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The fluidity and the effective surface tension of Al-Ductile iron (Fe-3.2C-8.5Al-0.05Mg) were studied at different pouring temperatures. A fluidity test comprising strips 1, 2, 4-and 8-mm-thickness were molded in sodium silicate-bonded silica sand. Our results quantify the increase of melt fluidity with increasing casting modulus and pouring temperature and show that the effective surface tension of the alloy is about 1.8 N/m. Comparing this value with the surface tension of Si-ductile irons and Al-ductile irons of lower Al contents, we conclude that the high level of Al used in this study results in the formation of an alumina film on the advancing liquid front, which raises the effective surface tension, reduces fluidity by 100 mm and reduces the ability of the metal to enter narrow sections from 0.4 to 1.0 mm thickness.

Mono-mineral and ultramafic geochemical reference materials (GRMs) were analyzed using a 2.5 MeV broad-beam proton-induced X-ray emission (PIXE) system designed to emulate the PIXE branch of the Mars Science Laboratory alpha-particle X-ray spectrometer (APXS). These samples supplement the heterogeneous rocks of our earlier emulation work. The results confirm that PIXE provides high-accuracy concentrations for major and minor elements in homogeneous samples, and the results agree well with those from mono-minerals in the original APXS calibration. A second X-ray detector was added to the system to extend the emulation to the X-ray fluorescence branch of the APXS. The GRMs analyzed using this detector have relatively high concentrations of the trace elements copper, zinc, rubidium, strontium or zirconium. Our PIXE concentrations were compared to values provided by both the GRM suppliers and an independent corporation. Agreement was good in most cases, with individual deviations attributed mainly to inaccuracies in these reference concentrations.

An unalloyed thin-wall ductile cast iron with the chemical composition of C = 3%, Si = 4.15%, Mn = 0.36%, Mg = 0.04%, and carbon equivalent of 4.39% was cast by both conventional and ablation casting in sections of nominal thickness 1, 2, 4, 8 mm. The effect of various commercial inoculants (based on Zr, Sr, and Ba) on microstructure and mechanical properties of the samples was investigated. In the ablation casting, the mold is eroded away by water, and the water becomes responsible for the heat transfer from the casting. In our work, ablative cooling could not be applied quickly enough to have any influence on the microstructure of the 1 mm thick cast section. In the 2-mm-thick specimen, ablation increased the nodule count from 497 to 1484 and graphite nodularity from 76 to 93%. In the 8 mm section, the tensile yield strength increased from 539 to 624 MPa and elongation was increased from 4.9 to 10.2%.

Ablation casting is a relatively new casting technique which can improve the mechanical properties of castings. In this paper, the microstructure and mechanical properties of plain carbon steel castings produced by conventional casting and ablation casting techniques were compared. It was found that ablation increased the ultimate strength from 638 to 1094 MPa and tensile elongation increased from approximately 13 to 16% as a result of severe microstructural changes from ferrite and pearlite to acicular ferrite, bainite, and martensite. Ablation increased the core hardness of the casting from 182 to 467 HB and the surface hardness from 205 to 508 HB.

This paper presents a personal view by the author of the role of bifilms in Al alloys. The mantra ‘microstructure determines properties’ is widely accepted as a truism, but is here critically assessed and found wanting. The case is made that bifilms from the casting process, while often invisible in the microstructure, are usually at least as important, if not of far greater importance, because they are often present as a dense population of cracks throughout the metal. The bifilm population controls the morphology of many features of cast and wrought structures. For cast alloys, bifilm control of pore morphology and Si morphology in Al–Si alloys is discussed, as is dendrite arm spacing (DAS). The tensile property benefits of grain refinement are seen to be mainly bifilm controlled. The properties ductility and fatigue appear to be especially dominated by bifilm content, as are invasive corrosion processes such as pitting, intergranular corrosion, hydrogen blistering and cracking. Bifilm control is proposed as a new concept permitting the improvement and control of metallurgical properties.

There appear to be two main growth mechanisms for graphite in cast iron i) Coupled eutectic growth forms of gray irons which are classical growth modes of simultaneous parallel growth of graphite and austenite, not reliant on a bifilm mechanism. These are necessarily fine structures as a result of their control by the rate of diffusion of carbon in the liquid. These structures are well understood. (ii) Uncoupled eutectic mechanisms which appear to be much less well understood, including (a) growth of graphite on silica bifilm substrates floating freely in the melt, forming such structures as A-type graphite flakes. This prediction appears to have now been confirmed by direct observation. The transition to (b) nodular morphology occurs by Mg eliminating the silica bifilms by an exchange reaction. In this way the substrates for flake growth are instantly removed, and graphite can now wrap completely around nuclei, thereby growing as a nodule. Graphite structures in heavy sections such as chunky graphite may now be understandable in terms of the reorganisation by flotation of bifilms and/or nuclei.

