In-situ observation of dislocation and analysis of residual stresses by FEM/DDM modeling in water cavitation peening of pure titanium

Transcription

1 IOP Conference Series: Materials Science and Engineering PAPER OPEN ACCESS In-situ observation of dislocation and analysis of residual stresses by FEM/DDM modeling in water cavitation eening of ure titanium To cite this article: D Y Ju and B Han 2015 IOP Conf. Ser.: Mater. Sci. Eng Related content - Decaying dark matter and the tension in 8 Kari Enqvist, Seshadri Nadathur, Toyokazu Sekiguchi et al. - Analysis of Residual Stress by Diffraction Using Neutron and Synchrotron Radiation ed M E Fitzatrick and A Lodini - An accurate method for determining residual stresses with magnetic nondestructive techniques in welded ferromagnetic steels P Vourna View the article online for udates and enhancements. Recent citations - Residual Stress Modification and Mechanisms of Bearing Steel with Different Microstructures during Water-Jet Cavitation Peening Shan Miao et al This content was downloaded from IP address on 10/12/2018 at 04:42

2 In-situ observation of dislocation and analysis of residual stresses by FEM/DDM modeling in water cavitation eening of ure titanium DY Ju 1,2 and B Han 2 1 Deartment of Material Science and Engineering, Saitama Institute of Technology, Jaan 2 Deartment of Mechanical Engineering, University of Science and Technology Liaoning, Qianshan load 184, Anshan, China dyju@sit.ac.j Abstract. In this aer, in order to aroach this roblem, secimens of ure titanium were treated with WCP, and the subsequent changes in microstructure, residual stress, and surface morhologies were investigated as a function of WCP duration. The influence of water cavitation eening (WCP) treatment on the microstructure of ure titanium was investigated. A novel combined finite element and dislocation density method (FEM/DDM), roosed for redicting macro and micro residual stresses induced on the material subsurface treated with water cavitation eening, is also resented. A bilinear elastic-lastic finite element method was conducted to redict macro-residual stresses and a dislocation density method was conducted to redict micro-residual stresses. These aroaches made ossible the rediction of the magnitude and deth of residual stress fields in ure titanium. The effect of alied imact ressures on the residual stresses was also resented. The results of the FEM/DDM modeling were in good agreement with those of the exerimental measurements. 1. Introduction Cavitation imact has historically attracted attention due to its costly damage to hydraulic mechanical arts, such as hydrofoil surfaces, turboum imellers, ums, and valves. As such, many researchers [1-5] have traditionally focused their investigations into the damage mechanism of cavitation. More recently, cavitation imact has been successfully develoed as a means to imrove the fatigue erformance of mechanical comonents by introducing residual stress into the suerficial layer of metallic comonents, in a manner similar to the more conventional rocess of shot eening. Ju and Qin et al. [16] designed a new ventilation nozzle, through which suitable air can be aerated into the extra high-velocity flow in the nozzle throat, thereby forming a tremendous ressure gradient between the ustream and downstream flows. This secific method is referred to as water cavitation eening (WCP) [6, 7]. Sahaya Grinsan and Gnanamoorthy successfully emloyed another cavitation rocess by injecting a high-seed oil jet into an oil-filled tank. This method, referred to as oil jet eening (OJP), effectively reduces the erosion of mechanical comonents [8-10]. Several recent investigations have revealed that WCP imroves fatigue erformance by introducing comressive residual stresses in the surface of metallic comonents [7-9, 11, 12. The method has also been demonstrated to induce high uniform comressive residual stresses in gear tooth surfaces, since comlicated and narrow surfaces can be more easily eened [16]. Comared with conventional shot eening, WCP can obtain the smoother surfaces. The distributions of imact ressure are isotroic, therefore, rocess caability of WCP is uniform at different incidence angle [16]; however, it is unknown if WCP can induce changes in the microstructures within the strengthened layers of metallic comonents, as shot eening does [18-21]. Residual stresses in materials are often the result of metallurgical rocesses, such as casting, Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by Ltd 1

