Main Defects in Aluminum Alloy Castings and Analysis of Their Causes    During the production of aluminum alloy castings, the formation of various defects directly affects the mechanical properties, air tightness and qualification rate of products. Accurate defect identification and understanding of their root causes form the foundation for process optimization and quality control.

I. Porosity Defects

This category of defects mainly stems from gas entrapment in molten metal or solidification shrinkage, representing one of the most common issues in casting.

1. Gas Porosity

Gas Entrapment: Improper design of the gating system or excessively high injection speed causes turbulence in the molten metal, entrapping and encapsulating air within the mold cavity.

Poor Venting: Insufficient area, inappropriate location or blockage of mold vent grooves prevents the smooth discharge of gas from the mold cavity.

Gas Generation from Coatings: Excessive mold release agent or die coating vaporizes instantaneously upon contact with high-temperature molten metal, generating a large volume of gas.

Moisture in Raw Materials: Foundry returns, aluminum ingots or coatings contain moisture and are put into use without sufficient drying.

Insufficient Melt Degassing: Refining processes during smelting (e.g., rotary degassing) fail to effectively remove hydrogen dissolved in the molten aluminum.

Morphological Characteristics: Smooth inner walls of voids, presenting a round or oval shape. Usually detected via X-ray inspection or subsequent machining.

Formation Mechanism:

Gas Entrapment: Improper design of the gating system or excessively high injection speed causes turbulence in the molten metal, entrapping and encapsulating air within the mold cavity.

Poor Venting: Insufficient area, inappropriate location or blockage of mold vent grooves prevents the smooth discharge of gas from the mold cavity.

Gas Generation from Coatings: Excessive mold release agent or die coating vaporizes instantaneously upon contact with high-temperature molten metal, generating a large volume of gas.

Moisture in Raw Materials: Foundry returns, aluminum ingots or coatings contain moisture and are put into use without sufficient drying.

Insufficient Melt Degassing: Refining processes during smelting (e.g., rotary degassing) fail to effectively remove hydrogen dissolved in the molten aluminum.

2. Shrinkage Cavity and Shrinkage Porosity

Morphological Characteristics: Rough inner walls and irregular shapes of voids, often concentrated at hot spots, heavy sections or areas near ingates of castings.

Shrinkage porosity is a network structure composed of densely distributed micro shrinkage cavities.

Formation Mechanism:

Insufficient Feeding: Aluminum alloy undergoes approximately 6% volume shrinkage during solidification. Shrinkage cavities form if the solidifying zone cannot be supplemented with sufficient molten metal.

Out-of-Control Solidification Sequence: The ideal sequence involves directional solidification from regions farthest from the gate towards the gate. A reversed sequence results in isolated liquid zones in heavy sections with no access to feeding.

Defective Riser and Gating System Design: Unreasonable sizing, positioning or feeding channel design of risers fails to provide effective feeding pressure and pathways.

Improper Pouring Temperature: Excessively high pouring temperature increases the volumetric shrinkage of the metal and prolongs solidification time, hindering the establishment of effective temperature gradients for directional solidification.

Morphological Characteristics: Rough inner walls and irregular shapes of voids, often concentrated at hot spots, heavy sections or areas near ingates of castings. Shrinkage porosity is a network structure composed of densely distributed micro shrinkage cavities.

II. Surface Defects

These defects directly impact the appearance quality of castings, and their causes are mostly associated with the mold filling process.

1. Flow Marks and Cold Shuts

Excessively Low Mold or Molten Metal Temperature: Reduces the fluidity of molten metal and causes premature solidification of the flow front.

Insufficient Filling Speed: Low pouring or injection speed results in inadequate kinetic energy of the molten metal.

Excessive Resistance in Gating System: Poor gating system design leads to overly long filling paths or small cross-sectional areas.

High Mold Back Pressure: Poor venting increases gas pressure inside the mold cavity, impeding steady filling of molten metal.

Morphological Characteristics: Wavy streaks or distinct bonding marks on the casting surface, caused by incomplete fusion of two molten metal flow fronts.

2. Die Scratches and Soldering

Insufficient draft angle of the mold.

Damage or undercuts present on the mold cavity surface.

