INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 15, No. 3, pp. 503-512
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DOI: 10.1007/s12541-014-0364-3
Design Method for Intermediate Roll in Multi-Stage Profile Ring Rolling Process: The Case for Excavator Idler Rim Kyung-Hun Lee1, Dae-Cheol Ko2, Dong-Hwan Kim3, Seon-Bong Lee4, Nag-Mun Sung5, and Byung-Min Kim6,# 1 PNU-IFAM Joint Research Center, Pusan National University, San 30, Jangjeon-dong, Geumjeong-gu, Busan, South Korea, 609-735 2 ERC/ITAF, Pusan National University, San 30, Jangjeon-dong, Geumjeong-gu, Busan, South Korea, 609-735 3 Department of Mechanical and Automotive Engineering, International University of Korea, 965, Dongburo Munsaneup, Jinju, Gyengnam, South Korea, 660-759 4 Faculty of Mechanical and Automotive Engineering, Keimyung University, 1000, Sindang-dong, Dalseo-gu, Daegu, South Korea, 704-701 5 Production Department, KALTEK, 1064-1, Toerae-ri, Hanlim-myun, Kimhae, Gyeongnam, South Korea, 621-873 6 School of Mechanical Engineering, Pusan National University, 30, Jangjeon-dong, Geumjeong-gu, Busan, South Korea, 609-735 # Corresponding Author / E-mail:
[email protected], TEL: +82-51-510-3074, FAX: +82-51-581-3075 KEYWORDS: Multi-stage profile ring rolling, Roll design, Uniform volume distribution element technique, Extremum rolling ratio, Excavator idler rim
In this paper, the design method for intermediate rolls has been developed to manufacture excavator idler rims in multi-stage profile ring rolling process, based on the uniform volume distribution element technique (UVDET) and the extremum rolling ratio(λmax). UVDET is presented as the ratio of the volume change of ring blanks to that of undeformed rings per unit height. λmax is defined as the ratio of the cross-sectional area of blanks to that of rolled rings. The theoretical value of intermediate roll shapes were determined by UVDET and λmax, i.e. the outer diameter and cross-sectional shape of the intermediate ring, respectively. FE simulations and profile ring rolling experiments using AISI 1035 steel alloys were performed to prove the validity of proposed design method. In order to investigate the deformation behavior in profile ring rolling of excavator idler rims, the expanding rule of plastic zone should be carried out firstly. The proposed roll design method led to the successful ring shape and the highest dimensional precision. Manuscript received: August 30, 2013 / Revised: December 10, 2013 / Accepted: January 7, 2014
1. Introduction Ring rolling is a universally applicable metal forming process to manufacture seamless rolled ring parts. Application for seamless rolled rings includes a wide range of mechanical parts, such as gear rims, powder generation plants, aircraft engines and large cylindrical vessels. Ring rolling has many advantages, such as high productivity, superior mechanical and high temperature properties, smooth surface, uniform quantity, close tolerance, considerable saving in material cost and shorter cycle time of additional machining.1,2 Most recently there is an increasing commercial interest in complicated cross-sectional shaped rings manufactured directly by plastic deformation instead of machining. Profile ring rolling is very difficult forming process that reduces the wall thickness, enlarges the diameter of the ring blank and forms the section of the ring. Moreover, net shaping of idler rims with the non-symmetric section is much more difficult due to the bending and under-filling of ring, generated because of different deformation velocities at a section and different sectional area at each position of ring.3 Typically, the cross sections in profile rings are developed through
© KSPE and Springer 2014
