Role of reinforcement/matrix interfacial strength in fatigue crack propagation in particulate SiC reinforced aluminum alloy 8090

A study has been made of the influence of the reinforcement/matrix interfacial strength on fatigue crack propagation in a powder metallurgy aluminum alloy 8090-SiC particulate composite. The interfacial region has been altered by two separate routes, the first involving aging of the 8090 matrix, with the subsequent formation of precipitate free zones at the boundaries, and the second consisting of oxidizing the surface of the SiC particles before their incorporation into the composite. In the naturally aged condition, oxidation of the SiC leads to a reduction in fatigue crack growth resistance at higher values of stress intensity range ΔK. This is due to a proportion of the crack growth occurring through voids formed in association with many of the weak SiC interfaces which have retained a layer of thick surface oxide after processing. On overaging no difference in crack growth rate is discernible between the oxidized and unoxidized SiC composites. It is proposed that this is due to similar levels of interfacial weakening having occurred in both composites, indicating that this is an important factor in the reduction of the high ΔK crack growth resistance of the unoxidized SiC composite on aging.

Dry sliding wear behavior of Al2O3 fiber reinforced aluminum composites

Dry sliding wear behavior of die-cast ADC12 aluminum alloy composites reinforced with short alumina fibers were investigated by using a pin-on-disk wear tester. The Al2O3 fibers were 4 mu m in diameter and were present in volume fractions (T-f)ranging from 0.03 to 0.26, The length of the fiber varied from 40 to 200 mu m. Disks of aluminum-alumina composites were rubbed against a pin of nitrided stainless steel SUS440B with a load of 10 N at a sliding velocity of 0.1 m/s. The unreinforced ADC 12 aluminum alloy and their composites containing low volume fractions of alumina (V-f approximate to 0.05) showed a sliding-distance-dependent transition from severe to mild wear. However, composites containing high volume fractions of alumina ( V-f > 0.05) exhibited only mild wear for all sliding distances. The duration of occurrence of the severe wear regime and the wear rate both decrease with increasing volume fraction. In MMCs the wear rate in the mild wear regime decreases with increase in volume fraction: reaching a minimum value at V-f = 0.09 Beyond V-f = 0.09 the wear rate increasesmarginally. On the other hand, the wear rate of the counterface (steel pin) was found to increase moderately with increase in V-f. From the analysis of wear data and detailed examination of (a) worn surfaces, (b) their cross-sections and (c) wear debris, two modes of wear mechanisms have been identified to be operative, in these materials and these are: (i) adhesive wear in the case of unreinforced matrix material and in MMCs with low Vf and (ii) abrasive wear in the case of MMCs with high V-f. (C) 2000 Elsevier Science Ltd. All rights reserved.

Quasicrystalline and related crystalline phases in a rapidly solidified 2024-2Li aluminium alloy

The formation and decomposition of quasicrystalline and crystalline phases in as-rapidly solidified and annealed commercial AISI 2024 aluminum alloy containing 2 wt% Li have been investigated by detailed transmission electron microscopy, including a combination of bright field and dark field imaging, selected area diffraction pattern analysis and energy dispersive X-ray microanalysis. The microstructure of as-melt spun 2024-2Li consists of alpha-Al cells, containing small coherent delta' precipitates, and particles or a continuous network of the icosahedral phase at the cell boundaries. After annealing at 300-degrees-C, the intercellular particles of the icosahedral phase coarsen progressively and assume a more faceted shape; after annealing at 400-degrees-C, particles of the decagonal and crystalline O phases precipitate heterogeneously on preexisting particles of the icosahedral phase; and after annealling at 500-degrees-C, the icosahedral and decagonal phases dissolve completely, and small particles of the crystalline O phase remain together with newly precipitated plates of the T1 phase. The icosahedral phase in melt spun and melt spun/annealed 2024-2Li belongs to the Al6CuLi3 class of icosahedral phases, with a quasilattice constant of 0.51 nm, a stoichiometry of (Al, Si)6(Cu, Mn, Fe) (Li, Mg)3 and an average composition of Al-24.1 at.% Cu-6.4 at.% Mg-1.7 at.% Si-0.3 at.% Mn-0.5 at.% Fe as-melt spun and Al-21.9 at.% Cu-6.3 at.% Mg-1.0 at.% Si-0.5 at.% Fe as-heat-treated. The decagonal phase in melt spun/annealed 2024-2Li belongs to the Al4Mn class of decagonal phases, with a periodicity of 1.23 nm along the 10-fold symmetry axis, a stoichiometry of Al3(Cu, Mn, Fe) and an average composition of Al-10.3 at.% Cu-13.8 at.% Mn-2.3 at.% Fe. The crystalline O phase in melt spun/annealed 2024-2Li has an orthorhombic structure with lattice parameters of a = 2.24 nm, b = 2.35 nm and c = 1.23 nm, a stoichiometry of Al3(Cu, Mn, Fe) and an average composition of Al-11.0 at.% Cu-14.8 at.% Mn-3.9 at.% Fe. Detailed analysis of selected area diffraction patterns shows a close similarity between the icosahedral, decagonal and crystalline O phases in melt spun and melt spun/annealed 2024-2Li. In particular, the decagonal phase and crystalline O phases have a similar composition, and exhibit an orientation relationship which can be expressed as: [GRAPHICS] suggesting that the orthorhombic O phase is an approximant structure for the decagonal phase.

