Elsevier

Optics & Laser Technology

Volume 84, October 2016, Pages 9-22
Optics & Laser Technology

Full length article
Effects of laser processing parameters on thermal behavior and melting/solidification mechanism during selective laser melting of TiC/Inconel 718 composites

https://doi.org/10.1016/j.optlastec.2016.04.009Get rights and content

Highlights

  • Cooling time during powder delivery was taken into account to simulate the actual production process.

  • Thermal behavior and further melting/solidification mechanism were proposed.

  • Forming mechanism of the cross-sectional configuration of the molten pool was analyzed.

  • Experimental study was implemented to validate the obtained results in simulation.

Abstract

A three-dimensional finite element model is proposed to study the effects of laser power and scan speed on the thermal behavior and melting/solidification mechanism during selective laser melting (SLM) of TiC/Inconel 718 powder system. The cooling time during powder delivery is taken into account to simulate the actual production process well. It shows obviously the existence of heat accumulation effect in SLM process and, the tailored set of cooling time of 10 ms during powder delivery alleviates that effectively. The maximum temperature gradient in the molten pool slightly increases from 1.30×104 °C/mm to 2.60×104 °C/mm as the laser power is increased from 75 W to 150 W. However, it is negligibly sensitive to the variation of scan speed. There is a positive corresponding relationship between the maximum rate of temperature change and processing parameters. A low laser power (75 W) or a high scan speed (300 mm/s) is more energy efficient in Z-direction of the molten pool, giving rise to a deep-narrow cross section of the pool. Whereas, a high laser power (150 W) or a low scan speed (50 mm/s) causes a shallow-wide cross section of the molten pool, meaning it is more energy efficient in the Y-direction of the melt. The combination of a laser power of 125 W and a scan speed of 100 mm/s contributes to achieve a sound metallurgical bonding between the neighbor layers and tracks, due to the proper molten pool size (width: 109.3 µm; length: 120.7 µm; depth: 67.8 µm). The SLM experiments on TiC/Inconel 718 powder system are performed to verify the reliability and accuracy of the physical model and, simulation results are proved to be correct.

Introduction

Nickel-based superalloys are widely used as high-temperature service parts in industrial and aerospace fields in which perfect combination of elevated temperature workability and mechanical properties are required [1]. Inconel 718, the typical representative of nickel-based superalloys, due to its outstanding oxidation resistance, hot corrosion resistance and fatigue resistance, has been recognized as the most promising candidate for many applications such as turbine wheel blades, rocket motors, nuclear reactors and fossil fuel components at elevated temperature [2]. Furthermore, Inconel 718 based metal matrix composites (MMCs), reinforced with discontinuous TiC reinforcements, show the superiority compared with the pure parts for significantly increased not only of strength and hardness, but also of elasticity modulus and wear resistance, amplifying effectively the serviceable range of Inconel 718 [3]. However, the poor wetting ability between the reinforcing particles and the matrix extremely reduces the particle/matrix interfacial bonding due to the formation of microcracks or pores, thus resulting in the premature failure of MMCs during mechanical loading [4]. Considerable research efforts are required to study the liquid wetting effect of powder particles for alleviating the pores and microcracks between the particle/matrix interface and, thus, improving their bonding ability. Meanwhile, the conventional manufacturing processes, such as casting process, easily cause the grain coarsening and shrinkage cavity/porosity and, they are difficult to manufacture Inconel 718 composites with complex geometry [5]. These put forward new requirements on the material forming techniques.

Selective laser melting (SLM), as one of the most important branches of additive manufacturing (AM) techniques, exhibits a great potential for direct fabricating 3D parts with flexibility in materials and shapes [6], [7], [8], [9]. During the SLM process, a large amount of powder comes up to the build table and a roller/blade spreads powder at the metal substrate in order to deposit powder layers with well designed thickness. Fully dense parts are created by a laser beam with a high intensity in a special scanning strategy, local melting and subsequent solidification of the powder bed in successive layers [10]. As the laser beam is scattered through the powder bed, the energy is absorbed by powder particles via both powder-coupling and bulk-coupling mechanisms [11]. The powder system experiences a rapid cooling rate (up to 106–7 °C/s) during the interaction between the powder particles and the laser beam, which has a substantial effect on the generation of fine and uniform microstructure, as well as resultant mechanical properties of final components [12], [13]. The materials in SLM process have a tendency to experience a significant non-equilibrium physical and chemical metallurgical process, exhibiting multiple modes of heat, mass and momentum transfer [14]. The typical defects associated with SLM such as “balling effect” , residual porosity, stress induced microcracks, warpage induced dimensional accuracy and delamination tend to occur under inappropriate processing parameters (laser power and scan speed) [15], [16]. Therefore, to obtain the desired SLM-fabricated parts, the significant research efforts are required to study the relationship between processing parameters and melting/solidification mechanism. However, the experimental measurements of thermal-physical statistics during SLM process are considered to be difficult for it involves the fast moving of laser energy source, the limited liquid lifetime of the molten pool and the extremely high cooling rate of the elevated-temperature melt. Consequently, the numerical simulation approach is typically chosen as an alternative to solve the problems mentioned above.

