Abstract: The relationships between the morphologies of primary silicon and undissolved silicon particles in melt of Al-Si hypereutectic alloys are studied by holding the melt from 600 to 1100°C and then quench interrupting. The variation of growth process and mechanism of primary silicon is also investigated. The results show that undissolved silicon particles in the melt would become the nuclei of primary silicon precipitated in solidification and there is a close relationship between the shape of primary silicon and undissolved silicon particles. The growth of silicon would follow not only twin plane re-entrant edge (TPRE), but also layer mechanism as well. Meanwhile, the shape of primary silicon also relied on kinetic surroundings, such as solute transportation. At higher overheating temperatures of the melt, the primary silicon takes star-like and tree-like shapes.
Abstract: 2 mm thick 6061-T6 aluminum alloy sheets were I square butt welded using 3kW Nd:YAG laser. Filler wires of 1 mm diameter, 5183A(Al-4wt.%Mg), 4043A(Al-5wt.%Si) and 4047A (Al-12wt.%Si) were used. The welds made with 4047A wire showed the lowest solidification cracking among the welds investigated. Abundant amount of Al-12wt.%Si eutectic which was observed at the grain boundaries of the 4047A wire feed welds was closely related with the reduced
solidification cracking susceptibility. Yield and tensile strength, and formability of the welds made with 4047A wire were improved compared to the welds made with other filler wires, which is attributed to the reduced cracking susceptibility in the welds.
Abstract: The refining of MG-silicon (MG-Si) is closely related to the cost and purity of solar-grade silicon (SoG-Si) as well as semiconductor-grade silicon (SeG-Si). Plasma arc refining of MG-silicon is one of the alternative and effective route to remove the impurities in silicon. In this study, a 60KW transfer-arc plasma melting furnace operated in105Pa was used to purify the MG-Si by different kinds of working gas, which was composed of 100%Ar, 95%Ar+5%O2, 95%Ar+5%H2, and
70%Ar+30%H2 respectively. During the processing, an optical spectrometer was used to monitor the changes of compositions. The experimental results show that the removal rate of impurities of aluminum, calcium, sodium, barium...etc. in silicon with plasma working gas containing oxygen, and hydrogen are higher than pure Ar plasma. Especially with 30% H2 plasma, the removal rate of the Na and Ba could reach 100% and the removal rate of Ca and Al could also achieve to 99.5% and 89.5% respectively. For the impurities of boron in the MG-Si, the elimination rate of hydrogen-mixed plasma could be as high as 76%.The in-situ monitoring of plasma refining is accomplished with the monochromators in the range of visible light’s wavelength. From the results of chemical analysis and optical spectrograph, it revealed that elimination rate of Fe and Al was higher in hydrogen-contained plasma arc than in pure Ar plasma, As to the refining of carbon, the hydrogen and oxygen mixed plasma arc are also efficient to reduce the carbon content in silicon, which could be decreased from 310 ppm to 60~70 ppm.
Abstract: The effect of aging treatment on the damping capacity of the high strength damping
aluminum alloy prepared by RS/PM process was investigated. The damping properties of the alloy were examined with dynamic mechanic thermal analyzer (DMTA). The damping capacity, as well as the dynamic Young’s modulus was measured at different temperatures and different loading frequencies. The analysis of microstructure characteristics was performed using transmission electron microscopy (TEM) and X-ray diffraction (XRD). It was found that the damping capacity of the alloy did not change significantly after aging treatment, but changed remarkably with the variation of measurement temperature and loading frequency. Grain boundaries in the alloy became clear and sharp with aging treatment proceeded and contributed to the total damping capacity.
Abstract: The images of the growing crystal in the growth process of the undercooled droplets of silicon were lively recorded by using a high-speed camera. The number of crystal that nucleated spontaneously from the undercooled liquid was found to decrease to 1 when the undercooling was higher than 5K. The morphology of the growing single crystal of silicon was a thin plate. A model for predicting the critical undercooling of growing single crystal of silicon from undercooled liquid has been developed. The theoretically predicted value of the undercooling from present model for silicon is in agreement with the experimentally measured result.
