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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.

500

400

300

200

100 Residual stress(MPa) 0

-100 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Depth (mm) 

Fig. 4 Thermal distribution of grinding Fig. 5 Distribution of residual stress without added heat source 508 Wang Pei-zhuo et al. / Procedia Engineering 174 ( 2017 ) 504 – 511

4. Results and discussion

4.1. Effect of feed rate on grinding temperature

Fig.4 shows the temperature distribution of grinding when vs=20 m/s, ap=0.01 mm, and vw=50 mm/s. The temperature in grinding zone increases gradually along the cutting direction. This is because the material in the front of the cutter subjected to severe extrusion, friction, and shear, result in the temperature increases of grinding zone. Then with the cutter tip away from the workpiece, the temperature decreases by heat transfer. While after the grinding wheel pass the workpiece, the surface temperature of grinding zone rapidly decreased from 780K to about 500K in 0.1s, and then the decreased tendency is slow down and reduce to the initial temperature. Also find the temperature of grinding zone changes significantly with different feed speed vw, the highest temperature of the grinding zone is 1200k and 900K, 750K when vw respectively to 10 mm/s, 30 mm / s and 60 mm / s.

4.2. Residual stress without added heat source

Fig.5 shows the distribution of residual stress without added heat source when vs=20m/s, ap=0.01mm, vw =30 mm/s. It can be seen that the surface residual stress is tensile without added heat source. And the residual stress first decreases with the increase of depth, then reach the minimum at a certain depth, and finally tend to zero. L=10mmǃH=4mmǃl=4mmǃD=3.4mm

500 Qv=6 x 109 W/m3 Qv=9 x 109 W/m3 400 9 3 Qv=12 x 10 W/m 300

200

100

0 Residual stress (MPa) -100

-200

0246810 12 Depth (mm)

Fig. 6 Thermal distribution during grinding with the heat source Fig.7 Residual stress at different Qv

4.3. Residual stress with added heat source

When the surface temperature of the workpiece increases during grinding, the internal temperature increases rapidly under the action of added heat source during grinding at same time. After grinding, the added heat source and the grinding heat stop working at same time, so the surface temperature of the workpiece rapidly decreases due to heat exchange with the outside, but the internal temperature of the workpiece decreases slowly, which lead to the compressive residual stress is generated on the surface of the workpiece. Fig.6 shows the temperature distribution with added heat source when vs=20m/s, ap=0.01mm, vw =30 mm/s. It can be seen that the superficial layer experiences a higher temperature than the subsurface layer in the grinding zone. The temperature decreases from the center to the edges, and the highest temperature is up to 897K at the center of the added heat source. Wang Pei-zhuo et al. / Procedia Engineering 174 ( 2017 ) 504 – 511 509

4.4. Effect of Qvǃ Dǃ lǃ L and H on the residual stress

L=10mmǃH=4mmǃl=4mmǃQv=12x109W/m3 L=10mmǃD=3.4mmǃl=4mmǃQv=12x109W/m3 500 D=0.8mm 500 D=3.4mm l=4.0 mm 400 D=6.0mm 400 l=7.0 mm l=12.0mm 300 300

200 200

100 100

0 0 Residual stress (MPa) Residual stress (MPa) -100 -100

-200 -200

0246810 12 0246810 12 Depth (mm) Depth (mm)

Fig.8 Residual stress at different depth D Fig.9 Residual stress at different l

9 3 H=4mmǃD=3.4mmǃl=4mmǃQv=12x10 W/m L=10mmǃD=3.4mmǃl=4mmǃQv=12x109 W/m3 500 500 H=4mm 400 L=10mm 400 H=6mm L=20mm H=8mm L=40mm 300 300

200 200

100 100

0 0 Residual stress (MPa) Residual stress Residual stress (MPa) -100 -100

-200 -200

0246810 12 0246810 12 Depth (mm) Depth (mm)

Fig.10 Residual stress at different L Fig. 11 Residual stress at different H

The distribution of residual stress along the depth under different heat source density Qv is shown in Fig.7 when the size and the location of the heat source is certain. It can be seen that with the increase of the density of heat source, the surface residual stress decreases, meanwhile, the residual stress at the center of the heat source increases. This is because, under a constant size of the heat source, Qv increases will make the heat flux increases and lead to the internal temperature of workpiece rises. In cooling process, the surface temperature decreases faster than the internal temperature due to surface exchange the heat with external medium. After cooling, tensile residual stress is generated at the center of added heat source, while compressive residual stress is generated under the surface. When 9 3 Qv =12u10 W/m , the compressive residual stress layer in 2mm is generated under the surface. Fig.8 shows the distribution of residual stress under different heat source depth D of 0.8 mmǃ3.4 mmǃ6.0 mm. It can be seen that with the increase of D the surface residual stress decreases at first and then increases. This is because the surface is at the edge of internal high temperature zone when D=0.8mm, and the surface layer is slight squeezed by internal layer in cooling process, therefore, the generated compressive surface residual stress is generally small. And when D=6.0mm, the internal expansion area is too far away from the surface, and it’s not 510 Wang Pei-zhuo et al. / Procedia Engineering 174 ( 2017 ) 504 – 511

