous coating of nickel over the fibre surface. Beyond 0.6 m thickness, nickel deposited in den-drite form over the continuous coating. For continuously coated fibres, the ultimate tensile properties of electroless coated fibres were near to uncoated carbon fibres suggesting adherent and defect free coating; while fibres coated by electrolytic and cementation process exhibited lower ultimate tensile strength (UTS) properties. The tensile fracture of the cementation coated fibres suggested degradation of the fibres. In composites, prepared by dispersing the coated fibres in pure aluminium matrix, no appreciable fibre-metal interaction was observed. NiAl₃ intermetallics were observed around and adjacent to the carbon fibres. Sometimes carbon fibres were found embedded in massive NiAl₃ intermetallics suggesting that fibre surface can also act as nucleating centre for these precipitates.     

Titanium diboride particle-reinforced aluminium with high wear resistance

A TiB₂ particle (61 vol%, 4 m mean size) reinforced aluminium fabricated by liquid-aluminium infiltration was subjected to unlubricated rolling wear and was found from the weight loss to be 1.5 times more wear resistant than 17-4 ph stainless steel, twice as wear resistant as 1020 steel, 7.5 times more wear resistant than 2024 aluminium, and 12.8 times more wear resistant than the aluminium matrix. This wear resistance is attributed to the lack of particle pull-out and the ability of the TiB₂ particles to protect the softer underlying matrix from abrasion. This composite was approximately three times more wear resistant than AlN particle (50 vol%)-reinforced aluminium. The greater wear resistance of Al/TiB₂ compared to Al/AlN is due to the slow wear of the TiB₂ particles and the AlN particle pull-out. A slight decline in tensile strength and no effect on the modulus was observed in Al/TiB₂ after heating at 300 or 600°C for 240 h. This high-temperature stability is attributed to the lack of reactivity between TiB₂ and the aluminium matrix.     

层状PPGr/PP复合材料的结构和电磁屏蔽性能

利用共挤出设备,制备了导电层(石墨填充聚丙烯PPGr)和绝缘层(聚丙烯PP)叠合的层状复合物,同时制备了PPGr普通共混物。比较了层状材料与普通共混材料在结构形态、电学性能、力学性能和电磁屏蔽性能方面的差异。研究发现,普通共混物电阻率为2.04×103Ω.cm时,在低频区(500 MHz~2 GHz)和中间频段区(2 GHz~18 GHz)内的最大屏蔽值都为34dB,而层状材料电阻率为2.79×105Ω.cm时,在上述频率区间的屏蔽效果分别达到49dB和56dB。力学性能测试表明层状材料的断裂伸长率和拉伸强度也优于普通共混材料。文中提出了层状复合物中电磁波的内部多重反射机理来解释这一实验现象。     

The effect of metal wires on the fracture of a brittle-matrix composite

The behaviour of stainless steel, work-hardened nickel and annealed nickel wires bridging a crack in a brittle-matrix has been studied as a function of the length and orientation of the wire. The pull-out stress for stainless steel wire in epoxy resin increases less than linearly with wire length, following the behaviour predicted by Takaku and Arridge [6]. Wires inclined at 20° and 40° to the tensile axis gave pull-out stresses some 30% higher than wires parallel to the tensile axis, this increase being attributed mainly to enhanced friction on the bent wire near its point of exit from the matrix. Work-hardened nickel wires fractured when their length exceeded a critical value, and the critical length was significantly shorter for inclined wires than for wires parallel to the tensile axis. In contrast, annealed nickel wires, no matter how long, did not fracture but pulled out at a limiting stress which was slightly higher for inclined wires than for wires parallel to the tensile axis. The results show that, in some cases, there does not exist a critical length above which an embedded wire will fracture rather than pull out of the matrix.     

