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Experimental study of through-depth residual stress in explosive welded Al–Cu–Al multilayer
Materials and Design 37 (2012) 577–581
Contents lists available at SciVerse ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Short Communication
Experimental study of through-depth residual stress in explosive welded Al–Cu–Al multilayer M. Sedighi, M. Honarpisheh ⇑ School of Mechanical Engineering, Iran University of Science and Technology, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 31 July 2011 Accepted 13 October 2011 Available online 6 November 2011
a b s t r a c t Explosive welded multi-layers sheet metals are widely used in recent years. Evaluation of residual stresses of this new merging material is necessary for better understanding of its mechanical behavior. In this paper, incremental hole-drilling (IHD) is used to measure through-depth non-uniform residual stress gradient in explosive welded Al–Cu–Al multilayers. At first, the multilayer sheets at two different initial thicknesses were made by using explosive welding. Then, through-depth residual stress gradient was obtained with the IHD method. In addition, calibration coefficients have been obtained to evaluate the residual stress by using finite element method (FEM) analysis. Results show that multilayers surface is subjected to high tensile residual stress. Also there is an intense gradient of residual stress at the interface of multilayers. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Explosive welded multilayers are used in different sectors such as aerospace and food industries. This new materials have superior properties such as corrosion and wear resistance with proper mechanical properties. Explosive welding is used as an excellent alternative for joining dissimilar metals and alloys at solid state [1]. Although, other techniques can be used to weld dissimilar metals [2,3] but explosive welding process can bond materials such as aluminum, titanium, copper and stainless steel [4]. This process can be joined wide variety of both similar and dissimilar metals [5]. High strength of bonding is one of the main advantages of the explosive welding process [6]. In this method, the impact causes work-hardening [7]. Kacar and Acarer [8] evaluated mechanical behaviors of explosive cladding of 316L stainless steel-DIN-P355GH steel. Gulenc [9] has investigated the interface properties and weld ability of aluminum and copper plates made by explosive welding method. Acarer and Demir [10] studied mechanical and metallurgical properties of explosive welded aluminum-dual phase steel. Through-depth residual stress can be created in explosive-welded multilayers due to different linear expansion coefficients of dissimilar materials [11]. The residual stress in the explosive welded multilayers affects on the behavior of multilayers. Also, the tensile pattern of residual stress on the surface is particularly undesirable, since they cause an increased susceptibility to fatigue and stress corrosion. Therefore, it is necessary to evaluate residual stresses in explosive welded multilayers. Residual stresses in engineering structures influence crack initiation, crack growth and fracture [12]. As one of the residual stress assessment method, hole-drilling method was first ⇑ Corresponding author. Tel.: +98 21 77491228; fax: +98 21 77240488. E-mail address: [email protected] (M. Honarpisheh). 0261-3069/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2011.10.022
carried out in the 1930s. Schajer and Yang [13] provided the first generalized finite element analysis of the hole-drilling method. Measurement of residual-stress distribution by the incremental hole-drilling method was carried out by Niku-lari et al. [14]. They proposed a new method of calibration and it is shown how the finite element analysis can be used for the determination of the correlation coefficients. Development of the high-precision incremental-step hole-drilling method for the study of residual stress in multi-layer materials was performed by Montay et al. [15]. They obtained the residual stress gradient at the test specimen. Pokataev et al. [16] have performed calculations and experimental determination of residual deflections of bimetallic components produced by explosion welding. Jianjun and Huaining [17] simulated welding residual stresses and the explosion shock waves action on welding residual stresses. They showed that the explosion treatment can indeed effectively reduce welding residual stresses. Numerical evaluation of residual stress in an explosive welded multilayer was performed by Wang et al. [18]. They predict that maximum residual stress occurred at the interface of multilayer. The incremental hole-drilling (IHD) method has been explained in ASTM E837-08 [19]. Therefore the IHD method could be used as an standard approach to estimate the non-uniform residual stress in the depth of explosive welded multilayers. To the best of authors’ knowledge, experimental measurement of through-depth residual stress in the explosive welded multilayers is not reported so far. Therefore, the aim of the present study is to measure the residual stress gradient in Al–Cu–Al multilayers in two different initial thicknesses by using IHD method. In this paper, firstly material and explosive welding process were defined and theory of measurement of residual stress has been discussed. Then, through-depth residual strain was measured using strain indicator. Also calibration coefficients have been obtained using
578
M. Sedighi, M. Honarpisheh / Materials and Design 37 (2012) 577–581
Fig. 1. Boundary condition to obtain (a) An calibration coefficient and (b) Bn calibration coefficient.
