Boundary element method predictions of the influence of the electrolyte on the galvanic corrosion of AZ91D coupled to steel

This research investigated the galvanic corrosion of the magnesium alloy AZ91D coupled to steel. The galvanic current distribution was measured in 5% NaCl solution, corrosive water and an auto coolant. The experimental measurements were compared with predictions from a Boundary Element Method (BEM) model. The boundary condition, required as an input into the BEM model, needs to be a polarization curve that accurately reflects the corrosion process. Provided that the polarization curve does reflect steady state, the BEM model is expected to be able to reflect steady state galvanic corrosion.

Development of a non-intrusive heat transfer coefficient gauge and its application to high pressure die casting Effect of the process parameters

A heat transfer coefficient gauge has been built, obeying particular rules in order to ensure the relevance and accuracy of the collected information. The gauge body is made out of the same materials as the die casting die (H13). It is equipped with six thermocouples located at different depths in the body and with a sapphire light pipe. The light pipe is linked to an optic fibre, which is connected to a monochromatic pyrometer. Thermocouples and pyrometer measurements are recorded with a data logger. A high pressure die casting die was instrumented with one such gauge. A set of 150 castings was done and the data recorded. During the casting, some process parameters have been modified such as piston velocity, intensification pressure, delay before switch to the intensification stage, temperature of the alloy, etc.... The data was treated with an inverse method in order to transform temperature measurements into heat flux density and heat transfer coefficient plots. The piston velocity and the initial temperature of the die seem to be the process parameters that have the greatest influence on the heat transfer. (c) 2005 Elsevier B.V. All rights reserved.

Understanding the crystallography of the eutectoid microstructure in a Zn-Al alloy using the edge-to-edge matching model

The orientation relationship (OR) between the beta(Zn) phase and the alpha(Al) phase and the corresponding habit planes in a Zn-Al eutectoid alloy were accurately determined using convergent beam Kikuchi line diffraction patterns. In addition to the previously reported OR. [11 (2) over bar0](beta)parallel to[110](alpha), (0002)(beta)parallel to ((1) over bar 11)alpha, two new ORs were observed. They are: [11 (2) over bar0](beta)parallel to [110], ((1) over bar 101)(beta) 0.82 degrees from (002)(alpha) and [(1) over bar 100](beta)parallel to[112](alpha), (0002)(beta) 4.5 degrees from (111)(alpha). These ORs can be explained and understood using the recently developed edge-to-edge matching model. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Light alloy castings for automotive applications: The case of Al vs Mg

The economical and environmental effects of mass reduction through Al and Mg primary alloys substitutions for cast iron and steel in automotive components are discussed using MF. Ashby's penalty functions method The viability of Mg alloy substitutions for existing Al alloy cast components is also considered. The cost analysis shows that direct, equal-volume, Al alloy substitutions for cast iron and steel are the most feasible in terms of the CAFE liability, followed by substitutions involving flat panels of prescribed stiffness. When the creation of CO2 associated to the production of Al and Mg is considered, the potential gasoline savings over the lifespan of the car compensate for the intrinsic environmental burden of Al in all applications, while electrolytic Mg substitutions for cast iron and steel are feasible for equal volume and panels only. Magnesium produced by the Pidgeon thermal process appears to be too primary energy intensive to be competitive in structural applications. Magnesium substitutions for existing Al alloy beams and panels are generally unviable. The current higher recycling efficiency of Al casting alloys confers Al a significant advantage over Mg alloys.

Sviluppo e applicazione di nuovi criteri per la verifica a fatica multiassiale di componenti strutturali in leghe di acciaio e alluminio

