Investigation on Double Oxide Film Initiated Pore Formation in Aluminum Casting Alloys

Abstract

The most common, and in fact, the most deleterious defects of aluminum casting alloys are the so-called double oxide films or bifilms, which have a central role in porosity formation, as they can easily unfurl and inflate into pores during the solidification of the alloys. Sr addition is generally used in the foundry industry for the modification of the eutectic Si phase of hypoeutectic Al-Si alloys. However, Sr microalloying usually leads to an increased pore formation tendency. As bifilms are preferred sites for pore formation, it should be expected that Sr additions have a significant effect on the number and/or the structure of double oxide films present in the melt. In this work, the relationship between Sr-concentration and the susceptibility to pore formation has been investigated through the evaluation of melt quality of melts which had different levels of Sr. The bifilm content of the melts was investigated by the analysis of K-mold specimens and X-ray computed tomography (CT) of reduced pressure test (RPT) samples. It was found that liquid alloys with a higher Sr concentration had a significantly greater tendency to pore formation, which can be explained by the presence of a larger number of bifilm defects in the liquid alloy.

Numerical Simulation and Experimental Validation of Melt Flow in the Naturally Pressurized Gating System

“…• SEM-EDX analysis showed presence of oxide layer on the inner surface of pores. It is consistent with works [27,30].…”

“…The oxide layer presence is also indicated by EDX analysis, which detected the higher presence of oxygen on inner pore surface, as shown in Figure 16. It corresponded with the work [27], where authors claimed that the pore formation is related to entrained double oxide layer and double oxide layer is initiator of pore formation.…”

Bifilm Inclusions in High Alloyed Cast Iron

“…Campbell proposed that these oxides are not taut marbles, cubes, rods, etc., but rather films or bifilms with a lower Stokes velocity, which lets them remain in the alloy suspension for a long time. Other researchers came to similar conclusions [ 25 , 26 ]. When there is no Mg, the oxysulfide particles nucleate on oxide bifilms rich in silica, and then graphite nucleates and grows on oxysulfide inclusions, forming flake graphite.…”

Effect of Gating System Design on the Quality of Aluminum Alloy Castings

“…The oxide layer can deteriorate the final quality of the casting if it is entrained into the melt by the turbulence of the melt surface and solidifies in the casting [1][2][3][4][5]. Several studies in recent years indicate that entrained double oxide layers in the casting act as porosity initiators and that hydrogen contributes to pore formation by diffusing between the layers, thereby enlarging the pores [6][7][8].…”