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.     

High cycle fatigue behaviour of microsphere Al2O3–Al particulate metal matrix composites

The high-cycle stress-life (S–N) curve and fatigue crack growth threshold (ΔK_th) behaviour of COMRAL-85^TM, a 6061 aluminium–magnesium–silicon alloy reinforced with 20 vol.% Al₂O₃-based polycrystalline ceramic microspheres, and manufactured by a liquid metallurgy route, have been investigated for a stress ratio of R = −1 (fully reversed loading). Fatigue testing was conducted on both smooth round bar (S–N) specimens and notched round bar (fatigue threshold) specimens. Unreinforced Al 6061-T6 also processed by a liquid metallurgy route and six powder metallurgy processed composites with particle volume fractions ranging between 5% and 30% were also studied. S–N data revealed that the powder metallurgy processed composites generally gave longer fatigue lives than the matrix alloy, whereas COMRAL-85^TM exhibited a reduced fatigue life. The fatigue threshold results were very similar for all the composites, being lower than for Al 6061-T6. Fatigue failure mechanisms were determined from examination of the fracture surfaces and the crack profiles.     

Effect of heat treatment on strength and abrasive wear behaviour of Al6061-SiC<Subscript>p</Subscript> composites

In recent years, aluminum alloy based metal matrix composites (MMC) are gaining importance in several aerospace and automobile applications. Aluminum 6061 has been used as matrix material owing to its excellent mechanical properties coupled with good formability and its wide applications in industrial sector. Addition of SiC_p as reinforcement in Al6061 alloy system improves its hardness, tensile strength and wear resistance. In the present investigation Al6061-SiC_p composites was fabricated by liquid metallurgy route with percentages of SiC_p varying from 4 wt% to 10 wt% in steps of 2 wt%. The cast matrix alloy and its composites have been subjected to solutionizing treatment at a temperature of 530°C for 1 h followed by quenching in different media such as air, water and ice. The quenched samples are then subjected to both natural and artificial ageing. Microstructural studies have been carried out to understand the nature of structure. Mechanical properties such as microhardness, tensile strength, and abrasive wear tests have been conducted both on matrix Al6061 and Al6061-SiC_p composites before and after heat treatment. However, under identical heat treatment conditions, adopted Al6061-SiC_p composites exhibited better microhardness and tensile strength reduced wear loss when compared with Al matrix alloy.     

Fracture toughness of microsphere Al2O3–Al particulate metal matrix composites

The fracture toughness and behaviour of COMRAL-85^TM, a 6061 aluminium–magnesium–silicon alloy reinforced with 20 vol% Al₂O₃-based polycrystalline ceramic microspheres, and manufactured by a liquid metallurgy route, have been investigated. Fracture toughness tests were performed using short rod and short bar (chevron-notch) specimens machined from extruded 19 mm diameter rod, heat treated to the T6 condition. The fracture toughness in the R–L orientation was found to be lower than in the C–R or L–R orientations owing to the presence of particle-free bands in the extrusion direction. Short rod tests were also conducted for the R–L orientation on six powder metallurgy composites with particle volume fractions ranging between 5% and 30%. It was found that the fracture toughness decreased progressively with particle volume fraction, but at a decreasing rate. A detailed examination of the fracture behaviour was made for both the liquid metallurgy and powder metallurgy processed composites.     

Investigation on the cutting quality characteristics of abrasive water jet machining of AA6061-B4C-hBN hybrid metal matrix composites

Aluminum metal matrix composites (AMMCs) explicitly show better physical and mechanical properties as compared to aluminum alloys and results in a more preferred material for a wide range of applications. The addition of reinforcements embargo AMMCs employment to industry requirements by increasing order of machining complexity. However, it can be machined with a high order of surface integrity by nonconventional approaches like abrasive water jet machining. Hybrid aluminum alloy composites were reinforced by B₄C (5–15?vol%) and solid lubricant hBN (15?vol%) particles and fabricated using a liquid metallurgy route. This research article deals with the experimental investigation on the effect of process parameters such as mesh size, abrasive flow rate, water pressure and work traverse speed of abrasive water jet machining on hybrid AA6061-B₄C-hBN composites. Water jet pressure and traverse speed have been proved to be the most significant parameters which influenced the responses like kerf taper angle and surface roughness. Increase in reinforcement particles affects both the kerf taper angle and surface roughness. SEM images of the machined surface show that cutting wear mechanism was largely operating in material removal.     

