An investigation on forging loads and metal flow in conventional closed-die forging of preforms obtained by open-die indentation

Transcription

1 Indian Journal of Engineering & Materials Sciences Vol. 11, December 2004, pp An investigation on forging loads and metal flow in conventional closed-die forging of preforms obtained by open-die indentation Yilmaz Can*, M Tahir Altinbalik & H Erol Akata Department of Mechanical Engineering, Faculty of Engineering and Architecture Trakya University, 22030, Edirne, Turkey Received 28 August 2003; accepted 5 September 2004 In this paper, an experimental study has been carried out to investigate the effectiveness of the usage of the open die indented specimen as preforms of conventional closed die forging. Cylindrical billets having two different aspect ratios (H 0 /D 0 ) are indented with two different ratios and thereafter forged using a conventional closed-die set-up. Effects of indentation and aspect ratios on forging load and on die filling have been observed and compared with reported results. IPC Code: B21K Forging is one of the important metal forming processes in which metal is plastically reshaped using dies. Forged parts are used in wide range of environment where high strength and reliability are important considerations. Although forging processes are performed in many ways, closed die forging is still quite most used method in industry. Closed die forging is used to obtain the final product in close tolerance. Forging process is governed by many factors such as friction, complexity of parts, die shape and temperature of die and billet. Flash formation is a characteristic property of conventional closed die forging and it is formed as a consequence of lateral metal flow towards die land. Flash formation causes material loss about 30% of the initial billet volume and requires subsequent trimming operation as reported elsewhere 1. In addition to this, flash generation causes both an increase of the surface being forged and to get harder to extrude metal laterally into converging gap between upper and lower dies. Desired dimensions of finished product can be obtained as die gap is completely filled. Complete die filling is achieved by restricting the metal flow to lateral direction. Therefore, research on flash land geometry and size, initial billet dimensions and perform design may help to obtain complete filling with less energy consumption and material loss. Although various types of parts are forged in industry, shape classification of commonly produced *For correspondence( ycan@trakya.edu.tr) parts was done first by Spies 2. Geometric complexity of the axisymmetric forged part was formulized by Teterin 3. Many investigations have been carried out to accomplish the desired forging part with lesser amount of metal loss and with less forging load. Early studies on dimension of flash land and metal flow into flash land in the conventional closed die forging were introduced by Lange 4, Vieregge 5, Altan 6 and The 7. Lyapunaov 8 defined filling coefficient based on volume of die gap and obtained product and investigated height/width ratios on die filling in plane strain of a long work-piece and maximum filling was obtained when height/width ratio is 1.4. Sözöz 9 made a similar work using cylindrical billets having different aspect ratios and better die filling was obtained for an aspect ratio of about 1.6. Akata 10 investigated the effect of flash land geometry on forging load and metal flow experimentally using three different types of flash land geometry and parts with different shape complexity factors. His experiments showed that there are always some metal flow into flash land even billet volume is 10% less than the volume of die cavity. Akata 11 also studied the effect of aspect ratio of billets on forging load and die filling ratio. Çapan 12 developed the backward can extrusion approach proposed by Akata 11 to analyze force requirement in forging of cylindrical parts. Akata 13 investigated variations of the amount of metal flow into flash land with respect to flash thickness for different flash-land forms and different shape complexity factors. Akata 14 conducted a series of experimental works about preform design by forging

