Application of hot workability studies to extrusion processing. Part III: Physical and mechanical metallurgy of Al-Mg-Si and Al-Zn-Mg alloys5

The conditions for homogenisation and cooling to improve the extrusion velocity and surface quality are derived from analysis of the influence of solidification segregation and constitutive particles in lowering the minimum melting temperature. Over-aged particles of Mg₂Si do not reduce surface quality as do large equilibrium ones and do not raise the strength as much as Mg and Si in solution. A Si level above the Mg₂Si ratio augments the hot strength less than excess Mg and strengthens the extrudate while diminishing toughness. During homogenisation cooling and pre-heating, the precipitates in both Al-Mg Si and Al-Zn-Mg alloys must be limited in size to permit solution as they pass through the extrusion zone. Both alloys have low quench sensitivity suiting them for rapid cooling upon exit from the press so that the extrudate is ready for age hardening. The influence of additional elements such as Cu, Mn, Zr and Cr on both extrusion process and product are reviewed.     

增材制造TiAl基合金的研究进展

TiAl基合金具有优异的高温性能,是一种极具竞争力的新型轻质高温结构材料,在汽车、军工、航空航天等领域具有广阔的发展潜力和应用前景.然而,TiAl基合金室温脆性较大,成形困难,是阻碍其发展与应用的主要瓶颈之一.增材制造基于"离散+堆积"的成形思想,以激光、电子束、电弧等作为高能热源,通过熔化丝材或者粉末,逐层堆积实现零...     

New Records of Freshwater Sponges (Porifera) for Southern Lake Michigan

Four species of freshwater sponge were identified in southern Lake Michigan: Spongilla lacustris (Linneaus), Eunapius fragilis (Leidy), Ephydatia fluviatilis (Linneaus), and Ephydatia muelleri (Lieberkuhn). Samples were collected from artificial substrates in Calumet (IL), Hammond (IN), and Michigan City (IN) harbors and represent the first reports of sponges in these waters. Numerical densities for all species combined were as high as 698 sponge colonies/m² on the iron hull of the permanently moored 110-m-long ship Milwaukee Clipper in Hammond Harbor. These densities were lowest at 1-m depth and greater at 2- and 3-m depths. A positive correlation (r² = 0.74) was found between the density of sponge colonies and the angle of the hull as it changed from near vertical at the water line to overhanging at greater depth (toward the keel). Ephydatia muelleri was the most common species based on frequency of occurrence and percent surface area covered. Observations of epizoic growth of sponges on live zebra mussels (Dreissena polymorpha Pallas) suggest that an ecological interaction exists between these two groups of organisms on these artificial substrates.     

Simulation of temperature-compensated voltage limit curves for aerospace NiCd batteries using a first principles' model

Temperature-compensated voltage limits (V/T limits) are routinely used in Low Earth Orbit (LEO) satellites to permit fast charging of the NiCd battery with minimum overcharge and without the problems of thermal runaway during overcharge. The voltage limits are experimentally determined from extensive testing of cells for a proper design of the charge control system to achieve the desired charge/discharge ratio in orbit. Here, we demonstrate the use of first-principles', mathematical models to construct the V/T curves theoretically. The predicted charge/discharge ratios under various orbit conditions such as different states of charge, in-rush currents and temperatures are compared with the experimental data.     

Composite of polyaniline containing iron oxides with nanometer size

A composite of polyaniline (PANI) containing iron oxides with nanometer size was synthesized by a chemical method. The structure, electrical and magnetic properties of the resulting PANI-Fe_xO_y composites depend on the pH value of the reaction medium and temperature. For the basic preparation conditions (e.g. pH 14), the resulting PANI-Fe_xO_y, composite can be attracted by a magnet at room temperature, and its magnetization with the applied magnetic field exhibits a hysteresis loop with H_c = 0. It has been demonstrated that nanocrystalline ferromagnetic particles of Fe₃O₄ with 14.0 nm size correspond to super-paramagnetic properties observed from PANI-Fe_xy composites. For the acidic conditions (e.g. pH1), on the other hand, the electrical properties of the resulting PANI-Fe_xO_y composite show a semiconducting behavior. By comparison with PANI-Cl⁻¹, the lower conductivity (about 10⁻⁴ S/cm) for PANI-Fe_xO_y composite may be due to the presence of iron oxides in the composite, although its doping degree is the same as that of PANI-Cl⁻.     

An advanced equipment for single-fibre pull-out test designed to monitor the fracture process

A newly designed single-fibre pull-out machine with optimized stiffness in all components was developed, in order to obtain stable crack propagation during the fibre pull-out. On the basis of a mathematical model, which simulates the debonding process during a single-fibre pull-out experiment, calculations of the stress distribution, the force-displacement traces and the fracture propagation were made. Some of these results are compared with pull-out measurements using glass fibres embedded in thermoplastic matrices. The agreement between simulation and test results is good, and shows that stable crack propagation is achievable. Because friction has an important influence on pull-out forces, the interpretation of single-fibre test results has to be revised.