Hot rolling behaviour of stir-cast Al 6061 and Al 6082 alloys - SiC fine particulates reinforced composites

Rolling of wrought aluminium matrix composites with hard phase-reinforcements such as SiC, is interesting to produce sheets for engineering constructions due to their light weight combined with good strength and wear resistance. In this work, the hot rolling behaviour is studied for stir-cast composites with matrix of Al 6061 and Al 6082 alloys and fine SiCp particulates with size of 15 μm and 8 μm and volume fraction up to 30%. For composite casting, optimum casting procedures and materials pre-treatment has been applied for successful insertion of particles into the melt, better particles/matrix wetting and particles distribution, minimized SiC/Al reaction. From thermomechanical simulation, step rolling is defined to be suitable at a strain rate of 1 s⁻¹ rate for each step, using intermediate heat treatment at 450 °C for a period of 10 s to 1 h. Generally, the quality of rolled product was improved with improving casting quality. Successive hot rolling resulted in decreasing void and the agglomeration clusters and hence enhanced mechanical properties are achieved. The flow behaviour under rolling of Al-particulate metal matrix composites, PMMCs, is analysed and the product is characterised for its mechanical properties.     

Effect of boron on microstructure and mechanical properties of thermomechanically processed near alpha titanium alloy Ti-1100

The effect of 0.2 wt.% of boron on the mechanical properties of Ti-1100, a near α titanium alloy, was evaluated at room temperature and at 600 °C in the as cast and thermomechanically processed (α-β rolled) condition after subjecting it to different heat treatments. Boron addition in Ti-1100 significantly refined the microstructure in the as cast condition but the mechanical properties did not show any improvement. However, in the thermomechanically processed (α-β rolled) and standard heat treatment condition, the yield strength (YS) and ultimate tensile strength (UTS) of the boron containing alloy increased significantly without any drop in elongation-to-failure as compared to the base alloy at both room temperature and 600 °C. No discernible trend was seen in YS and UTS in boron containing alloy with change in solution treatment temperature either at room temperature or at 600 °C.     

Hot Deformation Behavior of 6061 and 7108 Al-SiCp Composites

Hot deformation behavior of Al 6061- and Al 7108-SiC particulated composites (Al-PMMCs), prepared by stir casting with SiC particulates (SiCp) size of 8 and 15 μm and volume fraction from 0 to 20% is studied by uniaxial compression test carried out at temperature range from room temperature to 500 °C. The flow stress, work hardening behavior, and Young’s modulus are determined. Dynamic recrystallization is also studied. Work hardening and Young’s modulus are directly correlated with composite constituents, whereas the flow stress is greatly influenced by the porosity and SiCp agglomeration. The role of the SiCp in increasing the flow stress decreases by increasing the deformation temperature. The dynamic recrystallization process is stimulated by refining the SiCp and increasing their fraction in soft Al matrix. On the other hand, the PMMCs with Al6061 matrix has more potential for strain hardening than that with Al 7108 matrix. The strain hardening rate is influenced by the matrix type more than the SiCp volume fraction and size.     

Effect of SiC volume fraction on the age-hardening behavior in SiC particulate-reinforced 6061 aluminum alloy composites

The age-hardening behavior in the SiC particulate-reinforced 6061 aluminum alloy composites (SiCp/6061 Al alloy composites) was investigated using hardness measurement, calorimetric technique and transmission electron microscopy. The aging time for the peak hardness in 10%SiCp/6061 Al and 20%SiCp/6061 Al alloy composites was shorter than that of the unreinforced 6061 Al alloy, and the age-hardening was most accelerated in the 20%SiCp/6061 Al alloy composite. It is induced from the high density of dislocations in SiCp/6061 Al alloy composites which act as heterogeneous nucleation sites of precipitates and as a high diffusivity path of alloy elements. The precipitation sequence and the aspect of precipitates in the 6061 Al alloy cannot be affected considerably by the presence of SiC particulate and the volume fraction of SiC particulate. The activation energies for the formation of the intermediate β' phase in the unreinforced 6061 Al alloy, 10%SiCp/6061 Al and 20%SiCp/6061 Al alloy composite were 142.6 kJ/mol, 127.3 kJ/mol, 106.1 kJ/mol, respectively.     

