Xinyang Li

Bio: Xinyang Li is an academic researcher from RWTH Aachen University. The author has contributed to research in topics: Surface finish & Surface roughness. The author has an hindex of 2, co-authored 7 publications receiving 12 citations.

Finite-Element Modeling of Multi-Pass Forging of Nickel-Base Alloys Using a Multi-Mesh Method

Abstract: Nickel-base alloys are mostly used for high-temperature applications, many of which are heavily loaded safety components. The material properties highly depend on the microstructure, which, in turn, depends on the metal forming process and the heat treatment. FEM integrated microstructure models can satisfactorily describe the grain size development due to dynamic and static recrystallisation during a metal forming processes and the heat treatment. The simulation results obtained from modeled compression experiments are very promising so that consequently, simulations of more sophisticated processes, like multi-pass open die forging or radial forging, is the next reasonable goal. However, the computation times for the simulation of these processes are still unsatisfactorily long and thus, their application is deterred. To accelerate the simulations, a multi-mesh algorithm was implemented to the Finite-Element simulation package PEP & LARSTRAN/SHAPE. This method uses a Finite-Element mesh that is fine in the deformation zone and coarse in the remaining areas of the workpiece. Due to the movement of the tools during the simulation, the deformation zone moves across the workpiece and thus, necessitates a remeshing with a transition of the finely meshed area. A second mesh, which is fine over the entire volume of the workpiece, is used to store the nodal data and simulation results, which get transferred to the simulation mesh every time a remeshing operation becomes necessary. In combination with an adopted data transfer algorithm, this second mesh is used to minimize the loss of accuracy, if a previously finely meshed area becomes a coarsely meshed area. This simulation model can be used to optimize forging process chains with respect to grain size distribution as well as cost effectiveness and energy consumption.

Influence of process conditions and pore morphology on the closure rate of pores in hot rolling of steel

Abstract: Steel sheets are manufactured from slabs produced in continuous casting, which inevitably results in a porous initial microstructure. These pores are nuclei for ductile damage and need to be closed during rolling in regard of the strict performance requirements of todays advanced high strength steels. Due to the beneficial shape factor of the roll gap, recrystallization and high diffusion rates pore closure and elimination occurs primarily during hot rolling. In this paper, relevant influencing factors on pore closure are investigated based on representative volume elements (RVE). First, the load regime of typical multi-pass hot rolling schedules is calculated via macro scale FEM and then applied to micro scale RVEs. Within the RVE, shape changes of a given pore are tracked and the closure ratio is calculated. This paper elucidates the effect of different roll diameters, absolute reductions per pass, pore shape and pore orientation on the closure rate. The results show that the majority of pores can be closed during the initial hot rolling stages. However, pore closure can be accelerated through suitable process parameters. Furthermore, shape and orientation of pores should be taken into account, if a precise knowledge of the total height reduction, leading to complete pore closure, is desired.

Influence of process conditions and pore morphology on the closure rate of pores in hot rolling of steel

Abstract: Steel sheets are manufactured from slabs produced in continuous casting, which inevitably results in a porous initial microstructure. These pores are nuclei for ductile damage and need to be closed during rolling in regard of the strict performance requirements of todays advanced high strength steels. Due to the beneficial shape factor of the roll gap, recrystallization and high diffusion rates pore closure and elimination occurs primarily during hot rolling. In this paper, relevant influencing factors on pore closure are investigated based on representative volume elements (RVE). First, the load regime of typical multi-pass hot rolling schedules is calculated via macro scale FEM and then applied to micro scale RVEs. Within the RVE, shape changes of a given pore are tracked and the closure ratio is calculated. This paper elucidates the effect of different roll diameters, absolute reductions per pass, pore shape and pore orientation on the closure rate. The results show that the majority of pores can be closed during the initial hot rolling stages. However, pore closure can be accelerated through suitable process parameters. Furthermore, shape and orientation of pores should be taken into account, if a precise knowledge of the total height reduction, leading to complete pore closure, is desired.

