Effect of Sr addition on porosity formation in directionally solidified A356 alloy

Abstract It is well-known that the better the control of the liquid aluminium allows obtaining of better properties. One of the most important defects that is held responsible for lower properties has been the presence of porosity. Porosity has always been associated with the amount of dissolved hydrogen in the liquid. However, it was shown that hydrogen was not the major source but only a contributor the porosity. The most important defect that causes porosity is the presence of bifilms. These defects are surface entrained mainly due to turbulence and uncontrolled melt transfer. In this work, a cylindrical mould was designed (Ø30 x 300 mm) both from sand and die. Moulds were produced both from sand and die. Water cooled copper chill was placed at the bottom of the mould in order to generate a directional solidification. After the melt was prepared, prior to casting of the DC cast samples, reduced pressure test sample was taken to measure the melt quality (i.e. bifilm index). The cast parts were then sectioned into regions and longitudinal and transverse areas were investigated metallographically. Pore size, shape and distribution was measured by image analysis. The formation of porosity was evaluated by means of bifilm content, size and distribution in A356 alloy.

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The role of the pressure index in porosity formation in A356 alloy castings

Cast Metals ◽

10.1080/09534962.1995.11819182 ◽

1995 ◽

Vol 7 (4) ◽

pp. 219-225 ◽

Cited By ~ 10

Author(s):

Sen-Tien Kao ◽

Edward Chang

Keyword(s):

A356 Alloy ◽

Porosity Formation ◽

Pressure Index ◽

Alloy Castings

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Microstructure and properties of twinned dendrites in directionally solidified A356 alloy

Materials Science and Engineering A ◽

10.1016/j.msea.2018.07.079 ◽

2018 ◽

Vol 734 ◽

pp. 7-19 ◽

Cited By ~ 5

Author(s):

Yang Li ◽

Shuangming Li ◽

Luyan Yang ◽

Hong Zhong

Keyword(s):

A356 Alloy ◽

Directionally Solidified ◽

Microstructure And Properties

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Effect of Solidification Rate and Rare Earth Metal Addition on the Microstructural Characteristics and Porosity Formation in A356 Alloy

Advances in Materials Science and Engineering ◽

10.1155/2017/5086418 ◽

2017 ◽

Vol 2017 ◽

pp. 1-19 ◽

Cited By ~ 3

Author(s):

M. G. Mahmoud ◽

A. M. Samuel ◽

H. W. Doty ◽

S. Valtierra ◽

F. H. Samuel

Keyword(s):

Rare Earth ◽

Solidification Rate ◽

A356 Alloy ◽

Earth Metal ◽

Atomic Radius ◽

Shrinkage Porosity ◽

Minor Effect ◽

Porosity Formation ◽

Agent Interaction ◽

Eutectic Si

The present study was performed on A356 alloy with the main aim of investigating the effects of La and Ce additions to 356 alloys (with and without 100 ppm Sr) on the microstructure and porosity formation in these alloys. Measured amounts of La, Ce, and Sr were added to the molten alloy. The results showed that, in the absence of Sr, addition of La and Ce leads to an increase in the nucleation temperature of the α-Al dendritic network with a decrease in the temperature of the eutectic Si precipitation, resulting in increasing the freezing range. Addition of 100 ppm Sr results in neutralizing these effects. The presence of La or Ce in the casting has a minor effect on eutectic Si modification, in spite of the observed depression in the eutectic temperature. It should be noted that Ce is more effective than La as an alternate modifying agent. According to the atomic radius ratio, rLa/rSi is 1.604 and rCe/rSi is 1.559, theoretically, which shows that Ce is relatively more effective than La. The present findings confirm that Sr is the most dominating modification agent. Interaction between rare earth (RE) metals and Sr would reduce the effectiveness of Sr. Although modification with Sr causes the formation of shrinkage porosity, it also reacts with RE-rich intermetallics, resulting in their fragmentation.

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