The mechanical properties of the liquid/solid regime during the freezing of metals have been studied repeatedly, but, in general, without the knowledge of the presence of bifilms. (The oxide cracks are introduced into the metal by turbulence during casting processes.) The flow of liquid through the “pasty zone” has been simulated in theoretical models, which, at this time, appears far removed from reality as a result of the blockage of flow channels by bifilms. The opening of “hot tears” is seen to be the opening of bifilms. In addition, the elimination of hot tears is seen to be by the elimination of bifilms, particularly bifilms formed during the final pour of the liquid metal. It is predicted, and demonstrated, that even in a highly hot tear prone alloy subjected to freezing in a mold designed to encourage maximum tearing, that tears will not form if bifilms are absent. Thus, in the partly solidified condition, the occurrence of fracture is controlled by bifilms.

A significant recent revision of the solubility of oxygen in liquid aluminum has prompted a re-evaluation of the presence of alumina in the solidified metal. A simple analysis based on thermodynamics and diffusion results in an interesting configuration of nanosized alumina particles distributed in Al.

Kim and Withey discuss the formation of an ‘intermediate layer’, formed between grains, which they observe in some Ni-base superalloys. This author proposes that the layer is the result of the presence of a bifilm, a double film probably of oxide or nitride, which appears to be capable of providing a coherent explanation of all the interesting observations reported by the authors.

Fuchs and Kaplan carried out experiments in an attempt to ascertain whether oxide bifilms were present in a vacuum-cast Ni-base superalloy but concluded negatively. Although this author challenged their interpretation of their findings, both parties had overlooked the presence in the alloy of boron which is now known to inhibit bifilm formation. However, even though boron can help significantly, improved filling system designs remain important if other damaging entrainment defects are to be avoided.

The Alpha Particle X-ray Spectrometer (APXS), a field instrument onboard four martian rovers, measures largely unprepared, in situ samples on Mars. The APXS has high precision that enables the determination of elemental concentrations in a wide range of geologic materials. However, lack of sample preparation can lead to heterogeneous matrix effects, and understanding the associated uncertainty is essential for interpreting APXS data. Here we use Particle Induced X-ray Emission spectrometry (PIXE) to analyze a suite of geologic samples from Hawai'i as an analogue study to better understand APXS analyses of martian samples. Wavelength-Dispersive X-ray Fluorescence (WDXRF) analyses of fused glass beads establish higher-accuracy standards for the Hawaiian samples. Sulfate-silicate mixtures were made to evaluate sulfur analysis by PIXE. Results show that the PIXE concentrations for most major elements have 2–6% accuracy, which is comparable to the APXS. However, the PIXE concentrations are systematically high in Al and low in Mg, resulting in lower accuracy (13% and 20%, respectively). Olivine-phyric lavas and most of their altered products have the largest discrepancies with Al concentrations up to 25% high and Mg up to 35% low. Sulfur is systematically high (up to 30% in a basalt matrix) compared to gravimetric S concentrations in the sulfate-silicate mixtures. These systematic deviations in Mg, Al, and S are linked to heterogeneous matrix effects, because PIXE and APXS analyses assume all atoms in a sample to be homogeneously mixed on the sub-micrometer scale, which is not the case. Two key implications for APXS results are: (1) Olivine-bearing samples likely have reported concentrations of Mg that is too low and Al that is too high. Thus, olivine-phyric basalts in Gusev crater and the basaltic sand and soil at three landing sites may have Mg and Al concentrations closer to those of the olivine-phyric shergottites and modelled martian crust than previously thought. (2) Sulfate-silicate mixtures may have overestimated S concentrations reported, resulting in greater uncertainty in the stoichiometry of Ca-sulfates, which is used to deduce the geochemical associations of sulfur in samples.

Most of the major defects in Al alloy castings are the result of entrainment processes. The entrainment of the surface of the liquid creates bifilm defects, and the entrainment of air creates bubbles and bubble trails. Occasionally the entrainment of foreign inclusions, such as sand inclusions, can also be a problem. Bifilms form the initiators of gas porosity, shrinkage porosity, hot tears, and cracks. Since bifilms can be controlled by improved melting and casting techniques, all these defects are controllable. In addition, bifilms control the mechanical properties of castings, particularly tensile elongation, toughness, and fatigue. The other important effects caused by bifilms such as invasive corrosion behavior including pitting, stress corrosion cracking, and possibly hydrogen embrittlement, are beyond the scope of this review.

