Freezing Range, Melt Quality, and Hot Tearing in Al-Si Alloys
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ring is one of the most important defects in aluminum alloys. It has been reported that the main reasons that cause hot tearing are the contraction in mushy zone, restricted shrinkage, and lack of feeding.[1,2] It is not easy to estimate this defect because of some complex events occurring simultaneously during solidification. Although there are several studies on hot tearing,[3–8] yet, it cannot be fully explained.[1] The ‘‘uncertainty’’ in the characteristic properties of hot tearing has been listed as follows: Occurs as messy in the form of branched cracks. Main tearing and its extensions are observed to be
Eskin et al.[10] have extensively studied the characterisation of hot tearing phenomena and claimed that the stage of separation between dendrites is crucial and sensitive for the alloys that have a wide solidification range. Hot tearing formation was divided into four levels as described below: i. ii.
intergranular.
Defect surface has a dendritic morphology. Defect surface is usually packed with heavy oxides. Generally located on hot spots where shrinkage
iii.
takes place.
Not always seen under the same conditions. Specific to certain alloys; not seen in all alloys. Campbell[9] recommend that this problem can be controlled by
MUHAMMET ULUDAG˘ is with the Metallurgical and Materials Engineering Department, Bursa Technical University, Bursa, Turkey; Contact e-mail: [email protected] REMZI C¸ETIN is with the Materials Engineering Department, KTO Karatay University, Konya, Turkey. DERYA DISPINAR is with the Metallurgical and Materials Engineering Dept., Istanbul University, Istanbul, Turkey. Manuscript submitted August 7, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
Chill applications Grain refinement Working with different alloys Using suitable mold design Proper runner and sprue design (controlled filling)
iv.
Mass feeding where liquid and solid act freely. Interdendritic feeding where remaining liquid has to pass dendritic network. Network permeability is still very large to prevent pore formation. Interdendritic separation: as solid fraction is increased, liquid is trapped in interstitial spaces or immobilized because of surface tension. At this level, shrinkage occurs where solid network permeability decreases and material shrinks. Solid feeding where only the liquid pockets remain and the cast part has a significant strength on the last level of solidification (fs > 0.9). Hot tearing can occur at this level.
Eskin et al.[10] focused on the last two levels because interdendritic separation is quite sensitive to hot tearing in alloys that have a large solidification range. A study that measures hot tear length using a ‘‘U’’ shape mold was used to investigate the effect of silicon ratio on hot tearing in Al-Si alloys.[11] It was found that silicon can start to initiate hot tearing until 1.9 wt pct Si. However, the hot tearing can be decreased by silicon
additions after 1.9 wt pct. Pumphrey[12] studied hot tearing using six different alloys: Al-Si, Al-Cu, Al-Mg, Al-Fe, Al
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