Effect of a novel thixoforming process on the microstructure and fracture behavior of A356 aluminum alloy

In the present study, a novel thixoforming process for semi-solid deformation of A356 aluminum alloy is introduced using a continuous hot deformation process to the temperature being lower than the eutectic temperature of the alloy. A new hypothesis was introduced and the deformation mechanism of the alloy was investigated using the presented hypothesis. Microstructure and fracture surfaces of thixoformed samples were investigated using image analyzing technique and scanning electron microscopy. Obtained results indicated that this novel thixoforming process produces fine and compact silicon particles, dispersed uniformly in the microstructure of the alloy, compared to those produced by conventional thixoforming and gravity-cast processes with large and integrated morphology for silicon particles. The production stages of these silicon particles in this process were well documented by mentioned hypothesis. In order to investigate the effect of this novel process on mechanical properties of A356 alloy, tensile tests were conducted on produced samples. It was found that morphological changes of silicon particles as well as increasing the density ratio of samples in this process have a remarkable effect on enhancing the mechanical properties of produced alloy in comparison with other production routes. A new combination parameter, i.e. silicon density ratio (SDR) index was introduced. This parameter correlates the mechanical properties of samples to morphological properties of silicon particles and density ratio of them. Results of the study also indicated that samples with low SDR index have superior mechanical properties and consequently intergranular fracture mode.     

Effects of applied pressure on microstructure and mechanical properties of squeeze cast ductile iron

In this study, the effects of applied pressure during solidification on the microstructure and mechanical properties of cylindrical shaped ductile iron castings were investigated. Magnesium treated cast iron melts were solidified under atmospheric pressure as well as 25, 50 and 75 MPa external pressures. Microstructure features of the castings were characterized using image analysis, optical microscopy, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) techniques. Tensile properties, toughness and hardness of the castings were also measured. The results showed that average graphite nodule size, free graphite content and ferrite content of the castings decreased and pearlite and eutectic cementite contents increased as the applied pressure was raised from 0 to 75 Mpa. Graphite nodule count was first increased by raising the applied pressure up to 50 MPa and then decreased. The highest graphite nodule count was obtained at 50 MPa applied pressure. The microstructural changes were associated with the improved cooling rate and the expected changes in the corresponding phase diagram of the alloy under pressure. The ultimate tensile strength (UTS), yield point strength (0.2% offset) and fracture toughness of the castings were improved when the applied pressure was raised from 0 to 50 MPa. Further increase of the applied pressure resulted in slight decrease of these properties due to the formation of more cementite phase in structures as well as reduced graphite nodule count. Hardness of the castings continuously increased with increasing the applied pressure.     

Fabrication of NiCr alloy parts by selective laser melting: Columnar microstructure and anisotropic mechanical behavior

NiCr alloy, because of its wide applications in electrical elements and dental field was widely studied in the past. In this work, NiCr cubes and tensile specimens were fabricated by using a new processing technique-selective laser melting (SLM). Microstructural and mechanical behavior characterization of SLM-processed NiCr components was performed. An unusual columnar microstructural architecture composed of 〈1 0 0〉 texture (corresponding to (2 0 0) plane) oriented the building direction was observed. Moreover, it was found that the columnar grain growth across the melt pools occurred during the SLM process and the growth trend became stronger with the decrease of the laser scanning speed. Associated with the microstructural characteristic, an anisotropic mechanical behavior at different reference planes (i.e., at the horizontal and vertical surfaces) was demonstrated for the samples fabricated using different processing parameters. The results showed that with increasing the laser scanning speed, the microhardness at the horizontal surface decreased, while at the vertical surface it increased; an increase of the yield strength (YS) and the ultimate tensile strength (UTS) was observed.     

Effect of heat treatments on mechanical properties and fracture behavior of a thixocast A356 aluminum alloy

The effect of different heat treatments (T5 and T6) on mechanical properties, fracture behavior and damage evolution of A356 Thixocast aluminum alloy have been examined in detail in the present work. Tensile tests of the material have been performed in the as cast and as treated conditions in order to observe the different fracture behavior in consequence of the heat treatments. Optical and scanning electron microscopy techniques have been used to characterize the microstructure and fracture surfaces of the specimens. Finally, the precipitation processes of the material have been analyzed by hardness and electrical conductivity measurements and EDS analysis has been used to characterize the different phases in the as-thixo and as-treated conditions.     

