Acta Polytechnica https://doi.org/10.14311/AP.2024.64.0420 Acta Polytechnica 64(5):420–429, 2024 © 2024 The Author(s). Licensed under a CC-BY 4.0 licence Published by the Czech Technical University in Prague INVESTIGATION OF THE IMPACT OF PRODUCT THICKNESS AND STRAIN ON COLD FORGING PROCESSES Samya A. Nazala, Mohammed A. Nasserb, Muammel M. Hanona,∗, Hani A. Ameenb a Middle Technical University, Baquba Technical Institute, Al Tahreer Street, 32001 Baquba, Diyala, Iraq b Middle Technical University, Technical Engineering College – Baghdad, Muasker Al Rashid Street, 10074 Baghdad, Iraq ∗ corresponding author: muammel.m.hanon@mtu.edu.iq Abstract. This study investigates the impact of sheet metal thickness and strain generated on a cold forging process. A cold forging die was manufactured based on a numerically modelled product design that considered stress concentration die profiles. Experiments involved cold forging various sheet metals (lead, brass, aluminium, and steel) with thicknesses ranging from 2 mm to 6 mm to assess their influence on the thickness and strain. An assumption was formulated using Solidworks to determine the appropriate sheet metal thickness based on the material type. This assumption, based on volume considerations, showed practical applicability for selecting sheet metals with thicknesses close to those designed in the numerical modelling. The results revealed that softer materials, such as lead and aluminium, closely approximated the design thickness, particularly in the central region of the die profile, with error deviations of 1.14 % and 4.36 %, respectively. In contrast, harder materials demonstrated larger deviations from the design, with minimum errors of 4.94 % for brass and 5.73 % for steel. All sheet metals underwent compressive strain in the central region of the die profile due to the stamping operation. This work demonstrates a practical approach for selecting sheet metals based on material and thickness considerations, providing insights into the behaviour of different materials during the cold forging process. Keywords: Cold forging die, sheet materials, product thickness, strain analysis. 1. Introduction Forging, an ancient metalworking technique with ori- gins dating back to at least 4 000 BC, was initially used for crafting jewellery, coins, and various imple- ments through the process of hammering metal tools made out of stone [1]. Predominantly characterised as a hot working method, forging takes place at tem- peratures of up to 2 300 °F, however, a variant called cold forging has emerged in the form of impression die forging [2]. Cold forging encompasses a wide array of processes, including bending, cold drawing, cold head- ing, coining, extrusions, just to name a few, yielding a diverse range of part shapes [3]. The temperature of metals subjected to cold forging can span from room temperature to several hundred degrees. It is categorised into three primary types, namely, Cold forging, Warm forging, and Hot forging, depending on the temperature at which these processes are carried out [4]. Forging have been used since ancient times, and it was during the era of the Industrial Revolution that the demand for forged components increased significantly, prompting the development of new tech- nologies aimed at improving production output and material quality [5]. Currently, forged parts are con- sidered superior to their cast counterparts due to their distinctive properties, particularly grain flow [6]. Cold forging contributes to the reinforcement of metal strength by hardening it at room temperature, while hot forging results in optimal yield strength, reduced hardness, and increased ductility by hardening metal at exceedingly high temperatures [7]. Several researchers have considered sheet metal thickness and strain rate in their studies. Ahmad Baharuddin Abdullah (2007) [8] provided a summary of current developments in die design for the forging process and the associated systems, aiming to improve design processes and die efficiency. The research also addressed future challenges in the die design domain, proposing the establishment of an experimental sup- port system to meet the consumer demands. In their study, Shrikant Jain and R. K. Ranjan (2017) [9] investigated the deformation characteristics of an aluminium-copper composite under varying strain rates, influenced by ram velocity, within the context of cold forging. The study involved forging samples at different ram velocities, namely, 1.5, 50, 100, and 150 mm min−1, and assessed the forgeability by crack initiation at a corresponding percentage reduction in height. The researchers developed a mathematical model for an “upper bound” analysis of the average forging pressure on the platen during cold forging at different ram velocities, graphically depicting the rela- tive variation in average forging pressure with the per- centage reduction in height of these composite samples. 