Papers by Keyword: Cold Forging

Abstract: The production of tailored hollow shafts usually requires multiple manufacturing processes such as multi-stage forming processes and subsequently several machining operations, resulting into high costs and high manufacturing times. To address these challenges, a novel cold forging process featuring an adjustable forming zone was developed by the authors. This new approach enables the production of tailored hollow shafts with varying cross-sections in their length direction as well as internal undercuts within one stroke of the ram. In order to achieve the desired target geometry of a hollow shaft, a specific tool kinematic is required to precisely adjust the cross-section of the forming zone during the process. Currently, determining geometry-specific tool kinematics requires a time consuming iterative numerical procedure. In this paper, a machine learning approach for the prediction of the tool kinematics for a given target geometry of a tailored hollow shaft with variable wall thickness in its length direction is presented.

Abstract: Lubrication plays a crucial role in cold forging, influencing key factors such as material flow, surface quality and tool wear. The current state of the art presents conventional mineral-oil-based lubricants as one of a range of effective solutions; however, they generate residues, require cleaning and pose environmental concerns. This work explores CO₂ snow as a potential residue-free, sustainable alternative for cold forming of mild steels. Miniature spike tests were conducted to characterise friction behaviour under varying lubrication and surface conditions. The study demonstrates that CO₂ snow can effectively reduce friction, promote favourable material flow and achieve surface finishes comparable to conventional oils, while eliminating residue and post-processing requirements. These findings suggest that CO₂ snow represents a promising eco-friendly lubrication strategy, offering both technical performance and environmental benefits for sustainable cold forging operations.

Abstract: The Guided Material Flow (GMF) process is an advanced variant of the Samanta process designed for the net shape cold extrusion of gears. The GMF process employs a modified die geometry to control material flow and significantly reduce maximum tool loads, effectively overcoming traditional process limitations. Key advantages include enhanced tooth tip strength and a reduction in face end deformations, which are characteristic defects in the conventional Samanta process. Minimising these deformations reduces the requirement for subsequent machining and enhances overall material efficiency. A numerical dataset was generated to train and validate data driven surrogate models, facilitating rapid process analysis without the computational cost of continuous Finite Element Analysis (FEA). The models developed in this paper enable the precise prediction of critical process outputs, including maximum punch force, die filling behaviour, material utilisation and strain hardening at the tooth tip. This paper details the numerical data acquisition, the specific training and validation methodologies of the machine learning models and demonstrates their capability to accurately predict complex process outcomes when varying the geometry of the die active surface in the GMF process.

Abstract: Forming processes significantly influence the product properties of a formed workpiece. Next to the effects of work hardening and residual stresses, the influence of ductile damage determines the final performance of a formed component. Thus, precise damage models are crucial for designing new forming process sequences. In general, this is achieved by modelling the evolution of damage as a function of hydrostatic and deviatoric stress, characterized by the stress triaxiality and the Lode-parameter. However, calibrating damage models to the effects of triaxiality and the Lode-parameter is not trivial, since experiments usually represent a combination of both influences. A recent experimental approach by the authors offers the possibility to vary the Lode-parameter in extrusion experiments while keeping the triaxiality constant. This paper aims to use this data of the isolated deviatoric effect on damage to calibrate a damage evolution equation. The model is calibrated to void area fraction measurements obtained by scanning electron microscopy of extruded case-hardening steel 16MnCrS5. For validation, the model predictions for non-constant Lode-parameter histories are compared to corresponding experiments. The model and experiments are in good agreement.

Abstract: This work analyses the influence of batch-to-batch variability on both strain hardening and ductile fracture behaviour of a 42CrMoS4 steel under cold forging conditions. Mechanical testing combined with full-field strain measurements and finite element simulations is used to characterise material response and fracture under different stress states. Batch-dependent hardening laws are identified, and ductile fracture initiation is described using the Hosford–Coulomb criterion, calibrated independently for each material batch. The identified fracture strains and fracture surfaces exhibit a measurable variability between batches, even for similar stress-state conditions. The results provide quantitative evidence of batch-dependent material behaviour relevant for forming simulations

