Investigations on the effect of friction stir processing on Cu-BN surface composites

The main aim of this investigation focuses on fabrication of copper surface composites through friction stir processing (FSP) reinforced with boron nitride (BN) particles of varying volume fractions (5%, 10%, and 15%). Surface composites developed through single pass FSP were characterized for its microstructural, mechanical and tribological properties. Microstructural characterization indicated that developed surface composites were of good quality with reduced grain size and the SEM characterization confirmed good bonding between copper matrix and BN with uniform dispersion. Micro hardness survey of the developed surface composites showcased minimal deviation in the stir zone with increased trend in respect to the volume fraction of BN. The ultimate tensile strength, yield strength and percent elongation of FSPed composites was found to have reduced when compared with that of pure copper. BN dispersion in surface composite was effective in reducing the ductility and so maximum volume percent (15%) of BN dispersed composite prompt to have higher strength. The wear rate and friction coefficient of the developed surface composite was found decreasing with respect to increase in the dispersion of BN. Amongst the FSPed copper surface composite, specimen with 15?vol% of BN has shown the least wear rate with low coefficient of friction.     

Effect of graphite exfoliation through multipass friction stir processing on microstructure and mechanical properties of aluminium 7075-silicon carbide-graphite composites

In this study, friction stir processing techniques has been utilized to fabricate graphite-reinforced aluminium matrix composite. Silicon carbide will be mixed together with graphite flakes as reinforcement with the composition of hybrid ratio 60 : 40. The manipulated variable of this project is the number of passes used during friction stir processing (1, 2 and 3 passes). Three different number of passes are used to investigate its effect onto the reinforcement dispersion inside the aluminium metal matrix composites. Microstructural analysis has shown that with increasing number of passes, the distribution of reinforcement particles becomes more uniform and homogeneous. Nanoreinforcement particle dispersions are observed and analysed. Result shows that the size of agglomerations across the composite surface decreases as the number of passes increases. Further observation and analysis of on the particles has confirmed that graphite flakes have been successfully shear exfoliated into layers to cover more surface area. The overall mechanical properties have also observed significant increase with increasing number of passes. Sample with highest number of pass (3 pass run) managed to achieve maximum tensile strength and elongation percentage of 304.04 MPa and 2.54 %, respectively, which is the highest value among all the samples tested.     

Friction stir processing strategies to develop a surface composite layer on AA6061-T6

Friction stir processing (FSP) has been used to produce metal matrix composites by incorporating reinforcement particles in an AA6061-T6 matrix. Two types of particles (Al₂O₃ and SiC) were tested. Powder was placed into a mechanized square section groove on a plate surface and then sealed before FSP. This study investigates the effect of several strategies for reinforcement (number and direction of FSP passes) on the wear resistance behavior of friction stir-processed Al-SiC/Al₂O₃ composites. The distribution and size of the particles in the friction stir-processed zone were studied by optical and scanning electron microscopy. Ball-on-disk test was performed on both base material and surface metal matrix composites (SMMCs), and both friction coefficient and specific wear rate (SWR) were correlated with particle distribution and metallurgical effects on the metallic matrix. For all strategies and for both types of reinforcing particles used in this study, the friction coefficient decreases with respect to the base material. Moreover, the SWR is reduced for the conditions of one single FSP pass and two passes with opposite directions, when SiC are used. However, this positive effect has not been detected with Al₂O₃. Wear mechanisms in base metal and in SMMCs are compared and discussed in detail.     

Strengthening mechanisms of aluminum matrix composite containing Cu-coated SiC particles produced by friction stir processing

ABSTRACT

A friction stir processing (FSP) method has been developed to fabricate a locally reinforced aluminum matrix composite (AMC) by stirring electroless-copper-coated SiC particles into AA6061 matrix. The interfacial bonding between particulate reinforcement and the matrix was enhanced by the copper coating. Effective improvement in hardness and in tensile strengths has been proved. Microstructural investigation and analyses were conducted to correlate the microstructural evidences with the possible strengthening mechanisms. The effect of copper coating on the bonding between SiC particles and Al-matrix; the role of the dispersed Cu debris and the increased Cu content in solid solution on the strengthening; and the effect of friction stir on dislocation density and on the recrystallization behavior were analyzed. Multiple strengthening mechanisms due to diffusion between copper film and matrix; dispersion of fine copper debris and Al-Cu intermetallic compounds (IMCs) in the matrix; solid solution due to increased copper content and dislocation punching were four major mechanisms in interpreting the strengthening phenomena in AMC containing copper coated SiC reinforcements.     

