Effect of Spraying Condition and Material Properties on the Residual Stress in Plasma Spraying

J. Mater. Sci. Technol., Vol.20 No.2, 2004 149 Effect of Spraying Condition and Material Properties on the Residual Stress in Plasma Spraying Xiancheng ZHANG 1), Jianming GONG 1) and Shandong TU 1,2) 1) Department of Mechanical Engineering, Nanjing University of Technology, Nanjing 210009, China 2) Department of Mechanical Engineering, East China University of Science and Technology, Shanghai 2002379, China [ Manuscript received March 27, 2003, in revised form June 24, 2003] The thermomechanical behavior and the distribution of residual stresses due to thermal spraying of NiCoCrAlY coating were studied by thermomechanical finite element analysis. The effects of phase transformation due to solidifying process of coating particles, thickness and material properties of coating on the residual stresses were discussed. Results showed that residual stress decreases little with the stress relaxation due to the phase transformation. For the substrates with the same thickness, the residual stress increases with the increase in coating thickness. The state of residual stresses relates to the material properties of coating and substrate closely. The stress-induced failure model of coating is also discussed. KEY WORDS: Coating, Residual stress, Plasma spray, Phase transformation, Finite element method 1. Introduction Plasma spraying has been one of the most rapidly developing areas of thermal spraying and gained wide industrial acceptance in the recent years. The substrate and coating will deform with temperature in the process of plasma spraying. Residual stress will be induced due to the differential thermal contraction between substrate and coating during cooling, which usually cause spallation, distortion or the generation of defects such as cracks. Therefore, it is very important to understand the mechanism of causing residual stresses and their distribution. The formation of residual stresses is extremely complex in the process of plasma spraying and lots of factors, such as the coating thickness, coating material, spraying condition etc, influence spraying qualities [1]. Residual stresses depend not only on the thermal history during deposition of coating particles but also on the deposition characteristics [2,3]. Residual stresses are unavoidable in spraying and the effects of these stresses on coating performance cannot be disregarded. For the prediction of the residual stresses induced by spraying, previous investigations have developed several experimental methods, including diffraction methods, eflection methods and material removal methods, but most of these techniques impose severe limitations on specimen geometry [4] and the testing procedure is very dull. With the development of computer techniques, the capacity of finite element method (FEM) for analyzing the thermomechanical behavior during the formation process of coating has been further enhanced. For plasma spraying, Khor et al. [5] discussed the effect of residual stresses on the performance of plasma sprayed functionally graded ZrO 2/NiCoCrAlY coatings using the finite element code ANSYS, it is found that the residual stress was the lowest for the five-layer functionally graded coating compared to that of the duplex coating and three-layer coating with the same coating thickness. Furthermore, the effect of coating material and size on the residual stresses distribution in the coating and along the interface of coating and substrate had been analyzed by other authors [6] via FEM. Ju et al. [7] developed a continuum micromechanical model to analyze the residual stresses of the spraying coating process including the interfacial combinations between sprayed layer and substrate. However, up to now, most of these studies of the thermal transfer processes were steady state and did not consider the effect of phase transformation or solidification of coating on the final residual stresses and the effect of properties of coating material on the state of residual stresses in the coating. In fact, the different states of residual stresses can Prof., Ph.D., to whom correspondence should be addressed, E-mail: gongjm@njut.edu.cn. induce different failure modes to coatings. In the present paper, the residual stresses of NiCoCrAlY coating in plasma spraying are evaluated by the nonlinear thermomechanical finite element analysis. Furthermore, the effect of phase transformation due to solidification, the coating thickness and physical properties of coating material on the distribution of residual stresses are discussed. 