Page 320 - Biaxial Multiaxial Fatigue and Fracture

304 El LINO ET AL.

Three factors can lead to the presence of porosity: shrinkage, coupled with a lack of
interdendritic feeding during mushy zone solidification, evolution of hydrogen gas bubbles due
to a sudden decrease in hydrogen solubility during solidification, and collapsed air [l, 21.
Aluminium diecasting alloys present very interesting properties, namely: good
machinability, low weight, low transformation cost with the possibility of obtaining complex
shapes, and, moreover, they are recyclable. However, they are very prone to present casting
defects. Although the recent developments in pressure diecasting industry (use of low injection
velocities, special feeding [2, 31, vacuum and “true isostatic pressure” [I]) and the use of
simulation processes contributed to the improvement of the aluminium cast parts quality
(possibility of structural parts production), it is almost impossible to avoid the presence of
defects in the parts that are supplied to the customers [4-61. Considering this, foundry
companies follow international standard criteria (for example, ASTM Standard E 505 [7]) and
also develop internal standards to classify the parts as acceptable or unacceptable. Frequently, a
location or size of one defect is not critical in one part, but is unacceptable in other type of
parts. This is especially important in structural parts, where the loading type during service can
conduct to fatigue initiation. Pressure aluminium diecasting alloys have extremely low ductility
(1-3%) [4, 81, which means that once initiated, cracks easily propagate until the failure of the
component [9 3.
In the light of the above, it is obvious the interest, emphasized by current studies [l, 5, IO],
in evaluating the effect of the presence of defects on fatigue life reduction of pressure
diecastings. For example, some authors have been trying to determine the relationship between
casting conditions and the amount of porosity in a casting. The majority of the models capable
of providing a qualitative description of the level of microporosity fail to give accurate values
because the prediction of microporosity requires a detailed understanding of pore nucleation
and growth in the melt [ 11.
In the aluminium pressure diecasting industry, the main type of defects susceptible of being
observed are oxidized surfaces, foreign material inclusions (oxide), gas cavities (hydrogen and
gas porosity), macroshrinkage (cavities and sponginess), and microshrinkage (feathery, sponge,
intercrystalline or interdendritic) [6, 1 I].
Gas pores, shrinkage pores and gas-shrinkage pores represent the main types of porosity that
are detected in diecastings. In gas pores, liquid aluminium reacts with water vapour in the
atmosphere to produce aluminium oxide and hydrogen gas. Gas porosity arises during the
solidification, due to the difference in solubility of hydrogen gas in liquid and solid aluminium.
If a casting is poorly fed during solidification, shrinkage will cause a hydrostatic stress in the
liquid metal. This stress increases until a pore forms with the aid of a nucleus. Gas-shrinkage
pores results from the fact that gas evolution and shrinkage occur in the same volume of liquid
metal at the same time [ 11.
In this paper, the effect of the presence of porosity on fatigue life reduction of aluminium
pressure diecastings is quantified. Such an effect will then be used in a software, under
development, which will be able to predict fatigue life of high strength castings [12]. One of
the advantageous of this study is the fact that all the fatigue tests are conducted in samples
removed from real components or in real components and not in samples pressure diecasted
separately in the metallic moulds. These last samples frequently have a section thickness that
does not represent the usual parts thickness obtained in pressure diecasting.