J. Cent. South Univ. (2014) 21: 493−499 DOI: 10.1007/s11771­014­1966­8 New type of groove used to improve friction in roll forging JIA Zhi(贾智) , ZHOU Jie(周杰), JI Jin­jin(姬金金) School of Materials Science and Engineering, Chongqing University, Chongqing 400044, China © Central South University Press and Springer­Verlag Berlin Heidelberg 2014 Abstract: A FEM model for a failed industrial example of roll forging was established to analyze the generation mechanisms of the mismatch of size and shape of two spring board. To demonstrate the formulation of these defects, the bites condition and contact status between rectangular groove and workpiece during rolling the first and second spring boards were analyzed. Then, a new oval­diamond groove combining oval groove and diamond groove was presented to eliminate these defects. By analyzing field variables under the same deformation degree, the larger friction can be obtained on the contact surface of workpiece and the oval­diamond groove. The physical experiment validates that the oval­diamond groove can eliminate these defects effectively, and the size of part is in good agreement with design requirement. Key words: roll forging; spring board; oval­diamond groove; rectangular groove; automobile front axles 1 Introduction Roll forging is a process for reducing the cross­sectional area of heated bars or billets by passing them between two driven rolls that rotate in opposite directions and have one or more matching grooves in each roll. Compared to other conventional forming processes, the roll forging possess has remarkable advantages, such as greater productivity, greater material utilization ratio, better product quality, more simple equipment structure, longer life of rolling dies, an improved environment and automation and lower production costs. For a successful roll forging process, it is necessary to consider the roll separation force, the spread, the elongation, and the appropriate geometry of the roll groove. A lot of factors, such as metal flow, stresses, rolling­separation force, temperature and elastic deflection of the rolls, must be taken into account [1−2]. Several publications in the literature analyzed the forming mechanism with physical testing and numerical simulation theory. KNAPINSKI [3] found that the roll geometry underwent considerable distortion over the whole roll length during rolling, and this would result in incorrect dimensions of the finished product. BENASCIUTTI et al [4] proposed a simplified approach to computate thermal stresses in work roll of hot rolling mills, and found that the largest thermal gradient developed within a small region close to work roll surface. YOO et al [5] presented an analytical model to predict the surface profile of a workpiece in a round­to­2­R oval groove and a square section­to­2­R oval groove rolling sequence, and found that the surface profile of the workpiece in the 2­R oval groove is smaller than that of the 1­R oval groove [5]. The effects of the penetration rate of rolling tool on the fatigue strength and the residual stress of groove­rolled products were investigated by KIM et al [6]. BYON et al [7] studied the effect of roll gap adjustment on the exit cross sectional area variation of workpiece in a two­stand groove rolling process, and found that the previous stand has a more strong influence on this area of workpiece than the next stand. TIAN et al [8] developed a new rolling force model and a torque model to analyze the rolling pressure in asymmetrical rolling process, and found that the length of cross shear region increases with the increase in the speed ratio. Many researchers have paid attentions to roll forging and studied the groove design and die parameters. NA et al [9] presented a design method of forming groove­to­separating groove rolling sequence used in slit rolling process. SHAHANI et al [10] studied the temperature distribution, stress, strain and strain rate field in the roll die, and used the artificial neural network to predict the behavior of the slab during the hot rolling process. The wear resistance of hot rolling was evaluated on the basis of a tribological test aimed at reproducing the damage mechanisms by PELLIZZARI et al [11]. Not only shaft parts but also billets for precision die forging can be produced through roll forging. Hence, this Foundation item: Project(51275543) supported by the National Natural Science Foundation of China; Project(cstc2009aa3012­1) supported by the Key Program of Chongqing Science and Technology Foundation, China Received date: 2012−09−24; Accepted date: 2013−11−10 Corresponding author: JIA Zhi, PhD Candidate; Tel: +86−13996399957; E­mail: [email protected]
