Refinement of microstructure and improvement of mechanical properties of Al/Al2O3 cast composite by accumulative roll bonding process

Some important problems associated with cast metal matrix composites (MMCs) include non-uniformity of the reinforcement particles, high porosity content, and weak bonding between reinforcement and matrix, which collectively result in low mechanical properties. Accumulative roll bonding (ARB) process was used in this study as a very effective method for refinement of microstructure and improvement of mechanical properties of the cast Al/10 vol.% Al₂O₃ composite. The average particle size of the Al₂O₃ was 3 μm. The results revealed that the microstructure of the composite after eleven cycles of the ARB had an excellent distribution of alumina particles in the aluminum matrix without any noticeable porosity. The results also indicated that the tensile strength and elongation of the composites increased as the number of ARB cycles increased. After eleven ARB cycles tensile strength and elongation values reached 158.1 MPa and 7.8%, which were 2.54 and 2.36 times greater than those of the as-cast MMC, respectively.     

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.     

Microstructure development and texture evolution of ME20 sheets processed by accumulative roll bonding

Magnesium alloy ME20 sheets were produced by accumulative roll bonding (ARB) up to 8 cycles. The rolling microstructure was significantly refined during the first 2 cycles and remained homogeneous up to 6 cycles. After 8 cycles (ε ~ 6.4) the homogeneous equiaxed microstructure was replaced by a very fine shear band microstructure. With increasing ARB cycles, the texture intensity of basal poles decreased leading to a higher sheet tensile strength but with decreasing ductility.     

Evolution of reinforcement distribution in Al–B4C composites during accumulative roll bonding

The distribution of reinforcement particles in the matrix of a composite is one of the most important microstructural features affecting properties. In this study, nanostructured Al–B₄C composite sheets were processed by accumulative roll bonding (ARB), and the effect of the number of ARB cycles on the distribution of the B₄C particles in the Al matrix was evaluated. From optical microscopic studies accompanied by the radial distribution function analysis, it was realized that the microstructure uniformity is improved by increasing the number of ARB cycles. It was in good agreement with bulk hardness measurements in which the standard deviation of the hardness values was decreased by progression of the ARB process. In addition, the X-ray diffraction peak profile analysis revealed that the area weighted mean crystallite size of the Al matrix decreases to the nanometric scale (114 nm) after seven ARB cycles.     

Effects of rheocasting and heat treatment on microstructure and mechanical properties of A356 alloy

The effects of the rheocasting process and T5 heat treatment on microstructure and mechanical properties of A356 alloy were investigated. The results show that the temperature range for the solid-liquid state is roughly between 560 °C and 630 °C, and the solid fraction increases from 0% to 100% with decreasing temperature. The finer microstructure in rheocasting in comparison with the one in conventional casting was attributed to pressure breaking down the secondary dendrite arms, especially for specimens around 600-610 °C. It was proved that rheocasting specimens have improved mechanical properties over the conventional casting ones. Furthermore, the result shows that T5 heat treatment can strengthen A356 alloy, while the plasticity was reduced at the same time.     

Effect of a two-step solution heat treatment on the microstructure and mechanical properties of 332 aluminium silicon cast alloy

This paper investigated the effect of a two-step solution heat treatment on the mechanical properties and silicon-rich phase of 332 aluminium alloy. Traditional single-step T6 solution treatment (495 °C/6 h) increased the hardness value of the alloy by 5.96%, increased the tensile strength by 20.42% and reduced the elongation by 3.97%. Two-step solution treatment of the alloy (495 °C/2 h followed by 515 °C/4 h) increased the hardness value by 6.64%, increased the tensile strength by 16.01%, and reduced the elongation by 4.67% compared to the as-cast samples. Both solution treatments were followed by hot water quenching (75–90 °C) and artificial aging at 250 °C for 4 h. The difference in mechanical properties after heat treatment can be linked to the refinement and the spheroidisation of the silicon-rich phase in the alloy.     

