Laser surface treatment of Inconel 718 alloy: Thermal stress analysis

doi:10.1016/j.optlaseng.2010.03.012

Optics and Lasers in Engineering

Volume 48, Issues 7–8, July–August 2010, Pages 740-749

https://doi.org/10.1016/j.optlaseng.2010.03.012 Get rights and content

Abstract

Laser heating of Inconel 718 alloy is considered and the resulting temperature and stress fields are predicted using the finite element method (FEM). An experiment is carried out to treat the alloy surface by a laser beam at high pressure nitrogen environment. The metallurgical and morphological changes in the irradiated region are examined using the Scanning Electron Microscope (SEM), optical microscope, and X-ray Diffraction (XRD). It is found that the surface hardness of the alloy improves after the laser heating process, which is due to the microstructural changes and γ-phase nitride formation in the surface region. The maximum value of the residual stress predicted in the irradiated region is close to the yielding limit of the alloy.

Introduction

Inconel 718 alloy is a nickel based superalloy and it is widely used in power industry due to its high resistance to harsh environments. However, niobium segregation resulted in large scatter in the mechanical properties. One way of avoiding the segregation is to form the fine structures through the laser controlled melting. Although the alloy has excellent resistance to oxidation, the use of the assisting gas in laser treatment process is necessary to prevent the formation of oxide species at elevated temperature in the laser irradiated region. The assisting gas is usually an inert gas and it prevents the high temperature exothermic oxidation reactions. In general, nitrogen is used as an assisting gas in laser surface treatment process. However, the formation of nitride species is possible during the heating process. In addition, high heating and cooling rates during the laser processing results in high thermal stress field in the treated region. The microhardness of the alloy can be improved through forming fine microstructures via non-equilibrium fast solidification during laser melting process. However, the large-scale Nb segregation should be avoided in the re-melted region, since it results in freckles and large-scale phases which causes micro-fissures in the irradiated region. Consequently, investigation into laser gas assisted heating of Inconel 718 and the formation of thermal stress field in the irradiated region becomes necessary.

Considerable research studies are carried out to examine the treatment of Inconel alloys by a laser beam. Laser annealing of Inconel 718 alloy was studied by Liufa et al. [1]. They showed that the matrix-strengthening phases (γ″ and γ′) in the surface layers were dissolved by the laser irradiation. The analysis of laser-induced grain boundary liquation in the heat-affected zone of Inconel 617 alloy was carried out by Luo et al. [2]. They indicated that intergranular and transgranular presence of massive NbC product phase occurred in the base metal. The effect of cooling rate on the solidification of Inconel 718 was investigated by Antonsson and Fredriksson [3]. They indicated that the effective cooling coefficient increased with increasing cooling rate. The microstructure and mechanical properties of Inconel 718 electron beam welds were investigated by Ram et al. [4]. They showed that a considerable amount of interdendritic niobium segregation and brittle intermetallic phase occurred in the laser re-melted regions. Laser assisted machining of Inconel 718 alloy was investigated by Shi et al. [5]. They introduced the heating and stress models to predict temperature and stress fields during the laser processing. The optimization study on welding of Inconel 718 alloy was carried out by Xiao et al. [6]. They indicated that mechanically sound welds with narrow fusion and heat-affected zones could be produced using the optimal processing scheme. The mechanical and microstructural characteristics of the laser-deposited Inconel alloy were studied by Blackwell [7]. He showed that increased grain size was associated with the reduced strength and the fine second phase particles (oxides, carbides) tend to inhibit the grain growth. The microstructure and mechanical properties of the laser formed shapes from Inconel 718 alloy were studied by Zhao et al. [8]. They indicated that the presence of the porosities in laser formed Inconel 718 samples resulted in the low ductility and stress rupture properties, which promoted the occurrence of the micro-porous coalescence failure in the tensile samples. The microstructures and mechanical properties of laser welded Inconel 718 were examined by Hong et al. [9]. They showed that the ductility of welds was considerably lower than that of the base material, which was attributed to the detrimental phases such as lanes on fusion zone.

In the present study, laser gas assisted re-melting of Inconel 718 alloy is carried out. Temperature and stress fields in the irradiated region are predicted using the finite element method (FEM). The experiment is carried out to examine the metallurgical and morphological changes in the laser irradiated region. The nitrogen species formed in the irradiated region is examined using the X-ray Diffraction (XRD). The microhardness variation in the surface region is measured.

