Investigation of the required clamping force at multidirectional undercut-forging

This introduction paper is based on the paper "Investigation of the required clamping force at multidirectional undercut-forging" published by "Production Engineering, German Academic Society for Production Engineering (WGP)".

1. Overview:

• Title: Investigation of the required clamping force at multidirectional undercut-forging
• Author: Jonathan Ross, Jan Langner, Malte Stonis, Bernd-Arno Behrens
• Year of publication: 2018
• Journal/academic society of publication: Production Engineering, German Academic Society for Production Engineering (WGP)
• Keywords: Forging · Undercut · FEA · Multidirectional · Clamping force · Tool design

2. Abstract:

A hot forging process allows to produce parts of excellent quality and technical properties. Nevertheless, it is not possible to forge undercut geometries like piston pin bores, it is usually necessary to manufacture them in subsequent processes. Thus, an undercut-forging process was newly developed. Such a process requires a multidirectional forming tool, which is challenging due to a high clamping force of the tool during the process. With the research results, the requirements to the crucial tool components of heavy springs diminish, allowing using standard spring devices instead of large and expensive custom designed devices. The aim of this study is to analyze the clamping force, its origin, and influencing factors in order to facilitate the tool design. Therefore, in forming simulations the input parameters press velocity, initial temperature, and punch shape were investigated, and their effect on the clamping force was statistically evaluated. The press velocity has the major impact on the resulting clamping force. The initial part temperature and the shape of the punch tool showed minor but still significant effects. This combination of input parameters reduces the load and the stress on the tool, enabling to perform the process on smaller forging presses. Eventually, forging trials validated the results.

3. Introduction:

Forging is a vital manufacturing process for producing high-quality heavy-duty and lightweight components, particularly for automotive applications using versatile steel grades. However, conventional die forging is limited in geometrical flexibility, with a significant constraint being the inability to form undercut part geometries (Fig. 1). Parts requiring undercuts, such as piston pin bores, typically necessitate subsequent machining, which reduces material efficiency and increases process time.
To address this, a novel undercut-forging process has been developed, exemplified by its application to steel pistons with undercut piston pin bores (Fig. 2). This process utilizes a multidirectional tool mounted on a forging press. During this process, high forces act upon the upper die. If the clamping force holding the die closed is insufficient, die opening can occur, leading to underfilled part geometries. Therefore, maintaining a clamping force greater than or equal to the vertical force on the upper die is imperative. This study focuses on investigating this critical clamping force, its origins, and the parameters that influence it.

4. Summary of the study:

Background of the research topic:

Hot forging is a preferred method for producing parts with excellent mechanical properties. However, its application is limited when undercut geometries are required. A newly developed multidirectional undercut-forging process aims to overcome this limitation but introduces challenges, primarily the high clamping force required to keep the tool closed during operation. Effectively managing this clamping force is crucial for successful tool design, potentially allowing the use of standard, less expensive tool components (e.g., springs) and enabling the process on smaller forging presses.

Status of previous research:

While alternative methods exist for producing undercut components (such as additive manufacturing, machining, die casting, and combinations of cold forging with other processes), they often have limitations for mass-scale hot forging of steel parts, particularly in terms of production time or material properties (Section 1.1.1). Research into multidirectional forming has primarily focused on improving material properties or shaping parts without complex undercut geometries. Although multidirectional forging tools have been investigated, studies specifically addressing hot undercut-forging tools and the associated clamping force challenges were lacking prior to this research. The paper notes, "A multidirectional tool for hot undercut-forging is not available yet and studies regarding this topic do not exist" (Section 1.1.1).

Purpose of the study:

The primary aim of this study, as stated in the abstract, is "to analyze the clamping force, its origin, and influencing factors in order to facilitate the tool design." A further goal was to identify a combination of input parameters that minimizes the required clamping force, thereby enabling a higher forming degree for a maximum permitted tool load (Section 1.3 Hypothesis).

Core study:

The core of the study involved an investigation into how key input parameters—specifically press velocity (vₚ), initial part temperature (Tᵢ), and punch shape (PS)—affect the maximum required clamping force (MRCF) in the multidirectional undercut-forging process. This was primarily achieved through Finite Element Analysis (FEA) simulations, where the effects of these parameters were statistically evaluated. The findings from these simulations were then validated through experimental forging trials.

