Available online at www.sciencedirect.com ScienceDirect Procedia Engineering 174 ( 2017 ) 504 – 511 13th Global Congress on Manufacturing and Management, GCMM 2016 Control of Grinding Surface Residual Stress of Inconel 718 Wang Pei-zhuo , He Zhan-shu, Zhang Yuan-xi, Zhao Shu-sen School of Mechanical Engineering, Zhengzhou University, Zhengzhou 450001, Henan, China Abstract The grinding surfaces of the nickel-based superalloy usually generate tensile residual stress, which may reduce fatigue life of components. In order to transform tensile stress to compressive stress, this research develops a new technology that embeds a heat source inside the workpiece during grinding. The grinding process of Inconel718 is simulated by using COMSOL Multiphysics 5.0, and the distributions of residual stress with and without the added heat source are obtained. In addition, the influence of the density heat source, the length of heat source, the height of heat source and the distance between heat source and grinding zone on the residual stress are studied. The results are as follows: (1) The surface tensile residual stress can be transformed to the residual compressive stress by embedding a heat source in the workpiece. (2) The surface compressive residual stress is most sensitive to the density of heat source. (3) The maximum surface compressive residual stress is obtained by adjusting the density and position parameters of heat source. © 20172016 The The Authors. Authors. Published Published by Elsevierby Elsevier Ltd. LtdThis. is an open access article under the CC BY-NC-ND license (Peerhttp://creativecommons.org/licenses/by-nc-nd/4.0/-review under responsibility of the organizing). committee of the 13th Global Congress on Manufacturing and Management. Peer-review under responsibility of the organizing committee of the 13th Global Congress on Manufacturing and Management Keywords: nickel-based superalloy; grinding heat; residual compressive stress; simulation 1. Introduction Nickel-based superalloys are widely employed in aerospace jet engines and various industrial gas turbines due to their high-temperature strength, fatigue resistance, thermal stability and high corrosion resistance. In order to ensure high precision and low surface roughness of nickel-based superalloy components, grinding process is usually applied as the final material removal step [1,2]. Meanwhile, unwanted residual tensile stress generate on the surface after grinding. It may easily leads to the micro crack on the surfaceˈand then reduces the fatigue life of the components. * Corresponding author. Tel.: +86 187-3743-5732. E-mail address:[email protected] 1877-7058 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 13th Global Congress on Manufacturing and Management doi: 10.1016/j.proeng.2017.01.174 Wang Pei-zhuo et al. / Procedia Engineering 174 ( 2017 ) 504 – 511 505 Moreover, the tensile residual stress may also reduce the dimension precision of components and bring great difficulties for the subsequent assembly process [3]. Even with the low grinding speed and the feed rate, the residual tensile stress is unavoidable due to the existence of grinding heat [4,5]. In order to improve the service life, the grinding nickel-based superalloy components require pre–existing compressive surface residual stresses. M.Y Tan, et al.[6,7] studied the residual stress state of workpiece surface machined by pre-stress cutting, and it is found that the pre-stress cutting can effectively improve the compressive residual stress of the machined surface; The thermal-mechanical coupling flied of grinding surface was simulated and analyzed by X.M. Zhang[8],the size and distribution state of residual stress were obtained and thermal- mechanical’s influence on the residual stress was discussed and uncovered. Y.H. He [9] studied the influence of the thickness, temperature, stress ratio on fatigue crack growth of nickel-based superalloy direct aging GH4619. In many cases, it is often necessary to carry out some post processing methods to control the residual stress. W.F. He et al [10] studied the effect of laser impact on the nickel-based superalloy GH742 fatigue properties, and found that the depth of residual compressive stress layer after laser shock peening reach 110 mm, and the tensile fatigue life of nickel based superalloy was prolonged by more than 316 times. However, these post-processing methods are often costly or time-consuming, even sometimes damage the surface finish and bring some unwanted distortion to finely machined parts, which may be not applicable to components with tight tolerances, like crank journals, pistons, seal surfaces, bearing bores and cylinder walls 䭉䈟!