Section snippets

Experimental

The CO₂ laser (LC-ALPHAIII) delivering nominal output power of 2 kW was used to irradiate the workpiece surface. The nominal focal length of the focusing lens was 127 mm employed. The laser beam diameter focused at the workpiece surface will be ∼0.3 mm. Nitrogen assisting gas emerging from the conical nozzle and co-axially with the laser beam will be used. Laser treatment conditions are given in Table 1.

Inconel 718 in sheet form with 2.5 mm thickness is used in the experiments. Material

Mathematical analysis of temperature and stress fields

Fig. 1(a) shows the schematic view of the laser heating situation. The Fourier heat transfer equation pertinent to the laser heating process can be written as $ρ \frac{DE}{Dt} = (\nabla (k \nabla T)) {+ S}_{o}$ where E is the energy gain by the substrate material, k is the thermal conductivity, and S_o is the heat source term resembling the laser beam, i.e. $S_{o} = I_{o} (1 - r_{f}) e^{(- (x^{2} + y^{2} / a^{2}))}$

I_o is laser power peak density, a is the Gaussian parameter, r_f is the surface reflectivity, ρ is the density, and x and y are the axes while the laser

Numerical simulation

Finite element discretization was carried out using the ABAQUS software [10]. The simulation is performed in Abaqus/Standard and consists of sequential thermal-stress analysis. In the sequential thermal-stress analysis, 91 572 elements are used to create the model using two element types; for the heat transfer analysis, mesh used elements of type DC3D4 (4-node Linear heat transfer tetrahedron) and stress analysis used C3D4 (4-node Linear 3D stress tetrahedron). Fig. 1(b) shows the mesh used in

Results and discussion

Laser gas assisted re-melting of Inconel 718 alloy is studied. Temperature and stress fields are predicted using the finite element model (FEM) during and after the laser treatment process. The experiment is conducted and the microstructural and morphological changes in the laser irradiated region are examined.

Fig. 2 shows temperature contours in the laser heated region while Fig. 3 shows temperature distribution along the x-axis, the laser scanning axis, at locations y=0 and z=0. High

Conclusion

Laser gas assisted melting of Inconel 718 alloy is carried out. Temperature and stress fields in the laser irradiated region are predicted using the FEM. The metallurgical and morphological changes in the laser irradiated region are examined using SEM, optical microscope, and XRD. The microhardness prior and after the laser treatment is measured. It is found that temperature decays sharply around the irradiated spot particularly inside the workpiece. Initial heating of the workpiece along the x

Acknowledgements

The authors acknowledge the support of King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia.

References (15)

P.L. Blackwell
The mechanical and microstructural characteristics of laser-deposited IN718
Journal of Materials Processing Technology
(2005)
X. Zhao et al.
Study on microstructure and mechanical properties of laser rapid forming Inconel 718
Materials Science and Engineering A
(2008)
J.K. Hong et al.
Microstructure and mechanical properties of inconel 718 welds by CO2 laser welding
Journal of Materials Processing Technology
(2008)
J. Sengupta et al.
Quantification of temperature, stress, and strain fields during the start-up phase of direct chill casting process by using a 3D fully coupled thermal and stress model for AA5182 ingots
Materials Science and Engineering A
(2005)
L. Liuf et al.
Effects of laser annealing on microstructures and hardness of Inconel 718
Rare Metal Materials and Engineering
(2005)
X. Luo et al.
Theoretical analysis of grain boundary liquation in heat affected zone of Inconel 718 alloy: study of laser weldability of Ni-base supperalloys (3rd report)
Welding Research abroad
(2002)
T. Antonsson et al.
The effect of cooling rate on the solidification of Inconel 718
Metallurgical and Materials Transactions A (Physical Metallurgy and Materials Science)
(2005)

There are more references available in the full text version of this article.

