Causes and Process Improvement Measures for Shrinkage Porosity, Shrinkage Cavities, and Cracks in Cast Aluminum-Nickel-Cobalt (AlNiCo) Magnet Rough Parts

1. Introduction

Aluminum-nickel-cobalt (AlNiCo) alloys are widely used in permanent magnets, sensors, and precision instruments due to their excellent magnetic properties, high Curie temperature, and good thermal stability. However, during the casting process, defects such as shrinkage porosity, shrinkage cavities, and cracks often occur, severely affecting the mechanical properties, magnetic performance, and yield of the rough parts. This article systematically analyzes the root causes of these defects and proposes targeted process improvement measures to provide technical support for high-quality AlNiCo casting production.

2. Causes of Defects

2.1 Shrinkage Porosity and Shrinkage Cavities

Shrinkage porosity and shrinkage cavities are internal voids formed during the solidification of AlNiCo alloys due to insufficient liquid metal feeding. Their formation mechanisms and influencing factors are as follows:

2.1.1 Solidification Shrinkage Characteristics

AlNiCo alloys exhibit a wide solidification range (liquidus-solidus temperature difference), leading to a prolonged mushy zone during solidification. During this period, dendritic arms form, blocking the feeding channels and preventing the liquid metal from compensating for volumetric shrinkage, resulting in dispersed shrinkage porosity or centralized shrinkage cavities.

2.1.2 Inadequate Riser Design

• Insufficient Riser Volume: The riser fails to store enough liquid metal to compensate for solidification shrinkage.
• Improper Riser Location: The riser is not placed at the hot spot (the last solidification region), leading to localized feeding failure.
• Premature Riser Solidification: The riser solidifies before the casting, cutting off the feeding path.

2.1.3 Unreasonable Casting Structure

• Thick-to-Thin Transitions: Sharp changes in section thickness cause localized rapid cooling, forming hot spots that are prone to shrinkage defects.
• Sharp Corners and Fillets: Stress concentration at sharp corners inhibits liquid metal flow, exacerbating shrinkage porosity.

2.1.4 Improper Pouring Parameters

• Low Pouring Temperature: Reduces the fluidity of the liquid metal, impairing feeding efficiency.
• High Pouring Speed: Causes turbulence and air entrapment, leading to gas-assisted shrinkage porosity.
• Short Holding Time: Insufficient time for gas and inclusions to float, increasing the likelihood of porosity.

2.1.5 Mold Cooling System Defects

• Uneven Cooling Rate: Excessive temperature gradients between different sections of the casting induce thermal stress, promoting shrinkage cavity formation.
• Insufficient Cooling in Thick Sections: Slow cooling in thick regions prolongs the mushy zone, increasing shrinkage porosity risk.

2.2 Cracks

Cracks in AlNiCo castings are primarily caused by thermal stress or mechanical stress exceeding the material's strength during solidification or cooling. The main types and causes are:

2.2.1 Hot Tears (Thermal Cracks)

• Formation Mechanism: Occur during the late stages of solidification when the casting has limited ductility but is still subject to tensile stress due to uneven cooling or mold restraint.
• Influencing Factors:
  • Wide Solidification Range: Prolongs the mushy zone, increasing susceptibility to hot tearing.
  • High Thermal Expansion Coefficient: Amplifies thermal stress during cooling.
  • Improper Mold Restraint: Excessive friction or pressure from the mold restricts shrinkage, inducing cracks.
  • Sharp Corners and Thin Walls: Cause stress concentration, promoting crack initiation.

2.2.2 Cold Cracks

• Formation Mechanism: Occur after solidification due to residual stress from uneven cooling or external mechanical loads.
• Influencing Factors:
  • High Residual Stress: Caused by rapid cooling or improper heat treatment.
  • Low Ductility: The presence of brittle phases (e.g., excessive carbides) reduces crack resistance.
  • Mechanical Impact: During ejection or handling, localized stress exceeds the material's strength.

3. Process Improvement Measures

3.1 Optimizing Riser Design

1. Riser Volume and Location
   • Use numerical simulation (e.g., MAGMAsoft, ProCAST) to accurately predict the last solidification region and place the riser accordingly.
   • Increase the riser volume by 10–20% compared to theoretical calculations to ensure sufficient feeding.
   • Adopt side risers or multiple risers for complex castings to improve feeding efficiency.
2. Riser Type Selection
   • Use exothermic or insulating risers to delay solidification and extend the feeding time.
   • For thick-section castings, consider pressure-assisted feeding systems to enhance liquid metal flow.
3. Riser Neck Design
   • Optimize the riser neck dimensions to balance feeding pressure and solidification time. A narrow neck can reduce feeding resistance but may solidify prematurely, while a wide neck ensures feeding but may reduce yield.

