Key Engineering Materials Vols. 611-612

Abstract: For the numerical simulation of sheet metal forming processes, the commercial finite element software packages are among the most commonly used. However, these software packages have some limitations; in particular, they essentially contain phenomenological constitutive models and thus do not allow accounting for the physical mechanisms of plasticity that take place at finer scales as well as the associated microstructure evolution. In this context, we propose to couple the Abaqus finite element code with micromechanical simulations based on crystal plasticity and a self-consistent scale-transition scheme. This coupling strategy will be applied to the simulation of rolling processes, at different reduction rates, in order to estimate the evolution of the mechanical properties. By following some appropriately selected strain paths (i.e., strain lines) along the rolling process, one can also predict the texture evolution of the material as well as other parameters related to its microstructure. Our numerical results are compared with experimental data in the case of ferritic steels produced by ArcelorMittal.

Abstract: Aluminium is a potential light weight alternative to steel for deep drawn sheet components, but generally does not compare well to steels in terms of formability. Research in polycrystalline plasticity indicates applying shear to rolled fcc alloys improves their deep drawability by favourably modifying their crystallographic texture. Such processing could be realised industrially by cold asymmetric rolling (ASR), but in order to gain detailed understanding of the influence of process parameters on the evolution and through thickness homogeneity of the texture a validated full field multi-scale model of the process is required. This study examines the ability of a hierarchical multi-scale approach to predict evolved textures for aluminium sheet subjected to a mechanical test exhibiting a deformation mode relevant for ASR, namely simple shear. The homogeneity of the deformation field is assessed with full field strain measurement by digital image correlation, and macrotexture is measured by x-ray diffraction. The discrepancies are discussed and further work to validate the modelling approach for simulation of texture evolution in the ASR process is briefly outlined.

Abstract: The on-going trend in product miniaturization, together with the increasing quality and reduction of costs of micro-components, leads to the need of a robust process design, which might additionally avoid the occurrence of defects in the workpiece. Processes like microforming are affected by variations which can be foreseen but not totally mastered in the design stage. One approach is seen in an adaptive control system based on a metamodel processing the data of online measuring. This approach is grounded on in-depth knowledge based on correct and precise process modelling. This paper presents both experimental and simulative study of a microforging process, part of a more complex forming chain. It consists of six parallel ribs on metal strip. The ribs have a width of 250 µm and are spaced by a gap of 150 µm. The process has been studied by different punch forces, analyzing the final geometry of the workpiece. In particular the rib height is considered as critical to quality parameter. The simulations show reliable results that can be used for the design of the model interfacing measurement and control of the whole microforming process.

Abstract: The goal of this work was to investigate formability of AZ31B magnesium alloy during incremental equal channel angular pressing (I-ECAP). Square billets were processed using different routes of I-ECAP at temperatures varying from 125 °C to 250 °C. The billets were obtained from commercially available coarse-grained, hot-extruded rod and fine-grained, hot-rolled plate. A strong influence of the initial microstructure on processing temperature was reported. Fine-grained samples were successfully processed at 200 °C, while coarse-grained ones must have been heated up to 250 °C to avoid fracture. A gradual temperature decrease with subsequent passes allowed successful pressing at 150 °C. Processing using various routes of I-ECAP showed that a billet rotation before the last pass had strong influence on mechanical properties. The results of experiments were plotted on the diagram of allowable processing temperature for AZ31B. It was found that the relation between the minimum temperature in I-ECAP and the initial grain size could be described by a logarithmic equation.

Abstract: Flexible Pad Laser Shock Forming (FPLSF) is a new microforming process using laser-induced shock pressure and a flexible pad. This process involves high strain-rate (~10⁵ s^-1) plastic deformation of metallic foils along with the hyperelastic deformation of the flexible elastomer pad over which the foil is positioned. This paper studies the influence of flexible pad on the shockwave propagation behavior and the plastic deformation of metal foil in FPLSF using finite element analysis. The effect of flexible pad materials such as silicone rubber, polyurethane rubber and natural rubber on the deformation of copper foils has been analysed in detail. An increase in crater depth is observed with the reduction in flexible pad hardness. However, it is found that there exists an optimum hardness of the flexible pad to achieve perfect hemispherical craters on metal foils, as bending of foils at non-deformed region is observed with softer pads whereas flattening of crater surface occurs with harder pads. The effect of flexible pad thickness on the foil deformation was analyzed at six different thickness levels: 300 μm, 600 μm, 900 μm, 1200 μm, 1500 μm, and 2000 μm. Similarly, there exists an optimum flexible pad thickness to maximize the crater depth and achieve the hemispherical shapes. Analysis of flexible pad thickness indicates that the pad thickness influences the elastic recovery of the flexible-pad and hence the plastic deformation of the metallic foils.