420 https://doi.org/10.14311/AP.2024.64.0420 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 64 no. 5/2024 Investigation of the impact of product thickness and strain . . . The experimental results pertaining to forgeability, at different ram velocities, considering percentage reduc- tion in height, forging stress, compressive strength, percentage increase in bulge diameter, and linear hoop strain displayed a strong correlation with the theoret- ical results. Masakazu Kobayashi et al. (2018) [10] conducted measurements of 3D inhomogeneous plas- tic strain in cold-rolled aluminium-magnesium alloys containing small lead particles, a local strain measure- ment technique, with a specific focus on the impact of magnesium content on local strain and microstructure development. Synchrotron radiation microtomogra- phy was employed to obtain these measurements, with the observed local deformation significantly influenc- ing the mechanical properties of metallic materials, including strength and formability. It was noted that microstructural development varied with the magne- sium content in the aluminuim alloy, although no difference in shear strain development was observed. The strain distributions were compared to an investi- gated texture development conducted via SEM/EBSD. Additionally, J. Paulo Davim (2015) [11] discussed the wide range of manufacturing processes involved in material formation and machining, emphasising re- search and development in these areas. Gupta and Davim (2021) [12] explored advanced welding-joining and deforming techniques, covering theoretical devel- opments and practical improvements in manufacturing processes. Dixit et al. (2011) [13] provided a com- prehensive review of material behaviour modelling in metal forming and machining processes, highlighting its influence on process design and product outcomes. Petkar et al. (2023) [14] investigated the applications of natural computing algorithms in optimising cold forging backward extrusion processes, aiming to en- hance punch service life and mechanical strength of forged components, particularly in automotive pro- duction. Praveenkumar M. Petkar et al. (2020) [15] employed a multi-layered feed-forward artificial neu- ral network (ANN) model to analyse the influence of various parameters, such as billet size, reduction ratio, punch angle, and land height, in the context of cold forging backward extrusion processes. They examined the effects of effective stress, strain, strain rate, and punch force for AISI 1010 steel. The ANN model analysis revealed that increasing the billet size led to an increase in effective stress and strain with the reduction ratio, while decreasing the punch angle and land height impacted material flow and resulted in decreased process parameters, namely the effective stress, strain, strain rate, and punch force, ultimately reducing punch forces and improving the punch life. The proposed ANN model, based on finite element sim- ulation, provided insights into the forming behaviour of the cold forging process, offering valuable guidance for process design and streamlining efforts in the cold forging industry in terms of cost and time. Felix Kolpak et al. (2021) [16] delved into the effects of anisotropic work-hardening on the cold forging process, with a particular focus on material behaviour follow- ing strain path reversals. Three steel work-hardening behaviours were characterised for large monotonic strains (up to 1.7) and subsequent strain path re- versals (up to 2.5). Tensile and compressive tests conducted on the extruded material revealed a non- monotonic work-hardening behaviour. The study anal- ysed anisotropic work-hardening phenomena, includ- ing the Bauschinger effect, work-hardening stagnation, and permanent softening across all investigations, with these phenomena intensifying with pre-strain. The experimental work assessed the influence of strain path changes, investigating cross-hardening effects. The study achieved good agreement between the ex- perimental and numerical data by using a modified Yoshida-Uemori model capable of capturing all ob- served anisotropic work-hardening phenomena. Nu- merical constitutive models were integrated into sim- ulations for single and multi-stage cold forming pro- cesses, revealing the impact of anisotropic hardening on projected component properties and process forces. While previous studies have explored various as- pects of cold forging, such as die design, deformation characteristics of specific materials, and the effects of anisotropic work-hardening, the unique contribution of this study lies in its exploration of the interplay between sheet metal thickness and strain induced in cold forging procedures. Delving into the nuances of cold forging die geometry and the selection of mate- rials, it seeks to shed light on the intricate dynamics governing preproduction forging processes. This in- dicates a focus on understanding the early stages of forging, including the planning and design phases, which may differ from the emphasis of previous stud- ies that primarily focus on the manufacturing aspects of forging. 2. Materials and Methods The methodology chapter details the selection of prod- uct design with diverse geometrical profiles and the subsequent elaboration of the cold forging die design, including various components. The manufacturing of die components involved medium carbon steel, with material properties verified through testing. The cold forging process utilised multiple sheet metals, includ- ing lead, brass, and aluminium, with Solidworks soft- ware aiding in the die setup simulation and thickness assessment. Comparison of calculated and selected thicknesses validated the efficacy of the chosen thick- nesses for different metals, reinforcing the cold forging design assumption. Further elaboration on these as- pects is provided in the subsequent sections. 