Microstructural, texture, plastic anisotropy and superplasticity development of ZK60 alloy during equal channel angular extrusion processing

In this study, equal channel angular pressing (ECAP) was exploited to refine the grain size of a ZK60 magnesium alloy in multi-processing steps, namely at temperatures of 250˚C, 200˚C and 150˚C, producing an ultrafine-grained (UFG) structure. The microstructural development and texture evolution during ECAP were systemically investigated by electron backscattered diffraction (EBSD) analysis. The microstructure of the ECAP processed alloy was remarkably refined to an average grain size of 600 nm. During ECAP process the original fiber texture of the as-extruded alloy was gradually weakened and eventually replaced by a stronger texture component coinciding with ECAP shear plane. The ECAP processed material showed a proper balance of tensile as well as compression strength and tensile ductility at room temperature. Yield strength of 273 and 253 MPa in tension and compression, respectively, ultimate tensile strength of 298 MPa and fracture elongation of about 30% were obtained in the UFG alloy. A transition from ductile–brittle to ductile fracture consisting of very fine and equiaxed dimples was also found in the ECAP processed material. Compared to the as-received alloy, a combination of grain refinement and texture development in the UFG alloy gave rise to a notable reduction in mechanical asymmetric behavior at room temperature. The superplastic behavior of the as-extruded and ECAP processed alloy was also investigated at 200˚C with strain rate of 1.0×10-3 s-1. The concurrent effect of grain boundary sliding and favorable basal texture in the UFG alloy led to an achievement of elongation value of about 300% while, under similar testing conditions, the elongation of about 140% was obtained in the as-extruded alloy

Quality mapping of aluminium alloy diecastings

The effect of casting defects on mechanical properties was investigated for a highpressure die-cast aluminium alloy. A series of U-shaped components were cast using a fully controlled cold chamber high- pressure diecasting machine with different process parameters. Defects existed in gas and shrinkage pores as well as oxide inclusions. An X- ray equipment was used for a preliminary quality control but showed its limits in detecting defects. The castings were sectioned and tensile bars extracted from different locations in the castings in order to map the distribution of the mechanical properties. The decrease in mechanical properties correlated with the area fractions of defects revealed on the fracture surface of the tensile specimens. A quality mapping approach showed how the final properties depend on the position of the casting and the process parameters adopted

Sta Cast : Linee guida per la progettazione di getti in lega di alluminio

StaCast (New Quality and Design Standards for Aluminium Alloys Cast Products) è un progetto europeo dedicato allefonderie dell’alluminio con l’obiettivo di sviluppare una nuova classificazione dei difetti strutturali nei getti e di definiredei limiti di accettabilità di questi difetti a seconda della destinazione finale prevista. Il raggiungimento di questo obiettivoaiuterà significativamente le fonderie a ridurre il costo della non-qualità, migliorando nel contempo il loro margine dicompetitività grazie al conseguimento di importanti vantaggi, per esempio nei costi dell’energia. StaCast ha condotto unavasta indagine tra le fonderie europee per conoscere le caratteristiche principali della loro produzione, in quale misuraesse utilizzano gli Standard CEN, il bisogno di nuovi strumenti normativi riferiti ai difetti, alle proprietà meccaniche e allaprogettazione meccanica di getti in lega d’alluminio. Questo articolo presenta i principali risultati di tale indagine, che hacoinvolto circa 80 aziende, e l’impostazione dei documenti normativi che sono stati elaborati sulla Classificazione deiDifetti e sul Potenziale Meccanico di getti in lega d’alluminio

