Weld heat input/cooling rate (affected by welding process, parameters, technique, tooling, etc.) was found to influence the microstructural characteristics and segregational features in alloy 718 welds, with low heat inputs proving beneficial. Laves phase formed in the interdendritic regions of the weld metals as a result of segregation. The morphology and composition of Laves phase depended strongly on heat input/cooling rate and influenced its response to subsequent homogenization post-weld heat treatment. The various factors affecting the formation and control of Laves phase in alloy 718 welds are highlighted.

Ch Radhakrishna

Kalvala Prasad Rao

Heat treatment

microstructure

Morphology

Segregation (metallography)

Welding

Welds

LAVES PHASES

POST-WELD HEAT TREATMENT

superalloys

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

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IIT Madras

The formation and control of Laves phase in superalloy 718 welds

Journal of Materials Science

Microstructural characterization of alloy 718 fusion zone welded with both solid solution and age hardenable filler metal has been done. The microsegregation and the aging response were studied by employing three levels of weld cooling rate. Gas Tungsten Arc welding process was used. The fusion zone of solid solution filler metal has been responding to the aging treatment due to the weld process conditions and weld metal chemistry. However the weld metal composition was modified due to the higher molybdenum (Mo) content in solid solution filler metal. The effect of this modification on the phase reaction temperatures was studied and the same was compared with the conventional filler metal. © 2014 Elsevier Inc.

Materials Characterization

Laves phase in alloy 718 fusion zone - Microscopic and calorimetric studies

Even though alloy 718 is the best for welding among all nickel-base superalloys, the formation of the Laves phase in welds is a major concern. The presence of this phase drastically degrades mechanical properties of the welds. To study the influence of weld cooling rate on microstructure and mechanical properties of alloy 718 weldments, two distinct welding processes were adopted - gas tungsten arc (GTA) and electron beam (EB) welding. The EB welding resulted in finer and relatively discrete Laves phase in lower quantity due to higher cooling rates prevailing in this process. On the other hand, due to lower cooling rates, GTA weld fusion zones exhibited coarse Laves with higher niobium. Depletion of the primary strengthening element niobium in the surrounding regions of Laves promoted crack propagation. Because EB welds had finer and lower amount of Laves, EB weldments exhibited superior mechanical properties compared with GTA weldments. © The Minerals, Metals &amp; Materials Society and ASM International 2008.

Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science

Influence of weld cooling rate on microstructure and mechanical properties of alloy 718 weldments

Autogenous full penetration electron beam welds were made on alloy 718 with and without beam oscillation technique. Weldments were subjected to two types of post-weld heat treatments: direct aging (DA) and solution treatment at 980°C followed by aging (STA). When the welds were prepared using different heat inputs in the welding processes with and without beam oscillation, the influence of beam oscillation could not be studied in isolation but the coupled effect of heat input and beam oscillation was studied. Laves particles were finer in size and lower in amount in unoscillated welds compared with those in beam oscillated welds subjected to DA condition, δ phase needles were observed around Laves particles in the welds subjected to STA condition. The amount of Laves particles was less and that of δ phase was more in unoscillated welds compared with those in beam oscillated welds subjected to STA condition. Unoscillated weldments exhibited longer fatigue lives compared with beam oscillated weldments in both DA and STA conditions owing to less amount of Laves In the former. Weldments In STA condition had longer lives compared with those in DA condition. The role of δ phase needles in fatigue life could not be identified. © 2006 Institute of Materials, Minerals and Mining.

Science and Technology of Welding and Joining

Coupled effect of heat input and beam oscillation on mechanical properties of alloy 718 electron beam weldments

The aim of the present work is to study the effect of magnetic arc oscillation and current pulsing on the microstructure and high temperature tensile strength of alloy 718 tungsten inert gas weldments. The magnetic arc oscillation technique resulted in refined Laves phase with lesser interconnectivity. The full benefits of current pulsing in breaking the dendrites could not be realized in the present study due to relatively higher heat input used in the welding process. In the direct aged condition weldments prepared using magnetic arc oscillation technique exhibited higher tensile strength due to the presence of refined and lesser-interconnected Laves particles. In the solution treated and aged condition, magnetic arc oscillated weldments exhibited lower tensile strength compared with the weldments made without arc oscillation due to the presence of large amounts of finer δ needles. © 2006 Elsevier B.V. All rights reserved.

Materials Science and Engineering A

Influence of magnetic arc oscillation and current pulsing on microstructure and high temperature tensile strength of alloy 718 TIG weldments

Bead on plate, full penetration electron beam welds were produced in 2 mm thickness sheets of Inconel 718 in the solution treated condition. Welds were subjected to an aging treatment with and without post-weld solution treatment. Weld microstructures, high temperature tensile properties and stress rupture properties were evaluated. The as welded fusion zone showed a considerable amount of interdendritic niobium segregation and brittle intermetallic Laves phase. The tensile and stress rupture properties of the welds after post-weld aging treatment were found to be inferior in relation to the base metal. Post-weld solution treatment at 980°C was found to result in partial dissolution of Laves phase, some reduction in niobium segregation and the formation of δ phase needles around the Laves particles. The use of 980°C solution treatment was found to improve the weld properties to some extent, although not to the level of the base metal. The reasons for this behaviour are discussed, correlating microstructures, fracture features and mechanical properties. © 2005 Institute of Materials, Minerals and Mining.

Materials Science and Technology

Microstructure and mechanical properties of Inconel 718 electron beam welds

Materials at High Temperatures

Studies on creep/stress rupture behaviour of superalloy 718 weldments used in gas turbine applications

Metallurgical Transactions A

A model for the formation and solidification of grain boundary liquid in the heat-affected zone (HAZ) of welds

Superalloys 1992 (Seventh International Symposium)

The Structure of Rene' 220C

Microfissuring of Alloy 718 in the Weld Heat-Affected Zone

Journal	Data powered by TypesetJournal of Materials Science
Publisher	Data powered by TypesetSpringer Netherlands
ISSN	00222461
Open Access	No