In the present investigation, the microstructure and tensile properties of aluminium die-cast alloys, based on A380, are studied in details as a function of the iron and manganese content and solidification rate. One set of experiments was designed to examine the ?solely? effect of Fe content, which varied from 0.1 up to 1.6 wt % and another set with manganese additions, Mn:Fe ~ 1:2. Three solidification rates corresponding to ~ 10, 25 and 60 ?m in secondary dendrite arm spacing, SDAS, respectively were employed by using the unique gradient solidification technique. Microstructure analysis reveals that at relatively high cooling rates and iron levels, the iron-rich precipitates are suppressed to some extent and the strength is maintained at high levels, but the ductility is gradually decreased. The amount of iron-rich intermetallics does not appear to influence the size and area fraction of porosity and consequently the result suggests that tensile properties have not been adversely affected by porosity level. These results offer additional insight into commonly discussed microstructure features and their role in the determination of the quality and soundness of Al-Si cast alloys.
The non-metallic inclusions are metallurgical defect always present within the steel bulk. Although high cleanliness levels can be reached by the most up dated techniques, their presence is not avoidable, so the users of steel have to learn a correct management of such defects. This problem has become even more significant with the use of resulphurised steels, in which the sulphur is added to cause the precipitation of an abundant MnS population, which makes the steel more workable by the cutting tools. However, the presence of a not proper inclusional population can forbid the implementation of certain technological processes to be applied, i.e. welding, hot working etc.. Moreover, the sulphide inclusions improve the workability, but they remain within the product also after the technological transformations and they can weaken the whole material, so their use has to be decided only after a proper choice of the technological route. Thus, some indications about this topic have been obtained on the basis of some significant cases on which a failure analysis has been applied.
The yield stress (0.2%) of 5 mm thick high pressure die-cast AZ91 and AM60 alloys has been compared to the respective Vickers hardness. The yield stress and hardness were measured in as-cast specimens, and in specimens directly aged (from the as-cast condition) at 120oC for up to 10000 h. Unlike a range of sand cast and permanent mould cast Mg-Al alloys and an AZ91 alloy, for which the hardness increase proportionally to the yield stress, in high pressure die-cast magnesium alloys the hardness tends to be lower than expected from their yield stress.
The paper reviews experintenîol resulîs published in literature about the influence oJ'material strength antl surface.finish on fatigue behaviour of sîeels. The increasing imporîance of achieving accuruîe and homogeneous sutJace.finishes cts îhe tn.tîerial strength is increased antl,/or its toughneis is ktwered is rtcknowledged. Furîher, it is retnarked that, when extremely smooth .surface t:onditions can not be reliably obtained, the selection of high strength naterials Jitr.fatigue loading could become inadequuîe. Tw-o case stttdies are reported in order to show the possibte harmful consequences of unsuitctble material selection. The fir,st example reJers to fatigue.failures experienced by spring,s operating cú relatively low temperatures. The springs, quenchecl arul tempered to obtain high strength level, were shot-peened to improve their Jatigue behaviour. This latter îreQîment gave rise to surJace tlefecîs that acted as nucleation sites .for fatigue crucks which rapidly propagated in a moterial entbrittled by a number of Jàctor,s. The seconcl series oJ examined ntpÍures concerns rolls operating in a paper-mill plantThe cracks leadíng to.failures were nucleated at sutJace pits brought about by environmental corrosive atteck and propagated in a steel microstructure unsuitable îo sustain the highJatigue Loading. Riassunto Intendendo considerare f influenza della resistenza del materiale e della finitura superficiale sul comportamento a fatica degli acciai sono state riesaminate alcune serie di dati ripoúate in letteratura. È stato messo in evidenza che I'ottenimento dipezzi con finiture accurate assume un'importanza sempre crescente a1l'aumentare della resistenza e/o al diminuire della tenacita dell'acciaio utilizzato. È stato inoltre sottolineato il fatto che Ìa scelta di acciai ad alta resistenza per condizioni di carico ciclico può rivelarsi inadeguata nei casi in cui non possano venìre realizzate superfici estremamente levigate. Sono stati inoltre riportati due casi per mostrare le possibili conseguenze dannose di una errata scelta del materiale o 6elle sue condizioni microsfutturali. Un primo esempio si riferisce a rotture per fatica avvenute in molle operanti a temperature relativemente basse. Le molle, temprate e rinvenute per ottenere durezze elevate, erano state successivamente pallinate per migliorare il comportamento a fatica. Difetti e ripiegature superficìali lasciati da quest'ultimo trattamento avevano originato cricche rapidamente propagatesi ne1 materiale infiagilito. Una seconda serie di rotture ha invece riguardato dei rulli operanti in impiamti per Ìa produzione di carta. Le cricche che avevano determinate le rotture avevano in questo caso avuto origine in corrispondenza di vaiolature superficiali causate dall'ambiente conosivo e si erano propagate per fatica in una microstruttura non adatta a sostenere elevate condizioni di carico ciclico.