L’analisi termica di Fourier applicata alla modifica eutettica di una lega AlSi7

La “Fourier Thermal Analysis” (FTA) è una tecnica avanzata di analisi termica, sviluppata negli anni’80-’90 per determinare il calore latente rilasciato in fase di solidificazione, allo scopo di indagare sullecinetiche di nucleazione ed accrescimento delle varie fasi coinvolte. I risultati delle indagini condotte conquesta metodologia hanno contributo allo sviluppo dei codici di calcolo per la modellazione dei fenomenidi solidificazione di leghe da fonderia come le ghise e le leghe Al-Si. Scopo di questo lavoro è la verificadelle potenzialità della FTA nello studio della modifica eutettica di una lega Al-Si, e più in generale nelladinamica della solidificazione delle leghe Al-Si. La modifica eutettica viene impiegata allo scopo diincrementare le caratteristiche di resilienza e resistenza a fatica delle leghe Al-Si, l’efficacia deltrattamento è vincolata alla presenza di una percentuale minima di elemento modificante. L’analisitermica può essere impiegata per verificare il livello di modifica della lega, e quindi l’efficacia deltrattamento. È stata presa in considerazione una lega della classe A356, modificata con Na metallico, esono state confrontate le curve fs(t) e fs(T) ottenute con diversi livelli di modifica, allo scopo di correlarele caratteristiche delle curve di frazione di fase solida rispetto al tempo ed alla temperatura con lecaratteristiche microstrutturali della lega

Fourier thermal analysis applied to sodium eutectic modification of an ALSI7 alloy

The so-called "Fourier Thermal Analysis" (or FTA) is an evolution of the "integral thermal analysis", which is actually used as a process control in Aluminium and cast iron foundries. It has been developed since late 80´s in order to investigate nucleation and growing kinetics of the various phases in multi-component alloys. FTA is based on the evaluation of the thermal gradient in one-dimensional thermal field that arises in a cylindrical solidifying specimen. During the last twenty years, both the traditional thermal analysis and FTA have been applied to the experimental determination of the solid fraction during solidification, in order to assess results from numerical simulation. Nevertheless, FTA has not been applied to foundry process control or optimisation.Eutectic modification is extensively used in low-pressure permanent mould processes, in order to improve tensile properties and toughness of Al-Si alloys. The effectiveness of the treatment is subjected to the presence a minimum amount of modifying elements, such as Sr, Na or other elements. Traditional thermal analysis is useful in determining modification level of the alloy, then to control the modification treatment. Aim of this work is to verify the potentiality of gradient-based thermal analysis method, such as FTA, in eutectic modification investigation. An A356-type hypoeutectic Al-Si alloy has been modified with metallic sodium at four different modification levels. Two-thermocouple thermal analysis curves have been recorded, in order to perform FTA analysis. Fraction solid versus time (fs(t)) and temperature (fs(T)) have been determined at different modification levels. Microstructural characterization has been made using automatic image analysis. Average values of dimension and roundness of eutectic Si have been compared to thermal analysis results.A significant correlation between the so-called "eutectic depression" and silicon morphology has been observed. More relevant is the behaviour of the fraction solid curves, from which it is possible to note a significant delay in the start of eutectic reaction. This delay has been observed not only in time but also in temperature and fraction solid domain

PROPOSAL OF A CLASSIFICATION OF DEFECTS OF HIGH-PRESSURE DIECAST PRODUCTS

The paper gives an introduction to the one of the current actions of the WG “Quality of High Pressure DieCast (HPDC) products” within the TC “High pressure diecasting” of the Italian Association of Metallurgy, The preparation and dissemination of common tools to allow foundries to define a proper, comparable, quality standard is in fact one of the current targets of this WG. The basic tool, to which this paper refers, is a common terminology and classification of defects of HPDC products. An initial survey of literature and industrially adopted classifications of defect in components cast in metallic dies, revealed that the geometry and origin-based approaches are often mixed giving rise to a wide range of hybrid classifications. The proposed classification of defects of HPDC products, discussed within the WG with the contribution of several foundries, is a multi-level, hybrid-type classification. In the first level defects are grouped on the basis of their position (surface/internal and geometry defects) according to the typical control operations during which they can be detected and to the effects of defects on the functionality of the parts. The second level of the classification groups defect in classes according to their general metallurgical origin. The proposed classification doss not specify defect/cause correlations, but gives starting points to the identification of their specific causes. In order to better specify these features the analogies between the origin of internal/surface defects are highlighted by corresponding names for Level II classes: gas-related, shrinkagerelated, filling-related, thermal contraction defects and undesired phases. In addition the class of metal/die interaction defects is proposed for surface defects. The specific defects identified are those included in the third level of the classification. In the present paper an introduction to the general classes of defects defined in Level II is proposed. The action of the WG concerning defect classification of HPDC parts will be completed with the compilation of the official document including the proposed classification and multi-language terminology equivalences