The Ten Rules are a checklist of the conditions necessary for the production of successful castings, particularly dealing with the metal quality, with a view to achieving a casting with minimal, preferably zero, faults. The first five Rules specify the conditions to avoid entrainment defects, particularly bifilms and bubbles. The remaining Rules deal with the provision of feeding, avoidance of convection, chemical segregation, and residual stress, and the provision of pickup locations for machining.

The concept of the bifilm is now nearly 20 years old. It has been clearly shown to control structural defects in castings such as porosity and hot tearing, plus, in solid wrought products, mechanical properties such as ductility and fatigue. It appears to explain for the first time the structures of Al–Si alloys and cast irons. Furthermore, although there are a number of proposed mechanisms for crack initiation in metals, it seems most probable that these are invalid. It follows that there is probably no intrinsic crack failure mechanism for most engineering metals; metals should never crack. The only crack-initiating feature appears to be the bifilm, introduced accidentally during casting. By appropriate casting techniques, the introduction of bifilms can be avoided. For this reason, failure by fatigue and creep may be eliminated, together with invasive corrosion such as pitting, stress corrosion cracking and possibly other failure modes.

The benefits of Sr additions to Al–Si alloys to modify the eutectic are often impaired by the development of porosity, sometimes to the degree that benefits are negated. Experimental reports are reviewed in this paper, suggesting an explanation in terms of the oxide population in the melt. The unmodified silicon particles are nucleated by AlP, which has in turn nucleated on oxide bifilms. The oxide bifilms, which are essentially cracks, are straightened by the crystalline growth of Si particles, leading to increased crack size and consequently reduced mechanical properties. The addition of Sr improves properties by suppressing the formation of Si on bifilms and thereby preventing the straightening of the pre-existing cracks. Si is now forced to precipitate at a lower temperature as a coral-like eutectic. Unfortunately, the bifilms are now freed (the primary Si particles no longer exist to grow around and sequester the bifilms), remaining in suspension in the liquid metal, allowing them to act to block interdendritic flow and aid the initiation of the formation of pores, countering the benefits of the improved structure.

Most of our engineering metals are formed in the liquid state, but suffer mixing with air during pouring into molds. The result is the creation of doubled-over oxide films from the surface of the liquid, entrained in suspension in the metal, which, as a result of the lack of bonding between the opposed faces of the films, act as cracks. Multiple and additive populations of cracks are introduced during the processing of many metals, ensuring a high density of defects which survive in the solidified metal, and further survive plastic working by forging or rolling, etc. Many of our current engineering metals are therefore seriously impaired. This traditional impairment by these defects, called bifilms, is currently accepted as the norm. However, entrainment of the surface oxides during processing is not necessary, and its avoidance can deliver substantially defect-free metals for the first time. Improved casting technology to deliver these improvements is being successfully adopted for light alloys and steels.

During their processing from ores, all metals go through a particulate stage. These separate pieces have to be consolidated usually with heat and pressure to form useful engineering forms. In the case of metals which undergo a melting stage, the pouring and stirring actions which are commonly employed disintegrate the liquid into splashes and droplets which appear to mutually assimilate, creating a consolidated bulk liquid. However, in every case, whether consolidation takes place in the solid or liquid states, the consolidation mechanism naturally incorporates an oxide-to-oxide interface (although occasionally other surface films such as nitrides and pure carbon are involved). The concept of the meeting of oxides is of course assumed to be trivial and therefore almost universally overlooked. However, the consequences seem to be far from trivial. The creation of opposed, double (never single), unbonded oxide films, called ‘bifilms’ by the author, acts as cracks. They can survive extensive plastic working and seem to be prolific throughout metallurgy. They appear to exert significant control over the failure properties of metals, by both cracking and corrosion. They are proposed to constitute the Griffith cracks required for the failure of metals by fracture and fatigue. Bifilms may now be eliminated from some of our liquid processing routes, enabling for the first time the production of crack-resistant and corrosion-resistant metals.

