A Review on Porosity Formation in Aluminum-Based Alloys

Abstract

The main objective of this review is to analyze the equations proposed for expressing the effect of various parameters on porosity formation in aluminum-based alloys. These parameters include alloying elements, solidification rate, grain refining, modification, hydrogen content, as well as the applied pressure on porosity formation in such alloys. They are used to establish as precisely as possible a statistical model to describe the resulting porosity characteristics such as the percentage porosity and pore characteristics, as controlled by the chemical composition of the alloy, modification, grain refining, and the casting conditions. The measured parameters of percentage porosity, maximum pore area, average pore area, maximum pore length, and average pore length, which were obtained from statistical analysis, are discussed, and they are supported using optical micrographs, electron microscopic images of fractured tensile bars, as well as radiography. In addition, an analysis of the statistical data is presented. It should be noted that all alloys described were well degassed and filtered prior to casting.

Keywords: grain refining; hydrogen; modification; porosity; solidification rate; statistical modeling.

Figure 1

SEM electron images of porosity in 319 alloy (a) a mixture of gas and shrinkage, (b) a high magnification micrograph of shrinkage reveals dendrites.

Figure 2

Optical microstructure of porosity in 319 alloy (a) low hydrogen 0.12 mL/100 g Al, (b) high hydrogen 0.25 mL/100 g Al, (c) and (d) fractured surfaces of tensile bars corresponding to (a) and (b), respectively.

Figure 3

(a) Optical micrograph of 354 alloy containing 0.4% Zr - inset is an (Al,Si)₃(Zr,Ti) phase particle (500×), arrowed; (b) backscattered electron micrograph of 413 alloy showing the branching of a gas pore into a shrinkage pore during the solidification process.

Figure 4

RPT test taken from melts of (a) as-received 319 alloy; (b) after degassing; (c) melt containing 0.25 mL/100 g Al.

Figure 5

(a–c) Radiographic images of porosity distribution at different H₂ levels: (a) 0.12, (b) 0.18, (c) 0.22 mL/100 g Al, (d,e) schematic diagram (all dimensions are in mm), (e) actual mold configuration, and (f) typical casting.

Figure 6

Temperature–time curves obtained at different distance from the chill end. Thermocouples were placed along the centerline of the mold through the refractory block into the cavity.

Figure 7

Comparison of melt treatment and hydrogen level on the pore size obtained in three Al-Si based alloys, highlighting the effect of oxides as a function of the total amount of added Sr and grain refiner and hydrogen level.

Figure 8

Variation in shrinkage porosity as a function of Si content: (a) 0.5% (220 alloy), (b) 8.5% (380 alloy), and (c) 11.5% (413 alloy).

Figure 9

(a) Precipitation of coarse porosity in the interdendritic region and (b) microporosity at edges of the Al grains (arrowed black).

Figure 10

Optical microstructure of 319 alloy: (a) before grain refining; (b) after grain refining (using 0.0.15%Ti in the form of Al -5%Ti-1%B); (c) grain size in (a); (d) grain size in (b); (e) Porosity-H₂-Ti relationship.

Figure 11

Example of porosity in Sr-treated alloy: (a) backscattered electron image showing Sr-rich particles within a pore; (b) X-ray of Sr distribution in the particle marked X in (a). Note the rounded shape of the Sr-rich particles in (a).

Figure 12

Optical micrographs of 319 alloy obtained at (a) solidification rate ~8 °C/s and (b) solidification rate ~0.35 °C/s.

Figure 13

Effect of total amount of added Sr and grain refiner on the average pore size in three Al–Si based alloys.

Figure 14

Lognormal distributions of pore sizes in Al-7%Si-0.35%Mg (A356) alloy after 10- and 30-min holds: (a) for non-treated alloy; (b) with Al-5Ti-1B addition; (c) with Al-3B addition. In all cases, the alloy melts were degassed to avoid entrapment of oxide films.

Figure 15

Lognormal distributions of pore sizes for Al-15Sr addition after 10- and 30-min holds.

Figure 16

Variation in (a) porosity percentage and (b) density of porosity as a function of distance from the chill end.

Figure 17

Porosity distribution in 319 alloy containing (H = 0.1 mL/100 g Al, Sr = 250 ppm, Ti = 0.2%) as a function of distance from the chill end.

Figure 18

Porosity distribution in 319 alloy containing (H = 0.24 mL/100 g Al, Sr = 250 ppm, Ti = 0.2%) as a function of distance from the chill end.