2. Model Description 2.1 Thermomechanical model Spraying residual stresses were calculated by finite element method (FEM, ANSYS 6.0). Figure1 presents the analysis procedures. In order to model the distribution of residual stresses after plasma spraying, the used model represents a shape Ni substrate of 50 mm in diameter and 25 mm in height with a coating deposited to the top surface. The analytical model is assumed to be a perfect elastic body without plastic deformation in the whole analysis procedure. The geometry model is shown in Fig.2. An axial symmetric problem is chosen in order to reduce the data processing time. The meshes in the zone near the coating/substrate interface are refined to improve the accuracy of calculation. 2.2 Thermal model A total of 500 load steps increasing to 100 s were to complete the whole heat transfer process in the thermal analysis. The time increments were automatically optimized for each time step by the computer program. The modified Newton- Raphson method was used in each step for the heat balance iteration. The thermal element PLANE 55 and axisymmetric element behavior were selected. The convective heat transfer coefficients on the surfaces were estimated to be 20 Wm 2 K 1 based on the engineering formulae for natural convection. Thermal radiation was not considered in this calculation. 2.3 Mechanical model The temperature history obtained from the thermal analysis was input as a thermal loading into the structure model in the mechanical analysis. The element type PLANE 42 was achieved automatically and the axisymmetric element behavior was also selected. The steady state analysis was performed to calculate the distributions of residual stresses in the coating and the substrate. The proposed method is compared with finite element and experimental results taken from Khor et al. [5]. Khor et al. computed the residual stress in functionally graded ZrO 2/NiCoCrAlY coating using the steady state solution method via ANSYS. Figuer 3 shows the residual radial stress

150 J. Mater. Sci. Technol., Vol.20 No.2, 2004 Fig.1 Flow diagram of analysis procedure time in which the heat-transfer process is complete. Fig.2 Schematic description of the geometry used in the finite element model distributions along radius in the interface between the fivelayer coating and substrate.the solid line and the broken line in Fig.3 represent the radial stress computed by Khor et al. and present method, respectively using the same finite element model and materials. According to Fig.3, the maximum stress calculated by the present method is larger than Khor et al., but the average residual stresses computed by the method proposed here show certain degree of agreement with those by previous FEM. 3. NiCoCrAlY Coating with a Thickness of 1 mm Because the coating thickness is much smaller than the substrate thickness, the temperature gradients in the coating after spraying are not considered, which had little influence on the final value of residual stresses greatly [8]. The initial temperature of substrate and coating is 25 C (room temperature) and 1300 C, respectively, and the temperature of the entire specimen is about the home temperature after the calculated 3.1 Material properties of coating and substrate The material properties of coating and substrate as a function of temperature from 25 C to 1200 C and 800 C, respectively is shown in Table 1 [5]. It can be seen that the heat expansion coefficient of NiCoCrAlY coating and Ni substrate increases whereas the elastic modulus decreases with an increase in temperature,and α Ni < α NiCoAlY. 3.2 Residual stresses analysis Figure 4(a) shows the typical contour plot of radial stress distribution near the edge of specimen after cooling to room Fig.3 Residual stress computed by Khor et al. FEM and present work Table 1 Material properties of coating and substrate [5] Temperature Elastic Density Coefficient of Possion s Thermal Specific modulus thermal expansion rate conductivity heat / C / 10 5 MPa /(kg/m 3 ) 10 5 /(W/mK) /(J/kg K) Ni 25 2.07 8880 1.27 0.312 90.5 461 400 1.82 8880 1.64 0.312 65.3 460 800 1.50 8880 0.312 73.9 460 NiCoCrAlY 25 2.25 7320 1.40 0.300 4.3 501 400 1.86 7320 2.40 0.300 6.4 592 800 1.47 7320 4.70 0.300 10.2 781 1200 0.90 7320 7.10 0.300 16.1 764 ZrO 2 25 0.53 6037 0.72 0.250 1.5 500 400 0.52 6037 0.94 0.250 1.2 576 800 0.46 6037 1.60 0.250 1.2 637 1200 0.48 6037 2.20 0.250 1.1 656