494 process has been applied widely. SEDIGHI and MAHMOODI [12] studied the precision roll forging process of compressor blade, and the effect of elastic behavior of the machine structure as well as rolls deflection on the material flow and roll separating force. The blank­making roll forging die of front axle, pre­molding roll forging die and final forming roll forging die were designed respectively by LI et al [13]. ERVASTI and STAHLBERG [14] facilitated the development of adequately shaped grooves in the reducer rolling mill so that the flash volume obtained in production was reduced during forging process of automobile front axles. CAI [15] studied the forming process of automobile front axles, and the rolling die was designed. Because the material deformation styles during roll forging are different from die forging, there are some defects during roll process, and the design of rolling groove is very difficult. With the development of computers, the design of roll forging process has been optimized with the implementation of interactive graphics programs and simulation methods based on finite elements analysis. In this work, a complicated industrial example with defects was analyzed, and the experiments and FEM analysis were adopted to research the generation mechanisms of defects in roll forging. A new type of groove (oval­diamond groove) was presented to overcome the defects, such as the mismatch and undersize of two spring board. The field variable was analyzed to explain the difference between oval­diamond groove and rectangular groove. Finally, the physical experiment was carried out to validate the feasibility of this research. J. Cent. South Univ. (2014) 21: 493−499 Fig. 2 Automobile front axle after finishing roll forging 1) The shape of two spring board is mismatched. 2) A fold is located in spring board. 3) Size cannot meet the design requirements. After many times of trial, it was found that these defects changed during the forging process, and firstly appeared at the second step roll. So it could be supposed that these defects appeared because of the unreasonable design of the first and second step rolls, as shown in Fig. 3. The first­step roll was used to initially distribute material for each characteristic segment of the part, and an adequate longitudinal distribution of metal was obtained after this step. The second­step roll was designed to provide an approximate shape of spring board for final operation, and the fold and undersize were aroused by unreasonable design of the second­step roll. So, the mismatch of two spring board was originated from the first­step roll, and the fold was attributed to the second­step roll. 2 Failed industrial example In this section, a complicated roll forging example was presented, in which the rolled forging was used to prepare the billets for forming the automobile front axles. The technological process of roll forging for a front axle is given in Fig. 1. However, it failed in industrial application because of some defects located in spring board. The defects are summarized below, and shown in Fig. 2. Fig. 1 Technological process of forming of automobile front axle Fig. 3 Failure workpiece after roll forging: (a) After first­step roll; (b) After second­step roll To resolve these defects, it is needed to design reasonable die groove for spring board of the first­step and second­step rolls. In this failed industrial application, the cross­sections of the grooves are rectangular in first­step dies, and “hat­shaped” in the second­step die, as shown in Fig. 4. In this work, more attention has been paid on first­step dies. Firstly, it is needed to analyze the forming mechanism of these defects. But in actual production, field variables like strain and stress are very difficult to test, and it is very luxury using physical research method
J. Cent. South Univ. (2014) 21: 493−499 495 Fig. 4 Cross­section of groove at first­step and second­step roll (Unit: mm): (a) Rectangular groove; (b) Hat­shaped groove to optimize these variables. Fortunately, the finite element method has been accurate enough to substitute the physical research method. 