The effect of extrusion parameters on the fretting wear resistance of Al-based composites produced via powder metallurgy

The work reported in this paper is aimed at establishing the relationship between processing and the wear resistance of the metal matrix composites (MMCs) based on a novel alloy, Al-20Si-5Fe-3Cu-1Mg. The MMCs were processed via a commercially viable powder metallurgy (PM) route, i.e. through mixing the atomized matrix alloy powder with 10 vol% SiC or Al₂O₃ particles, cold isostatic pressing, degassing and hot extrusion. It has been found that the extrusion window of the MMCs is greatly narrowed due to their increased deformation resistance on one hand and incipient melting of their matrix on the other. For a sound MMC extrudate, a reduction ratio over a critical value must be applied. However, a further rise of this ratio leads to deterioration of local interfacial cohesion between the ceramic phase and the matrix dispersed with a high volume fraction of silicon crystals and intermetallic dispersoids, thus degrading the MMCs in tensile properties. Furthermore, fretting wear tests at room and elevated temperatures and with dry and wet contacts show that the MMCs extruded at a higher reduction ratio has a higher mass loss and an increased friction coefficient. The work points to the direction of further research, i.e. on MMCs containing spherical reinforcement instead of commonly used angular particles.     

Effect of ceramic reinforcements on microwave sintered metal matrix composites

Metal matrix composites (MMCs) using Aluminum Alloy 2900 and 2024 as matrix material with silicon carbide and alumina as reinforcement have been fabricated through powder metallurgy route for investigation. The average particle size of matrix metal and reinforcement material considered in this research is 10?µm. AA-SiC and AA-Al₂O₃ composites with 3, 6, and 9 weight percentage (wt%) of SiC and Al₂O₃ are fabricated. The Rockwell hardness and Compressive strength of AA-SiC and AA-Al₂O₃ composites were found to increase with an increase in the wt% of reinforcement when the samples were microwave sintered. AA 2024 with 6?wt% Al₂O₃ reinforced MMCs samples were exhibiting improved hardness results, strength behavior, and stress-strain behavior when the samples are microwave sintered. AA 2900 with 6?wt% Al₂O₃ exhibited good ductility and formability properties. Good Microstructural bonding was observed in the MMCs, which is attributed to finer Al₂O₃ particulate used as reinforcement and the microwave sintering.     

A study on microstructure and mechanical properties of Al 6061-TiB2 in-situ composites

In situ reinforced aluminium based metal matrix composites (AMMCs) are emerging as one of the most promising alternatives for eliminating the inherent defects associated with ex situ reinforced AMMCs. Researchers in recent past have attempted various processing techniques for the development of in-situ composites, of which liquid metallurgy is the most widely adopted technique. Development of in-situ composites via liquid metallurgy route using master alloys is a relatively new processing technique. Very little information is available providing the usage value of these reinforcing materials.The present study is an attempt to explore the processing and characterization of in situ AMMCs using master alloys as reinforcement materials. Al 6061-TiB₂ in-situ composites were fabricated by liquid metallurgy route using Al 6061 as the matrix material and Al-10%Ti and Al-3%B as reactive reinforcements. Tests carried out on the fabricated composites include XRD, metallographic studies, EDAX analysis, microhardness, grain size analysis and tensile strength tests. The developed composites exhibited superior structural properties when compared with base alloy.     

Mechanical behavior of Al-based matrix composites reinforced with Mg58Cu28.5Gd11Ag2.5 metallic glasses

Al-based composites reinforced with Mg₅₈Cu_28.5Gd₁₁Ag_2.5 glassy particles have been synthesized by powder metallurgy. Powder consolidation was carried out by uniaxial hot pressing at temperatures within the super-cooled liquid region of the reinforcement to take advantage of the viscous flow of the glassy particles. The composites have improved yield and compressive strength compared to the unreinforced Al matrix without deteriorating the plasticity of the material. The relationship between mechanical properties and structure of the composites was investigated and described through the modified shear lag and mixture models.     

Processing of Ni-WC-8Co MMC casting through microwave melting

The melting and casting of nickel-based powder metal matrix composites (MMCs) through microwave energy is carried out in the present work. The nickel powder was mixed with 5% and 10% volume fraction of the WC-8Co reinforcement powder and processed in a microwave oven at 2.45?GHz and 900?W. The developed castings revealed complete melting of the nickel powder within 25 minutes of microwave exposure. The processing mechanism of MMC castings through microwave is explained and the developed castings were subjected to the microstructure and mechanical characterizations. The results of XRD analysis revealed the formation of some hard intermetallics such as NiSi and Cr₂₃C₆. The back-scattered scanning electron microscopy images of castings microstructures revealed the formation of nearly equiaxed grains of the matrix. It was observed that WC particles within the matrix were in agglomerated patterns, which were randomly dispersed. The presence of hard phases of WC reinforcement and formed intermetallic carbides enhanced the microhardness (788?±?52?HV) of the developed composites.     

