Materials Science Forum Vols. 730-732

Abstract: In the context of a R&D project concerning the new Alcácer do Sal composite railway bridge, a study of the fatigue crack growth on samples of its base material and weldments was performed. For this purpose, tests were carried out on CT specimens designed according to ASTM E647 standard, using the approximate thickness (B) of a structural detail of interest, B=32mm. The choice of B led to a relatively large specimen and was justified by the desire to better simulate service conditions, which would not be possible with smaller specimens, particularly in the case of weldments. The test matrix used included three values of R ratio (maximum/minimum load), 0.1, 0.4 and 0.7, and three material conditions, namely base material (BM), heat affected zone (HAZ) and weld metal (WM). When the nominal range of the stress intensity factor (DK) is used, the measured data displays a strong effect of the weldments on the FCG rates, with the base material presenting higher da/dN values. An evaluation of opening load behaviour was carried out, and it showed extensive closure caused by residual stresses in the HAZ and WM specimens. The investigation included the full field measurement of the residual stress perpendicular to the crack plane, using the contour technique. When the opening load effect was taken into consideration it was found that the da/dN vs. ∆K of the BM, HAZ and WM specimens is approximately identical. Furthermore if loading effects are considered, no significant difference is found for the three R values used, even if, as expected, higher R corresponds to higher da/dN values.

Abstract: Scanning electron microscopy (SEM) observation of the fracture surface of CT specimens was carried out at CEMUP in the context of a FEUP R&D programme for the characterization of fatigue crack growth (FCG) of base material and welded steel CT specimens, carried out under constant load range, reported in a companion paper. The inclusion of specimens of welded material in the FCG characterization programme justified the need for relatively large size specimens. The thickness of the specimens (B=32mm) was approximately identical to the thickness of the structural application of interest, since with thinner specimens the possible effects of residual stress fields would be at least partially lost. The CT specimens were designed according to ASTM E647 standard. The FEI QUANTA 400 FEG / EDAX Pegasus X4M available at CEMUP was used for the SEM observations, carried out in the fatigue fracture surfaces of base and welded (HAZ) steel. Both in base material as well as in welded specimens, it was found that there are regions of the fracture surfaces where no striations are observed, mixed with other regions (patches) with striations. Some regions are featureless (ropey appearance). It was not possible to find a preferential orientation of the striation spacing, that might be directly related with the macroscopic direction of crack propagation; on the contrary, the regions presenting striations seem to have a mainly random orientation. This fact contributes to the absence of correlation between the distance between striations (s) measured using SEM, and the macroscopic fatigue crack growth rate da/dN. Only for substantial values of the crack length/specimen width (a/W) ratio a reasonable agreement between s and da/dN is found. The relationship s/(da/dN) presents values that decrease up to approximately one, as a/W increases. For low values of a/W, s is one or two orders of magnitude greater than da/dN. In the specimens analyzed, fatigue crack growth rates below approximately 2 to 3 x10^‑7 m/cycle are associated with approximately constant s values. This border is over the transition between regions I and II of the typical sigmoidal log-log plot of the da/dN vs. DK (I corresponding to the near threshold regime and II to the Paris law regime), and this fact should be accounted for in failure analysis involving SEM of fracture surfaces. Finally, the distance between striations s proved to be insensitive to the presence of important residual stresses in the case of the welded specimens.

Abstract: Density Functional Theory (DFT) calculations were performed. They were firstly implemented to optimize the structure and refine the stoichiometry of the only ternary compound, CuLi_0.08Mg_1.92 of the Cu-Li-Mg system. Furthermore using DFT, several possible structures of CuMg₂H_x were optimized. Since most of the hydrides are cubic structures or can be considered as distortions of a cubic structure, we have started calculations for CuMg₂H_x (x = 4 - 6) with tetragonal and monoclinic structures, similar to those of the hydrides formed by the nearest neighbors of Cu and Mg in the periodic table: NiMg₂H₄ and CoMg₂H₅ (e.g. monoclinic C2/c and tetragonal P4/nmm, respectively). It can be concluded that the most stable configuration corresponds to CuMg₂H₅ with C2/c structure. We have performed several neutron scattering experiments that are in agreement with the first principles calculations.

Abstract: In this work, three water-cooled experimental solidification devices were developed, and experiments were carried out with an Al-1.5wt%Fe alloy. The three experimental setups consist of vertical cylindrical steel molds with each of them having different zones cooled by water. For the inward solidification, a cooled tube is used having its upper and bottom part thermally insulated. For the outward solidification, a cooled tube, forming an inner part, is concentrically placed inside a cylindrical mold, which is thermally insulated from the environment, by using insulating materials. For the upward solidification, the bottom part of the mold is water-cooled and consists of a thin (3 mm) disc of carbon steel, whilst the cylindrical surface is covered with insulating material to avoid lateral heat losses. A numerical solidification model based on the finite difference method is applied for the simulation of the three aforementioned cases of solidification from the chilled surface considering transient heat flow conditions. Experimental thermal readings in the castings have been used for the determination of the transient overall metal/coolant heat transfer coefficient, h, through a numerical-experimental fit of casting thermal profiles based on inverse heat transfer calculations. It was found a significant variation of h as a function of time during solidification in the three cylindrical set-ups experimentally examined, including a remarkable increase in h during the outward solidification. Introduction