Al–Li alloys are characterized by a strong anisotropy in mechanical and microstructural properties with respect to the rolling direction. In the present paper, 4mm sheets of 2198 Al–Li alloy were joined via friction stir welding (FSW) by employing a rotating speed of 1000mm/min and a welding speed of 80mm/min in parallel and orthogonal direction with respect to the rolling one. The joints mechanical properties were evaluated by means of tensile tests at room temperature. In addition, fatigue tests were performed by using a resonant electro-mechanical testing machine under constant amplitude control up to 250Hz sinusoidal loading. The fatigue tests were conducted in axial control mode with R=σmin/σmax=0.33, for all the welding and rotating speeds used in the present study.
The problem of sheet metal stamping in general was analyzed, defining the parameters involved and the objectives to be reached for process optimization. D.O.E. (Design of experiments) and statistical analysis (ANOM, ANOVA) techniques were applied to the case of a component which, though of geometrically simple configuration, was representative of the process. The objective of the work was methodological: through this application, it was possible to develop a series of procedures and obtain positive initial results, particularly as regards sensitivity analysis for the parameters involved.
High-temperature plastic deformation and dynamic recrystallization were investigated in an extruded and heat-treated AZ31alloy in the temperature range between 200 and 400°C. High-temperature straining resulted in partial dynamic recrystallization above 250°C; at 400°C recrystallization was complete and a moderate grain growth was observed. The peak flow stress dependence on temperature and strain rate was described by means of the conventional sinh equation; calculation of the activation energy for high temperature in the whole range of temperature deformation gave Q=155 kJ/mol, i.e. a value that is reasonably close to, but greater than, the activation energy for self-diffusion in Mg. When the data obtained at the lowest temperature were excluded from the calculation, the activation energy increased to 180 kJ/mol. This difference in the activation energy value can be explained by the occurrence of dynamic recrystallization in the high-temperature regime; this observation was substantially confirmed by the plots of strain-hardening rate as a function of stress that were used to identify the onset of dynamic recrystallization.
It was studied the corrosion behaviour of two joints, obtained by Friction Stir Welding (FSW), of aluminium alloy metal matrix composites (MMCs), reinforced with alumina particles. W6A20A and W7A10A composite joints were examined. Corrosion tests were performed in EXCO or 3.5% NaCl solutions. The corrosion rates of MMCs and joints were compared. Corrosion tests were performed on both the nugget and the thermally affected zones of the joints. Pitting potentials were measured on electrodes prepared by isolating the different zones. Short-circuit currents were measured between the nugget, the thermally affected zone and the base. The results indicated the different behaviour of the two joints: the corrosive attack on the W7A10 joint was concentrated on the nugget and in its adjacent zones both in EXCO and 3.5% sodium chloride solution; in the case of W6A20A, the nugget remained protected while the adjacent zones and the base material were corroded. Electrochemical measurements confirmed these results. However, FSW, which produces welds without the formation of a fused zone, causes structural modifications in zones near the nugget, due to the higher temperature produced by the presence of the reinforcement. This negatively affects the corrosion resistance of the composite, also in relation to the base alloy composition.
Evaluation of hot formability of an AL-4.6ZN-0.8MG alloy by increasing-strain-rate torsion tests M. El Mehtedi, N. Ryum, S. Spigarelli, E. Evangelista and B. Ronning The hot formability of an experimental Al-4.6%Zn-0.8%Mg alloy was studied by torsion testing. Conventional constant strain-rate tests were carried out to investigate material response in selected condition of temperature, T, and strain rate, ©. In order to determine the relationship relating flow stress with strain rate and temperature, the conventional procedure, based on straining at constant strain rate, requires a relatively large number of tests. The scope for reducing the number of tests by increasing linearly , i.e. , where K is a constant, from 0 to a definite value, was explored in this work and a series of increasing strain-rate tests were carried out. Nine tests, with K ranging from 0.05 to 0.2 s-1, were performed at 500 oC; the resulting curves were used to recalculate the isostrain rate curve; comparison between experimental and calculated curve gave encouraging results, confirming the reliability of both testing procedures. The microstructure of torsioned samples was investigated by light (LM) and scanning electron microscopy (SEM). EBSD patterns were obtained to analyse grain size distribution and the presence of substructures. The results are discussed in the light of the more recent theories of high-temperature deformation of Al alloys.
The preliminary results of microstructural characterization and creep behaviour of an innovative TiAl 8% atomic Nb alloy are exposed in this paper. Two different batches of this material, with same nominal chemical composition, but produced through different solidification processes and heat treatments, have been studied. The production processes generated different ? + ?2 microstructures in the two batches, both analyzed through X ray diffractometry (XRD) and transmission electron microscopy (TEM). Constant load creep tests have been performed on both batches at the same temperatures (700?C and 850?C) and loads in order to compare the creep behaviours. XRD and TEM analyses have been carried out after creep tests in order to determine microstructural evolution of the materials and to establish correlation with creep behaviours.