2 488 INDIAN J ENG. MATER. SCI., DECEMBER 2004 different parts having different shape complexity factors. Complete die filling was obtained when the parts having smaller shape complexity factor was used as preform for subsequent forging steps. However, forging load and process time was relatively high. Recently, FEM is widely used in analyzing forging process because of the rapid development of computer technology. Zaho 15 studied about preform die shape using a FEM based backward tracing method. Shape complexity factor defined by Teterin 3 was used as a criterion for controlling boundary conditions. Zhao 16,17 and Wright 18 developed finite element based sensitivity analysis for preform die design for plane strain forgings. They presented preform die surface by using B-splines and B-splines coefficients were used as design parameters. Lapovok 19 offered an approach to preform design in metal forming process based on FEM. He examined the perform design of the forged H beam section by selecting coefficient of friction as main parameter defining the metal flow and the strain distribution. Different preforms were obtained by using H/h ratios and flash formation occurred for friction coefficient bigger μ=0.15. Open die indentation can be used for obtaining preforms in closed-die forging studies. Studies on open die piercing have been done date back to end of 19 th century. Since then, researchers studied open die piercing to investigate deformation mechanism and process parameters theoretically and experimentally In this work, cylindrical parts having two different aspect ratios were open die indented in two different ratios to make a preform for subsequent closed die forging operation. Indented parts then were forged using the conventional closed die to get a product of which shape complexity factor is Specimens were machined from extruded lead bars. The effect of indentation ratio and aspect ratio on die filling and forging load was investigated experimentally. The present results are compared with the reported results. Experimental Procedure Indentation and forging tests were carried out by using a hydraulic press at a ram speed of 5 mm/s. The specimens were cut and machined to required dimensions from extruded round bars having two different diameters as 23 and 21.5 mm. Dimensions of die cavity were designed and manufactured so that shape difficulty factor of a forged part with complete filling is It has been known from reported literature that complete die filling couldn t obtained in single step by using these billet dimensions with shape complexity factor of The volume and shape of the die cavity were defined as taking the distance between the die halves as 2 mm. In order to make a comparison between results of presented study and the results of ref 14, where the complete die filling was obtained in 4 forging steps using a billet having 10 per cent excess material volume than die gap volume, 10 per cent excess volume of material is used. The specimens were machined providing 10 per cent excess material than that of die cavity. There are two reasons to select the diameter of specimen as 23 and 21.5 mm. The first reason is that the indented part must easily be placed into die gap prior to forging operation; the second is that to make a comparison between results given by previous studies 8,9. Smaller specimen diameters were not used in order to avoid buckling. Indentation of the specimens Four different indentation punches were used in the indentation stage of the experimental work in order to obtain the preforms of closed-die forging. Characteristic dimensions of the punches, billets and indented parts are shown in Fig. 1 and Table 1. Maximum diameter of an indented preform must not exceed the maximum diameter of the lower die half in order to meet the first condition mentioned above and to provide placing it into die gap easily. To avoid having bigger indented billet diameter, indentation ratios are chosen as 0.25 and Indentation ratio (IR), using the dimensions of Fig. 1, is defined as: H IR = ln H o (1) A schematic presentation of the experimental setup for indentation experiments is shown in Fig. 2. Vertical displacement of the punch was controlled Table 1 Specimens and the punch dimensions D d p (Preform groups) 8 (A1) 13.5 (A2) 16 (A3) 18.5 (A4) 8 (B1) 13.5 (B2) H 0 /D 0 (Aspect ratio) (B3) 18.5 (B4)

3 CAN et al.: FORGING LOADS AND METAL FLOW IN CONVENTIONAL CLOSED DIE FORGING 489 automatically and stopped at predetermined levels as calculated by Eq. (1). Forging of the indented preforms Conventional closed die forging experiments were carried out using preformed specimens that were indented before. Forging sequences were shown in Fig. 1 The punch and the billet dimensions used in the indentation stage Fig. 2 Configuration of the die assembly for indentation Fig. 3. Preforms were put into the lower die half and then upper die half was pressed toward the lower die half. Closing the die halves during forging process was controlled electronically in order to obtain the desired flash thickness. Forging operations were stopped when the flash thickness is reached to 2 mm. Load values were measured and recorded electronically by using an x(t)-y(t) recorder. It was also designed and produced a trimming die for eliminating the flash after forging was completed. At the end of each forging operation, weights of the net forged part without flash (G n ) were measured after trimming of flash carefully with a trimming die. Filling ratios were calculated as follows: G FR = 100 G n (2) i where G i is the ideal forged part with complete filling. Results and Discussion Calculated die filling ratios (FR) and measured maximum forging loads during forging experiments are shown in Figs 4 and 5 respectively for two different indentation ratios (IR). In these figures, variations of calculated filling ratios and measured loads were plotted against related indenter diameter for two different indentation ratios. Each point in these figures represents mean value of four experimental results. It can be seen from Figs 4(a) and (b) that the best die filling was obtained for a preform of which aspect ratio is 1.5 and indented with punch having mm diameter which is designated by preform group of A2 as shown in Table 1. The die filling for aspect ratio of 1.5 is approximately 5 per cent higher than that of Fig. 3 Closed-die forging of the indented performs (a) die assembly with indented part, (b) end of the forging stage, (c) final product with flash before trimming, and (d) trimmed part

