Effect of Gas Bubbling Filtration Treatment on Microporosity Variation in A356 Aluminium Alloy

doi:10.1007/s40195-016-0434-x

Abstract

Abstract:

In the present study, the contribution of the gas bubbling filtration (GBF) process to the microporosity variation, microstructural characteristics and tensile properties of A356 aluminium alloy was investigated. The test specimens were fabricated through gravity casting in terms of the process variables: the degassing time, the impeller rotation and the aperture size of gas inlet hole. The density measurement and scanning electron microscope fractography analyses were conducted to evaluate the variation of the volumetric porosity and fractographic porosity with the GBF process, respectively. The fractographic porosity of the specimens can be minimised under specific GBF conditions in terms of the buoyant velocity and the absorbing capacity of gas bubbles, the inclusion of oxide films, whereas the volumetric porosity can be wholly reduced on the lapse of degassing time. The ultimate tensile strength (UTS) and elongation at optimal conditions were improved to approximately 30 MPa and 1.5% compared with no GBF treatment. Even though an extension of the degassing time and/or excessive stirring action of the melt may induce the inclusion of bifilm oxides and the increase of fractographic porosity, the tensile properties of over-treated specimens were maintained to a level which is similar to those that did not undergo GBF treatment due to the grain refinement accompanying with the GBF process. In addition, the defect susceptibility of UTS and elongation to microporosity variation could be remarkably improved at an optimal GBF condition.

Key words: Aluminium alloy, Microporosity, Gas bubbling filtration process (GBF), Tensile property

Choongdo Lee, Taeil So, Kwangseon Shin. Effect of Gas Bubbling Filtration Treatment on Microporosity Variation in A356 Aluminium Alloy[J]. Acta Metallurgica Sinica (English Letters), 2016, 29(7): 638-646.

Figures/Tables 14

Table 1 Chemical composition of A356 aluminium alloys

Si	Mg	Mn	Cu	Fe	Ti	Sr	Al
7.3	0.33	0.055	0.09	0.291	0.053	0.02	Bal.

Fig. 1 Dimensions of crucible and impeller used for melt treatment

Fig. 2 Typical microstructural views of gravity-casted A356 alloys after GBF treatment; a no degassing and b 5-min degassing treatment

Fig. 3 Average SDAS value with the lapse of degassing time for GBF treatment of A356 alloy

Table 2 Typical values for microstructural change on GBF treatment

Aperture/GBF time		Grain size (μm)	SDAS (μm)	Eutectic Si particles
Aperture/GBF time		Grain size (μm)	SDAS (μm)				Area fraction (%)	Circularity	Aspect ratio
No GBF		225.3	41.1	18.4	0.92	1.93
2 mm diameter	5 min	196.1	37.4	18.3	0.76	2.19
	10 min	193.8	31.3	17.6	0.74	2.21
	15 min	185.2	23.2	16.3	0.73	2.28
1 mm diameter	5 min	194.5	34.6	18.3	0.75	2.09
	10 min	185.6	29.6	18.1	0.74	2.21
	15 min	178.6	20.3	16.9	0.73	2.17

Fig. 4 Variation of volumetric porosity with the degassing time

Fig. 5 Variation of fractographic porosity measured in the tensile fractured surface on degassing time

Fig. 6 Variation of tensile strength and elongation with the degassing time under difference aperture sizes of the gas inlet of 1 mm diameter a and 2 mm diameter b

Fig. 7 SEM images of the fractured specimens; no GBF a, high magnification b, c for A- and B-positions in a, 10-min GBF treatment with impeller of 2-mm-dia. aperture d and spectral results e, f of EDS analysis for A- and B-positions in a

Fig. 8 Comparison of the porosity a and the tensile properties b with melt stirring by rotation of impeller

Fig. 9 Defect susceptibility of the tensile strength a and elongation b for the microporosity variation with regard to the GBF treatment conditions

Table 3 Defect susceptibility of the tensile properties to microporosity variation and maximum values of the tensile properties achievable under defect-free conditions

Treatment	Defect susceptibility		Maximum values achievable in defect-free condition (f = 0)		Strain-hardening exponent
Treatment	UTS	Elongation	UTS (MPa)	Elongation (%)	Strain-hardening exponent
No GBF	1.03	5.27	190.2	3.7	0.23
10 min (2 mm diameter)	0.42	3.72	209.8	5.1	0.26
5 min (1 mm diameter)	0.43	2.38	212.3	5.1	0.25

Table 4 Variation of typical chemical element with GBF treatment

Treatment	Si	Mg	Fe	Ni	Cr	Al
No GBF	7.31	0.33	0.291	0.042	0.026	Bal.
15-min GBF	7.25	0.43	1.08	0.178	0.171	Bal.

Fig. 10 Comparison of XRD patterns between the GBF and non-GBF-treated alloys

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