The 60 years of solidification research since the publication of Chalmer’s constitutional undercooling in 1953 has been a dramatic advance of understanding which has and continues to be an inspiration. In contrast, 60 years of casting research has seen mixed fortunes. One of its success stories relates to improvements in inoculation of gray irons, and another to the discovery of spheroidal graphite iron, although both of these can be classified as metallurgical rather than casting advances. It is suggested that true casting advances have dated from the author’s lab in 1992 when a critical surface turbulence condition was defined for the first time. These last 20 years have seen the surface entrainment issues of castings developed to a sufficient sophistication to revolutionize the performance of light alloy and steel foundries. However, there is still a long way to go, with large sections of the steel and Ni-base casting industries still in denial that casting defects are important or even exist. The result has been that special ingots are still cast poorly, and shaped casting operations have suffered massive losses. For secondary melted and cast materials, electro-slag remelting has the potential to be much superior to expensive vacuum arc remelting, which has cost our aerospace and defense industries dearly over the years. This failure to address and upgrade our processing of liquid metals is a serious concern, since the principle entrainment defect, the bifilm, is seen as the principle initiator of cracks in metals; in general, bifilms are the Griffith cracks that initiate failures by cracking. A new generation of crack resistant metals and engineering structures can now be envisaged.

Tensile testing along with nondestructive methods has been traditionally used to assess the quality of cast Al alloys. However, clear guidelines on what remedial steps can be taken in case of low quality indicated tensile data are not available to foundry engineers. Quality indices in the literature were developed as a measure to quantify quality to assist foundry engineers in their pursuit of higher quality. A brief review of those quality indices developed previously is provided. The quality indices can be grouped into two: (1) those with no physical meaning and (2) those that measure what fraction of the expected tensile performance is achieved. The quality index developed by the authors is based simply on the ratio of measured elongation to maximum achievable elongation, portrayed as a percentage of what is achievable. This quality index is recommended over the others. In principle the approach is applicable to all alloys. Step-by-step guidelines on its use and interpretation are provided.

At normal engineering stresses and temperatures cavitation in metals appears to be theoretically impossible. It is proposed that the only common viable initiating mechanism for cavitation in both liquid and solid metals is an atomically unbonded mesoscale interface. Such features are provided by entrainment defects from an original consolidation process such as casting or powder metallurgy. In most metals entrainment defects occur as a result of turbulence during the original melting and pouring of the liquid metal. The most common entrainment defects are bubbles and bifilms. Both are present in large populations in most metals and present easy initiation sites for cavitation and for the precipitation of second phases which are thereby made to appear brittle as a result of the enshrined bifilm crack. The promise of metals free from entrainment defects is briefly reviewed.

The Mars Science Laboratory Curiosity rover carries a basalt calibration target for monitoring the performance of the alpha particle X-ray spectrometer. The spectrum acquired on Sol 34 shows increased contributions from Mg, S, Cl and Fe relative to laboratory spectra recorded before launch. Mars Hand Lens Imager images confirm changes in the appearance of the surface. Spectra taken on Sols 179 and 411 indicate some loss of the deposited material. The observations suggest deposition of a surface film likely consisting of dust mobilized by impingement of the sky crane’s terminal descent engine plumes with surface fines during Curiosity’s landing. New APXS software has been used to model the thin film that coated the calibration target on landing. The results suggest that a film of about 100 nm thickness, and containing predominantly MgO, Fe 2 O 3 , SO 3 , Cl and Na 2 O could give rise to the observed spectral changes. If this film is also present on the alpha particle sources within the APXS, then its effect is negligible and the terrestrial calibration remains appropriate.

In the design of runner systems, ceramic foam filters are used for reducing the velocity of liquid metal in order to avoid so called bifilm defect resulting from a high gating velocity (over its critical gating velocity) in aluminum gravity casting. In this study, two types of runner systems incorporated with the ceramic foam filters were designed. In order to observe the flow phenomena in these two runner systems with and without the filters, a water analogy experiment in a transparent plastic mold was utilized. Furthermore, in order to understand the effect of the filters used in these systems, an aluminum sand casting experiment was conducted. The quality of the cast metal in the outlet area of the filter for these systems was investigated. The defect content of this casting sample was measured by the re-melt reduced pressure test (re-melt RPT) and followed by measuring its bulk density. An optimized runner system with the filter was suggested in this study. Keywords: ceramic foam filter, runner system design, gravity casting, critical gating velocity, bifilm defect.