4 490 INDIAN J ENG. MATER. SCI., DECEMBER 2004 Fig. 4 Variation of die filling against indenter diameter (a) IR=0.25 and (b) IR=0.33 Fig. 5 Variation of forging load against indenter diameter ( a) IR=0.25 and ( b) IR=0.33 aspect ratio 1.2, when indentation ratio kept as 0.33, as shown in Fig 4b. Figs 5(a) and (b) show that the smallest forging load value is obtained for the aspect ratio of 1.5 and indentation ratio of It is observed from Figs 4 and 5 that the easiest metal flow in closed die forging is obtained for the preform that was indented by the punch designated by number A2 as shown in Table 1. It is clearly seen from these figures that A1 and B1 preform groups have no considerable effect on both die filling and forging load. During the forging of perform groups of A3-B3 and A4-B4, material beneath the hollow region flows radially instead of vertically depending on indenter diameter. Radial metal flow increases the flash. Therefore, forging load increases and die filling ratios decreases due to flash amount. It is known that, a successful forging can be done if the following two conditions are met just at the beginning of flash formation in a conventional closeddie forging process: (a) an adequate volume of material to fill the remaining die gap must be trapped in the die and (b) the extrusion of material through the narrowing flash land must be more difficult than the filling of more intricate details of the die gap 6. First condition is related to the shape of die gap and billet or preform to be forged. Relating with the presented study, this condition can easily be provided if the maximum diameters of the indented forms are kept as small as possible with higher indentation ratios. Flash formation begins later for preforms having smaller maximum diameters than that of greater ones. For this reason the second condition is indirectly related with the second condition. In this study, it can be concluded that the preform group of A2 with IR=0.33 is fulfilled the above conditions. [ In this work, it is observed that the maximum die filling is obtained for the forging of pre-forms of A groups having aspect ratio of 1.5. For this reason, variations of the filling ratios and forging load against indenter diameter for two different indentation ratios are obtained and shown in Figs 6 and 7 respectively. From these figures, it can be seen that specimens experienced more preforming give higher die filling ratios and forging loads. As shown in Fig. 6, maximum die filling ratio is obtained using punch number A2 with indentation ratio of In Fig. 7, it can also be seen that the forging load obtained from the experiment which were preformed using the specimens pierced by punch A2 with indentation ratio of 0.33 is only 5 per cent higher than that obtained with indentation ratio of