The dislocation mechanisms of dynamic recovery (DRV) in metals of high stacking fault energy (SFE) give rise to steady-state straining dependent on temperature and strain rate due to development of constancy in the spacings of subgrain boundaries (wS, SGB) and of the dislocations within both the SGB and the subgrains. When the grains are large compared to subgrains, the interactions of grain boundaries with SGB are limited to s e rration formation but when one-grain dimension is reduced to about twice the SGB spacing, the interactions begin to define a minimum grain dimension. However, the cellular size defined by the mixture of SGB and GB remains constant at wS along with the stress. In metals of low SFE, the above is seldom attained since dynamic recrystallization (DRX) intervenes to provide new grains more than twice the subgrain size. On a larger scale, transition boundaries between deformation bands lying between diff e rently slipping and rotating bands, become permanent and rapidly rise in angle and take on GB behavior in both DRV (serrations) and in DRX (nucleation sites). The evidence became more precise as techniques advanced through polarized optical, scanning and transmission electron and orientation imaging microscopies; however, deficiencies in each technique often created confusions that were resolved only through detailed comparison of the evidence.
The hot torsion test has proven very versatile in determining high temperature (> 0.5 T m ) characteristics at constant strain rates of l0 -3 to 10 2 s -1 .The stress-strain and strain hardening curves are suitable bases for constitutive equations giving temperature and strain rate dependencies. Critical points in the above curves in association with metallography clarify the progress of the restoration mechanism and the influence of solutes, precipitates and inclusions. The ductility is related to how the softening mechanisms delay the fracture mechanisms ssmming from the alloy structure. The rate of static softening after hot deformation is determined by comparing the yield stress on reloading after an interval to that before it; many measurements for different times can be carried out on a single specimen by recrystallizing it between each stage in the torsion apparatus. The physical simulation of rolling schedules having many passes with declining temperature and rising strain rate is possible on computerized machines
In this manuscript we review some characterization tools for Al foundry alloys. Castability is the ability of an alloy to be cast without formation of defects such as cracks, pores or misruns. Being able to measure, hence to control, these defects is fundamental and plays a critical role for the development of numerical models for castability. Also melt quality influences these defects formation. Alloy dependent phenomena that determine castability are, among others, fluidity and porosity. This manuscript focuses on the main characterization tools for measuring fluidity, porosity and melt quality. RIASSUNTO
A preliminary experimental comparison of the behaviour of aluminium and magnesium alloys subjected to Liquid Hot Isostatic Pressing (LHIP) is proposed. The two metals melt at approximately the same temperature. However, as a consequence of a larger deformability of magnesium at elevated temperatures, the choice of LHIP parameters – and especially the temperature at which the pressure is applied – in the present exploratory case was constrained to values far smaller than those one would like to select in order to improve the ultimate tensile stress and the elongation to fracture.
The High Pressure Diecasting process is very attractive to the casting buyer, offering fast production rates coupled to optimised production costs. As castings become more complex and wall sections become heavier so the advantage of the very rapid cooling rate is reduced and casting defects more commonly seen in gravity diecasting and sand casting are now being experienced. The need to heat treat and weld castings also means that casting quality levels must be improved. A fresh approach to the treatment of the Aluminium melt is therefore required and this paper describes the design of a test piece and a test programme to examine the influence that metal treatment can have on casting quality. Simulation is used in the design of the test piece and various non-destructive tests are used to measure the quality of the Aluminium melt, prior to casting. Finally X-Ray, SEM and image analysis and mechanical testing are used to assess the quality of the resulting castings.
Despite the large number of existing alloys and alloy databases, identifying proper alloys for specific applications still remains a challenge. In order to facilitate the selection and prediction of aluminum die casting alloys and their properties, an electronic database - ?i-Select-Al? - has been developed by the Advanced Casting Research Center (ACRC) and the North American Die Casting Association (NADCA). The key to the predictions is the determination of a relationship between alloy properties, chemical composition, and processing variables. Theoretically, these relationships can be ?accurately? determined using fundamental physical principles. However, in practice, the underlying mechanisms are not fully understood and difficult to be utilized. In this case, approximate empirical models are considered. In version 1.0 of the software trend equations have been generated. The nature of these trend equations limits the applicability and prediction ability of the software. In order to improve the prediction power; relationships based on an artificial neural network (ANN) were exploited in version 2.0. ANN has proven to be a highly flexible tool, suitable to treat multiple-input conditions and nonlinear phenomena with complex relationships between input and output variables. This article presents the working mechanisms, the programming, and the application of ANN in this project. The results show that ANN is a valuable modelling tool for predicting properties- fromcomposition and composition- from-properties for aluminum die casting alloys.
Preliminary results are reported of the temperature variations in a fatigue pre-cracked compact tension specimen ofAISI 316, subjected to a uniformly increasing cross-head displacement. The temperature variations in the regions surrounding the crack tip are sensed by three point-like thermistors. At the thermoelastic-plastic instability the load vs. time characteristic exhibits a barely perceptible kink. Conversely, the initial thermoelastic cooling is followed by a sudden temperature rise near the crack tip and a delayed, smoother rise at greater distances. This experimental procedure, based on " thermal emission " allows one to identify with precision the formation and development of the plastic zone
