Materials Science Forum Vols. 675-677

Abstract: Nowadays, researches concerned magnetic abrasive finishing (MAF) are becoming increasingly popular. Polishing head is one of the most important factors in the whole polishing system in which magnetic abrasive act directly on the workpiece. In this paper, two kinds of polishing heads applied in the polishing of free form surface are proposed. They are hard polishing head and soft polishing head, respectively. And some important factors are contrasted such as selfsharpening capacity, morphology distribution, and removal uniformity. Moreover, the finishing force is the key factor which is different between the two polishing heads having a great influence on polishing effect. Through comparison and analysis, the soft polishing head is better to finish free form surface. And the experimental results reflect its superiority in polishing free form surface.

Abstract: Al2O3/TiAl in situ composites were successfully prepared by vacuum hot-pressing method with elemental powder mixtures of Ti, Al, TiO2 and Fe2O3. The effect of Fe2O3 addition on the microstructures and mechanical properties of Al2O3/TiAl composites has been investigated. The results show that the Rockwell hardness, flexural strength and fracture toughness of the composites increase with increasing Fe2O3 content. When the Fe2O3 content is 0.84 wt %, the flexural strength and the fracture toughness reach the maximum value of 624 MPa and 6.63 MPa·m1/2, respectively. The improvement of mechanical properties is firstly associated with a more homogeneous and finer microstructure developed by addition of Fe2O3; secondly, it is related to the increase of the ratio of α2-Ti3Al/γ- TiAl matrix phases.

Abstract: Quasi-static and dynamic axial crushing tests were carried out to investigate the energy absorption capacity of aluminum foam with several densities. First, aluminum foam samples with three densities were crushed quasi-statically to study their compression characteristics, the stress strain relationship curves and energy absorption capacities were achieved from the test results; then dynamic crushing tests were carried out to check the crushing behaviors of the corresponding samples; finally, the energy absorption capacities of tested samples were compared. According to the tests results, all the aluminum foam samples showed very good isotropy and very high mass specific energy absorption capacities. For the crushing tests, the dynamic crushing behaviors were very similar to the static ones, which meant aluminum foam was insensitive to the crash velocity tested.

Abstract: The poor formability of aluminum alloy at room temperature limits its use in some products with complex shapes, hence the warm forming process is intended to overcome this problem by using an elevated temperature. Now, the warm formability of AA5086 has not previously been well investigated in the literatures, especially at a rapid forming rate. In this paper, a numerical method has been developed to investigate the warm formability of an AA5086 sheet. Firstly, the dynamic tensile test was carried out under different forming temperatures and forming rates to identify an appropriate constitutive law for the sheet. The inverse analysis was performed to identify the parameter values in the constitutive law. Then based on the commercial finite element program ABAQUS, the Marciniak test was simulated to evaluate the sheet formability of by implementing a user-defined material subroutine UHARD. The effects of forming temperature and forming rate on sheet formability were investigated and it is shown that the formability of AA5086 seems to be insensitive to the forming temperature and forming rate.

Abstract: In this paper, a method based on the displacement-traction map is developed to calculate the bandgaps of transverse waves propagating in a 2D phononic crystal composed of nanosized circular holes in a square lattice. The Young-Laplace equation is employed to take into account of the surface effects of the nanosized holes. Detailed calculations are performed for the system with nanosized circular holes in an aluminum host with or without the surface effect. The result shows that all bands descend with the first bandgap becoming wider due to the existence of the surface effects.

Abstract: The purpose of this investigation is to detect damage from stress distribution in the surface of near pre-crack tip by using X-ray diffraction technique during biaxial tension test. An measurements apparatus to measure stress distribution along pre-crack direction was fabrication by use of a biaxial tensile test device and a stress analyzer based on single exposure technique with one position sensitive proportional counter. Stress distribution with different tensile applied stress ratios were measured during biaxial tension test. As results, the shape of actual stress was keeping increase with increasing tensile applied stress. At maximum applied stress, the residual stress increases with the increasing distance from the crack tip; after reaching a maximum it gradually diminish.

Abstract: In order to deeply investigate the contact stress state of two balls, a new fractal contact model of two ball’s surfaces is set up on the basic of traditional contact theory and new fractal contact model theory. It is proved to be true through the predicting result and the contact example respectively by using the traditional Hertz theory and the fractal theory. This model can generally reflect the infections of contact stress by the micro-factors and macro-factors of two balls, so it can ensure veracity and certainness of stress analysis. The establishment of the model supplies a basis and theory foundation for further practical application (such as contact stress calculation of rolling bearing and sliding bearing) and experimental research.

Abstract: In this work, the effects of centrifugal radius and mould rotation speed on the tensile strength, yield strength, specific elongation, and microhardness on Al-Cu alloy castings are investigated. The results show that, with increasing the centrifugal radius or mould rotation speed, the mechanical properties increase gradually. With increasing the centrifugal radius, the variation amplitude of mechanical properties of Al-Cu alloys at mould rotation speed 600rpm is greater than that at 300rpm. This is due to the finer microstructure and the strengthened grain boundary and then resulting in the increase of the resistance to dislocation slipping.

Abstract: This paper investigated the self-thickening effects on fabricating close cell aluminum foam directly using semi-solid melt by mechanical stirring without adding other particles. Using the high viscosity character of semi-solid melt as well as mechanical stirring technique, blowing agents (TiH2) could be homogeneously distributed without other thickening additions such as CaO or TiC which were commonly used to thicken aluminum melt in metal foam industry. It was observed that the semi-solid melt could reach a proper condition for the blowing agents to separate and foam after self-thickening process and the solid fraction played a key role in self-thicken effects compared to the stirring time and speed. The self-thickening effects could meet the purity requirement of melt as well as to fabricate aluminum foam with small pores and good structure using precursors. Aluminum foam was a multi-functional material with features of ultra-light, high-toughness, impact resistance, high specific strength, high specific stiffness, heat isolation and fire retardant [1]. The viscosity of melt was extremely important in the process of fabricating aluminum foam. Jin.I and Kenny.LD did some research on the effects of adding solid particles to thicken the melt and their study showed that the best thickening effect could be obtained when the particle size of 5-20 mm and the proportion of 10~20% of liquid aluminum [2]. Miyoshi.T also did some research on the stirring time in the thickening process and showed that adding Ca particles of 1.5~3%(wt) could effectively controlled the viscosity [2]. Research by J.Banhart showed shat 1-5 %(wt) Al2O3 and SiC adding particles could also thicken the semi-solid melt very effectively[3] [4]. Berry.CB and Hall.CG’s research showed that blowing air, oxygen or other gases could also reach thickening effect [2]. There were several methods to thicken the melt but the same ultimate goal was to obtain a suitable melt environment for distribution and foaming of blowing agents. This paper mainly investigated the effects of the self-thickening semi-solid melt on the distribution behavior of blowing agents. Taking the advantage of the high viscosity of semi-solid melt as well as mechanical stirring technique, without adding other thickening particles, blowing agents (TiH2) could be homogeneously distributed in the semi-solid melt. The parameters such as solid fraction, stirring time and speed were investigated in this paper.

Abstract: The supercell based plane wave expansion method is used to study the effects of random disorders on the band structures of a two-dimensional (2D) solid-fluid phononic crystal. Phononic systems with steel scatterers embedded in a water matrix are calculated in detail. The radius disorder and location disorder are concerned. The influences of the disorder degree on the first band gap are investigated. The localization phenomenon is discussed by computing the displacement fields in the supercell.