ᵚ᢮ࡠᕅ⭘ⓀDŽ. Therefore, in order to control the grinding surface residual stress of nickel-based superalloy, this paper take the grinding process of inconel 718 as the study object, and develop a new technology that embeds a heat source into the workpiece by a add-on induction heating during grinding process. 2. Principle of the proposed technology 2.1. Generation mechanism of surface residual stress Fig. 1 (a) residual stress generation in the traditional technology; (b) residual stress generation in the proposed technology. 506 Wang Pei-zhuo et al. / Procedia Engineering 174 ( 2017 ) 504 – 511 It’s generally believed that the existence of residual stresses in a ground specimen is due to the combined action of mechanical and thermal effect. But the residual stress generated by mechanical is very small compared with thermal stress [12]. So the effect of mechanical on residual stress is neglected in this study, only considering the effect of thermal on residual. As shown in fig.1 (a), the thermal expansion of hotter material closer to the surface is partially constrained by the cooler subsurface material in grinding zone. The compressive thermal stresses are generated near the surface, which will cause plastic flow in compression if sufficiently large. And during subsequent cooling, the volume of plastically deformed material tends to reduce in comparison to the beneath subsurface material, so the requirement of material continuity causes tensile stresses near the surface. As shown in fig.1 (b), in the proposed technology, a ‘hot’ inner layer will be introduced in a controllable way before the grinding process occurs. Before the grinding wheel engaging with the workpiece, the subsurface layer is pre-heated, so this part of the work material intends to expand. During the grinding process, the superficial layer experiences a higher temperature than the subsurface layer, causing more severe plastic deformation and compressive stress than the subsurface layer. While after the grinding wheel pass the workpiece, the surface is subjected to rapid cooling, but the subsurface is still with a higher temperature. The distribution of this temperature gradient will maintain the superficial residual stress distribution to the final status. 2.2. Couple model of grinding-added heat Z qmax i X 2mm *ULQGLQJKHDW L D l H $GGHGKHDW Fig.2 Grinding heat and added heat Fig.3 Finite element mesh of workpiece Many thermal models have been applied to analyze the grinding processes until now, and the profiles of the heat source are mainly assumed to be a rectangular or triangular. Further studies have shown that the triangular heat flux distribution agrees better with the measured temperature distribution than the simple uniform heat flux assumption [13,14]. Therefore, for simplicity, a right angled triangle is selected to be the heat source profile in this study. Fig.2 shows the couple model of grinding -added heat. The heat flux of a random point i at grinding zone x 2HP x qi qmax u u 0 d x d Lc Lc Lcb Lc Where, the contact length LC ad s ;H is the energy partition coefficient to the workpiece, it can be calculated by u uch H= ,u is total energy and uch is energy carried away by chips; b is the width of cut; P is the net grinding u power, P can be obtained from the measured power by subtracting the idling power, which can be monitored by using a Hall-effect transducer. Wang Pei-zhuo et al. / Procedia Engineering 174 ( 2017 ) 504 – 511 507 Different heat source arrangement will affect the distribution of residual stress. The influence of the parameters ˖heat source depth D, the heat source height H, the distance between the heat source and the grinding area l and the heat source length L on the residual stress was studied to obtain the maximum surface compressive residual stress. 3. Finite element model 3.1. Model parameters Tab.1 Thermal properties T(K) Heat capacity(J/kg·K) Thermal conductivity(W/m·K˅) 100 455 10.8 200 483 12.8 300 495 15.5 400 515 17.5 500 528 18.8 600 558 20.9 700 570 21.8 800 685 26.8 900 640 26 1000 640 26.5 1100 640 27.4 1200 640 28 The grinding process of nickel-based superalloy Inconel718 is simulated by using COMSOL Multiphysics 5.0. The size of workpiece is 40 mm u20 mmu 6 mm. Inconel 718 is assumed to be isotropic elastic-perfectly plastic material, and the stress of each point is zero before grinding. Tab.1 shows the heat capacity and thermal conductivity of Inconel 718 at different temperatures, respectively. 3.2. Mesh Generation Since the grinding zone is moved with the grinding wheel, and the temperature gradient greatly in the region, therefore, only refining the mesh of grinding zone and the shallow surface layer of the workpiece. The refined mesh shows in Fig.3 6 2 The initial temperature of the workpiece T=300 K, the maximum heat flux qmax=34u10 W/m , the convective heat transfer coefficient h=1500 W/m2K.