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Zhao longzhi, Chen binghui, Yangmin, Yan Hong УДК 669.71:629.014+629.014
INVESTIGATION OF EQUAL CHANNEL ANGULAR EXTRUSION OF 6061 ALUMINUM ALLOY USED IN HIGH-SPEED
TRAIN*
1 Introduction
With the development of high speed train, the weight reduction of the train is becoming the scientific focus, which can decrease the energy consume. Due to high ratio of strength to density, Aluminum insteading of the convention steel can need the demand of the high speed train, which attract a great deal of attention in recent years. The main challenges of aluminum train are the process for train body, substructure and frame.
It is well known that the mechanical properties and ductility can be improved by grain refinement. Thus, the application of equal channel angular extru-sion(ECAE)[1], which is one of the severe plastic deformation techniques, has the potential in refining the microstructure of bulk materials[2-9]. In addition, it has many unique features such as large strains, high strain rates, no change of shape, minimal load requirements etc.. In its most typical deform, the ECAE process involves shear deformation of the workpiece through a die containing two channels of equal cross section that meet at a predetermined angle(Fig.1). Deformation occurs in the immediate vicinity of the plane lying at the intersection of the two channels. Although conceptually simple, the straining within the deformation zone where the channels intersect is complex. A number of approaches have been taken to describe the kinematics of the deformation, including those based on the slip-line field technique and related flow models, the finite element method (FEM), the physical models, and the related simulative tech-niques[10,11]. DEFORM-2D finite element analysis,
* Foundation item: Sponsored by supported by Key Laboratory of Ministry of Education for Conveyance and Equipment (East China Jiaotong University) (No.01306016) and(No.01307148), Young Science foundation of Jiangxi Provine Education Office(No.09497), the National Natural Science Foundation of China(No. 50765005).
the most powerful metal forming process simulation software is widely using in the world currently and. In recent years, it has been widely used in the numerical simulation of metal forging, extrusion, rolling and other plastic processing technology. In this study, DEFORM-2D was used to simulate the ECAE process of 6061 aluminum alloy, the influence of mold inside corner radius on the ECAE process by the change of stress and strain was discussed.
2 Simulation preparation
2.1 Die structure
In the cuboid sample, with the width of m and the length of l, strain along the thickness direction is zero, so the process of ECAE can be regarded as two-dimensional plane strain process. ECAE mold shown in Figure 1, intersection angle y and lateral angle y of the two-channel are 90°. Channel width is d=10mm, inside corner radius is r=5mm, outer corner radius is R=8mm.
Fig. 1. Schematic of ECAP mould
2.2 The choice of simulation parameters
The commercil finite element software DE-FORM-2D is selected to simulate the ECAE, which have two ways to read the flow stress. One is put the data points of flow stress under the deformation parameter ^¿/r) into software, and then the software
механика. транспорт. машиностроение. технологии
automatically interpolation calculation according to the entered data; the other is put the empirical or semi-empirical formula of flow stress into software. The former was used in this paper. The material used for the simulation is 6061 aluminum alloy, Figure 2 shows its stress-strain curves. The material of die is H13, and the process parameters of ECAE simulation shown in Table 1. The simulation ignores the friction and the plastic deformation generated from the heat, deformation process can be as the isothermal process, so the simulation do not taken into account the temperature field distribution, and the die and punch is considered as rigid body.
Fig. 2. Stress-strain curves of 6061 aluminum alloy
3 Results and discussion
3.1 Process of ECAE deformation
Figure 3 shows the load-time curve of ECAE process, the results show that the load driving the deformation varies with the displacement during the following five steps: rapid increase, slowly increase, another rapid increase, stabilizing and decrease.
Fig. 3. The load-time curve
The characteristics of the five stages were discussed on the effective stress varying with increasing the displacement by simulation. Figure 4 shows the relation between the effective stress and the displacement. The front of billet has passed the corner, the parts occured shear deformation increasing with the deformation process, the internal stress and the load of billet have also been increased, as shown in Figure 4(a). Figure 4(b) shows the billet has passed shear deformation, and then bended toward the top of channel. It has not subjected too many restrictions before it contacts with the top of channel, so in this stage the load increases slowly. The billet contacts with the channel at this time, die restrict the deformation, so the load increased rapidly. It is noteworthy that the billet has not filled completely the channel in this stage, but formed a narrow and long slit with the top of channel. The load basically maintains invariable with extrusion advance, as shown in Figure 4(d).With the tail part of billet withdraws from the main deformation range, the load decreases, as shown in Figure 4(e).