Sigma phase precipitation modelling in a UNS S32760 superduplex stainless steel

Sigma phase precipitation is known to have detrimental effects on mechanical and corrosion propertiesin stainless steels. Accordingly, heat treatments and welding must be performed carefully. For this reason,computer models which predict the evolution of secondary phases with the temperature are very usefulfor a good planning of industrial heat treatments or welding. A semiempirical model, based on the pioneerwork of Wilson and Nilsson, describing the microstructural evolution of UNS S32760 superduplex stainlesssteels was developed. The computer model, based on the modified Johnson–Mehl–Avrami-Kolmorgorov (JMAK)type of equation, uses information from isothermal experiments of sigma-phase transformation and predictsthe transformation kinetics during a cooling process. A good agreement was found between the predicted CCTdiagramand the experimental tests

Correlation between microstructure and mechanical properties of Al-Si diecast engine blocks

In spite of weight reduction of passenger car, the cylinder block is the heaviest component among many automotive engine parts and plays a key role in fuel efficiency and the drivability of vehicles. In hypoeutectic aluminium alloys, the final mechanical properties are strictly connected to microstructural features such as the distribution, the morphology and dimensions of primary ?-Al phase and eutectic Si particles, as well as, type of iron-bearings and defects. The microstructure, in turn, depends on filling process and solidification dynamic. In this work, a HPDC 4-cylinders-inline cylinder block was exhaustively analysed and mechanical properties were correlated to microstructural features. Mechanical properties are affected by microstructure. The best values of UTS and elongation to fracture are obtained for low secondary dendrite arm spacing (SDAS) values and small and more compact eutectic Si particles. If the combined effect of ?-Al phase and eutectic Si particles is taken into account, a linear correlation between UTS and product of SDAS, equivalent diameter and aspect ratio of Si particles is observed, while the elongation to fracture shows an exponential trend, suggesting an high sensitivity on microstructural variation

Simulation of fluidity in AL-SI alloys

In this study, MAGMA soft® commercial software package was used to simulate the fluidity of A356 alloy. Established an optimum mesh generation, the influence of important metallurgical parameters, such as heat transfer coefficient, casting temperature and coherency temperature, on fluidity were simulated. The simulation results were compared with fluidity laboratory tests carried out with spiral-shaped sand moulds and the results from the simulations were found to be consistent with the experiments. Therefore, this study sets a basis for more extensive use of simulations as a means for predicting and optimizing the fluidity of aluminium alloys. In addition, the results from the spiral-shaped mould tests were compared with vacuum fluidity tests carried out using an A356 alloy and the two techniques showed consistent results

Design and realization of an experimental cold crucible levitation melting system for light alloys

Thanks to their properties of ultra-lightness and high strength/weight ratio, Mg and Al alloys find increasingemploy in aerospace, automotive and biomedical applications. These alloys can be formed using all theconventional technologies used for other materials, like casting and forming. However, the mechanicalproperties of the final components are significantly influenced by the quality of the starting liquid metal.In fact, the quality of the starting liquid metal has been substantially increased in recent years, thanks to theimprovement of cleaning technologies. To this purpose, electromagnetic processing of materials has evolved asan important experimental technique in the fields of material processing, associated with applications such asshape controlling, flow driving, online detecting, controlled heat generation, inclusion removing, magneticlevitation. In particular, electromagnetic levitation, as a promising technique, can be helpful to create somenew phenomena and discoveries, especially in melting process. This work describes the design, optimizationand realization of a cold crucible levitation melting (CCLM) system for light alloys. Electromagnetic models areused and applied in FEM codes to numerical simulate the working range of the CCLM.The simulation results show good agreement with experimental data