A Review Study on the Main Sources of Porosity in Al‐Si Cast Alloys

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.

β Grain refinement by yttrium addition in Ti-6Al-4V Wire-Arc Additive Manufacturing

“…The change in solidification behaviour due to the addition of Y, that leads to an improved grain structure by providing a larger melting point range and a wider mushy zone with more developed dendrites, will also unfortunately cause a greater gas concentration in the inter-dendritic liquid [74]. Oxide particles are further known to aid nucleation of gas pores within melts [75] and shrinkage porosity will equally be promoted by there being a larger pressure drop from poor liquid feeding in the wider mushy zone in the Y doped solidifying melt pool, through the thin inter-dendritic channels produced by the low partition coefficient of Y when using low addition levels. For example, the images in Figure 5 suggest that liquid feeding through such narrow channels is being blocked during the latter stages of solidification by the formation of the Y2O3 particles.…”

Intermetallics Formation during Solidification of Al-Si-Cu-Mg Cast Alloys

“…Therefore, the increase in amount of porosity in the Sr-modified alloy, i.e., RM alloy, may be interpreted in terms of an increased pore size and density. It is inferred from the work of Samuel et al [35] that increasing the pore size is more harmful to the alloy's mechanical properties, which is another parameter to be considered in the use of grain refining, which may lead to a reduction in the porosity, as the application of grain refining is reported to be effective in reducing the pore size [36].…”

Crystallography and Interface Structures in As-Arc Melted and Laser Surface-Remelted Aluminum–Silicon Alloys with and without Strontium Addition

“…Aluminum-silicon (Al-Si) alloys find extensive applications across, for example, automotive, aerospace, electronics, power generation, construction, and sports industries due to their lightweight nature, good mechanical properties, and corrosion resistance [1][2][3][4]. Eutectic Si in conventionally cast Al-Si alloys exhibits the form of branched clusters of plates or flakes, known as a wheatsheaf configuration [5].…”