Ramin Raiszadeh

A method to study the history of a double oxide film defect in liquid aluminum alloys

Entrained double oxide films have been held responsible for reductions in mechanical properties in aluminum casting alloys. However, their behavior in the liquid metal, once formed, has not been studied directly. It has been proposed that the atmosphere entrapped in the double oxide film defect will continue to react with the liquid metal surrounding it, perhaps leading to its elimination as a significant defect. A silicon-nitride rod with a hole in one end was plunged into liquid aluminum to hold a known volume of air in contact with the liquid metal at a constant temperature. The change in the air volume with time was recorded by real-time X-ray radiography to determine the reaction rates of the trapped atmosphere with the liquid aluminum, creating a model for the behavior of an entrained double oxide film defect. The results from this experiment showed that first oxygen, and then nitrogen, was consumed by the aluminum alloy, to form aluminum oxide and aluminum nitride, respectively. The effect of adding different elements to the liquid aluminum and the effect of different hydrogen contents were also studied.

Fabricating Al Foam from Turning Scraps

This article describes the sintering-dissolution process (SDP) that is employed for fabricating aluminium foams from AA336 turning scraps. The effect of sintering temperature, chip size, and chip weight fraction in the NaCl/Al mixture on the mechanical properties of the foams produced was also investigated. The results indicate that, in contrast to commercial-purity Al foams, which (as stated in the literature) can be produced in SDP by either liquid- or solid-state sintering, solid-state sintering does not take place between the AA336 chips, and so melting of the chips is essential to form a strong network structure in the preform. A chip weight fraction of 60%, chip size of about 0.5 mm, and a sintering temperature of 600°C were determined to be the optimum parameters for a successful sintering dissolution process for the fabrication of Al foams from AA336 turning scraps.

Effect of stirring on behaviour of double oxide film defects in A356 aluminium melt

Abstract The effect of stirring of melt on the behaviour of double oxide film (bifilm) defects in A356 melt was investigated using a reduced pressure test (RPT) technique. The melt was poured from a height into a crucible to introduce bifilms into the melt. The melt was then either remained stagnant or mechanically stirred, and RPT samples were taken from the melt at 8 min intervals. The RPT samples were then characterised by determination of their porosity parameters and examination of the internal surfaces of the pores using a scanning electron microscope. The results showed that stirring of the melt facilitated the diffusion of H in the melt into the atmosphere of the defects and hence accelerated the removal of the defects from the melt by floatation. This effect was attributed to the increase in the stress induced to the defects, which increased the rate of formation of cracks on their layers.

Study of double oxide film defect behaviour in liquid Al–Mg alloys

Abstract The change in the composition of oxide layers and the possibility of the formation of bonding between the layers of a double oxide film defect when held in Al melts containing 0·7, 1 and 2 wt-%Mg were investigated. The defect was modelled by maintaining two aluminium oxide layers in contact with one another in the melt at 1023 K for times between 5 s and 50 h. Any changes in the composition and morphology of these layers were studied by SEM and energy dispersive X-ray. The results showed that in all the alloys, the initial Al2O3 layer first transformed to MgAl2O4 and then to MgO. These transformations caused the two layers to bond together gradually. The results confirmed that, in all the Al alloys containing 0·3–4·5 wt-%Mg, oxide films have the chance to heal if held in the liquid metal for a long enough period of time.

New analytical estimation of solid shell thickness at length of continuous casting mould

Abstract A new analytical formula for the estimation of solid shell thickness over the length of a continuous casting mould has been developed. The precision of this formula was experimentally compared to that of the classic analytical solution of Stefan and the results of finite difference (FDM) and finite point (FPM) models of the process. The results showed that the new approach estimated the thickness of the solid shell along the continuous mould with greater precision than the Stefan solution and that the precision of the new approach was comparable to those of the FDM and FPM approaches. Therefore, despite its simplicity, the equation has potential to be used as a quick, inexpensive and relatively accurate tool to estimate the thickness of the solid shell in continuous casting moulds and some other solidification processes.

