Advanced Materials Research Vols. 89-91

Abstract: Ultrasonic welding between SS400 mild steel sheet and aluminum alloy A5052 sheet containing magnesium was conducted. In this study, authors investigated the influence of ultrasonic welding condition on the mechanical properties and the interface microstructure of a joint, and examined the effect of an insert metal to improve the joint strength. The main results obtained in this study are as follows. It was possible to ultrasonically weld SS400 mild steel sheet and A5052 sheet. When the clamping force was varied keeping the welding time constant for 1s, the joint strength showed the maximum at the clamping force of 588N and it decreased with increasing the clamping force because the frictional action at the interface decreased with it. When the clamping force was kept constant at 588N and the welding time was varied, the joint strength reached the maximum at the welding time of 2.5s, followed by decreasing in the joint strength welded at 3s welding time due to the formation of Fe2Al5 intermetallic compound at the interface. Using insert metal of commercially pure aluminum successfully improved the joint strength and the strength of the joint welded at 3s welding time was about three times larger than that without the insert metal.

Abstract: Tb0,3Dy0,7Fe1,9 composites attract much attention due to their large magnetostriction, and small eddy current losses. In this work intelligent polymer matrix composite materials consisting of Tb0,3Dy0,7Fe1,9 particles was obtained by mixing epoxy resin and Tb0,3Dy0,7Fe1,9 powder with grains from 38 to 106 μm. The relationships among the manufacturing technology of these materials, their microstructure, as well as their magnetostriction were evaluated. Resulting composites can extend the possibilities of application the magnetostrictive materials simultaneously reduce the cost of Tb0,3Dy0,7Fe1,9.

Abstract: Ti and Ti-6Al-4V coatings were deposited by cold gas dynamic spray process using nitrogen as propellant gas. For Ti, the inlet gas temperature and pressure were varied for two different powder morphologies in such a way that the average particle velocity ranged from ~600 to 850 m/s. In addition, the nozzle traverse speed was varied. For all conditions, the deposition efficiency, the porosity, and the microhardness were measured. It is shown that the porosity level decreased as the gas temperature and pressure increased, whereas the velocity, deposition efficiency, and microhardness increased. Furthermore, it is observed that a lower nozzle traverse speed engendered a softer coating. The coating adhesion on a grit 24 Al2O3 blasted mild steel surface was established to be greater than 78.8 MPa. In addition, a Ti-6Al-4V coating was produced and was determined to be slightly more porous compared to a CP Ti coating.

Abstract: The catalytic activity of oxidation-reduction pre-treated Ni3Al powder for methane steam reforming was examined. The oxidation-reduction pre-treatment consisted of two steps: oxidation in air at various temperatures from 973 to 1373 K, and then followed by reduction in H2 at 873 K. It was found that the oxidation-reduction treatments significantly reduced the onset temperature of activity, i.e., improved the activity of Ni3Al powder at low temperatures. The characterization of Ni3Al surface showed that an outer surface layer of fine NiO particles were formed on the surface of Ni3Al after oxidation. These NiO particles were reduced to metallic Ni by the subsequent reduction treatment, resulting in the high activity for methane steam reforming. These results indicate that the Ni3Al can form highly active surface structure with oxidation-reduction treatment, having excellent heat resistance.

Abstract: Large-diameter steel pipes are produced by induction seam-welding followed by induction-assisted heat treatment. The microstructure and distribution of crystal orientations have been studied and related to the mechanical properties of the welded regions. The welding and heat-treatment process leads to a microstructure, a simple observation of which can not explain the observed variations in toughness in the vicinity of the welding joint, because the crystallographic grain size, which represents the scale of similarly oriented adjacent grains, is much coarser than the ordinary grain size. Furthermore, heating the affected zone into the austenite phase field followed by cooling does not completely eliminate the coarse regions of similarly oriented grains. The consequences of this on mechanical properties are discussed.

Abstract: Generally, three typical behaviors are recognized in hot-deformation of austenite. However, considering that those austenite grains involved in single-pass deformation are mostly on the scale of dozens of micrometers or even much larger than that, it is meaningful to investigate hot-deformation behaviors of austenite grains smaller than 10μm. In the current paper, austenite grains of different sizes were prepared through repetitive treatment of rapid reheating and quenching with changing the holding temperature and/or holding time. Kinds of true stress-true strain curves at 900oC and 950oC indicate that austenitic deformation can be gradually coordinated by grain boundary behaviors, such as grain boundary sliding and/or diffusion. Simultaneously, the macroscopic deformation is more likely to be dominated through co-operation grain boundary sliding (CGBS).

Abstract: The ferrite phase strengthened by the highly dense nanometer-sized carbides is named super ferrite. The effects of Ti, Ti-Mo, and Ti-Nb micro-alloy additions on the precipitation strengthening in three experimental high-strength low-alloy steels have been investigated by using transmission electron microscopy and micro-hardness measurement. The objective of this work was to study the carbide precipitation under the conditions of isothermal transformation. It was found that interphase precipitation of nano-sized carbide in Steel Ti-Mo had very strong contribution to the hardness of the steel. The distribution of interphase precipitated carbides is associated with both the sheet spacing and the inter-particle spacing, which have been studied. The result implies that (Ti, Mo)C particles possess an excellent effect on precipitation hardening.

Abstract: This work utilizes thermoelectric power (TEP) measurement to track secondary precipitation during the interrupted ageing cycle for AA2024 alloys as well as the subsequent re-ageing cycle. Clear evidences of secondary precipitation were observed which affect the further precipitation sequence during re-ageing. The TEM results confirm that S and S” phases co-exist upon re-ageing.

Abstract: Surface repair experiments with Nd:YAG coaxial laser cladding of NiCoMo maraging powder were made on specimens from maraging steel (EN 10027-2, mat. no. 1.2799). The influences of different modes of laser-beam guidance with various powder mass flows and with different degrees of overlapping of individual traces on the dilution and the repair area were determined. The micro and macroscopic analyses of microsections of fusion zone (FZ), heat affected zone (HAZ) and through-depth microhardness were analysed after cladding and after subsequent solution and precipitation annealing. The microchemical (EDS) analysis was performed at various depths. The residual stresses in the clad face and in the clad toe were determined and compared, using the hole-drilling method.

Abstract: By the finite element method, the N-S momentum, energy equations and continuum equations, etc. which describe the fluid flow and heat transfer in the slope of the rheocasting-rolling for Semi-solid Magnesium Alloy were computed. The influence of the different slope parameters( pouring temperature, length of slope, lean angle)on the metallographic structure of semi-solid magnesium alloy was analyzed. The simulative results can provide effective data to confirm optimized slope size and technical parameter.