Integrated Robotic Plasma Spraying System for Advanced Materials Processing

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Integrated Robotic Plasma Spraying System for Advanced Materials Processing Weisheng Xia1,2 , Haiou Zhang2 , Gui-Lan Wang1 , Yunzhen Yang1 , Guangchao Han3 , and Haiping Zou1 1 State Key Laboratory of Materials Processing and Die & Mould Technology Huazhong University of Science and Technology, Wuhan 430074, China 2 State Key Laboratory of Digital Manufacturing and Equipment Technology Huazhong University of Science and Technology, Wuhan 430074, China 3 China University of Geosiences, China

Abstract— During atmospheric plasma spraying (APS), to control the time dependent D.C. plasma jet behavior requires the comprehensive understanding of its electric, magnetic, thermal, thermodynamic phenomena. In this paper, influence of particles injection with the form of suspension on the fluctuation of plasma jet is analyzed, and a control approach is presented to eliminate the effect. Moreover, an integrated robotic plasma spraying system for advanced materials processing, i.e., rapid metal tooling and solid oxide fuel cell (SOFC), is developed, which combines a PC-based controller with a six-axis robot. The intelligent adaptive adjustment of robot spraying trajectories and self-dispatch of manufacturing strategies were carried out by the resultant system according to the feedback of the temperature and thickness of sprayed coatings and other information during plasma praying. The flexibility of the forming system was promoted by integrating plasma spray forming with robot motion control. Excellent control performance is observed and this system can be effective to meet the requirements of different materials processing techniques.

1. INTRODUCTION

Atmospheric plasma spraying (APS) has already been applied in widespread industries for a variety of applications due to its low cost and simplicity [1]. It can rapidly manufacture coatings with almost all kinds of materials, such as metals, alloys, ceramics and polymers. Hence, APS has successfully shifted from a traditional technology for surface engineering to a versatile material processing one, and drawn keen attention especially for its noteworthy application of solid oxide fuel cells (SOFCs) [2, 3]. Meanwhile, there is an increasing use of robot in industry domains because of its high flexibility and broad operating region [4, 5]. For further study on rapid tooling using plasma thermal spray technology, we conducted developmental research of combining the industry robotic technology and plasma thermal spray technology to establish integrated robotic rapid spray metal tooling process for manufacturing injection molding tools and sheet metal forming dies. 2. INTEGRATED ROBOTIC PLASMA SPRAYING SYSTEM 2.1. System Design and Setup

As a line-of-sight process, APS can fabricate satisfying coatings with complex geometries through holding spray guns by a robot. It is very easy and simple to generate a spray trajectory or modify processing parameters by using a programmable robot, such as spray angle, spray distance, scanning velocity and step, etc. Moreover, robotic plasma spraying has proved a feasible and high-efficient solution to ensure a high accuracy in process and coating repeatability. System make-up of robotic plasma spraying system is shown in Fig. 1, which includes plasma jet generator, six-axis industrial robot and turning platform, powder injector, and PC+PLC based control system. Siemens S7-300 PLC was connected to the system through MPI (Multi-Point Interface), and the opening OPC (Ole for Process Control) protocol was applied for the data communication between PC and PLC. Communication between PC and UP-20 robot was established based on the data transmission medium of Ethernet, and the robot control software was developed for the host control and the exchanging of working jobs by PC. PC was chose as the central controller of the system with the help of process monitoring software with COM (Component Object Model) technology to meet with the multiple requirements of process monitoring and processing

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optimization, information storage, computing resources and so on. Hence, the superiorities of PLC over the stable and safe control of field apparatus and that of PC over process monitoring, data storage, control algorithm were effectively integrated to enhance the system function. Finally, the robotic plasma spray forming system was established accompanied with forming process monitoring, intelligent adaptive adjustment of robot spraying trajectories, real-time control of robot and other functions. The intelligent adaptive adjustment of robot spraying trajectories and self-dispatch of manufacturing strategies were carried out by the resultant system according to the feedback of the temperature and thickness of sprayed coatings during plasma praying [6].

