Influence of the laser source pulsing frequency on the direct laser deposited Inconel 718 thin walls

https://doi.org/10.1016/j.jallcom.2020.158095Get rights and content

Highlights

  • Different pulsed laser frequencies affect the microstructure and surface integrity.

  • The surface roughness improves upon decreasing the pulsed laser frequency.

  • The cooling rate increases when low pulsed laser frequencies are used resulting in a finer microstructure.

  • The use of the optimum frequency results in reduced segregation and fine microstructure.

Abstract

The Direct Laser Deposition (DLD) process has shown significant results in manufacturing due to its relevant flexibility to refurbish high-performance components (e.g. turbine blades or disks) or fabricating complex shaped parts. The solidification microstructure during the DLD process, is known to be controllable using different process parameters that induce changes in the grain structure, micro-segregation, and phase transformations. This work focuses on the effect the frequency of the pulsed laser has on the metallurgical characteristics of deposited thin walls. More specifically, the effect of three different pulsing rates (10 Hz, 100 Hz, 1000 Hz) during the deposition of the Nickel-based superalloys Inconel 718 has been studied and the results compared with parts produced by continuous wave laser mode. This work highlights how the pulsing rate significantly affected the thermal history, melt pool shape, grain size and its morphology, segregation region (Nb-enriched), and hardness. Finally, the microhardness was also evaluated and a correlation between the metallurgical characteristics and the pulsing rate was established.

Introduction

Additive Manufacturing (AM) processes are creating interesting opportunities in producing high-performance structures, together with specifically designed metallurgical features. Among all the AM process, the Direct Laser Deposition (DLD) technique has been widely used for repairing or cladding mechanical components (e.g. turbine blade and blisks, hard coating for complex tools for mining). Moreover, the DLD also enables the fabrication of 3D near-net-shape parts directly from CAD models [1], [2], [3].

