L18: Solidification defects

Learning Outcomes:

• Explain the influence of the solidification mode of an alloy on the distribution of porosity.

• Derive the shape of the shrinkage pipe formed in a pure metal solidifying in a cylindrical mould.

• Understand why the liquid becomes supersaturated in dissolved gas during solidification, and how this leads to porosity.

• Describe the origin of common types of macro segregation.

Influence of Solidification Mode on Porosity Distribution

• Key Concept: The way an alloy solidifies (planar, columnar, or equiaxed) affects where and how porosity forms.

• Planar Growth:

  • Porosity forms at the top of the casting.

  • Caused by shrinkage as the solidification front moves downward.

• Equiaxed Growth:

  • Porosity forms in interdendritic spaces (between dendrites).

  • Results in microporosity due to isolated liquid pockets shrinking.

• Columnar Growth:

  • Porosity forms between columnar grains.

  • Caused by shrinkage and gas evolution, leading to macropores.

• Heat Flow Direction:

  • Vertical heat flow: Porosity forms at the top.

  • Radial heat flow: Porosity forms in a parabolic shape.

  • Lateral heat flow: Porosity forms along the sides.

2. Shape of the Shrinkage Pipe in a Cylindrical Mould

• Key Concept: When a pure metal solidifies in a cylindrical mould, a shrinkage pipe forms due to volume contraction.

• Derivation:

  • Use conservation of mass: Mass loss from the top = mass gain in new solid.

  • Formula for vertical heat flow:

    

    Where:

    • h = height of the shrinkage pipe.

    • ρs​ = solid density.

    • ρl​ = liquid density.

    • s = solidification distance.

  • For radial heat flow, the shrinkage pipe shape is parabolic:

    

    Where:

    • r = radius.

    • r0​ = initial radius.

3. Supersaturation of Dissolved Gas and Porosity Formation

• Key Concept: Metals dissolve more gas in the liquid state than in the solid state, leading to gas porosity.

• Process:

  1. During solidification, gas (e.g., hydrogen) is partitioned into the remaining liquid.

  2. The liquid becomes supersaturated with gas.

  3. When supersaturation exceeds a critical level, gas bubbles nucleate, forming pores.

• Combined Effects:

  • Shrinkage porosity: Caused by volume contraction during solidification.

  • Gas porosity: Caused by gas evolution.

  • Together, they create voids in the solidified metal.

4. Origin of Common Types of Macro segregation

• Key Concept: Macro segregation refers to compositional inhomogeneities at the scale of the casting (mm or larger).

• Types and Causes:

  1. Gravity-Induced Segregation:

     • Denser solid phases settle under gravity.

     • Results in lower solute content at the bottom of the casting.

  2. Solidification Shrinkage-Induced Segregation:

     • As columnar grains grow, shrinkage causes liquid to be sucked toward the mould wall.

     • Leads to higher solute content near the surface.

  3. Convection-Induced Segregation:

     • Thermal and solute convection in the liquid redistributes solute.

     • Causes macro segregation patterns.

• Extreme Case: In some castings, solute-rich liquid can accumulate at the top, creating a strong compositional gradient.

Additional Notes for Revision

• Porosity Avoidance:

  • Degassing: Bubble argon gas through the liquid to reduce hydrogen content.

  • Feeders: Use feeders to supply liquid and compensate for shrinkage.

  • Design: Ensure porosity forms outside the casting (e.g., in runners or feeders).

• Thermo-Mechanical Effects:

  • Solidification defects (porosity, macro segregation, cracking) are influenced by:

    • Thermal contraction.

    • Gas evolution.

    • External loads (e.g., in high-pressure die casting).