Nano-sized aluminum oxide reinforced commercial casting A356 alloy matrix: Evaluation of hardness,wear resistance and compressive strength focusing on particle distribution in aluminum matrix

Achieving a uniform distribution of reinforcement within the matrix is a challenge which impacts directly on the properties and quality of the composite material. In the present study a fabrication and evaluation approach was used focusing on particle distribution in metal matrix. Al and Cu powders were separately milled with nano-Al₂O₃ particles and incorporated into A356 alloy via vortex method to produce cylindrical A356/nano-Al₂O₃ composites. The stirring was carried out in various durations. The variations of density, hardness, compressive strength, and wear resistance were measured throughout the cylindrical samples. The evaluation of mechanical properties and microstructural studies showed that an increase in stirring time led to a more uniform dispersion of particles in the matrix and also led to a decrease in mechanical properties due to an increase in porosity content of the composites compared with those of the samples stirred for shorter durations. Moreover, milling process affected particle distribution. Nanoparticles more uniformly dispersed in the Al₂O₃–Cu reinforced samples compared with that of the samples reinforced with Al₂O₃–Al or pure alumina powders.     

Thermal fatigue behavior of squeeze cast, discontinuous alumina-silicate fiber-reinforced aluminum alloy (A356) composite

Microstructural damage mechanisms owing to thermal cycling and isothermal exposure at elevated temperature are studied for a short alumina-silicate fiber-reinforced aluminum alloy (A356) composite produced by pressure casting. The tensile strength of the metal matrix composite is found to degrade considerably in each case. An X-ray double-crystal diffraction method is employed to study the mechanisms of recovery in the matrix. The fractal dimension of the X-ray “rocking curves” for individual grains in the composite reflects the substructure formation owing to the rearrangement of dislocations into subdomain walls. Recovery by polygonization is more pronounced in the case of thermal cycling than for equivalent isothermal exposure. The residual stresses in the matrix that provide the fiber clamping force undergo more relaxation in the case of isothermal exposure. The two competing damage mechanisms, thermally activated recovery by polygonization and relaxation of clamping stresses in the matrix, result in identical strength degradation (25%) for both thermal cycling and isothermal exposure.     

Compressive and wear behaviors of bulk nanostructured Al2024 alloy

Compressive and wear properties of bulk nanostructured Al2024 alloy prepared by mechanical milling and hot pressing methods were investigated. Al2024 powders were subjected to high-energy milling for 30 h to produce nanostructured alloy. As-milled powders were compacted at 500 °C under 250 MPa in a uniaxial die. Consolidated sample had an average hardness and relative density values of 207.6 HV and 98%, respectively. Uniaxial compression tests at strain rates in the range of 1.67 × 10⁻⁴–1.67 × 10⁻² s⁻¹ were performed using an Instron-type machine. The wear behavior of nanostructured sample was investigated using a pin-on-disk technique under an applied load of 20 N. The compression and wear experiments were also executed on samples of commercial coarse-grained Al2024-O (annealed) and Al2024-T6 (artificially-aged) alloys, for comparison. The structure of consolidated Al2024 was characterized by X-ray diffraction (XRD). The yield strength and compressive strength of nanostructured Al2024 reached a value of 698 MPa and 712 MPa at strain rate of 1.67 × 10⁻⁴ s⁻¹, respectively, which was considerably higher than those for coarse-grained Al2024-O and Al2024-T6 counterparts. Worn surfaces and the wear debris were analyzed by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and XRD. Nanostructured Al2024 revealed a low friction coefficient of 0.3 and a wear rate of 12 × 10⁻³ mg/m, which are significantly lower than those obtained for Al2024-O and Al2024-T6 alloys. This enhanced wear resistance was mainly caused by nanocrystalline structure with high hardness value. The dominating wear mechanism of nanostructured Al2024 appeared to be delamination mechanism.     

Deformation behavior of aluminum alloy matrix composites reinforced with few-layer graphene

Microstructure and mechanical properties of aluminum alloy 2024 (Al2024)/few-layer graphene (FLG) composites produced by ball milling and hot rolling have been investigated. The presence of dispersed FLGs with high specific surface area significantly increases the strength of the composites. The composite containing 0.7 vol.% FLGs exhibits tensile strength of 700 MPa, two times higher than that of monolithic Al2024, and around 4% elongation to failure. During plastic deformation, restricted dislocation activities and the accumulated dislocation at between FLGs may contribute to strengthening of Al2024/FLG composites.     

