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Influence of laser polishing on surface roughness and microstructural properties of the remelted surface boundary layer of tool steel H11

Paint appearance is an important factor in the overall product quality of steel sheet, especially in the automotive industry. The present paper deals with the functional behaviour of the surface texture according to paint appearance. The main results on the improvement of paint appearance of outer car body panels, obtained with coated and uncoated cold rolled steel, textured with EBT-rolls, is reviewed. There are different methods to relate steel sheet surface texture to the appearance after painting. From the literature it is found that waviness has a detrimental effect on the surface appearance after painting. Different roughness and waviness parameters are compared in order to find a good relation with paint appearance. The following surface texture parameters are evaluated in this study: roughness parameters Ra, peak count Pc, waviness parameters Wca and amplitude between 0.5 – 5mm (Fourier analysis) and envelope parameters motif W and Ra-macro, measured with a spherical stylus of radius 1.5mm. Classical parameters such as Ra and Pc are only able to evaluate paint appearance for one single texturing process, and within very limited range. The ability to predict paint appearance by Ra-macro and Wca also seems to be limited, because of their dependency on the roll texturing technique. Based on extensive testing, it is suggested that Fourier analysis might be the best approach to predict the appearance after painting, and this independently of the texturing process. The results clearly show the improved appearance of Sibetex textured sheets compared to conventional textured sheets. This is due to the fact that, with Sibetex sheet, a reduction in waviness can be achieved.

Assessment of steel surface roughness and waviness in relation with paint appearance.

Soft actuators, as an important part of soft robotics, have attracted significant attention due to their inherent compliance, flexibility and safety However, low capacity in force and load limits their applications Prestored elastic energy can improve the capacity in output force and load of soft actuators This work introduces a soft pneumatic bistable reinforced actuator inspired by the Venus Flytrap's bistable mechanism that allows for the storage of elastic energy The proposed actuator consists of two separated 3D-printed actuation chambers that are attached to a central prestressed steel shell to achieve bistability The pressure triggering the pneumatic bistable reinforced actuator from one stable state to the other is derived and validated by experiments Further experimental comparisons between the proposed actuator and a dual chamber actuator show that the proposed design significantly improves the block tip force, load capacity and stiffness The pneumatic bistable reinforced actuator also demonstrates superior performance in the actuation speed and bending angle under the same input pressure Finally, a two-finger gripper is developed using the proposed actuator, which is demonstrated to grasp and hold various objects

A soft pneumatic bistable reinforced actuator bioinspired by Venus Flytrap with enhanced grasping capability.

Influence of sheet metal texture on the adhesive wear and friction behaviour of EN AW-5083 aluminum under dry and starved lubrication

In order to reduce friction and wear in cold forging of aluminium and its alloys a tremendous amount of lubricants is used The main task of the liquid lubricants is the separation of tool and workpiece surfaces to avoid adhesion and abrasion From an ecological and economical point of view, a reduction of the consumption of lubricants is meaningful Novel surface modification techniques have been investigated to enable dry cold forming In previous work, a reduced wear by adhesion of aluminium to an AISI H11 ESR tool steel surface was detected for a combination of laser polishing and a subsequent surface functionalization Here, structured surfaces combined with surface functionalization are investigated regarding their wear behaviour Therefore, different structures are realized on AISI H11 ESR tool steel surfaces by laser structuring These structures are aligned parallel and perpendicular to the material flow direction Furthermore, different structure depths are generated It is evident that the laser induced surface melting causes both, a reduced micro roughness as well as a homogenized elementary distribution Moreover, the surfaces are functionalized with an octadecylphosphonic acid selfassembled monolayer (C18PA-SAM) The SAM’s distal alkyl termini functionalize the tool steel surface und thus only weak van-der-Waals interactions are present in the aluminium-tool steel interface rendering an interaction, which is expected to be similar to the interaction in conventionally lubricated aluminium forming Finally, the surfaces are appraised regarding their wear behaviour in a compression-torsion-wear tribometer The different experimental results document the potential of the here investigated combined tool surface modification Although, the performance of a lubricated process is not completely reached a reduction in friction and wear can be achieved compared to unmodified tools

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Probing the potential of structured and surface functionalized tools for dry cold forging of aluminium