Processing, microstructure and mechanical properties of nickel particles embedded aluminium matrix composite

Nickel particles were embedded into an Al matrix by friction stir processing (FSP) to produce metal particle reinforced composite. FSP resulted in uniform dispersion of nickel particles with excellent interfacial bonding with the Al matrix and also lead to significant grain refinement of the matrix. The novelty of the process is that the composite was processed in one step without any pretreatment being given to the constituents and no harmful intermetallic formed. The novel feature of the composite is that it shows a three fold increase in the yield strength while appreciable amount of ductility is retained. The hardness also improved significantly. The fracture surface showed a ductile failure mode and also revealed the superior bonding between the particles and the matrix. Electron backscattered diffraction (EBSD) and transmission electron microscopy analysis revealed a dynamically recrystallized equiaxed microstructure. A gradual increase in misorientation from sub-grain to high-angle boundaries is observed from EBSD analysis pointing towards a continuous type dynamic recrystallization mechanism.     

Investigating effects of process parameters on microstructural and mechanical properties of Al5052/SiC metal matrix composite fabricated via friction stir processing

Friction stir processing (FSP) is a novel process for refinement of microstructure, improvement of material’s mechanical properties and production of surface layer composites. In this investigation via friction stir processing, metal matrix composite (MMC) was fabricated on surface of 5052 aluminum sheets by means of 5 μm and 50 nm SiC particles. Influence of tool rotational speed, traverse speed, number of FSP passes, shift of rotational direction between passes and particle size was studied on distribution of SiC particles in metal matrix, microstructure, microhardness and wear properties of specimens. Optimum of tool rotational and traverse speed for achieving desired powder dispersion in MMC was found. Results show that change of tool rotational direction between FSP passes, increase in number of passes and decrease of SiC particles size enhance hardness and wear properties.     

A novel in-situ polymer derived nano ceramic MMC by friction stir processing

Friction stir processing (FSP) is a solid state technique used for material processing. Tool wear and the agglomeration of ceramic particles have been serious issues in FSP of metal matrix composites. In the present study, FSP has been employed to disperse the nanoscale particles of a polymer-derived silicon carbonitride (SiCN) ceramic phase into copper by an in-situ process. SiCN cross linked polymer particles were incorporated using multi-pass FSP into pure copper to form bulk particulate metal matrix composites. The polymer was then converted into ceramic through an in-situ pyrolysis process and dispersed by FSP. Multi-pass processing was carried out to remove porosity from the samples and also for the uniform dispersion of polymer derived ceramic particles. Microstructural observations were carried out using Field Emission Scanning Electron Microscopy (FE-SEM) and Transmission Electron Microscopy (TEM) of the composite. The results indicate a uniform distribution of ~ 100 nm size particles of the ceramic phase in the copper matrix after FSP. The nanocomposite exhibits a five fold increase in microhardness (260HV₁₀₀) which is attributed to the nano scale dispersion of ceramic particles. A mechanism has been proposed for the fracturing of PDC particles during multi-pass FSP.     

Influence of friction stir processing parameters on surface modified 90Cu-10Ni composites

Friction Stir Processing (FSP) has emerged as a distinctive and pioneering solid state technique to produce surface composites. The objective of the present research is to produce reinforced 90/10 Copper–Nickel surface composites with different carbide-based ceramic particles through FSP and study the relationship of its dynamic parameters including tool rotational speed, tool traverse speed, and width of the groove over the surface behavior. Responses such as sliding wear, microhardness, and surface modified area in the friction stir processed region are modeled using polynomial, nonlinear, multiple regression based on the central composite design of experiment. Analysis of the developed models showed that the FSP parameters; traverse speed, rotational speed, and groove width have significant influence on both the sliding wear and microhardness of developed surface composite. And furthermore, tool rotational speed and tool traverse speed, simultaneously control dispersion of reinforcement in the surface. To validate the abovementioned noteworthy results and to study the microstructural aspects, selected specimens were carried over metallurgical analysis and the obtained results were put forward in detail in this paper.     