a multi-stage ring rolling operation with shaped rolls. The aim is to roll gradually the preform into a larger ring part that has a more complex cross-section than the original. The preform is typically a smaller cylindrical shape with a central hole and an outside diameter in the same range as the source ingots. Profiled contours can be obtained by subsequent mechanical processing or rolling with shaped tools. Intermediate ring geometries and kinematic relationship between different sets of rolls must be controlled carefully, in order to control the cross-sectional shape and the outer diameter of the ring accurately. The design of roll dies and preforms is established through practice and relies heavily on designer experience and simple analytical tools. However, to move ahead, a new paradigm in designing dies and preforms is required. The new approach is based on a shift to scientific techniques such as finite element analysis and quick-execution computer methods. These techniques have the potential to increase process efficiency and reduce materials waste in the ring rolling process. The earlier studies for profile ring rolling were carried out based on the theoretical and experimental method. Mamalis et al. studied the cavity formation in rolling profiled rings based on the slip line method and experimental works.4 Moussa and Hawkyard investigated the multi-
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stage ring rolling of aluminum bicycle wheel rims. During the design stages an analysis was made of the flange folding mechanism, to determine suitable conditions for avoiding hinge formation at the loading point.5 Over the last decade, many researches have sought to apply FEM to profile ring rolling process. Lim et al. investigated strain distribution of V-section profiled rings and observed that plastic strain is concentrated on the inner and outer diametrical surfaces.6 Davey and Ward studied an ALE approach for finite element ring rolling simulation of profiled rings.7 Yeom et al. studied a process design of profile ring rolling for turbine diaphragm using the dynamic materials model and FE simulation.8 New techniques have been developed by Souza et al. in order to lower the cost and improve the buy-to-fly ratio of rings and cases in aerospace engines through advances in preform design and processing.9 Kim et al. investigated manufacturing processes of Ti-6Al-4V plane and profiled ring products using three-dimensional FEM simulation and experimental analysis.10 Li et al. revealed the interactive influences between the feed rate of mandrel and rotational speed of main roll on T-shaped cold ring rolling by 3D-FE numerical simulation.11 Hua et al. explained the expanding rules of plastic zone in roll gap and deformation behavior in L-section profile cold ring rolling by FE simulation.12 Qian et al. researched on gripping conditions and provided theoretical basis for technical design in profile ring rolling of raceway groove.13 Zhao and Qian studied the influence of rolling ratio on groove-section profile ring rolling by theoretical analysis and FE simulation.14 Deformation behavior of spirogyrate ring for cold ring rolling and the expansion law of the ring diameter were investigated by Xu et al.15 All these studies have focused on simple profile ring rolling like Lsection, T-section, groove-section and its plastic deformation behavior. It is difficult to apply to the actual production for lack of the research on intermediate roll design, especially the excavator idler rim with profile in both sides. The purpose of this study is to develop intermediate roll design method of multi-stage profile ring rolling for the excavator idler rims with both radial surface contours, based on the uniform volume distribution element technique (UVDET) and extremum rolling ratio (λmax). UVDET is presented as the ratio of the volume change of a ring blank to that of undeformed ring per unit height. Extremum rolling ratio is defined as the ratio of appropriately sectional area of blank to that of rolled ring, respectively. To verify the proposed method, we carried out FE simulations by developing reliable 3D-FE models using Forge, a commercially finite-element platform. The analytical results were inspected by profile ring rolling experiments performed using an AISI 1035 alloy steel ring with an initial outer diameter of 337 mm. The proposed method was evaluated in terms of the shape accuracy and geometries of rolled rings.