La presente Tesi ha per oggetto lo sviluppo e la validazione di nuovi criteri per la verifica a fatica multiassiale di componenti strutturali metallici . In particolare, i nuovi criteri formulati risultano applicabili a componenti metallici, soggetti ad un’ampia gamma di configurazioni di carico: carichi multiassiali variabili nel tempo, in modo ciclico e random, per alto e basso/medio numero di cicli di carico. Tali criteri costituiscono un utile strumento nell’ambito della valutazione della resistenza/vita a fatica di elementi strutturali metallici, essendo di semplice implementazione, e richiedendo tempi di calcolo piuttosto modesti. Nel primo Capitolo vengono presentate le problematiche relative alla fatica multiassiale, introducendo alcuni aspetti teorici utili a descrivere il meccanismo di danneggiamento a fatica (propagazione della fessura e frattura finale) di componenti strutturali metallici soggetti a carichi variabili nel tempo. Vengono poi presentati i diversi approcci disponibili in letteratura per la verifica a fatica multiassiale di tali componenti, con particolare attenzione all'approccio del piano critico. Infine, vengono definite le grandezze ingegneristiche correlate al piano critico, utilizzate nella progettazione a fatica in presenza di carichi multiassiali ciclici per alto e basso/medio numero di cicli di carico. Il secondo Capitolo è dedicato allo sviluppo di un nuovo criterio per la valutazione della resistenza a fatica di elementi strutturali metallici soggetti a carichi multiassiali ciclici e alto numero di cicli. Il criterio risulta basato sull'approccio del piano critico ed è formulato in termini di tensioni. Lo sviluppo del criterio viene affrontato intervenendo in modo significativo su una precedente formulazione proposta da Carpinteri e collaboratori nel 2011. In particolare, il primo intervento riguarda la determinazione della giacitura del piano critico: nuove espressioni dell'angolo che lega la giacitura del piano critico a quella del piano di frattura vengono implementate nell'algoritmo del criterio. Il secondo intervento è relativo alla definizione dell'ampiezza della tensione tangenziale e un nuovo metodo, noto come Prismatic Hull (PH) method (di Araújo e collaboratori), viene implementato nell'algoritmo. L'affidabilità del criterio viene poi verificata impiegando numerosi dati di prove sperimentali disponibili in letteratura. Nel terzo Capitolo viene proposto un criterio di nuova formulazione per la valutazione della vita a fatica di elementi strutturali metallici soggetti a carichi multiassiali ciclici e basso/medio numero di cicli. Il criterio risulta basato sull'approccio del piano critico, ed è formulato in termini di deformazioni. In particolare, la formulazione proposta trae spunto, come impostazione generale, dal criterio di fatica multiassiale in regime di alto numero di cicli discusso nel secondo Capitolo. Poiché in presenza di deformazioni plastiche significative (come quelle caratterizzanti la fatica per basso/medio numero di cicli di carico) è necessario conoscere il valore del coefficiente efficace di Poisson del materiale, vengono impiegate tre differenti strategie. In particolare, tale coefficiente viene calcolato sia per via analitica, che per via numerica, che impiegando un valore costante frequentemente adottato in letteratura. Successivamente, per validarne l'affidabilità vengono impiegati numerosi dati di prove sperimentali disponibili in letteratura; i risultati numerici sono ottenuti al variare del valore del coefficiente efficace di Poisson. Inoltre, al fine di considerare i significativi gradienti tensionali che si verificano in presenza di discontinuità geometriche, come gli intagli, il criterio viene anche esteso al caso dei componenti strutturali intagliati. Il criterio, riformulato implementando il concetto del volume di controllo proposto da Lazzarin e collaboratori, viene utilizzato per stimare la vita a fatica di provini con un severo intaglio a V, realizzati in lega di titanio grado 5. Il quarto Capitolo è rivolto allo sviluppo di un nuovo criterio per la valutazione del danno a fatica di elementi strutturali metallici soggetti a carichi multiassiali random e alto numero di cicli. Il criterio risulta basato sull'approccio del piano critico ed è formulato nel dominio della frequenza. Lo sviluppo del criterio viene affrontato intervenendo in modo significativo su una precedente formulazione proposta da Carpinteri e collaboratori nel 2014. In particolare, l’intervento riguarda la determinazione della giacitura del piano critico, e nuove espressioni dell'angolo che lega la giacitura del piano critico con quella del piano di frattura vengono implementate nell'algoritmo del criterio. Infine, l’affidabilità del criterio viene verificata impiegando numerosi dati di prove sperimentali disponibili in letteratura.