3 FEM analysis for failed industrial example 3.1 FEM model In order to further understand the formation mechanism of the defects, finite element method was employed to simulate the roll forging process. One of the primary advantages of finite element analysis is its ability to parametrically analyze the forming process which is difficult to obtain experimentally. Design of roll forging die is very complicated owing to the varied surface and multiple segments of the die. The parametric design has been realized using the UG6.0 software for reducing the time of model building. Dies designed in UG can be directly imported into the finite element software by the interface between them. The analysis of the roll forging process adopts the rigid­plastic FEM based on the flow formulation of the penalized form of the incompressibility. In this work, the workpiece material is AISI 5140, and the material flow behavior is determined by the flow stress data. The true stress−strain curves used in this simulation is presented in Fig. 5. In this two­step roll forging process, constant shear friction model is used for the bulk­forming simulation. Fig. 5 Stress−strain curves of AISI 5140: (a) At 800 °C; (b) At 1000 °C; (c) At 1200 °C In order to reduce CPU time and eliminate the effect of irrelevant factors, the following assumptions are developed: 1) The dies are defined as rigid bodies, and the billet is plastic material. 2) The friction factor on the material/die interface is constant. The shear friction model is used in this simulation, and it states that the friction is a function of the yield stress of the deforming body. 3) The rotational velocity of roller is constant during the rolling process. 4) The material is isotropic and the yielding behavior follows the Von Mises yield criterion. 5) The mesh type of the billet is tetrahedral. The rectangular­hat groove system model for roll
496 J. Cent. South Univ. (2014) 21: 493−499 forging is shown in Fig. 6, and the model consists of a part and two dies. The workpiece is deformed from a round billet to a rectangle one in the first pass, and then enters the second pass after rotating 90° around its axis. The main rolling conditions used and the information of billet are given in Table 1. Fig. 7 Workpiece deformation processes during first­step roll: (a) t=0.35 s; (b) t=0.5 s; (c) t=0.65 s; (d) t=0.8 s; (e) t=0.85 s; (f) t=0.9 s; (g) t=1.1 s; (h) t=1.3 s; Fig. 6 Finite element model of roll forging Table 1 Main parameters of rolling Parameter Value and description Billet material AISI 5140 Billet diameter/mm 140 Billet length/mm 855 Friction coefficient 0.5 −1 Rotational speed of roll/(rad∙s ) 1.8 Initial temperature of workpiece/°C 1100 Die temperature/°C 20 Environment temperament/°C 20 Heat transfer coefficient/ (N∙mm∙s −1 ∙°C −1 ) 11 Convection coefficient/ (N∙mm∙s −1 ∙°C −1 ) 0.02 Mesh number for billet 50000 3.2 Analysis of mechanism of defects In this section, the mechanism of defects during rolling forging process was analyzed, and complete attention has been paid on the reason why the size and shape of two spring boards are mismatched during first­step roll. Figure 7 shows the workpiece deformation process during first­step roll. By comparison, the FEM results basically accord with the observed results. Workpiece contour is the enveloping surface of rolling die and vice versa, while rolling die contour is the enveloping surface of forging piece [15]. From Figs. 7(a)−(c), when rolling the first spring board, the die is forced to bite into the workpiece, which brings thrust on workpiece along the x direction. So, the circumferential velocity of the roller surface is appr oximat el y equa l t o the a xial vel ocit y of th e workpiece at neutral plane, and the slip of workpiece along the die groove can be eliminated. For this reason, the dimension of first spring board can meet the design requirement after first­step roll. The deformation process of second spring board is shown in Figs. 7(d)−(g). The dimension of second spring board cannot meet the designed requirement, as shown in Fig. 7(h). There are two reasons for this phenomenon. Firstly, the cross­sections of the grooves are rectangular at the second spring board and the rectangular groove only exerts a force on the top and bottom surface of workpiece, as shown in Fig. 4(a). So, the friction between die and workpiece is insufficient to make workpiece follow the movement of the die. Secondly, during the initial stage of rolling the second spring board, a thrust along the –x direction is imposed on workpiece by the die, and it can increase the relative sliding between workpiece and die. So, the relative motion between roll and workpiece is not consistent, as shown in Fig. 7(e). As discussed above, the bites condition and contact status of die and workpiece are different during rolling the first and second spring boards. To solve these problems, it is necessary to increase the die−workpiece contact area in the entry of deformation zone during the second spring board roll. 4 Die structure optimization In order to resolve the problems above, a new groove shape is presented as a substitute for the rectangular groove, and it is called oval­diamond groove, as shown in Fig. (8). This groove has a slope on each of its two sides, and the top of the groove is a circular which is tangent to two slopes. As a combination of oval groove and diamond groove, the oval­diamond groove puts pressure on the side of billet, and keeps space between die and workpiece on top and bottom. So, this