In fact, some heat conduction three-dimensional models have been established to investigate the thermal behavior and further melting/solidification mechanism during SLM process recently. Gusarov et al. [17] simulated and analyzed the temperature distribution of steel 316 L powder bed during SLM process. The analysis of the capillary stability of the segmental cylinder applied to the calculated melt pool estimated the stability of the process depending on the scanning velocity, powder layer thickness, and the material optical and thermal properties in the developed model. The results showed that the thermal-physical phenomenon was highly dependent on the processing parameters (laser power, scan speed, thickness of powder layer, properties of materials, etc.). Roberts et al. [18] proposed a three dimensional thermal model for fabrication of TiAl6V4 parts, considering the temperature-dependent thermal-physical properties, which is beneficial to improve the computational accuracy. This model involving multiple layers was equally of great importance because the thermal interactions of successive layers affected the temperature gradients, which governed the heat transfer and thermal stress development mechanisms. The work predicted the transient temperature distribution during the production. It was found that the laser region experienced rapid thermal cycles accompanied with commensurate stress cycles. Ali Foroozmehr et al. [19] conducted the finite element simulation of steel 316 L SLM process considering optical penetration depth and, each calculating step is divided into some smaller sub-steps where the change in the materials properties from “powder” to “solid” occurred for improving calculation precision. They paid attention to the influences of the different scan speeds on the melt pool depth, width, and length. It suggested that the melt pool size varied from the beginning of a track to its end and from the first track to the next one. The melt pool size, however, reached a stable condition after a few tracks. To date, although there are some numerical simulations have been carried out to investigate the thermal behavior during laser processing of metal powder system, the rare researches focus on the thermal behavior and melting/solidification mechanism during SLM of TiC/Inconel 718 powder system. Meanwhile, the simulation three-dimensional models are needed to be further perfected and optimized for more precise calculation and simulating the actual fabrication process excellently.

In this investigation, a three-dimensional finite element model was established based on the models mentioned above to predict the relationship between melting/solidification mechanism and processing parameters, using ANSYS 13.0 software. Latent heat of phase change, multiple heat transfer mechanisms, temperature-dependent thermal physical properties and cooling time during powder delivery were considered in order to precisely simulate the actual production process, as well as improve the calculation accuracy. Moreover, the movement of Gaussian laser source and the laser energy loading of multi-layer and multi-track were realized using APDL secondary development language. The effects of laser power and scan speed on the thermal behavior and the forming mechanism of cross-sectional configuration of the molten pool were analyzed. The formation mechanisms of processing defects, such as “balling effect” , warpage, pores, microcracks, delamination, etc. were discussed in order to optimize the processing parameters and obtain desired SLM-produced parts. Meanwhile, the corresponding experiments were also implemented to investigate the microstructure of the SLM-produced components with different laser processing conditions for verifying the reliability and accuracy of the model proposed in this paper.

Section snippets

Physical description of SLM

Fig. 1 depicts the schematic overview of the interaction zone between laser radiation and powder. As the top surface of the powder bed is irradiated by the incident laser beam, a small fraction of laser energy is dissipated by radiation and convection. The remainder, the vast majority of laser energy, is absorbed by powder particles, leading to the rapid heating and resultant localized melting. After the moving Gaussian laser heat source leaves the melt region, which is called molten pool,

Powder materials

The powder used in this study as starting materials includes the spherical Inconel 718 powder with a size distribution of 15–45 µm, which is produced by gas atomization method, the TiC nanopowder with polygonal shape and its size in an average of 50 nm. The chemical compositions of Inconel 718 powder are shown in Table 4. The Inconel 718 and TiC components with a weight ratio of 0.75:0.25 are mixed homogeneously in a Fritch Pulverisette 6 planetary ball mill (Fritsch GmbH, Germany) with a