Development and evaluation of a non-destructive adhesion test

The object of this work was to further develop the idea introduced by Muaddi et al (1981) which enables some of the disadvantages of earlier destructive adhesion test methods to be overcome. The test is non-destructive in nature but it does need to be calibrated against a destructive method. Adhesion is determined by measuring the effect of plating on internal friction. This is achieved by determining the damping of vibrations of a resonating specimen before and after plating. The level of adhesion was considered by the above authors to influence the degree of damping. In the major portion of the research work the electrodeposited metal was Watt's nickel, which is ductile in nature and is therefore suitable for peel adhesion testing. The base metals chosen were aluminium alloys S1C and HE9 as it is relatively easy to produce varying levels of adhesion between the substrate and electrodeposited coating by choosing the appropriate process sequence. S1C alloy is the commercially pure aluminium and was used to produce good adhesion. HE9 aluminium alloy is a more difficult to plate alloy and was chosen to produce poorer adhesion. The "Modal Testing" method used for studying vibrations was investigated as a possible means of evaluating adhesion but was not successful and so research was concentrated on the "Q" meter. The method based on the use of a "Q" meter involves the principle of exciting vibrations in a sample, interrupting the driving signal and counting the number of oscillations of the freely decaying vibrations between two known preselected amplitudes of oscillations. It was not possible to reconstruct a working instrument using Muaddi's thesis (1982) as it had either a serious error or the information was incomplete. Hence a modified "Q" meter had to be designed and constructed but it was then difficult to resonate non-magnetic materials, such as aluminium, therefore, a comparison before and after plating could not be made. A new "Q" meter was then developed based on an Impulse Technique. A regulated miniature hammer was used to excite the test piece at the fundamental mode instead of an electronic hammer and test pieces were supported at the two predetermined nodal points using nylon threads. This instrument developed was not very successful at detecting changes due to good and poor pretreatments given before plating, however, it was more sensitive to changes at the surface such as room temperature oxidation. Statistical analysis of test results from untreated aluminium alloys show that the instrument is not always consistent, the variation was even bigger when readings were taken on different days. Although aluminium is said to form protective oxides at room temperature there was evidence that the aluminium surface changes continuously due to film formation, growth and breakdown. Nickel plated and zinc alloy immersion coated samples also showed variation in Q with time. In order to prove that the variations in Q were mainly due to surface oxidation, aluminium samples were lacquered and anodised Such treatments enveloped the active surfaces reacting with the environment and the Q variation with time was almost eliminated especially after hard anodising. This instrument detected major differences between different untreated aluminium substrates.Also Q values decreased progressively as coating thicknesses were increased. This instrument was also able to detect changes in Q due to heat-treatment of aluminium alloys.

Nonmetallic inclusion modification and its effect on the final properties of a linepipe steel

Five linepipe type steels were produced in order to study the effect of calcium and magnesium injection on their final properties. Two of these steels were at the extremes of the sulphide range i.e. 0.003 and 0.017% sulphur with no injection attempted; thereby, providing standards to compare with the injected steels. The oxygen level varied from 21 to 63 p.p.m. The cast ingots were controlled-rolled and isothermally rolled in order to study the deformation characteristics of the residual non-metallic inclusions. The structure and cleanliness of these steels was evaluated metallographically using the light microscope, SEM, and image analysis and the results related to their Charpy toughness and HIC resistance. Increasing sulphur levels decreased final properties of the steel. In the untreated state, with as little as 0.003% sulphur, test orientation was highly influential. Modification of sulphur bearing steels was achieved with low modifying element to sulphur ratios provided that the oxygen content was very low. Injection of calcium into steel caused interaction with oxide and sulphide inclusions which was biased toward oxide reduction relative to sulphur removal. Magnesium again reduced oxides and appeared to be linked with aluminia containing inclusions in the final product. It produced improved toughness values relative to a similar sulphur containing calcium treated steel. The results of this work could be extended to establish the mechanism of inclusion modification with magnesium additions to sulphur bearing steels.

Electrodeposition of corrosion resistant zinc alloy coatings

With the increase use of de-icing salts on roads for safety, the need for improved corrosion resistance of the traditional galvanized automobile bodies has never been greater. In the present work, Zn alloy coatings (Zn-Ni and Zn-Co) were studied as an alternative to pure Zn coatings. The production of these deposits involved formulation of various acidic (pH of about 5.5) chloride based solutions. These showed anomalous deposition, that is, alloys were deposited much more easily than expected from the noble behaviour of Ni and Co metals. Coating compositions ranging from 0 to about 37% Ni and 20% Co were obtained. The chemical composition of the coatings depended very much on the electrolytes nature and operating conditions. The Ni content of deposits increased with increase in Ni bath concentration, temperature, pH and solution agitation but decreased considerably with increase in current density. The throwing power of the Zn-Ni solution deteriorated as Ni metal bath concentration increased. The Co content of deposits also increased with increase in Co bath concentration and temperature, and decreased with increase in current density. However, the addition of commercial organic additives to Zn-Co plating solutions suppressed considerably the amount of Co in the coatings. The Co content of deposits plated from Zincrolyte solution was found to be more sensitive to variation in current density than in the case of deposits plated from the alkaline Canning solution. The chromating procedures were carried out using laboratory formulated solution and commercially available ones. The deposit surface state was of great significance in influencing the formulation of conversion coatings. Bright and smooth deposits acquired an iridescent colour when treated with the laboratory formulated solution. However, the dull deposits acquired a brownish appearance. The correlation between the electrochemical test results and the neutral salt spray in marine environment was good. Non-chromated Zn-Ni coatings containing about 11-14% Ni increased in corrosion resistance compared to pure Zn. Non-chromated Zn-Co deposits of composition 4-8% were required to show a significant improvement in corrosion resistance Corrosion resistance was improved considerably by conversion coating. However, the type of conversion coating was very important. Samples treated in a laboratory solution performed badly compared to those treated in commercial solutions. Zn alloy coatings were superior to pure Zn, the Schloetter sample (13.8% Ni) had the lowest corrosion rate, followed by the Canning sample (1.0% Co) and then Zincrolyte (0.3% Co).Neither the chromium content of the conversion films nor the chromium state was found to have an effect on corrosion performance of the coatings.