Characteristics of several carbon fibrereinforced aluminium composites prepared by a hybridization method

The properties and microstructures of several high-strength and high-modulus carbon fibrereinforced aluminium or aluminium alloy matrix composites (abbreviated as HSCF/Al and HMCF/Al, respectively, for the two types of fibre) have been characterized. The composites evaluated were fabricated by pressure casting based on a hybridization method. It was found that the strength degradation of high-modulus carbon fibres after infiltration of aluminium matrices was not marked and depended upon the type of aluminium matrix. However, the strength of high-strength carbon fibres was greatly degraded by aluminium infiltration and the degradation seemed to be independent of the type of aluminium matrix. The longitudinal tensile strength (LTS) of CF/Al composites was very different between HMCF/Al and HSCF/Al composites. The HMCF/Al composites had LTS values above 800 MPa, but the HSCF/Al composites had only about 400 MPa. In contrast, the transverse tensile strength of the HSCF/Al composites, above 60 MPa, was much higher than that of the HMCF/Al composites, about 16 MPa. Chemical reactions were evident to the interface of high-strength carbon fibres and aluminium matrices. There was no evidence of chemical products arising between high-modulus carbon fibres and Al-Si alloy and 6061 alloy matrices. However, it was considered that some interfacial reactions took place in pure aluminium matrix composites. Fracture morphology observation indicated that the good LTS of CF/Al composites corresponded to an intermediate fibre pull-out, whereas a planar fracture pattern related to a very poor LTS and fibre strength transfer. The results obtained suggested that interfacial bonding between carbon fibres and aluminium matrices had an important bearing on the mechanical properties of CF/Al composites. An intermediate interfacial bonding is expected to achieve good longitudinal and transverse tensile strengths of CF/Al composites.     

Submicron diameter nickel filaments and their polymer-matrix composites

Discontinuous nickel filaments of diameter 0.4 m and having a carbon core of diameter 0.1 m were fabricated by electroplating nickel on discontinuous carbon filaments. They exhibited a grain size of 0.016 m and electrical resistivity of about 5 × 10^–6 ·cm. In an amount as low as 7 vol.% in a polymer (polyether sulfone) matrix, they resulted in a composite exhibiting electromagnetic interference shielding effectiveness of 87 dB and reflection coefficient 0.95 at 1–2 GHz, tensile strength 52 MPa, tensile ductility 1.0%, and density 1.87 g/cm³.     

Tensile and Impact Properties of Shielded Metal Arc Welded AISI 409M Ferritic Stainless Steel Joints

The present study is concerned with the effect of filler metals such as austenitic stainless steel, ferritic stainless steel and duplex stainless steel on tensile and impact properties of the ferritic stainless steel conforming to AISI 409M grade. Rolled plates of 4 mm thickness were used as the base material for preparing single pass butt welded joints. Tensile and impact properties, microhardness, microstructure and fracture surface morphology of the joints fabricated by austenitic stainless steel, ferritic stainless steel and duplex stainless steel filler metals were evaluated and the results were reported. From this investigation, it is found that the joints fabricated by duplex stainless steel filler metal showed higher tensile strength and hardness compared to the joints fabricated by austenitic and ferritic stainless steel filler metals. Joints fabricated by austenitic stainless steel filler metal exhibited higher ductility and impact toughness compared with the joints fabricated by ferritic stainless steel and duplex stainless steel filler metals.     

Rate of formation of intermetallic compounds in aluminium matrix-carbon fibre composites

This paper describes the formation of aluminium carbide at the interface between aluminium and carbon fibres as a result of annealing for 4 h at 550°C or above, or for 1 h at 620°C or above. For a quantitative analysis of the reaction, measurements were made of the weight reductions of carbon fibre that occurred as a result of annealing, these reductions being proportional to the quantity of aluminium carbide formed. The relationship between the amount of aluminium carbide formed, the tensile strength and the surface characteristics of carbon fibres is discussed.     

