**1. Introduction**

Alloy 718 was developed almost 60 years ago [1] and it has been the most widely used Ni-based superalloy to date. Being a precipitation-strengthened superalloy, it has been broadly used for the manufacturing of both land-based energy and aircraft turbine components, showing an outstanding performance at working temperatures up to 700 ◦C under high structural loading and corrosive conditions [2].

While other precipitation-hardened Ni superalloys have relatively high amounts of gamma prime (γ') former elements, i.e., Al and Ti, alloy 718 is based on the addition of Nb, which forms metastable gamma double-prime (γ") precipitates of Ni3Nb. The precipitation

**Citation:** Álvarez, P.; Cobos, A.; Vázquez, L.; Ruiz, N.; Rodríguez, P.P.; Magaña, A.; Niklas, A.; Santos, F. Weldability Evaluation of Alloy 718 Investment Castings with Different Si Contents and Thermal Stories and Hot Cracking Mechanism in Their Laser Beam Welds. *Metals* **2021**, *11*, 402. https://doi.org/10.3390/ met11030402

Academic Editors: João Pedro Oliveir and Zhi Zeng

Received: 3 December 2020 Accepted: 9 February 2021 Published: 1 March 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

kinetics of γ" is slower compared to γ (Ni3Al, Ni3Ti and Ni3(Ti, Al)), which contributes to improve castability, hot working, and weldability. The improvement is basically due to the fact that alloy 718 remains in a softer state during these manufacturing processes, avoiding the build-up of internal stresses. In terms of weldability, the sluggish precipitation kinetics of alloy 718 minimises strain age cracking (SAC) after welding and during post-welding heat treatment [2].

Alloy 718 investment castings are usually melted and poured inside vacuum furnaces and subsequently heat treated by hot isostatic pressing (HIP) to ensure highest performance. Intermetallic phases such as NbC and Nb-rich Laves phase can be found in alloy 718 castings due to segregation of chemical elements during slow solidification [3,4]. These secondary phases solidify at low temperatures (e.g., γ/Laves eutectic at temperatures down to 1180 ◦C) and they are usually concentrated along grain boundaries. Melting of Laves phases that are formed during the terminal solidification has been identified as the origin of the higher cracking susceptibility of castings during welding in comparison with wrought alloy 718 [4,5]. The reason for this is that Laves phase is readily melted upon heating in contrast with constitutional liquation of NbC that requires a dissolution reaction to form a liquid [2,6]. The latter is the predominant liquation mechanism identified in welds of wrought parts [2,6,7].

Incipient melting of Laves phase gives rise to a liquid which is distributed along grain boundaries, drastically reducing the strength of the material and its capability to withstand stresses. Therefore, sophisticated thermal treatments have been developed with the aim of reducing the amount of Laves phases and consequently heat-affected zone (HAZ) liquation cracking susceptibility of alloy 718 castings [4,8,9]. The aim of these treatments is to solubilise deleterious Laves phase and reduce compositional gradients in the as-cast microstructure. In fact, the concentration of several residual elements such as B, P, and S in grain boundaries can promote HAZ liquation cracking to a higher degree by decreasing even more the initial melting temperature and modifying the wetting characteristics of the intergranular liquid [4,10–12].

Formation of γ/Laves eutectic can also cause fusion zone (FZ) cracking in alloy 718 welds. This eutectic solidifies at a much lower temperature than the bulk matrix and wi-dens the solidification temperature range [2,4]. Wider solidification temperature range is directly associated with higher hot cracking susceptibility due to a longer coexistence of solid and liquid phases.

In order to study hot cracking susceptibility of Ni superalloys, researchers have defined and implemented different weldability assessment trials [2]. They are usually classified into three categories.

Self-restrained or representative tests use the inherent strain of the welding to induce cracking. They try to reproduce real joint configuration and residual stress levels. The drawback of this type of test is that it does not give quantitative values of cracking susceptibility and the result only indicates if the weld cracks pop up or not. Sometimes circular welding paths are applied to induce higher residual stresses.

In simulative tests, either a tension or bending deformation is externally applied du-ring welding. The Varestraint test is probably the best known test in this category, entai-ling the application of bending deformation along the longitudinal direction of the weld. The development of the Varestraint testing method from its origin was thoroughly reviewed by Andersson et al. [13]. Deformation enhances hot cracking and its extension, i.e., number and length of cracks, which can be determined at different strain levels. In this way, cracking susceptibility of different materials can be compared [14,15]. However, in real welding applications, residual stresses are predominant in contrast with plastic strain.

Finally, both strength and ductility are directly measured at high temperatures in hot ductility tests. In these tests, the temperature at which the material loses complete its strength (nil strength temperature (NST)) is determined by heating up testing samples under a constant tensile load. Additionally, nil ductility temperature (NDT), corresponding to the peak heating temperature at which the area reduction of the broken surface

is 0%, and the ductility recovery temperature (DRT), at which 5% of area reduction is reco-vered after cooling down from a temperature close to NST, are computed. NDT and DRT are determined from on-cooling curves and the strain is only applied when the testing temperature has been reached. Hot ductility behaviour is linked to hot cracking susceptibility since cracks are generated when the material cannot accommodate stresses and strains induced during welding [2,9].

In a recent paper [16], the current authors compared the hot cracking susceptibility of wrought and investment casting alloy 718 by Varestraint test while applying pulsed and continuous tungsten inert gas (TIG) welding and LBW. It was concluded that hot cracking was enhanced in LBW samples due to extended centre line fusion zone (FZ) cracking showing a fishbone-like cracking pattern. Minor influence of pulsation mode and grain size was observed and, in fact, casting samples with grain sizes 30 times coarser showed slightly better performance than wrought material. It must be noted that Laves phases were not observed in investment casting samples and only some traces of needle-like delta (δ) phase and Mo sulphide were detected in base material.

Pulsed current can refine solidification microstructure and reduce the amount of Laves phase and Nb segregations in TIG welds according to [17,18]. Moreover, Bai et al. [19] recently investigated the potential benefits of combining high-frequency micro-vibration and LBW. Under particular vibration frequencies, the length of the liquation cracks in HAZ was reduced, but not completely avoided. The authors performed bead-on-plate tests and obtained weld cross sections with nail or mushroom shape. LBW has also been studied in alloy 718 parts by other researchers [20–25]. These investigations targeted the influence of LBW parameters and energy input on porosity, microstructure, and mechanical properties of these welds. A deep analysis of HAZ cracking susceptibility of this alloy during LBW was performed in particular by [20,22]. Impact of weld shape morphology, grain size, pre-weld heat treatment, and boron segregation were investigated.

In this work, weldability and hot cracking susceptibility in laser beam welds of five alloy 718 investment castings were investigated. Influence of chemical composition, Si content, solidification rate, and pre-weld heat treatment was studied by carrying out three different weldability tests. Cracking behaviour was compared, and results were completed with the detailed microstructural analysis after welding tests. Additionally, influence of preweld heat treatment on microstructure of parent material is discussed. An analysis about the correlation of weldability assessment test results, i.e., Varestraint and hot ductility, with cracking trend observed in real bead-on-plate LBW trials was performed. Fundamentals of the mechanism that triggers HAZ cracking in bead-on-plate test are explained.