The Effect of Strontium on the Strength of Layers of Double Oxide Film Defects

The change in the strength of layers of oxide film (bifilm) defects when held in Al-0.05 and 0.1 wt pct melts was evaluated indirectly by comparing the porosity, average pore roundness, and pore number density results obtained from a reduced pressure test of the commercial purity and Sr-containing Al melts subjected to different holding and vibration regimes. Any changes in the composition of the pores were also studied using scanning electron microscopy and energy-dispersive X-ray spectroscopy. The results showed that the Al2O3 layers of a newly formed oxide film defect, when held in Al-0.05 or 0.1 wt pct Sr melt, transformed gradually into an Sr-containing oxide. This transformation caused the strength of the oxide layers to increase and the rate of the expansion of its atmosphere and removal of the defect from the melt to decrease. This might influence the increase in the porosity of Sr-modified Al castings.

Effect of vibration on behaviour of bifilms in A356 and A357 melts

The effect of applying vibration to a melt on the behaviour of bifilm defects in A356 and A357 melts was studied using a reduced pressure test technique. The results showed that vibrating a melt can have a dual effect on bifilms. This effect depends on the rate of phase transformations that occur in the oxide films. If the transformation occurs fast enough then the vibration would facilitate the formation of bonding between the layers of bifilm defects by causing the atmosphere of the defects to be consumed faster. Otherwise, the vibration might facilitate the diffusion of hydrogen into the atmosphere of the defects, and hence prevent or delay the formation of bonding between the oxide layers.

Weibull verification of reduced pressure test as a tool for assessing Al melt quality

Abstract The credibility of reduced pressure test (RPT) as a substitute for Weibull analysis for benchmarking bifilm defects in Al alloys was confirmed by comparing the RPT parameters (2D and 3D porosity, and Bifilm Index) with the Weibull moduli for UTS values for pure Al and Al-0.05Sr alloys. The porosity of RPT samples was found to have an exponential decay relationship with Weibull modulus. A reasonable trend for Bifilm Index could not be found. 3D porosity of RPT samples was found to be more favourable than 2D porosity or Bifilm Index. Some evidence was also found of connections occurring between the sides of bifilm defects at the surfaces of the fractured tensile test bars of Al-0.05Sr alloy which had a relatively high UTS.

Decrease in the rate of diffusion of hydrogen through layers of bifilm defects in Al melt in the presence of Sr

The change in the diffusion rate of H through layers of bifilm defects in Al-0.05 wt% Sr melt was investigated by studying the change in the porosity of samples that were held in furnace for different times, subjected to vibration and then solidified under reduced pressure. The results showed that the vibration accelerated the rate of formation of cracks on the layers of the defects, the rate of diffusion of H into their atmosphere and the rate of consumption of Sr in the melt. These rates decreased gradually as the melt was held in the furnace before applying the vibration. This decrease was attributed to the transformation of alumina to Sr-containing oxides, which caused the strength of the layers to increase and the rate of formation of cracks on them to decrease. This behaviour may have an impact on the increase in porosity of Al castings modified by Sr.

Removal of Bifilms from Al Melts by Stirring in Unbaffled and Baffled Crucibles

The free surface of aluminum melt, during stirring under different rotation speeds in unbaffled and baffled crucibles, was simulated numerically using a RANS turbulence model coupled with a particle-tracking method. The effect of mechanical stirring of Al melts containing 0.3, 0.7, and 4.5 wt pct Mg in unbaffled and baffled crucibles on the rate of elimination of bifilms from the melt was also investigated using a reduced pressure test. The results showed that the stirring increased the rate of removal of bifilms from the melt due to an increase in the rate of formation of cracks on the bifilms. Using baffles in the crucible accelerated the rate of removal of bifilms by changing the pattern of movement of bifilms in the melt. The critical stirring speeds, above which the oxide layer on the Al melt could submerge into the bulk liquid, were determined by the simulation. It was confirmed by the experimental results to be about 158 and 420 RPM for the unbaffled and baffled crucibles, respectively.