Tensile behaviour of squeeze cast and forged short alumina fibre reinforced AA 6061

Abstract

The tensile properties of a composite consisting of 20 vol.-% short δ alumina fibres in an aluminium matrix (AA 6061) prepared by squeeze casting have been investigated, before and after 30% reduction by forging. By annealing the composite before forging, a 30% forging reduction could be achieved at room temperature, without crack formation. A reduction in mean fibre length from about 65 to 15 μm was observed but most fibre breaks were filled by matrix. By heat treating the composites after forging, their elongation to fracture was increased to about twice that of a similarly heat treated unforged composite of comparable strength. The improvement of ductility is attributed to break-up of the fibre skeleton structure inherited from the fibre preform. A model is presented that predicts that for these fibres an optimum effective reinforcement is achieved at fibre lengths of about 100 μm, which explains why the reduction in fibre length caused by forging does not result in significant strength loss.

MST/1720     

不锈钢/铝合金异种金属激光-MIG熔钎焊工艺研究

分别采用激光-MIG复合焊和单MIG焊,实现了2mm厚的304不锈钢和6061铝合金对接接头的熔钎焊,对比了不同焊接热源对接头显微组织、界面层化合物及力学性能的影响。结果表明,采用激光-MIG复合焊可以获得性能良好的不锈钢-铝对接接头。激光-MIG复合焊接头的界面层化合物为FeAl_2和Fe_4Al_(13),厚度约为5μm;而单MIG焊接头的界面层化合物厚度约为3μm,主要为Fe_4Al_(13)。激光-MIG复合焊接头的抗拉强度为105MPa,比单MIG焊接头提高了10.8MPa,达到铝合金母材的33.9%。接头试样拉伸断裂均起裂于钎焊界面处,并向余高处扩展,且由脆性断裂转变为韧性断裂。     

Low voltage electrical properties of polypropylene filled with stainless steel fibres and a model of sample conductivity based on fibre geometry

The d.c, electrical properties of 80×80×3 mm³ polypropylene plaques filled with 6.5 μm diameter stainless steel fibres have been studied for volume fractions in the vicinity of a critical threshold at which the volume resistivity changes very rapidly with filler concentration. By the use of very low power inputs to eliminate any possibility of local temperature changes, the samples have been established to be ohmic conductors with resistivities ranging from 12 to 0.61 Ω cm for fibre volume fractions of 1 to 3%. It is suggested that percolation conditions i.e. continuous chains of metal fibres are produced at low volume fraction of filler because of a special fibre geometry i.e. a substantial proportion of the fibres are three dimensionally folded into shapes of roughly helical form, thus enhancing the probabilities of contact between adjacent fibres. For simplicity a model structure of perfect helices with identical diameters and pitch has been examined. The model leads to a critical volume fraction at the percolation threshold, which is in good agreement with experiment and proportional to the square of the ratio of fibre diameter to helix diameter. The threshold resistivity range predicted by the model is also a function of fibre and helix diameter and this resistivity also decreases with mean fibre length. It is argued further that there exists an optimum value of fibre aspect ratio for which the critical volume fraction is a minimum. The predicted threshold resistivity is in good agree ment with experiment providing that a small amount of the size coating is assumed to have been removed during manufacture of the plaques, thus allowing a small fraction of the fibre fibre contacts to be conducting.     

Effect of holding time on vacuum brazing for a stainless steel plate–fin structure

This paper presents a vacuum brazing of 304 stainless steel plate–fin structures with nickel-based BNi-2 filler metal. The effect of brazing holding time on tensile strength and microstructure has been investigated, aiming to obtain the optimal brazing holding time. The microstructure in brazing joint consists of diffusion-affected zone (DAZ), interface reaction zone (IRZ), isothermally solidified zone (ISZ) and athermally solidified zone (ASZ). The structure in the fillet is composed of solid solution, nickel silicon, nickel boron compound and a mixture with nickel silicon and nickel boron. The tensile strength increases along with the increase of holding time, but decreases when the holding time is over 25 min. A maximum tensile strength of 65.1 MPa is obtained with 25 min holding time. Too short holding time will make boron diffuse insufficiently and generate a great deal of brittle boride components, and too long holding time will make the base metal dissolve into the filler metal excessively and creates more corrosion voids.     