Simulation parameters
Table.1
Billet tem- Die tempera- Extrusion Friction Nodes Units Step length,
perature, /C ture, /C speed,/mm • s"1 factor /mm
400 350 C 5 0.3 2583 2475 0.25
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(e)
Fig. 4. Relation between effective stress and displacement
3.2 Analysis of strained condition
Figure 5 shows the relation between effective strain and time at different location of billet, with the advance of the deformation the strain of each points increased and finally achieve to the stable stage, the effective strain of point 6 and point 7 less than other central points, and the effective strain values in the width direction of billet are also different. Effective strain at the bottom of billet less than the middle and top, and the time reach to a steady effective value longer.
Fig. 5. Relation between effective strain and time at different sites of sample
3.3 Analysis of stress state
Semiatin etal[14] suggested that ECAE is a pure shear deformation in plane through the research on its deformation mechanism. But this study shows that when the inner corner radius is not zero, the billet in the extrusion process is not only subject the shear stress, but also subject the compression or pull stretch effect in different extrusion stages and different parts, that is, the stress in extrusion process is closely related to the inner corner. In order to research the stress state of ECAE process, point 1 and point 2 on the inside corner and outside corner (Figure 6) were selected. From Figure 6(a), both P1 and P2 subject pressure stress in the beginning of extrusion, as the punch downward movement, the billet occurs plastic deformation when it enter into the channel bend, at this time the stress of P1 and P2 are beginning to get bigger because of the billet subject resistance by the outer corner. When the punch continued downward movement, the plastic deformation increases, and the variation of stress in the inner and outer corner, the compressive stress of P2 gradually converted to tensile stress and the value increased, at this time the stress of P1 still being compressive stress but its absolute value continues to increase. Billet continues to deform, P2 first enter into the corner, its tensile stress begin to decrease after reaching its maximum tensile stress, when the point reach 45°slope of channel bend, the tensile stress begin convert to compressive stress and this state remain to the end of deformation; P1 enter into the corner behind P2, when it reach to 45° slope of channel bend, the compressive stress convert to tensile stress and this state remain to the end of deformation. As it can be seen ,the stress of inside and outside corners are opposite, and the 45° slope of channel bend is separatrix, outer conner firstly suffer tensile stress, and then compressive stress, the inner conner firstly suffer compressive stress, and then tensile stress. The analysis of the maximum stress as shown in Figure 6(b) receive the similar results. the size and change trend of effective stress as shown in
(a) Maximum stress
(b) Mean stress
Time/s
(c) Effective stress Fig. 6. Relation of inner and outer corner between stress and time
Figure 6(c) is similar to Figure 6(a), it indicating the plastic deformation of P1 and P2 are the same.
4 Conclusion
(1) In the single stroke ECAE of 6061 aluminum alloy, the load driving the deformation varies with the displacement increase during the following five stages: rapid increase, slowly increase, another rapid increase, stabilizing and decrease.
(2) Inhomogeneous deformation exists in the width direction of the billet, and the plastic deformation at the end of the sample is smaller than that at top and in center.
(3) In the extrusion process, the mean stress and maximum principal stress of the inner corner and outer corner is different, the outer conner is the first ten-siled, and then compressed, on the contrary, the stress at the inner conner reverses, the change trend of effective stress is similar.
(4)The extrusion process is closely related to the inner corner, when the inner corner radius is not zero, the
billet in the extrusion process is not only subject to the shear stress,but also the compression; while the inner corner is zero, it only subject to the shear stress.
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Zhang Jian, Li Deying, Zhao Longzhi
YßK 669.719:629.014+629.014
SIMULATION OF 3-D TRANSIENT TEMPERATURE FIELD IN SELECTIVE LASER SINTERING*
1 Introduction
Since the advent of aluminum industry in 1888, aluminum has been in close contact with the railway industry for its features of light weight, corrosion resistance and good overall durability. Aluminum is not only applied in goods vehicles, light rail trains and intercity transport trains, subways, fast trains, intercity passenger trains as a mature material, but also in today's high-speed trains, such as Ace-la^ TGV Transrapid, Japanese Shinkansen and Pendolinotype, and in future maglev trains [1]. Therefore, the prospect of aluminum applied in the railway industry is very broad.
* Sponsored by Foundation of ECJTU Science and Re-search(01308013)
Selective Laser Sintering (SLS) is a rapid prototyping technology, which sinters the metal powder with high-power laser layer by layer to form 3-D part according to the established paths. It has many advantages, such as short cycle to manufacture, insensitivity to the shape complexity and wide range of materials [2,3]. SLS technology is applied not only to produce accurate models and prototypes, but also of direct functional metal part with reliable structure. As aluminum has strong reflectance to the light and heat, and oxidizes easily in the air to produce Al2O3 with high MP, so it is difficult to control in the selective laser sintering process. Therefore, the studies of aluminum and aluminum matrix composites introduced to SLS have been rare reports.
In SLS, the distribution of temperature field has a direct impact on sintering mechanisms, and then
