Elsevier

Acta Materialia

Volume 57, Issue 18, October 2009, Pages 5419-5429
Acta Materialia

In situ thermal imaging and three-dimensional finite element modeling of tungsten carbide–cobalt during laser deposition

https://doi.org/10.1016/j.actamat.2009.07.038Get rights and content

Abstract

Laser deposition is being used for the fabrication of net shapes from a broad range of materials, including tungsten carbide–cobalt (WC–Co) cermets (composites composed of a metallic phase and a hard refractory phase). During deposition, an unusual thermal condition is created for cermets, resulting in rather complex microstructures. To provide a fundamental insight into the evolution of such microstructures, we studied the thermal behavior of WC–Co cermets during laser deposition involving complementary results from in situ high-speed thermal imaging and three-dimensional finite element modeling. The former allowed for the characterization of temperature gradients and cooling rates in the vicinity of the molten pool, whereas the latter allowed for simulation of the entire sample. By combining the two methods, a more robust analysis of the thermal behavior was achieved. The model and the imaging results correlate well with each other and with the alternating sublayers observed in the microstructure.

Introduction

Laser deposition processes are widely reported to result in unique combinations of microstructure and properties [1], [2], [3], [4], [5]. The Laser Engineered Net Shaping (LENS®) process is a specific laser deposition method that links rapid solidification with the direct fabrication of alloy parts. This process has been used to deposit a broad range of materials, including stainless steels, nickel-based superalloys, copper alloys and titanium alloys with enhanced physical and mechanical properties [6], [7], [8], [9]. The microstructural features that control these observed property improvements have also been well documented, with a focus on the rapid solidification aspects, which lead to structural refinement [7], [8], [10]. These findings do not, however, directly apply when the material system is more complex, such as for a cermet material, which consists of both a metallic phase that can be melted by the laser and a nonmetallic phase that is not completely melted. In our previous work, we have found that LENS®-deposited cermets, such as tungsten carbide–cobalt (WC–Co) and titanium–tungsten carbide–nickel ((Ti,W)C–Ni), exhibit inhomogeneous microstructure leading to inhomogeneity in the mechanical properties [11], [12]. In order to better understand the source of the inhomogeneity, it is essential to investigate the thermal behavior within the cermet material during laser deposition. The thermal behavior can be studied by both experimental methods and numerical simulation.

Two experimental methods to characterize thermal behavior during LENS® deposition have been reported: contact measurements, such as with thermocouples, and non-contact measurements, such as with a radiation pyrometer. Thermocouple measurements provide a direct and efficient means of measuring temperature. Thermal history has been studied by inserting thermocouples into samples during deposition [8], [13], [14]. However, LENS® deposition is a rapid solidification process that occurs under non-equilibrium conditions, and solidification is completed in a few milliseconds. Recording temperature variation over such short times is a challenge with the thermocouple method, given limitations in response time as well as temperature resolution. Another limitation of this method is the inability to acquire temperature information for the molten pool and its surrounding area during the deposition process. Therefore, non-contact thermal imaging measurement, such as visible and near-infrared radiation pyrometry, has been applied within the past decade to obtain the temperature information for the area of interest for SS316 and SS410 single-pass wall builds [7], [15], [16], [17]. A high-speed single-wavelength digital charge-coupled device (CCD) video camera system was used to monitor the thermal images of an SS316 single-pass wall build during the deposition process by Hofmeister et al. [15]. A ThermavizTM two-wavelength imaging pyrometer system was used to study the thermal behavior during deposition of SS316 and SS410 by Wei et al. [16] and Wang et al. [17], respectively. Both systems can be used to investigate the thermal behavior of a deposited material in terms of temperature profile, temperature gradient and cooling rate in the molten pool and the surrounding area. Thermal imaging of cermet deposition has not, however, been reported previously.

Although the thermal imaging method can be applied to investigate the thermal changes during deposition, the spatial area that a thermal imaging system can record has a limited range if the same optical path of the laser is used, mainly focusing on the area in and around the molten pool. Thermal behavior outside the molten pool is also critical in understanding the development of the microstructure, properties and residual stress of the entire part. Accordingly, numerical simulation, such as with the finite element method (FEM) and the finite difference method (FDM), can be effectively implemented to understand the thermal history of the entire sample during deposition. A number of studies have simulated the thermal behavior during LENS® deposition [15], [18], [19], [20], [21]. An initial attempt with FEM was reported by Hofmeister et al. in 1999 on an SS316 single-pass wall part [15]. Other researchers have reported similar work on other types of steels with finer finite elements using commercial software such as Sysweld and Abaqus [18], [19], [20], [21]. However, simulations of three-dimensional components, such as a thick-wall part with multiple passes or a part with complex geometries, have not been previously reported, nor have simulations of cermet deposition.

Our prior studies on LENS®-deposited WC–Co thick-wall components showed interesting alternating sublayers in the microstructure, which led to variability in mechanical properties [11]. This inhomogeneous microstructure is believed to derive from the thermal behavior within the WC–Co materials during deposition under certain conditions, and the presence of both a molten phase (Co) and a solid phase (WC). The purpose of this study was therefore to use in situ high-speed thermal imaging in combination with three-dimensional FEM to provide a better understanding of the thermal behavior of three-dimensional cermet components during laser deposition and its influence on the inhomogeneity in microstructure.

Section snippets

Experimental methods

A high-speed digital camera system was applied in this work to record the thermal images of a WC–Co thick-wall sample during LENS® deposition. Fig. 1 shows the experimental setup for this thermal imaging system. A digital CCD video camera was placed above the LENS® system, which was stationary and always in focus, viewing the molten pool regardless of the X, Y and Z positions. A 55 mm macro lens and broad band pass filter centered at 650 nm were used in the image path. The camera, lens and filter

Numerical simulation

To establish the foundation for the numerical simulation of the thermal conditions during deposition, the material is assumed to obey Fourier’s law of heat conduction in a vector form in three-dimensional space:q=-k·T=-kxTxx-kyTyy-kzTzzwhere q (W m−2) is the heat flow conducted per unit area along the direction of (x,y,z), k (W m−1 K−1) is the thermal conductivity with the corresponding component of kx, ky and kz to the principal axes x, y, and z, T (degrees K) is the

Results and discussion

Fig. 4 shows a thermal image for the LENS®-deposited WC–Co thick-wall sample when layer 5 was being built in the direction of the white arrow. The image is colorized to show temperature in degrees K. Note that the calculated temperature results are within an accuracy of ±15 K. The green circle indicates the solid–liquid interface at the eutectic temperature (1593 K) of WC–Co. Thus, the diameter of the green circle represents the size of the molten pool. The green “gradient” curser indicates where

Conclusions

An in situ high-speed digital camera system was used in combination with three-dimensional FEM to study the thermal behavior of a three-dimensional WC–Co cermet component during the LENS® process. The area and length of the molten pool, together with the highest temperature within the molten pool, were observed by thermal imaging to increase with sample height until a steady-state was achieved. The minimum temperature gradient and cooling rate were observed near the solid–liquid interface, with

Acknowledgements

This paper is based upon work supported by the United States National Science Foundation under Grant No. DMI-0423695. Work by Sandia is supported by the United States Department of Energy under contract DE-AC04-94AL85000. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy. The authors would like to thank Professor Jean-Pierre Delplanque at UC Davis for suggestions on the modeling work, and Dr. Baolong Zheng

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