Laser controlled melting of pre-prepared inconel 718 alloy surface

https://doi.org/10.1016/j.optlaseng.2011.06.002Get rights and content

Abstract

Laser treatment of Inconel 718 alloy surface is carried out. The alloy surface is coated with a carbon layer containing 7% TiC particles prior to the laser treatment. The carbon coating provides increased absorption of the incident laser beam and holds TiC particles. The microstrutural and morphological changes in the laser treated region are examined using optical and scanning electron microscopes, energy dispersive spectroscopy, and X-ray diffraction. The microhardness of the surface is measured and the residual stress formed at the surface vicinity is determined from the XRD technique. It is found that partial dissolution of carbide particles takes place at the surface. The composition of fine grains at the surface vicinity, nitride compounds formed, and dissolution of Laves phase at the surface region enhances the hardness at the treated surface. In addition, laser treated surface is free from the micro-crack network and cavities.

Highlights

► Partially dissolution of carbide particles takes place at the surface. ► Composition of fine grains at the surface vicinity, nitride compounds formed. ► Dissolution of Laves phase at the surface region enhances the hardness at the treated surface. ► Laser treated surface is free from the micro-crack network and cavities.

Introduction

Inconel 718 alloy is widely used in gas turbine industry and nuclear power plants due to its mechanical properties at high temperatures. The hardness and wear resistance of the alloy surface can be improved through surface processing techniques. Laser treatment of the alloy surface offers considerable advantages over the conventional methods. Some of these advantages include fast processing, precision of operation, and low cost. In addition, laser surface treatment technique is involved with controlled melting, through which the hard particles such as TiC can be injected at the surface vicinity during the treatment process. However, niobium segregation, Laves phase, and microfissures could occur in the vicinity of the heat affected zone after the laser processing, which limits the practical applications of the laser processed surfaces. Consequently, investigation into laser treatment of Inconel 718 alloy and microstructural changes in the treated region becomes essential.

Considerable research studies were carried out to examine laser treatment of Inconel alloys. Microstructural and mechanical properties of laser welded Inconel 718 workpieces were examined by Hong et al. [1]. They showed that modified cyclic solution treatment reduced microfissures and improve tensile and fatigue properties. The microstructure and mechanical properties of laser treated Inconel 718 alloy were studied by Zhao et al. [2]. The results revealed that a continuous thin film of Nb rich film was present and carbides along the grain boundaries at the fracture surface caused crack initiation and propagation along the fracture path. The mechanical and microstructural characteristics of laser deposited Inconel 718 alloy were investigated by Blackwell [3]. He indicated that prior to hot isostatic processing, Inconel 718 deposit exhibited anisotropic properties; however, hot isostatic processing reduced the anisotropy within Inconel 718 deposit. Microstructure and tensile properties of Inconel 718 laser welds were investigated by Ram et al. [4]. They showed that solution treatment at 980 °C resulting in considerable dissolution of Laves phase leading to some improvement in properties of weld section. In addition, solution treatment caused precipitation of needle like δ-phase around partially dissolved Laves particles. Fracture toughness analysis of laser beam welded Inconel 718 and Inconel 625 alloys was carried out by Yeni and Kocak [5]. The ductile crack growth analysis revealed that laser beam welded region exhibited no significant changes in mechanical properties; however, slight decrease in fracture toughness of the fusion and heat affected zones was observed. Laser assisted machining of Inconel 718 alloy sheets was studied by Anderson et al. [6]. The results predicted from the economic analysis of machining revealed that laser assisted machining of Inconel 718 had significant benefits over conventional machining. Laser assisted finish turning of inconel 718 was carried out by Tavakoli et al. [7]. They indicated that a significant drop in the cutting force was observed when thermal softening caused by the laser power was in effect. The recast layer formed during the laser trepan drilling into Inconel 718 was investigated by Chien and Hou [8]. The results indicated that the assist gas pressure, the peak power, and the focal position exerted the greatest influence on the recast layer thickness. The effect of laser scanning path on microstructures and mechanical properties of laser solid formed Inconel 718 alloy was examined by Liu et al. [9]. They showed that the inhomogeneity of grain size was considered to be the prime reason for the worse ductility of the laser scanned workpieces. Laser treatment of Inconel alloy surfaces and the potentiodynamic response of the resulting surface in electrolytic liquid were investigated by Yilbas et. al. [10]. They found that laser treatment reduced the corrosion resistance of the treated surface because of the irregular surface texture resulted after laser treatment process. Laser gas assisted surface treatment of Inconel 718 was studied by Yilbas et. al. [11]. They showed that the hardness of the treated surface increased notably because of the grain refinement and nitride species formed at the surface vicinity.

In the present study, laser treatment of Inconel 718 alloy surface is carried out. The workpiece surfaces are pre-prepared prior to laser treatment process. In the surface pre-preparation cycle, carbon film containing 7% of TiC particles is formed using the water soluble phenolic resin. The carbon film provided improved absorption of the incident film and formation of carbonitride compounds at the surface vicinity during the laser treatment process. The microstructural and morphological changes in the laser treated region are examined using optical and scanning electron microscopy (SEM), energy, dispersive spectroscopy (EDS), and X-ray diffraction analysis. The residual stress formed is measured after incorporating the XRD technique.

Section snippets

Experimental

A CO2 laser (LC-ALPHA III) delivering a nominal output power of 2 kW in pulse mode with different frequencies was used to irradiate the workpiece surface. The nominal focal length of the focusing lens was 127 mm. The laser beam diameter focused at the workpiece surface was 0.3 mm. Nitrogen assisting gas was fed through a conical nozzle co-axial with the laser beam. The laser treatment parameters are given in Table 1.

Inconel 718 samples of 15 mm×13 mm×3 mm were used in the experiments. The water

Results and discussion

Laser controlled melting of Inconel 718 alloy surface is carried out. A carbon film of about 40 μm thickness and containing 7% TiC particles is formed at the workpiece surface prior to laser treatment process. The microstructure and morphological changes in the laser treated region is examined.

Fig. 2 shows SEM micrograph of the top view of the laser treated workpiece surface. It can be observed that laser treated surface consists of closely spaced regular laser tracks. The width of each track is

Conclusion

Laser treatment of Inconel 718 surface at high pressure nitrogen gas environment is carried out. The carbon film containing 7% TiC particles is formed at the workpiece surface prior to laser treatment process. The formation of carbon film provides enhancement of laser beam absorption at the surface and hosting TiC particles. The microstructural and metallurgical changes in the laser treated region is examined using optical and scanning electron microscopes, energy dispersive spectroscopy, and

Acknowledgments

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

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