Bn ¼
e1n e3n 2Dhn ðr1n r2n Þ cos 2hn
Or
ð2Þ 1 n
2 n
Bn ¼
e e Dhn ðr1n r2n Þðsin 2hn þ cos 2hn Þ
Fig. 2. Parallel arrangement of experimental set-up of explosive welding process.
An ¼
e1n þ e3n 2Dhn ðr1n þ r2n Þ
finite element method. Finally, through-depth residual stresses values were extracted.
Or
2. Through depth residual stress measurement In this section, firstly the theory of measurement method is explained. Then the material and explosive welding condition is defined. Finally the experimental measurement process of residual stress is explained. 2.1. Theory In the incremental hole-drilling method, calculation is based on the measured experimental strains ðe1n ; e2n ; e3n Þ at each increment of sheet depth and calibration coefficients called An and Bn [19]. If a 45° rosette is used the principal stresses r1n and r2n can be calculated from Eq. (1):
8 e1n ðAn þ Bn sin 2hn Þ e2n ðAn Bn cos 2hn Þ > > > < r1n ¼ 2An Bn ðsin 2hn þ cos 2hn ÞDhn 2 > e ðA þ Bn cos 2hn Þ e1n ðAn Bn sin 2hn Þ > n > : r2n ¼ n 2An Bn ðsin 2hn þ cos 2hn ÞDhn
ð1Þ
A 3D finite element model can be used to compute the An and Bn coefficients. The determination of calibration coefficients has been done in two steps. Firstly, two FEM analyzes have been carried out. Then, the strain data has been read from FEM outcome to calculate calibration coefficients. In FEM step of the procedure, different loads are applied in the model shown in Fig. 1. Fig. 1a can provide data for calculation of An and Fig. 1b for calculation of Bn. The finite element models were built with C3D4 elements. The FEM analysis is intended to start with no hole model loaded with proper boundary condition. To simulate the drilling operation, the elements in the hole are removed from the mesh and the presented load in Fig. 1 is applied for each increment. In the second step, the calibration coefficients in each increment are calculated using Eqs. (2) and (3).
ð3Þ
e1n sin 2hn þ e2n cos 2hn An ¼ Dhn ðr1n þ r2n Þðsin 2hn þ cos 2hn Þ hn ¼
1 tan1 2
e1n 2e2n þ e3n e1n e3n
ð4Þ
where e1n ; e2n and e3n are strains in the finite element model, r1n and r2n are applied load in the model at each increment and hn is the direction of maximum residual stress deviated from direction 3. 2.2. Material definition and explosive welding process In this study, a parallel arrangement was used for experimental setup of explosive welding process (Fig. 2). The structure of the composite laminate was made from the copper layer setting in the middle with aluminum alloy layers on both sides, manufactured by double explosive welding process. The explosive used in this study was a powder type, AMATOL, of detonation velocity equal to 2500 m/s (detonation wave speed through the explosive) and density equal to 800 kg/m3. The thickness of the explosive was equal to 14 mm. In this study, the measurement of residual stress in the Al–Cu–Al multilayers has been performed in two different initial thicknesses using the IHD method. The multilayer strips produced by explosive welding in two different cases; Case 1: the initial thickness of strip was equal to 3.78 mm (tal = 1.35 mm, tcu = 0.97 mm and tal = 1.46 mm) and Case 2: the initial thickness of strip was equal to 4.85 mm (tal = 1.86 mm, tcu = 0.91 mm and tal = 2.08 mm). Also the true stress–strain behaviors of aluminum and copper have been shown in Fig. 3 obtained by tensile tests. 2.3. Experimental measurement approach Measurement of through-depth residual stresses in the explosive welded multilayers has been performed in the mentioned two cases. The experimental strain results were obtained by using
M. Sedighi, M. Honarpisheh / Materials and Design 37 (2012) 577–581
579
Fig. 3. True stress–strain behaviors of aluminum and copper layers.
Fig. 4. Strain gage rosette arrangement for determining residual stress [15].
