Entrained defects and mechanical properties of aluminium castings

The presence of entrained double oxide films, known as bifilms, has been identified as a contributing factor to the variability in mechanical properties observed in aluminium castings. These bifilms consist of folded-over oxide films containing gas-filled crevices and are formed due to turbulence on the liquid metal's surface during handling and pouring. Additionally, it has been suggested that hydrogen dissolved in the aluminium melt can permeate these defects, causing them to expand and leading to the formation of hydrogen porosity. This, in turn, exacerbates the detrimental effects on the mechanical properties of the castings. In this study, the ultimate tensile strength (UTS) and percentage elongation of sand cast bars were compared under various casting conditions. These parameters were chosen as indicators of casting reliability, which was expected to be influenced by the presence of oxide films. The results indicated that incorporating filters in the gating system and reducing the runner height led to a noticeable improvement in tensile strength and elongation. This improvement was attributed to enhanced mold filling conditions, which reduced the likelihood of oxide film entrainment. The findings of this research provide valuable insights into the factors that affect the properties of light metal alloy castings. By understanding these influences, it becomes possible to develop improved practices that result in healthier castings with enhanced mechanical properties

Influence of bifilm defects generated during mould filling on the tensile properties of Al–Si–Mg cast alloys

Entrapped double oxide film defects are known to be the most detrimental defects during the casting of aluminium alloys. In addition, hydrogen dissolved in the aluminium melt was suggested to pass into the defects to expand them and cause hydrogen porosity. In this work, the effect of two important casting parameters (the filtration and hydrogen content) on the properties of Al–7 Si–0.3 Mg alloy castings was studied using a full factorial design of experiments approach. Casting properties such as the Weibull modulus and position parameter of the elongation and the tensile strength were considered as response parameters. The results suggested that adopting 10 PPI filters in the gating system resulted in a considerable boost of the Weibull moduli of the tensile strength and elongation due to the enhanced mould filling conditions that minimised the possibility of oxide film entrainment. In addition, the results showed that reducing the hydrogen content in the castings samples from 0.257 to 0.132 cm3/100 g Al was associated with a noticeable decrease in the size of bifilm defects with a corresponding improvement in the mechanical properties. Such significant effect of the process parameters studied on the casting properties suggests that the more careful and quiescent mould filling practice and the lower the hydrogen level of the casting, the higher the quality and reliability of the castings produced

Enhanced heterogeneous nucleation on oxides in Al alloys by intensive melt shearing

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.Aluminium alloys, including both foundry and wrought alloys, have been extensively used for light-weight structural and functional applications. A grain refined as-cast microstructure is generally highly desirable for either subsequent processing ability or mechanical properties of the finished components. In this thesis, the grain refined microstructures in Al alloys have been achieved by intensive melt shearing using the melt conditioning by advanced shearing technology (MCAST) without deliberate grain refiner additions. Such grain refinement has been attributed to the enhanced heterogeneous nucleation on the dispersed oxide particles. It has been established that the naturally occurring oxides in molten Al alloys normally have a good crystallographic match with the a-Al phase, indicating the high potency of oxide particles as the nucleation sites of the a-Al phase. The governing factors for these oxide particles to be effective grain refiners in Al alloys have been proposed, including the achievement of good wetting between oxide particles and liquid aluminium, a sufficient number density and uniform spatial distribution of the dispersed oxide particles, and near equilibrium kinetic conditions in liquid alloys. In the present study, near equilibrium kinetic conditions can be achieved by intensive melt shearing using a twin screw mechanism, which has been confirmed by the observed equilibrium a-AlFeSi phase in a cast Al alloy and the transformation from g- to a-Al2O3 at 740±20oC under intensive shearing. For different alloy systems, depending on the alloy system, and melting conditions, due to the particular types of oxide formed and its crystallographic and chemical characteristics, the nucleation site of the nucleated phase is different. Specifically, MgAl2O4 relative to MgO, and a-Al2O3 relative to g-Al2O3, have higher potency as heterogeneous nucleation sites of a-Al phase in Al alloys. In future, the modification of the crystallographic match, and of the other surface characteristics related to the interfacial energy between the specific oxides and nucleated phase by trace alloying addition through segregation to the interface between oxides and nucleated phases combined with physical melt processing (such as intensive shearing in the present study) should be investigated in more detail.This study was funded by the EPSRC and Norton Aluminium Limited, UK

CORROSION BEHAVIOUR OF THE AZ31 MAGNESIUM ALLOY AND SURFACE TREATMENTS FOR ITS CORROSION PROTECTION

Nowadays environmental conservation forces the transportation industry to manufacture lighter and low emissions transport vehicles. In this contest, magnesium alloys have found increasing attention by the automotive industry because of their low density associated with good mechanical properties. However the low corrosion resistance of magnesium alloys in wet environments is still a limiting factor against their widespread diffusion. The aim of this research is both studying the correlation between microstructure and corrosion behaviour of a AZ31 magnesium alloy and developing eco-friendly protective technologies. In particular inhibitors and surface pretreatments with different organic compounds have been investigated. The effect of microstructure on corrosion resistance of AZ31 alloy has been analyzed through a comparative study between the electrochemical behaviour of as-cast and hot rolled AZ31. Environmentally friendly sodium salts of mono-carboxylic acids have been studied as inhibitors of AZ31 alloy corrosion in a standard saline solution. Moreover, long chain sodium mono-carboxylates have been tested as promoters of conversion coatings for this alloy. Finally, significant improvements have been achieved by modifying the protective coatings obtained by 3-mercapto-propyl-trimethoxysilane through cerium nitrate or oxide nanoparticle additions

An Investigation in to Alternative Material and Manufacturing Solutions for Lightweighting Interior and Exterior Automotive Brightware