Abstract: Cold forming is characterized by high dimensional and shape accuracy as well as energy and cost efficiency in the series production of highly stressed components. Cold forming is characterized by high tribological loads. Complex lubrication systems are necessary to ensure fault-free production despite the high tribological loads. In the course of increasing demands on environmental compatibility, the disadvantageous zinc-phosphate-based lubricant systems have been replaced by more environmentally friendly single-layer lubricant systems. However, their functionality is strongly dependent on temperature, so that exact knowledge of the prevailing temperatures in the forming zone is necessary for optimum design of the lubricants [1]. Due to the high tribological stress, established measuring methods based on thermocouples can only approach the forming zone up to 10 mm. Previous works of the authors have shown that sensory lubricants based on thermochromic indicators are in principle capable of measuring temperatures directly in the forming zone [2,3]. Their functionality is based on the irreversible color change as a function of temperature [4]. The aim of this study is to develop a standardized test methodology for calibrating the sensory lubricants, which enables an exact correlation between temperature and color value. In addition, suitable indicators are to be identified and their influence on the tribological system analyzed. The test methodology developed uses inductive heating to heat the samples coated with the sensory lubricant to as high as 500 °C within 1 s. The temperature of the surface is determined by the temperature of the lubricant. By determining the surface temperatures reached as well as the color values under diffuse illumination in an integrating sphere, defined temperature ranges can be assigned to the color values of the indicators. With three indicators, which were identified as suitable, it was possible to detect temperatures in the contact zone of a full forward extrusion process and in the contact zone of the sliding compression test that reflect the simulated temperatures. In addition, the sliding compression test showed that the indicators have no influence on the tribological system up to an indicator concentration of 4 %.

Abstract: Wire-arc additive manufacturing is a method of 3D printing metal using welding techniques. However, due to heat, the mechanical properties of the deposited material may be affected. Various methods have been proposed to mechanically improve the properties. In this study, cold deformation was introduced to enhance the properties. The effects of a few parameters, including welding speed, wire feed rate, heat input, thickness ratio, and types of material, were studied. Based on the result, the hardness, tensile, and wear properties of the manufactured part improved, while other properties, like impact toughness, had a lower value. Based on the preliminary result, cold deformation shows potential alternatives for part repair or reconstruction of worn or broken parts.

Abstract: Manufacturing of hollow components with local functional internal surfaces often requires complex process routes with several individual operations. By using a special hollow cold forging process with an adjustable deformation zone it is possible to manufacture hollow shafts with varying wall thickness over its axial length parts just within a single stroke of the press. Combining this principle with a splined mandrel allows manufacturing of tailored hollow shafts with local internal splines. However, the impact of geometry of mandrel and other process parameters on the shape of the cold forged internal splines have not been investigated yet. Furthermore, an underfilling phenomenon can occur on the outer surface of shaft during specific process states. In this contribution, several mandrel geometries and their impact on the part shape and filling / underfilling phenomenon in the mentioned process are investigated. To determine those effects, a numerical investigation has been conducted. Besides the geometry of the splined mandrel, also other parameters such as die geometry and tool kinematics were considered. The numerically calculated workpiece geometries are compared to the ideal geometries using a deviation analysis.

Abstract: This paper describes the tribological investigation of the cold forming and ejection process during ironing of hollow components with helical internal geometry. The experimental tests on a test tool are intended to evaluate the tribological system by varying the surface pretreatment of the blanks and the lubricant. For the evaluation of the tribological effects the ironing and ejection force are measured. In addition, the die filling as well as the surface quality of the pressed parts are determined, allowing the correlation of the test results with the tribological conditions. Furthermore, a numerical investigation of the process was performed. The numerical results are presented and compared with the experimental results.

Abstract: To improve the sensitivity of the steady combined forward and backward extrusion test proposed in previous work, an optimization job based on the finite element simulations was carried out. A raw material of 0.45% carbon steel was tested under different stain rates from 0.001s^-1 to 1s^-1 and different temperatures from 30°C to 400°C, and the material flow stresses were modelled by Hensel-Spittel equation. The deformation degree of the forward extrusion was set as 50%. The key parameters including the deformation degree of the backward extrusion, the ratio between the radius of the punch nose and the radius of the punch, the taper angle of the punch, the die angle, the sizing lands of the punch and the die were optimized. The sensitivity of the optimal design is improved about 20% compared with previous design when the friction factor is assumed as 0.03~0.15. The new group of calibration curves presents more scatter than the old group. The sensitivity improvement is also validated by the experimental works.