FRS-2-23 gauge (made by TML Company) with a gauge circle diameter of 5.13 mm (Fig. 4). The drilling operation was carried out using a 27000 rpm high velocity drilling machine and using a 2 mm central drill. The high speed drilling allows to minimize the stresses induced by the operation. The strain data were measured using a Vishay strain indicator (Fig. 5). 3. Results and discussion To investigate the residual stress distribution in the multilayers, it is necessary to obtain the calibration coefficient in multilayers as Eqs. ((2) and (3)) and using FEM analysis. Figs. 6 and 7 show the variation of these coefficients in the depth of the multilayers. The distribution of residual stress in the Al–Cu–Al multilayers strip have been presented in Figs. 8 and 9 by using the measured strain data, the calibration coefficients and Eq. (1). Different linear expansion coefficients of dissimilar materials create residual stresses in explosive welded multilayers. After the explosive welding process, residual strains are remained in the layers. These strains act like a stress source on the explosive-welded multilayer. It leads to residual stress creation in the multilayer. It is necessary to mention that the residual stress distribution in the explosive-welded multilayer has direct relation with such parameters like; materials properties of each layer, explosive welding
Fig. 5. Instrumentation of residual stress measurement.
parameters and especially layers arrangement. In this study, the experimental measurement of residual stress shows that the surface (aluminum layer) is subjected to high tensile residual stress state. The arrangement of layers in this study creates tensile residual stress on the surface of the multilayer. Although there are limited number of reports in the literature about residual stresses distribution in explosive-welded multilayers, but numerical evaluation of residual stress [18] shows the tensile residual stress at the surface (titanium layer) in the multilayer welded joint of titanium–tantalum. Tensile residual stresses in the products are generally undesirable because they lower the elastic limit of the products and also increase the tendency to warpage during subsequent machining operation. The mechanism of creation of this tensile residual stress pattern at the surface can be explained as follow: explosion waves moves in the direction of explosion, so the aluminum layer is subjected to tensile stress. It is obvious that the tensile stress transmit to the copper layer after contacting by the aluminum layer. The elastic elongation of the aluminum layer is higher than the copper layer due to their different elasticity modules. When the layers return to the equilibrium condition without external load, the aluminum layer remains at the tensile residual stress state. This may cause cracking on the surface of explosive-welded multilayer. So, the user should be aware of the surface tensile residual stress when any external load is applied. Also an intense gradient of residual stress occurs in the interface of multilayer. There is a higher
580
M. Sedighi, M. Honarpisheh / Materials and Design 37 (2012) 577–581
Fig. 6. Through-depth calibration coefficients in Al–Cu–Al explosive welded multilayer; Case 1.
Fig. 9. Through-depth residual stress in Al–Cu–Al explosive welded multilayer; Case 2.
that the initial thickness of Al–Cu–Al multilayers does not play an important role on the surface residual stresses. In these figures gradient of maximum residual stress in aluminum layer is similar for two presented cases. Also it could be seen from the results that in both mentioned cases the tensile residual stress decreases in the aluminum layer when getting far from surface. It is necessary to mention that, in the incremental hole drilling method, results of residual stresses at the depth greater than 1 mm have some errors [19]. As it can be observed from Figs. 6 and 7, the calibration coefficients have sharp variation at the depth greater than about 1 mm. This fact can affect the residual stress accuracy for the depth greater than 1 mm as shown in Figs. 8 and 9. 4. Conclusions Fig. 7. Through-depth calibration coefficients in Al–Cu–Al explosive welded multilayer; Case 2.
Explosive welding is a technique to bond similar and dissimilar metals with large surface areas. In addition, evaluation of throughdepth residual stress in explosive welded multilayers is necessary due to its effects on material behavior. In this study measurement of through-depth residual stress in Al–Cu–Al multilayers has been performed by using incremental hole-drilling method. The following points can be highlighted: 1. Calibration coefficients An and Bn were calculated by using FEM analysis to obtain residual stress in multilayers sheets. 2. There was an intense variation of residual stress in interface of Al–Cu–Al multilayer. 3. The tensile residual stress has been occurred on the surface of multilayers. 4. Initial thickness of Al–Cu–Al multilayers does not play an important role on the values of residual stresses at the surface.