J. Mater. Sci. Technol., Vol.20 No.2, 2004 151 Fig.4 Typical contour of stress distribution of NiCoCrAlY coating (a) radial stress, (b) shear stress Fig.5 Stress distribution of NiCoCrAlY coating (a) radial stress, (b) shear stress, (c) axial stress temperature. It can be seen that there is a remarkable stress concentration at or close the edge of coating/substrate interface. The stress concentration may cause the spallation of coating and the crack at the interface[5]. Figure 5(a) shows the radial stress distribution along radius on the surface of coating, in coating and at the interface between coating and substrate. It can be observed that the stress is usually tensile in the coating and at the interface and the tensile stresses decrease with an increase in the distance from the coating surface to the interface. The tensile stresses decrease abruptly near the edge of the specimen. The tensile stresses will make the through-thickness cracks develop from preexisting defects in the coating and generate the decohesion of coating[8]. The tensile radial stresses on the coating surface change to compressive with an increase in the radius and the maximal compressive stress of 5 MPa is produced near the edge of specimen, which may cause the distortion of coating[5,8]. Figure 4(b) shows the typical contour plot of shear stress distribution near the edge of the specimen. It can be seen that the maximal shear stress (13.5 MPa) is produced near the edge of interface. The large compressive stress in coating tends to promote microcrack propagation along the interface and provide a driving force for bulking and eventual spallation[8]. The shear stresses distribution in the coating is shown in Fig.5(b). The shear stresses show a sharp discontinuity and concentration at or near by the edge of the specimen. Figure 5(c) shows the axial stresses distribution along radius in the coating. Remarkable stress concentration is also observed at the edge of the specimen and there is a large compressive stress at the edge of the interface. Thus it can be seen that stress discontinuity and concentration in the coating and at the interface between coating and substrate may cause the spallation of the coating[9]. sary to consider the effect of solidification because the residual stresses are related to the moving interface between solid phase and liquid phase on the progress of phase transformation. The thermal analysis on the phase transformation is a strongly nonlinear problem and the melting latent heat is defined by the variation of enthalpy of coating material as a function of temperature. Figure 6 shows the distribution of radial stress at the interface along radius with and without considering phase transformation in the solidifying process. It can be seen that the value of radial residual stress decreases because of the stress relaxation due to phase transformation, but the pattern of stress distribution is not influenced. If phase transformation is taken into account, the maximum stress decreases 3 MPa and the stress concentration occurs at the edge of the specimen. The axial stress and shear stress decreased only a little. In fact, the residual stresses also decrease in the coating. 4.2 Effect of coating thickness For the NiCoCrAlY coating with a thickness of 500 µm, 1 mm and 1.5 mm, the residual stresses distributions are similar, but the stress concentration becomes more severe for the 4. Results and Discussion 4.1 Effect of phase transformation In the process of plasma spraying, stress relaxation due to phase transformation or solidification of coating has some contribution on the final residual stresses[8,10,11], which is less considered in the previous simulations[3]. It is neces- Fig.6 Effect of phase transformation on the radial stress of NiCoCrAlY coating

152 J. Mater. Sci. Technol., Vol.20 No.2, 2004 Fig.7 Typical contour plot of stress distribution of ZrO2 coating (a) radial stress, (b) axial stress, (c) shear stress in Fig.7(b) and (c) respectively. It can be observed that the remarkable stress concentration occur near the edge of interface. There is rather a large axial stress of 21 MPa at the edge of interface, which will cause the spallation and distortion of coating. The coating failure mode depends on the stress state in the coating. Residual tensile stress may induce the fracture of coating normal to interface, but residual compressive stress can cause buckling and eventual spallation of coating, as shown in Fig.8. It can be seen that the compressive stress can prompt the crack initiation and propagation along the interface, whether spallation will occur or not is determined by the relative fracture strengths of the interface and coating[8]. 5. Conclusions Fig.8 Failure model of a thin coating under residual stress (a) bulking and spallation (compressive stress), (b) perpendicular microcrack (tensile stress)[8] 1.5 mm thick coating near the edge of interface. The value of shear stresses is almost same for different coating thickness, but the radial stress and axial stress decrease obviously with the decrease of coating thickness. The maximum radial stress and axial stress for 500 µm thick coating are only 12.04 MPa and 7.32 MPa, respectively at the interface. 