Effect of cold-rolling on tensile strength of SiCw/Al composite

SiCw/Al composite was fabricated through a squeeze cast route and cold rolled to about 30%, 50% and 70% re-duction in thickness, respectively. The length of whiskers in the composite before and after rolling was examined using SEM. Some of the rolled composites were recrystallization annealed to remove the work hardening of matrix alloy. The tensile strength of the rolled and annealed SiCw/Al composites was examined and then associated with the change of the whisker length and the work hardening of matrix alloy. It was found that the tensile strength is a function of the degree of cold rolling. For the cold rolled composites, with the increase in the degree of cold rolling, the tensile strength increases at first, and decreases when the degree of cold rolling exceeds 50%. For the annealed ones, however, the tensile strength de-creases monotonously with the increase in rolling degree. The different changes in tensile strength between the rolled and annealed composites could be attributed to the result of     

Carbon nanotube reinforced aluminum composite fabricated by semi-solid powder processing

Semi-solid powder processing (SPP) is a promising technology that combines the benefits of semi-solid forming and powder metallurgy. In this study, carbon nanotube (CNT) reinforced aluminum alloy 6061 (Al6061) composite was synthesized by SPP for the first time. Mechanical alloying was used to disperse the CNTs in the matrix phase. The effects of the processing temperature (600 °C, 620 °C and 640 °C) on the microstructure, hardness, fracture surface and composition of the Al6061-CNT composite were investigated. Overall, the Al6061-CNT composite showed full densification above 99% with 100 MPa of pre-compaction and 50 MPa of pressure during consolidation in the semi-solid regime. Microstructure and the fracture surface analyses showed that the CNTs were uniformly dispersed throughout the Al6061 matrix. Higher density composite was obtained at higher liquid content, although the highest composite hardness was achieved when processed at 620 °C. It was speculated that the formation of carbides at higher temperatures affected the interface bonding between the matrix and CNT. The study showed feasibility of manufacturing CNT reinforced metal composites by SPP.     

Effect of ageing on microstructure and mechanical properties of bulk, cryorolled, and room temperature rolled Al 7075 alloy

The effect of ageing on mechanical properties and microstructural characteristics of a precipitation hardenable Al 7075 alloy subjected to rolling at liquid nitrogen temperature and room temperature are has been investigated in the present work employing hardness measurements, tensile test, XRD, DSC, and TEM. The solution-treated bulk Al 7075 alloy was subjected to cryorolling and room temperature rolling to refine grain structures and subsequently ageing treatment to simultaneously improve the strength and ductility. The solution treatment combined with cryorolling up to a true rolling strain of 2.3 followed by low temperature ageing at 100 °C for 45 h has been found to be the optimum processing condition to obtain fine grained microstructure with improved tensile strength (642 MPa) and good tensile ductility (9.5%) in the Al 7075 alloy. The combined effect of suppression of dynamic recovery, partial grain refinement, partial recovery, solid solution strengthening, dislocation hardening, and precipitation hardening are responsible for the significant improvement strength-ductility combination in the cryorolled Al 7075 alloy subjected to peak ageing treatment. The cryorolled and room temperature rolled Al 7075 alloy, upon subjecting to peak ageing treatment, have shown higher strength and ductility in the former than the latter. It is due to presence of high density of nanosized precipitates in the peak aged cryorolled sample.     

The effect of partially carbonized fibers on the mechanical properties of carbon/carbon composites

The carbon/carbon (C/C) composite with satisfactory mechanical properties were obtained through introduction of partially carbonized fibers as a precursor. Applying this procedure the production cost of C/C composites may be significantly reduced. Stabilized PAN fibers were partially carbonized at temperatures ranging from 400 to 1000 °C and reinforced with phenolic resin, resole type. Cured composite were carbonized up to 1000 °C in an inert atmosphere. Monofilament tensile test strength, Young's modulus and tensile strength of partially carbonized fibers were determined. Mechanical properties of carbon/carbon composites (flexural strength and flexural modulus) determined by using three-point bending test. The effect of partially carbonized fibers on the mechanical properties of C/C composites was examined by scanning electron microscope (SEM) through analysis of the fracture surface. The C/C composite reinforced with partially carbonized fibers at 600 °C showed quite satisfactory flexural strength. This confirms assumptions that through co-carbonization of partially carbonized fibers and resin C/C composite with suitable mechanical properties could be obtained.     