References
Fig. 8. Through-depth residual stress in Al–Cu–Al explosive welded multilayer; Case 1.
stress level at the interface due to differences in material properties. The results show that the materials properties have a significant effect on the magnitude and distribution of the residual stress in the layers and their interface. On the other hand, Figs. 8 and 9 show
[1] Akbari-Mousavi SAA, Farhadi Sartangi P. Experimental investigation of explosive welding of cp-titanium/AISI 304 stainless steel. Mater Des 2009;30:459–68. [2] Zhu ML, Xuan FZ. Effects of temperature on tensile and impact behavior of dissimilar welds of rotor steels. Mater Des 2010;31:3346–52. [3] Zhu ML, Xuan FZ. Correlation between microstructure, hardness and strength in HAZ of dissimilar welds of rotor steels. Mater Sci Eng A 2010;527:4035–42. [4] Findik F. Recent developments in explosive welding. Mater Des 2011;32:1081–93. [5] Crossland B. An experimental investigation of explosive welding parameters. Met Technol 1976;3:8. [6] Akbari-Mousavi SAA, Al-Hassani STS, Atkins AG. Bond strength of explosively welded specimens. Mater Des 2008;29:1334–52.
M. Sedighi, M. Honarpisheh / Materials and Design 37 (2012) 577–581 [7] Mamalis AG, Vaxevanidis NM, Szalay A, Prohaszka J. Fabrication of aluminium/ copper bimetallics by explosive cladding and rolling. J Mater Process Technol 1994;144:99–117. [8] Kacar R, Acarer M. An investigation on the explosive cladding of 316 L stainless steel-din-P355GH steel. J Mater Process Technol 2004;152:91–6. [9] Gulenc B. Investigation of interface properties and weldability of aluminum and copper plates by explosive welding method. Mater Des 2008;29:275–8. [10] Acarer M, Demir B. An investigation of mechanical and metallurgical properties of explosive welded aluminum-dual phase steel. Mater Lett 2008;62(25):4158–60. [11] Ghosh M, Das S, Banarjee PS, Chatterjee S. Variation in the reaction zone of solid state bonded commercially pure titanium and 304 stainless steel. Mater Sci Eng A 2005;390:217–26. [12] Mahmoudi AH, Truman CE, Smith DJ. Using local out-of plane compression (LOPC) to study the effects of residual stress on apparent fracture toughness. Eng Fract Mech 2008;75:1516–34. [13] Schajer GS, Yang L. Residual-stress measurement in orthotropic materials using the hole-drilling method. Exp Mech 1994:324–33.
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[14] Niku-Lari A, Lu J, Flavenot JF. Measurement of residual-stress distribution by the incremental hole-drilling method. J Mech Work Technol 1985;11: 167–88. [15] Montay G, Cherouat A, Lu J, Baradel N, Bianchi L. Development of the highprecision incremental-step hole-drilling method for the study of residual stress in multi-layer materials: influence of temperature and substrate on ZrO2–Y2O3 8 wt.% coatings. Surf Coat Technol 2002;155:152–60. [16] Pokataev EP, Trykov YP, Kraev AS. Calculations and experimental determination of residual deflections of bimetallic components produced by explosion welding. Weld Int 1999;13(6):482–4. [17] Jianjun G, Huaining C. Numerical analysis of welded joint treated by explosion shock waves. Front Mater Sci China 2007;1(2):197–202. [18] Wang Y, Cal H, Ma N. Measurement of residual stresses in a multi-layer explosive welded joint with successive milling technique. Strain 1999:7–10. [19] ASTM E837-08. Standard test method for determining residual stresses by the hole drilling strain-gauge method. West Conshohocken: American Society for Testing and Materials; 2008.