2. Design Method for Intermediate Roll of Multi-stage Profile Ring Rolling 2.1 Mechanism of profile ring rolling Ring rolling is a typical incremental forming process with high flexibility. Fig. 1 shows the schematic illustration of a profile ring rolling mill. The ring rolling process not only holds the attributes of the plate
Fig. 1 Schematic illustration showing operation of radial-axial ring rolling machine
rolling, asynchronous rolling and multi–way rolling, but also relates to the feed movement of the mandrel and axial rolls, the rotating movement of the main rolls, the guiding movement of the centering rolls as well as the rotation of the ring itself with the diameter expanding movement. Movement control for each roll is as follows; (1) The main roll rotates at a rotational speed during whole process. (2) The mandrel advances gradually towards the main roll and runs idle because of the friction on the contact surface. (3) The set of axial rolls rotates with rotating of the ring and upper conical roll simultaneously squeezes the ring height. (4) Two centering rolls grip the ring outer diameter to ensure the circularity of the ring and control the axial roll rotation speed. The movement of the centering roll is controlled by outer diameter of ring measured by a tracer roll during the process. Some cases of the profile ring with both inside and outside contours require many kinds of main rolls and mandrels. So several stages may be needed corresponding to a roll change. The ring is typically reheated between these stages. During profile ring rolling process, metal flows in three different ways in bite region: axially, radially, and circumferentially. Unlike closed-die forging, in which flow is restricted by the die cavity, in ring rolling an increase in roll forces causes material to flow in the circumferential direction. Flow in the circumferential direction causes ring growth, and the rate of such growth can vary along the cross section. Some regions can actually thin out in preference to others, depending on the roll interaction, and this can results in local underfills.10 To minimize deleterious under-fills during the final stage in ring rolling, research on intermediate process and roll design in multi-stage profile ring rolling is necessary.
2.2 Process classification of ring cross section shapes Ring rolling process has classified them into two large groups according to the cross section of rings, whether the section is profiled or not. Fig. 2 shows a range of typical contoured shaped cross sections that can be produced by ring rolling.16 When the profiled ring is a symmetric and small size, mandrel makes the square section into symmetric section. However, if the profiled ring is non-symmetric or large size, the type of mandrel control system cannot be used because the rigidity of mandrel is poor or two mandrels need to be exchanged during ring rolling process. Fig. 3 shows the procedure for process classification and roll design
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contours, the multi-stage profile ring rolling is recommended as shown in Fig. 2 (18, 19, 20, 21, 22, 23). Among these contours, trapezium shape may be fabricated by single-stage profile ring rolling using pre-forged ring blanks.
2.3 Calculation of initial ring blank size The first step of a roll design for the profile ring rolling process is to find initial blank dimensions and calculate rolled ring dimensions. First, the dimension of final profiled ring shapes should be determined from the target product by adding the allowable tolerance (Z) for each of the ring surfaces, and checked based on the size and weight limitations of the ring mill system capacity. The allowable tolerance is expressed as: Fig. 2 Typical rolled ring cross sections: rings with rectangular cross sections (1~3), rings with inside contours (4~10), rings with outside contours (11~17), and rings with both side contours (18~23)16
in ring rolling. This figure shows that (a) the presence or absence of surface contours and symmetry planes, (b) the number of surface contours and symmetry planes, and (c) the direction of symmetry axis could classify the ring rolling process into three groups, a plain-, a single stage profile- and multi stage profile ring rolling. And then, (d) UVDET, the extremum rolling ratio determines the number of stage and intermediate roll shapes. With contoured cross sections, the behavior of materials being worked is even more difficult to predict than with rectangular cross sections. Many contours can be rolled from regular rectangular blanks, especially axisymmetric shapes with thinner wall sections at the center (double flange outside diameter or inside diameter) and with