Key Engineering Materials Vols. 504-506

Abstract: Forward two-hole extrusion of Al has been investigated with the purpose of studying how metal flow inside the billet is influenced by the location of the holes in the dies, i.e. whether they are position near to or far apart from each other. The study has been conducted by means of finite element analysis (FEA) using the software DEFORM 3D® and validation of simulation results are done by comparison with grid pattern experiments performed long time ago by one of the authors. The analysis shows that the experimental conditions are well reproduced by FEA. New insight into the metal flow phenomena in two-hole extrusion is also gained thanks to the analysis. It is shown, for instance, that moving the holes far apart from each other brings about a distinct shift in the metal flow. The deformations subjected to the peripheral outer shear zones of the billet material then become much more localized than when the two holes are close.

Abstract: The necessity to simulate extrusion processes requires the knowledge of the material in terms of the constitutive equations. The present investigation deals with the development of a methodology based on the multivariable regression analysis in order to predict the flow behaviour of a ZEK200 Mg alloy in the interval of temperature varying between 150 and 450°C. The proposed models relate the flow curve levels with temperature, strain rate and deformation with the variables selected using the statistical method. It has been found that the proposed methodology allows the modeling of the flow behaviour of the Mg alloy under investigation from the beginning of the plastic deformation down to the softening phase taking into account the peak value. In addition, it is comparable, in terms of the temperature intervals considered in the modeling, with the Sellars and Tegart-based models.

Abstract: The paper presents the latest development of the numerical model for extrusion of industrial profiles having complex shapes. The model provides the material flow analysis coupled with mechanical and thermal problems in the tooling set. The simulation predicts possible shape deterioration due to uneven material flow through the bearing zone and helps to equalise it by means of optimisation of the bearing design, chamber and feeding channels. The locations of welding zones in the die are clearly predicted. It allows to modify the die design for better welding conditions and to provide optimal location of welding seams in the product. Die stress analysis shows the ways to extend the tool life by means of reducing of fatigue failure and selecting proper die materials as well as to correct the influence of the die deformation on the material flow. The described model is implemented in an especially dedicated program QForm-Extrusion that effectively simulates the extrusion of hollow and solid profiles with very high elongation ratios. The experimental verification of the model is illustrated by model and industrial experiments as well as by series of case studies performed in production environment.

Abstract: In this study the cold extrusion of Al alloys will be investigated by finite element analysis (FEA) using DEFORM 2D. Besides, theoretical calculations to find the required extrusion force will be done. Several FE-simulations have been performed for two different extrusion geometries and for different conditions of friction including high and low friction, to quantify the influence of friction on the extrusion force. The results from FEA and theory for cold extrusion are compared to see if there is good agreement. The correlation between the data obtained by theory and FEA is discussed.

Abstract: The accurate simulation and the optimization of extrusion processes can be a helpful technique to ensure producibility of complex aluminum profiles, for example for the automobile industry. Currently, the die designing is based on expert’s knowledge and cost-intensive prototyping. The paper deals with numerical investigations based on finite element simulations as well as experimental investigations of an industrial extrusion process. A newly developed method for longitudinal seam weld prediction is applied to analyze the position of the longitudinal welding line and the welding quality.

Abstract: Aim of the work is to investigate different strategies in balancing material flow during direct extrusion through porthole dies. Two AA6082 hollow profiles were simultaneously extruded by a single die with different portholes extrusion ratio, dissimilar welding chambers and different bearing lengths. A strict process control was realized by measuring thermal conditions in the die by means of 6 thermocouples and on the profiles by a self calibrating pyrometer for aluminum alloy applications. Several billets were extruded at different ram speeds (2 to 7 mm/sec) and the effect of die design on surface quality, profile lengths and thermal field was recorded. The profiles were then sectioned and the position of the seam welds in the profiles identified and compared also with the profiles tip.

Abstract: In extrusion of hollow Al-profiles two kinds of pressure welds are present inside the extrusion. One is called the charge weld (CW) and forms across the boundary interface between two billets extruded in sequence. The other is the seam weld (SW) which extends longitudinally along the extruded profile and the extrusion metal behind each die bridge. It is considered to form because of the splitting of the extrusion metal over the die bridge into metal streams which flow past the bridge and rejoin as they encounter behind the bridge. Over the time attempts have been made to explain the mechanics of extrusion welding for both the CW and the SW. Still there is lack of understanding of how these welds form, the main reasons for this is that the deformation conditions around a die bridge are complex and difficult to investigate. Because of the recent advancement of two technological fields, experimental grid pattern analysis and simulation of metal flow by FEA; new tools for analysis of the mechanics of formation of the SW and the CW are now available. The simplest possible case of 2D-extrusion seam welding is considered here and an attempt is made to describe the fundamental deformation mechanisms present when this weld forms behind a butt-ended die bridge.

Abstract: In last decades, national legislations have become even more restrictive with respect to the application and the disposal of hazardous lubricants in sheet metal forming. The introduction and the continuous improvement of environmental friendly solutions has seen a continuous growth and attention, but today the usage of traditional hazardous lubricants is still significant, requiring expensive cleaning operations and harmful cleaning agents to remove them. The use of solid organic lubricants can reduce or eliminate this drawback, despite their performances may be significantly affected by specific process parameters and the presence of debris. In this paper the performances of a solid organic lubricant have been compared to traditional liquid lubricants applied to sheet metal forming applications. Different surface topographies have been reproduced and the effects in terms of frictional behaviour have been investigated.

Abstract: In the last decades, Physical Vapour Deposition (PVD) and Chemical Vapour Deposition (CVD) processes have been significantly improved and optimized for the applications on dies for sheet metal forming processes. However, due to the different contact conditions at dies-blank interfaces, and the wide range of applied contact pressures, the selection of the correct coating may be still affected by trials-and-error approaches. Although many methods to evaluate the tribological performances of such coatings can be found in scientific literature, they often suffer of limitations in reproducing the interface conditions typical of industrial processes. The objective of the present research work is to investigate the tribological behaviour of two coatings deposited by PVD magnetron sputtering technique. Both investigations in laboratory and industrial conditions were performed: the former to evaluate their tribological characteristics, the latter to test the performances of coatings in production lines. The results, in terms of wear resistance, outline the comparison of the new technology with the performances of traditional dies.

Abstract: Theoretical solutions for several rigid plastic models used to describe plastic flow in metal forming processes are singular in the vicinity of maximum friction surfaces. In particular, velocity gradients and the equivalent strain rate approach infinity near such surfaces. Such singular behavior can be excluded from consideration by choosing another friction law or material model. However, a different approach is proposed in the present paper. The starting point of this approach is that many experiments show that velocity gradients are very high in the vicinity of surfaces of high friction and that a narrow material layer is formed near such surfaces whose properties are very different from the properties in the bulk. Taking into account that the equivalent strain rate has a significant effect on the evolution of material properties, this experimental fact suggests that a theory based on the singular plastic solutions can be developed to describe the formation of the aforementioned material layer. In the present paper such a theory is proposed to describe the evolution of grain size. It is assumed that, in addition to the equivalent strain rate, the material spin has an effect of the evolution of grain size. It is then shown that the solutions for the material spin are singular as well. The interrelation between the present theory and strain gradient theories of plasticity is discussed. It is shown that it is necessary to account for the strain rate gradient to propose a more adequate theory to deal with the material flow near surfaces of high friction. Some experimental results on the formation of the narrow layer of ultra-fine grains in the vicinity of the fraction surface in extrusion are presented. An illustrative example to relate these experimental results and the new theory is given.