3 forging, welding, and quenching. Residual stresses from metallurgical rocesses usually deend on changes in thermal sources and volume accomanying the hase transformation. Generally, two tyes of residual stresses should be considered, that is, macro-residual stresses and micro-residual stresses [23]. Macro-residual stresses deend on the lastic deformation of solid materials due to raid non-uniform cooling, while micro-residual stresses are caused by strain and deformation due to hase transformation and changes in microstructure. We also know that distortions due to thermal and elastic-lastic deformation and strain, as well changes in hase transformation and texture in manufactured materials, are imortant regardless of the tye residual stresses. Unfortunately, since the ost-wcp distortion rocess is minimal, it is not sufficiently clear why the WCP rocess creates such large comressive residual stresses, similar to traditional eenings. Therefore, it is imortant to clarify the cause of residual stress generation through basic material roerties research. In order to aroach this roblem, secimens of ure titanium were treated with WCP, and the subsequent changes in microstructure, residual stress, and surface morhologies were investigated as a function of WCP duration. The influence of water cavitation eening (WCP) treatment on the microstructure of ure titanium was investigated. The microstructural evolution in the near-surface of ure titanium as a function of WCP time was characterized by X-ray diffraction (XRD), otical microscoy (OM), scanning electron microscoy (SEM), and transmission electron microscoy (TEM). After WCP treatment, changes in the microstructure, as well as residual stress and surface morhologies as functions of WCP time, were recorded using a novel exerimental design involving an in-situ observation function. The obtained results indicate that twinning lays an imortant role in the lastic deformation and residual stresses of hexagonal close-acked (HCP) structured metal materials, and therein, that the deformation twinning and twinning interaction were induced by WCP in the strengthening layer. A stable comressive residual stress layer was found in the near-surface of the investigated ure titanium. In this aer, a novel combined finite element and dislocation density method (FEM/DDM), roosed for redicting macro and micro residual stresses induced on the material subsurface treated with water cavitation eening, is also resented. A bilinear elastic-lastic finite element method was conducted to redict macro-residual stresses and a dislocation density method was conducted to redict micro-residual stresses. These aroaches made ossible the rediction of the magnitude and deth of residual stress fields in ure titanium. The effect of alied imact ressures on the residual stresses was also resented. The results of the FEM/DDM modeling were in good agreement with those of the exerimental measurements. 2. Exerimental rocedures 2.1. Test secimen and WCP conditions Fig. 1 deicts a schematic of the test secimen and clam. The test secimens consisted of ure titanium in as-annealed rocess state. The size of each test secimen was 4 mm 6mm 32 mm. The secial clam deicted in Fig. 2 was used to ideally combine the secimens (A) and (B) into one art. The key rocess conditions of WCP include aeration flux, standoff distances (SOD), and WCP duration. All of these attributes significantly imact the degree of enhancement of the rocess, and therein, it is necessary to determine ideal rocess conditions rior to treating the test secimens. According to recent observations[15] regarding imact ressure distribution measurements, an aeration flux of 0.4 L/min and a SOD of 85 mm were demonstrated to induce a tremendous imact ressure. As shown in Fig. 2, the secimens were treated by WCP in the bottom center of the test water tank. WCP durations of 15 min, 30 min, 45 min, and 60 min were investigated in this study. 2

4 WCP direction Observation (a) Secimen A Secimen and clam Secimen B Clam SOD Nozzle Water Fig. 1: Schematic of the test secimen and the clam. Fig. 2: Schematic of the WCP rocess 2.2. OM observation of microstructures The secimens A and B were first ground and olished, and the surfaces of the microstructure observation oints were eroded with a solution of 10% HF + 5% HNO % H 2 O. The microstructure hotos showing the different otimal oints were recorded by OM. As deicted in Figs. 1 and 2, the same two secimens were joined by the secial clam in the WCP rocess. Finally, the microstructures at the same oints were again recorded by OM after the WCP treatment. In the receding method, microstructural changes in the suerficial layer were recorded at each ste for each WCP duration TEM observation of microstructures The TEM (JEOL2100) was used to examine secimen microstructures at a deth of 30 to 40 μm from the surface in order to further investigate any microstructural changes due to the WCP treatment. In order to obtain a clear image of the microstructure, samles were observed using SEM after etching in a solution containing 10%HF + 5%HNO3 +85%H2O. A Twin-Jet Electroolisher (FISCHIONE-140 Digital Power Control) and a solution of 5% HClO4 + 95% CH3OH was used to etch the secimens to an otimal thickness for TEM analysis Residual stress measurement The surface residual stress and deth distribution of residual stress in the near-surface layer were measured by X-ray diffraction stress analysis using the conventional sin 2 Ψ method. A secial otical system jointly extracted recise information concerning the residual stresses in areas as small as 0.15 mm in diameter. In this study, a side inclination method was also used for stress measurement. The X-ray tube (Co-Kα tye) was oerated at 30 kv and 10 ma, with a slit diameter of 2 mm. The shift of the α-ti (114) diffraction rofile was detected at angles φ = 10, 20, 30, 40, and 45. The diffractive angle 2θ 0 was The stress constant of the X-ray diffraction analysis was 180 MPa/. Moreover, Vickers hardness was measured on the eened surface (eened for 45 min) and uneened secimens with a load of 9.8 N. In order to investigate the deth distribution of the residual stress, the near-surface layer of the test oint was removed by ste-by-ste electrolytical olishing using a Proto Electrolytic Polisher-Model 8818 (Proto Manufacturing Ltd.). The deth of the electrolytic olishing was aroximately 10 to 25 µm at each ste, and was adjustable via the voltage control, timer control, and flow rate of the electrolyte control. The deth distribution of the residual stress was obtained by measuring the residual stress at each ste. 3. Analysis Model by FEM/DDM Considering not only macro-lastic strain but also micro-lastic strain, the total residual stress σ induced by WCP is given by residual 3

