Papers by Keyword: Hypereutectic

Abstract: The Present study was undertaken to investigate the porosity defects induced by semi-solid isothermal heat treatment in cast hypereutectic Al-18% Si alloy for different holding time. All specimens were heated up to 585 °C then hold for 10, 20, 30 and 40 min. in an electrically heated resistance furnace with heating rate of 10 °C.min^-1. After the semi-solid heat treatment, the samples were taken out immediately for water quenching. Bulk density, total porosity and pore sizes at center and edge of each treated sample were investigated. It was found that the porosity in final treated samples increases by increasing the holding time. The pore size increases and the density decreases steeply with increasing the semi-solid isothermal holding time. Above 20 min., holding time the pore size at the edge shows higher value compared with that measured at the center of sample. It might be recommended that shorter holding time during semi-solid isothermal heat treatment should be selected in order to obtain minimum porosity percentage and higher density Al-18% Si alloy castings.

Abstract: By introducing ZrO₂ (4Y) powder into the thermit, the solidified Al₂O₃-ZrO₂ (4Y) ceramic composites with eutectic and hypereutectic microstructures were prepared via combustion synthesis in high gravity field, and the microstructures and mechanical properties of the solidified ceramic composites were discussed. XRD, SEM and EDS showed that the Al₂O₃-33%ZrO₂ (4Y) as the eutectic were composed of random-orientated rod-shaped colonies consisting of a triangular dispersion of orderly submicron-nanometer t-ZrO₂ fibers, surrounded by inter-colony regions consisting of spherically-shaped micrometer t-ZrO₂ grains, whereas Al₂O₃-45%ZrO₂ (4Y) as the hypereutectic were comprised of spherically-shaped micron-meter t-ZrO₂ grains, surround by irregularly-shaped α-Al₂O₃ grains and a few colonies. Compared to the directionally solidified Al₂O₃-ZrO₂ (Y₂O₃), the increase in hardness and flexural strength of the eutectic obtained in current experiment was due to high densification, small-size defect and high fracture toughness induced by residual stress toughening and transformation toughening mechanisms; meanwhile, in despite of the moderate decrease in hardness, high flexural strength of the hypereutectic was considered to be a result of small-size defect and high fracture toughness induced by transformation toughening and microcrack toughening mechanisms.

Abstract: In this study the effect of cerium addition on the microstructure of hypereutectic Al-15Si alloy was investigated. Experiments have been conducted on Al15Si alloys with Ce content in the alloy varied from 0 to 2.51 wt.%. The alloys were produced by casting in a permanent mould. Optical microscopy, X-Ray Diffraction (XRD) and Vickers microhardness were used in this investigation. The results showed that the addition of 0 to 2.5 wt.% of cerium led to the formation of precipitation of rod-like Al₃Ce phase in the Al-matrix. The microhardness of the alloys increases with the increase in cerium content as a result of the Al₃Ce precipitation.

Abstract: Abstract. Aluminum high silicon alloys have concerned many researchers due to their high wear resistance, lightness, high corrosion resistance and low thermal expansion. Casting of high silicon Al-Si alloys (i.e. Si content greater than 17 wt.%) will generate large degrees of segregation and coarse microstructures due to the low rates of solidification. The problems associated with ingot casting of hypereutectic Al-Si alloys (i.e. segregation, coarse microstructures and porosity) may be overcome by rapid solidification processing such as spray, weld, and chill methods (e.g. melt spinning). The alloys under consideration here contain Al, Si, Zr, Cu, Mg, Fe and Ni. These alloys were produced by rapid solidification i.e. melt spinning. The aim of this paper is to characterise the hardness of material produced by rapid solidification at various stages of production. Several alloy variants were examined and relate the hardness to the microstructure. Piston A390 made by casting was examined for comparison.

Abstract: This research aims at studying the structural refinement of 15%Cr-2%Mo hypo- and hypereutectic white irons through dynamic solidification and Nb-alloying (1-2%Nb). Solidification behavior was studied using DSC. Microstructure investigation and analysis were performed using optical, image analyzer as well as SEM, EDX and XRD analysis. Refinement mechanisms proposed involved both enhancing nucleation and altering the growth process of the primary and eutectic phases either by fragmentation of the primary growing phase in the case of dynamic solidification or by consuming the available carbon from the liquid and hence the amount and size of M7C3 will decrease in the case of addition of Nb-alloying. Nb has higher affinity to carbon during solidification, higher formation temperature than the main microstructure constituents and forms fine carbide so Nb achieves refinement of the microstructure.

Abstract: A tough hypereutectic high chromium white iron has been developed by considering the alloy as a double in-situ fibrous composite and applying the principles of composite theory. The eutectic in the ternary Fe-Cr-C system solidifies as M7C3 carbide rods of irregular hexagonal cross section in a matrix of austenite. The carbide rods are intermittently joined together with no misorientation at the joint. The colonies (eutectic grains) of a 27%Cr alloy solidify with a flat solid – liquid interface. In hypereutectic high chromium white irons the primary carbide solidifies as single crystal rods with no branching. Depending on the thermal conditions in the mould the primary carbides are nucleated only on the mould surface, or repeatedly at the solid – liquid interface. In 27% Cr, 4.5%C hypereutectic alloys, the as-solidified microstructure is one of long parallel aligned primary carbides in a eutectic matrix of carbide rods in austenite, or short randomly orientated primary carbides in a eutectic matrix. In either case the microstructure can be described as a double in-situ fibrous composite. The application of composite theory has resulted in hypereutectic high chromium white iron castings with improved fracture toughness similar to that of heat-treated alloy steel but with superior wear resistance.

Abstract: The influences of low frequency electromagnetic field on microstructures and macro-segregation in direct chill casting process were investigated in the experiments，Al-19.2%Si alloys were semi-continuously cast into ingots with 100 mm in diameter. Microstructures and macrostructures of samples taken from different part of the cast with different electromagnetic field conditions were characterized by optical microscopy. The results showed that low frequency electromagnetic field refines the microstructures and reduces macro-segregation. Unlike casting without electromagnetic field, the primary Si grains were homogeneous with fine dimensions and the morphology of the primary silicon exhibited small blocky structures or near-spherical structures. Further more, decreasing frequency is beneficial to the improvement. In the range conditions in the experiments, the optimum frequency is found to be 15Hz. During casting, the temperature of the liquid metal in the sump was monitored. The results shown, under LFEC the width of the liquid-solid region became narrowed and the temperature field in the sump also became homogeneous.

Abstract: In the present study, a Fe-Cr-C hypereutectic alloy was prepared from industry-grade materials and subjected to modification and fluctuation, through which new types of particle reinforced composites, hypereutectic in-situ composite, was generated. The structures of the composite modified or not with the range of fluctuation addition from 0% to 2.8wt.%, were investigated. The primary carbides were refined with the addition of modifying agents and fluctuations. Increasing the amount of fluctuation resulted in finer primary carbides. At 1380oC, with the addition of modifying agents and 2.8wt.% fluctuation addition, the structure was well modified.

Abstract: This paper investigates the effect of bubble on the primary Si size in Al-18wt.%Si alloy. The primary Si size observed was varied with bubble size and bubbling time in bubble process. The effect of the holding temperature of the melt in bubbling process was also investigated. In water model, as the injecting bubble size decreases, the residual bubble size in the water decreases and the residual bubble conservation time in the water increases. Also in the experiment of Al melt, the primary Si size decreases, as the injecting bubble size decreases and as the bubble processing time increases. Pore was observed at the center of primary Si. This pore was observed at many Si phases. So, this experiment suggests that the bubble can be used as nucleation sites of primary Si.