4.3 Effect of material properties The value and state of residual stresses are closely related to the material properties of coating and thermal expansion coefficient influences the state and distribution of residual stresses directly. The residual stresses are also calculated for ZrO2 coating and Ni substrate without considering the influence of phase transformation. The material properties of ZrO2 are shown in Table 1. It can be seen that αni > αzro2. The calculating results indicate that the radial stresses in coating and at interface are also almost tensile and the remarkable stress concentration also appears near the edge of interface, but the value of radial stresses was lower than it in NiCoCrAlY coating. This phenomenon is not completely unexpected, that may be induced to the rapid cooling of coating particles. The compressive stresses in a thin coating can improve the coating durability in a certain extent[11], but the tensile stresses will make the through-thickness cracks develop from preexisting defects in the coating and generate the decohesion of coating[8]. Figure 7(a) shows a typical contour plot of radial stress distribution for 1.0 mm ZrO2 coating near the edge of the specimen. The tensile radial stresses occur in the coating and on the coating surface. The maximum stress is 33 MPa, which may cause the induction of surface crack. The tensile radial stresses change to compressive stresses near the interface and in the substrate. The typical contour plot of axial stress and shear stress distribution near the edge of specimen are shown (1) the residual stresses in plasma sprayed NiCoCrAlY coating on Ni substrate are simulated by finite element method. The results indicate that the radial stresses in the coating are compressive and a large stress concentration exists at the edge of the interface which may cause the spallation of coating. If the phase transformation in spraying process is considered, the value of residual stresses is reduced due to stress relaxation. (2) the residual stresses in the coating decrease with the decrease in coating thickness but the state of residual stresses is almost not changed for the same coating and substrate material. The material properties of coating influence the state and value of residual stresses in the coating greatly. For the ZrO2 coating on Ni substrate, although the coefficient of thermal expansion of the coating is lower than the substrate, the tensile stresses still formed in the coating and the remarkable stresses concentration exists near the edge of interface. But the value of tensile radial stress was lower than it in NiCoCrAlY coating. While, the failure model of coating is affected by the state of residual stresses in coating. Acknowledgement The authors are grateful to the support provided by the National Natural Science Foundation of China (Contract No.10172046) and Science and Technology Project of China Petrochemical Co. (Contract No.02JSNJYZ101001). REFERENCES [1 ] Xiancheng ZHANG, Jianming GONG and Shandong TU: Pressure Vessel Technol., 2003, 20(1), 33. (in Chinese) [2 ] O.Knotek, R.Elsing and U.Balting: Surf. Coat. Technol., 1988, 36(1-2), 99. [3 ] J.D.Lee, H.Y.Ra, K.T.Hong and S.K.Hur: Surf. Coat. Technol., 1992, 56(1), 27. [4 ] J.Matejicke, S.Sampath, P.C.Brand and H.J.Prask: Acta Mater., 1999, 47(6), 607. [5 ] K.A.Khor and Y.W.Gu: Mater. Sci. Eng., 2000, A277(1-2), 64. [6 ] Xiancheng ZHANG, Jianming GONG, Shandong TU and Juan FENG: J. Nanjing University of Technol., 2003, 25(1), 63. (in Chinese)

J. Mater. Sci. Technol., Vol.20 No.2, 2004 153 [7 ] D.Y.Ju, M.Nishida and T.Hanabusa: J Mater. Process. Technol., 1999, 92-93(8), 243. [8 ] V.Teixeira: Vacuum., 2002, 64(3-4), 393. [9 ] G.Knuyt, K.Vandierendonck, C.Quaeyhaegens, M.Van Stappe and L.M.Stals: Thin Solid Films., 1997, 300(1-2), 189. [10] S.Kuroda and T.W.Clyne: Thin Solid Films, 1991, 200(2), 49. [11] S.C.Gill and T.W.Clynez: Metall. Trans., 1990, 21B, 377. Nomenclature ρ density h f film coefficient C specific heat T A temperature at the surface of model T temperature T B bulk temperature t time {T e} nodal temperature vector {q} heat flux {Q e} nodal thermal flux vector Q rate of internal heat generation [K] heat conductivity matrix η unit outward normal vector { T e} nodal temperature increment matrix α coefficient of thermal expansion Research on Thermal Deep-drawing Technology of Magnesium Alloy (AZ31B) Sheets Shihong ZHANG, Kun ZHANG, Zhongtang WANG, Chuanfu YU, Yi XU and Qiang WANG Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China [ Manuscript received April 18, 2003, in revised form June 24, 2003] Forming technology of Mg alloy (AZ31B) sheets can be investigated by thermal deep drawing experiments. In the experiments, the blank holder and die contacting with the blank were heated to the same temperature as the blank by using the heating facility. The circular blank heated in an oven is formed at a temperature range of 100 400 C to obtain the optimum forming temperature range and the effects of major technical parameters on the workpiece quality. It is found that the blank is brittle at temperatures lower than 200 C. Temperatures higher than 400 C are not suitable for forming of the sheets because of severe oxidation and wrinkling. AZ31B shows an excellent formability at temperatures from 300 to 350 C and can be formed into a workpiece with good quality. When the blank holder force is 9 kn, extruded sheets with a thickness of 1 mm can be formed into cups without wrinkling. Workpieces show strong anisotropic deformation behavior on the flanges. KEY WORDS: Magnesium alloy (AZ31B), Thermal deep drawing, Blank holder force, Wrinkling 1. Introduction Magnesium alloys with the characteristics of light weight and recycling are increasingly becoming the ideal materials for modern industrial products. They have been applied widely for light structural and functional parts, especially in the automotive and electronic industries. Because of lower density, better collision safety property and electromagnetic interference shielding capability, Mg-alloys are available for producing some structural parts such as the coverings of mobile telephones, notebook computers and portable mini-disks (MD). In the past, the demand for this alloy as a structural material was not high because of its less availability commercially as well as limited manufacturing methods. In recent years, die casting of Mg-alloys has been the prevailing method for making parts in the automotive industry. However, this process is not ideal in making thin-walled Mg structures because of excessive amount of waste materials. So sheet metal forming processes (such as thermal deep-drawing process, isothermal gas forming [1] ) are developed to make thin-walled parts with good mechanical property and surface quality to avoid the defects above. It is commonly recognized that Mg possesses poor formability at room temperature because of its hexagonal closed packed structure. Fortunately, formability of Mg-alloys can be effectively improved by enhancing forming temperature, for example, up to above 300 C [2]. However, temperatures higher than 400 C are not suitable for forming because of severe oxidation and grain growth. Sheet metal forming processes of Mg-alloys, which need be improved continually, have being developed and industrilized in Japan, Germany, Taiwan and Singapore [3 5]. The research group including the authors has studied Prof., Ph.D., to whom correspondence should be addressed E-mail: shzhang@imr.ac.cn. sheet metal forming processes of Mg-alloys in recent years [6 7]. In this paper, thermal deep drawing experiments of the Mg-alloy (AZ31B) sheets were utilized to obtain the optimum forming temperature range and to discuss the effects of major factors on the workpiece quality. At the same time, a finite element analysis was performed to simulate technical defects of blanks. 2. Experimental Conditions 2.1 Experimental equipments and facilities A set of thermal deep drawing experimental facilities were designed to analyze formability of the Mg-alloy sheets, the tool setup is shown in Fig.1. The experiments have been carried out using a 100 tons four-post multifunctional hydraulic press. Spring components were applied on the blank holder to regulate the blank holder forces to avoid wrinkling of sheets. Circular blanks were heated in an oven. The blank holder and die contacting with the blank were heated to the same temperature as the blank by electric heating elements. The blank taken out of the heating oven was put on the surface of the die and formed by the punch (to strengthen the drawn wall part of the blank using the heating exchange, the punch should not be heated.) [4]. The dimensions of main tool used for the deep drawing are summarized Table 1. 2.2 Experimental materials Materials used in the experiments are extruded sheets of Mg alloy AZ31B. The chemical composition are shown in Table 2. The initial diameters of blanks are 110 mm and 120 mm with a sheet thickness of 1 mm. 2.3 Processing conditions Process conditions include the forming temperature, the preheating temperature of tools, the forming speed and the lubricant. The forming temperatures and preheating temper-