Cracking in the welding of cupro-nickel alloys

The problem of variation in weld crack susceptibility caused by small variations in alloy and impurity elements for the 70-30 cupro-nickel alloy has been investigated. Both wrought and cast versions of the alloy have been studied, the main techniques employed being the Varestraint test and weld thermal simulation. In the wrought alloys, cracking has been found to occur mainly in the weld metal, whilst in the cast alloys cracking is extensive in both weld metal and heat affected zone. The previously reported effects of certain impurities (P,S,Si) in increasing cracking have been confirmed, and it has also been shown that Ti and Zr may both have a crack promoting effect at levels commonly found in cupro-nickels, whilst C can interact with several of the other elements investigated to produce a beneficial effect. The testing carried out using the weld thermal simulator has shown that a relationship does exist between hot ductility and weld cracking. In particular, the absence of the peak in ductility in the range 1100°C-900°C on cooling from a temperature near to the solidus is indicative of a highly crack susceptible alloy. Principal practical implications of the investigation concern the relationship of weld metal cracking to alloy composition, especially the level of certain impurities. It would appear that the upper limits permitted by the alloy specifications are unrealistically high. The introduction of lower impurity limits would alleviate the current problems of variability in resistance to cracking during welding.

Defects in steel investment castings

A general investigation was performed, in an industrial environment, of the major types of defect specific to investment castings in steel. As a result of this work three types of metallurgical defect were selected for further study. In the first of these, defects in austenitic stainless steel castings were found to result from deoxidation by-products. As a result of metallographic investigation and the statistical analysis of experimental data, evidence was found to support the hypothesis that the other two classes of defects - in martensite stainless and low alloy steels -both resulted from internal or grain boundary oxidation of the chromium alloy constituent This was often found to be followed by reaction between the metal oxides and the ceramic mould material. On the basis of this study, proposals are made for a more fundamental investigation of the mechanisms involved and interim suggestions are given for methods of ameliorating the effect in an industrial situation.

The influence of non-metallic inclusions upon the properties of linepipe steels

The principal aim of this work was to determine the role of non-metallic inclusions in the process of hydrogen stepwise cracking (SWC). Additionally, the influence of inclusions upon the notch ductility of hydrogen charged (HC) and uncharged (UN) tensile specimens was examined. To obtain a basis for experiment a series of low carbon-manganese steels were prepared by induction melting. In order to produce variations in the composition, morphology, volume fraction, size and distribution of the inclusions the steel chemistry was adjusted prior to casting by additions of deoxidiser and Ca-Si injection. Sections of each ingot were hot rolled. Metallography, image analysis, mechanical tests and hydrogen SWC tests were then carried out. The volume fraction, morphology, and shape of inclusions influenced the tensile ductility of the steels. Marked anisotropy was found in the steels containing type II MnS inclusions at all rolling temperatures, whereas the fully Ca treated steel was isotropic. It was found that several inclusion parameters (projected length PL, mean free distance MFD, nearest-neighbour distance NND) correlated with fracture strain. An increase in inclusion volume fraction and/or the dimension of inclusions on a plane parallel to the plane of fracture led to a decrease in fracture strain. The inclusion parameters did not correlate with the fracture strains for the HC tensile specimens. However, large or clusters of inclusions acted as the principal sites for crack initiation. `Fisheyes' or areas of `flat' fracture were often found on these fracture surfaces. The criteria for SWC initiation was found to be either large inclusions or clusters of inclusions. As the PL of inclusions increased the probability of large SWCs occurring increased. SWC initiation at inclusions was believed to occur at a critical concentration of hydrogen. Factors which assisted the concentration of hydrogen at inclusions were discussed. None of the proposed mechanisms of hydrogen embrittlement could be identified as the single cause of SWC.