Generally, the process consists of a laser source that creates a melt pool on a substrate, while the metallic powder is delivered from a nozzle into the melt pool and fully melted. The laser head can be mounted on a cartesian system able to move on x, y and z directions, coupled with rotational support, in order to have 5-axis of degree of freedom, alternatively it is simply positioned at the end of an arm robot. In this contest, the Nickel base superalloy Inconel 718 is widely used in aerospace applications to produce high-performance components because of its good formability, high corrosion resistance, and excellent mechanical properties up to 650 °C [4], [5]. The Inconel 718 typically consists of a matrix, the γ phase (face-centered cubic FCC) structure that provides appropriate ductility and toughness at high temperatures. Furthermore, the strengthening of the alloy is guaranteed by the presence of the precipitation phase gamma prime γ′ (Ni3Al with spherical morphology) and gamma double prime γ″ (Ni3Nb with disc morphology). This later has a metastable body-centred tetragonal structure that can be transformed into a coarse platelet stable orthorhombic δ phase, if exposed for a certain time at temperatures above 650 °C [6]. During the DLD process, the material undergoes rapid cooling and solidification, resulting in a refined dendritic microstructure. Concerning the Inconel 718, the fast Nb diffusion at the interface between the liquid and the solid phase during solidification leads to the formation of significant interdendritic Nb segregation regions. These spots in the material are characterized by brittle intermetallic, and usually undesirable Laves phase (eutectic Laves phase/γ in the last stage of the solidification) [7]. The presence of the microsegregation is unavoidable during the welding process of the Inconel 718, as reported by Radhakrishna and Prasad Rao [8]. Indeed, the formation of the Laves phase is easier due to the microsegregation of elements present in the alloy, such as Nb, Ti, and Mo because of the non-equilibrium solidification conditions. The Laves phase is characterized by a hexagonal close-packed crystallographic structure and is usually represented by a concentration of Nb ranging from 10% to 30%. It is generally known that the presence of this phase is not particularly desired due to the deleterious effect on the mechanical properties [9]. Different studies have shown that the formation and the presence of the Laves phase in the alloy deplete the matrix of the principal alloying elements (e.g. the Nb), which strongly contributes to the formation of the strengthening phases (e.g. γ′ and γ″) of the material [4], [9], [10]. Moreover, the Laves phase, and in particular the interface region between this later and the matrix,represents a nucleation point of cracks leading to the failure of the material when subjected to high or low cycle fatigue [11]. Due to the important role of the DLD process in the industrial environment, several studies were conducted to define a process strategy able to reduce and control the microsegregation formation. Antonsson and Fredriksson in 2003 studied the effect of the cooling rate on the solidification of Inconel 718, focusing their attention on the solidification sequence and the microstructural changes, as well as the Nb segregation. They discovered that the cooling rate has a significant effect on the type of microstructure that develops from the liquid phase, as well as on the segregation of the Nb. In detail, the coarseness of the matrix structure (γ phase) decreased when the cooling rate was increased, and the morphology changed from dendrites to cells. They also measured the Nb segregation depending on the cooling rate, and showed that the percentage of Nb was lower at this higher cooling rate due to the improved solubility of this phase into the γ matrix [12]. Zhang et al. investigated the effect of ultrarapid cooling on the microstructure when the Inconel 718 alloy was used in the cladding process. They observed that the high cooling rate led to a reduction of the Laves phase and a variation in its morphology which was clearly refined and widely dispersed. In detail, the ultrarapid cooling induced a significant reduction of the constitutional supercooling, limiting the Nb segregation into the Laves phase [13]. Other researchers, such as Qi et al. and Zhang et al. analysed the effect of the different heat treatment on the microstructure, mechanical properties, and Nb segregation of the Inconel 718 produced by laser cladding. Qi et al. observed that the direct age could improve the strength of the material, due to the formation of the strengthening phases, but the ductility was strongly compromised due to the presence of the Laves phase. The homogenized solution treatment and ageing were able to completely dissolve the Laves phase into the matrix, but these treatments compromised the microstructure due to the significant grain growth [14]. Zhang et al. obtained similar results which also showed the amount of the Laves phase was different from the region close to the substrate, and the top region of the deposited material [13]. As highlighted by Ma et al. and Zhai et al., the process parameters such as scan speed and the power can affect the microstructure and the microsegregation [15], [16]. Zhai et al. showed that a combination of power and scan speed could change the cooling rate, leading to fast cooling and an exceptionally fine microstructure with lower Nb segregation. Ma et al. defined a map that showed the morphology changes from cells to dendritic microstructure, depending on the power and scan speed, as well as the hardness variation depending on the energy density. Kong et al. studied the effect of different Nb-content in the IN718 alloy on the metallurgical characteristics and the mechanical properties. They noticed that if the Nb content within the alloy is lower than 3–4%, there is no formation of Laves phase and significant segregation regions. Moreover, the increase in the Nb content enhanced the formation of finer microstructure as well as the presence of the Laves phase improving thus the strength of the material [17]. New studies revealed the effectiveness of the pulsed laser during the laser metal deposition in terms of Nb segregation modification, as well as the surface roughness in the produced parts. Gharbi et al. analysed the use of pulsed regime laser, versus the continuous regime on the surface showing that the use of a pulsed laser can generate smoothing effects and improve the surface finish [18]. They did not provide any information about the effect of the pulsed regime of the laser on the microstructure, and segregation as well as mechanical properties. Xiao et al. studied the effect of the laser modes (continuous and pulsed waves) on Nb segregation and Laves formation during laser deposition of Inconel 718. They claimed that the pulsed regime promoted the formation of fine equiaxed dendrites, reduced Nb segregation, and fine discrete Laves phase due to the high cooling rate [19]. Further studies of Xiao et al., of the continuous and pulsed regime of the laser during metal deposition, highlighted that the samples produced via QCW responded better to the heat treatment and, therefore, higher hardness values were achieved [20]. Recently, Xiao et al. investigated the effects of two different frequencies when the pulsed laser was used to deposit the Inconel 718. They noticed finer microstructure and more randomly distributed segregated regions when low frequencies were used. In particular, two values of frequencies were investigated but no comparison was carried out with the continuous wave laser mode [21]. Ram et al. showed that the current pulsing during the Inconel 718 Gas Tungsten Arc Weld can control the Laves phases, and in particular its morphology (long chains of interconnected Laves phase versus smaller and well separated interdendritic region) [22]. Wang et al. observed a higher ultimate tensile strength of the samples produced by pulsed regime laser metal deposition, than the ones produced by the continuous wave laser mode. They also tested different values of duty cycle (interval of time in which the pulse is on or off), however, the results are related to stainless steel (AISI 316L) [23]. Finally, an interesting study has been carried out by Li et al. that describe, through numerical simulation, how the pulsed or continuous laser affects the thermal field, as well as the melt pool, its shape, and the microstructure [24]. Although the study showed interesting results in terms of microstructure changes and hardness due to the different laser regime, no results depending on the different frequency of pulsed laser were reported. In the previously mentioned works, the authors used a pulsed regime, but the process parameters were different and although they observed similar phenomena, the results were slightly different from each other suggesting that the pulsing parameters affect the process. Therefore, this work aims to assess the effect of different pulse frequencies of the laser on the microstructure, as well as on the segregation distribution of the Inconel 718 thin walls produced by DLD. A comprehensive analysis of the morphology and texture of the grains carried out by electron microscopy is reported and discussed. The results obtained by the thermal analysis are used to discuss the surface roughness, and the melt pool changes due to the process parameter modification (pulsed laser frequency). Moreover, the mechanical properties in terms of microhardness analysis are shown and correlated with the microstructure and the pulse frequency.