5 CAN et al.: FORGING LOADS AND METAL FLOW IN CONVENTIONAL CLOSED DIE FORGING 491 Fig. 6 Variation of die filling ratio against punch diameter for different indentation ratios (d 0 =21.5 mm) Fig.8 Comparison of present and previous works in terms of die filling ratio versus excess material Fig. 7 Variation of forging load against punch diameter for different indentation ratios (d 0 =21.5 mm) In this case, priority must be given to either better die filling or lower forging load by production engineers in forging die design. Comparison of the results of presented study with those of earlier works 11,14 is made by Figs 8 and 9. The results obtained from this study in terms of die filling are better than the others results except of Ref. 14 where 100 per cent die filling was obtained in four forging steps. Increasing forging steps raises both tooling cost and lead time. Fig. 9 shows the variation of forging load with filling ratio obtained from this and earlier works 11,14. An interesting result of the present work could be seen in Fig. 9. Forging load which obtained with best die filling from this work and the forging loads obtained from earlier works 11,14 are compared in Fig. 9. As seen in Fig. 9 minimum forging load which measured for best die filling of presented study is about 13 per cent lower than that obtained for the best die filling in Ref. 14. Fig.9 Comparison of present and previous works in terms of forging load versus die filling ratio Conclusions It can be concluded from the results of presented study that, open die indented specimen can successfully be used as preforms for conventional closed die forging operations. Properly indented preforms can reduce the number of preforming steps which reduce the forging load and lead time and increase the die life. It can also be concluded that, increasing indentation and aspect ratios help to provide the better die filling in general. However, billet and indentation punch diameters, and indentation ratios have considerable effects on the effectiveness of the approach in terms of the forging loads and the filling ratios. On the other hand, experimental studies of this

6 492 INDIAN J ENG. MATER. SCI., DECEMBER 2004 work were limited to using only single shape complexity factor, single material difference ratio and two aspect and indentation ratios. For this reason, some additional experimental works must be conducted using more different shape complexity factors, aspect ratios and excess material in order to get more fruitful results. References 1 Lee J H, Kim Y H & Bae W B, J Mater Process Technol, 72 (1997) Spies K, Preforming in forging and the preparation of reducer rolling, Ph.D.Thesis, Technical University, Hannover, Teterin G P, et al., Kuznechno-Stampovochnoe Proizvodstvo, 7 (1966) 6. 4 Lange K, Metal Treatment, 29 (1963) Vieregge K, A contribution to flash design in closed-die forging, Ph.D.Thesis, Technical University, Hannover, Altan T & Henning H J, Metall Met Form, 39 (1972), The J H L & Scrutton R F, J Eng Ind, Trans ASME, (August 1972) Lyapunaov N & Kobayashi S, Metal flow in plane-strain closed-die forging, 5 th NAMRC, Amherst, Massachusetts, May 1977, Sözöz H, Salman S & Yükler İ, Machine Technology, (Oct.1998), 50 (in Turkish). 10 Akata H E, Çan Y & Çapan L, The effect of flash land form to load and material flow in the closed-die forging, 5 th Nat Conf Machine Design and Production, Middle East Technical University (METU) (1992), Announcements Book, 183. (in Turkish) 11 Akata H E, Çan Y & Altınbalık M T, An approach to perform design in the closed-die forging, The Second Biennial European Conf. on Engineering Systems Design and Analysis, London, England, July , ASME, Çapan L & Baran O, J Mater Process Technol, 102 (2000) Akata H E & Altınbalık M T, The effect of flash form to material flow in the closed-die forging, 6 th Material Symp, Pamukkale University, (1995), Announcements Book, 338 (in Turkish). 14 Akata H E & Altınbalık M T, The effect of the perform design to load and material flow in the closed-die forging, 7 th Int Conf Machine Design and Production, Middle East Technical University (METU), (1996), Announcements Book, 11 (in Turkish). 15 Zhao G, Wright E & Grandhi R, Int J Machine Tools Manufact, 35 (9) (1995) Zhao G, Wright E & Grandhi R, Int J Machine Tools Manufact, 37 (9) (1997) Zhao G, Wright E & Grandhi R, Int J Numer Methods Eng, 40 (1997) Wright E & Grandhi R, Int J Product Process Improv, 1 (1) (1999) Lapovok R Y & Thomson P F, Int J Machine Tools Manufact, 35 (11) (1995) Seghal M & Kobayashi S, J Eng Ind Trans ASME, (Nov.1972) Chitkara N R & Patel U, Plastic flow in deep indentation during open die piercing of thick circular blocks of aluminium by rigid mandrel and ring type piercers, Adv. Tech. Plasticity, Proc. 1 st. Int. Conf., (1984), Dudra S & Im Y T, J Mater Process Technol, 21 (1990) 143.