Investigations of the influence of a superimposed oscillation on the fatigue strength

Within the scope of the transregional collaborative research centre TCRC73, the effects of an oscillation superimposed forming process for the production of a demonstrator component are investigated. Previous studies in this field were limited to a consideration of the process-related parameters such as the influence of the plastic work and the friction or the component-related parameters such as the influence of the surface quality and the mold filling. This research concentrates on the consideration of the mechanical vibration resistance of components that were manufactured superimposed oscillated. For this purpose, Wöhler tests are conducted in which the fatigue strength of superimposed oscillation pre-stretched test samples and oscillation-free pre-stretched test samples are investigated. First, Wöhler curves are generated in the tensile threshold range for tensile samples made out of the steels DC04 and DP600. Subsequently, tensile specimens are pre-stretched superimposed oscillated and oscillation-free. These specimens are subjected to a tensile threshold load until they break. The influence of the superimposed oscillation forming on the long-term fatigue of components is derived from the comparison of the bearable load cycles. Investigations of the microstructure of the specimens are conducted in order to draw conclusions about the influence on the long-term strength. © 2020 by the authors. Licensee MDPI, Basel, Switzerland

Material Characterization and Modeling for Finite Element Simulation of Press Hardening with AISI 420C

The process of press hardening is gaining importance in view of the increasing demand for weight reduction combined with higher crash safety in cars. An alternative to the established manganese-boron steel 22MnB5 is hot-formed martensitic chromium steels such as AISI 420C. Strengths of 1850 MPa and elongations of 12% are possible, exceeding those of 22MnB5. In industrial manufacturing, FE-simulation is commonly used in order to design car body parts cost-efficiently. Therefore, the characterization and the modeling of AISI 420C regarding flow stress, phase transformations as well as failure behavior are presented in this paper. Temperature-depended flow curves are determined, showing the low flow stress and hardening behavior at temperatures around 1000 °C. Cooling experiments are carried out, and a continuous cooling diagram is generated. Observed phases are martensite and retained austenite for industrial relevant cooling rates above 10 K/s. In addition, tests to investigate temperature-dependent forming limit curves are performed. As expected, the highest forming limit is reached at 1050 °C and decreases with falling temperature. Finally, a simulation model of a press-hardening process chain is set up based on the material behavior characterized earlier and compared to experimental values. The forming force, phase transformation and forming limit could be calculated with good agreement to the experiment

Numerical Investigations on Stresses and Temperature Development of Tool Dies During Hot Forging

Hot-forming tools are subjected to high thermal and mechanical stresses during their application. Therefore, a suitable design of the tool die is important to ensure a long tool life. For this purpose, numerical simulations can be used to calculate the occurring stresses and the temperature development in the tools during the course of a stroke or over several forging cycles. The aim of this research is to investigate the effect of different radii on the resulting stresses in the lower die of the forming tools. Furthermore, the temperature evolution over several cycles is analysed to determine their effect on the temperature. When investigating the stress, it was found that a larger radius leads to a reduction in stresses. In addition, it could be numerically proven that the base temperature of the die levels off after a certain number of cycles. These findings will be used in further research dealing with the service life calculation of dies subjected to thermo-mechanical alternating stresses

Process analyses of friction drilling using the Smoothed Particle Galerkin method

As a cost-effective hole production technique, friction drilling is widely used in industrial and automotive manufacturing. Compared with the traditional bolted connection, it enables the fastening of thin metal sheets and thin-walled tubular profiles. Friction drilling results in higher thread length and joint strength, thus better fulfilling the demand for lightweight structures. However, in the numerical simulation of friction drilling, the traditional finite element method encounters difficulties caused by the extreme deformation and complex failure of the material. A large number of elements are usually deleted due to the failure criterion, which significantly reduces the solution accuracy. The development of meshless methods over the past 20 years has alleviated this problem. Especially the Smoothed Particle Galerkin (SPG) method proposed in recent years and incorporating a bond-based failure mechanism has been shown to be advantageous in material separation simulations. It does not require element removal and can continuously evolve each particle's information such as strain and stress after the material failure. Therefore, the SPG method was used in this research for the simulation of frictional drilling of HX220 sheet metal. First the particle distance and the friction coefficient were varied to investigate the applicability of the SPG method to the friction drilling process. Predicted and experimental results were compared and found to be in high agreement. Furthermore, the influence of input parameters, such as sheet thickness, feed rate and rotational speed, on axial force as well as torque of the tool and the surface temperature of the workpiece during friction drilling was investigated numerically

Hematite vs. Clays: their potential as red pigments and their use in three sites at the Puna of Jujuy (Argentina)