Cited by (83)

Analysis of laser-induced surface damage of single-crystal Ni-based superalloy towards improving machinability
2024, Journal of Manufacturing Processes
Single-crystal (SC) nickel-based superalloys provide improved high-temperature strength, and resistance to creep, fatigue and oxidation, compared to equiaxed and columnar-grained components due to their higher solid solution strengthening, precipitation hardening and the absence of grain boundaries. However, the SC nickel-based superalloys are ‘difficult-to-cut’. To enhance the machinability, a concept of laser-induced surface damage (LISD)-assisted machining is being studied. Laser surface modification (LSM) experiments were been performed by employing the design of experiments (DoE) strategy L18, OA (orthogonal array). The LSMed geometries were characterized to identify various laser-induced surface defects including solidification cracks, microcracks, micropores, microstructural changes, phase changes, recrystallization, and deformation. Further, parametric interactions and optimization of the process parameters were performed to manufacture a larger LISD layer. The parameter-structure-property relationship was derived systematically and the mechanisms of LISD on the SC nickel-based superalloy were described in details. The results indicate that the LISD increase with an increase in laser power followed by a decrease in scan speed and beam diameter. LISD geometries possess 1 % to 4 % of crack density and lower hardness of around 27 % than the base metal. Slot milling experiments were then conducted on the LISD specimens, which showed up to a 40 % reduction in cutting forces when compared to untreated specimens. It is evident that this methodology can be adopted to further improve the machinability of various difficult-to-cut materials.
Laser ultrasonic detection of submillimeter artificial holes in laser powder bed fusion manufactured alloys
2024, Optics and Laser Technology
In the development of additive manufacturing (AM) technology, laser powder bed fusion (LPBF) is one of the important processing methods. However, the hole defects in the fabricated samples limit the development. Laser ultrasonic (LU) technology plays a major role in the detection of LPBF parts with tiny defects, which has the advantages of non-contact and non-destructive. In this work, the detection of submillimeter internal defects in four typical LPBF alloys by LU technology is studied numerically and experimentally. A multiphysics simulation model of LU detection is established to investigate the propagation characteristics of excited ultrasonic waves in different LPBF alloys and their interaction with submillimeter artificial defects. Simulation results show that the amplitude of longitudinal (L) wave at the defect is the largest in AlSi10Mg alloy, and the amplitude of L wave in the 316L alloy, Ti6Al4V alloy and In718 alloy are very close, but their phases are slightly different. The amplitude of L wave tends to decrease nearly linearly with the increase in defect diameter. Then, four typical LPBF alloys are fabricated and measured by the LU through-transmission detection. The geometric information of artificial holes with a diameter larger than 0.2 mm are clearly characterized by the LU C-scan results, indicating the prominent applicability and feasibility of LU detection on different materials fabricated by LPBF.
Modeling fatigue behavior of additively manufactured alloys with an emphasis on pore defect morphology
2023, Journal of the Mechanics and Physics of Solids
Additively manufactured (AM) materials are prone to porosity, which limits their widespread adoption in fatigue-limited applications. Experimental campaigns are vital in understanding the effects of porosity on fatigue resistance but are costly and time consuming. This work presents a modeling framework to predict fatigue life debits in AM materials due to representative pore defects including keyhole, lack of fusion, and linearly aligned/planar pores. Materials characterization data is leveraged from specimens of alloy 718 (also known as IN718) that have been intentionally seeded with pore defect structures via a systematic modification of the process parameters. Statistically equivalent microstructure (SEM) models are generated as analogues to the experimental specimens and undergo crystal plasticity simulations. A Bayesian inference framework is used to calibrate a critical value of a damage parameter, which is used to predict the fatigue lives of SEMs seeded with pores. A strategy based on the macro-scale treatment of notches is proposed to regularize micromechanical field variables in large stress gradients in the presence of pores, or more generally, geometric discontinuities, of various sizes and morphology. The regularization volume is inversely proportional to the local pore radius of curvature and adequately smooths micromechanical fields. The framework presented here can be used to augment experimental data and rapidly evaluate the detrimental effects of porosity on fatigue resistance in AM components.
An efficient transient three-dimensional thermomechanical modeling of dynamic thermal stress building and releasing in a selective laser melting process
2022, Journal of Materials Processing Technology