5. Research Methodology

Research Design:

The research employed a combination of Finite Element Analysis (FEA) simulations and experimental validation. The FEA simulations were designed to investigate the impact of three input parameters on the maximum required clamping force (MRCF) during the multidirectional undercut-forging of a 42CrMo4 steel piston:

1. Initial part temperature (Tᵢ): 1100 °C and 1250 °C.
2. Press velocity (vₚ): 13.35 mm/s and 26.7 mm/s.
3. Punch shape (PS): Conical and Spherical.
   These parameters, each at two levels, resulted in eight distinct parameter combinations for simulation (Table 1). The target geometry was a steel piston with undercut prebores (Fig. 3). Experimental forging trials were subsequently conducted, focusing on the most influential parameter (press velocity), to validate the simulation outcomes.

Data Collection and Analysis Methods:

Data was collected through:

• FEA Simulations: Performed using Forge NxT 2.1 software. Material behavior of 42CrMo4 was modeled using the Hensel and Spittel equation (Section 2). The primary output analyzed was the force on the upper die (representing MRCF) as a function of the punch path (Fig. 11). Sufficient mold filling was defined as a contact distance of ≤ 0.2 mm in the critical area (Fig. 10) and was determined manually for each simulation.
• Experimental Trials: Two preforms were forged with varied press velocities (vₚ_min and vₚ_max) under otherwise similar conditions.

Data analysis methods included:

• Statistical Analysis of Simulation Data: This involved generating a normal probability plot for MRCF (Fig. 15), a Pareto chart of standardized effects to identify significant variables and interactions (Fig. 16), and main effects plots to quantify the impact of each parameter (Fig. 17).
• Analysis of Force-Stroke Curves: Comparison of MRCF-punch stroke curves for different parameter combinations.
• Metrological Analysis: The parts from experimental trials were subjected to 3D-analysis to compare their surface distance to the target geometry derived from simulations (Fig. 19).

Research Topics and Scope:

The central research topic was the investigation of the required clamping force in a multidirectional undercut-forging process. The scope was specifically focused on:

• A steel piston (42CrMo4) as the sample part (Fig. 3).
• The influence of three input parameters: press velocity (vₚ), initial part temperature (Tᵢ), and punch shape (PS) on the maximum required clamping force (MRCF).
• Identifying optimal parameter settings to minimize MRCF, thereby aiding tool design and enhancing process efficiency.
  The study concentrated on the formation of prebores, not complete piercing of the piston bores (Section 1.1.2), and did not consider the flash removal process (Section 1.1.2).

6. Key Results:

Key Results:

The study yielded several key findings regarding the factors influencing the maximum required clamping force (MRCF) in multidirectional undercut-forging:

• All three investigated input variables—press velocity (vₚ), initial part temperature (Tᵢ), and punch shape (PS)—demonstrated a statistically significant effect on MRCF (Fig. 16, Fig. 17).
• Press velocity (vₚ) had the most substantial impact. Increasing vₚ from 13.35 mm/s (vₚ_min) to 26.7 mm/s (vₚ_max) reduced the mean MRCF by approximately 39% (from 4571 kN to 2784 kN).
• Initial part temperature (Tᵢ) was the second most influential factor. Increasing Tᵢ from 1100 °C (Tᵢ_min) to 1250 °C (Tᵢ_max) led to a reduction in mean MRCF by about 31% (from 4341 kN to 3015 kN).
• Punch shape (PS) exhibited a minor yet significant effect. The conical PS resulted in a lower mean MRCF (3340 kN) compared to the spherical PS (3956 kN). Switching from conical to spherical PS increased the MRCF by approximately 17%.
• A significant interaction effect between Tᵢ and vₚ on MRCF was also observed (Fig. 16).
• The underlying mechanism for these effects was identified as thermal loss. Lower press velocities, lower initial part temperatures, and the spherical punch shape (associated with longer process times or different contact evolution) resulted in greater heat loss from the part to the tooling. This increased the material's flow stress, consequently leading to higher MRCF (Section 3.3, Fig. 18).
• The optimal parameter combination for minimizing MRCF was found to be a high initial part temperature (Tᵢ_max = 1250 °C), high press velocity (vₚ_max = 26.7 mm/s), and a conical punch shape. This combination yielded the lowest MRCF of 2165.8 kN among the simulated scenarios (Table 4, combination no. 8).
• Experimental forging trials validated the simulation findings, particularly showing that a higher press velocity (vₚ_max) resulted in better mold filling, especially when actual part temperatures were lower than planned due to handling time (Fig. 19).
• The required clamping force versus press stroke curves consistently showed a characteristic pattern: an initial phase with a low gradient, followed by a sharp increase in force up to the MRCF as the die cavity approached full filling (Fig. 14).
• Simulations indicated that conical punches achieved sufficient mold filling with a shorter final punch path (Sfinal_conical = 12.9 mm) compared to spherical punches (Sfinal_spherical = 15.2 mm) (Section 3.1).