Different reinforcement strategies of hybrid surface composite AA6061/(B4C+MoS2) produced by friction stir processing

Aluminum surface composites have gained huge importance in material processing due to their noble tribological characteristics. The reinforcement of solid lubricant particles with hard ceramics further enriches the tribological characteristics of surface composites. In the current study, friction stir processing was chosen to synthesize hybrid surface composites of aluminum containing B₄C and MoS₂ particles with anticipated improved tribological behavior. B₄C and MoS₂ powder particles in 87.5: 12.5 ratio were reinforced into the AA6061 by hole and groove method. Microstructural observations indicated that reinforcement particles are well distributed in the matrix. The hardness and wear resistance of hybrid surface composites improved as compared to the base material, due to well distributed abrasive B₄C and solid lubricant MoS₂ particles in AA6061. The hybrid surface composites achieved ∼32 % increased average hardness as compared to the base material. Hole method revealed ∼13 % better wear resistance compared to the groove method for friction stir processed hybrid surface composite, attributing to an improved homogeneity of particle distribution shown by zigzag hole pattern. Moreover, friction stir processed AA6061 without reinforcement particles exhibited reduced hardness and wear resistance due to loss of strengthening precipitates during multi-pass friction stir processing.     

Strategical parametric investigation on manufacturing of Al–Mg–Zn–Cu alloy surface composites using FSP

Friction stir processing (FSP) is a unique approach being presently researched for composite fabrication. In the present investigation, Al-B₄C surface composite was fabricated through FSP by incorporating B₄C powder particles into Al–Mg–Zn–Cu alloy (AA 7075) matrix. The influence of varying powder particle reinforcement strategies on the microstructure, powder distribution, microhardness, and wear resistance of the surface composite is reported. In addition, AA 6061/B₄C composites were prepared using the same parameter set and the powder distribution in the composite was compared to that in the AA 7075/B₄C composite. More homogeneous dispersion of B₄C powder was observed in AA 6061 as compared to AA 7075 substrate. Among the prepared AA 7075/B₄C composites, the best B₄C powder distribution was detected in samples processed using fine powder and incorporating the change in stirring direction between passes. The hardness and wear resistance of the prepared composites were almost doubled attributing to several strengthening mechanisms and B₄C powder distribution in the AA 7075 matrix.     

Nickel particle embedded aluminium matrix composite with high ductility

Nickel particulate reinforced aluminium matrix composite was processed without formation of deleterious Al₃Ni intermetallic by friction stir processing (FSP). FSP resulted in uniform dispersion of nickel particles in the aluminium matrix with excellent interfacial bonding and also lead to grain refinement of the matrix. The composite exhibited a threefold increase in the yield stress (0.2% proof stress). The most novel feature of the composite is that an appreciable amount of ductility is retained while the strength increases significantly. The microstructure evolution was studied by transmission electron microscopy and electron backscattered diffraction analysis. EBSD analysis showed a dynamically recrystallized equiaxed microstructure having a considerable fraction of low-angle boundaries. TEM observations revealed that these low-angle boundaries are essentially subgrain boundaries formed by dislocation rearrangement and absorption during friction stir processing.     

Fabrication of AA6061/Al2O3 nano ceramic particle reinforced composite coating by using friction stir processing

Nano ceramic particle reinforced composite coatings were created by incorporating Al₂O₃ ceramic particles into the surface of AA6061-T6 alloy plate with multiple pass friction stir processing (FSP). Optical microscopy and Micro-Vickers hardness tests were employed to investigate the influence of axial force and the number of FSP pass on the distribution of the ceramic particles and the hardness of the generated nano ceramic particle reinforced composite coating. Results show that the composite coating is as deep as the length of the pin probe. No distinct interface was developed between the coating and the base metal. The composite region becomes greater as the axial force and the number of FSP pass increased. At the same time, the distribution of the ceramic particles became more homogeneous. Nano particles in the coating have no significant effect on the macro-hardness of AA6061-T6 aluminum alloy even in the composite zone due to the softening of matrix material resulted from overaging. Spindle torque of the tool increased with increasing axial force, while it became less variable and smaller in subsequent pass compared to that in the first pass.     