Impact behavior related to crack initiation and growth of cast A357 (Al–7Si–0.7Mg) alloy was experimentally investigated. Dynamic fracture toughness for various artificial aging conditions were evaluated by exploiting instrumented impact Charpy V-notch experiments; impact tests were preferred instead of the standard fracture toughness tests in order to have a fast assessment of the effect of artificial aging on the fracture toughness behavior. Characteristic points of impact load–deflection curves related with cracking were identified and post-correlated with different artificial aging conditions. Absorbed impact energies for stable and unstable crack growth were calculated for 28 different aging conditions; resulting cracking velocity at the unstable crack regime (rebound compliance) was also calculated and it was found that rebound compliance is inversely related to ductility. Cracking initiation load values were identified by exploiting the compliance changing rate method; both stress intensity factor and J-integral analysis were employed to calculate critical dynamic fracture toughness values. They were found to be slightly higher than corresponding quasi-static properties and they are both changing with varying aging conditions. Limitations and results of both notch-fracture toughness analyses were discussed by taking into account the material aging condition. Performed scanning electron microscopy on the fracture surfaces revealed the fracture mechanism that is correlated with respective microstructure.

The fatigue life of 6061-T6 alloy, normally used in its wrought form, was investigated in this study in cast form from parts produced by the new ablation casting process. All specimens were excised from military castings. Unidirectional tensile test results yielded elongation values comparable to forgings and extrusions. A total of 39 fatigue specimens were tested by the rotating cantilever beam technique at five maximum stress levels. Moreover nine specimens excised from a forging were also tested for comparison. Results revealed that the fatigue life of ablation-cast 6061-T6 (i) follows a three-parameter Weibull distribution, and (ii) is comparable to data from the 6061 forging and is superior to conventionally cast Al-7% Si–Mg alloy castings published in the literature. Analysis of the fracture surfaces of ablation-cast 6061-T6 via scanning electron microscopy showed the existence of fracture surface facets and multiple cracks propagating in different directions.

In this research a simple, yet accurate model of a single cell, needed for safety assessment of batteries under mechanical abuse conditions, was developed. Extensive testing was performed on a 18650 lithium ion cell, including indentation by a hemispherical punch, lateral indentation by a cylindrical rod, compression between two flat plates, and three-point bending. The batteries were tested in an environmental chamber at a 10% SOC. A finite element model was developed, composed of shell elements representing outside casing, and solid elements for the active material with a binder lumped together with the current collectors and the separator. The jelly roll is modeled as a homogenized and isotropic material. The homogenous model assumes different properties in tension and compression, but does not account for the effect of structural anisotropy caused by the layered nature of the jelly roll. Very good correlation was obtained between LS Dyna numerical simulation and test results in terms of load–displacement relations, deformed shape of the battery, and initiation and propagation of a crack in the shell casing. The FE model was also capable of predicting the onset of short circuit of the cell.

The necessity to stop the pouring of liquid metals is becoming urgent. Pouring is the main source of entrained bifilms. Bifilms are the fundamental mechanisms which initiate all our familiar casting problems including porosity, hot tears, low mechanical properties, corrosion initiation etc. As lattice dislocations explain plasticity, bifilms explain pore initiation and fracture initiation. Only when pouring is eliminated (or, at second best, sufficiently controlled) will casting processes start to achieve their potential to deliver routinely sound and reliable castings (with the additional potential to make healthy profits!).

The Ni-base superalloys, which are normally melted and cast in a vacuum, entrain their surface-oxide film during turbulent pouring of the melt; unfortunately at this time, this process is universally practiced for investment castings of these materials. The entrained film becomes a bifilm crack automatically, so that cast alloys have a large population of cracks that controls their failure behavior. The problems of the growth of single crystals and the welding of polycrystalline alloys are reviewed to illustrate the central role of bifilms in the cracking of turbine blades, the heat-affected zones of welds, and the reliability of properties. It has been demonstrated that improved gravity pouring systems can reduce these problems significantly, but only countergravity filling of molds is expected to result in defect-free castings. Recent cases in which turbine blades failed in service are examined, and the central role of bifilm defects in these failures is discussed.

Fatigue potential has been studied in cast aluminum alloys with regard to the fatigue crack initiation mechanism at the casting defects, particularly surface and subsurface defects. The significance of facets is interpreted as the presence of defects in the interior of castings. Furthermore, two varieties of facets have been identified, one originating as a dendrite-straightened bifilm (type I facet) and the other originating from a slip plane mechanism around casting defects (type II facet). The fatigue life potential of castings is reexamined based on the involvement of defects during the formation of both types of facets. It is proposed that the true fatigue life potential of defect free castings has yet to be observed, i.e., if castings can be produced without defects, then their fatigue performance will be significantly higher than even the best performances observed so far.