Figure 1: Schematic diagram of robotic plasma spray forming system. 2.2. Influence of Suspension Injection on the Fluctuation of Plasma Jet

During plasma spraying, to control the time dependent D.C. plasma jet behavior requires the comprehensive understanding of its electric, magnetic, thermal, thermodynamic phenomena. The arc fluctuations already have an important influence on particles velocities and temperatures. This influence is by far more important with the suspension injection because fluctuations act on drops or jet penetration, fragmentation, particle trajectories, heating and acceleration. Plasma properties (e.g., velocity, specific enthalpy, gas mass density) vary continuously along the plasma jet radius i.e., along the suspension penetration path toward the plasma jet axis [7]. The instabilities of the arc root of the D.C. torch involve high transient voltage fluctuations and thus dissipated power fluctuations, resulting in plasma jets varying continuously in length and position [8] with strong variations of their velocities in the axial direction.

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Figure 2: Experimental set-up of (a) SPS and (b) the atomization profile of suspension.

Figure 2(a) presents the scheme of the experimental system for SPS using in this study. The liquid feedstock system is composed of three stainless steel tanks, in which suspensions are stored, and stirred and mixed continually by the propeller fixed in the tank. During the process of SPS, tanks are pressured with compressed air (N2 ) monitored and controlled with a rotor flow meter

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and pressure regulating valve. It is worth noting first that the momentum of the liquid droplets has to be high enough to ensure their penetration into the core of the plasma jet. Hence, the atomized nozzle was self-designed with three inlets for suspensions and one inlet for the atomizing gas. Picture of suspension drops by the atomized nozzle is shown in Fig. 2(b). Hence, the liquid feedstock system can be used for the SPS process of three different suspension compositions, and the pressure of the tank of the suspension feeders can be varied to modify the drop velocities. In this paper, images of plasma jet are collected by a charge-coupled device (CCD), and then grey values are calculated through the formal image processing and grey transform, finally, temperature field of plasma jet was obtained through the Abel transformation on the basis of the relationship between the grey value of images and the radiation intensity of plasma jet, which was established through the calibration experiment [9]. From the temperature field information, it is easy to establish optimal processing parameters, and also would further reduce costs and make spraying tooling more attractive, and furthermore, it can keep the forming process and quality of coatings. Different profiles of plasma jet during suspension injection are shown in Fig. 3. After the adjustment, the particle suspension flow fluently from the tank to the injector and it is also easy to penetrate into the center zone of plasma jet. This case is beneficial to enhance the deposition efficiency and the utilization ratio of powders.

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Figure 3: Different profiles of plasma jet influenced by suspension injection (a) during and (b) after adjustment process. 3. ADVANCED MATERIALS PROCESSING 3.1. An Application of SOFC

Figure 4 illustrates the surface line-scan photograph of the planar PEN (Positive-ElectrolyteNegative) coatings. The PEN was composed of anode, graded layer, electrolyte, graded layer and cathode. In the two graded layers between the respective electrode and the electrolyte, the material components gradually vary and every component layers contact tightly. The porosity of the anode graded layer changes gradually from high to low, and the porosity of the cathode graded layer gradually changes from low to high. The thicknesses of the anode, the electrolyte and the cathode are 200, 60, and 100 µm, respectively. In order to avoid increasing the resistance of cell, the graded layer only has the thickness of 20 ∼ 30 µm. According to the AC complex analysis results and comparison with the electrical conductivity of the PEN without the graded layers, the electrical conductivity of the one with the graded layers increased sharply.

Figure 4: SEM of the planar PEN.