Abstract: The alloying of the zinc with some additional elements in specific quantities could be reliably used in the hot-dip coating process. These elements form specific intermetallic compounds that conferred substantially improved performance to the end-product specially by improving of the corrosion resistance. The paper presents the research that has developed to analyze the influence of aluminum, bismuth and tin like alloying elements added in the zinc on corrosion resistance of galvanized steels. Steel samples were galvanized by the hot-dip method in micro-alloyed zinc baths. The influence of the alloying elements on the formation of the different phases and on the diffusion process is discussed. The corrosion experiments were carried out in a simulated environment of accelerated corrosion conditions. The kinetics and the mechanism of corrosion of the samples were studied in correlation with the effect of alloying elements on nature and characteristics of protective layers.

Abstract: Friction Stir Channelling (FSC) is an innovative technique of manufacturing integral and continuous channels (also referred as conformal channels) in monolithic plates in a single step. The process is capable of producing non-linear channels in solid components with similar control of the parameters applied during Friction Stir Welding. The tool geometry and concept plays a major influence in the results from FSC. During FSC a controlled amount of workpiece material flow-out from the processed zone producing the internal channel. The heat energy that softens the workpiece material is generated from dissipation during plastic deformation, internal viscous dissipation during the material flow and dissipation from frictional work between the tool and the workpiece. This research work is based on a detail metallographic, geometric and hardness analysis of integral and continuous FS channels produced in a monolithic plate of the aluminium alloy AA7178‑T6 with 13mm of thickness, typically used in structural aircraft applications. In this paper the metallographic characterization and the hardness distribution in the processing zone that surrounds the channel are presented and discussed. Channel’s geometric characterization is also presented and discussed. The channel microstructure and its roughness features were obtained using optical microscopy and SEM.

Abstract: The paper presents the research that has developed to clarify the zinc microalloying process with nickel, aluminum, bismuth and tin. We have analyzed the parameters and factors that influenced the dissolution process of alloying solid elements in the liquid phase (zinc bath) in respect to compositional controlling process. In accordance we analyzed the most important parameters that influenced this process. There have established the optimum making technology of the zinc alloys used into hot dip galvanized process and also the correction technology of the bath composition into continuous process.

Abstract: In the present study a hypomonotectic Al-0.9wt%Pb alloy was directionally solidified under transient heat flow conditions and the microstructure evolution was analyzed. The solidification thermal parameters such as the growth rate, the cooling rate and the temperature gradient were experimentally determined by cooling curves recorded by thermocouples positioned along the casting length. The monotectic structure was characterized by metallography and a microstructural transition was observed. From the casting cooled surface up to a certain position in the casting the microstructure was characterized by well-distributed Pb-rich droplets in the aluminum-rich matrix, followed by a mixture of fibers and strings of pearls from this point to the top of the casting. The interphase spacing (λ) and the diameter of Pb-rich particles were also measured along the casting length and experimental growth laws relating these microstructural features to the experimental thermal parameters are proposed.

Abstract: Intermetallics and superalloys brazing development is a current topic owing the extending use of these alloys in industrial applications. In this work a γ-TiAl alloy was joined to Inconel 718 by active metal brazing, using Incusil-ABA as filler. Joining was performed at 730 °C, 830 °C and 930 °C, with a 10 min dwelling time. The interfaces were characterized by Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS) and Electron Backscatter Diffraction (EBSD). For all processing conditions, the reaction between the base materials and the braze alloy produced multilayered interfaces. For all processing temperatures tested (Ag), (Cu), AlNi₂Ti and AlCu₂Ti were identified at the interface. Raising the brazing temperature increased the thickness of the interface and coarsened its microstructure. The increase of the extension of the interface was essentially due to the growth of the reaction layers formed near each base material, which were found to be mainly composed of intermetallic compounds. The mechanical behavior of the joints, at room temperature, was assessed by microhardness and shear tests. For all processing conditions the hardness decreases from periphery towards the Ag-rich centre of the joints. Brazing at 730 °C for 10 min produced the joints with the highest average shear strength (228±83 MPa). SEM and EDS analysis of the fracture surfaces revealed that fracture of joints always occurred across the interface, preferentially through the hard layer, essentially composed of AlNi₂Ti, resulting from the reaction between Inconel 718 and the braze alloy.

Abstract: For the correct simulation of solidification and temperature evolution in the continuous casting of steel, the determination of boundary conditions describing the heat-transfer phenomena through the strand surface, in each cooling zone of the casting machine, is extremely important. These boundary conditions are usually expressed as heat fluxes or heat transfer coefficients. In the present study, the surface temperature of the steel billet was experimentally determined in a steelmaking plant by infrared pyrometers positioned along the secondary cooling zone during real operation of a continuous casting machine. These data were used as input information into an Inverse Heat Transfer Code, implemented in this work, in order to permit the heat transfer coefficients of each spray cooling zone to be determined. The resulting simulations of temperature evolution during continuous casting have shown that the solidification was not complete at the unbending point and that there was a risk of breakout at the mold exit under the adopted operating conditions.