Effect of tempering upon the tensile properties of a nanostructured bainitic steel

The tensile properties of a nanostructured carbide-free bainitic steel formed at 200–250 °C are compared against those after tempering sufficiently to remove the retained austenite. Although significant ductility is observed following tempering, a comparison of tempered and untempered samples shows that it is in fact reduced when a comparison is made at identical strength. The shape of the stress–strain curves shows clear evidence that the capacity for work hardening is reduced with the loss of austenite. The nanostructure of the steel transformed at 250 °C is examined by transmission electron microscopy, to compare the as-transformed to the tempered structure. In this case after tempering at 500 °C the energy absorbed during the tensile test is lower, due to the lower strength. Reduction of strength is caused by the slight coarsening of the bainite plates, and lower dislocation density after tempering. Considering the formation of carbide particles in high strength steel, impressive ductility is exhibited even in the tempered condition.The authors would like to thank Professor A. L. Greer for the provision of laboratory facilities at the University of Cambridge and also express their gratitude to the Scholars Rescue Fund of International Institute of Education in Washington DC, for supporting Hala Salman Hasan’s work in Cambridge.This is the accepted manuscript version. The final published version is available from Elsevier at http://www.sciencedirect.com/science/article/pii/S0921509314009642

A Cellular Automaton / Finite Element model for predicting grain texture development in galvanized coatings

Hot-dipping galvanizing process is a widely used and efficient way to protect steel from corrosion. We propose to master the microstructure of zinc grains by investigating the relevant process parameters. In order to improve the texture of this coating, we model grain nucleation and growth processes and simulate the zinc solid phase development. A coupling scheme model has been applied with this aim. This model improves a previous two-dimensional model of the solidification process. It couples a cellular automaton (CA) approach and a finite element (FE) method. CA grid and FE mesh are superimposed on the same domain. The grain development is simulated at the micro-scale based on the CA grid. A nucleation law is defined using a Gaussian probability and a random set of nucleating cells. A crystallographic orientation is defined for each one with a choice of Euler's angle (Ψ,θ,φ). A small growing shape is then associated to each cell in the mushy domain and a dendrite tip kinetics is defined using the model of Kurz [2]. The six directions of basal plane and the two perpendicular directions develop in each mushy cell. During each time step, cell temperature and solid fraction are then determined at micro-scale using the enthalpy conservation relation and variations are reassigned at macro-scale. This coupling scheme model enables to simulate the three-dimensional growing kinetics of the zinc grain in a two-dimensional approach. Grain structure evolutions for various cooling times have been simulated. Final grain structure has been compared to EBSD measurements. We show that the preferentially growth of dendrite arms in the basal plane of zinc grains is correctly predicted. The described coupling scheme model could be applied for simulated other product or manufacturing processes. It constitutes an approach gathering both micro and macro scale models.The authors would like to thank the International Lead Zinc Research Organization (ILZRO) for the financial support supplied to this research project

Modélisation des processus de germination / croissance des grains de zinc pour l’étude du procédé de galvanisation

Près de la moitié des 10 millions de tonnes de zinc produites annuellement à travers le monde sont utilisées pour la galvanisation de pièces d’acier. Le dépôt d’un film adhérent de zinc obtenu par ce procédé constitue une anode sacrificielle limitant fortement la corrosion du métal. Dans le but de mieux appréhender la formation de l’état de surface final du matériau galvanisé, un modèle couplant des approches automates cellulaires et éléments finis a été développé. Il permet de simuler la germination des cristaux à la surface du substrat et la croissance des différentes branches dendritiques. Le développement préférentiel des grains dans le plan basal et l’évolution de leur vitesse de croissance selon leur orientation spatiale sont montrées. De plus, la confrontation de ces résultats avec les analyses EBSD menées parallèlement sur des échantillons de tôles minces, montre une bonne prédiction de la texture du film protecteur

Modélisation des processus de germination / croissance des grains de zinc pour l’étude du procédé de galvanisation