Debenzylation by Ni-A1 alloy

Nickel-aluminum alloy in aqueous sodium hydroxide-ethanol medium brings about facile hydro-enolrsis of benzyl ether function in substrates carrying a vicinal methoxy group. Simplicity of the reaction conditions and cheapness of the reagent are some of the advanrages of the procedure. Additionally, carbonyl functions when present are untouched and alkaline medium of the reaction makes it compatible with the presence of acid-sensitite groups.

Grain Floatation During Equiaxed Solidification of an Al-Cu Alloy in a Side-Cooled Cavity: Part I-Experimental Studies

Experimental studies were performed to investigate the role and influence of grain movement on macrosegregation and microstructure evolution during equiaxed solidification. Casting experiments were performed with a grain-refined Al-Cu alloy in a rectangular sand mold. For the aluminum alloy studied, the equiaxed grains are lighter than the bulk melt and thus float up. Experiments were designed to investigate floatation phenomena of equiaxed grains in the presence of thermosolutal convection. Cooling curves were recorded at key locations in both the casting and the chill. Quantitative image analysis and spatial chemical analysis were performed on the solidified casting to observe the chemical and microstructural inhomogeneity created by the melt convection and solid floatation. Several notable features that can be attributed to grain movement were observed in temperature histories, macrosegregation patterns, and microstructures. In our experiments, the floatation of grains influences the thermal conditions and the overall flow direction in the casting cavity. In some cases, the induced flow resulting from the grain movement caused a flow reversal. This in turn influences the solidification direction, microstructure evolution, and the overall macrosegregation behavior.

Texture and formability studies on AA7020 Al alloy sheets

This paper deals with the influence of crystallographic texture on room temperature mechanical behavior of the sheets of the aluminum alloy AA7020 processed to different thicknesses. Three different thicknesses of the alloy sheet, namely 1, 1.85, and 3.6 mm, corresponding to different textures were investigated. Tensile tests were carried out at 0°, 45° and 90° with respect to sheet rolling direction and the resulting in-plane anisotropy in 0.2 proof stress, work hardening and plastic strain ratio (r-value) were determined. Texture derived r-values are also calculated and discussed vis-à -vis the experimentally obtained r-values. Finally the formability of the optimal alloy was studied using forming limit diagrams. Effect of natural aging, with a simulated heat treatment of 70 °C for 2 h on FLD was studied and compared with the as solutionized samples. It was observed that, the strain levels in the bi-axial region of the FLD were not much affected by the heat treatment. © 2012 Elsevier B.V. All rights reserved.

Effect of Annealing Temperature in Al 1145 Alloy Sheets on Formability, Void Coalescence, and Texture Analysis

This paper deals with a combined forming and fracture limit diagram and void coalescence analysis for the aluminum alloy Al 1145 alloy sheets of 1.8 mm thickness, annealed at four different temperatures, namely 200, 250, 300, and 350 A degrees C. At different annealing temperatures these sheets were examined for their effects on microstructure, tensile properties, formability, void coalescence, and texture. Scanning electron microscope (SEM) images taken from the fractured surfaces were examined. The tensile properties and formability of sheet metals were correlated with fractography features and void analysis. The variation of formability parameters, normal anisotropy of sheet metals, and void coalescence parameters were compared with texture analysis.

Semisolid Processing of A380 Al Alloy Using Cooling Slope

This study is aimed toward obtaining near spherical microstructural features of Rheocast A380 aluminum alloy. Cooling slope (CS) technique has been used to generate semisolid slurry from the superheated alloy melt. Spherodization of primary grains is the heart of semisolid processing which improves mechanical properties significantly in the parts cast from semisolid state compared to the conventional casting processes. Keeping in view of the desired microstructural morphology, i.e., rosette or spherical shape of primary alpha-Al phase, successive slurry samples have been collected during melt flow and oil quenched to investigate the microstructure evolution mechanism. Conventionally cast A380 Al alloy sample shows dendritic grains surrounded by large eutectic phase whereas finer, near spherical grains have been observed within the cooling slope processed slurry and also in the solidified castings which confirms the effectiveness of semisolid processing of the alloy following cooling slope technique. Grain refiner addition into the alloy melt is found to have favorable effect which leads to the generation of finer primary grains within the slurry with higher degree of sphericity.