Significant improvement of semi-solid microstructure and mechanical properties of A356 alloy by ARB process

Accumulative roll bonding (ARB) process was used in this study as an effective method for manufacturing high-strength, finely-dispersed and highly-uniform A356 alloy. It was found that when the number of ARB cycles was increased, the uniformity of silicon particles in the aluminum matrix improved, the particles became finer and spheroider and therefore, the tensile strength (TS) and ductility of the samples improved. The microstructure of the manufactured A356 alloy after five ARB cycles indicated a totally modified structure such that it's TS and elongation values reached 269 MPa and 5.3% which were 2.6 and 2.5 times greater than those of the as-cast material, respectively. Also, the hardness value increased from 55.4 (for as-cast sample) to 100.2 HV (after the fifth cycle of ARB), and registered 81% increase.     

The effects of thermal treatment on the mechanical properties of wild Pear (Pyrus elaeagnifolia Pall.) wood and changes in physical properties

The aim of the study was to investigate the effects of thermal treatment on the mechanical and physical properties of wild pear wood. The results obtained for thermal treatment at 160 °C for 2 h showed that the modulus of elasticity was increased about 5%, while bending strength and compression strength decreased by 7.42% and 7.55%, respectively. The physical properties of wild pear wood were improved as 2.6%, 5.3%, 8.5% and 0.8% swelling in tangential, radial and longitudinal sections and 1.7%, 1.1% and 0.9% at 50, 65 and 85 Rh% and changes in ΔEab^* was 8.50%, respectively. It was determined that the changes ratio of these properties increased as the temperature and durations were increasing. Therefore, wild pear wood can be used as an alternative for tropical woods in decoration and veneer industry.     

Acoustic emission and fracture behavior of GFRP woven laminates at cryogenic temperatures

This paper describes an experimental and analytical study on fracture and damage behavior of GFRP woven laminates at cryogenic temperatures. CT (compact tension) tests were carried out at room temperature, liquid nitrogen temperature (77 K) and liquid helium temperature (4 K) to evaluate the critical values of the fracture mechanics parameters. During the CT tests, AE (acoustic emission) method was implemented. AE signals can identify the critical load at which gross failure occurs. A FEA (finite element analysis) was also applied to calculate the fracture mechanics parameters. The failure criteria (Hoffman criterion and maximum strain criterion) or the damage variable based on the continuum damage mechanics was incorporated into the model to interpret the experimental measurements and to study the damage distributions within the specimen. Several methods of calculating J-integral are discussed.     

Tensile and impact-toughness behaviour of cryorolled Al 7075 alloy

The effects of cryorolling and optimum heat treatment (short annealing + ageing) on tensile and impact-toughness behaviour of Al 7075 alloy have been investigated in the present work. The Al 7075 alloy was rolled for different thickness reductions (40% and 70%) at cryogenic (liquid nitrogen) temperature and its mechanical properties were studied by using tensile testing, hardness, and Charpy impact testing. The microstructural characterization of the alloy was carried out by using field emission scanning electron microscopy (FE-SEM). The cryorolled Al alloy after 70% thickness reduction exhibits ultrafine grain structure as observed from its FE-SEM micrographs. It is observed that the yield strength and impact toughness of the cryorolled material up to 70% thickness reduction have increased by 108% and 60% respectively compared to the starting material. The improved tensile strength and impact toughness of the cryorolled Al alloy is due to grain refinement, grain fragments with high angle boundaries, and ultrafine grain formation by multiple cryorolling passes. Scanning electron microscopy (SEM) analysis of the fracture surfaces of impact testing carried out on the samples in the temperature range of −200 to 100 °C exhibits ductile to brittle transition. cryorolled samples were subjected to short annealing for 5 min at, 170 °C, and 150 °C followed by ageing at 140 °C and 120 °C for both 40% and 70% reduced samples. The combined effect of short annealing and ageing, improved the strength and ductility of cryorolled samples, which is due to precipitation hardening and subgrain coarsening mechanism respectively. On the otherhand, impact strength of the cryorolled Al alloy has decreased due to high strain rate involved during impact loading.     