Section snippets

Material and methods

The gas-atomised IN718 powder supplied by LPW (Carpenter Technology Corporation) was employed in this experimental work, and the certified chemical composition is reported in Table 1. The average particle size of the powder is 85 µm, with 90% of the particles falling within the size range of 43–106 µm. As showed in Fig. 1a, the powder particles are mostly spherical, showing some fine satellite particles. Analysing the cross-section (Fig. 1b), some internal porosities were also observed, and

Thermal analysis of the deposited thin walls

The thermal analysis aimed to evaluate the thermal profile during the deposition, and the temperature distribution on the entire deposited parts depending on the process parameters set, for example laser mode and frequencies. The thermal analysis results were reported in Fig. 3. This demonstrates in detail the temperature cycle on a single point positioned in the middle region of the wall, and the comparisons among the CW and the different frequencies of the pulsed laser are also shown. Fig. 3b

Effect of pulsing frequency on the thermal gradient

The laser mode used during the DLD showed a significant effect on the thermal field. This developed due to the interaction among the powder, heat source and substrate seen in the initial steps of the deposition. The transition of the frequency from 10 Hz to 1000 Hz of pulsed laser was used until the CW showed an increment of heat within the melt pool, and in general in the overall deposition process. The use of the pulsed laser showed a melt pool characterized by a “heart-beat” behaviour, and,

Conclusion

In this work, the effect of different pulsed wave laser frequencies, in comparison to continuous wave laser, on the surface roughness, microstructure, segregation and hardness was investigated during DLD. Infrared imaging was used to estimate the cooling rates induced by the laser mode and frequency. The surface quality and the roughness were evaluated by SEM and laser confocal microscopy. Micro-segregation was characterised using SEM and EDX analyses while the microstructure characteristics

CRediT authorship contribution statement

Stano Imbrogno: designed the study and performed the experimental tests and the analysis of the results. Writing – review and editing under the supervision of Moataz Attallah. Abdullah Alhuzaim: contribution to the experimental tests and analysis of the results under the supervision of Moataz Attallah. Reviewing the paper. Moataz Attallah: support on experimental feasibility and infrastructure, writing – review and editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors would like to acknowledge the European research project that founded this research. The project belongs to Horizon 2020 research and innovation programme Novel ALL-IN-ONE machines, robots, and systems for affordable, worldwide and lifetime Distributed 3D hybrid manufacturing and repair operations (Project ID: 723795). AA would like to acknowledge the funding by the Royal Commission for Jubail and Yanbu (Kingdom of Saudi Arabia) for funding his Ph.D. project.

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