Effects of pulsed magnetic field on microstructures and morphology of the primary phase in semisolid A356 Al slurry

A novel technique for introducing pulsed magnetic field (PMF) during isothermal holding period of A356 Al alloy has been employed to prepare semisolid slurry. The effects of vibration frequency of PMF on morphology of the primary phase in semisolid A356 Al alloy slurry were studied, and some characteristic parameters characterized the morphology and grain size of the primary phase were obtained. The results show that the primary α-Al particles become smaller and rounder with the increase of vibration frequency. The slurry with primary α-Al particle average diameter of about 79 μm and average shape factor of about 0.58 can be prepared under the action of a PMF at 10 Hz and 600 °C for 4 min. It is feasible to use PMF processing to prepare semisolid alloy slurry, because of its strong forced convection within the whole bulk melt.     

Anisotropic compressive behavior of Al–Mg alloy foams manufactured through accumulative roll-bonding process

Closed-cell Al–Mg alloy (A5052) foams are produced through accumulative roll-bonding (ARB) process. The preform plate containing uniform titanium hydride (TiH₂) particles is manufactured after six ARB cycles. Macroscopic porosity is as large as 47% in the condition of infrared heating at 913 K. Compressive tests are carried out in the condition of different loading axes. Yield stress in the normal direction (ND) was lower than other loading directions. Anisotropic deformation behavior is due to the anisotropic cell morphology which was induced during the ARB process.     

Effects of modification on microstructure and properties of ultrahigh carbon (1.9 wt.% C) steel

The effects of a modifier that contains Rare Earths (RE), low melting point alloy (Al-Bi-Sb) and Ca-Si alloy on an ultrahigh carbon steel containing 1.9 wt.% C were studied. Microstructure characterization was carried out with optical microscopy (OM) and scanning electronic microscopy (SEM) combined with energy-dispersive spectrometry (EDS). Upon modification, the continuous eutectic carbide network structure was broken up and changed to a partly isolated and finer blocky structure in the as-cast alloy. Differential scanning calorimetry (DSC) revealed that the eutectoid temperature increased and the eutectic temperature decreased for the modified alloy. Modification also improved the impact toughness of the tempered steel, with a significant increase from 6.5 to 12.6 J cm⁻², despite the hardness remaining around 66 HRC. Furthermore, in pure sliding under loads of 20, 60 and 100 N for 600 s against a zirconia ball, the modified alloy shows slightly higher friction coefficient at all loads than the non-modified one. In addition, the friction coefficient for the steel specimen decreased with load from 20 N to 100 N attributing to a reduction in metallic wear and the formation of a thicker oxide film on the surfaces.     

Production of high strength Al-Al2O3 composite by accumulative roll bonding

Recently accumulative roll bonding has been used as a novel method to produce particle reinforced metal matrix composites. In this study, aluminum matrix composite reinforced by submicron particulate alumina was successfully produced and the effects of number of ARB cycles and the amount of alumina content on the microstructure and mechanical properties of composites were investigated. According to the results of tensile tests, it is shown that the yield and tensile strengths of the composite are increased with the number of ARB cycles. Scanning electron microscopy (SEM) reveals that particles have a random and uniform distribution in the matrix by the ARB cycles and a strong mechanical bonding takes place at the interface of particle-matrix. It is also found that the tensile strength of the composite, as a function of alumina content, has a maximum value at 2 vol.%, which is 5.1 times higher than that of the annealed aluminum.     

Damping capacities and microstructures of magnesium matrix composites reinforced by graphite particles

Magnesium matrix composites reinforced by graphite particles were fabricated using stir casting with graphite particle size of 50 μm and graphite particle volume fractions of 5%, 10%, 15% and 20%, respectively. A dynamic mechanical analyzer was used to measure the damping capacities of as-cast composites. The experimental results reveal that the graphite particles play an important role on the damping capacities of as-cast composites. The strain amplitude independent damping of as-cast composites increases significantly as the graphite particle volume fraction increases from 0% to 15%, but decreases when the volume fraction exceeds 15%. The damping values of as-cast composites rise slowly with increasing temperature from room temperature to 125 °C, and have no obvious change with increasing temperature from 125 °C to 250 °C, but rise rapidly with increasing temperature from 250 °C to 400 °C.     

