Microstructure and tensile properties of Inconel 718 pulsed Nd-YAG laser welds

doi:10.1016/j.jmatprotec.2004.09.081

Journal of Materials Processing Technology

Volume 167, Issue 1, 25 August 2005, Pages 73-82

https://doi.org/10.1016/j.jmatprotec.2004.09.081 Get rights and content

Abstract

Pulsed Nd-YAG laser welding characteristics of Inconel 718 sheet material (2 mm thick) were investigated. Welds were subjected to three different post-weld heat treatments: direct aging, 980 °C solution treatment + aging (980STA), and 1080 °C solution treatment + aging (1080STA). Weld microstructures and room temperature tensile properties were evaluated. Weld tensile properties in direct aged condition were found to be inferior in relation to the base metal due to the presence of Nb-rich brittle intermetallic Laves phase in fusion zone microstructure. Solution treatment at 980 °C was found to result in considerable dissolution of Laves phase, leading to some improvement in weld properties, though not to the level of base metal. The treatment was also found to result in precipitation of needle-like δ-phase around partially dissolved Laves particles. Solution treatment at 1080 °C was found to result in complete dissolution of Laves phase, but also in significant grain coarsening in the base metal, which led to considerable deterioration in base metal properties.

Introduction

Superalloy Inconel 718 is widely used for high temperature applications in aerospace, power and nuclear industries, which often involve joining by fusion welding. The alloy's weldability is generally considered to be good, largely because of its resistance to strain-age cracking due to the sluggish precipitation kinetics of its principal strengthening precipitate γ″ (b.c.t. Ni₃Nb) [1]. The alloy exhibits reasonably good resistance to weld solidification cracking, but it is prone to microfissuring in heat-affected zone (HAZ) [1], [2], [3], [4]. Another concern is segregation of Nb and consequent formation of Nb-rich Laves phase, a brittle intermetallic compound represented as (Ni, Cr, Fe)₂ (Nb, Mo, Ti), in the interdendritic regions during weld metal solidification. It is now well recognized that Laves phase is detrimental to weld mechanical properties, particularly with respect to tensile ductility, fracture toughness, fatigue and creep rupture properties as it aids in easy crack initiation and propagation, in addition to consuming significant amounts of useful alloying elements [5], [6], [7], [8], [9], [10].

While the Laves problem in alloy 718 fusion welds necessitates a thorough insight into weld microstructures (especially with regard to Nb segregation and Laves phase in quantitative terms) and mechanical properties with a sound understanding on welding process influences and structure–property–fracture correlations, existing knowledge on the same is not comprehensive enough. Available information on alloy 718 weld microstructures and mechanical properties is mostly concerned with GTA and EB welds. Radhakrishna and Prasad Rao have compared Nb segregation and Laves formation in GTA and EB welds and highlighted the relative advantages of using lower heat inputs and low heat input processes for welding alloy 718 [9], [10]. Some success has also been reported in controlling Nb segregation and Laves formation in EB and GTA welds. For example, Murthy has successfully employed beam oscillation techniques for controlling Nb segregation and Laves formation in alloy 718 EB welds [11]. Similarly, the effectiveness current pulsing in improving fusion zone microstructure and mechanical properties in alloy 718 GTA welds has been recently demonstrated [12]. In contrast, very little published work is available on laser welding of alloy 718. Much of the work published on laser welding of alloy 718 centers on welding parameter optimisation for producing sound welds with special reference to HAZ microfissuring and weld porosity [13], [14], [15] and the information available on laser weld microstructures and mechanical properties is very scanty.

Use of post-weld solution treatments is another way of approaching the Laves problem. Although some information is available in this regard [7], [10], [15], the factors influencing the extent of Laves dissolution and the various other microstructural changes and their influence on weld properties are not fully understood. In the current work, therefore, an attempt has been made to evaluate fusion zone microstructures and tensile properties of alloy 718 pulsed Nd-YAG laser welds. It is also envisaged to study the effect of post-weld solution treatment at 980 °C and 1080 °C on weld microstructure and mechanical properties.

Section snippets

Experimental details

Inconel 718 cold rolled sheets (2 mm thick) in 980 °C solution treated condition were used. The chemical composition of the base material is given in Table 1. Autogenous bead-on-plate full-penetration welds were produced in base material coupons, with weld bead running parallel to rolling direction, using a pulsed Nd-YAG laser welding machine (LUMONICS, UK) with 400 W mean power rated capacity. The welding conditions employed for making the welds are listed in Table 2, which were arrived at after

Base material microstructures

The microstructure of the base material in 980 °C solution treated condition consisted of fine equiaxed austenitic grains (ASTM 9–10) (Fig. 1a) decorated with a small amount of fine globular δ-phase (Fig. 1b). A number of MC type primary carbides and carbonitrides were also observed in the base material. Solution treatment at 1080 °C resulted in considerable grain coarsening (ASTM 3–4) and in complete dissolution of δ-phase (Fig. 1c). The primary carbides and carbonitrides were not affected by

Microstructures

Laser welding is characterised by extremely high cooling rates (of the order of 10⁴ °C/s against 100 °C/s in GTA welding), which influence several aspects of weld metal solidification. Alloy 718, being a heavily alloyed material, solidifies in dendritic mode. It is well known that the scale of dendritic structure is inversely proportional to the solidification cooling rates [16]. Thus, the rapid weld metal cooling rates inherent in laser welding are responsible for the formation of very fine

Conclusions

•
Laser welding results in lower amount of Laves phase in alloy 718 weld microstructure with fine, discrete particle morphology and with lower Nb concentration claiming significant microstructural advantages over GTA welding.
•
Post-weld solution treatment at 980 °C results in dissolution of much of the Laves phase, but do not completely eliminate it. The treatment also results in precipitation of needle-like δ-phase takes place around the Laves particles.
•
Post-weld solution treatment at 1080 °C

Acknowledgements

The authors gratefully acknowledge Mishra Dhatu Nigam Limited, Hyderabad, India, for providing Inconel 718 sheet material for this investigation. The authors are grateful to CE (A), CEMILAC, Bangalore, India, for giving permission to publish this work. The authors are thankful to Dr. Sreekanth Joshi, ARCI, Hyderabad, for his guidance in welding experiments.

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Microstructure and tensile properties of Inconel 718 pulsed Nd-YAG laser welds

Abstract

Introduction

Section snippets

Experimental details

Base material microstructures

Microstructures

Conclusions

Acknowledgements

J. Mater. Process. Technol.

Welding of Inconel 718 – a historical review

Microfissuring of Inconel 718 in the weld HAZ

J. Met.

Progress in joining of advanced materials

Int. Mater. Rev.

Effect of minor elements on weldability of nickel base superalloys

Int. Mater. Rev.

Some problems in welding Inconel 718

Welding J.

Fatigue crack growth in Inconel 718 weldments at elevated temperatures

Welding J.

Effect of heat treatment on the tensile and fracture toughness behaviour of Inconel 718 weldments

Welding J.

The effect of Laves phase on the mechanical properties of wrought and cast + HIP Inconel 718