3.2 Improving Casting Structure

1. Wall Thickness Uniformity
   • Avoid sharp changes in section thickness; use gradual transitions (e.g., fillets with radii ≥ 5 mm) to reduce thermal gradients.
   • For thick sections, incorporate internal chills or cooling ribs to accelerate solidification and minimize hot spots.
2. Stress Relief Features
   • Add stress-relief grooves or ribs at sharp corners to distribute stress and prevent crack initiation.
   • Use hollow or ribbed structures to reduce mass and improve cooling uniformity.
3. Gating System Optimization
   • Design the gating system to ensure smooth liquid metal flow with minimal turbulence.
   • Use tapered runners and gates to control flow velocity and prevent air entrapment.
   • Position gates at thick sections to promote directional solidification toward the riser.

3.3 Controlling Pouring Parameters

1. Pouring Temperature
   • Maintain an optimal pouring temperature (typically 10–20°C above the liquidus) to ensure good fluidity without excessive shrinkage.
   • For AlNiCo alloys with high nickel content, slightly higher temperatures may be required to compensate for their high viscosity.
2. Pouring Speed
   • Use a moderate pouring speed (0.5–1.0 m/s) to avoid turbulence and air entrapment.
   • For large castings, adopt a multi-stage pouring technique to gradually fill the mold and reduce thermal shock.
3. Holding Time
   • Allow sufficient holding time (3–5 minutes) in the ladle for gas and inclusions to float before pouring.
   • Use argon shielding or covering agents to prevent oxidation during holding.

3.4 Enhancing Mold Cooling

1. Cooling Channel Design
   • Incorporate conformal cooling channels in the mold to achieve uniform cooling rates across the casting.
   • Use water-cooled inserts or external cooling plates for thick sections to accelerate solidification.
2. Thermal Insulation and Chills
   • Apply thermal insulation coatings to thin sections to slow cooling and balance thermal gradients.
   • Use external chills (e.g., copper or steel inserts) at thick sections to promote rapid solidification and reduce shrinkage porosity.
3. Mold Material Selection
   • Choose mold materials with high thermal conductivity (e.g., H13 steel) for thin sections to enhance heat dissipation.
   • For thick sections, use materials with lower thermal conductivity (e.g., graphite) to slow cooling and reduce hot tearing risk.

3.5 Reducing Thermal Stress

1. Controlled Cooling Rates
   • Implement a slow cooling rate (≤ 5°C/min) during the solidification range to minimize thermal gradients.
   • Use furnace cooling or insulating blankets to maintain uniform temperature distribution.
2. Stress Relief Heat Treatment
   • Perform a stress-relief annealing treatment (e.g., 500–600°C for 2–4 hours) after solidification to reduce residual stress.
   • For large castings, consider a multi-stage annealing process to gradually relieve stress without inducing new cracks.
3. Mold Restraint Minimization
   • Design the mold with sufficient draft angles (≥ 1°) to facilitate easy ejection and reduce mechanical stress.
   • Use ejector pins with appropriate size and location to distribute ejection forces evenly.

3.6 Material and Melting Process Control

1. Chemical Composition Optimization
   • Adjust the nickel and cobalt content to narrow the solidification range and improve feeding efficiency.
   • Limit the content of impurities (e.g., sulfur, phosphorus) that promote hot tearing.
2. Melting Practice
   • Use dry and clean charge materials to reduce hydrogen pickup and porosity.
   • Employ degassing techniques (e.g., rotary impeller degassing) to remove dissolved gases before pouring.
   • Control the melting temperature to avoid excessive oxidation and nitrogen absorption.
3. Grain Refinement
   • Add grain refiners (e.g., titanium or boron) to promote equiaxed grain formation, which improves feeding and reduces hot tearing susceptibility.
   • Use electromagnetic stirring during melting to achieve a uniform grain structure.

4. Case Study: Process Improvement for an AlNiCo Magnet Casting

A manufacturer of AlNiCo permanent magnets encountered severe shrinkage porosity and hot tearing in a complex-shaped casting. The original process used a single riser with inadequate volume, and the mold lacked cooling channels, leading to uneven cooling and high residual stress.

Improvement Measures:

1. Riser Redesign: Replaced the single riser with two side risers of increased volume, positioned at the hot spots identified by simulation.
2. Cooling System: Added conformal cooling channels in the mold to achieve uniform cooling rates across the casting.
3. Pouring Optimization: Adjusted the pouring temperature to 10°C above the liquidus and reduced the pouring speed to 0.7 m/s.
4. Stress Relief: Implemented a stress-relief annealing treatment at 550°C for 3 hours after solidification.

Results:

• Shrinkage porosity was reduced by 80%, and hot tearing was eliminated.
• The yield of acceptable castings increased from 65% to 92%.
• The magnetic properties of the final product improved due to the reduced defect density.

5. Conclusion

Shrinkage porosity, shrinkage cavities, and cracks are common defects in AlNiCo castings, primarily caused by inadequate feeding, thermal stress, and improper process parameters. By optimizing riser design, improving casting structure, controlling pouring parameters, enhancing mold cooling, reducing thermal stress, and refining material and melting practices, these defects can be significantly reduced or eliminated. Numerical simulation tools and systematic process optimization are key to achieving high-quality AlNiCo castings with improved mechanical properties and magnetic performance.