Près de la moitié des 10 millions de tonnes de zinc produites annuellement à travers le monde sont utilisées pour la galvanisation de pièces d’acier. Le dépôt d’un film adhérent de zinc obtenu par ce procédé constitue une anode sacrificielle limitant fortement la corrosion du métal. Dans le but de mieux appréhender la formation de l’état de surface final du matériau galvanisé, un modèle couplant des approches automates cellulaires et éléments finis a été développé. Il permet de simuler la germination des cristaux à la surface du substrat et la croissance des différentes branches dendritiques. Le développement préférentiel des grains dans le plan basal et l’évolution de leur vitesse de croissance selon leur orientation spatiale sont montrées. De plus, la confrontation de ces résultats avec les analyses EBSD menées parallèlement sur des échantillons de tôles minces, montre une bonne prédiction de la texture du film protecteur

A review about Friction Stir Welding of metal matrix composites

Although Friction Stir Welding (FSW), a rather recent solid state welding process, has been proven able to join metal matrix composites (MMCs), it is still subjected to many challenges that need to be overcome to extend its applications. The current review aims to establish a global state-of-the-art of Friction Stir Welding of metal matrix composites. Many points of interests will be discussed, namely the microstructure of the joint and more especially the behavior of the reinforcements during joining, the friction stir weld properties, the advantages of using FSW to weld MMCs by comparison with other joining processes and the issue of tool wear. Some ways of improvement and development are finally proposed

State of the art about dissimilar metal friction stir welding

Friction stir welding is a rather recent welding process (patented in 1991 by Thomas et al., ‘Improvements to friction welding’ UK patent application no. 9125978.8, US Patent 5460317, 1995) that has shown great potential for welding dissimilar materials even of different metallic nature, e.g. Al to steel, Mg to steel, Al to Ti, Mg to Ti, Al to Cu, Al to Mg. This review presents the specific microstructural features and mechanical properties, in particular tensile strength, of such welds. A focus will be on the material flow and welding defects, on the intermetallic compounds, on constitutional liquation, on particularities related to dissimilar lap welding and finally on process modifications to improve dissimilar friction stir weldability

Atomic scale characterization of a pure Al – galvanized steel spot magnetic pulse joint interface

International audienceThe bonding interface of pure Al – galvanized steel spot magnetic pulse welds was characterized at the atomic scale. The Al-Zn interface microstructure is worthy of attention as it contains a few micrometers thick biphased layer. This singular layer consists of an aggregate of grains with a 200 to 400 nm size containing islands of either Al or Zn solid solutions oversaturated in Zn or Al, respectively. Inside a grain, some mottled contrasts with a size in-between 5 and 10 nm correspond to fluctuant solute contents. Besides, some native oxides have been dissolved at the interface suggesting the incorporation of oxygen atoms in the biphased layer. Large and elongated (Al,Fe,Si) particles primitively contained in the base Al were also dissolved which gives rise to some round shaped Si particles in the biphased layer. All these microstructural features and changes are consistent with a ballistic origin. Due to both the absence of solidification defects and the rather planar morphology of the bonding interface, the process is expected to occur at the solid state

Arresting the grain growth of nano grains in Copper for effective heat evacuation applications

Copper has been well known for its ability to conduct heat and/or electricity and is equally appreciated in automobile as well as electronic industries. However, at higher temperatures copper suffers from grain growth and hence subsequent decrease in thermal conductivity. Hence, stabilization of the microstructure becomes an important issue especially since copper finds applications in brake pads as heat evacuators. Friction Stir Processing (FSP) is a solid state processing technique which is effective in producing severely deformed microstructures and is capable of mixing/welding/processing nearly any combination of materials. FSP has been adapted to process copper with Yttria particles to obtain nanosized Yttria particles uniformly distributed in copper. Yttria being stable up to 1400oC, it is expected to anchor the grain boundary migration of the copper at medium to high temperatures. Grain size evolution as a function of the number of passes is studied with a simulative heat treatment that raises the samples temperature to 200oC and subsequently quenches it . Thermal conductivity will be correlated to the microstructure