5 σ σ + σ residual = (1) FEM DDM where, σ FEM is the resolved residual stress induced by macro-lastic deformation, which can be obtained from the finite element method; and σ DDM is the resolved residual stress induced by macro-lastic deformation, which can be obtained from the dislocation density method. Under uniaxial strain conditions, the highest elastic stress level in the imact wave roagation is defined as the Hugoniot Elastic Limit (HEL). According to Hook s law [27], the surface lastic strain ε can be exressed as ε 2HEL P = ( 1) 3λ + 2µ HEL (2) where, P is the imact ressure and a function of ressure ulse duration, λ is Lame s constants, and µ is Lame s constants. The residual stress σ FEM induced by WCP can be written as µε (1 + ν ) 4L (1 + ν ) σ FEM= σ 1 (3) 0 (1 ν ) + σ 0 πr where, r is the radius of circular imact zone on the target, σ 0 is initial residual stress, and L is the deth of lastic affection. σ A µ b N + (4) DDM = m σ DDM σ 0 where, is the residual stress induced by dislocations; A is the constant caused by unknown sources; µ is the shear modulus; Nm is the density of mobile dislocation ;b is the magnitude of Burgers vector, and σ 0 is the original stress. According to Eq. 2 and Eq. 1, the total residual stress σ residual can be written as µε (1 ν ) 4 (1 ν ) + L + σ residual = 1 Aµ b N + σ (5) m 0 (1 ν ) + σ 0 πr Based on Eq. 1, the total residual stress σ residual induced by WCP can be obtained by the combined finite element method and dislocation density method (FEM/DDM) aroach. 4. Results and discussion 4.1. Microstructures The microstructural changes of the suerficial layers as a function of WCP duration, at the same observation oints, are shown in Fig.3. Figure 3 (a) deicts tyical equiaxial α grains of annealed ure titanium, with grain sizes in the range of 20 to 100 μm. From Fig. 3 (b-d), the microstructural evolution of the titanium secimens at WCP durations of 15 min, 30 min and 45 min can be clearly observed. The deformation twinning induced by WCP at a deth of 30 to 40 μm from the surface can be observed via the OM micrograhs in Fig. 4 (a,b). The density and quantity of deformation twinning increase gradually with increasing WCP duration, and decrease gradually with increasing layer deth from the treated surface. The density and quantity of deformation twinning no longer significantly increase when the WCP time exceeds 45 min and when the deth of deformation twinning reaches aroximately 150 μm. The trend in microstructural change is similar to that of the comressive residual stress. At the same time, the integrity of the treated surface is maintained, roving that WCP can roduce a smoother surface with minimal structure losses. The TEM microstructures at a deth of 30 to 40 μm are deicted in Fig. 5. The interaction of deformation twinning in two different directions, as well as the high density dislocations among the bands of twinning, can be seen in Fig. 5 (a). Three tyes of twinning systems cross at the same oint, as shown in Fig. 5 (b). 4

