Technical paper
Laser cutting of alumina tiles: Heating and stress analysis

https://doi.org/10.1016/j.jmapro.2012.08.001Get rights and content

Abstract

Laser cutting of alumina tiles is carried out, and the temperature and stress fields developed in the cutting section are predicted numerically using ABAQUS finite element code. The morphological changes along the cut edge surfaces are examined using the optical and scanning electron microscopes. The residual stress formed in the cutting section is obtained after incorporating the XRD technique. The residual stress predicted is compared with the experimental data. It is found that the residual stress predicted agrees well with the experimental results. The dross attachment at the kerf exit is observed, which is associated with the high melting temperature of the workpiece.

Introduction

High power lasers are widely used as a machine tool due to their precision of operation, easiness of process control, net shaping, and fast and cost-effective processing. Laser cutting involves with gas assisted processing and localized heating. The phase change takes place during the localized heating process. The assisting gas is inert in general and has two-fold effects: (a) provides protection of the irradiated surface from the oxidation reactions and (b) purges the molten material from the cutting section. During the localized heating, the high temperature gradients are formed in the vicinity of irradiated region. Although, the elastic modulus of the substrate material reduces considerably at elevated temperatures, high temperature gradients result in excessive stress levels in the irradiated region, which limits the practical applications. Moreover, ceramics are difficult to machine using the conventional techniques because of their high toughness, surface hardness, and brittleness. Laser machining can offer possible solution to the machining problem associated with the mechanical properties of ceramics. The model study of laser heating and thermal stress gives insight into the physical process, which takes place during the cutting process. Consequently, investigation into laser cutting process and thermally induced stress development in the cutting section becomes essential.

Considerable studies were carried out to examine the laser processing of ceramics. The comparison of volumetric and surface heating sources in modeling of laser melting of ceramic materials is carried out by Li et al. [1]. They showed that that the model incorporating the volumetric heating source was more accurate than that incorporating the surface heat source. The extensive review on laser machining of the structural ceramics was presented by Samant and Dahotre [2]. They indicated that laser machining emerged as a potential technique for machining the structural ceramics. Laser surface processing of metals and ceramics for industrial applications was examined by Bergmann et al. [3]. They indicated that micromachining using the high power excimer laser was not restricted to planar geometries and the excimer lasers produced clean cuts with no burning as compared to mechanical processes. The damaged-reduced laser cutting of thick ceramics using a simultaneous pre-score approach was carried out by Pereles-Santiago et al. [4]. They indicated that the method adopted was capable of dictating the path when the fracture could not be oxidized. Laser cutting of alumina with a control fracture technique was investigated by Tsai and Chen [5]. They showed that the tensile stress generated at the surface due to laser beam separated the material along its path. Nd:YAG laser micro-cutting of alumina–aluminum interpenetrating phase composition was examined by Biswas et al. [6]. They performed the response surface methodology based on optimal parametric analysis to achieve the minimum surface roughness of the micro-cut section. The cutting of mullite–alumina ceramic plates with CO2 laser was examined by Quintero et al. [7]. They demonstrated that the cut quality could be improved when an assisting gas injection system consisting of off-axis De Laval nozzle was used. Laser inert gas cutting of thick-section stainless steel and medium-section alumina was investigated by Wandera et al. [8]. They showed that the clean cut surfaces with the minimal dross attachment were achieved after proper setting of the cutting parameters. In addition, they indicated that the surface roughness of the cut section was reduced through increasing the assisting gas pressure. Laser cutting of aluminum nitride and thermal stress induced fracture were examined by Molian et al. [9]. They showed that the thermal stress model introduced offered significant benefits in laser machining, such as improved precision, good cut quality, high cutting speed and low energy losses. A statistical analysis of striation formation during laser cutting of ceramics was introduced by Wee et al. [10]. They showed that striation wavelength and upper and lower striation lengths were most influenced by the interaction time and the irradiance. Laser cutting of alumina using active stressing technique for fracture delaying was carried out by Akarapu and Segall [11]. They used the probabilistic fracture mechanism to quantify the influence of the induced compressive stresses on the time and nature of the fracture.

Although laser cutting of alumina was investigated previously, the thermal stress field and residual stress formation due to circular cutting of large diameter holes were not considered in the previous studies [9], [10], [11]. Since the thermal stress developed around the cut edges dependents on temperature gradients and thermal expansion/contraction of the cut edges, the stress field around the circular cut differs from the straight cuts [9], [10], [11]. In the present study, laser circular cutting of alumina tiles is carried out. Temperature and stress fields are predicted using the finite element model. The morphological changes in the cut sections are examined using scanning electron and optical microscopes. The residual stress formed within the cut surfaces is obtained using the XRD technique.

Section snippets

Heating and thermal stress analysis

Fig. 1 shows the schematic view of the laser circular cutting and the coordinate system. In the analysis, the solid body heat conduction with temperature-dependent conductivity, internal energy (including latent heat effects), and convection and radiation boundary conditions are considered. The Fourier heat transfer equation pertinent to the laser heating process can be written as:ρDEDt=((kT))+Sowhere E is the energy gain by the substrate material, k is the thermal conductivity, and So is the

Numerical simulation

Finite element discretization was carried out using the ABAQUS software [13]. The simulation is performed in ABAQUS/Standard and consists of sequential thermal-stress analysis. In the sequential thermal-stress analysis, 142,468 elements are used while 132,140 hexahedral elements are used for the thermal-stress analysis. In addition, for the heat transfer analysis, mesh used elements of type DC3D8 (8-node linear heat transfer brick) and stress analysis used C3D8 (8-node linear stress brick).

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 ∼0.9 mm. Nitrogen assisting gas emerging from the conical nozzle and co-axially with the laser beam was used. Laser cutting experiments were repeated incorporating the different levels of laser cutting parameters. The level of laser cutting parameters

Results and discussion

Laser circular cutting of alumina is carried out and temperature and stress fields developed in the cutting section are predicted using ABAQUS code [13]. The morphological changes in the cutting section were examined using the optical and scanning electron microscopes. The residual stress formed in the cutting section is obtained using the XRD technique. Since the circular cutting is investigated, the arc generated during the cutting and locations, at which temperature stress fields are

Conclusion

Laser circular cutting of alumina tiles is carried out. Temperature and stress fields developed during the cutting process are predicted using ABAQUS numerical code. The morphological changes at the cut edge surfaces are examined using optical microscope and SEM. The residual stress developed in the cut edge surface is obtained after incorporating the XRD technique. It is found that temperature at the surface reaches slightly above the melting temperature of the substrate material. The

Acknowledgements

The authors acknowledge the support of King Fahd University of Petroleum and Minerals and Karmetal AS in Turkey. Dhahran, Saudi Arabia for this work.

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