Contents lists available at SciVerse ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Short Communication
Experimental study of through-depth residual stress in explosive welded Al–Cu–Al multilayer M. Sedighi, M. Honarpisheh ⇑ School of Mechanical Engineering, Iran University of Science and Technology, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 31 July 2011 Accepted 13 October 2011 Available online 6 November 2011
a b s t r a c t Explosive welded multi-layers sheet metals are widely used in recent years. Evaluation of residual stresses of this new merging material is necessary for better understanding of its mechanical behavior. In this paper, incremental hole-drilling (IHD) is used to measure through-depth non-uniform residual stress gradient in explosive welded Al–Cu–Al multilayers. At first, the multilayer sheets at two different initial thicknesses were made by using explosive welding. Then, through-depth residual stress gradient was obtained with the IHD method. In addition, calibration coefficients have been obtained to evaluate the residual stress by using finite element method (FEM) analysis. Results show that multilayers surface is subjected to high tensile residual stress. Also there is an intense gradient of residual stress at the interface of multilayers. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Explosive welded multilayers are used in different sectors such as aerospace and food industries. This new materials have superior properties such as corrosion and wear resistance with proper mechanical properties. Explosive welding is used as an excellent alternative for joining dissimilar metals and alloys at solid state [1]. Although, other techniques can be used to weld dissimilar metals [2,3] but explosive welding process can bond materials such as aluminum, titanium, copper and stainless steel [4]. This process can be joined wide variety of both similar and dissimilar metals [5]. High strength of bonding is one of the main advantages of the explosive welding process [6]. In this method, the impact causes work-hardening [7]. Kacar and Acarer [8] evaluated mechanical behaviors of explosive cladding of 316L stainless steel-DIN-P355GH steel. Gulenc [9] has investigated the interface properties and weld ability of aluminum and copper plates made by explosive welding method. Acarer and Demir [10] studied mechanical and metallurgical properties of explosive welded aluminum-dual phase steel. Through-depth residual stress can be created in explosive-welded multilayers due to different linear expansion coefficients of dissimilar materials [11]. The residual stress in the explosive welded multilayers affects on the behavior of multilayers. Also, the tensile pattern of residual stress on the surface is particularly undesirable, since they cause an increased susceptibility to fatigue and stress corrosion. Therefore, it is necessary to evaluate residual stresses in explosive welded multilayers. Residual stresses in engineering structures influence crack initiation, crack growth and fracture [12]. As one of the residual stress assessment method, hole-drilling method was first ⇑ Corresponding author. Tel.: +98 21 77491228; fax: +98 21 77240488. E-mail address: [email protected] (M. Honarpisheh). 0261-3069/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2011.10.022
carried out in the 1930s. Schajer and Yang [13] provided the first generalized finite element analysis of the hole-drilling method. Measurement of residual-stress distribution by the incremental hole-drilling method was carried out by Niku-lari et al. [14]. They proposed a new method of calibration and it is shown how the finite element analysis can be used for the determination of the correlation coefficients. Development of the high-precision incremental-step hole-drilling method for the study of residual stress in multi-layer materials was performed by Montay et al. [15]. They obtained the residual stress gradient at the test specimen. Pokataev et al. [16] have performed calculations and experimental determination of residual deflections of bimetallic components produced by explosion welding. Jianjun and Huaining [17] simulated welding residual stresses and the explosion shock waves action on welding residual stresses. They showed that the explosion treatment can indeed effectively reduce welding residual stresses. Numerical evaluation of residual stress in an explosive welded multilayer was performed by Wang et al. [18]. They predict that maximum residual stress occurred at the interface of multilayer. The incremental hole-drilling (IHD) method has been explained in ASTM E837-08 [19]. Therefore the IHD method could be used as an standard approach to estimate the non-uniform residual stress in the depth of explosive welded multilayers. To the best of authors’ knowledge, experimental measurement of through-depth residual stress in the explosive welded multilayers is not reported so far. Therefore, the aim of the present study is to measure the residual stress gradient in Al–Cu–Al multilayers in two different initial thicknesses by using IHD method. In this paper, firstly material and explosive welding process were defined and theory of measurement of residual stress has been discussed. Then, through-depth residual strain was measured using strain indicator. Also calibration coefficients have been obtained using
578
M. Sedighi, M. Honarpisheh / Materials and Design 37 (2012) 577–581
Fig. 1. Boundary condition to obtain (a) An calibration coefficient and (b) Bn calibration coefficient.
Bn ¼
e1n e3n 2Dhn ðr1n r2n Þ cos 2hn
Or
ð2Þ 1 n
2 n
Bn ¼
e e Dhn ðr1n r2n Þðsin 2hn þ cos 2hn Þ
Fig. 2. Parallel arrangement of experimental set-up of explosive welding process.
An ¼
e1n þ e3n 2Dhn ðr1n þ r2n Þ
finite element method. Finally, through-depth residual stresses values were extracted.
