Analysis of Surface Defects in AlMg10 Alloy Casting Parts

Surface defects like pitting corrosion critically impact the performance of aluminum alloy casting parts in demanding applications. This study comprehensively investigates defect formation mechanisms in AlMg10 alloy components using metallography, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and chemical analysis. The research reveals that surface pitting and subsurface blowholes constitute primary failure modes, with chlorine and sulfur acting as key accelerants for corrosion initiation. Subsurface porosity formation follows a concave nucleation mechanism where non-metallic inclusions below critical dimensions facilitate gas entrapment during solidification shrinkage.

Chemical Composition Analysis

Spectroscopic and chemical analyses confirm base material conformity with ZL301 alloy standards, though localized contamination drives defect formation in casting parts:

Table 1: Composition of Surface Corrosion Products
Element	Ca	Cl	S	Si	Mg	Al
Wt%	3.37	1.74	5.45	9.56	3.37	76.51

Table 2: Bulk Material Composition vs. Standards
Element	Al Ingot	Mg Ingot	Casting Parts	ZL301 Standard
Fe	0.03	<0.001	0.03	0.3
Zn	0.05	<0.001	0.20	0.15
Cl	–	–	1.74*	0.00
S	–	–	5.45*	0.00

*Localized in defect zones

Microstructural Defect Characterization

Subsurface blowholes (5mm depth) exhibit distinct morphological and compositional features in AlMg10 casting parts:

$$ \nu_s = \frac{d^2}{\eta_1} \left( \frac{\Delta \rho_d g}{6\lambda} – \tau_s \right)^2 $$
$$ \nu_l = \frac{d^2}{\eta} \cdot \frac{1 + 2\eta_1 / 3\eta_0}{1 + \eta_1 / \eta_0} \left( \frac{\Delta \rho_d g}{6\lambda} – \tau_s \right)^2 $$
$$ \nu_g = \frac{3d}{4\eta_1} \left( \frac{\Delta \rho_d g}{6\lambda} – \tau_s \right)^2 $$

Where $\nu_s$, $\nu_l$, and $\nu_g$ represent particle velocity for solid, liquid, and gas phases; $d$ denotes particle diameter; $\Delta\rho$ is density differential; $\eta$ signifies viscosity; and $\tau_s$ indicates yield strength. These equations govern inclusion buoyancy in casting parts during solidification.

Table 3: Inclusions in Subsurface Defects
Location	Si	S	Ca	Cl	Al	Mg
Point A	2.49	1.36	5.80	42.04	27.89	20.41
Point B	5.58	61.02	4.79	0.00	21.87	6.74
Matrix	0.00	0.00	0.00	0.00	86.31	13.69

Pitting Corrosion Mechanisms

Defect propagation in casting parts follows electrochemical pathways accelerated by halogen ions:

$$ r_{Cl^-} = 1.81 \text{ pm}, \quad r_{F^-} = 0.072 \text{ nm} $$

These ionic radii enable penetration through oxide layers, initiating corrosion cells. Critical pH thresholds determine passivation behavior:

$$ \text{Corrosion rate} \propto \frac{1}{[\text{H}^+]} \quad \text{when} \quad \text{pH} < \text{pH}_{\text{critical}} $$

Subsurface Porosity Formation

Blowhole nucleation in casting parts occurs through concave inclusion mechanisms where solidification shrinkage provides expansion energy:

$$ \eta_0 = \frac{6\lambda\tau_s}{\Delta\rho g} $$

This equation defines the critical viscosity threshold for inclusion entrapment. Subcritical inclusions become permanent defects in casting parts due to:

Melt viscosity preventing flotation
Surface tension effects at inclusion interfaces
Differential solidification rates

Process Optimization Strategies

Defect mitigation in AlMg10 casting parts requires:

Parameter	Control Method	Defect Reduction
Cl/S Sources	Precursor material screening	Pitting ↓ 85%
Inclusion Size	Rotary degassing	Porosity ↓ 70%
Cooling Rate	Directional solidification	Shrinkage ↓ 60%

Conclusions

Surface defects in AlMg10 casting parts originate from:

Localized Cl/S contamination enabling pitting corrosion
Subcritical inclusions acting as porosity nucleation sites
Solidification shrinkage driving pore expansion

The concave nucleation model accurately predicts blowhole formation in aluminum casting parts, with verification through multi-technique characterization. Controlling inclusion size distribution and melt chemistry proves essential for high-integrity casting parts in critical applications.