Microstructural stability and high-temperature mechanical properties of AZ91 and AZ91 + 2RE magnesium alloys

The effects of 2 wt.% rare earth element addition on the microstructure evolution, thermal stability and shear strength of AZ91 alloy were investigated in the as-cast and annealed conditions. The as-cast structure of AZ91 consists of α-Mg matrix and the β-Mg₁₇Al₁₂ intermetallic phase. Due to the low thermal stability of this phase, the strength of AZ91 significantly decreased as the temperature increased. The addition of rare earth elements refined the microstructure and improved both thermal stability and high-temperature mechanical properties of AZ91. This was documented by the retention of the initial fine microstructure and ultimate shear strength (USS) of the rare earth elements-containing material after long-term annealing at 420 °C. The improved stability and strength are attributed to the reduction in the volume fraction of β-Mg₁₇Al₁₂ and retention of the thermally stable Al₁₁RE₃ intermetallic particles which can hinder grain growth during the annealing process. This behavior is in contrast to that of the base material which developed a coarse grain structure with decreased strength caused by the dissolution of β-Mg₁₇Al₁₂ after exposure to high temperature.     

Homogeneous corrosion of high pressure torsion treated Mg-Zn-Ca alloy in simulated body fluid

Magnesium alloys have got extensive attention as biodegradable implant materials due to their biodegradability in the physiological environment and similar elastic modulus to natural bone. But their poor corrosion resistance is a dominant problem that limits their clinical application due to the inhomogeneous distribution of the second phase. Nevertheless, after high pressure torsion (HPT) treatment, the second phase became nano-sized particles and distributed uniformly in grain interiors instead of along grain boundaries. The immersion tests indicated that the HPT-treated sample exhibited homogeneous corrosion resulting from the uniform distribution of the second phase. The results of the potentiodynamic polarization experiments showed that, compared with the as-cast alloy, the corrosion current density of the HPT-treated alloy decreased from 5.3 × 10^− 4 A/cm² to 3.3 × 10^− 6 A/cm².     

Vacuum brazing of aluminium metal matrix composite (55 vol.% SiCp/A356) using aluminium-based filler alloy

Aluminium matrix composites with high volume fractions of SiC particles, as the reinforcements, are potentially suitable materials for electronic packaging. These composites, due to their poor weldability, however, have very limited applications. The microstructure and shear strengths of the bonds made in 55 vol.% SiC_p/A356 composite, using an aluminium based filler alloy containing Cu, Si, Mg and Ni, were investigated in this paper. The brazing temperature had a clear effect on the bond integrity, and the samples brazed at 560 °C demonstrated good bonding between the filler alloy and the SiC particles. The maximum shear strength achieved in this work was 102 MPa.     

Microstructure and mechanical properties of commercial purity titanium severely deformed by ARB process

Commercial purity titanium was deformed by accumulative roll-bonding (ARB) process up to 8 cycles (equivalent strain of 6.4) at ambient temperature. This is the first study on ultra-high straining of h.c.p. metals by the ARB process. The microstructure of the ARB-processed specimens showed two kinds of characteristic ultrafine microstructures. One was the lamellar boundary structure elongated along RD, which has been also reported in the ARB-processed cubic metals. The lamellar boundary spacing decreased with increasing ARB strain and reached about 80 nm after 5 ARB cycles. The other microstructure was the equiaxed grains having mean grain size of 80–100 nm. Such a fine and equiaxed grain structure has not yet been reported in the as-ARB-processed materials before. The fraction of the equiaxed grains increased as the ARB process proceeded, and 90% of the specimen was filled with the equiaxed grains after 8 ARB cycles. As the number of the ARB process increased, the tensile strength increased and the total elongation decreased gradually. After 6 ARB cycles, the specimen exhibited almost the same mechanical properties as that of commercial Ti-6Al-4V alloy.     

In-situ observation of the nucleation kinetics and the mechanism of grain refinement in Al-Si alloys (Part I)

The nucleation behavior of primary aluminium phase in a hypoeutectic Al-Si foundry alloy is studied using the 3DXRD microscope during the liquid-solid phase transformation for continuous cooling. Grain nucleation and grain growth for few different casting conditions of a commercial aluminium alloy (A356: Al-7Si-0.4 Mg-0.1Fe-0.1Ti wt.%) were investigated using three dimensional X-ray diffraction microscope (3DXRD) located at ID11 at European Synchrotron Radiation Facility (www.ESRF.eu). To conduct the study a monochromatic hard X-ray beam (energy of 70 keV) with a beam size of 200×200 µm² was used and using a special furnace the microstructure evolution during solidification of a commercial Al-Si foundry alloy (A356) was monitored in-situ. Results gathered from solid fraction information showed adding 0.1 wt.% Ti (as Al-3Ti-B) changes the primary aluminium nucleation temperature and aluminium grain size. Furthermore, it showed that at slower cooling rate (0.04-0.1 K/s) grain refiner can alter the primary aluminium nucleation temperature by 20 °C, whereas at higher rates (2 K/s) this figure was reduced down to 5 °C.     