6 4.2. Residual stress The tyical residual stress distributions in the suerficial layers, as functions of WCP duration, are deicted in Fig.6. From Figure 6, it can be observed that the maxima of the comressive residual stresses due to WCP are at the surfaces of the secimens for all of the investigated WCP treatment durations. This observation result differs from that of shoteened. The maximum residual stress is usually observed below the surface during the shot-eening rocess. The distributions of comressive residual stresses deend on material characteristics and eening conditions. Furthermore, the load mode of WCP is an imact wave ressure due to cavitation collase. The obvious macro-nonuniform lastic deformation is not induced by WCP, as comared to the deformation one would observe in other traditional methods. Crystalline defects, such as vacancies, interstitials, dislocations, and twinning, are the rimary causes of comressive residual stress distributions during WCP rocess. The maximum of comressive residual stress observed was 620 MPa at a WCP duration of 60 min. As the deth from the surface was increased to 150 µm, the comressive residual stress was observed to gradually decrease. In comarison to residual stress distributions of WCP-treated secimens, those of original secimens essentially show zero residual stress throughout the entire near-surface. Since the comressive residual stress was observed to slowly increase after 45 minutes, this can be considered as the saturation time of comressive residual stress. This result indicates that the comressive residual stress can be induced in the near-surface of the secimen through WCP. Vickers hardness measurements were conducted on the eened (45 min) and uneened secimen surfaces to evaluate their resective hardnesses. Table 2 deicts the changes in surface hardness of the eened and uneened secimens, and therein, the average hardness of the eened secimens was about 17 HV higher than that of the uneened secimens, indicating strengthening due to WCP Analysis results of residual stress by FEM/DDM model According to the redicting method roosed in this aer, the residual stress distributions were redicted after 45 min with the different alied imact ressures. Figure 7 shows the redicted results of residual stress after 45 min with the different alied imact ressures. The black line indicates the redicted results based on the EFM model, while the red line reresents the rediction results based on the DDM model. The green line shows the total residual stresses based on the FEM/DDM model. In Fig. 7, the redicted results with different alied imact ressures after 45 min and the exerimental results measured by XRD method are comared. The redicted results considering the macro-residual stress and the micro-residual stress with alied imact ressure around 800 MPa are in good agreement with the exerimental results by XRD method. These results indicate that the new model could otentially be a useful tool in redicting the residual stresses and investigating the effect of rocessing materials arameters on the distribution of residual stress. 4.4 Strengthening modeling and mechanism of WCP Fig. 8 illustrates the roosed mechanism of WCP, and deicts the strengthening model of the surface microstructure for a ure titanium late during the WCP rocess. The modeling is based on the residual stress concentration generated by a ile-u caused by a dislocation activation of the imact source [21-26], such as that shown in Fig. 8. The amount of dislocation-induced ile-u is determined by the distance L between the dislocation lines created by the source and barrier, and the alied stress. The local stress in front of the barrier is equal to the roduct of the initial residual stress and the number of iled-u dislocations. The unique twinning that is created will strongly deend on micro-residual stresses and dislocation-induced ile-u. The initiation and roagation of the twinning in a neighboring grain are also shown in Fig.8. The crystal structure of ure titanium is a tyical HCP structure at room temerature. The basal lanes (0001) are the close-acked lanes, and there are three close-acked directions <1120>; hence, in HCP, there are three sli 5

7 systems. This low number of sli systems means it is difficult to lastically deform an HCP metal. It can be shown that five sli systems must oerate in each grain of a olycrystal if it is to deform in a manner comatible with the deformations of its neighbors. In light of these constraints, the deformation of an HCP olycrystal must occur via mechanical twinning to avoid cracking at the grain boundaries. The dislocation sources in crystal grains are continuously induced in the near-surface of secimens due to the shear stresses. The dislocation density increases gradually, and many dislocation ile-us form. When lastic deformation occurs, deformation twinning is induced at the location of the dislocation block due to the low number of sli systems. Imact waves caused by the collase of a cavitation bubble can roduce significant strain, a high strain rate, and numerous direction cycle loads, which all induce the overlaing and interaction of deformation twinning. This indicates that the enhancement mechanism of WCP might be related to the activity of the deformation twinning and the high density of dislocations. At the same time, the evolution of microstructures can also induce comressive residual stress in the near-surface of the secimens. From the results in this aer, we conclude that the mechanism corresonding to the model roosed in this aer is aroriate. Since it accurately describes the systems investigated here, we feel it may be useful for designing WCP rocess conditions, and erhas for harnessing the ossible reinforcing effect observed in the WCP-rocessed secimens. (a) 0min (b) 15min WCP surface WCP (c) 30min (d) 45min Fig. 3: The microstructural evolution rocess of ure titanium with WCP duration, at the same observation oint. 6