Thermally stable microstructures and mechanical properties of B4C-Al composite with in-situ formed Mg(Al)B2

B₄C particulate-reinforced 6061Al composite was fabricated by powder metallurgy method. The as-rolled composite possesses high tensile strength which is comparable to that of the peak-aged 6061Al alloy. More importantly, the microstructures and mechanical properties are thermally stable during long-term holding at elevated temperature (400 °C). The microstructual contributions to the strength of the composite were discussed. Transmission electron microscopy (TEM) analysis indicates that the in-situ formed reinforcement Mg(Al)B₂, as products of the interfacial reactions between B₄C and the aluminum matrix, show not only good resistance to thermal coarsening but also strong pinning effect to the grain boundaries in the alloy matrix.     

Studies on mechanical and dry sliding wear of Al6061–SiC composites

Particulate reinforced Al-MMCs exhibits better mechanical properties and improved wear resistance over other conventional alloys. In the present paper, the experimental results of the mechanical and tribological properties of Al6061–SiC composites are presented. The composites of Al6061 containing 2–6 wt% SiC were prepared using liquid metallurgy route. The experimental results showed that the density of the composites increase with increased SiC content and agrees with the values obtained through the rule of mixtures. The hardness and ultimate tensile strength of Al6061–SiC composites were found to increase with increased SiC content in the matrix at the cost of reduced ductility. The wear properties of the composites containing SiC were superior to that of the matrix material.     

Processing of heat-treated silicon carbide-reinforced aluminum alloy composites

The present study investigates the processing of heat-treated silicon carbide (SiC) particle-reinforced 6061 aluminum alloy (AA) composites. As-received SiC powders were heat treated at 1300ºC, 1400ºC, and 1500ºC in nitrogen atmosphere for 2 h, and the 6061 AA–SiC composites were developed by spark plasma sintering at 560ºC and 60 MPa for 5 min in argon atmosphere. The amorphous silicon nitride is found to form in SiC particles as a result of heating at 1400ºC. The microstructure of the composites exhibited uniform distribution of SiC or SiC/Si₃N₄ particles in 6061 AA matrix. Further, the heat-treated SiC-reinforced 6061AA composites exhibited improved mechanical properties. A typical combination of UTS of 240 MPa and elongation of 21% is obtained for the 6061 AA composites prepared using SiC powders heated at 1400ºC.     

6061 Al reinforced with zirconium diboride particles processed by conventional powder metallurgy and mechanical alloying

The homogenous distribution of the reinforcement phase is an essential condition for a composite material to achieve its superior performance. Powder metallurgy (PM) can produce metal matrix composites in a wide range of matrix reinforcement compositions without the segregation phenomena typical of casting processes. Particularly, mechanical alloying can be used to mix the matrix and reinforcement particles, enhancing the homogeneity of the reinforcement distribution. This work investigates the production of aluminium 6061 reinforced with zirconium diboride by mechanical alloying followed by cold pressing and hot extrusion, and compares the results with the same composite produced by conventional PM and hot extrusion. The incorporation of the ZrB₂ particles produces only a small increase in the material hardness, but a small decrease in the UTS when conventional PM is employed. Mechanical alloying breaks the reinforcement particle clusters, eliminates most of the cracks present in the surface of the reinforcement particles, decreases its size and improves its distribution. This enhancement of the composite structure, in addition to the metallurgical aspects promoted by mechanical alloying in the matrix, brings approximately 100% improvements in the composite UTS and hardness, compared with the composites obtained by PM.     

Microstructure,tensile and fatigue properties of AA6061/20 vol.%Al2O3p friction stir welded joints

Metal matrix composites reinforced with Al₂O₃ particles combine the matrix properties with those of the ceramic reinforcement, leading to higher stiffness and superior thermal stability with respect to the corresponding unreinforced alloys. However, their wide application as structural materials needs proper development of a suitable joining processes. The present work describes the results obtained from microstructural (optical and scanning electron microscopy) and mechanical evaluation (hardness, tensile and low-cycle fatigue tests) of an aluminium alloy (AA6061) matrix composite reinforced with 20 vol.% fraction of Al₂O₃ particles (W6A20A), welded using the friction stir welding process. The mechanical response of the FSW composite was compared with that of the base material and the results were discussed in the light of microstructural modifications induced by the FSW process on the aluminium alloy matrix and on the ceramic reinforcement. The FSW reduced the size of both particles reinforcement and aluminium grains and also led to overaging of the matrix alloys due to the frictional heating during welding. The FSW specimens, tested without any post-weld heat treatment or surface modification showed lower tensile strength and higher elongation to failure respect to the base material. The low-cycle fatigue life of the FSW composite was always lower than that of the base material, mainly at the lower strain-amplitude value. The cyclic stress response curves of the FSW composite showed evidence of progressive hardening to failure, at all cyclic strain-amplitudes, while the base material showed a progressive softening.     