Figure Name List:

• Fig. 1 Undercuts at a die impression (left) and at a piston (right)
• Fig. 2 Multidirectional undercut-forging schematic principle of operation (sectional view)
• Fig. 3 Part deformation in multidirectional undercut-forging
• Fig. 4 Forming simulation of the undercut-forging process in cross sectional view
• Fig. 5 Origin of the force on upper die at early (left) and late stage of the process
• Fig. 6 Multidirectional undercut-forging tool’s schematic principle of operation in chronological sequence (1–4)
• Fig. 7 Forging tool mounted in hydraulic press
• Fig. 8 Flow curves of 42CrMo4 steel at different temperatures
• Fig. 9 Conical (above) and spherical punch shape (below) in different views
• Fig. 10 Definition of point of sufficient mold filling in forming simulations; color indicates the distance in surface between part and die impression
• Fig. 11 Required clamping force-press stroke-curves of each parameter combination’s simulation; combinations with conical punch shape (red frame) showing shorter final punch path than ones with spherical punch shape (blue frame)
• Fig. 12 Different material displacement due to punch shape; in total (left) and enlarged display at final punch path (right)
• Fig. 13 Required clamping force-relative press stroke-paths for each simulation
• Fig. 14 Qualitative curve of the clamping force-press stroke-path of each simulation
• Fig. 15 Normal probability plot; response variable is maximum required clamping force [kN]
• Fig. 16 Pareto chart of standardized effects highlights the impact the input variables and their interactions have on maximum required clamping force (with a significance level = 0.05)
• Fig. 17 Effect of press velocity, initial part temperature, and punch shape on mean maximum required clamping force
• Fig. 18 Cross sectional view on the final part temperature distribution at the point of mold filling in the prebore area for each parameter combination nos. 1–8; combinations of higher MRCF show lower temperatures in the punch contact area
• Fig. 19 Results of metrological 3D-analysis of the pistons from forging trials (above); colored scale referring to surface distance between forged parts and target geometry; comparison of parts forged both at high initial temperature with conical punch at low press velocity (left) and high press velocity (right); zoom on critical region; part at low press velocity shows larger surface areas of insufficient mold filling (blue and red) than part at high press velocity

7. Conclusion:

This study successfully investigated the impact of initial part temperature (Tᵢ), press velocity (vₚ), and punch shape (PS) on the maximum required clamping force (MRCF) for a newly developed multidirectional undercut-forging process of a steel piston. All three parameters, along with the interaction between vₚ and Tᵢ, were found to have a significant effect on MRCF. Press velocity (vₚ) exerted the largest influence, followed by initial part temperature (Tᵢ), and then punch shape (PS).
Specifically, increasing press velocity (e.g., from 13.35 to 26.7 mm/s) reduced MRCF by approximately 39%, while increasing initial part temperature (e.g., from 1100 to 1250 °C) reduced it by about 31%. Employing a conical punch shape instead of a spherical one was also beneficial, reducing MRCF (or rather, spherical increased it by ~15% compared to conical). The optimal combination for minimizing MRCF involves high press velocity, high initial part temperature, and the use of conical punches. This approach reduces the main load on the forging tool, which can lead to the use of standard heavy springs instead of custom-designed ones, enable the process on smaller presses, or allow for higher forming degrees when press forces are limited.
A critical finding was the strong correlation between the effects of the input variables on MRCF and the thermal loss experienced by the part during forging. Thermal loss acts as a dependent variable; minimizing it is paramount for producing high-quality undercut-forged parts with minimum press force.
Future research should focus on refining simulation models to better account for thermal loss due to real handling times. Additionally, investigations into the final grain flow and properties of the undercut-forged pistons are warranted. The feasibility of this undercut-forging process for other part geometries also needs to be demonstrated to expand its applicability.

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9. Copyright:

• This material is a paper by "Jonathan Ross, Jan Langner, Malte Stonis, Bernd-Arno Behrens". Based on "Investigation of the required clamping force at multidirectional undercut-forging".
• Source of the paper: https://doi.org/10.1007/s11740-018-0830-3

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