thicker wall section at the center (rings having shaped–protrusion), as shown in Fig. 2 (5, 7, 8, 10, 12, 14, 15, 17). Once the rolling ratio between crosssectional areas of initial rectangular ring blank and final ring product exceeds the extremum rolling ratio, the depth of the groove or protrusion that can be rolled with significant overall shape distortion are progressively reduced. In this case, the multi-stage profile ring rolling is required. On the other hand, when the rolling ratio is smaller than the maximum value, the product can be rolled by single-stage profile rolling. When it is found that a rectangular or open-die blank will not yield the desired contour, i.e., final ring product with one-side contour without a symmetry plane, blank preforming must be used, as shown in Fig. 2 (4, 6, 9, 11, 13, 16). Typically, the starting point for a new contour shape (from a preformed blank) is the application of the volume conservation factor and UVDET between final ring products to initial blanks. For example, flanges are asymmetric rings with outer profile showing a very different volume distribution from top to bottom. The flange neck has a sleeve-type shape whereas the flange body is more disc-shaped. Due to the highly different volumes distribution a profiled blank shape is required. The ring is divided into a number of axial slices, or disks, and the volume of each slice is calculated. By knowing the diameter of the rolling mandrel to be used, and therefore the inside diameter of the blank, and a theoretical blank outside diameter can be calculated for each slice (assuming no height change). The theoretical blank outside diameter shape is generated by the aggregate of the individual slice outside diameters. In order to manufacture profiled rings with both inside and outside
Z = C1 + C2 + C 3 + C4
(1)
where, C1 = 2.54 mm (allowance for unavoidable defects based on DIN 7527), C2 = 0.579 mm (growth factor for ring diameter), C3 = 0.209 mm (growth factor for ring height or wall thickness, whichever is larger),16 and C4 is variable allowances caused by a scale loss.18 In order to induce the uniform deformation of the profiled ring and reduce the applied load the final product was prepared by multi-step processes. The first step is to make a donut-shape preform that is usually formed by upsetting and piercing processes. The second step is to form the plain shape by the ring rolling process. To prevent the surface defects such as fishtail and fold the relationship between the wall thickness and ring height as shown in Eq. (2) was used.17 S H0 S ------ = 1 – -----1- D2 + -----0H1 H1 H1
(2)
where, the subscripts 0 and denote preform and plain shape ring dimensions, H is the height, S is the wall thickness, respectively. The inner diameter of preform, D0,i is determined by the piercing punch size. Applying the constant volume condition, H1, S1, D1,o and D1,i as the desired plain ring dimensions, S0 can be obtained from the following equation.17 D1, o + D1, i S0 S 2 S -------------------------= ----- 1 – -----1- D + -----02 ( D0, i + S0 ) S1 H1 H1
(3)
where, D is the diameter, the subscripts i and o stand for the inner and outer diameters, respectively.
2.4 Uniform volume distribution element technique (UVDET) The objective of formulating the volume distribution is to design the intermediate roll shape.18 Basic concept of UVDET is to break up the blank into a number of regions like line elements. During the process, the cross-sectional area of the ring reduces at each time step, resulting in a corresponding increase in diameter of the ring due to the material incompressibility. The material flows across the boundary of an element into its neighboring element, while satisfying volume constancy. The total volume of a ring is calculated by summing up the volume per unit height for all the line elements. The optimal roll shape for intermediate ring rolling stages is obtained by minimizing the volume distribution per unit height that is calculated by the volumes of the undeformed and rolled ring:
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Fig. 3 Flow chart of process design for ring rolling
Vi, h – Vi + 1, h hh = -------------------------= 0 , ( 0#i Vi, h 2
2
2
h)
(4) 2
radial contours is shown in Fig. 4. According to geometry relationships, sectional areas are
Vi, h = p (Ri, h – ri, h ) , Vi + 1, h = p ( Ri + 1, h – ri + 1, h )
A1 = S1 H1
(6)
where, ηh is the reduction in volume per unit height, i.e., the volume distribution. V and h are, respectively, the volume and height of ring blanks. The subscript i denotes the number of stages in multi-stage profile ring rolling process. So Vi,h represents the volume per unit height of intermediate roll shapes.