PLA/algae composites: Morphology and mechanical properties

Bio-composites with poly(lactic) acid as matrix and various algae (red, brown and green) as filler were prepared via melt mixing. Algae initial size (below 50 μm and between 200 and 400 μm) and concentration (from 2 to 40 wt%) were varied. First, algae morphology, composition and surface properties are analysed for each algae type. Second, an example of algae particle size decrease during processing is given. Finally, tensile properties of composites are analysed. The surface of algae flakes was covered with inorganic salts affecting filler–matrix interactions. The Young’s modulus of composites increased at 40 wt% load of algae as compared with neat PLA although the strain at break and tensile strength decreased. In most cases the influence of algae type was minor. Larger flakes led to better mechanical properties compared to the smaller ones.     

Transient liquid-phase insert metal bonding of Al2O3 and AISI 304 stainless steel

Transition liquid-phase insert metal bonding of Al2O3 and AISI 304 stainless steel based materials is investigated. This joining technique allows the continuous replenishment of the active solute which is consumed by the chemical reaction that occurs at the ceramic/filler metal interface. Replenishment is facilitated by employing a sandwich of filler materials comprising tin-based filler metal and amorphous Cu50Ti50 or NiCrB interlayers. During Al2O3/AISI 304 stainless steel bonding, the highest shear strength properties are produced using a bonding temperature of 500 °C. Thick reaction layers containing defects form at the ceramic/filler material interface when higher bonding temperatures are applied. Bonding at temperatures above 500 °C also increases the tensile residual stress generated at the periphery of Al2O3/AISI 304 stainless steel joints. The shear strength of joints produced using NiCrB interlayers markedly increased following heat treatment at 200 °C for 1.5 h. Heat treatment had little influence on the shear strength of the joint produced using Cu50Ti50 interlayers. This revised version was published online in November 2006 with corrections to the Cover Date.     

Comparison of fibres for creep strengthening of zinc-aluminium foundry alloys

A comparative evaluation is made of a variety of possible fibrous reinforcements for strengthening zinc-aluminium foundry alloys. The composites are processed by squeeze casting, using preforms of alumina, carbon, stainless steel or low carbon steel fibres. A drastic improvement of the creep strength is achieved with the use of alumina or steel fibres. However, an acceptable level of fracture toughness is maintained only in the composites reinforced with steel fibres. This property results from the low interface adhesion which allows bridging of the crack by the fibres. Low carbon steel fibres do not exhibit more interface reaction than stainless steel fibres. It is concluded that low carbon steel fibres provide a better compromise when taking into account the creep strength, the fracture toughness and the cost of the composite.     

Tensile behaviour of stainless steel fibre/epoxy composites with modified adhesion

In this study we investigate the tensile behaviour of unidirectional and cross-ply composites reinforced with ductile stainless steel fibres and modified adhesion to the epoxy matrix. Results show that annealed stainless steel fibres have a potential in designing tough polymer composites for structural applications. The stiffness of the UD composites made from these fibres is 77GPa combined with the strain-to-failure between 15% and 18% depending on the level of adhesion. Silane treatments were used to modify the adhesion. By treating the stainless steel fibres with different silane coupling agents, an increase of 50% in the transverse 3-point-bending strength was realised. Increasing the adhesion by 50% leads to a higher tensile strength and strain-to-failure in both UD and cross-ply laminates and a higher in-situ strength of the 90° plies. It also delays formation of matrix cracks and hinders growth of debonding.     

Vacuum hot roll bonding of titanium alloy and stainless steel using nickel interlayer

Abstract

Vacuum hot roll bonding of titanium alloy and stainless steel using a nickel interlayer was investigated. No obvious reaction or diffusion layer occurs at the interface between stainless steel and nickel. The interface between titanium alloy and nickel consists of an occludent layer and diffusion layers, and there are the intermetallic compounds (TiNi₃, TiNi, Ti₂Ni and their mixtures) in the layers. The total thickness of intermetallic layers at the interface between titanium alloy and nickel increases with the bonding temperature, and the tensile strength of roll bonded joints decreases with the bonding temperature. The maximum tensile strength of 440·1 MPa was obtained at the bonding temperature of 760°C, the reduction of 20% and the rolling speed of 38 mm s^–1.