Effect of hot extrusion on microstructures and mechanical properties of SiC nanoparticles reinforced magnesium matrix composite

Particulate reinforced magnesium matrix nanocomposite prepared with semisolid stirring assisted ultrasonic vibration was subjected to extrusion at 350 °C with an extrusion ratio of 12:1. Extrusion of the SiCp/AZ91 nanocomposite induced large scale dynamic recrystallization resulting in a fine matrix microstructure. There were two kinds of zones in the extruded nanocomposite: SiC nanoparticle bands parallel to the extrusion direction and refined-grain zones between the SiC nanoparticle bands. In the SiC nanoparticle bands, there were SiC nanoparticles along the boundaries of refined grains. The distribution of SiC nanoparticles was uniform although some agglomerates of SiC nanoparticles still existed in the SiC nanoparticle bands. The ultimate tensile strength, yield strength and elongation to fracture of the SiCp/AZ91 nanocomposite were simultaneously improved by extrusion. Results from the extruded SiCp/AZ91 nanocomposite tensile testing at different temperatures (75, 125, 175 and 225 °C) revealed an increase of the tensile strength and ductility values compared with the unreinforced and extruded AZ91 alloy.     

Microstructure and mechanical properties of Mg-Al-Mn-Ca alloy sheet produced by twin roll casting and sequential warm rolling

Microstructural evolution and mechanical properties of twin roll cast (TRC) Mg-3.3 wt.%Al-0.8 wt.%Mn-0.2 wt.%Ca (AM31 + 0.2Ca) alloy strip during warm rolling and subsequent annealing were investigated in this paper. The as-TRC alloy strip shows columnar dendrites in surface and equiaxed dendrites in center regions, as well as finely dispersed primary Al₈Mn₅ particles on interdendritic boundaries which result in the beneficial effect on microstructural refinement of strip casting. The warm rolled sheets show intensively deformed band or shear band structures, as well as finely and homogeneously dispersed Al-Mn particles. No evident dynamic recrystallization (DRX) takes place during warm rolling process, which is more likely attributed to the finely dispersed particle and high solid solution of Al and Mn atoms in α-Mg matrix. After annealing at 350 °C for 1 h, the warm rolled TRC sheets show fine equiaxed grains around 7.8 μm in average size. It has been shown that the present TRC alloy sheet has superior tensile strength and comparative elongation compared to commercial ingot cast (IC) one, suggesting the possibility of the development of wrought magnesium alloy sheets by twin roll strip casting processing. The microstructural evolution during warm rolling and subsequent annealing as well as the resulting tensile properties were analyzed and discussed.     

Understanding the effect of nitrogen in austenitic stainless steel on the intergranular stress corrosion crack growth rate in high temperature pure water

Understanding the effect of nitrogen content on the crack growth rate (CGR) due to intergranular stress corrosion cracking (IGSCC) in high temperature (288 °C) pure water, in non-sensitised and strain-hardened stainless steel (SS) type 304 LN was the focus of this study. Non-sensitised SS containing two different levels of nitrogen (0.08 and 0.16 wt.%) in the solution annealed condition was strain-hardened by cross-rolling at 200 °C (warm rolling). It has earlier been reported that SS with a higher nitrogen level in the warm rolled condition has a higher CGR in high temperature pure water. Tensile testing was carried out using both the SS in the warm rolled as well as in the solution annealed condition at 288 °C. Samples were prepared for transmission electron microscopy (TEM) from the warm rolled SS and from the tensile tested (at 288 °C) specimens. TEM studies indicated that twinning and shear band formation were the major modes of deformation due to rolling at 200 °C and these feature were observed to terminate at grain boundaries, leading to regions of higher strain and stresses at grain boundaries. Higher nitrogen SS has higher grain boundary strain and stresses making the grain boundary regions more susceptible to IGSCC, resulting in higher CGR values. At 288 °C dislocation entanglement and cross-slip were the predominant modes of deformation.     