2.1. Product design In this work, the product design under study features various geometric profiles, including grooves, sharp edge, dome, and fillets. The detailed design of the product sample, along with dimensional drawings and isometric views, is presented in Figure 1. 421 S. A. Nazal, M. A. Nasser, M. M. Hanon, H. A. Ameen Acta Polytechnica Figure 1. The details of the product design. Figure 2. The design and manufacture of the cold forging die. 2.2. Design of cold forging die Although forging commonly entails shaping metal by compressing it between dies, it is crucial to under- stand that forging techniques can be used for different types of metal, such as bars and sheet metal. This study specifically focused on the cold forging process applied to sheet metal forming. The term “forging die” in the current research refers to the tool used in the sheet metal forming process that applies lo- calised compressive loads to shape the sheet metal into the desired form. In the experimental setup, the die design comprises several components, each serving a specific function, as listed below. The design and manufacture of the assembly, including the main die parts, is illustrated in Figures 2 and 3: (1.) Upper and lower jaws: These components are responsible for shaping or approximating the shape of the manufactured workpiece during the forging process. (2.) Upper and lower die bases: These parts provide support and fixation to the die jaws. (3.) Guide pillars: These elements ensure a smooth direction and alignment of the upper and lower jaw. (4.) Spring: This component is used to lift the upper jaw away from the manufactured workpiece after the forging process is complete. (5.) Bolts and nuts: These fasteners are used to secure the die parts together. 422 vol. 64 no. 5/2024 Investigation of the impact of product thickness and strain . . . (a). Upper jaw. (b). Lower jaw. (c). Lower base. (d). Upper base. (e). Guide. (f). Physical appearance of the guide. Figure 3. The detailed design and manufacturing of the upper jaw, lower jaw, lower base, upper base, guide and physical appearance of the guide. 2.3. Manufacturing of the die The cold forging die components are manufactured using medium carbon steel (CK45) in accordance with German standards (DIN) as is it readily available on the market. To ensure the desired material properties, test samples were analysed for chemical composition to determine the exact material type and mechanical properties required for the forging process. These testing specimens were obtained by cutting steel sheet with dimensions based on ASTM-68 standard for the tensile testing. The chemical composition and mechan- ical properties of the die material were provided by the General Company for Engineering Examination and Qualification, as shown in Tables 1 and 2. 423 S. A. Nazal, M. A. Nasser, M. M. Hanon, H. A. Ameen Acta Polytechnica Wt. % C % Si % Mn % P % S % Cr % Mo % Ni % Al % Cu % Fe % Standard [17] 0.42–0.5 0.17–0.37 0.5–0.8 ≤0.035 ≤0.035 ≤0.25 Tested 0.469 0.197 0.586 0.0159 0.0698 0.028 0.0045 0.0377 0.0011 0.0527 Bal Table 1. The standard and tested chemical composition of the material. Sample No. Yield stress [MPa] Ultimate stress [MPa] Elongation [%] Hardness [VC] Standard [17] 355 610 20 1 362 615 24 275 2 378 625 26 275 3 375 615 26 315 Average 371 618.33 25.33 Table 2. The standard and tested mechanical properties of the material. Metal alloy Calculated Sheet thickness Thickness thickness [mm] chosen [mm] assessment Lead 4.18 3 Failed 4 Failed 6 Succeed Brass 4.88 2 Failed 5 Failed 6 Succeed Aluminum 4.59 2 Failed 3 Failed 6 Succeed Steel 3.69 2 Failed 3 Succeed Table 3. Thickness assessment of sheet metals for cold forging process. 2.4. Cold forging process Several sheet metals are used for manufacturing the product to analyse the influence of the product thick- ness and strain obtained during the forging process, which depends on the type of the sheet metal and the shape design of the die jaws. Different types of sheet metal with varying thicknesses have been used in the cold forging processes. The metals used in forg- ing production include lead, brass, aluminium, and steel with a thickness range of 2–6 mm. Solidworks is a comprehensive design software widely used for mod- elling parts and assemblies, including die structures. In this study, Solidworks was used with an assump- tion to determine the clearance between the jaws of the die, which is crucial for understanding the range of thicknesses achievable for various materials. By simulating the die setup in Solidworks, the available thicknesses of each material were accurately assessed. The assumption, based on the idea that the volume of the selected sheet metal type, represented as a cir- cular plate, is equal to the designed product volume obtained from the Solidworks product model plus the volumetric extension, as represented by the equation: Vselected sheet = (1 + El %) · Vdesign product. (1) The calculated sheet thickness for different types of metal is then determined by the equation: th = 4(1 + El %) · Vdesign product π · d2 , (2) where Vselected sheet = π·d2 4 , Vdesign product = 25 250.13 mm3, El % the percentage elongation of the product metal as shown in Table 3. d the sheet metal diameter before the forging process is 100 mm. The selected thickness of the sheet metals used before the forging process, based on the calculated thickness for each type of the sheet material, is shown in Table 3. The definitions of success and failure for the sheet metal thicknesses in Table 3 were based on the forg- ing process outcomes. A successful thickness resulted in the desired product shape without defects, while a failure led to issues like bending or tearing. Success or failure was determined through a visual inspection and comparison with design specifications, considering deviations from the calculated values. 