Advanced Materials Research Vols. 750-752

Abstract: Zn-Al filler metal wettability tests were performed on pure aluminum surface. The test results show the spreading area of Zn-Al filler metal gets large on pure aluminum surface gradually with the increase of Al content in Zn-Al filler metal under the match of CsF-AlF₃ flux. When Al content in the Zn-Al filler metal is 15 wt.%, the spreading area of 85Zn-15Al filler metal is biggest on pure aluminum surface. Accordingly, the dendritic eutectoid structure size is smaller near the interface between the Zn-Al filler metal and the pure aluminum base metal by metallographic analysis. When Al content in the Zn-Al filler metal exceeds 15wt.%, the spreading area of filler metal begins to become small. At the same time, the eutectoid structure presents circle shape near the interface between the Zn-Al filler metal and the pure aluminum base metal.

Abstract: The direct diffusion bonding of TiAl-based intermetallics and the diffusion bonding of TiAl-based intermetallics with hydrogenated Ti6Al4V alloys interlayer containing 0.5wt% hydrogen were carried out. The effects of hydrogen on diffusion bonding were investigated by SEM, EPMA, XRD, TEM and TG/DSC. The good joint was formed at 850°C for 15 min under a pressure of 15MPa at the diffusion bonding of TiAl-based intermetallics with hydrogenated Ti6Al4V alloy interlayer containing 0.5wt% hydrogen, and the room temperature shear strength was up to 290MPa. Relatived to direct diffusion bonding of TiAl-based intermetallic, the bonding parameters decreased prodigiously. According to the experimental observations, the Ti6Al4V alloy hydrogenated 0.5 wt% consisted of close-packed hexagonal structure α′ martensite phase, face-centered cubic structure of δ-phase, α and βH structure. The lamellar δ hydride and βH phase disappeared after bonding, and the lamellar (α+β) structure were formed. Because of the dehydrogenation during bonding, metastable hydride containing low hydrogen appeared. The remaining hydrogen in Ti6Al4V alloy at high temperatures enhanced the capacity of the plastic deformation and the diffusion ability of the alloy elements, which helped to improve the spread of the atom.

Abstract: Mg-Y-Nd-Gd-Zn-Zr magnesium alloys with six different compositions are prepared through sand casting. A research shall be conducted on the casting structure and mechanical properties at room temperature of this magnesium alloys and the effect of main alloying rare earth elements of Y, Gd and Nd is researched through analysis on the test data of mechanical properties. It is seen from the research: In as-as-cast alloys, Y and Gd have a larger solid solubility, which are tend to uniform distribution in the alloy but there is much more Nd to precipitate. The second phase is mainly covered by Mg₁₂Nd and a few of Mg₂₄Y₅ and Mg₅Gd which are in form of block, globularity and strip. After T6 treatment, a large number of intermetallic compounds of Mg₂₄Y₅ with Y are precipitated from the alloy and the second phase has changed significantly in size and morphology which is uniformly and widely distributed in the grain and grain boundary with a good strengthening effect. When a few of Gd is added, the strength and plasticity of matrix alloy are improved. With the adding of Gd, the precipitated phase of microstructure is also increased which is tend to coarsening and segregation so as to have an effect on its mechanical properties.

Abstract: The effect of Al-10Sr master alloy modifier with different content on the microstructure and properties of as-cast Al-12.6Si-0.35 Mg alloy was investigated. The results show that, eutectic silicon varies from original needle and flake to fibrous shape; The shape, size, and quantity of α -Al all changed; When Sr content is 0.023%, the modification effect is most ideal, α -Al shows the highest quantity and smallest size. Meanwhile, the tensile strength and elongation of the alloy reach to the optimum values of 248.2 MPa and 7.2% respectively. These two values are 1.34 and 2.77 times higher than the 185 MPa and 2.6% before modification respectively.

