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.

M. Kamaraj

Department of Metallurgical and Materials Engineering

Alloys

Calorimetry

Differential scanning calorimetry

Electric arc welding

Electric welding

Filler metals

Fillers

Gas metal arc welding

Gas welding

Ionization of gases

Metals

Solid solutions

Tungsten

Welding

Welds

X ray diffraction

AGING RESPONSE

AGING TREATMENT

ALLOY 718

Gas tungsten arc welding

GAS TUNGSTEN ARC WELDING PROCESS

LAVES-PHASE

MICRO-SEGREGATION

Micro-structural characterization

Molybdenum

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.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Materials Characterization

Abstract: The micro-segregation of niobium and Laves phase formation during Gas Tungsten Arc (GTA) welding of alloy 718 were studied by employing different cooling techniques which resulted in weld cooling rates ranging from 43.2 °C/s to 508.7 °C/s. The weld cooling rates were achieved by the combinations of modified pulse wave form, argon and helium shielding gases, copper heat sink and liquid nitrogen cooling. A combination of compound current pulse (CCP) waveform with helium shielding and liquid nitrogen cooling resulted in lesser niobium segregation and discontinuous, finer Laves particles in the interdendritic regions compared to that of the conventional constant current GTA weld metal. This process yielded better ageing response and improved high temperature mechanical properties of the weldments. © 2016 Informa UK Limited, trading as Taylor & Francis Group.

Materials at High Temperatures

High temperature mechanical properties of cryogenically cooled alloy 718 weldments

The microsegregation in alloy 718 fusion zone during gas tungsten arc welding affected the aging response and the mechanical properties. The average weld cooling rate was enhanced in gas tungsten arc (GTA) welding process by employing liquid nitrogen during the process. The combination of in-process and external cooling methods reduced the microsegregation of Nb in alloy 718 fusion zone. Detailed characterization of the weld microstructures in the as-welded and the direct aged conditions was conducted. The dendritic structure have been refined to a level of 10-20 μm dendrite arm spacing and Laves particle sizes ranging from 0.04 to 1.43 μm due to the enhanced weld cooling rate. Similarly, the average Laves volume fraction was reduced from 45 to 0.1 % when compared to the conventionally cooled GTA welding process. The microsegregation was quantified through the computed solidification time through the DSC analysis and the volume fraction of the eutectic phases. It was found that the solidification time was reduced significantly from 2.499 s to as low as 0.36 s with the liquid nitrogen-cooled GTA welding process. The microhardness survey revealed an improved aging response of the fusion zone. [Figure not available: see fulltext.] © 2016, International Institute of Welding.

Welding in the World

Effect of enhanced cooling on microstructure evolution of alloy 718 using the gas tungsten arc welding process

The present investigation deals with the weldability, structure – property relationship of dissimilar metallurgical joints involving Nickel based superalloy Inconel 718 and ferritic stainless steel AISI 430 using pulsed current gas tungsten welding (PCGTAW) process employing ER2553 and ERNiCrMo-4 filler wires. NDT analysis and macrostructure examination corroborated for defect free weldments. Microstructure studies revealed the presence of unmixed zone at the weld interface of Inconel 718 whereas grain coarsening effect was observed at the heat effected zone (HAZ) of AISI 430. Both cellular and equiaxed dendrites were dominated at the fusion zone of ERNiCrMo-4 weldment; whereas different types of ferrite morphologies such as delta and widmanstätten ferrite were observed at the cap and middle zones of ER2553 fusion zone respectively. It was opined that the tensile failures occurred at the parent metal of AISI 430 for both the fillers. Charpy V-notch studies showed that ERNiCrMo-4 weldment exhibited better impact toughness than ER2553 weldment. Similarly the bend test also recommended the use of ERNiCrMo-4 filler due to the acquaintance of excellent bending strength and soundness. This study also addressed the structure – property relationships of these weldments. The outcomes of the study would be highly beneficial to the nuclear, aerospace and petrochemical industries. © 2017 Portuguese Society of Materials (SPM)

Ciência & Tecnologia dos Materiais

Investigations on the structure – property relationships of PCGTA welds involving Inconel 718 and AISI 430

Inconel 718 (2 mm thick) was welded using argon and helium gas shielded tungsten arc welding process with a filler metal. Both constant current and compound current pulse modes were applied and the cooling rates calculated. The dependence of Laves phase formation, dendrite arm spacing and niobium segregation ratios in fusion zone on the nature of shielding gases and current was studied. The maximum instantaneous weld cooling rate was achieved for the combination of Helium shielding gas and compound current pulse mode. This ultimately resulted in reduction of laves phase, segregation of niobium and dendrite arm spacing in the fusion zone. © 2013 Elsevier B.V. All rights reserved.

Journal of Materials Processing Technology

Effect of weld cooling rate on Laves phase formation in Inconel 718 fusion zone

The detrimental laves formation in fusion zone during welding of Inconel 718 is controlled with compound current pulsing technique along with helium shielding gas. Also solid solution filler wire is used to minimize the niobium segregation. Welds were produced in 2mm thick sheets by GTA welding process and subjected to the characterization techniques. The results show, refined fusion zone microstructure, reduced amount of laves phase, minimum niobium segregation and softer fusion zone in the as welded condition. © (2012) Trans Tech Publications.

Materials Science Forum

Laves phase control in inconel 718 weldments

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

The objective of the present work is to study the influence of magnetic arc oscillation and current pulsing on the fatigue behavior of alloy 718 tungsten inert gas (TIG) weldments in two different post-weld heat treatment conditions. For reference base metal data have also been presented. As the base metal did not have undesirable Laves phase, it exhibited superior fatigue resistance than the weldments. In the direct aged condition, continuous current weldments (made without magnetic arc oscillation or current pulsing) exhibited the highest fatigue resistance. This was attributed to lower % Nb in Laves of these welds despite the welds had lumpy and large amount of Laves with more interconnectivity. As magnetic arc oscillated welds had refined and lower amount of Laves with less interconnectivity, they had better fatigue resistance than the pulsed current weldments. In the solution treated and aged condition, in spite of having more Laves with higher % Nb, the continuous current weldments exhibited the highest fatigue resistance due to lesser number of δ-needles. As pulsed current welds had the largest amount of Laves with high % Nb and δ-needles, they exhibited the least fatigue resistance. As relatively higher heat input was used in pulsed current technique in the present study to get full penetration welds, the benefits of current pulsing were not realized. © 2006 Elsevier B.V. All rights reserved.

Materials Science and Engineering A

Influence of magnetic arc oscillation and current pulsing on fatigue behavior of alloy 718 TIG weldments

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.

Journal of Materials Science

The formation and control of Laves phase in superalloy 718 welds

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

Journal	Data powered by TypesetMaterials Characterization
Publisher	Data powered by TypesetElsevier Inc.
ISSN	10445803
Open Access	No