Af = Sf Hf – SG, f ⋅ HG, f – 2SP, f ⋅ HP2, f – SP, f ⋅ HP1, f
(7)
2.5 Extremum rolling ratio The rolling ratio, I is defined as the ratio of a sectional area of previous blank to that of deformed ring, which is regarded as a decisive parameter for the technical design because it directly influences the blank dimension and ring deformation degree.14,18 A I = -----1 Af
(5)
where, A1 and Af are sectional area of blank and rolled ring, respectively. The dimension of the plain ring blank and deformed ring with both
Due to the axial restriction of the closed rolling gap, the axial height of the ring is approximately unchangeable. Therefore it can be assumed that H1 = H f
(8)
Substituting Eqs. (6), (7) and (8) into Eq. (5), the ring wall thickness, S1, is obtained as follow: λA λA S1 = --------f = --------f H1 Hf
(9)
Allowing for the volume constancy in metal plastic deformation, then, the volume factor, C, can be following. C = ( 2R1 – S1 ) ⋅ S1 ⋅ H = Lc, f ⋅ Af
(10)
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Fig. 5 Dimension of ring cross sections: (a) plain ring blank and (b) profile ring for an excavator idler rim
Fig. 4 Plain ring blank and rolled profile ring product: (a) plain ring blank and (b) deformed ring with rectangular groove- and protrusionsection profile where, Lc,f is a distance between center of sectional area of blank. Substituting Eq. (9) into Eq. (10), the relationships between the rolling ratio and the blank dimensions are obtained. lAf C C lAf R1 = ---------- + -------- , r1 = ---------- – -------2lAf 2Hf 2lAf 2Hf
(11)
The ultimate condition for the ring blank satisfied for the stable rolling of profiled ring with rectangular groove and protrusion sections is shown in Eq. (12), where the radius of mandrel is not greater than the inner radius of blank. S1 + Rmandrel ≤ R1
(12)
Substituting Eq. (11) into Eq. (12), the following Eq (13) can be drawn. 2
– Rmandrel + Rmandrel + C/Hf lmax ≤ ----------------------------------------------------------------Af /Hf
(13)
The theoretical value of intermediate roll shape design is determined using Eqs. (4) and (13), i.e., the outer diameter and cross-sectional shapes of the intermediate ring, and extremum rolling ratio could be calculated, respectively.
2.6 Roll design of excavator idler rims An excavator idler rim has both inside-groove and outside-protrusion contours, and then, multi-stage profile ring rolling is recommended to manufacture this type of rings, as shown in Fig. 3. Fig. 5(a) shows the cross section of a plain ring and Fig. 5(b) is the cross-sectional geometry of excavator idler rim, which is based on a constant volume. The cross-sectional profile marked by a dashed line and solid line represent the idealized representation of actual idler rims and rolled ring product considering the tolerance. The designed outer diameter of profile ring is 579 mm, the inner diameter is 453.7 mm, and the height in axial direction is 202 mm, respectively. Using the above calculation method for initial ring blank size, i.e., Eq (3), the plain ring
Fig. 6 Non-uniform volume distribution between the plain and final profile ring
was determined as the outer diameter (D1,o) of 337 mm, wall thickness (S1) of 61 mm and ring height (H1) of 202 mm, respectively. Non-uniform volume distribution between the plain and final profile ring is shown in Fig. 6. There are two typical areas: the blue-hatched area (Part B, as shown in Fig. 5(a)) and the red-shade area (Part A, as shown in Fig. 5(a)) having negative and positive value of volume distribution, respectively. First, the hatched area can actually thin out in preference to others and this can result in local under-fillings. Second, the shade area can result in significant bulges at outer diameter. This bulge is axially rolled on thus causing an overlap at the inner diameter of idler rims or causes excessive ring growth at the center phase. The volume conservation law in metal forming indicates that blank volume is constant during ring rolling, but it does not demonstrate that volumes of each ring part A, B and C remain constantly. If there is material flow between each part it possibly interferes with their normal deformations, so the rolled ring does not obtain the specifically dimensional requirements, blank dimensions needs to be adjusted to compensate for the additional material. Under this condition, in order to satisfy dimension requirements of the rolled ring, blank dimensions need to be adjusted to compensate for the additional material. Fig. 7(a) represents the uniform volume distribution between the intermediate ring and final product by the above Eq. (4). There are many intermediate ring shapes which are satisfied the uniform volume distribution condition, as shown in Fig. 7(b)-(d). To choose an optimum one among various intermediate roll shapes in multi–stage ring rolling,
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Fig. 7 Dimension of intermediate ring cross sections and volume distribution: (a) uniform volume distribution between the intermediate and final profile ring, (b) intermediate ring-I with rolling ratio of 1.51, (c) intermediate ring-II with rolling ratio of 1.34, and (d) intermediate ring-III with rolling ratio of 1.14
the additional standard is required. Another standard to determine the intermediate roll shape is a rolling ratio, λmax. Using Eqs. (5) and (13) the rolling ratio (λ) between the initial plain ring and final profile ring, and the theoretical value of extremum rolling ratio (λmax) are 1.83 and 1.34, respectively. Because the rolling ratio exceeds the maximum value, idler rims cannot be manufactured through single-stage profile ring rolling. The intermediate roll shape should be satisfied the uniform volume distribution condition and λ ≤ λmax. The proposed intermediate roll shape is shown in Fig. 7(c) and its outer and protruding inner diameters are 427.3 mm and 300.6 mm, respectively. To verify the effectiveness of roll design method, we carried out FE-simulations by developing reliable three-dimensional finite element models using Forge platform and experiments for multistage ring rolling of excavator idler rims using AISI 1035.