424 vol. 64 no. 5/2024 Investigation of the impact of product thickness and strain . . . Figure 4. The lead sheet metal products. Figure 5. The brass sheet metal products. Figure 6. The aluminium sheet metal products. The choice of sheet thickness for all metals in Ta- ble 3 depends on the availability of metal types on the market, as detailed in the table. According to the selected thicknesses of the sheet metals, some of them, especially the softer materials like lead, brass, or aluminium, succeeded with higher thicknesses than calculated, while lower thicknesses failed. Conversely, for harder materials such as steel, the selected sheet thickness that were nearly or slightly lesser than cal- culated agreed with this assumption and succeeded in production, while those significantly lower failed. Figure 4 shows the lead sheet metal products, where the thicknesses less than (3 mm) or nearly close to (4 mm) the calculated value of 4.18 mm failed due to bending inside the product, while higher thicknesses (6 mm) succeeded. Similarly, for brass sheet metal, a lower thickness (2 mm) than the calculated failed due to wrinkling, while a thickness close to the calculated one (5 mm) failed to produce the correct stamping shape, but a higher thickness (6 mm) did, as shown in Figure 5. For aluminium sheet metal, a smaller thick- ness (2 mm) failed due to a tearing in the middle, while a sheet with a thickness of 3 mm failed to produce the correct stamping shape; however, a higher thickness (6 mm) succeeded, as shown in Figure 6. In the case of steel sheet metal, two thicknesses were used due to its hard nature; the lower thickness (2 mm) failed to produce the correct stamping shape, but a thickness of (3 mm), closest to the calculated value, succeeded, as shown in Figure 7. These results support the assumption in cold forging design, making it easier to select sheet metal thick- ness based on the product shape design (derived from numerical modelling) and the mechanical properties of the metal used. 425 S. A. Nazal, M. A. Nasser, M. M. Hanon, H. A. Ameen Acta Polytechnica Figure 7. The steel sheet metal products. 3. Results and discussions In this research, the analysis of forging focuses on the effects of two factors: the thickness of the product and the generated strain. The discussion of the first factor, thickness, is based on the product design created numerically using Solid- works, aiming to achieve the desired product shape with the design thicknesses, as depicted in Figure 8. This figure presents the cross-sectional thickness of the designed product at specific distances from the centreline, with sections taken at critical locations susceptible to deformation due to the forging pro- cess or the complexity of the design. To assess the successfully manufactured products through the cold forging process, the products are cut into two halves, as illustrated in Figures 4–7, and the thicknesses are measured at the same locations identified in Figure 8. The measured thicknesses of the successful products for different types of material are plotted against the design thickness, as shown in Figure 9. Figure 9 illustrates that lead and aluminium sheet metals closely match the designed thickness, particu- larly in the central region from the centre of the die profile, with error deviations of 1.14 % and 4.36 %, respectively, as calculated using the error relation formula: err = thmax − thmin thmax × 100 %, (3) where thmax is the maximum thickness of the design, or practically measured, Figure 8. The design thicknesses of the product at specific distances. thmin is the minimum thickness of the design, or practically measured. This phenomenon occurs due to the height of the upper jaw, which, combined with the softness of the materials, exerts high pressure, pushing the sheet into the lower jaw and resulting in a close thickness up to the middle section of the die jaws profile. It is also noteworthy that the sheet thickness used exceeds the design. However, this high pressure causes the softer materials to flow upwards, increasing the upper part of the product thickness even further, with an error deviation of 62.4 % for lead and 55.69 % for aluminium. This behaviour diminishes when using harder and less soft sheet metals, as observed for brass sheet metal, which exhibits a higher thickness than the design, resulting in a minimum error of 4.94 % and a maximum of 37.33 %. Conversely, the combined effect of hard material with less sheet thickness causes the product thickness to significantly deviate from the design, particularly evident for steel sheet, where the minimum error is 5.73 % and the maximum is 51.73 %. The recorded data of the thicknesses are listed in Table 4. The second factor to be analysed is the strain of the sheet metal during the forging operation of product manufacturing. The strain calculation is based on the following relation: ε = thmeasured − thdesigned thdesigned , (4) where tmeasured is the practically measured thickness of the sheet metals, tdesigned is the designed thickness obtained from the numerical modelling. 