Hot Cracking

Hot cracking refers to the defective tears occurring at the later phases of die-casting solidification. It’s characterized by crooked cracks with irregular surface fractures. It’s one of the most common casting defects in malleable iron and steel castings, as well as some light alloy castings.

Despite shrinkage being the primary antagonist to cause the defect, several other factors contribute significantly to its appearance. These physical and mechanical reasons form surface and even interior tears. So, it’s necessary to review all the involved factors to diagnose this defect properly.

This article briefly discussed the characteristics, types, causes, effects, and remedial measures for hot cracks during a die-casting process.

Characteristics of Hot Cracking

Hot cracks may vary in length, color, shape, and depth. But they mostly have the following features –

• Oxidized color without any metallic luster.
• Irregular, uneven, and tortuous/tearing shape.
• Wide opening with gradually narrowing inside.
• Initiating and propagating along the grain boundary.
• Fractures can run through the entire casting.
• Dendrites often affect the cross-section.
• The interior crack doesn’t extend to the surface.

Differentiating hot cracks from cold ones is possible by looking into the fracture pattern and tear characteristics. The torn surface often appears black on steel or iron castings, whereas aluminum die castings have greyish tears.

Types of Hot Cracks

Depending on the location of the hot tears or crack on castings, the defect may be divided into the following types:

1. Outer Cracks:Cracks visible on the casting surface are the exterior ones. They mainly occur at the corners or edges, sections with sharp thickness change, and slower modification process. External tears may appear distributive at the grain boundary.
2. Inner Cracks:Inner cracks are formed within the casting and require more sophisticated inspection methods for detection. Highly irregular interior tears may occur at the final solidification stages.

Causes of Hot Cracking

Several theoretical and practical reasons lead to the formation of hot tears or cracks, most notably –

1. Internal Tension: More time for the mold to solidify develops internal tension, leading to cracks. The failed contraction intensifies the tension as the mold holds the molten but solidifying cast.
2. Sharp Corners: If an inside corner comes last to solidify, that spot remains hotter than other plain spots. And the structure becomes vulnerable at that sharp edge, resulting in eventual cracks.
3. Shrinkage Porosity: Spots too hot to solidify evenly close to surfaces develop shrinkage porosity. It’s a common error in the die-casting process to cause surface fractures or external tears.
4. Thermal Control: The thermal control shouldn’t affect the cast’s dwelling time, specifically for thin-wall specimens. Poor cooling leads to excessive contraction, contributing to warm cracks.
5. Die Movements: Negligible die shifts are difficult to detect but occur when the dies are separated. Several factors, individually or together, can trigger cracks due to faulty dragging force.
6. Uneven Ejection: Drags or sticking issues with the ejection system while moving the casting out can twist the solid. This initiates uneven ejection, causing cracks on upper or non-sided surfaces.
7. Flowing Patterns: The casting process with flow patterns may have a thin oxide layer as a surface defect, mostly in aluminum alloys. This layer hinders the solidifying process, causing fractures.
8. Incorrect Proportion: The incorrect proportioning range of the minor alloy constituents triggers uneven solidifying. Excessive modifiers do the same, contributing to hot tears during casting.
9. Excessive Variation: Variations in the die casting process are often used for desired outcomes. However, excessive distortion may form hot cracks in castings for unaccounted factors.

Spots Prone to Hot Cracking

Shrinkage stress remains concentrated in specific regions of a casting mentioned below. These spots mostly experience hot cracks during the process.

• Uneven wall thicknesses
• Small interior angles
• Large linear shrinkage
• Lap joint bifurcations
• Larger or smaller riser

Effects of Hot Cracking in Casting

These solidification fractures negatively affect some of the top casting requirements, including –

1. 1. Poor Aesthetics: Exterior hot tears can effectively diminish the overall aesthetics of the casting. Surface imperfections may lower or even discard the value in certain industries.
   2. Compromised Strength: Hot cracks in castings often have serious structural integrity issues. The affected section easily fails on shear or torque, concerning its large-scale usability.
   3. Improper Sealing: Cracks on thinner walls provide minimal sealing protection for specific uses. Castings for hydraulics or automotive become worthless due to severe leakage.
   4. Increased Waste: Furcated or branched fractures lead to the complete dismay of the die-cast parts. It not only contributes to further production costs but also extra metal waste.

Preventing Hot Tears

Multiple mechanical and metallurgic countermeasures exist to avoid hot tears in die casting. But, adopting the following precautions can reduce the possibility by a significant margin.

1. Improved Casting Mold –Design improvements like providing uniform wall thickness, and avoiding sharp corners can reduce stress concentration in the casting that could form hot cracks.

2. Superior Alloy Melting –Mechanical processes like degassing or refining can exclude oxides or gases. Controlling harmful impurities can reduce cracks.

3. Correct Casting Process –The exact measurements can initiate and maintain proper cooling and subsequent solidification, lessening fractures. It primarily includes –

• Minimal dwelling or holding time to remove the casting from the mold (die).
• Properly designed thermal control to exert no effect on the dwelling time.

4. Proper Alloy Constituents –Iron (Fe), Copper (Cu), and Silicon (Si) should be in the perfect range for aluminum alloys without excessive modifiers.

5. Slowing Down the Process –Careful observation can prevent the formation of cracks. And slowing down the process often helps detect irregularities.

6. Instant Fixing Actions –Putting pressure on hotter spots can reverse the effects. Heating the surrounding cold spots may speed up the recovery as well.

It’s not always possible to craft crack-free casting all the time. Still, preventive measures can provide excellent results in controlling such defects. However, the actual result depends on the alloy chosen and the mold (die) used.