3. Verification of Proposed Design Method 3.1 FE-analysis model FE simulations were performed to prove the validity of proposed roll design method, under Forge software environment. The 3D FE model has feature as follows: (1) Rolls are set as rigid bodies. Main roll rotates at a constant rotation speed around axial direction. Mandrel is free to rotate around axial direction and move along feed direction. The centering roll option triggers. (2) Coulomb friction model is adopted to friction of contact surfaces between rolls and ring, and the friction coefficient of contact surfaces between forming roll and ring, mandrel and ring is taken 0.178.18 (3) An ALE approach and a coupled thermo-mechanical tetrahedron
Fig. 8 3-D FE model for profile ring rolling of excavator idler rims: (a) 2nd profile ring rolling and (b) 3rd profile ring rolling element with 4 nodes are adopted for model calculation to avoid convergence problem and reduce computation times. Based on the intermediate roll design method, two kinds of 3D-FE simulation models of profile ring rolling are established by a coupled thermo-mechanical FE method. First, the 3D-FE model for profile ring rolling of excavator idler rims is shown in Fig. 8. FE- model for 1st plain ring rolling process was skipped. Fig. 8(a) represents FE-model for 2nd profile ring rolling process, profile mandrel (#2), and a cross section of initial ring blanks. FE model for 3rd profile ring rolling process, profile mandrel (#3), and a profiled cross section are shown in Fig. 8(b). The profiled cross-section after 2nd profile ring rolling is fully the same as the intermediate ring II (see Fig. 7(c)). Relevant parameters needed for the model are shown in Table. 1. Used Material of model is AISI 1035 steel, its density, elastic modulus and Poisson’s ratio are 7850 kgm-3, 200 GPa and 0.3, respectively. The initial temperature of ring blanks is 1250oC and the stress-strain curves are shown in Fig. 9. The friction coefficient defined on the interfaces
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Table 1 Input parameter for FE-analysis and experiments of excavator idler rims Parameter Ring material Friction coefficient Radii of plain main roll RPlain main Radii of profile main roll RProfile main (Outer/Inner) Radii of 1st plain mandrel RPlain mandrel #1 Radii of 2nd profile mandrel RProfile mandrel #2 (Outer/Inner) rd Radii of 3 profile mandrel RProfile mandrel #3 (Outer/Inner) Radii of centering roll RCentering Cone angle of axial rolls α Outer diameter of plain ring D1,o Inner diameter of plain ring D1,i Axial height of plain ring H1 Temperature of ring blank T Rotational speed of main roll NMain
Value AISI 1035 0.178 425 mm 425 mm / 388 mm 65 mm 102.64 mm / 63.35 mm 116.55 mm / 74.4 mm 145 mm 40° 337 mm 215 mm 202 mm 1250°C 30 rpm
Fig. 10 Variation curves of feed amount in profile ring rolling of excavator idler rims
Fig. 9 Effective stress-strain curves of AISI 1035 under different temperatures and strain rates: (a) 0.1 s-1, (b) 1 s-1, and (c) 10 s-1, and (d) hot compression test between the ring and main roll is 0.178.18 The friction coefficients on the mandrel and axial rolls are zero because they are assumed as smooth surfaces.19 The used temperature-dependant physical properties (thermal conductivity specific heat, etc) of the alloy are derived from Metal Supplier Online of America: specific heat 778 J·kg-1·oC-1, conductivity 35.5 W·m-1·C-1, emissivity 0.88. All the rolls are treated as a rigid body. The draft of a mandrel and upper axial roll are calculated based on the feasible forming condition and shown in Fig. 10.18,19
3.2 Result of FE-analysis Ring plastic penetration means that plastic zone penetrates the ring section, and ring produces the plastic deformation by reducing the thickness and expanding the diameter in the rolling process. The expanding rule of plastic zone can be clearly revealed in the process of
