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Abstract: The fusion zone and heat affected zone (HAZ) microstructures of electron beam welded superalloy 718PLUS™ (718 Plus) that has been newly developed by ATI ALLVAC were examined. The microsegregation pattern during solidification of the fusion zone indicated that while Fe, Co, W, and Cr segregated to the core of the gamma dendrites, Nb, Ti, and Al were extensively rejected into the interdendritic liquid. Electron diffraction and X-ray microanalysis using transmission electron microscopy (TEM) of the fusion zone showed that the major secondary phases that formed from the interdendritic liquid were gamma/MC type carbide eutectic and gamma/Laves eutectic constituents. HAZ microstructure showed partially melted zone immediately adjacent to the fusion zone and intergranular microfissuring associated with resolidified products which suggested that HAZ cracking in this alloy occurred by liquation cracking. Microstructural examination of the HAZ using analytical scanning electron microscope showed resolidified gamma/Laves eutectic on the cracked and backfilled grain boundaries. Fine resolidified MC type carbide particles were also observed in the HAZ. Causes of grain boundary liquation were identified and the solidification of intergranular liquid in the HAZ was discussed. [Copyright &y& Elsevier]

Abstract: The microstructure of laser beam weld-metal of an yttrium doped directionally solidified alloy IC 6A, with chemical composition Ni–16Al–8.5Mo–0.12B–0.05C–0.03Y (at.%) was studied. The dendritic microsegregation observed within the fusion zone indicated that dendrite cores were slightly depleted in molybdenum and aluminum and the interdendritic regions were also considerably enriched in yttrium. Severe cracking in the weld-metal was observed and was found to be closely associated with interdendritic eutectic-type microconstituents identified as consisting of γ, γ′ and Ni–Mo phases. An yttrium-rich phase (Ni3Y) was observed in some interdendritic regions containing the eutectic γ, γ′ and Ni–Mo products. Their formation was discussed in relation to plausible microsegregation induced alteration of primary solidification path during cooling from welding temperatures. [Copyright &y& Elsevier]

Abstract: The laser beam weld heat affected zone (HAZ) microstructure of a newly developed aerospace alloy, IC 6, was examined. HAZ microfissuring was observed and found to be associated with grain boundary liquation facilitated by subsolidus eutectic-type transformation of the alloy’s major phase, γ′ precipitates, and interfacial melting of M6C-type carbide and (Mo2Ni)B2-type boride particles. [Copyright &y& Elsevier]

Abstract: The fusion zone microstructure of laser welded alloy IC6 was examined. Extensive weld-metal cracking was observed to be closely associated with non-equilibrium eutectic-type microconstituents identified as consisting of γ, γ′ and NiMo (Y) phases. Their formation has been related to modification of primary solidification path due to reduced solutal microsegregation. [Copyright &y& Elsevier]

Microstructural examination of simulated HAZs and those of in an actual TIG welded Inconel 738 superalloy was conducted, and an alternate mechanism through which high volume fraction of constitutionally liquated γ′ phase contributes to the alloy''s low resistance to weld HAZ cracking by limiting liquid film migration process, is proposed. [Copyright &y& Elsevier]

Abstract: A microstructural study was performed on transient liquid phase (TLP) bonded Waspaloy superalloy with a Ni–Cr–B filler. The applicability of a diffusion model based on Fick''s second law of diffusion to determine the time required for complete isothermal solidification (t f) was investigated. Over the temperature range of 1065–1110°C, experimental observations of t f were in reasonable agreement with t f values predicted by the diffusion model. However, a notable deviation was observed in joints prepared between 1175 and 1225°C in that the rate of isothermal solidification was reduced at these temperatures resulting in the formation of a centerline eutectic-type microconstituent, which in contrast, was prevented from forming after holding the brazing assembly for an equivalent bonding time at a lower temperature of 1145°C. Boride particles were observed as part of the eutectic product, which suggested that diffusion of boron out of the liquated insert was also reduced at these higher temperatures. A decrease in solubility of the melting point depressing solute, boron, with increase in temperature is suggested to be an important factor contributing to the reduction in isothermal solidification rate observed at the higher bonding temperatures. [Copyright &y& Elsevier]

The effect of processing variables on the growth of a titanium aluminide layer produced from elemental titanium and aluminum foils was examined, with the intention of yielding a titanium–titanium aluminide composite structure. To produce the intermetallic phase in situ, two processing techniques were employed, diffusion bonding and combustion synthesis. Microstructural examination of the titanium aluminide layer revealed striking similarities in the resulting morphology yielded by both processing techniques, in particular for those samples diffusion annealed at lower temperatures (615 °C and below) and those reacted by combustion synthesis. In both cases, an exothermic chemical reaction was responsible for aluminide formation in specimens, and in those samples processed by diffusion bonding, this reaction was observed to be rate controlled by a diffusion mechanism. Chemical analysis of the product aluminide showed the layers to be composed completely of a single phase, TiAl3, which is believed to be the result of thermodynamic stability of this phase relative to other stable equilibrium phases in the titanium aluminum system. The results of these experiments are discussed with a view of utilizing these processes to optimize the mechanical behavior of titanium aluminide by incorporating it into a laminated metal composite. [Copyright &y& Elsevier]