The selective laser melting (SLM) process involves irradiation from a high-power laser to melt and sinter powdered material. The high temperature gradient near the laser spot generates high residual stress that can lead to failure of fabricated parts. Estimation of the residual stress for real transient SLM processes by simulation involves considerable computation effort which makes its application in real industrial SLM processes rather difficult. To tackle this challenge, a two-stage coupled transient thermomechanical model with a quasi-transient heat source was developed. In the first stage, a transient thermal analysis was performed to acquire temperature distribution of the full SLM process. The temperature distribution serves as thermal loading input for subsequent transient thermal stress analysis. This quasi-transient thermomechanical model can provide residual stress data consistent with that of a conventional full transient model in 68% less computation time. The data provided by the new model also includes the dynamic stress release due to re-heating and re-melting in overlapped laser scanned regions which was in close confirmation with experimental results. This efficient quasi-transient model is useful for rapid analysis and optimization of SLM processing parameters.
Mechanical response and wear behavior of graphene reinforced inconel 718 composite produced via hybrid accumulative roll bonding and gas tungsten arc welding process
2022, Journal of Materials Research and Technology
In this study, graphene reinforced Inconel 718 composites were produced using a novel hybrid accumulative roll bonding (ARB)–gas tungsten arc welding (GTAW) and the mechanical and wear resistance of the composite were assessed. The microstructure characterization was studied by XRD, SEM, TEM and Raman spectroscopy. The results display that GTAW approach was a sustainable way to produce Inconel 718 based composite while addition of graphene nanoplatelets (GNPs) leads to a substantial improvement in strength of superalloy matrix, as well as the wear performance enhancement. Both yield strength and tensile strength of composite specimen with 1.0 wt% GNPs was 35% and 45% higher than those of base Inconel material, and the friction coefficient and wear rate were 204% and 52% lower than those of non-reinforced Inconel 718. Such enhancement of tribological properties can be ascribed to the hardness increase of GNPs–containing specimens and the formation of GNPs protecting film on the worn exteriors.
Effect of thermal cycles on laser direct energy deposition repair performance of nickel-based superalloy: Microstructure and tensile properties
2022, International Journal of Mechanical Sciences
Solidification conditions and cooling rate can significantly affect the microstructure and the mechanical properties of Nickel-based superalloy GH4169 components repaired by laser direct energy deposition (L-DED). In this work, we employ inter-channel dwell time in the l-DED repairment of GH4169 superalloy groove in order to control the thermal cycles during deposition, thus regulating the microstructure and tensile properties of repaired GH4169 groove. The tensile properties of materials repaired by using different inter-channel dwell times were compared by tensile tests with digital image correlation (DIC) technique, and the microstructure and fracture mechanism were also investigated by scanning electron microscope (SEM). Results show that the dendrite structure, grain size of recrystallization, primary dendrite arm spacing, distribution of Laves and γ'' phases of the repaired material are significantly different under different inter-channel dwell times of 0 s, 4 s, 8 s and the associated different thermal cycles. The content of the Laves phases is lower, the grain size is smaller and distribution of γ'' phases are relatively uniform with the additional dwell time of 8 s. Owing to the difference in microstructure, the tensile strengths of both the repaired material and the substrate material increase with the increase of inter-channel dwell time. Moreover, the repaired GH4169 materials exhibit similar or even higher tensile strength than that of the substrate GH4169 material. Besides, due to the uneven precipitation of Nb element in repaired material and the associated change of Laves phases and carbides, the measured elongations of the substrate are larger than those of the repaired materials. The changes in microstructure under varying dwell times also lead to differences in strain localization and strain hardening exponent of the repaired materials. The present study paved a novel way to achieve high-quality repair of superalloys via laser direct energy deposition.

View all citing articles on Scopus

View full text

Laser surface treatment of Inconel 718 alloy: Thermal stress analysis

Abstract

Introduction

Section snippets

Experimental

Mathematical analysis of temperature and stress fields

Numerical simulation

Results and discussion

Conclusion

Acknowledgements

Journal of Materials Processing Technology

Materials Science and Engineering A

Journal of Materials Processing Technology

Materials Science and Engineering A

Effects of laser annealing on microstructures and hardness of Inconel 718

Rare Metal Materials and Engineering

Theoretical analysis of grain boundary liquation in heat affected zone of Inconel 718 alloy: study of laser weldability of Ni-base supperalloys (3rd report)

Welding Research abroad

The effect of cooling rate on the solidification of Inconel 718

Metallurgical and Materials Transactions A (Physical Metallurgy and Materials Science)