Fig. 11 Plastic zone distributions of different ring sections during 2nd profile ring rolling: (a) excavator idler rim and (b) axial section of ring
ring plastic penetration.12 In order to investigate the deformation behavior in profile ring rolling of excavator idler rims, the expanding rule of plastic zone should be carried out firstly. Fig. 11 shows the plastic zone distributions of different ring sections during 2nd profile ring rolling process. Blue areas indicate the elastic zone, and areas with other colors mean the plastic zone with different plastic strain. The plastic zones are first produced at the internal and exterior surfaces of ring part C (see Fig. 5(a)), when feed amount is little. With the increase of feed amount, the plastic zones of internal surface and exterior surface expand towards the central plan along radial direction at part B, and penetrate the ring wall at last, as shown
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Fig. 14 Profile ring rolling experiment of excavator idler rims
Fig. 15 Schematic illustration of forming sequence for manufacturing excavator idler rims
Fig. 12 Plastic zone distributions of different ring sections during 3rd profile ring rolling: (a) excavator idler rim and (b) axial section of ring
section (CG), when feed amount is little. As the feed amount increases, the plastic zone of CG expands towards the corner of outer protrusionsection (CP). Exterior surface of part A receives the stress of the profile main roll, but internal surface does not receive stress at early stage of ring rolling process. With the increase of feed amount, the plastic zone of CO expands towards the internal surface of part A. Fig. 13 shows the cross-sectional shapes of the deformed material excavator idler rim at each feed amount of 10 mm. After the draft of 72.02 mm it approaches the required final cross-sectional shape.
4. Experiment of Multi-Stage Profile Ring Rolling Process
Fig. 13 Comparison of cross-sections of excavator idler rim after each feed amount of 10 mm
in Fig. 11(b). In order to restrict metal flow in the axial direction and keep UVDET, the inclined part A of profile mandrel #2 comes into contact with internal surface of ring, and then, the plastic zone of internal surface expand toward exterior surface. Finally, plastic zone expands from part A, C towards the part B of ring in axial direction, and gradually penetrate the whole section of the profile ring. Fig. 12 shows the plastic zone distributions of different ring sections during 3rd profile ring rolling process. Due to different stress conditions, the expanding rules of plastic zone in radial sections of each part are different (see Fig. 5). Exterior surface and internal surface of part C receive stress of profile main roll and mandrel #3, respectively, so the plastic zones are firstly produced at the corner of inner groove-
4.1 Experimental procedure In order to verify the validity of the proposed design method for intermediate rolls experimentally, the multi-stage profile ring rolling experiment have been carried out using AISI 1035 alloy steel, as shown in Fig. 14. The rolling mill (Kaltek Co., Ltd.) used in this work has the manufacturing capacity of outside diameter in the maximum 3000 mm and the maximum rolling force of about 8000 kN. The related dimension of the rolls and ring blank, initial temperature of ring blank, and ring material are listed in Table 1 and Fig. 7(c). Fig. 15 represents the schematic illustration of the forming sequence in order to manufacture excavator idler rims. General experiment conditions are as follows: (1) 1st plain ring rolling for fabricating a cylindrical ring blank using plain mandrel #1 and main roll (2) Dimensions of rolled plain ring: outer diameter of 337 mm, inner diameter of 215 mm, and height of 202 mm (3) 2nd profile ring rolling for fabricating an intermediate ring using profile mandrel #2 and plain main roll (4) Dimension of rolled profile ring: outer diameter of 427.3 mm (5) 3rd profile ring rolling for manufacturing an excavator idler rims using profile mandrel #3 and profile main roll
