Evaluation of Liquation Cracking Behavior and Susceptibility in Heat-Affected Zone of CM247LC Superalloy Welds for Turbine Blade Application

In this study, the weldability of the as-cast CM247LC superalloy for turbine blade applications was metallurgically evaluated in terms of its hot cracking behavior and susceptibility. For this purpose, a real blade was manufactured using a directional solidification casting process, and gas tungsten arc welding was performed at the tip and cavity of the upper blade. Hot cracking was confirmed in the heat-affected zone (HAZ) of gas tungsten arc welds, and the cracks were characterized as liquation cracks, since a cobble or dropletshaped crack surface consistent with a liquid film was clearly confirmed. Microstructural analysis of the cracking surface and thermodynamic calculations helped elucidate the metallurgical mechanisms of the liquation cracking. In other words, the cracking was attributed to liquation of the γ-γ’ eutectic colony and the constitutional liquation of the MC-type carbides: these phases existed in the as-cast microstructure. In particular, it was calculated that liquation of the γ-γ’ eutectic colony during welding occurs at least at 1488 K and that constitutional liquation of MC-type carbides begins at 1411 K, while the equilibrium solidus temperature of the CM247LC alloy is 1530 K. Finally, the liquation cracking susceptibility was quantitatively evaluated through a spot-Varestraint test, and it was confirmed for the first time that the higher susceptibility of as-cast samples can be suppressed by employing a pre-weld heat treatment such as solution treatment.

Effect of Laser Cladding Stellite 6-Cr3C2-WS2 Self-Lubricating Composite Coating on Wear Resistance and Microstructure of H13

In order to prevent the wear failure of the hot-working die, the composite coatings of Stellite 6-Cr3C2-WS2 was fabricated on H13 hot-working die steel by laser cladding. The composite coating was prepared through the in-situ generation technology, that can give H13 the ability of self-lubricating at the working temperature (about 200 °C). The effect of the various WS2 percentages on the properties of the coating was studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), microhardness test, friction and wear test. In addition, the phase constitutions, microstructures and wear properties were also investigated systematically. The obtained hardness of the cladding coating is approximately 2.5 times higher than the substrate because of the constituents of γ-(Fe, Co)/Cr7C3 eutectic colony, (Cr, W)C carbide and dendritic crystals in the coating. Furthermore, the friction coefficient decreases to 70% of the substrate due to the CrS self-lubricating phase. The analyses results suggest that an 85% Stellite 6-10% Cr3C2-5% WS2 composite coating has excellent material properties.

Thermodynamic Functions of Fe3B Borides

In this work we study the structural properties of hypoeutectic and hypereutectic Fe-B alloys, depending on the temperature of heating above the liquidus line and the cooling rate. Experiments were carried out for the Fe-B system alloys with boron content of 2.0 - 4.5 % (wt.), the rest is iron. To determine physical properties of the alloys microstructural and X-ray diffraction analysis were used. It is shown that overheating of the molten alloy to 150 K above the liquidus line and aftercooling leads to complete suppression of the formation of primary iron crystals in the hypoeutectic alloys and partially to suppression of Fe2B formation in the hypereutectic alloys of the Fe-B system. For the first time it is shown that heating of Fe-B hypoeutectic alloys to 150 K above the liquidus line and cooling with a rate of 103 K/s lead to formation of Fe3B boride in as-cast state, which was present in the eutectic colony.

Microstructural evolution and mechanical properties of Mg–Cu–Zn ultrafine eutectic composites

Novel ultrafine eutectic composites containing structural and spatial heterogeneities have been systematically developed in an Mg–Cu–Zn ternary system. Microstructural investigations of the ultrafine eutectic composites revealed that the bimodal eutectic structure consists of a mixture of cellular-type fine (α-Mg + MgZn2) and anomalous-type coarse (α-Mg + MgZn2 + MgCuZn) eutectic structures. An Mg72Cu5Zn23 alloy composed of the bimodal eutectic structure without micron-scale α-Mg dendrites presents a strong improvement of yield strength up to 455 MPa with a decent plastic strain of 5%. The rotation of the bimodal eutectic colony along the interfaces is considered to be an effective way to dissipate the stress localization thus enhancing the macroscopic plasticity.

Ceramic materials of the quasi-binary LaB6-MoB2 system

Alloys of the quasi-binary LaB6-MoB2 system synthesized by melting in an atmosphere of helium have been studied. It has been established that the phase diagram of the system is eutectic and the phases do not interact. The eutectic colony crystallizes in form of the platelike eutectic on the basis of the LaB6 crystal of a cubic structure. It has been also shown that the crystallization from melt under conditions of crusibleless zone melting is accompanied by the MoB2 phase decomposes during cooling with precipitation of the submicron Mo2B5 and ?-MoB crystallites. This results in an increase of hardness. According to the Mo - B phase diagram, the decomposition temperature of the MoB2 increases with increasing the boron content. In addition, the degree of the decomposition is higher if the melting point of alloy is higher, since the intensity of diffusion process increases at high temperatures.