Abstract
The present study was performed on A356 and B319 alloys in mechanically stirred or degassed condition. Melts were Sr‐modified and grain‐refined. Hydrogen content was varied from less than 0.1 ml/100 g Al to ∼0.4 ml/100 g Al; Fe was increased to 0.8% in B319 alloy. Lanthanum and cerium were added as 99.5% pure metals. Two main techniques were used to investigate porosity formation: fracture surface of tensile or fatigue test bars, or reduced pressure test (RPT) method. Porosity type and shape were examined. The results show that pore size is more influential than small scattered ones from a mechanical point of view. Tensile testing is affected by porosity located at the center of the testing bar, whereas edge porosity is responsible for crack initiation in case of fatigue testing. Intermetallics precipitate in the form of intercepted platelets which restricts the flow of the molten metal, leading to formation of shrinkage cavities. Precipitation of clusters of compounds from the liquid state such as Al2Si2Sr, Mg2Sn, Al3Ti, or added Al2O3particles would as well act as nucleation sites for porosity formation. Most oxides were observed in the form of long branched strings. In some cases, bifilms were also reported in addition to SrO and MgO.
Highlights
Macroscopic cavities occurring in a casting are normally a shrinkage type due to poor feeding
Gas and Shrinkage Porosity. e solubility of hydrogen in pure aluminum, and 319 and 356 aluminum alloys, is shown in Figure 5[25], whereas Figure 6illustrates the porosity development during solidification [26]. e results of Boudreault et al [27] on the formation of gas porosity in 356 and 319 alloys solidified under different rates are controlled by the amount of absorbed hydrogen and solidification time, applying the same techniques described in the experimental section. e results are presented in Figures 7(a)and 7(b)
Figure 8demonstrates the presence of a mixture of gas and shrinkage cavities at the edge of a tensile bar of non-modified A356 alloy in the T6 condition, whereas long shrinkage cavities can be seen at the center of the sample
Summary
Macroscopic cavities occurring in a casting are normally a shrinkage type due to poor feeding. E effect of surface porosity on the tensile properties of the used A356 alloy in the T6 condition is illustrated in Figure 13(tensile and fatigue testing at room temperature).
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