Microstructure and tribological performance of an aluminium alloy based hybrid composite produced by friction stir processing

In this study, friction stir processing (FSP) was utilized to incorporate SiC and MoS₂ particles into the matrix of an A356 Al alloy to form surface hybrid composite. A constant tool rotation rate of 1600 rpm and travel speed of 50 mm/min with a tool tilt angle of 3° was used. The wear resistance of the processed samples improved significantly as compared to that of the as-cast alloy. Microstructural analyses showed a uniform distribution of reinforcement particles inside the nugget zone, and a MoS₂ rich mechanically mixed layer (MML) on the top of worn surface. This MoS₂ layer is considered to stifle plastic deformation and thus, to improve tribological properties of the alloy.     

Multipass FSP on AA6063-T6 Al: Strategy to fabricate surface composites

Surface composites were fabricated on AA6063-T6 base metal using silicon carbide (SiC) reinforcement particles by friction stir processing (FSP). Influence of multiple FSP passes was investigated on the SiC particle distribution, processed zone dimensions, and microhardness of fabricated composites. The rotational speed, traverse speed, and tool tilt were kept constant and the numbers of passes were varied at 2, 4, 6, and 8. The particle distribution in processed zone was analyzed using OM and SEM, while microhardness were evaluated by Vickers indentation test. The results reveal that with increase in FSP passes there is increase in processed zone dimensions and elimination of defects such as agglomeration of particles and void. The microhardness of reinforced region was increased uniformly with increasing passes which is attributed to homogeneous distribution of reinforcement particles. The peak microhardness value of 81.9 Hv was obtained in sample which is processed with eight numbers of FSP passes. Processed zone indicates good bonding with the substrate and grain refinement.     

搅拌摩擦加工制备颗粒增强铝基复合材料的研究现状及展望

铝合金作为现代工程和高新技术领域发展的关键材料之一,具有密度小、比强度和比刚度高、耐蚀性好等特点。通过在铝基体中添加增强相颗粒,制备得到的颗粒增强铝基复合材料既有铝合金良好的强度、韧性、易成形性等特点,又有颗粒的高强、高模等优点,是近年来应用最广的一类金属基复合材料。 目前,制备铝基复合材料的方法主要有粉末冶金法、铸造以及超声波法等,但这些方法在制备过程中需要较高的温度,颗粒与金属基体容易发生不良的界面反应,从而影响界面结合效果,降低复合材料的性能。搅拌摩擦加工(FSP)作为一种新型的固相加工技术,可同时实现材料微观组织的细化、致密化和均匀化。目前,FSP直接法已在铝基复合材料制备方面取得应用,主要是将增强相颗粒通过打盲孔或开槽的方式预置在金属基体内再进行FSP,进而制备出高致密度的颗粒增强铝基复合材料。因为FSP过程的温度低,颗粒与铝基体不会发生界面反应,所以该方法也被用于制备具有形状记忆效应(SME)的铝基功能复合材料。 近年研究结果表明,颗粒相对FSP制备的铝基复合材料晶粒细化起到显著作用,这有助于提高复合材料的拉伸强度、显微硬度及疲劳强度等力学性能。随着颗粒含量的增加和颗粒尺寸的减小,复合材料的力学性能得以增强。再者,减小颗粒尺寸有利于改善颗粒与基体之间的结合。另外,通过优化搅拌头的结构、形状和尺寸,以及FSP工艺参数,已经可以实现加工后颗粒相在基体中的均匀分布。 鉴于搅拌摩擦加工(FSP)直接法在制备颗粒增强铝基复合材料方面所具备的短流程、高效能以及基体与增强相颗粒界面无杂质等优势,本文对目前FSP直接法制备颗粒增强铝基复合材料的最新研究现状进行了总结。主要综述了FSP制备颗粒增强铝基复合材料过程中颗粒的含量、类型及尺寸对复合材料组织与力学性能的影响,并对颗粒分布均匀性以及颗粒与铝基体的界面问题做了阐述。文章最后深入分析了当前研究中的不足之处并展望了未来的研究方向。     

Friction stir processing of Al6061-SiC-graphite hybrid surface composites

Friction stir processing (FSP) of Al6061-SiC-Graphite hybrid composites is studied in detail via force analysis, spectroscopic, microstructural and indentation studies. Effect of various tool rotational speeds was assessed, and the axial force variation was monitored. The presence of graphite as a reinforcement influences the axial force fluctuations due to its flaky nature and high thermal conductivity. Variation in microstructure at different tool rotational speed is studied using scanning electron microscope. The tool rotational speed has a significant influence on the area of FSP zone, fragmentation and depth of penetration of particles, dispersion of agglomerates and grain refinement of the matrix material. Spectroscopic characterization of the processed samples was done using Raman analysis and X-Ray diffraction studies. A noticeable change in intensity and shift in the respective Raman peak positions were observed, indicating residual stresses and various disorders in the crystal structure of the reinforced particles. Influence of tool rotational speed and existence of SiC and Graphite particles on the mechanical properties were further evaluated using nano indentation testing. The hybrid composite shows the combination of best and uniform mechanical properties at an optimum set of processing parameters.     