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3.2. Rapid Metal Tooling

Rapid metal tooling has received widespread attention because die and mold-making of rapid tooling for both trial and mass production poses a problem in realizing the rapid development of new products. It has been demonstrated that thermal spraying process is an attractive method to manufacture metal molds of any size ranging from small to large. Our integrated robotic rapid spray metal tooling process was based on the 6-DOF (Degree of Freedom) industry robot. This industrial robot was employed to perform the central content of the rapid metal tooling procedure. First, a ceramic block was fabricated into ceramic prototype mould through robotic milling procedure. Second, the ceramic mould surfaces were coated with iron-nickel-chromium alloy layers by robotic plasma spraying procedure, and then bismuth alloy, which undergoes little thermal expansion, or zinc alloy was cast to make the backup of the sprayed layer. Finally, the ceramic pattern was broken and removed, so that robotic polishing of the sprayed layer proceeded and the completion of the metal tools for injection molding or sheet metal forming was obtained [10]. Using the methods of generating robotic trajectory of milling, spray and polishing, the ironnickel-chromium alloy spray automobile cover components die mould was experimentally made. Fig. 5 shows model and mould pictures of integrated robotic plasma spraying metal tooling process. The fabricated mould can satisfies the craft forming request completely.

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Figure 5: (a) Concave mould, (b) protruding mould assembling chart and (c) the product of the car panel. 4. CONCLUSIONS

1) In order to improve the process quality and reliability of plasma spray, an integrated RPS system is developed, which provides users with the high quality of spraying forming and coatings. This system can meet the requirement of advanced materials processing, and also has satisfying operation and fine control effects. 2) A control approach is presented to eliminate the influence of suspension injection on the plasma jet. It is helpful for users to achieve the optimal process parameters and develop process control in spraying process. 3) Fabrication experiments of the automobile cover component and the planar PEN of SOFC confirm the multipurpose uses of this system. It can enable spraying tooling large and medium mould with complex surface and the consistent production of those coatings. ACKNOWLEDGMENT

Authors gratefully acknowledge the contribution of the “863” project from the Ministry of Science and Technology (2007AA04Z142), as well as that of the National Nature Science Foundation of China (Grant No. 50675081). In particular, thanks are given to the Analytical and Testing Center of the Huazhong University of Science & Technology. REFERENCES

1. Fauchais, P., “Understanding plasma spraying,” J. Phys. D — Appl. Phys., Vol. 37, No. 9, R86–R108, 2004. 2. Hui, R., Z. Wang, O. Kesler, L. Rose, J. Jankovic, S. Yick, R. Maric, and D. Ghosh, “Thermal plasma spraying for SOFCs: Applications, potential advantages, and challenges,” J. Power Sources, Vol. 170, No. 2, 308–323, 2007.

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3. Henne, R., “Solid oxide fuel cells: A challenge for plasma deposition processes,” J. Therm Spray Technol., Vol. 16, No. 3, 381–403, 2007. 4. Nassenstein, K. and D. Luckenbach, “Progress in thermal spray processes,” Thermal Spray Connects: Explore Its Surfacing Potential!, 378–382, Basel, Switzerland, 2005. 5. Vergeest, J. S. M. and J. W. H. Tangelder, “Robot machines rapid prototype,” Ind. Robot, Vol. 23, No. 5, 17–20, 1996. 6. Xia, W.-S., H.-O. Zhang, G.-L. Wang, Y.-Z. Yang, and Y. Zou, “Open architecture robotic plasma spray forming system based on Ethernet,” Robot, Vol. 30, No. 1, 17–21, 2008 (in Chinese). 7. Etchart-Salas, R., V. Rat, J. Coudert, P. Fauchais, N. Caron, K. Wittman, and S. Alexandre, “Influence of plasma instabilities in ceramic suspension plasma spraying,” J. Therm Spray Technol., Vol. 16, No. 5, 857–865, 2007. 8. Coudert, J. F., M. P. Planche, and P. Fauchais, “Characterization of D.C. plasma torch voltage fluctuations,” Plasma Chem. Plasma Process., Vol. 16, No. 1, S211–S227, 1995. 9. Xia, W., H. Zhang, G. Wang, and D. Liu, “Plasma jet temperature field diagnostics for process control in rapid plasma spray tooling,” ICMA 2004 — International Conference on Manufacturing Automation: Advanced Design and Manufacturing in Global Competition, 771–776, Wuhan, China, 2004. 10. Zhang, H., G. Wang, Y. Luo, and T. Nakaga, “Rapid hard tooling by plasma spraying for injection molding and sheet metal forming,” Thin Solid Films, Vol. 390, No. 1–2, 7–12, 2001.