Improving sliding and abrasive wear behaviour of cast A356 and wrought AA7075 aluminium alloys by plasma electrolytic oxidation

An important limitation of aluminium alloys for mechanical applications is their poor tribological behaviour. In this study, surface treatment by plasma electrolytic oxidation (PEO) has been applied to two widely used aluminium alloys: A359 (hypoeutectic Al–Si–Mg) cast alloy and AA7075 (Al–Zn–Mg–Cu) wrought alloy, in order to improve their wear resistance, under sliding and abrasive wear conditions. The main aim of this work was the comparison of the properties and wear resistance of the oxide layers grown under the same PEO treatment conditions on two different aluminium alloys which might be coupled in engineered components. Significant differences in the phase composition, microstructure and mechanical properties measured by microindentation were observed in the oxide layers grown on the two substrates, and were ascribed to the effects of the different compositions and microstructures of the substrate alloys. Abrasion tests were carried out in a micro-scale abrasion (ball-cratering) test, with both alumina and silicon carbide abrasive particles. The results demonstrated the influence of the abrasive material on wear behaviour: whereas relatively aggressive SiC particles gave comparable results for both PEO treated and untreated samples, with the less aggressive Al₂O₃ abrasive the wear rates of the PEO treated samples, for both substrates, were significantly lower than those of the untreated substrates. In unlubricated sliding the PEO treatment significantly increase the wear resistance of both the aluminium alloys, at low applied load. In this condition the wear behaviour of the PEO treated alloys is strongly influenced by the stability of a protective Fe–O transfer layer, generated by wear damage of the steel counterpart. Under high applied loads however, the transfer layer is not stable and the hardness of the PEO layer, as well as the load bearing capacity of the substrate, become the main factors in influencing wear resistance.     

New methodology and apparatus for accelerated long-life testing of linear-drive cryocoolers

In this paper we are presenting a new approach which was developed for the accelerated tests of cryocoolers with a lifetime of 5–20 yr. In accordance with this approach the cryocooler is separated into critical units, each of which is tested under its own unique program. After completion of tests and assembly the cryocooler parameters are compared to the parameters of the system prior to the beginning of accelerated test.     

Influence of temperature on impact fracture behavior of an alloy steel

In this paper, the influence of temperature on impact toughness and fracture behavior of alloy steel (AISI Classification 8320) is presented and discussed. Impact toughness decreased with a decrease in test temperature. The extrinsic influence of temperature on impact toughness–fracture resistance relationships is rationalized in light of the conjoint and mutually interactive influences of intrinsic microstructural features, local stress states and macroscopic fracture behavior.     

Cold rolling and lubricated wear of 5083 aluminium alloy

The effect of cold deformation on the lubricated wear of 5083 aluminium alloy was investigated. SAE 10W was selected as liquid lubricant. The aluminium alloy was submitted to a cold rolling process, until the average thickness of the specimens was reduced by 7% and 15% respectively. From the experimental results obtained, the Stribeck curves for the as received and cold rolled aluminium alloy specimens were exacted. In all cases the three lubrication regimes were identified. In addition, the cold deformation process has led to a decrease of the friction coefficient of the tribosystem: 5083 aluminium alloy–410 stainless steel, for the same wear conditions (applied load, sliding speed and lubricant). The dominant wear mechanisms in each lubrication regime were studied via Scanning Electron Microscopy (SEM).     

Mechanical and fracture properties of an AZ91 Magnesium alloy reinforced by Si and SiC particles

A commercial AZ91 magnesium alloy (nominal composition Mg–9%Al; 1%Zn; 0.3%Mn, balance Mg in weight percent) reinforced with SiC particles and modified by the addition of Si has been used in this study. Formation of an “in situ” composite (Mg–Mg₂Si) results in strong bonding between Mg₂Si and the matrix interface. Samples were deformed in compression in the temperature interval from room temperature up to 300 °C. Stress relaxation tests were performed with the aim to reveal the thermally activated processes. Reinforcing effect of SiC and Mg₂Si particles decreases with increasing temperature. The estimated values of the activation volume as well as the activation enthalpy indicate that the main thermally activated process is connected with a rapid decrease of the internal stress. Fracture properties were studied in impact tests at various temperatures. A ductility enhancement was found at 200 °C and temperatures above 200 °C.