8 (a) (b) Fig. 4: OM microstructures at a deth of μm from the surface. (a) (b) Twinning Twinning Twinning interaction Twinning interaction Dislocation 800nm 400nm Fig. 5: TEM microstructures at a deth of μm from the surface. (a) The interaction of deformation twinning in two different directions, as well as the high density dislocations among the bands of twinning ; (b) Three tyes of twinning systems cross at the same oint. Table 2 : The hardness changes induced by WCP on the surface Process Vickers hardness (HV) Average (HV) Uneened Peened

9 Fig.6: Distributions of residual stress in the strengthening layer. Fig. 7: Predicted results of residual stress versus exerimental data by FEM/DDM. Fig. 8: Modeling the strengthening of surface microstructures during the WCP rocess. 4. Conclusions The results obtained in this investigation show that WCP can induce a stable comressive residual stress in the suerficial layer of titanium secimens. The deth of the comressive residual stress zone can be u to aroximately 150 μm. Deformation twinning and dislocations lay an imortant role in the micro-lastic deformation of HCP-structured metal materials. The following conclusions can be drawn from this aer concerning the residual stress generation mechanism. The dislocation density of the α-hase of ure titanium increases when the imact energy caused in the WCP rocess exceeds the internal energy of the ure titanium crystal in the initial WCP rocess stages. This was confirmed to generate twinning deformation via the dislocation-induced formation and develoment of a sliing zone. The deformation due to twinning, the interaction between these deformation twins and dislocations, and local micro-lastic deformation are all induced by WCP in the strengthening layer. These findings indicate that the enhancement mechanism of WCP might be related to both the activity of the deformation twinning, and to the dislocations. The redicted results of residual stress in the near-surface of ure titanium were in good agreement with those of the exerimental measurements 8

10 Acknowledgements This research receives ongoing suort from the High-Tech Research Center and Oen Research Center at the Saitama Institute of Technology. References [1] Hammitt FG and De MK Wear [2] Tomita Y and Shima A 1986 J. Fluid Mech [3] Chen YL, Kuhl T and Israelachvili J 1992 Wear [4] Sun Z, Kang XQ, Wang XH 2005 Mater. Des [5] Tang CH, Cheng FT and Man HC 2006 J Surf. Coat. Tech [6] Qin M, Ju DY and Oba R 2006 Surf. Coat. Tech [7] Han B, Ju DY and Jia WP 2007 Al. Surf. Sci [8] Sahaya Grinsan A and Gnanamoorthy, R., 2006 Al. Surf. Sci. 253, [9] Sahaya Grinsan A and Gnanamoorthy R 2006 Al. Surf. Sci [10] Sahaya Grinsan A and Gnanamoorthy R 2006 J Surf. Coat. Tech [11] Ramulu M, Kunaorn S, Jenkins M, Hashish M and Hokins J.2002 J. Press. Vess. Technol [12] Kunaorn S, Ramulu M, Jenkins M and Hashish M 2005 J. Press. Vess. Technol [13] Sahaya Grinsan A and Gnanamoorthy R 2007 J. Manuf. Sci. Eng [14] Sahaya Grinsan A and Gnanamoorthy R 2007 J. Eng. Mater. Technol [15] Han B, Ju DY and Nemoto T 2007 Mater. Sci. Forum [16] Ju DY, Qin M, Koubayashi T and Oba R 2006 Surf. Eng [17] Qin M, Ju DY and Oba R 2006 Surf. Coat. Tech [18] Martin U, Altenberger I, Scholtes B, Kremmer K and Oettel H 1998 Mater. Sci. Eng. A [19] Altenberger I, Scholtes B, Martin U and Oettel H 1999 Mater. Sci. Eng. A [20] Wu X, Tao N, Hong Y, Xu B, Lu J and Lu K 2002 Acta Mater [21] Harada Y, Fukaura K and Haga S 2007 J. Mater. Process. Tech [22] Wang TS, Lu B, Zhang M, Hou RJ and Zhang FC 2007 Mater. Sci. Eng. A [23] Noyan C and Cohen JB, 1987 Residual Stress Measurement by Diffraction and Interretation, Sringer-Verlag, New York, [24] Ju DY 2002 Handbook of Residual Stress and Deformation of Steel. In: Totten G, Howes M and Inoue T (Eds.), ASM International, Ohio, [25] Meyers MA, Vöhringer O and Lubarda VA 2001 Acta Mater [26] Gilman JJ 1997 Phil. Mag [27] Jordan AS, Caruso R and von Neida AR Bell Syst. Tech