Characteristics of temperature distributions in SLM process

Fig. 3 shows the transient temperature distribution on the top surface and longitudinal view of the molten pool as laser beam reaches Point 1, Point 2 and Point 3, respectively, with v of 100 mm/s and P of 125 W (Fig. 2(b)). At the center of the first track (Point 1, Fig. 2(b)), the isotherm curves on the top surface of the molten pool are similar to a series of ellipses and, the ellipses of fore part are more intensive than those at the back-end of it. Meantime, it is asymmetry of isotherms

Conclusions

The three-dimensional finite element model, considering the cooling time for powder delivery, temperature-dependent properties, multiple heat transfer mechanisms and latent heat of phase change, was established to mainly focus on the effects of processing conditions on the thermal behavior and further melting/solidification mechanism, as well as forming mechanism of the molten pool configuration, and the following conclusions were drawn.

  • (1)

    The maximum temperature, the width, the length, and the

Acknowledgments

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Nos. 51575267 and 51322509), the Top-Notch Young Talents Program of China, the Outstanding Youth Foundation of Jiangsu Province of China (No. BK20130035), the Program for New Century Excellent Talents in University (No. NCET-13–0854), the Science and Technology Support Program (The Industrial Part), Jiangsu Provincial Department of Science and Technology of China (No. BE2014009-2),

#1 Qimin Shi Nanjing University of Aeronautics and Astronautics, Nanjing, China. Qimin Shi is currently doing his master’s degree in College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics (NUAA), PR China. He received the bachelor degree in Materials Processing Engineering from NUAA in Jun. 2015. His principal research interest is laser additive manufacturing including selective laser melting (SLM), and laser metal deposition (LMD), especially majoring

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#1 Qimin Shi Nanjing University of Aeronautics and Astronautics, Nanjing, China. Qimin Shi is currently doing his master’s degree in College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics (NUAA), PR China. He received the bachelor degree in Materials Processing Engineering from NUAA in Jun. 2015. His principal research interest is laser additive manufacturing including selective laser melting (SLM), and laser metal deposition (LMD), especially majoring in the experiments and simulation of nickel-based composites processed by SLM.

#2 Dongong Gu (Corresponding author) Nanjing University of Aeronautics and Astronautics, Nanjing, China. Professor Dongdong Gu is currently a Full Professor in College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics (NUAA), PR China. He received the Ph. D. in Materials Processing Engineering from NUAA in Jun. 2007. From Sep. 2009 to Aug. 2011, he worked in Fraunhofer Institute for Laser Technology ILT as the Alexander von Humboldt Research Fellow. His principal research interest is laser-based additive manufacturing including selective laser melting (SLM), direct metal laser sintering (DMLS), and laser metal deposition (LMD). Prof. Gu has authored/co-authored 3 books and more than 100 papers in a number of internationally recognized peer-reviewed journals.

#3 Mujian Xia Nanjing University of Aeronautics and Astronautics, Nanjing, China. Mujian Xia is currently a Ph. D candidate in College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics (NUAA), PR China. He received the Master Degree in Materials Processing Engineering from Jiangsu University in Jun. 2012. Starting from Sep. 2015, he studied in College of Materials Science and Technology, NUAA. His principal research interest is laser-based additive manufacturing including selective laser melting (SLM) and direct metal laser sintering (DMLS), especially majoring in the experiments and simulation of nickel-based composite materials by SLM.

#4 Sainan Cao Nanjing University of Aeronautics and Astronautics, Nanjing, China. Sainan Cao is currently pursuing for a master’s degree in College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics (NUAA), PR China. She received the bachelor degree in Materials Processing Engineering from NUAA in Jun. 2014. Her principal research interest is laser additive manufacturing including selective laser melting (SLM), direct metal laser sintering (DMLS), and laser metal deposition (LMD). She has authored/co-authored 3 papers in internationally recognized peer-reviewed journals including J Laser Appl, Mater Sci Eng A, J Mater Res.

#5 Ting Rong Nanjing University of Aeronautics and Astronautics, Nanjing, China. Ting Rong is pursuing her master's degree in College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics (NUAA), PR China. She received the bachelor degree in Materials Processing Engineering from NUAA in Jun.2015. Her principal research interest is laser additive manufacturing including selective laser melting (SLM), and laser melting deposition (LMD), especially majoring in the experiments of nickel-based composites processed by SLM.

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