Existing literature for a variety of material and manufacturing techniques was reviewed, in order to find alternative solutions for lightweighting auto-motive brightware. Light alloys such as magnesium, aluminium and titanium were all investigated as alternatives to the current cast zinc parts. Additive layer manufacture and metal injection moulding were both found to have po-tential for lightweighting parts but were not suited overall to manufacturing brightware. After the initial literature review, using cast aluminium was in-vestigated in more detail and experimental testing was completed on a range of alloys. Alloys of Al-Si, Al-Mg and Al-Zn were tested on their visual appear-ance, mechanical performance and corrosion resistance against existing zinc brightware. Anodising was also attempted on some these aluminium alloys but found to be unsuitable for Al-Si alloys. The surface roughness, gloss and hardness of these alloys was competitive with existing brightware. Anodising of Al-Mg and Al-Zn alloys increased their corrosion resistance significantly but not to the level of the current zinc brightware. This was attributed to the poor optimisation of the anodising procedure. Despite this, these alloys showed potential for much greater performance after anodising. However, the literature review of these alloys suggested that the castability of the Al-Mg and Al-Zn alloys could be a major concern for use in brightware

Effect of runner thickness and hydrogen content on the mechanical properties of A356 alloy castings

Earlier studies demonstrated the detrimental effect of entrained bifilm defects on aluminum cast alloys’ tensile and fatigue properties. It was suggested that hydrogen has a contributing role as it diffuses into the bifilms and swells them out to form hydrogen porosity. In this study, the effect of the runner height and hydrogen content on the properties of A356 alloy castings was investigated using a two-level full factorial design of experiments. Four responses, the Weibull modulus and position parameter of both the ultimate tensile strength (UTS) and % elongation, were assessed. The results suggested that decreasing the runner height and adopting procedures intended to decrease the hydrogen content of the casting caused a considerable enhancement of the Weibull moduli and position parameters of the UTS and % elongation. This was reasoned to the more quiescent practice during mold filling, eliminating the possibility of bifilm formation as well as the decreased hydrogen level that eliminated the amount of hydrogen diffused into the bifilms and accordingly decreased the size of the entrained defects. This, in turn, would allow the production of A356 cast alloys with better and more reproducible properties

A review on selective laser sintering/melting (SLS/SLM) of aluminium alloy powders: Processing, microstructure, and properties

Manufacturing businesses aiming to deliver their new customised products more quickly and gain more consumer markets for their products will increasingly employ selective laser sintering/melting (SLS/SLM) for fabricating high quality, low cost, repeatable, and reliable aluminium alloy powdered parts for automotive, aerospace, and aircraft applications. However, aluminium powder is known to be uniquely bedevilled with the tenacious surface oxide film which is difficult to avoid during SLS/SLM processing. The tenacity of the surface oxide film inhibits metallurgical bonding across the layers during SLS/SLM processing and this consequently leads to initiation of spheroidisation by Marangoni convection. Due to the paucity of publications on SLS/SLM processing of aluminium alloy powders, we review the current state of research and progress from different perspectives of the SLS/SLM, powder metallurgy (P/M) sintering, and pulsed electric current sintering (PECS) of ferrous, non-ferrous alloys, and composite powders as well as laser welding of aluminium alloys in order to provide a basis for follow-on-research that leads to the development of high productivity, SLS/SLM processing of aluminium alloy powders. Moreover, both P/M sintering and PECS of aluminium alloys are evaluated and related to the SLS process with a view to gaining useful insights especially in the aspects of liquid phase sintering (LPS) of aluminium alloys; application of LPS to SLS process; alloying effect in disrupting the surface oxide film of aluminium alloys; and designing of aluminium alloy suitable for the SLS/SLM process. Thereafter, SLS/SLM parameters, powder properties, and different types of lasers with their effects on the processing and densification of aluminium alloys are considered. The microstructure and metallurgical defects associated with SLS/SLM processed parts are also elucidated by highlighting the mechanism of their formation, the main influencing factors, and the remedial measures. Mechanical properties such as hardness, tensile, and fatigue strength of SLS/SLM processed parts are reported. The final part of this paper summarises findings from this review and outlines the trend for future research in the SLS/SLM processing of aluminium alloy powders

The behaviour of double oxide film defects in the processing of liquid Mg alloys

The global demand for Mg alloys continually grew in the last 20 years, motivating a wide interest in the improvement of the mechanical properties of Mg-alloy castings. In addition, the existence of double oxide film defects, which were widely recognised as a major factor in the quality and reproducibility of the properties of light-alloy castings, has been demonstrated in Mg-alloy castings. Thus it became important to understand behaviour of double oxide film defects formed in Mg-alloys. In the work reported here, three different Mg alloys (commercial pure Mg, AZ91 alloy, and Mg-Y alloys) and two cover gases (SF6/air and SF6/CO2), were used, in order to involve different doubled oxide films which may have different behaviours. Direct and cross-sectional observations of the double oxide film defects formed the Mg-alloy castings protected by different cover gases were obtained via a Scanning Electron Microscopy (SEM), and the focus ion beam milling (FIB) technique. In addition, oxide films growing on the corresponding Mg-alloy melt surfaces were also investigated. Based on the observed film structures in conjunction with a thermochemical calculation, evolution processes of the different double oxide film defects were suggested. The quality of Mg-alloy castings was evaluated by the Weibull modulus, which is popularly used to discriminate “good” and “bad” castings. A shortcoming of the traditional Weibull estimation method (i.e. linear least square method) was demonstrated, and a new estimation method was therefore come up with. The Weibull modulus result revealed that air can confer an improvement in the quality of AZ91 castings, compared with CO2