Microstructure and Mechanical Properties of B4C/6061Al Nanocomposites Fabricated by Advanced Powder Metallurgy

In this study, B₄C/6061Al nanocomposites reinforced with various volume fractions of nano‐sized B₄C particles (B₄C/6061Al NCs) are successfully fabricated by a powder metallurgy route consisting of spark plasma sintering (SPS) and hot extrusion and rolling (HER). The microstructure evolution, phase composition, and mechanical properties of B₄C/6061Al NCs are experimentally investigate. The results show that nearly fully dense (maximum ≈99.21%) as‐SPSed NCs can be fabricated, and this can be attributed to joule heating at the particle contacts and tip spark plasma at the gaps. Nanosized B₄C particles mainly distributed in the 6061Al particles boundaries and formed inhomogeneous network materials in as‐SPSed NCs, while B₄C particles distributed relatively homogeneously in the 6061Al matrix after HER. No new phases are found in the B₄C/6061Al NCs over three deformation stages. The pin effect of the nanosized B₄C can suppress dynamic recovery and improve the driving force for dynamic recrystallization. The mechanical properties are further improved after HER, and the maximum ultimate tensile strength and yield strength for as‐rolled NCs are 305 and 168 MPa. The strengthening mechanisms mainly included load transfer strengthening, dislocation strengthening, Orowan strengthening, and fine‐grain strengthening.

     

On the role of matrix grain size and particulate reinforcement on the hardness of powder metallurgy Al–Mg–Si/MoSi2 composites

Six Al–Mg–Si composites reinforced with 15 vol.% of MoSi₂ intermetallic particles, together with three unreinforced monolith Al–Mg–Si (AA6061) alloys have been processed by powder metallurgy to quantify the roles of alloy matrix grain size and reinforcement particle on their solutionized hardness and ageing response. In the range studied, hardness of solutionized composites follows a Hall–Petch mechanism. Moreover, it can be rationalised as the sum of the hardness of the alloy matrix with the same matrix grain size (d) and a term H_R, that accounts for 17–27% of total hardness, is roughly constant and independent of reinforcing size and distribution. Matrix grain size is responsible for 50–65% of hardness, whereas the contributions of solid solution and Orowan strengthenings account for 17–26%. Upon heat treatment at 170 °C, hardening ability decreases linearly with d^?1/2, fitting all data points to a single equation independently of whether they correspond to the composites or to the monolith alloys.     

Production and mechanical properties of metallic glass-reinforced Al-based metal matrix composites

Al-based metal matrix composites were synthesized through powder metallurgy methods by hot extrusion of elemental Al powder blended with different amounts of metallic glass reinforcements. The glass reinforcement was produced by controlled milling of melt-spun Al₈₅Y₈Ni₅Co₂ glassy ribbons. The composite powders were consolidated into highly dense bulk specimens at temperatures within the supercooled liquid region. The mechanical properties of pure Al are improved by the addition of the glass reinforcements. The maximum stress increases from 155 MPa for pure Al to 255 and 295 MPa for the samples with 30 and 50 vol.% of glassy phase, respectively. The composites display appreciable ductility with a strain at maximum stress ranging between 7% and 10%. The mechanical properties of the glass-reinforced composites can be modeled by using the iso-stress Reuss model, which allows the prediction of the mechanical properties of a composite from the volume-weighted averages of the components properties.     

团簇对粉末热挤压铝基复合材料力学性能的影响

为明确晶须团簇行为对材料力学性能的影响,采用粉末热挤压法制备了硼酸镁晶须增强铝基复合材料,对不同含量的晶须增强铝基复合材料进行了力学性能测试,并基于载荷传递模式提出相应的模型对材料强度进行预测.结果表明:随着硼酸镁晶须含量的增加,团簇加剧;当晶须体积分数大于10%时,材料力学性能降低;提出的模型考虑了团簇因素,成功预测了复合材料的实验强度.