Abstract: Directional solidification of the undercooled melt which combines melt undercooling with conventional directional solidification is a new method of obtaining directional structure. This method is used to realize the directional solidification of the undercooled Ag70.3Cu29.7 melt. The experiment is performed in two steps: The undercooled melt is obtained by glass fluxing, and the above specimen is remelted and excited with Ga-In-Sn coolant. By this method, directional solidification dendrites of Ag70.3Cu29.7 alloy are achieved successfully. The columnar dendrites are straight and fine. The primary arm spacing is 21µm in average for a 65K undercooling, as compared to 38µm for a 32K undercooling. This confirms that higher undercooling promotes finer dendritic microstructures.
Abstract: Laser surface remelting experiments on Zn-4.0wt.%Cu hyperperitectic alloy have been performed on a 5kW CW CO2 laser with the scanning velocities between 6 and 1207mm/s. Microstructures in both longitudinal and transverse section of the molten pools have been analyzed by optical microscope and SEM technique, and the average composition in the molten pools has been measured by Electron Probe Microanalysis(EPMA). With the increasing of growth rate, the microstructures of Zn-4.0 wt.%Cu alloy change from planar interface to lamellar structures and then
cellular structures, and finally to absolute stability planar interface at a growth rate of 562mm/s, which shows reasonable agreement with that predicted by M-S theory.
Abstract: Laser Rapid Forming (LRF) is a new and advanced manufacture technology to
accomplish near net shape metal components with high performance for use in the aerospace, defense and chemical processing industries. In this paper the close-range continuous photography is used to take side views on the cladding zone in order to disclose inherent characteristic of the laser rapid forming process. The vivid molten pool shape is obtained by adopting image partition technology since the not molten area is eliminated from the photos. It is found that the molten pool size depended essentially on the LRF process parameters. With introducing the metal powders into the focus spot of the laser beam, the molten pool retains a segment arc raised outward and the molten pool free surface inclines to the axis of laser beam. Further results show that the molten pool inclination plays an important role on the microstructure of the LRF components.
Abstract: The pulsed electric current produced by discharging of capacitors pass through the
double solenoids with ferrosilicon core, the instant strong pulsed magnetic field with the oscillating and declining characters was made between two ferrosilicon cores. The effect of high-pulsed magnetic field on the unidirectionally solidified Al-Cu eutectic microstructure with 10µm/s withdrawn velocity was investigated. Under the high intensive pulsed magnetic field, the Al-Cu eutectic solidified morphology experience three evolution stages that are regular columnar structure to breaking off fine grain, coarsening dendrite to newly regularization columnar structure with
increasing of 0～15.5J charge energy. It is found that rich copper phase evidently come out inter eutectic cells and the eutectic spacing decrease in newly regularization specimen. The induced electric field caused by high pulsed magnetic field in metallic melt brought into oscillating solute electro-migration in front of solidification interface, which has the effect of promoting solute diffusion and reducing the constitutionally supercooling region.
Abstract: The microstructure and tensile mechanical properties of directionally Solidified
NiAl-15Cr alloy at various temperatures have been investigated in this paper. The results reveal that the microstructure consists of dendritic β-NiAl phase, interdendritic γ/γ’, γ’ phase transient layer and α-Cr precipitation in β-NiAl phase. With the increase of temperature obvious Brittle-Ductile-Transition (BDT) behavior is observed and the BDT temperature (BDTT) is sensitive to initial strain rate. When the initial strain rate increases by two-order magnitude, the BDTT has an approximate 150K increase. In the temperature range of 1123-1373K, the alloy exhibits good tensile plasticity and poor strength comparing with the NiAl/Cr(Mo,Hf) alloy. Superplastic-like deformation behavior with large elongation (exceed 200%) is achieved at 1323K. The balance between strain hardening (by dislocation glide) and strain softening (dynamic recover and recrystallization) is responsible for the large tensile elongation of this alloy.