A Semi-empirical Mathematical Model to Estimate the Duration of the Atmosphere within a Double Oxide Film Defect in Pure Aluminum Alloy

Melts of commercially pure liquid aluminum, and an Al-7Si-0.3Mg alloy, were cast into molds designed to produce entrainment of oxide film defects. The melts were held for periods of up to 20 minutes to investigate whether changes in the oxide film defects in the melt could occur, once sufficient time had elapsed for consumption of their internal atmosphere. The alloys were characterized by the determination of their Weibull modulus, examination of fracture surfaces under scanning electron microscopy (SEM), and determination of their porosity characteristics. The Weibull moduli of the ultimate tensile strength (UTS) values of the Al-7Si-0.3Mg alloy were reduced initially by holding in the liquid state for 10 minutes, but then the values increased after holding for 20 minutes. This high Weibull modulus was found despite oxide films being observed on the fracture surfaces. In the case of the commercial purity Al, the UTS Weibull moduli increased only slightly with holding for 20 minutes. The results suggested that holding of Al alloys in the liquid state influenced the scatter of mechanical properties by influencing the porosity content of the castings, which was related to their oxide film content. Some evidence for healing of a double oxide film defect with time was also found in the commercial purity Al alloy.

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

The Effect of Holding Liquid Aluminum Alloys on Oxide Film Content

Raiszadeh

Griffiths

2010

“…First oxygen and then nitrogen of the trapped atmosphere of the defect came into contact and react with the surrounding Al melt through these cracks to produce Al 2 O 3 and AlN, respectively. Using a semi-empirical mathematical model, [5] they suggested that this process took a few seconds to 2 minutes, depending on the size of the oxide defect and the severity of the turbulence in the melt.…”

Section: Formation Of a Double Oxide Film Defect Inmentioning

confidence: 99%

Effect of Ca on the Behavior of Double Oxide Film Defects in Commercially Pure Aluminum Melt

Khaleghifar

Raiszadeh

Doostmohammadi

2014

The change in the composition of oxide layers and the possibility of the formation of bonding between the two layers of a double oxide film defect when held in Al-0.01 and 0.05 wt pct Ca melt was investigated, and the thermodynamics of the transformations occurring in the Al-CaAl 2 O 3 system was studied. The defect was modeled experimentally by maintaining two aluminum oxide layers in contact with one another in the liquid metal at 1023 K (750°C) for times ranging from 5 seconds to 50 hours. Any changes in the composition and morphology of these layers were studied by scanning electron microscopy and energy-dispersive X-ray spectroscopy. The results showed that the initial Al 2 O 3 layer started to transform to a mixture of calcium aluminates in both alloys when the Al bars were still in the solid state. This transformation caused the two oxide layers to bond with each other gradually after the bars melted. The degree of bonding, however, was not consistent between the repetitions of the experiments, which was attributed to the difference between the initial H content of the melt on different experimental days.

“…The air trapped in a bifilm will be expected to lose its oxygen by continued oxidation of the matrix, followed by nitrogen that would react to form nitride, [53] leaving mainly argon. In this way, it seems possible that clouds of fine argon bubbles will be released into the melt.…”

Section: Spheroidal Graphite Iron (Sgi) (Ductile Iron)mentioning

confidence: 99%

A Hypothesis for Cast Iron Microstructures

Campbell

2009

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.