Investigation of microstructures and mechanical properties of A356 aluminum alloy produced by expendable pattern shell casting process with vacuum and low pressure

In this study, microstructures and mechanical properties of A356 aluminum alloy obtained by expendable pattern shell casting process with vacuum and low pressure (EPSC-VL) were investigated. Meanwhile, microstructures and mechanical properties of A356 alloy among EPSC-VL, expendable pattern shell casting process under gravity casting (EPSC-G), lost foam casting with vacuum and low pressure (LFC-VL) and lost foam casting under gravity casting (LFC-G) were compared. The results showed that the microstructure of A356 alloy fabricated by EPSC-VL was finer and denser than the products from EPSC-G, LFC-G and LFC-VL processes, and its grain size was only 147.2 μm compared to LFC-G (327.1 μm). Moreover, its porosity defects were greatly reduced (0.16%) compared to LFC-G (1.97%). The tensile strength, elongation and hardness of castings produced by EPSC-VL were all considerably higher than the products from EPSC-G, LFC-G and LFC-VL. The A356 alloy made by EPSC-VL exhibited morphology of dimple fracture that was very deep and well-distributed. In addition, the castings produced by EPSC-VL process had better surface quality compared to LFC castings.     

Structure and properties of AA3003 alloy produced by accumulative roll bonding process

The investigation of the microstructure and mechanical properties has been conducted on AA3003 alloy produced by a novel intense plastic straining process named accumulativev roll-bonding (ARB). The results show that ultra-fine grained 3003 alloy having mean grain size of 700–800 nm was successfully produced by the 250°C-ARB. The average grain sizes of 250°C-ARB samples were reduced greatly from about 10.2 m initially to 700–800 nm. After 6 cycles of ARB, the whole volume of the material was filled with ultra-fine grains with high angle boundaries. The tensile strength of the ARB processed 3003 alloy (after 6 cycles) is considerably higher than that of the initial material, and about 1.5 times higher than that of commercially available fully-hardened (H18) 3003 alloy. Strengthening in ARB processed 3003 alloy may be attributed to strain hardening and grain refinement hardening.     

Influences of Preparation Conditions and Melt Treatment Procedureson Melt Treatment Performance of Al-5Ti-Band Al-10Sr Master Alloys

The influences of preparation conditions of Al-5Ti-B （as-cast and hot-rolled） and Al-10Sr （as-cast and hotextruded） and melt treatment procedures on the grain refinement and modification performance of A356 alloy are experimentally studied. For the two master alloys, the 50% reduction is sufficient to meet the demands of the efficient grain refinement and modification of A356 alloy. When Al-STi-B is introduced into the melt prior to degassing, the grain refinement efficiency of Al-5Ti-B will be greatly increased due to the better dispersity of TiB2 particles. Al-5Ti-B master alloy is less prone to affect the modification effect of Al-10Sr when they are used together.     

Preparation and properties of Mg-Cu-Mn-Zn-Y damping magnesium alloy

This paper presents the successful preparation of a high-damping, high-strength Mg-Cu-Mn-Zn-Y alloy by alloying and extrusion. The damping capacity of the as-cast Mg-3Cu-1Mn alloy (alloy 1) displayed evident variations with changes in Y and Zn content. The as-cast Mg-3Cu-1Mn-2Zn-1Y alloy (alloy 2) exhibited excellent damping capacity and unusual damping growth in the high-strain amplitude stage; the damping capacity of alloy 2 exceeded that of alloy 1 and even approached that of pure Mg when the strain amplitude exceeded 5 × 10⁻⁴. This observation is believed to be related to the formation of long and parallel dislocation configurations and the interactions between these dislocations and plastic second-phase particles. The study also showed a remarkable improvement in the mechanical properties of as-extruded Mg-Cu-Mn-Zn-Y alloys. We attribute this finding to dynamic recrystallization, dispersion strengthening, and work hardening during hot extrusion.