As a result of the extremely strong interatomic bonds, pores and cracks are difficult to form in metals. They seem unlikely to be created intrinsically by the normal mechanisms involved in the formation of a solid by solidification from liquid, or condensation from vapor phases, or probably, by lattice mechanisms in the solid state. It is proposed here that initiation sites for pores and cracks for most failures of metals can only be initiated from unbonded interfaces. Such unbonded defects are introduced into metals only via extrinsic (entrainment) mechanisms resulting from production processes, particularly melting and casting. Only entrained inclusions, particularly bifilms, have unbonded interfaces that can be opened to constitute Griffith cracks and can explain the initiation of macroscopic fracture and related microscopic processes, such as a decohesion between the second phases and a matrix. In the absence of entrained defects, metals would be predicted to fail in tension only either (1) at high stresses probably in excess of 20 GPa or (2) by ductile flow to the point of 100 pct reduction in area. Improved melting and casting processes giving freedom from entrained defects promise unprecedented performance and reliability of engineering metals.

This study is undertaken to investigate the effect of different concentrations of Li (0.03–0.35%) on the microstructure and tensile properties of Al–15 wt.%Mg2Si in situ metal matrix composite. The results showed that 0.15 wt.% Li addition changes the morphology of primary Mg2Si from irregular or dendritic to polyhedral shape and its average particle size decreases from 32 μm to 4 μm. Microstructural observations also depicted that the morphology of the eutectic Mg2Si phase alters from flake-like to coral-like. Further investigations on tensile tests revealed optimum Li level (0.15%) for improving both UTS and elongation values. A study of the fracture surfaces via scanning electron microscope (SEM) revealed a brittle mode of failure in unmodified composite however the addition of 0.15%Li converts the fracture behaviour to ductile. The behaviour is explained in terms of the presence of oxide bifilms in the liquid Al alloy.

The Weibull statistics used in the analysis of mechanical data from castings are reviewed. The insight that can be gained from the three-parameter analysis is introduced, and in particular, the meaning of the threshold value in the three-parameter Weibull distribution from a casting process standpoint is discussed. Weibull mixtures, which are necessary when different defect populations are present in castings, are discussed in detail. Data sets from the literature are reinterpreted for demonstration of the guidelines. Step-by-step procedures are outlined for analysis of Weibull mixtures and for fatigue life data sets with run-outs. The results indicate that knowledge of the casting process is necessary for the accurate interpretation of Weibull probability plots. Because the filling systems of most castings currently are designed poorly, a mixture of Weibull distributions can be expected to be the norm rather than the exception. A single Weibull distribution can be expected only in castings whose filling systems were designed so that no new damage to liquid metal takes place during filling.

The benefits of Sr additions to Al–Si alloys to modify the eutectic are often impaired by the development of porosity, sometimes to the degree that benefits are negated. Experimental reports are reviewed in this paper, suggesting an explanation in terms of the oxide population in the melt. The unmodified silicon particles are nucleated by AlP, which has in turn nucleated on oxide bifilms. The oxide bifilms, which are essentially cracks, are straightened by the crystalline growth of Si particles, leading to increased crack size and consequently reduced mechanical properties. The addition of Sr improves properties by suppressing the formation of Si on bifilms and thereby preventing the straightening of the pre-existing cracks. Si is now forced to precipitate at a lower temperature as a coral-like eutectic. Unfortunately, the bifilms are now freed (the primary Si particles no longer exist to grow around and sequester the bifilms), remaining in suspension in the liquid metal, allowing them to act to b...

An analysis of the statistical distribution of the ultimate tensile strength (UTS) of cast Al2O3 containing aluminium matrix composite and its matrix alloy (A357) was carried out using different volume fractions of alumina particles (0, 5, 10 15 and 20%). The scatter of UTS data was assessed by the use of two parameter Weibull analysis. Results for the matrix alloy and metal matrix composites in the heat treated condition (T6) were compared. It was found that the addition of reinforcing particles to the matrix alloy resulted in lower UTS and ductility and increased scatter of results. Weibull modulus of cast MMC rods was found to be about 22 in the matrix alloy, falling to six in the 20% alumina MMC. Fractographic examinations of lower strength specimens showed that oxide films and the consequent porosity play an important role in fracture. The future potential, if MMCs can be manufactured without entrained oxides, is seen to be exciting.

The flow behaviour of aluminium alloy matrix composites containing Al2O3 and SiC particles is studied. A357 aluminium alloy was the matrix alloy common to both metal matrix composites (MMCs). Reinforcing particles were similar in shape and size (40 μm). Both MMCs were made by the stir casting process using parameters optimised so far as possible. The flowability of the MMCs was studied by measuring the fluidity of strips cast in a permanent mould. The results showed that the fluidity of both MMCs appeared to be reduced by increasing percentage addition of particles and further reduced by increasing times and temperature. It is proposed that much of this behaviour is not the result of the added reinforcing particles but the result of oxide films necessarily entrained by the stir casting process. The agglomeration of additions commonly seen in particulate MMCs is also attributed to the entrainment of clusters of additions as they enter the liquid through the surface oxide film.