Microstructure and Evolution of Mechanically-Induced Ultrafine Grain in Surface Layer of Al-Alloy Subjected to USSP

Experiments were conducted to investigate the ultrafine-grained (UFG) microstructures in the surface layer of an aluminum alloy 7075 heavily worked by ultrasonic shot peening. Conventional and high-resolution electron microscopy was performed at various depths of the deformed layer. Results showed that UFG structures were introdued into the surface layer of 62 μm thick. With increasing strain, the various microstructural features, e.g., the dislocation emission source, elongated microbands, dislocation cells, dislocation cell blocks, equiaxed submicro-, and nano-crystal grains etc., were successively produced. The grain subdivision into the subgrains was found to be the main mechanism responsible for grain refinement. The simultaneous evolution of high boundary misorientations was ascribed to the subgrain boundary rotation for accommodating further strains. Formed microstructures were highly nonequilibratory.  2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Growth Mechanism And Corrosion Behavior Of Ceramic Coatings On Aluminum Produced By Autocontrol Ac Pulse Peo

Ceramic coatings are produced on aluminum alloy by autocontrol AC pulse Plasma Electrolytic Oxidation (PEO) with stabilized average current. Transient signal gathering system is used to study the current, voltage, and the transient wave during the PEO process. SEM, OM, XRD and EDS are used to study the coatings evolution of morphologies, composition and structure. TEM is used to study the micro profile of the outer looser layer and inner compact layer. Polarization test is used to study the corrosion property of PEO coatings in NaCl solution. According to the test results, AC pulse PEO process can be divided into four stages with different aspects of discharge phenomena, voltage and current. The growth mechanism of AC PEO coating is characterized as anodic reaction and discharge sintering effect. PEO coating can increase the corrosion resistance of aluminum alloy by one order or two; however, too long process time is not necessarily needed to increase the corrosion resistance. In condition of this paper, PEO coating at 60 min is the most protective coating for aluminum alloy substrate. (C) 2008 Elsevier B.V. All fights reserved.

Welding of Al alloy plates using a novel high-average-power pulsed CO2 laser

A novel high-average-power pulsed CO2 laser with a unique electrode structure is presented. The operation of a 5-kW transverse-flow CO2 laser with the preionized pulse-train switched technique results in pulsation of the laser power, and the average laser power is about 5 kW. The characteristic of this technique is switching the preionized pulses into pulse trains so as to use the small preionized power (hundreds of watts) to control the large main-discharge power (tens of kilowatts). By this means, the cost and the complexity of the power supply are greatly reduced. The welding of LF2, LF21, LD2, and LY12 aluminum alloy plates has been successfully achieved using this laser. (c) 2005 Society of Photo-Optical Instrumentation Engineers.

Welding of Al alloy plates using a novel high-average-power pulsed CO2 laser

A novel high-average-power pulsed CO2 laser with a unique electrode structure is presented. The operation of a 5-kW transverse-flow CO2 laser with the preionized pulse-train switched technique results in pulsation of the laser power, and the average laser power is about 5 kW. The characteristic of this technique is switching the preionized pulses into pulse trains so as to use the small preionized power (hundreds of watts) to control the large main-discharge power (tens of kilowatts). By this means, the cost and the complexity of the power supply are greatly reduced. The welding of LF2, LF21, LD2, and LY12 aluminum alloy plates has been successfully achieved using this laser. (c) 2005 Society of Photo-Optical Instrumentation Engineers.

Corrosion resistance of hot-dipped zinc and zinc alloy coated sheet steels under offshore atmospheric environment

The application of hot-dipped zinc and zinc-aluminum alloy coatings were introduced. Exposure tests of the steels with these coatings were conducted in the offshore atmosphere in Qingdao and Xiamen for 12 years separately. Effects of the coating thickness, alloy composition and atmospheric environment on the corrosion performance were studied. Results of the onsite exposure tests were compared with the results of a previous indoor salt spray accelerated corrosion tests. The study supports that zinc-aluminum alloy coatings are useful in providing better corrosion resistance and can be further developed for future applications.

Electrochemical Behaviour of the AA2024 Aluminium Alloy Modified with Self-Assembled Monolayers/Polyaniline Double Films

The electrochemical characteristics of the AA2024 aluminium alloy modified with octadecyltrimethoxysilane (ODTMS) + polyaniline (PANi) and propiltrimethoxysilane (PTMS) + (PANi) were studied in the present work. The results show that the different protective coatings shift the values of corrosion and pit potentials to more positive values making the system nobler and indicate that the double film ODTMS + PANi present the best protection against corrosion characteristics, that is probably due to the two contributions: anodic protection associated with the barrier effect.