Modelling of void healing in hot rolling due to recrystallization

Abstract: Porosities, introduced into the material during continuous casting can impair the mechanical properties and performance of semi-finished material as well as final products. Healing of these porosities through closure and bonding of voids with subsequent dissolution of the interface in further hot working can reduce this detrimental effect. While large deformations under compressive stresses as well as high temperatures are beneficial for void healing, not all regions in the workpiece experience the same conditions. Gradients of these metrics in normal direction and over time create regions of varying probability for void healing. Knowledge on the process conditions and the void shapes in the respective regions is essential to predict void healing successfully and to produce a sound product, especially if the process window is limited. In the present work, a multiscale modelling approach is utilized to predict void closure and recrystallization along the normal direction in the roll gap. A practical void healing criterion is developed by combining these two mechanisms and then used to classify a typical workpiece in normal direction according to the likelihood of void healing. Optimal void healing conditions are found at a relative height of 80% while the center region (50%) of the workpiece exhibits the least favorable conditions for void healing.

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

Abstract: 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.

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

Abstract: 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.

Fast resolution of incremental forming processes by the Multi-Mesh method. Application to cogging

Abstract: A Multi-Mesh Multi-Physics (MMMP) method is developed to reduce the very long computational time required for simulating incremental forming processes such as cogging or ring rolling. It consists in using several finite element meshes on the same domain to solve the different physics of the problem. A reference mesh is used to accurately store the results and history variables, while the different computational meshes are optimized to solve each physic of the problem. The MMMP algorithm consists in two main key-steps: the generation of the different unstructured meshes and the data transfer between the meshes. The accuracy of the method is supported by using meshes that are embedded by nodes. The method is applied to the simulation of the cogging metal forming process for which it shows as accurate and more than ten times faster than the standard method with a single mesh.

Une méthode MultiMaillages MultiPhysiques parallèle pour accélérer les calculs des procédés incrémentaux

Abstract: L'objectif de notre travail est de reduire le temps de calcul des procedes multiphysiques incrementaux, tout en conservant avec precision l'histoire du calcul et en prenant en compte l'aspect multiphysique. Notre choix est tombe sur la methode MultiMaillages MultiPhysiques (MMMP). Le principe de la methode consiste a utiliser pour chaque physique un Maillage de Calcul qui lui est optimal, et a considerer un Maillage Reference pour le stockage des resultats. Etant donne que l'application principale de notre travail de these est le procede de martelage qui est un procede couple thermomecaniquement, on a applique la methode MMMP a ce procede en considerant 2 maillages : un maillage pour le calcul mecanique et un autre pour le calcul thermique que l'on a aussi utilise comme Maillage Reference. La particularite du procede de martelage est que la deformation plastique est localisee dans la zone de contact avec les outils, et a l'exterieure de cette zone la deformation est negligeable. Le maillage Mecanique est genere en se basant sur cette particularite : il est divise en deux zones, une zone qui a une taille de mailles fine, c'est la zone de deformation (zone de contact avec les outils) et une autre, constituee par le reste du maillage c'est-a-dire la ou il ne se passe presque rien ; dans cette zone on a considere une taille deraffinee egale a deux fois la taille fine. Pour ameliorer la qualite du transport qui est fait par la methode d'interpolation inverse on a utilise trois techniques : la premiere consiste a grader la zone de deformation dans le Maillage Mecanique telle qu'elle est dans le Maillage Reference, la deuxieme consiste a deraffiner la zone de faible deformation par un deraffinement emboite par nœuds, c'est a dire en eliminant des nœuds sons ajouter ou bouger les nœuds existants et la troisieme concerne les variables elementaires telles que la deformation generalisee et consiste a ne pas transporter cette variable mais a la recalculer sur le maillage d'arrivee a partir de la vitesse. Le cout eleve du transport est reduit a moins de 1 % du temps total par une technique de transport sans relocalisation qui consiste a faire la localisation du maillage d'arrivee dans le maillage de depart uniquement au premier increment et utiliser cette localisation pour les autres increments. Le partitionnement du Maillage Mecanique est fait independamment du Maillage Reference, ce qui ameliore l'efficacite parallele de la methode. L'acceleration MMMP est excellente, elle varie entre 4 et 18 en fonction du nombre de degres de liberte, du nombre d'increments et de la configuration de calcul. En parallele elle chute un peu puisque le Maillage Mecanique du calcul Multimaillage a moins de degres de liberte que le Maillage du calcul Monomaillage, cependant la methode continue a nous offrir des accelerations meme sur un tres grand nombre de processeurs.