Abstract: Faceted growth of primary Al₃Ni phase in the hypereutectic Al-Ni alloy in a high magnetic field was investigated. It was found that faceted growth of primary Al₃Ni phase was enhanced in the presence of the magnetic field. However, the fibrous to granular transition of Al-Al₃Ni eutectics occurred. The undercooling of primary and eutectic phases during solidification was measured using differential thermal analysis. It was showed that the undercooling of primary phase was hardly changed but that of eutectics markedly increased in the magnetic field. According to Cahn theory of crystal growth, the critical driving force was used to satisfactorily explain the morphology transition in the magnetic field.

Abstract: Titanium alloy is a premium metal with properties of low density, high hardness and resistance against high temperature and corrosion, the application of which has gained serious explorations in manufacture and military. Laser pulses of different power, duration and beam spot size are employed under room temperature to investigate the thermal stress variation of a titanium alloy plate with the aim of proposing ideal coefficients in laser processing of titanium alloy. The experiment was carried out as numeric simulations of thermal response and thermal dynamo coupling of the titanium alloy plate by using the COMSOL software.

Abstract: Ti6Al4V (TC4) slices were boronized with rare earth oxide (RE) addition in the agent at 950°C (below β phase transus of TC4). The morphology, phases, properties and structures of TC4 matrix and boride layers were studied. The results show that the boride layer on the surface of TC4 consists of TiB₂ and TiB dual compounds. The boride layer is compact, uniform and less porous compared with that obtained at 1050°C (above β phase transus of TC4), and the hardness profile and brittleness of the layers are improved to a certain extent. Lower temperature boronizing can effectively prevent the growth of coarse β phase grains. Duplex microstructure is obtained in TC4 matrix, and the work piece distortion is reduced.

Abstract: In order to settle the environmental pollution and improve the excellent rate of products, ZL109 piston alloy modified by Al9Fe3P master alloy was prepared in factory. The microstructures and mechanical properties of ZL109 piston alloy were invested respectively. The technological parameters and modifying effect of using Al9Fe3P alloy were researched too. Experiments show that high quality of ZL109 pistons can be produced by adding 0.4~0. 5wt% Al9Fe3P master alloy at 760-770°C.The maxim and average grain size of primary Si decrease largely, less than 66μm and 40μm respectively, and the mechanical properties were improved accordingly. It was proved that Al-Fe-P master alloys have advantages such as no pollution, no reaction slag, shorter modifying time and saving energy, which would reduce the total cost of piston products. So Al-Fe-P master alloy is a promising modifier to refine ZL109 piston alloys.

Abstract: The oxidation of a ternary Cu-Ni-Co alloy containing about 20 at.% Ni and 75 at.% Co, balance Cu (Cu-20Ni-75Co), was studied at 600-800°C in 0.1MPa pure O₂. The kinetic curves for the present alloy are irregular and deviate from the parabolic rate law and are composed of three quasi-parabolic stages. The oxidation rates decrease continuously with time more rapidly than required by the parabolic rate law. These indicate that the scales become more protective with the increment of time. The oxidation rates of the present alloy increase with the increment of temperature. Cu-20Ni-75Co alloy formed a dark external layer composed of Co₃O₄ followed by a slightly lighter internal layer of CoO. Obviously, adding 20 at% Ni to Cu-75Co alloy is able to decrease the critical Co content required to form the most stable oxide and finish a transformation from the mix internal oxidation to the formation of the exclusive external scales of Co oxides.

Abstract: Nanostructured Cu90Al10 alloy powders were prepared by high energy ball milling mechanical alloying (MA). Up to 10 at.% Al could be dissolved into copper by mechanical alloying at room temperature. Effects of milling time on phase transformation, structural characteristics, and mechanical property of powders were investigated by using X-ray diffraction (XRD), Optical Microscopy (OM) and microhardness tester. The results show: with increasing the milling time, the powder particle size increased gradually, and then it tended to be homogeneous. The grain size of the alloy decreased gradually, but the yield strength increased with the extension of the ball milling. After 30h milling, the grain size reached the minimum value of 9 nm, and the yield strength obtained the maximum value of 511 MPa.