Or
2. Through depth residual stress measurement In this section, firstly the theory of measurement method is explained. Then the material and explosive welding condition is defined. Finally the experimental measurement process of residual stress is explained. 2.1. Theory In the incremental hole-drilling method, calculation is based on the measured experimental strains ðe1n ; e2n ; e3n Þ at each increment of sheet depth and calibration coefficients called An and Bn [19]. If a 45° rosette is used the principal stresses r1n and r2n can be calculated from Eq. (1):
8 e1n ðAn þ Bn sin 2hn Þ e2n ðAn Bn cos 2hn Þ > > > < r1n ¼ 2An Bn ðsin 2hn þ cos 2hn ÞDhn 2 > e ðA þ Bn cos 2hn Þ e1n ðAn Bn sin 2hn Þ > n > : r2n ¼ n 2An Bn ðsin 2hn þ cos 2hn ÞDhn
ð1Þ
A 3D finite element model can be used to compute the An and Bn coefficients. The determination of calibration coefficients has been done in two steps. Firstly, two FEM analyzes have been carried out. Then, the strain data has been read from FEM outcome to calculate calibration coefficients. In FEM step of the procedure, different loads are applied in the model shown in Fig. 1. Fig. 1a can provide data for calculation of An and Fig. 1b for calculation of Bn. The finite element models were built with C3D4 elements. The FEM analysis is intended to start with no hole model loaded with proper boundary condition. To simulate the drilling operation, the elements in the hole are removed from the mesh and the presented load in Fig. 1 is applied for each increment. In the second step, the calibration coefficients in each increment are calculated using Eqs. (2) and (3).
ð3Þ
e1n sin 2hn þ e2n cos 2hn An ¼ Dhn ðr1n þ r2n Þðsin 2hn þ cos 2hn Þ hn ¼
1 tan1 2
e1n 2e2n þ e3n e1n e3n
ð4Þ
where e1n ; e2n and e3n are strains in the finite element model, r1n and r2n are applied load in the model at each increment and hn is the direction of maximum residual stress deviated from direction 3. 2.2. Material definition and explosive welding process In this study, a parallel arrangement was used for experimental setup of explosive welding process (Fig. 2). The structure of the composite laminate was made from the copper layer setting in the middle with aluminum alloy layers on both sides, manufactured by double explosive welding process. The explosive used in this study was a powder type, AMATOL, of detonation velocity equal to 2500 m/s (detonation wave speed through the explosive) and density equal to 800 kg/m3. The thickness of the explosive was equal to 14 mm. In this study, the measurement of residual stress in the Al–Cu–Al multilayers has been performed in two different initial thicknesses using the IHD method. The multilayer strips produced by explosive welding in two different cases; Case 1: the initial thickness of strip was equal to 3.78 mm (tal = 1.35 mm, tcu = 0.97 mm and tal = 1.46 mm) and Case 2: the initial thickness of strip was equal to 4.85 mm (tal = 1.86 mm, tcu = 0.91 mm and tal = 2.08 mm). Also the true stress–strain behaviors of aluminum and copper have been shown in Fig. 3 obtained by tensile tests. 2.3. Experimental measurement approach Measurement of through-depth residual stresses in the explosive welded multilayers has been performed in the mentioned two cases. The experimental strain results were obtained by using
M. Sedighi, M. Honarpisheh / Materials and Design 37 (2012) 577–581
579
Fig. 3. True stress–strain behaviors of aluminum and copper layers.
Fig. 4. Strain gage rosette arrangement for determining residual stress [15].
FRS-2-23 gauge (made by TML Company) with a gauge circle diameter of 5.13 mm (Fig. 4). The drilling operation was carried out using a 27000 rpm high velocity drilling machine and using a 2 mm central drill. The high speed drilling allows to minimize the stresses induced by the operation. The strain data were measured using a Vishay strain indicator (Fig. 5). 3. Results and discussion To investigate the residual stress distribution in the multilayers, it is necessary to obtain the calibration coefficient in multilayers as Eqs. ((2) and (3)) and using FEM analysis. Figs. 6 and 7 show the variation of these coefficients in the depth of the multilayers. The distribution of residual stress in the Al–Cu–Al multilayers strip have been presented in Figs. 8 and 9 by using the measured strain data, the calibration coefficients and Eq. (1). Different linear expansion coefficients of dissimilar materials create residual stresses in explosive welded multilayers. After the explosive welding process, residual strains are remained in the layers. These strains act like a stress source on the explosive-welded multilayer. It leads to residual stress creation in the multilayer. It is necessary to mention that the residual stress distribution in the explosive-welded multilayer has direct relation with such parameters like; materials properties of each layer, explosive welding
Fig. 5. Instrumentation of residual stress measurement.