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Fig. 17 Rolled excavator idler rim and its cross section
Fig. 18 Comparison of shape accuracy between rolled excavator idler rim and 3D-drawing
Fig. 16 Deformed ring configuration during multi-stage profile ring rolling: (a) 1st plain ring rolling, (b) 2nd profile ring rolling, and (3) 3rd profile ring rolling (6) Dimensions of rolled idler rims: ridge-outer diameter 579 mm, outer diameter 535 mm, inner diameter 453.7 mm, and the wall thickness of grove and ridge 15 mm
of 427.5 mm. There is no volume flow between the intermediate and final rings in the axial direction, and the excavator idler rim is formed well, as shown in Fig. 16(c). The outer diameter is 535 mm, and there are no bending defects and under-filling areas. The complete ring cross section must be acquired at the same moment the desired diameter is reached for the following reason. Immediately after the pass is filled, the thinner-wall section attempts grow faster circumferentially than the thicker-wall sections for a given decrease in roll gap. The thicker sections are stretched by the more rapid circumferential growth of the thin sections, and the contour begins to deteriorate in the thicker-wall sections. Therefore, it is very important to fill the inner and outer protrusion-section area at the same time. Fig. 17 represents rolled excavator idler rim and its cross section. Using non-contact 3D laser scanning (SURVEYOR DS-4060), comparison of shape accuracy between rolled excavator idler rims and 3D- drawing are shown in Fig. 18. The least, greatest and mean values of an observational error at the outer surface are 0.06 mm, 2.95 mm and 1.28 mm, respectively. The design method using the UVDET and extremum rolling ratio led to the successful ring shape and the highest dimensional precision.
5. Conclusions 4.2 Experimental results Fig. 16 shows the deformed ring configuration during the multi-stage profile ring rolling process. The outer diameter and axial height of a rolled plain ring are 337.5 mm and 202.4 mm after the 1st plain ring rolling, respectively. During the 2nd profile ring rolling process, the outer surface of the ring was fully contact with main roll, and there are no bending defects. The rolled profile ring has a particular outer diameter
In this study, a general roll design method for multi-stage profile ring rolling process was proposed. The effectiveness of proposed method was validated by 3D FE-analysis and profile ring rolling experiments of excavator idler rims. The main conclusions are as follows: (1) The proposed roll design method was posited based on the uniform volume distribution element technique, UVDET and the
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extremum rolling ratio, λmax: the condition is expressed by Eqs. (4) and (13). (2) Expanding rules of plastic zone were clearly revealed in hot ring rolling process with an arbitrary section profile. During the rolling process of excavator idler rims, exterior and internal surface of part A and C receive stress of profile main roll and mandrel #3, respectively, so the plastic zones are firstly produced at the corner of inner groove-section (CG) and the outer surface of part A (C0), when feed amount is little. With the increase of feed amount, the plastic zone of CG expands towards the corner of outer protrusion-section (CP). As feed amounts increase, the plastic zone of CO expands towards the internal surface of part A. (3) The proposed design method led to the successful ring shape and the highest dimensional precision. This research results provide valuable guidelines for the roll design in the actual profiled ring production.
ACKNOWLEDGEMENT This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2012 R1A5A1048294) and PNU-IFAM JRC.
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