Several indices are available in the literature to assess the structural quality of cast Al alloys, especially Al-7 pct Si-Mg alloys that are based on tensile test results. Some of these indices, most notably the one developed by Drouzy et al., provide a number that necessarily does not have a physical meaning, whereas the others are a measure of what fraction of the expected tensile performance is achieved. These indices are analyzed in depth, their similarities and shortcomings are discussed in detail, and recommendations are made.

The various microstructures of cast irons are reviewed, including carbidic and graphite forms (flake, compacted, spheroidal, and undercooled, etc.), exploring whether the presence of externally introduced defects in the form of oxide double films (bifilms) in suspension in melts seem to provide, for the first time, a uniform explanation for all the structures and their properties. Silica-rich oxide bifilms provide the substrates on which oxysulfide particles form, nucleating graphite. The presence of the film provides the favored substrate over which graphite grows, which leads to the development of flake graphite. The addition of limited Mg to form compacted graphite destroys all but a remnant of the silica-rich bifilms. The oxide film remnant is stabilized by the presence of the graphite nucleus, which causes the graphite to grow unidirectionally in a filamentary form. The addition of excess Mg destroys all traces of the oxide bifilms, leaving only the original nuclei, around which graphite is now free to entirely enclose, initiating the spherical growth mode. Undercooled graphite is the true coupled growth form, nucleated at even lower temperatures in the absence of favorable film substrates in suspension; the graphite adopts a continuous growth mode in a matrix of austenite. Carbides in mottled and white irons form on the oxide bifilms that often lie along grain and interdendritic boundaries, which explains the apparent brittleness of these strong, hard phases. In most cases of nonspheroidal growth modes (flake and misshaped spheroids), it is proposed that the impairment of the mechanical properties of irons is not strongly determined by graphite morphology but by the presence of oxide bifilms. Spheroidal graphite iron has the potential for high properties because of the absence of bifilms.

The L-shaped junctions in running and gating systems used in aluminum gravity casting have been investigated. Using computational modeling, a guideline for constructing two geometries of L-junctions was developed. The sequential filling profile of liquid metal along L-junction was confirmed by real-time X-ray video of an aluminum alloy sand casting. The change of flow direction through L-junctions can yield a high coefficient of discharge Cd, without entrapping detrimental oxide film defects because of the smooth flow, minimizing surface turbulence. A short “clear-up” time, the duration of filling a component, of L-shaped junction is also achieved. For the necessary of future application of this junction, its dimensionless equivalent lengths (LE/D) and loss coefficient K were estimated. The main aim of this work was to eliminate the trial and error approach as designing a multiple-gate system. From the guideline of L-junction, the two junctions can be assembled into a complex multiple-gate runner system. In this novel design of multiple-gate system, uniform distribution of flow through each gate into a mould cavity has been demonstrated. A high Cd value was also predicted.

The effects of both Li modification and cooling rate on the microstructure and tensile properties of an in-situ prepared Al-15 pct Mg2Si composite were investigated. Adding 0.3 pct Li reduced the average size of Mg2Si primary particles from ~30 to ~6 μm. The effect of cooling rate was investigated by the use of a mold with different section thicknesses from 3 to 9 mm. The results show a refinement of primary particle size as a result of both Li additions and cooling rate increases, and their effects are additive. Similarly, both effects increased ultimate tensile stress (UTS) and elongation values. The thin sections show somewhat unexpectedly low and scattered tensile results attributed to the casting defects observed in fracture surfaces. The Li-modified alloy displays serrated yielding behavior that is not fully explained here. The refinement by Li and enhanced cooling rate is explained in terms of an analogy with the effect of Sr and cooling rate in Al-Si alloys, and is ultimately attributed to the effect of the alkali and alkaline earth metals deactivating oxide double films (bifilms) suspended in Al melts as favored substrates for intermetallics.

