modifying equal channel angular pressing (ecap) for cu-al-ni
TRANSCRIPT
Modifying Equal Channel Angular Pressing (ECAP) for Cu-Al-Ni Shape Memory Alloys Processing
Hesam Ghourchibeigy Department of Mechanical Engineering K. N. Toosi University of Technology
Tehran, Iran [email protected]
Ali Shokuhfar Department of Mechanical Engineering K. N. Toosi University of Technology
Tehran, Iran [email protected]
Mohammad Sadegh Haghighat Nia Department of Mechanical Engineering
Shahid Rajaee Teacher Training University Tehran, Iran
Fatemeh Arabi Dentistry Department
Shahed University Tehran, Iran
Mehrdad Arjmand Department of Mechanical Engineering, K. N. Toosi University of Technology
Tehran, Iran [email protected]
Abstract— Different external parameters related to the ECAP process of Cu-Al-Ni shape memory alloys have been studied using ABAQUS 6.10 Software, which is widely used today. The parameters studied are outer corner angle and, die and billet temperature. Exploring optimum amounts for the influencing parameters, have made this work outstanding. Besides, there was no data for ECAPing Cu-Al-Ni shape memory alloys; so, the next work should be experimental study of the process. Based on the optimal strain homogeneity in the sample with lower dead zone formation, without involving any detrimental effects, outer corner angle was selected.
Keywords: Equal channel angular pressing, Cu-Al-Ni shape memory alloys, FEM analysis
I. INTRODUCTION Recently, the importance of severe plastic deformation
and equal channel angular pressing (ECAP) has been increasingly recognized due to the unique physical and mechanical properties inherent in various ultrafine grained materials [1–4]. Configuration of an ECAP die is shown if “Fig. 1”. In addition, Shape memory alloys show an important capability of acting both as sensors and actuators, which makes them outstanding multifunctional materials to be used in smart applications [5, 6]. It is quite well-known that, among the different families of shape memory alloys, Ti–Ni alloys (or Nitinol) have been traditionally used with great success for a wide variety of technological purposes, covering not only the biomedical field [7], but also several industrial applications [8,9]. However, Nitinol shows a higher cost than other alloys and a very strong limitation of
transformation temperatures (about 120 °C). Nowadays, there is a current increasing demand of shape memory alloys to be used as smart materials at higher temperatures (over 200 °C). Therefore, Cu–based alloys have been tested as a preferential alternative to Nitinol for high temperature applications, since they show a lower cost than Ti–Ni, high thermal stability and transformation temperatures as high as 240 °C [10]. Nevertheless, polycrystalline Cu–Al–Ni alloys produced by conventional casting are quite brittle [11], which is related to their large elastic anisotropy, large grain size and strong orientation dependence of transformation strain [12, 13]. For different reasons, the methods tried to improve the ductility of polycrystalline Cu–Al–Ni alloys, such as the addition of grain refiners [14] and the elaboration by melt spinning [15], have not given the expected results. Therefore, ECAP was considered to be a good alternative to these methods [16, 17].
In this paper, the effects of external factors on the deformation during ECAP are investigated. The finite element analyses are employed for this purpose. First, the separate effects of the outer corner, die and billet temperature and working temperature was considered.
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2010 International Conference on Nanotechnology and Biosensors IPCBEE vol.2 (2011) © (2011) IACSIT Press, Singapore
Figure 1. Configuration of an ECAP Die
II. FINITE ELEMENTS METHODS In order to investigate the effects of aforementioned
parameters on the deformation behavior of workpieces and to obtain optimum die design, a plane-strain finite element method (FEM) simulation of the ECAP process was carried out using the ABAQUS Software. The friction factor between the inner surfaces of the die channel and the specimen was assumed to be 0.1, corresponding to a value in hot metal forming [18]. A constant ram speed of 1 mm/s was employed. The numbers of initial meshes of a plane strain element are around 5000.