enough to produce compressive residual stress. So in order to generate the largest compressive surface residual stress, heat soure should neither too deep nor too shallow. In the simulation conditions, when D=3.4mm, the compressive surface residual stress reach the maximum. Fig.9 shows the residual stress distribution under different distance between the heat source and the grinding area l of 4mm, 7mm, and 12mm. It can be seen that with the increase of l, the surface residual stress increases but the internal residual stress decreases. This is because the increase of l will decreases the internal temperature of area under grinding zone, so that the thermal deformation is reduced, and the extrusion to surface is also reduced. Therefore, after cooling, the generated internal residual stress decreases, while under the action of grinding heat, depth of generated compressive residual stress layer also decreases, but the surface residual stress increases gradually. Thus, smaller l is in favour of obtaining compressive surface residual stress. Fig.10 shows the residual stress distribution under different heat source length L of 10mm, 20mm, and 40mm. Qv decreases with the increase of L when heat flux Qmax is constant, which lead to the temperature of added heat source decreases. Therefore, after cooling, the residual stress decreases at the added heat source area , while under the action of grinding heat, depth of compressive residual stress layer is also decreases, and the surface residual stress increases gradually. So the largest compressive residual stress is obtained when L=10mm. The residual stress along the depth is shown in Fig.11 when heat source height H=4mm, 6mm, and 8mm. When the other conditions remain unchanged, the increase of the H will also reduce the heat source density Qv, resulting in temperature decreases of the added heat source center, thus the thermal deformation is reduced, and the extrusion to surface is also reduced. So under the action of grinding heat, the residual stress at the center of added heat source is decreased and the surface residual stress is increased after cooling. So H should be 4mm to obtain the largest compressive residual stress.

5. conclusions

The grinding compressive surface residual stress is obtained by embedding a added heat source inside the workpiece, and adjusting the parameters such as the density, size and position of the heat source. With the increase of the heat source density Qv, surface residual stress changes from tensile to compressive. In order to obtain the surface residual compressive stress, the added heat source density should be large enough. In this experiment, the 9 3 maximum surface residual stress is -130 MPa when the Qv=12u10 W/m .Smaller distance between heat source and grinding zone l, smaller heat source length L, smaller heat source height H, and moderate heat source depth D are in favor of obtaining compressive surface residual stress. Of all the parameters been considered, the optimum parameters of is D=3.4mm, l=4.0mm, L=10mm, H=4mm, respectively.

Acknowledgements

The present research is supported by the National Natural Science Foundation of China (51305408), the Key Project of Higher Education of Henan Province(15A460029), China Postdoctoral Science Foundation(2015M582199), Foundation for The Excellent Youth Teacher of Zhengzhou university.

References

[1] C. Guo, Z. Shi, H. Attia, D. Mclntosh, Power and wheel wear for grinding nickel alloy with plated CBN wheels, Cirp Ann– Manuf Techn. 56 (2007) 343-346. [2] R.A. Waikar, Y.B. Guo, Effect of surface texture and white layer by hard turning vs. grinding on frictional performance, NAMRI of SME, 37 (2009) 1-8. [3] N.B. Fredj, H. Sidhom, C. Braham, Ground surface improvement of the austenitic stainless steel AISI 304 using cryogenic cooling, Surf. Coat. Technol, 200 (2006) 4846-4860. [4] S. Malkin, C. Guo, Thermal analysis of grinding, Cirp Ann–Manuf Techn, 56 (2007) 760-782. [5] A.D. Sosa, M.D. Echeverría, O.J. Moncada, J.A. Sikora, Roughness produced by grinding thin wall ductile iron plates, Int. J. Mach Tool Manu , 47 (2007) 229-235. [6] M.Y. Qin, B.Y. Ye, A.D.He, Investigation into residual stress of pre-stress cutting based on thermo-mechanical coupling analysis , 40(1) (2012)47-52. Wang Pei-zhuo et al. / Procedia Engineering 174 ( 2017 ) 504 – 511 511

[7] B.Y. Ye, R.T. Peng, X.Z. Tang, Z.W. Liang, Residual stress and surface morphology of pre-stress hard cutting, J.Sou China Univ Techno J.Sou China Univ Techno, 36(4) (2008)6-9. [8] X.M. Zhang; L.J. Liu; S.C. Xiu; B. Bai, Simulation analysis of ground surface residual stress with thermal-machaincal coupling principle, J.Northeast Univ, 35 (2014) 1758-1762. [9] Y.H. He, H.C. Yu, W.B. Guo, L.L Shen., B. Su, Experimental study on fatigue crack growth behavior of direct aging GH4169 superalloy, J. Aerospace Power, 21 (2009) 49-53. [10] W.F. He, Y.H. Li, Z.W. Zhou, Effects of laser shock processing on fatigue property of GH742 Ni-based superalloy, T. Mater Heat Treat, 30(3) (2009) 42-45. [11] J. Lindemann, C. Buque, F. Appel, Effect of shot peening on fatigue performance of a lamellar titanium aluminide alloy, Acta Mater, 54 (4) (2006) 1155-1164. [12] Y. Tian, B. Shirinzadeh, D. Zhang, X. Liu, D. Chetwynd, Effects of the heat source profiles on the thermal distribution for ultraprecision grinding. Precis Eng , 33 (2009) 447-458. [13] H.J. Kim, N.K. Kim, J.S. Kwak. Heat flux distribution model by sequential algorithm of inverse heat transfer for determining workpiece temperature in creep feed grinding, Int. J. Mach Tool Manu, 46 (2006) 2086-2093.