parameters and especially layers arrangement. In this study, the experimental measurement of residual stress shows that the surface (aluminum layer) is subjected to high tensile residual stress state. The arrangement of layers in this study creates tensile residual stress on the surface of the multilayer. Although there are limited number of reports in the literature about residual stresses distribution in explosive-welded multilayers, but numerical evaluation of residual stress [18] shows the tensile residual stress at the surface (titanium layer) in the multilayer welded joint of titanium–tantalum. Tensile residual stresses in the products are generally undesirable because they lower the elastic limit of the products and also increase the tendency to warpage during subsequent machining operation. The mechanism of creation of this tensile residual stress pattern at the surface can be explained as follow: explosion waves moves in the direction of explosion, so the aluminum layer is subjected to tensile stress. It is obvious that the tensile stress transmit to the copper layer after contacting by the aluminum layer. The elastic elongation of the aluminum layer is higher than the copper layer due to their different elasticity modules. When the layers return to the equilibrium condition without external load, the aluminum layer remains at the tensile residual stress state. This may cause cracking on the surface of explosive-welded multilayer. So, the user should be aware of the surface tensile residual stress when any external load is applied. Also an intense gradient of residual stress occurs in the interface of multilayer. There is a higher
580
M. Sedighi, M. Honarpisheh / Materials and Design 37 (2012) 577–581
Fig. 6. Through-depth calibration coefficients in Al–Cu–Al explosive welded multilayer; Case 1.
Fig. 9. Through-depth residual stress in Al–Cu–Al explosive welded multilayer; Case 2.
that the initial thickness of Al–Cu–Al multilayers does not play an important role on the surface residual stresses. In these figures gradient of maximum residual stress in aluminum layer is similar for two presented cases. Also it could be seen from the results that in both mentioned cases the tensile residual stress decreases in the aluminum layer when getting far from surface. It is necessary to mention that, in the incremental hole drilling method, results of residual stresses at the depth greater than 1 mm have some errors [19]. As it can be observed from Figs. 6 and 7, the calibration coefficients have sharp variation at the depth greater than about 1 mm. This fact can affect the residual stress accuracy for the depth greater than 1 mm as shown in Figs. 8 and 9. 4. Conclusions Fig. 7. Through-depth calibration coefficients in Al–Cu–Al explosive welded multilayer; Case 2.
Explosive welding is a technique to bond similar and dissimilar metals with large surface areas. In addition, evaluation of throughdepth residual stress in explosive welded multilayers is necessary due to its effects on material behavior. In this study measurement of through-depth residual stress in Al–Cu–Al multilayers has been performed by using incremental hole-drilling method. The following points can be highlighted: 1. Calibration coefficients An and Bn were calculated by using FEM analysis to obtain residual stress in multilayers sheets. 2. There was an intense variation of residual stress in interface of Al–Cu–Al multilayer. 3. The tensile residual stress has been occurred on the surface of multilayers. 4. Initial thickness of Al–Cu–Al multilayers does not play an important role on the values of residual stresses at the surface.
References
Fig. 8. Through-depth residual stress in Al–Cu–Al explosive welded multilayer; Case 1.
stress level at the interface due to differences in material properties. The results show that the materials properties have a significant effect on the magnitude and distribution of the residual stress in the layers and their interface. On the other hand, Figs. 8 and 9 show
[1] Akbari-Mousavi SAA, Farhadi Sartangi P. Experimental investigation of explosive welding of cp-titanium/AISI 304 stainless steel. Mater Des 2009;30:459–68. [2] Zhu ML, Xuan FZ. Effects of temperature on tensile and impact behavior of dissimilar welds of rotor steels. Mater Des 2010;31:3346–52. [3] Zhu ML, Xuan FZ. Correlation between microstructure, hardness and strength in HAZ of dissimilar welds of rotor steels. Mater Sci Eng A 2010;527:4035–42. [4] Findik F. Recent developments in explosive welding. Mater Des 2011;32:1081–93. [5] Crossland B. An experimental investigation of explosive welding parameters. Met Technol 1976;3:8. [6] Akbari-Mousavi SAA, Al-Hassani STS, Atkins AG. Bond strength of explosively welded specimens. Mater Des 2008;29:1334–52.
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