Prediction of mechanical and wear properties of copper surface composites fabricated using friction stir processing

Friction stir processing (FSP) has evolved as a novel solid state method to prepare surface composites. In this work, FSP technique has been successfully applied to prepare copper surface composites reinforced with variety of ceramic particles such as SiC, TiC, B₄C, WC and Al₂O₃. Empirical relationships are developed to predict the effect of FSP parameters on the properties of copper surface composites such as the area of the surface composite, microhardness and wear rate. A central composite rotatable design consisting of four factors and five levels is used to minimize the number of experiments. The factors considered are tool rotational speed, traverse speed, groove width and type of ceramic particle. The effect of those factors on the properties of copper surface composites is analyzed using the developed empirical relationships and explained in this paper taking into account the microstructural characterization of the prepared copper surface composites. B₄C reinforced composites have higher microhardness and lower wear rate.     

Toughening of a brittle thermosetting polymer: Effects of reinforcement particle size and volume fraction

Micron- and nanometer-sized aluminum particles were used as reinforcements to enhance the fracture toughness of a highly-crosslinked, nominally brittle, thermosetting unsaturated polyester resin. Both particle size and particle volume fraction were systematically varied to investigate their effects on the fracture behavior and the fracture toughness. It was observed that, in general, the overall fracture toughness increased monotonically with the volume fraction of aluminum particles, for a given particle size, provided particle dispersion and deagglomeration was maintained. The fracture toughness of the composite was also strongly influenced by the size of the reinforcement particles. Smaller particles led to a greater increase in fracture toughness for a given particle volume fraction. Scanning electron microscopy of the fracture surfaces was employed to establish crack front trapping as the primary extrinsic toughening mechanism. Finally, the effects of particle volume fraction and size on the tensile properties of the polyester-aluminum composite were also investigated. The measured elastic modulus was in accordance with the rule-of-mixtures. Meanwhile, the tensile strength was slightly reduced upon the inclusion of aluminum particles in the polyester matrix.     

Particulate migration behavior and its mechanism during selective laser melting of TiC reinforced Al matrix nanocomposites

A transient three-dimensional model for describing fluid flow characteristics and particles migration behavior within the melt pool during selective laser melting (SLM) of TiC/AlSi10Mg nanocomposites was developed. The powder-solid transition, variation of thermophysical properties, and surface tension were considered in the model. The influence of laser energy per unit length (LEPUL) on heat and mass transfer, melt pool dynamics, and particles rearrangement was investigated. It showed that the Marangoni convection became more vigorous with an increase of LEPUL, accordingly enhancing the thermal capillary force. The high laser energy input induced a sufficient liquid formation and an improved wettability, lowering the friction force exerting on TiC solids. Under this condition, the reinforcing particles can be well mixed within the matrix. The experimental study on the distribution state of TiC reinforcement in the SLM-processed Al matrix was performed. The results validated that the dispersion of TiC reinforcement changed from a severe aggregation to a uniform dispersion in the matrix with the increase of LEPUL. The TiC reinforcement experienced a microstructural variation from the standard nanoscale structure with a mean particle size of 70–90 nm to the relatively coarsened submicron morphology with an average particle size of 134 nm as LEPUL increased.     

Interface investigation of BN particle and aluminosilicate short fibre hybrid reinforced Al-12Si alloy composite

A BN particle and aluminosilicate short fibre reinforced hybrid Al-12Si composite was fabricated by squeeze casting. The interface microstructures between reinforcements and matrix were investigated by transmission electron microscopy (TEM). The experimental results showed that there is no reaction product formation at fibre/matrix and BN/matrix interface, and the matrix is in intimate contact with fibre and BN. Because of the addition of self-lubricant BN particles and good connection between BN and matrix alloy, the coefficient of friction of the hybrid composite is improved.