J. Cent. South Univ. (2014) 21: 493−499 497 Fig. 8 Cross­section of oval­diamond groove new groove can increase the contact area between die and workpiece, avoid the heterogeneous deformation, and increase the stability of billet at the same time. In order to test the effect of oval­diamond groove, the deformation process of workpiece is analyzed by using FEM method. In this analysis, the parameters of groove are given as follows: R1=10 mm, R2=32 mm, R3=140 mm, β=90° and H=79.5 mm. Figure 9 shows the workpiece deformed by oval­diamond groove roll. From Fig. 9, the size and shape of two spring board are almost the same. To analyze the difference between oval­diamond and rectangular groove, the field variables in the cross­ section A of workpiece are discussed. Fig. 10 Effective stress contours of cross­section A at 0.45 s: (a) During rectangular groove rolling; (b) During oval­diamond groove rolling Fig. 9 Workpiece after oval­diamond groove roll (Unit: mm) 4.1 Stress field Stress is defined as the force acting on a unit area of material. The stress field of cross­section A at 0.45 s is shown in Fig. 10. It can be seen during oval­diamond groove rolling and rectangular groove rolling, the effective stresses of workpiece are relatively high at contact area compared to other areas, and decrease with the reduction of distance from the contact area. From Fig. 10(a), for rectangular groove rolling, the minimum effective stress of section A is 49.5 MPa, and located at two sides, and the maximum effective stress is 72 MPa. From Fig. 10(b), for oval­diamond groove rolling, the minimum value of 40.9 MPa occurs in the center of Section A, and the maximum value is 92 MPa. From above analysis, the effective stress of workpiece is relatively high under oval­diamond groove rolling, which means that large friction can be obtained to improve the coordination of die rolling and the axial movement of the workpiece. 4.2 Strain field Strain is a measure of the deformation degree of an object. The strain distribution on cross­section A at 0.45 s is shown in Fig. 11. It can be observed from Fig. 11 that the maximum effective strain of workpiece occurs at the contact area, and decreases with the reduction of distance from the contact area. The maximum effective strain on Section A under oval­diamond groove rolling is 0.235 and the minimum value of 0.0588 occurs in the center. The maximum strain value in rectangular groove is almost the same as that under oval­diamond groove rolling.
From above analysis, the deformation degree of workpiece is almost identical during two kinds of rolling, but larger effective stress can be obtained in the contact surface between the workpiece and the oval­diamond groove. This means that the contact area between workpiece and die is expanded during oval­diamond groove rolling. The forming load during oval­diamond groove rolling is slightly larger than that during rectangular groove rolling, but less flow fluctuation can be obtained during oval­diamond groove rolling, as shown in Fig. 12.
498 J. Cent. South Univ. (2014) 21: 493−499 Fig. 13 Roll dies in physical experiment Fig. 11 Effective strain contours of cross­section A at 0.45 s: (a) During rectangular groove rolling; (b) During oval­diamond groove rolling roll forging machine. In order to save experiment funds, the experiment material is lead, which can be recovered and recycled. The workpieces at the end of each step are respectively shown in Fig. 14. Through measurement and comparison, the sizes of first and second spring board are almost the same, and all of them are in good agreement with design requirement. This groove can thoroughly eliminate the fold as well. Hence, less remanufacturing time and cost for automobile front axles are required. Furthermore, higher quality and lower cost products are generated. Fig. 14 Workpieces after two rolling pass: (a) First spring board; (b) Second spring board Fig. 12 Forming load during roll forging process 6 Conclusions 5 Physical experiment 1) The mechanism of mismatch during roll­forging process is analyzed, and the main reason is the different bites condition and contact status between die and workpiece during rolling the first and second spring boards.