The analysis was carried out on °=Φ 90 . The commercial finite element software, Abaqus/Explicit, was used to carry out the simulations. Billet with dimensions of 10×100 mm was modeled for 2D plane strain simulations with ideal-elastic material properties of Cu-Al-Ni with Ultimate Tensile Strength 700=yS , thermal expansion
coefficientC
e°
−= 1563.1α , Young’s
Modulus GPaE 65= , Density 312.7 cmg=ρ and
Poisson’s ratio 3.0=υ [19]. All simulations were
performed with a ram speed of smm1 , which generates
negligible heat, due to the plastic deformation. CPS4R elements for die and CPE4R elements for billet were used. Elements edge sizes vary from 0.3 to 6.
Several parameters were studied in this work. The first studied parameter was corner angle,Ψ , and the process was studied for °°°°=Ψ 36.29,2.23,53.17,34.12 . Second factor was process temperature. Three different conditions were studied in this section was: process at room temperature, process at C°400 and at last C°400 for die and C°600 for billet temperatures.
III. RESULTS AND DISCUSSION
A. Overal deformation behavior of the sample billet The deformed shape of the samples simulated with
different corner angles is shown in Fig. 2. It depicts the deformation behavior of the sample, dead zone/corner gap formation and thus gives broad idea on the generation of stresses and strains during the deformation step. The mesh of the sample simulated with 34.120°=ψ , shown in Fig. 2a, is more severely deformed than that with other angles (Fig. 2b–d). As the outer corner angle increases the front end of the sample starts to bend upwards (observable in Fig. 2c and d), and generates non-uniform deformation in the front end of the sample. This bending causes wastage of substantial volume of material during multi-pass ECAP. The amount of non-uniform deformation is observable in the form of equivalent plastic strain (PEEQ) distribution.
Figure 2. Stress contours for deformed billet at various corner angles.
The strain homogeneity and the amount of strain induced in the deformed samples were directly obtained from the effective strain distribution in the samples simulated with different tool angles as shown in Fig. 3. The PEEQ values are shown at the end of the deformation process to study the strain homogeneity achieved in the fully deformed sample. The PEEQ values achieved in the sample simulated with sharp outer corner angle (Fig. 3a) were mostly constant except at the outer side of the sample.
B. Die and billet temperature and working temperature To study die and billet temperature effects, four different
simulations was done. The first one simulated the situation at which the die temperature is C°400 and billet temperature is C°600 (case1). The second one was done with the same temperature for die and billet, C°400 (case2). Third case was carried out with room temperature for both die and billet,
C°27 (case 3). It is obvious that process temperature does not affect the
studied parameters significantly, and so it may just has effect in material flow, which was not considered in our analysis. According the [19], and based on figures 4-8, the process should be done at elevated temperatures, which will help us to do the ECAP process easier.
°= 34.12)ψa
°= 53.17)ψb
°= 2.23)ψc
°= 36.29)ψd
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Figure 3. PEEQ distribution of the samples simulated with different outer
corner angle.
Figure 4. Load- Displacement curve for 3 aforementioned cases.
Figure 5. Max. Equivalent plastic strain- Displacement curve for 3
aforementioned cases.
Figure 6. logarithmic shear strain at integration points- time curve for 3
aforementioned cases.
Figure 7. S. Mises for billet- time curve for 3 aforementioned cases.
Figure 8. S. Mises for die- time curve for 3 aforementioned cases.
(a
(b
(c
(d
°= 34.12ψ
°= 53.17ψ
°= 2.23ψ
°= 36.29ψ
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IV. CONCLUSION The best outer corner angle was explored using FEM
analysis, done with Abaqus 6.10 software. The samples simulated with °= 34.12ψ , behaved closest to ideal simple shear but generated a larger strain distribution and severe stress concentration at the sharp outer corner. Amongst all the outer corner angles, °= 36.29ψ and °= 2.23ψ showed greater non-uniformity in strain distribution than
°= 53.17ψ and °= 34.12ψ . °= 2.23ψ And °= 36.29ψ also yield greater tensile stresses in the
deformed samples. Thus, optimal strain homogeneity in the material, with lower dead zone and without involving detrimental effects, can be achieved with
°=Φ 90 and °= 34.12ψ .
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