Effect of vertical rolling at various temperatures on subsequent multi-pass severe rolling of AZ31B alloy sheet

AZ31B alloy was subjected to vertical rolling at various temperatures prior to multi-pass severe rolling processing including initial rolling including one 80% reduction pass and finish rolling at 300 °C and 350 °C, respectively. The depth and number of edge crack, microstructure evolution and tensile properties were examined. The results indicate that pre-vertical rolling at low temperature before severe rolling can significantly restrain edge crack, change relative frequency distribution of edge-crack depth, increase microstructure homogeneity and sharply change the intensity and distribution of basal texture of initial-rolled sheets. The level of edge crack increases with increased vertical rolling temperature above 100 °C mostly due to the combination of shear band density, microstructure homogeneity, grain size and texture of rolled sheets. Compared with conventional rolling, the effect of vertical rolling on final mechanical properties depends on the finish rolling temperature due to the combination of shear bands, twins and grain size. The variation trend of mechanical properties with increased vertical rolling temperature is also sensitive to finish rolling temperature. For as-rolled sheets, the severe rolling route with vertical rolling at 100 °C and finish rolling at 300 °C should be required.     

Influence of Ce surface treatments on corrosion behaviour of A3xx.x/SiCp composites in 3.5 wt.% NaCl

The influence of silicon carbide particles (SiCp) proportion and matrix composition on aluminium metal matrix composites A3xx.x/SiCp modified by cerium-based conversion or electrolysis coating was evaluated in 3.5 wt.% NaCl at 22 °C using potentiodynamic polarization and gravimetric measurements. Ce-treated surfaces presented better corrosion behaviour in chloride media than original composite surfaces without treatment. Both treatments preferentially covered the intermetallic compounds and SiCp. The electrolysis afforded a higher degree of protection than conversion treatment because the coating was more extensive. Coating microstructure and nature of corrosion products were analysed by scanning electron and atomic force microscopy (SEM, AFM) and low angle X-ray diffraction (XRD).     

Microstructure and mechanical properties of AM31 magnesium alloys processed by differential speed rolling

Ingot casted AM31 alloys were rolled at a warm temperature of 350 °C and subsequently rolled at 300 °C using equal speed rolling (ESR) and differential speed rolling (DSR) with speed ratios of top roll to bottom roll, 1.2 and 1.5, respectively. Microstructures, textures and mechanical properties of the as-rolled AM31 sheets were examined. Ductility was improved by DSR due to inclination of basal poles and weakened texture. The sheets produced by DSR with a speed ratio of 1.2 showed highest strength and ductility at room temperature, which can be attributed to homogeneous fine grain distribution and tilted basal texture.     

Enhanced Mechanical Properties of AA5083 Matrix Composite via Introducing Al0.5CoCrFeNi Particles and Cryorolling

High-entropy alloy particles (HEAPs) can markedly enhance the mechanical properties of metal matrix composites (MMCs). In this study, AA5083/Al_0.5CoCrFeNi HEAPs MMCs with different HEAPs contents (0, 1, and 3 wt%) were prepared via a stir-casting, and then these MMCs sheets were hot rolled (573 K) and cryorolled (77 K), respectively. The mechanical properties of the MMCs sheets were measured by tensile testing and microhardness test. Additionally, their microstructures were analyzed by scanning electron microscopy and transmission electron microscopy. Results revealed that the ultimate tensile strength (UTS) of the as-cast AA5083/Al_0.5CoCrFeNi HEAPs MMCs were improved from 203 to 257 MPa by adding 3 wt% HEAPs. And the mechanical properties of the MMCs sheets were improved after cryorolling. After cryorolling with 50% rolling reduction ratio, the MMCs with 1 wt% HEAPs had an UTS of 382 MPa, which was 1.9 times that of the MMCs before rolling. Finally, the strengthening mechanisms of HEAPs and cryorolling on the AA5083/HEAPs MMCs were discussed.     