El análisis de pinturas rojas en pictografías de tres sitios de la Puna jujeña puso de manifiesto el uso exclusivo de hematita en su realización. Este pigmento rojo fue utilizado en la región desde inicios de la ocupación humana y su reiterada presencia en representaciones pictóricas diacrónicas revela la especificidad de esa materia prima en Hornillos 2, Cueva Quispe y Tres Pozos. Los pigmentos rojizos recuperados de los niveles estratigráficos en dos de los sitios están constituidos por hematita y por arcillas rojas. El uso de uno u otro tipo de pigmento estaría relacionado con las propiedades intrínsecas de ambas especies minerales, por tal motivo, planteamos que fueron utilizadas con fines diferentes.Physical-chemical characterization of red paints from pictographs found at three sites in the Puna of Jujuy shows that hematite was the only pigment used in their execution. Hematite pigment was used in the region from the earliest human occupations and its recurring presence in diachronic rock paintings reveals the specificity of this material in Hornillos 2, Cueva Quispe and Tres Pozos. In addition to hematite, reddish pigments recovered from stratigraphic levels include clay pigments. As the use of one kind of pigment over another would be linked to its intrinsic properties, we propose that these different pigments were used for different purposes.Fil: Sola, Patricia. Universidad de Buenos Aires. Facultad de Filosofía y Letras. Instituto de Arqueología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Yacobaccio, Hugo Daniel. Universidad de Buenos Aires. Facultad de Filosofía y Letras. Instituto de Arqueología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Rosenbusch, Mariana Lidia. Comision Nacional de Energia Atomica. Centro Atomico Constituyentes; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Alonso, Maria Susana. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geociencias Basicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Basicas, Aplicadas y Ambientales de Buenos Aires; ArgentinaFil: Maier, Marta Silvia. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Unidad de Microanálisis y Métodos Físicos en Química Orgánica. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Unidad de Microanálisis y Métodos Físicos en Química Orgánica; ArgentinaFil: Vázquez, Cristina. Comision Nacional de Energia Atomica. Centro Atomico Constituyentes; ArgentinaFil: Cata, Maria Paz. Universidad de Buenos Aires. Facultad de Filosofía y Letras. Instituto de Arqueología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Numerical analyses of the influence of a counter punch during deep drawing

In the automotive sector, the demand for high crash safety and lightweight construction has led to an increased use of steels with higher strengths. However, the rising number of varying materials with different strengths and ductilities lead to an increasing complexity in productionmaking it more challenging to ensure robust processes. Therefore, the focus of current researches still lays on the further development and extension of forming processes to enable high productivity and reliable production. A powerful tool for an efficient optimisation and extension of forming processes is the Finite Element Method (FEM), which offers time-and cost saving potentials in the design phase. In deep drawing, the use of a counter punch offers the possibility oextending the process limits. By superimposing compressive stresses on the workpiece, the initiation of cracks can be delayed, thus higher drawing ratios can be achieved. The aim of this research is therefore the numerical investigation of a deep drawing process with a counter punch to analyse the influence on the crack initiation and identify optimisation potentials for the processFor this cause, the applied force as well as the position and geometry of the counter punch are varied and the influence on fracture initiation is evaluated. It is found that the applied force on the counter punch is the major influencing factor for crack initiation. Furthermore, it was concluded that the contact between the counter punch and the workpiece should be applied as soon as the bottom of the cup is shaped. A further improvement can be achieved if the counter punch is geometrically adapted to the bottom of the workpiece

Investigations of the Influence of a Superimposed Oscillation on the Fatigue Strength

Within the scope of the transregional collaborative research centre TCRC73, the effects of an oscillation superimposed forming process for the production of a demonstrator component are investigated. Previous studies in this field were limited to a consideration of the process-related parameters such as the influence of the plastic work and the friction or the component-related parameters such as the influence of the surface quality and the mold filling. This research concentrates on the consideration of the mechanical vibration resistance of components that were manufactured superimposed oscillated. For this purpose, Wöhler tests are conducted in which the fatigue strength of superimposed oscillation pre-stretched test samples and oscillation-free pre-stretched test samples are investigated. First, Wöhler curves are generated in the tensile threshold range for tensile samples made out of the steels DC04 and DP600. Subsequently, tensile specimens are pre-stretched superimposed oscillated and oscillation-free. These specimens are subjected to a tensile threshold load until they break. The influence of the superimposed oscillation forming on the long-term fatigue of components is derived from the comparison of the bearable load cycles. Investigations of the microstructure of the specimens are conducted in order to draw conclusions about the influence on the long-term strength

Simulation of Hot-Forging Processes with a Temperature−Dependent Viscoplasticity Model

Hot forging dies are subjected to high cyclic thermo-mechanical loads. In critical areas, the occurring stresses can exceed the material’s yield limit. Additionally, loading at high temperatures leads to thermal softening of the used martensitic materials. These effects can result in an early crack initiation and unexpected failure of the dies, usually described as thermo-mechanical fatigue (TMF). In previous works, a temperature-dependent cyclic plasticity model for the martensitic hot forging tool steel 1.2367 (X38CrMoV5-3) was developed and implemented in the finite element (FE)-software Abaqus. However, in the forging industry, application-specific software is usually used to ensure cost-efficient numerical process design. Therefore, a new implementation for the FE-software Simufact Forming 16.0 is presented in this work. The results are compared and validated with the original implementation by means of a numerical compression test and a cyclic simulation is calculated with Simufact Forming

Design, Characterisation and Numerical Investigations of Additively Manufactured H10 Hybrid-Forging Dies with Conformal Cooling Channels

Internal die cooling during forging can reduce thermal loads, counteracting surface softening, plastic deformation and abrasive die wear. Additive manufacturing has great potential for producing complex geometries of the internal cooling channels. In this study, hybrid forging dies were developed combining conventional manufacturing processes and laser powder bed fusion (L-PBF) achieving conformal cooling channels. A characterisation of the used hot-work tool steel’s AISI H10 powder material was carried out in order to determine suitable parameters for L-PBF processing and heat treatment parameters. Additionally, the mechanical properties of L-PBF-processed AISI H10 specimens were investigated. Furthermore, the influence of different internal cooling channels regarding a possible structural weakening of the die were analysed by means of a finite element method (FEM) applied to a hot-forging process. The numerical results indicated that the developed forging dies withstood the mechanical loads during a forging process. However, during the investigation a large dependency between the resulting stresses and the chosen parameters were observed. By choosing the best combination of parameters, a reduction of the equivalent stress by 1000 MPa can be achieved. Finally, a prototype of the hybrid-forging dies featuring the most promising cooling channel geometry was manufactured