Materials Science Forum Vol. 877

Abstract: The corrosion behavior of Al-B₄C metal matrix composites in H₃BO₃ solutions with different Cl^- contents was investigated using potentiodynamic polarization and zero resistance ammetry techniques. Results show that the corrosion of Al-B₄C composites in H₃BO₃ solution increases with increasing B₄C volume fraction in the composites. The main corrosion characteristic of Al-B₄C composites in H₃BO₃ solution is the galvanic corrosion between Al matrix and B₄C particles. In the galvanic couple, B₄C particle acts as cathode and Al matrix acts as anode. The cathodic reaction is hydrogen revolution reaction, which controls the corrosion mechanism of Al-B₄C composites. Pitting is not observed on the composite surface in the H₃BO₃ solution with zero Cl^-. However, with addition of Cl^- in H₃BO₃ solution, pitting occurs and the corrosion resistance remarkably decreases with increasing Cl^- content. The corrosion resistance of Al-B₄C composites in H₃BO₃ solutions is compared with that in the standard 3.5% NaCl solution.

Abstract: The initiation of localised corrosion in anodised AA2099-T8 alloy is simulated by using Cellular Automata (CA) method. For CA simulation, the system consisting of corrosive electrolyte, porous anodic alumina film and alloy matrix is described as a 500×300 two-dimensional square lattice. Eight types of cells are defined to construct the cellular space, with the Von Neumann neighbourhood (four cells neighbourhood) being selected. The cellular space is constructed such that a void generated by oxidation of the high-copper-containing Al-Fe-Mn-Cu-Li particle exists in the porous anodic alumina film consisting of a porous layer and a barrier layer. It is suggested that localised corrosion is preferentially initiated in the void defect region in the porous anodic film, which is related to increased corrosion product diffusion rate in this region compared with diffusion in other regions of the porous layer.

Abstract: Effect of heat treatments on the stress corrosion behavior of 7050 Al alloys in 3.5% NaCl solution has been investigated using slow strain rate tensile (SSRT) test. During the slow strain rate tensile process, electrochemical impedance spectroscopy (EIS) in real time was carried out to characterize the electrochemical behavior for different tempers 7050 Al alloys. The investigation shows that both the stress corrosion resistance of 7050 Al alloys is controlled by heat treatments due to the different precipitates state. The improvement of stress corrosion resistance is contributed to the tiny precipitates in matrix which are beneficial to corrosion potential and maintain passivation, and precipitates discontinuous distribution at grain boundary which obstruct intergranular crack connection. Moreover, base on the results, we find out retrogression and re-aging (RRA, i.e., T6 + 200 °C/ retrogression + water quench + T6) increases both tensile strength and stress corrosion resistance. The optimized of retrogression time is 30 minutes.

Abstract: P-doped γ-Al₂O₃ was found to be a potent substrate to nucleate primary silicon whilst good modification of the eutectic matrix is retained during solidification of hypereutectic Al-Si alloys. On using P-doped γ-Al₂O₃ could be a perfect and clean source of P without additional impurities. The optical micrographs show that the morphologies of primary silicon crystals in solidification of Al-18Si alloy are changed from irregular coarse morphologies to fine regular particles. The average particle size of primary Si decreased from 52 μm to 25 μm and 22 μm in adding P-doped α-Al₂O₃ and P-doped γ-Al₂O₃ respectively. It was clear that P-doped γ-Al₂O₃ led to good refinement of primary Si and the modification effect on eutectic Si was retained in solidification of commercial purity Al-18Si alloy. Adding P-doped γ-Al₂O₃ give a good primary Si refinement to Al-18Si alloy if compared with the addition of P and using finer P doped γ-Al₂O₃ powder give narrower particle size range similar to that of adding P.

Abstract: Aluminum (Al) alloys are increasingly used in the transportation industry to reduce the weight of vehicles due to their high strength-to-weight ratio. These applications unavoidably involve similar and dissimilar joining of an automotive grade 5754 Al alloy to manufacture multi-material vehicle body structures and parts. Ultrasonic spot welding (USW), an emerging and promising solid-state joining technology, can be suitably applied to join Al alloys. In this study, 5754 Al alloy was welded in similar (Al5754-Al5754) and dissimilar (Al5754-ZEK100 Mg alloy, Al5754-HSLA steel) configurations at varying levels of welding energy. It was observed that USW had a strong effect on the interface microstructure, with fine grains present at the weld interface via dynamic recrystallization in the similar welding, while an interface diffusion layer formed in the dissimilar welding. The tensile lap shear strength increased with increasing welding energy, reached its optimum value, and then decreased with further increasing welding energy. The strength of dissimilar Al5754-ZEK100 and Al5754-HSLA steel joints was about 55% and 88% of that of the similar Al-Al joints, respectively. The dissimilar Al5754-HSLA steel joints exhibited the longest fatigue life due to the reduced stress concentration and additional strengthening arising from the brazing effect of the squeezed-out Al-Zn eutectic structure at the nugget edge.