Bistable metal shells with a fully closed unfolded geometry are of great interest as lightweight construction parts which could be transported without housing and unfolded at the construction place. In order to achieve the effect of bistability in metallic shells, residual stresses with a specific distribution along the shell thickness are necessary. These residual stresses can be introduced in bending processes. The tools with specific bending radii are used to influence the curvature of the shell in the different stable states and thus determine whether a completely closed profile can be achieved. In addition to the forming process, the shell thickness and the shell material have an effect on the achievable geometries and stability. In order to manufacture bistable metallic cylindrical shells from different materials and shell thicknesses, it is necessary to be able to determine a promising process sequence and corresponding bending radii in advance. For this reason, this article presents a semianalytical model for the calculation of bistability and final curvatures. This model is applied to an incremental die-bending process using two bending operations with bending radii of 6 to 12 mm and a 0.2 mm thick steel shell of grade 1.1274 (AISI 1095). The calculation results show that bistability cannot be reached for all combinations of the two bending radii. Moreover, the model indicates that a bistable and fully closed shell is only achieved for a bending radii combination of R1 = 6 mm and R2 = 6 mm. With the aim of model verification, experiments with a closed-die incremental bending tool were performed. Calculated and experimental results show good correlation regarding bistability and curvature. In addition, X-ray diffraction measurement of the residual stresses shows a good qualitative agreement regarding the calculated and experimental results.

https://www.mdpi.com/2504-4494/3/1/22/pdf

A Semianalytical Model for the Determination of Bistability and Curvature of Metallic Cylindrical Shells

During the last decades, high manganese steels (HMnS) were considered as promising materials for crash-relevant automobile components due to their extraordinary energy absorption capability in tensile tests. However, in the case of a crash, the specific energy, absorbed by folding of a crash box, is lower for HMnS as compared to the dual phase steel DP800. This behavior is related to the fact that the crash box hardly takes advantage of the high plastic formability of a recrystallized HMnS during deformation. It was revealed that with the help of an alternative heat treatment after cold rolling, the strength of HMnS could be increased for low strains to achieve a crash behavior comparable to DP800. In this work, a multi-scale finite element simulation approach was used to analyze the crash behavior of different material conditions of an HMnS. The crash behavior was evaluated under consideration of material efficiency and passenger safety criteria to identify the ideal material condition and sheet thickness for crash absorption by folding. The proposed simulation methodology reduces the experimental time and effort for crash box design. As a result of increasing material strength, the simulation exhibits a possible weight reduction of the crash box, due to thickness reduction, up to 35%.

https://www.mdpi.com/2075-4701/9/7/772/pdf

Computer-Aided Material Design for Crash Boxes Made of High Manganese Steels

Friction has a significant influence on almost all metal forming processes. An in situ measurement of the friction stress within the forming process is in general difficult. Therefore, different experimental setups based on the indirect measurement of a friction dependent value are used to determine the friction conditions in laboratory experiments. For example the ring compression test and the conical tube-upsetting test are using the change of the geometrical shape of a specimen to investigate an averaged friction coefficient within the process. The essential advantages of conical tubes are the prevention of sticking friction and a homogeneous displacement and relative velocity along the contact surface depending on the friction conditions and the used cone angle. However, in both methods the development of the friction conditions during the upsetting process and the relative velocity between tool and workpiece are unknown. In this paper an extended setup of the conical tube-upsetting test is presented. The development of the specimen profile is detected by a laser sensor during the process at elevated temperatures. Experiments are conducted for different cone angles and the measured data is compared to FE-simulations. The time-dependent geometric data is used for the calculation of the relative displacement and relative velocity between tool and workpiece at the edge of the contact zone. A comparison with classical nomograms indicates a change of the friction conditions during the upsetting process. Finally, simulations are fitted to the experimental results by using a variable friction coefficient.

Extended Conical Tube-Upsetting Test to Investigate the Evolution of Friction Conditions

To produce cold-rolled steel strips with specific mechanical properties and surface roughness typically temper rolling is adopted. In most cases, a uniform roughness pattern on the strip surface is mandatory. Due to the wear of the textured work rolls, their surface roughness ( R a ) continuously reduces during the process, which should be accounted for process control. However, conventional temper rolling systems fail to guarantee a uniform surface roughness. In this work, the influence of strip tension on the imprinting of surface roughness during temper rolling is analyzed based on a multi-scale FE modeling concept to explore new ways for surface roughness control. This is done in simulation where, a macroscopic rolling model incorporating strip tension is coupled to a mesoscopic imprinting model and both models are validated using copper rolling trials. The influence of different thickness reductions, strip tensions and incoming strip's surface roughness on imprinting is modeled and compared. The numerical results reveal that a higher strip tension decreases the roughness transfer, which presents potential to control the roughness transfer ratio without changing other process parameters like the prescribed thickness reduction in the future.

Marco Teller

Papers

Probing the potential of structured and surface functionalized tools for dry cold forging of aluminium

A Semianalytical Model for the Determination of Bistability and Curvature of Metallic Cylindrical Shells

Computer-Aided Material Design for Crash Boxes Made of High Manganese Steels

Extended Conical Tube-Upsetting Test to Investigate the Evolution of Friction Conditions

Modeling and exploiting the strip tension influence on surface imprinting during temper rolling of cold-rolled steel