Effect of Sr modification on hot tearing of A380.1 alloy
Muhammet Uludağ Remzi Çetin Derya Dışpınar
AbstractSurface of most liquid metals is covered by a film that forms as a result of the reaction of atmosphere with the metal. These films are harmless as long as they remain on the surface. However, they can be entrained into the bulk of the melt when the surface is disturbed, such as with additions to the melt, and/or when new surfaces are created, such as during pouring. These entrained films are called bifilms because of their double, film-on-film nature [1]. In aluminum alloys, they remain in the liquid and have a deleterious effect on casting quality, mechanical properties and performance of castings. To produce high quality aluminum castings, it is essential to start with a melt that is substantially free from bifilms and bifilm-opening agents such as hydrogen dissolved in liquid aluminum and intermetallics such as Fe-containing phases [2]. When initial melt quality is poor and/or significant damage is given to liquid metal, casting suffer from defects, such as pores, inclusions and hot tears. Although pores and hot tears have been traditionally regarded as intrinsic defects [3], i.e., occurring as a result of metal properties, recent research [1] has shown that the root cause of pores and hot tears are bifilms. Moreover in situ experiments with solidifying transparent liquids have shown that hot tears are initiated by pores or inclusions. Strontium is added to Al-Si alloys so that the morphology of Si eutectic changes from acicular to fibrous with the intention of improving mechanical properties [4, 5]. However, Sr additions have been reported [6] to increase porosity in Al castings. Some users have even reported that the increased porosity more than negates any beneficial effects of Sr, and consequently they abandoned modification by Sr [7]. Bifilms in the melt and Sr additions interact to initiate pores in Al-Si alloys [8].