Abstract
Porosity occurs in cast solidifying metals and alloys due to negative pressures generated during solidification contraction, and pressure developed by gases dissolved in the motten metal. Both the above processes may act either together or separately to produce such shrinkage or gas defects (collectively termed pores). They are generally unwanted and constitute a major industrial problem. This paper is an attempt to review up-to-date knowledge of the conditions of pore formation in cast metals and alloys. Various mechanisms responsible for pore nucleation and growth are summarized, and experimentally evaluated using an unfed type of mould with aluminium alloy castings. The observations are in support of a non-nucleation mechanism of pore formation playing a major role in the occurrence of such defects in cast metals. Further, in gas-containing alloy melts the critical amounts of gas required for single and multiple pore nucleation have been determined quantitatively and are listed in the text. The gas contents of the melts were measured using an apparatus based on the “first bubble technique”. It is also experimentally observed that under poor feeding conditions more than one of the non-classical nucleation mechanisms may be functional at the same time for the formation of such defects.
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Abbreviations
- P e :
-
Sum of external forces which tend to collapse a pore
- P h :
-
Hydrostatic pressure
- P g :
-
Internat gas pressure in liquid metal
- P a :
-
Atmospheric pressure applied during solidification
- P s :
-
Shrinkage pressure
- γ:
-
Surface tension at gas-metal interface
- P f :
-
Fracture pressure of liquid metal
- 2γ/r :
-
Surface tension resistance
- V i :
-
Volume of initial gas content of liquid Metal
- P i :
-
Total internal pressure due to gas and/or shrinkage
- K s,K L :
-
Gas solubility constants of solid and liquid metal, respectively
- K′:
-
Ks/KL
- r :
-
Radius of liquid channel
- R :
-
Radius of cylindrical casting
- L :
-
Length of liquid channel
- L c :
-
Length of cylindrical casting
- f 1 :
-
Fraction of liquid left in the casting of pure and short freezing-range alloy, πr 2 L/πR 2 L c
- V i * :
-
Critical gas content
- t :
-
Tortuosity of channel
- n :
-
Number of channels (approximately number of dendrite arms per unit area)
- f L :
-
Fraction of liquid left in the casting of mushy freezing alloys, πr 2 nt/πR 2
- B :
-
K Lπnt(1 -K′)
- nb i 2 :
-
B2 (approximately)
- r*:
-
Critical radius for pore nucleation (pore belowr* size cannot exist)
- σ:
-
Surface tension of liquid
- N :
-
Avogadro's number
- k :
-
Boltzmann constant
- h :
-
Planck's constant
- T :
-
Temperature (K)
- a,b :
-
Inner and outer radii of solidified shell
- Y :
-
Uniaxial yield stress
- η, ηs :
-
Energy per unit area of solid-liquid and solid-vapour interface, respectively
- h :
-
Thermal diffusivity of solid metal
- Cp :
-
Specific heat of solid metal
- m,K 1,K 2 :
-
Constants
- α:
-
Fractional contraction of liquid phase on solidification [(solid density — liquid density)/solid density]
- α′:
-
α/(1 −α)
- Γ:
-
Vortex circulation factor
- d :
-
Density of liquid
- μ:
-
Metal viscosity
- h 1 :
-
Thermal conductivity of mould
- H :
-
Heat of fusion of metal
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Gupta, A.K., Saxena, B.K., Tiwari, S.N. et al. Pore formation in cast metals and alloys. J Mater Sci 27, 853–862 (1992). https://doi.org/10.1007/BF01197634
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DOI: https://doi.org/10.1007/BF01197634