Unreasonable Ejection System Design: Uneven distribution, insufficient quantity or unbalanced force of ejector pins.

Excessive Casting Shrinkage Stress: Improper mold cooling system design generates excessive gripping force of the casting on cores or mold inserts.

Poor performance of mold release agent or inappropriate spraying process.

Morphological Characteristics: Linear scratches on the casting surface; in severe cases, casting material adheres to the mold cavity surface.

Formation Mechanism:

Insufficient draft angle of the mold.

Damage or undercuts present on the mold cavity surface.

Unreasonable Ejection System Design: Uneven distribution, insufficient quantity or unbalanced force of ejector pins.

Excessive Casting Shrinkage Stress: Improper mold cooling system design generates excessive gripping force of the casting on cores or mold inserts.

Poor performance of mold release agent or inappropriate spraying process.

III. Crack Defects

Cracks are severe casting defects, typically related to stress concentration and material properties.

1. Hot Cracking

Alloy Hot Brittleness: A wide solidification temperature range of the alloy, or precipitation of low-melting eutectic phases at grain boundaries (e.g., iron-rich phases in Al-Si alloys), significantly reduces high-temperature strength.

Improper Casting Structure Design: Abrupt changes in section thickness leading to stress concentration.

Restrained Shrinkage: Poor deformability of cores or molds generates excessive mechanical resistance during the solidification and shrinkage stage of castings.

Morphological Characteristics: Tortuous and irregular cracks with oxidized fracture surfaces due to high-temperature formation.

Formation Mechanism:

Alloy Hot Brittleness: A wide solidification temperature range of the alloy, or precipitation of low-melting eutectic phases at grain boundaries (e.g., iron-rich phases in Al-Si alloys), significantly reduces high-temperature strength.

Improper Casting Structure Design: Abrupt changes in section thickness leading to stress concentration.

Restrained Shrinkage: Poor deformability of cores or molds generates excessive mechanical resistance during the solidification and shrinkage stage of castings.

2. Cold Cracking

Excessive Residual Stress: Internal stress induced by non-uniform cooling exceeds the tensile strength of the material.

External Mechanical Impact: External forces applied during shakeout, cleaning or handling processes.

Morphological Characteristics: Straight or smoothly curved cracks with clean, unoxidized fractures. Occurs after castings cool to an elastic state.

Formation Mechanism:

Excessive Residual Stress: Internal stress induced by non-uniform cooling exceeds the tensile strength of the material.

External Mechanical Impact: External forces applied during shakeout, cleaning or handling processes.

IV. Inclusion Defects

Inclusions disrupt the continuity of the matrix and cause severe damage to material properties.

1. Slag Inclusions

Incomplete Melt Purification: Excessively vigorous stirring during smelting increases oxidation, or improper operation of refining, settling and slag skimming processes.

Failure of Slag Trapping Function in Gating System: Oxide scale or molten slag on the melt surface is entrapped into the mold cavity during pouring.

Morphological Characteristics: Non-metallic inclusions such as alumina and flux slag present inside or on the surface of castings.

Formation Mechanism:

Incomplete Melt Purification: Excessively vigorous stirring during smelting increases oxidation, or improper operation of refining, settling and slag skimming processes.

Failure of Slag Trapping Function in Gating System: Oxide scale or molten slag on the melt surface is entrapped into the mold cavity during pouring.

2. Hard Spots

Precipitation of Iron-Rich Phases: Excessive impurity elements such as Fe and Mn in the alloy form coarse hard and brittle intermetallic compounds (e.g., β-Al₅FeSi).

Morphological Characteristics: Abnormally bright particles on the surface after machining, leading to rapid tool wear.

Formation Mechanism:    Precipitation of Iron-Rich Phases: Excessive impurity elements such as Fe and Mn in the alloy form coarse hard and brittle intermetallic compounds (e.g., β-Al₅FeSi).

V. Conclusion

The formation of defects in aluminum alloy casting is a multi-factor and systematic problem. Fundamentally solving these issues requires establishing a complete system covering raw material control in smelting, mold design and manufacturing, optimization of casting process parameters, and full-process quality monitoring. Accurate morphological identification and root cause analysis of defects are prerequisites for implementing effective corrective and preventive measures.