A new patented casting process is described, based on a precision aggregate mold, bonded with a water-soluble binder. For the first time the twin functions of the mold (i) defining shape and (ii) providing cooling have been successfully separated allowing the production of castings of all sizes and shapes (thin and thick walls) for all Al and Mg alloys. Because solidification and cooling are separately controlled by the application of water a number of advantages follow immediately. The mold is ablated (i.e. eroded) away by the water without fume or dust and the ‘air gap’ is eliminated by direct contact with the water, enhancing the rate of solidification to levels normally unattainable, resulting in significantly enhanced properties. The unusual microstructures of Al-Si alloys having large DAS but micron-sized eutectic silicon indicates, in agreement with earlier predictions, that DAS per se does not control strength or ductility. The process is currently proving itself in commercial operation, having the additional advantages of modest start-up and tooling costs as well as low competitive piece part costs because of the use of low cost materials and the recycling of aggregate and water.

The yield strength–elongation relationship in cast Al–Cu–Mg alloys (206) castings has been investigated by analyzing 13 datasets from the literature on premium quality castings. The data representing the elongation for a given yield strength have been found to fall within an envelope, the top limit of which represents the ductility potential of Al–Cu–Mg alloy castings. Moreover, using the fracture toughness equation by Hahn and Rosenfield, estimated intrinsic fracture toughness values have been found to be consistent with the data reported in the literature.

The yield strength–elongation relationship in cast Al-7 pct Si-Mg castings has been investigated by analyzing 18 datasets from the literature on premium quality castings. The data representing the elongation for a given yield strength have been found to fall within an envelope, the top limit of which represents the ductility potential of Al-7 pct Si-Mg alloy castings. Analysis of maximum elongation data indicated that secondary arm spacing has no effect on elongation. It has also been determined that the quality index of Drouzy et al. does not provide a particularly accurate representation of the trends in maximum ductility data. A new quality index based on those proposed previously by Caceres and Din et al. is proposed. Moreover, using the fracture toughness equation by Hahn and Rosenfield, intrinsic plane strain fracture toughness and intrinsic elongation values have been found to be correlated, indicating the internal consistency of the approach taken in this study. The implications of these results are discussed in the article.

The effect of structural integrity on the tensile deformation of A206-T71 castings was evaluated. To generate a wide range of structural integrity, some castings were subjected to hot isostatic pressing (HIP) at various conditions and the others received no remedial processing. The analysis of true stress–strain curves of all castings showed that A206-T71 follows the Kocks–Mecking Stage III work hardening model in both non-HIPed and HIPed conditions. Nevertheless distinct differences were found within and between the HIPed and non-HIPed conditions. Kocks–Mecking parameters were correlated to elongation and toughness (strain energy). Structural integrity or quality, as measured by relative toughness, was found to have two types of effects on the Kocks–Mecking parameters. These effects are discussed in the paper.

The effect of hot isostatic pressing (HIP) on fatigue properties of A206-T71 castings was investigated. Castings were (i) non-HIPed, or HIPed at (ii) typical HIP temperature, (iii) solution heat-treat temperature and (iv) eutectic melting temperature. HIP conditions were found to increase average and maximum fatigue life. However, the range in fatigue life also increased after HIP, which is attributed to the location of defects as well as HIP's ability to heal porosity and thin bifilms more effectively than thicker bifilms. The mechanism for healing bifilms by HIP appeared to be the fracture of bifilms and partial bonding across their surfaces. Fatigue life data were analyzed using three-parameter Weibull statistics, which showed that HIP significantly increases threshold fatigue life, but increasing HIP temperature has no effect on the threshold fatigue life.

Molten magnesium alloys oxidise rapidly during casting and handling if adequate prevention measures are not taken. New oxide films that form in a very short time during pouring seem to be one of the main reasons for dross-like defects. The characteristics of these new oxide films were observed. The magnesium alloy AZ91 was selected for the study. Samples were prepared based on the technique in which an oxide–metal–oxide sandwich was made by bubble impingement. After impingement the contact areas of two adjacent and entrapped bubbles were selected for the study. Features such as thickness, size, morphology and chemical composition of the oxide film have been examined and shown by SEM. Possible consequences of the morphology of the oxide film were discussed.

The effects of hot isostatic pressing (HIP) temperatures on bifilms and tensile properties of A206-T71 castings were investigated. Castings were: (i) non-HIPed, and HIPed at (ii) typical HIP temperature, (iii) solution heat-treat temperature and (iv) eutectic melting temperature. Tensile strength and elongation data were analyzed using Weibull statistics. In all cases, HIP was found to increase the threshold tensile properties. Also, average and maximum tensile properties, as well as variability in elongation were increased by HIP. This was attributed to HIP's ability to heal porosity and thin bifilms more effectively than thicker bifilms. The mechanism for healing bifilms by HIP appeared to be the fracture of thin bifilms and partial bonding across their surfaces. The effect of increased HIP temperature was masked by the presence of spinel bifilms, which resisted the healing effect of HIP.