Abstract: An orthogonal hardness study of 4 factors (the step one aging temperature (T1) and time (t1), and step two aging temperature (T2) and time (t2)) with 3 levels was designed to optimize an accelerated aging process for a newly developed conductive aluminum alloy (Al-0.4wt.%Si-0.3wt.% Mg-0.3wt.%Ce). Statistical analysis of the experimental results indicates that the factor significance ranked from high to low is t2, T2, T1, and t1. The optimal aging process is 140°C×5h+220°C×5h. Experimental validation confirms that all the strength, hardness, elongation and conductivity with the optimized aging process exceed the predictions. Compared with the commonly used T7 process (190°C×20h), the optimized aging process leads to a more superior precipitation effect and much less time. TEM observation indicates the accelerated aging process significantly expedites the precipitating transformation even with much shorter aging time.

Abstract: The hot deformation behavior of an Al-0.92Mg-0.78Si-0.20Cu-0.60Zn alloy was studied by isothermal compression in the temperature range from 350 to 500 oC with strain rates of 0.01-10s^-1 on Gleeble-1500 thermo-mechanical simulator. The microstructural evolution during hot deformation was investigated by electron back-scatter diffraction (EBSD). The results show that the strain rate and deformation temperature have significant influence on flow behavior, and the flow stress increases with increasing strain rate and decreasing deformation temperature. Processing map at the strain of 0.7 is obtained and exhibits three peak efficiency domains (380-420 oC at 0.01s^-1, 480-500 oC at 0.01s^-1 and 450-500 oC at 0.1-0.32s^-1). According to the processing map and microstructure observation, the optimized processing condition of hot deformation for the alloy is at 450-500 oC and strain rate of 0.1-0.32s^-1. The homogenized ingot is hot rolled at 480 oC with a strain rate of 0.1s^-1 based on optimized deformation parameters. The fraction of high-angle grain boundary is 35.2%, which is in accord with microstructure after hot deformed at 500 oC with a strain rate of 0.1s^-1.

Abstract: In this paper, precipitation of ScAl₃ phase during solutionizing and ageing and its impact to mechanical properties of hypoeutectic Al-Sc alloy and effect of Sc addition amount were investigated by TEM and SEM microstructure observation and mechanical property testing. During solutionizing at 640^oC for 24h, for Al-0.2wt.%Sc and Al-0.4wt.%Sc alloy, ScAl₃ particles formed in the course of solidification become smaller, indicating, as a whole, Sc is partially dissolved into Al solution. Simultaneously, precipitation of two types of ScAl₃ particles are observed by TEM. One has a size of 200~300nm, and the other is much small, 5~20nm in size. Though re-dissolution of Sc solute into Al solution and precipitation of large and fine ScAl₃ particles occur in the solutionizing course, but the strength and hardness are decreased. The key reason for it is thought to be the softening effect of high level of vacancies in matrix lattice from the high solutionizing temperature. After further aging at 300^oC for 3h, a great number of fine ScAl₃ particles are precipitated in the Al matrix, which leads to a considerable precipitation hardening effect, thus the strength and hardness are increased obviously. Increasing the Sc content in the alloy results in a considerable rise in as-cast and as-aged strength and hardness, due to the solution strengthening and precipitation hardening respectively.

Abstract: A series of Al-6.3Zn-2.3Mg-2.3Cu-0.15Zr alloys with different reduce of Zn, Mg, Cu and Zr were prepared by ingot-metallurgy processing. Effects of homogenization on the microstructure and properties of Al-Zn-Mg-Cu-Zr aluminium alloy were respectively studied by means of metallographic microscopy, electrical conductivity test, differential thermal analysis and X-ray diffraction phase analysis. The results indicated that the overheating temperature of these alloys is between 473°C and 477°C, and there was little difference to the overheating temperature of 7050 alloy. During homogenization process, using three kinds of developed heat treatment of homogenization of 7050 alloy, with the rising of homogenization temperature and the complication of the homogenization heat treatment, the electrical conductivity decreased and hardness gradually increased. The three-step homogenization has a better effect than single homogenization, as it can completely eliminate the endothermic peak of non-equilibrium phases. Many MgZn₂ phases are present in the ingot with three-step homogenization and slow cooling.

Abstract: A series of Al-6.3Zn-2.3Mg-2.3Cu-0.15Zr alloys with different reduce of Zn, Mg, Cu and Zr were prepared by ingot-metallurgy processing. The metallurgical structure and mechanical properties were investigated by optical microscope, scanning electron microscopy and other equipment. The results indicated that the ingot’s microstructures of the four alloys contain the phrases of η (MgZn₂) and θ (Al₂Cu), which mostly distribute at the grain boundaries in a shape of continuous network. After extrusion processing, the grain of laser welding aluminium alloy was elongated along the extrusion direction, therefore forming fibrous structures, and meanwhile the second phase particles with different degrees of fragmentation were arranged along the extrusion direction since the microstructure of extruded bars was inherited by the as-cast structure. Zr could significantly inhibit recrystallization of alloy; the recrystallization of the alloy with lower Zr was more obvious. As the content of Zr reduced, the tensile strength of alloy decreased, but the electrical conductivity and hardness increased. When the content of Cu was lower, the hardness were decreased.