Analysis of Microstructures and Mechanical Properties of Particle Reinforced AlSi7Mg2 Matrix Composite Materials

The present study examined the microstructures and mechanical properties (tensile and impact strength, hardness) of selected metal matrix composite materials. SiCp reinforced AlSi7Mg2 matrix composites were produced using gravity and squeeze casting methods and subsequently T6 heat treated. Some of the squeeze casted composites were shaped by extrusion. The extrusion generated an equiaxed matrix structure and SiCp caused a homogeneous distribution. The quasi absence of porosity in the squeeze casted composites led to improved mechanical properties. Whereas an increase in the SiCp ratio resulted in an increase of the tensile strength, it led to a decrease of the impact strength values. The enhancement of the mechanical properties following an applied heat treatment was better for materials shaped by extrusion.     

Effect of heat treatment on mechanical properties of particulate reinforced Al6061 composites

Mechanical tests were carried out to study the behavior of albite reinforced Al 6061 matrix composite produced through the liquid vortex method and heat treated to T6 treatment. The matrix material Al 6061 was also tested as a control material for comparison. The hardness, ultimate tensile strength (UTS), compression strength, and Young’s modulus were found to be higher than those of the control alloy. The reinforcement noticeably enhances the dislocation density in the matrix upon cooling from the solutionizing temperature. The dislocations that serve as heterogeneous nucleation sites for strengthening precipitates during subsequent aging treatments, after the precipitation kinetics of the matrix of the composite, were compared to the control alloy. A higher density of dislocations and a higher density of intermediate precipitates were observed. There was a marginal improvement of tensile strength, compression, and hardness with aging.     

Microstructures and mechanical properties of the in situ TiB-Ti metal-matrix composites synthesized by spark plasma sintering process

In situ synthesized TiB reinforced titanium matrix composites have been synthesized by spark plasma sintering (SPS) process at 950-1250 °C, using mixtures of 15 wt% TiB₂ and 85 wt% Ti powders. The effects of the sintering temperature on densification behavior and mechanical properties of the TiB-Ti composites were investigated. The results indicated that with rising sintering temperatures, relative densities of the composites increase obviously, while the in situ TiB whiskers grow rapidly. As a result, bending strength of the TiB-Ti composites increases slowly at the combined actions of the factors referred above. Fracture toughness of the composites is improved remarkably due to the large volume fraction of Ti matrix, the crack deflection, pull-out and the micro-fracture of the needle-shaped TiB grains. The results also suggested that TiB-Ti composite sintered at 1250 °C by SPS process exhibits the highest relative density of 99.6% along with bending strength of 1161 MPa and fracture toughness of 13.5 MPa m^1/2.     

Friction and wear behaviors of reciprocatingly extruded Al–SiC composite

This study focuses on the friction and wear behaviors of reciprocatingly extruded Al–SiC composites. To increase the strength of metal matrix composites and refine the grains of the matrix some deformation processes can be applied, such as reciprocating extrusion (RE). For this reason, RE was carried out on a 6061 Al matrix by a SiC (20 μm) reinforced composite one. The billets were extruded under a pressure of 17.5 MPa at 573 K with a 10:1 extrusion ratio. The reciprocating extrusions were carried out by using up to 15 passes.     

Abrasive wear behavior of Ni-P coated Si3N4 reinforced Al6061 composites

Ni-P coated Si₃N₄ reinforced Al6061 composites were fabricated by vortex method. Percentage of reinforcement was varied from 6 wt.% to 10 wt.% in steps of 2. Cast matrix alloy and developed composites were hot forged at a temperature of 500 °C using a 300T hydraulic hammer. Both as cast and hot forged matrix alloy and its composites were subjected to microstructure studies, grain size analysis, microhardness and abrasive wear tests. Microstructure studies reveal uniform distribution of silicon nitride particles with good bond between matrix and reinforcement in both as cast and hot forged condition. It is observed that, increased content of reinforcement in both as cast and hot forged composites do result in significant grain refinement. However, when compared with as cast matrix alloy and its composites hot forged alloy and its composites exhibits higher extent of grain refinement. Both as cast and hot forged composites exhibit improved microhardness and abrasive wear resistance when compared with the unreinforced alloys under identical test conditions. Abraded worn surfaces were examined using scanning electron microscopy (SEM) for possible wear mechanisms. Increased abrasive particle size and load has resulted in larger extent of grooving leading to increased abrasive wear loss for both the matrix alloy and developed composites.