2) The oval­diamond groove is presented as a substitute for rectangular one to reduce the defect. Oval­diamond groove puts pressure on the side of billet, and keeps space between die and workpiece on top and bottom. It can increase the contact area between die and
Based on above analysis, it can be concluded that oval­diamond groove is in favor of improving forging quality. Subsequently, the physical experiment is carried out to test and validate the feasibility of this work. The workpiece is deformed from a round bar to an oval­diamond one in the first step, and then enters into the hat­shape groove after rotating 90° around its axis. Figure 13 shows the roll die installed on the DR46­1000 J. Cent. South Univ. (2014) 21: 493−499 workpiece, avoid the heterogeneous deformation, and increase the stability of billet. 3) Under the same condition of deformation degree, the large friction can be obtained on the contact surface between workpiece and the oval­diamond groove. The forming load during oval­diamond groove rolling is slightly larger than that during rectangular groove rolling, but the less flow fluctuation can be obtained during oval­diamond groove rolling. 4) The physical experiment validates that the oval­diamond groove can resolve defect perfectly, and the size of part is in good agreement with design requirement. 499 [6] [7] [8] [9] [10] References [1] [2] [3] [4] [5] HU Zheng­huan, ZHANG Kang­shen, WANG Bao­yu, SHU Xue­dao, YANG Cui­ping. Formed technology and simulation of parts about the cross wedge rolling [M]. Beijing: Metallurgical Industry Press, 2004: 1−3. (in Chinese) MINUTOLO F C, DURANTE A, LANBIASE F, LANGELLA A. Dimensional analysis of a new type of groove for steel rebar rolling [J]. Journal of Materials Processing Technology, 2006, 175(1/2/3): 69−76. KNAPINSKI M. The numerical analysis of roll deflection during plate rolling [J]. Journal of Materials Processing Technology, 2006, 175(1/2/3): 257−265. BENASCIUTTI D, BRUSA E, BAZZARO G. Finite elements prediction of thermal stresses in work roll of hot rolling mills [J]. Procedia Engineering, 2010, 2(1): 707−716. YOO U K, LEE J B, PARK J H, LEE Y. Analytical model for predicting the surface profile of a work piece in round­to­2­R and square­to­2­R oval groove rolling [J]. Journal of Mechanical Science and Technology, 2010, 24(11): 2289−2295. [11] [12] [13] [14] [15] KIM W, KAWAI K, KOYAMA H, MIYAZAKI D. Fatigue strength and residual stress of groove­rolled products [J]. Journal of Materials Processing Technology, 2007, 194(1/2/3): 46−51. BYON S M, NA D H, LEE Y. Effect of roll gap adjustment on exit cross sectional shape in groove rolling­experimental and FE analysis [J]. Journal of Materials Processing Technology, 2009, 209(9): 4465−4470. TIAN Yong, GUO Yan­hui, WANG Zhao­dong, WANG Guo­dong. Analysis of rolling pressure in asymmetrical rolling process by slab method [J]. Journal of Iron and Steel Research, International, 2009, 16(4): 22−26. NA D H, CHO S H, LEE Y. Experimental and numerical studies for the forming groove and separating groove design in slit rolling process [J]. Journal of Mechanical Science and Technology, 2011, 25(9): 2439−2446. SHAHANI A R, SETAYESHI S, NODAMAIE S A, ASADI M A, REZARE S. Prediction of influence parameters on the hot rolling process using finite element method and neural network [J]. Journal of Materials Processing Technology, 2009, 209(4): 1920−1935. PELLIZZARI M, CESCATO D, DE FLORA M G. Hot friction and wear behavior of high speed and high chromium iron for rolls [J]. Wear, 2009, 267: 467−475. SEDIGHI M, MAHMOODI M. An approach to simulate cold roll­forging of turbo­engine thin compressor blade [J]. Aircraft Engineering and Aerospace Technology, 2009,81(3): 191−198. LI Ru­xiong, JIAO Song­hua, WANG Jin­lv. Roll­forging technology of automotive front axle precision performing and die design [J]. IERI Procedia, 2012, 1: 166−171. ERVASTI E, STAHLBERG U. A quasi­3D method used for increasing the material yield in closed­die forging of a front axle beam [J]. Journal of Materials Processing Technology, 2005, 160(2): 119−122. CAI Zhong­yi. Precision design of roll­forging die and its application in the forming of automobile front axles [J]. Journal of Materials Processing Technology, 2005, 168(1): 95−101.
(Edited by YANG Bing)