Abstract: FE analyses of backward extrusion of multiple micro-pins have been conducted to examine the effects of pin geometry, layout and spacing. The effect of friction on material flow and accuracy of pin geometry has been investigated. FE analysis was performed for a commercial grade Al1050 as well as its ultrafine grained version produced by severe plastic deformation allowing a comparison to be made. Pin layout plays a large role in achieving uniformity of pin height. Material starvation resulted in the central pins being shorter than the surrounding pins. Simulations without the centre pin were conducted, which resulted in increased pin height uniformity. Simulations with friction resulted in a greater uniformity of pin height but required a greater load. Using stronger ultrafine grained material increased the load and reduced the pin height uniformity.

Abstract: Tribology is one of the major issues in forming processes. It is influenced by many factors such as workpiece and tool material, lubrication, process parameters, geometric scale etc. Especially in microforming processes, friction plays an important role due to an increased surface to volume ratio and the domination of open over closed lubricant pockets. A simple and sensitive method to quantify the friction factor under realistic conditions of massive forming is the barrel compression test. The friction factor is calculated out of the friction-dependent barreling of cylinder samples while being compressed between two parallel tool surfaces.In these investigations, the barrel compression test was applied to determine the friction factor between cylindrically shaped samples made of the aluminium alloy AlMg4.5Mn0.7 (EN AW 5083) and polished surfaces made of the tool steel 1.3343. The specimen diameter was varied between 0,5 mm and 10 mm. The focus of investigations was the size-dependence of the friction factor under the variation of the parameters such as forming degree, lubrication conditions, and die velocity. In addition to the calculation of the friction factor, surfaces were evaluated by microscopy and roughness measurements.

Abstract: Within this paper the characterization of hybrid components consisting of selective electron beam melting (SEBM) additive structures and sheet metal of alloy Ti-6Al-4V will be presented. Key idea of the new production approach is the combination of the advantages of two different manufacturing processes. On the one hand the very high flexibility of the additive manufacturing process and on the other hand the economic production of conventional geometries by deep drawing operations. Main challenge within this new and innovative process is the identification and quality of the properties of the new hybrid components after the manufacturing process. The necessary evaluation consists of three parts: the analysis of the deep drawing blanks, the additive manufactured structure and finally the connection between both. Whereas standardized testing methods are available for the testing of the blanks and the additive structure, there are hardly scientific publication which deals with the investigation of the connection between them. Therefore, a new testing methods and consequently a new tool design was developed in order analyze the specimens in dependency of different strain- and stress conditions. At the end microstructural investigations were performed to identify the fundamental mechanisms which lead to the different properties on macroscopic scale. The result proofed that in particular the electron beam power has a high influence on the production process and thereby the connection quality.

Abstract: This paper presents a new procedure to estimate the material removal (MR) in such conditions or operations where small amount of material or wear occur. The monitoring of material removal is essential to understand the machining mechanisms of several processes such as super finishing ones. For example the study of some mass finishing (MF) operations, i. e. the barrel finishing (BF) and the spindle finishing (SF), have been always limited by the difficulty to measure the local surface modification. Thus there is no knowledge about the relationship between process parameters and obtainable surface quality. The procedure is based on profilometer measurements typically used to characterized local surface morphology. An algorithm automatically finds the most representative peak of the profile. The comparison between the Abbot-Firestone curves, related to peaks achieved in different condition, permits to measure the volume of material removed by the operation. This method overcomes the well-known problem to repositioning the instrument in the same place when the part is moved from machining process to measurement one. In the case of BF, experimental demonstrated the reliability of this methodology to provide the evolution of material removed as a function of working time. Moreover the graphical plot of the representative peak at different times gave important information about machining mechanism. In particular it allowed to verify assumptions regarding the plastic deformation and the peak cutting which takes place.

Abstract: A vertical type tandem twin roll caster equipped with a scraper for the clad strip was invented. This roll caster could cast three layers clad strip which base strip had lower melting point than that of the overlay strips. The base strip was cast by an upper twin roll caster and the overlay strips were cast by a lower twin roll caster. The scrapers were attached to the lower twin roll caster, and were innovated to cast this type of three layers of clad strip. Solidification layers those became the overlay strips were pulled from between the scraper and the roll. The melt of the alloy which was as same as the base strip was poured between the scraper and the base strip. This melt connect the overlay strip and the base strip. The base strip was not re-melted. The scraper enabled that the solidification layer of the overlay strip contact to the melt of the base strip without mixing of the melt of the base and overlay strip. In this way, the sound three layers clad strip which base strip had lower melting point than that of the overlay strip could be cast by the vertical type tandem twin roll caster equipped with a scraper.