426 vol. 64 no. 5/2024 Investigation of the impact of product thickness and strain . . . Figure 9. The relation between the design thickness and the measured thicknesses of different metal products. Location from Designed Lead product Aluminum product Brass product Steel product the center thickness thickness thickness thickness thickness line [mm] [mm] [mm] [mm] [mm] [mm] 0 5.26 5.2 5.5 5 5.58 6 6.23 6.2 5.78 4.9 3.23 10.49 7.5 7 6.3 4.7 3.62 13.02 6.82 6.5 6.13 4.65 3.11 26.17 6.82 7 6.35 5.2 3.14 28 2.82 7.5 6.26 5.14 3.8 35 1.82 7 6.2 4 3.7 43 3.5 6.5 7.9 4.42 3.26 Table 4. The recorded thicknesses for the design and different metal products. The strain results depicted in Figure 10 offer valu- able insights into the deformation behaviour of the sheet metal during the cold forging process. It is evident from the figure that all used sheet metals undergo compression strain in the middle region of the die profile from the centre. This phenomenon occurs primarily due to the downward movement of the upper jaw, which exerts pressure on the sheet metal, facilitating the stamping of the product profile. Notably, the brass sheet metal exhibits the highest compression strain among all materials. This can be attributed to its inherent hardness compared to lead and aluminium, as well as the use of a thicker sheet than the design specifications. Conversely, the steel sheet, characterised by its smaller thickness and higher hardness, demonstrates a relatively lower com- pression strain. However, an interesting observation emerges regarding the outer region of the product profile, particularly beyond 25 mm from the centre. Here, the strain distribution indicates the presence of tensile strains. This occurrence can be attributed to the complex shape of the die profile, which induces deformation and alters material behaviour. The push- ing and pressing action of the upper jaw on the sheets lead to material flow, particularly in the softer metals, causing bending in the outer regions of the product profile. This bending results in the development of tensile strains, as depicted in the figure. Overall, the strain analysis provides critical insights into the lo- calised deformation behaviour of the sheet metals, highlighting the influence of material properties and die geometry on the cold forging process. 4. Conclusion This study has investigated various factors influencing cold forging operations, as revealed through experi- mental and numerical analyses. The main findings and implications are as follows: (1.) The assumption used in calculating thickness proved successful in selecting sheet metals closely matching the intended design. This simplifies the process of choosing sheet metal thickness for cold forging based on the product form design model, while considering the ductility properties of the selected metal. 427 S. A. Nazal, M. A. Nasser, M. M. Hanon, H. A. Ameen Acta Polytechnica Figure 10. The strain effects of the product materials in the cold forging process. (2.) The choice of sheet metals significantly impacts the final product profile, with softer materials such as lead closely matching the design, followed by alu- minium, brass, and steel, which exhibit increasing deviations from the intended profile. (3.) Lead and aluminium sheet metals closely match the design thickness, particularly in the middle region of the die profile, with error deviations of 1.14 % and 4.36 %, respectively. However, the use of higher sheet thickness than the design, coupled with high pressure, leads to thicker upper product portions, evidenced by error ratios of 62.4 % for lead and 55.69 % for aluminium. (4.) The deviation from the design diminishes with harder and less soft sheet metals, as seen in the case of brass and steel. Brass, despite having a higher thickness than the design, exhibits a minimum error of about 4.94 % and a maximum error of approxi- mately 37.33 %. Conversely, steel, with its thinner sheet and harder material, has a minimum error of 5.73 % and a maximum error of approximately 51.73 %. (5.) All sheet metals undergo compression strain in the middle region of the die profile due to the press- ing action of the upper jaw during stamping. The degree of compression strain varies, with brass expe- riencing higher strain due to its hardness and greater thickness, and steel experiencing lower strain due to its thinner sheet and harder material. 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ASTM International, Pennsylvania, USA, 2016. https://doi.org/10.1520/DS67D-EB 429 https://doi.org/10.1504/IJMPT.2017.080560 https://doi.org/10.1016/j.promfg.2018.07.190 https://doi.org/10.1016/C2018-0-00909-3 https://doi.org/10.1016/j.matdes.2011.03.049 https://doi.org/10.3390/app13031276 https://doi.org/10.3390/met10111431 https://doi.org/10.1007/s12289-021-01641-9 https://doi.org/10.1520/DS67D-EB Acta Polytechnica 64(5):420–429, 2024 1 Introduction 2 Materials and Methods 2.1 Product design 2.2 Design of cold forging die 2.3 Manufacturing of the die 2.4 Cold forging process 3 Results and discussions 4 Conclusion References