The effects of oxide film characteristics on the bubble damage defect in Al5Mg and Al7SiMg alloys

There is a general agreement among the researchers that the surface oxide film of film forming alloy may have serious effects, on the soundness of castings, when they are entrained, as a result of surface turbulence. It has been suggested that the thickness of the surface oxide films is different alloys may varies an order of magnitude. In this work the effects of the surface oxide film thickness of two Al alloys, Al5Mg and Al7SiMg, and the velocity of molten metal, during mold filling, are reported. In order to investigate the effects these two variables, plates with thickness of 10 mm from Al5Mg and Al7SiMg alloys were cast, via a bottom-filling system, having the mold entry velocity of 0.5, 1, 1.5, 2, 2.5, and 3 m/s, respectively. The amount of entrapped bubbles was measured in the samples, using X-ray test. Then, three point bending test was carried out on plates. Finally, the fracture surface of samples was studied with electron microscope. Experiments showed that in the velocities higher than critical velocity (0.5 m/s), the amount of entrapped bubble is increasing while the volume percent of residual bubble in Al5Mg alloy is 5 to 10 times more than Al7SiMg alloy. The results show that there is, possibly, a connection between the thickness of surface oxide film and the quantity of bubble entrapment in the castings. Examining the fracture surface of samples indicate the presence of oxide films in residual bubble.

کلیدواژه ها:

Double Oxide Film ، Bubble Damage ، Critical Velocity ، Bubble Entrapment