Hot shortness primarily indicates the formation of permanent cracks in the casting during a semisolid state. It’s closely associated with hot cracking or hot tearing. Nonetheless, this commonly encountered defect is a pivotal concern in a metal’s castability.
There are several factors involved in causing hot shortness in aluminum die casting. The problem is so severe that eventual scrapping of aluminum alloys can be necessary. Hence, having a comprehensive knowledge of this casting defect is a must.
This article sheds a brief light on the causes, effects, and prevention of hot shortness in die-casting parts.
Types of Hot Shortness
There are two specific types to categorize the overall defect :
- Liquidation cracking– The defective cracks occur at the grain boundary when the metal is in an enriched liquid phase.
- Surface hot shortness– It’s the penetration of oxidized residual elements on the subsurface into the grain boundary.
The grain boundary is difficult to define in easy terms. But the interacting interface between the solid state and the liquid state can be considered as the grain boundary.
Causes of Hot Shortness
The formation of hot shortness is complex due to the involvement of the internal fluidity mechanism. But the antagonists to trigger this issue are relatively simple, including –
- Imbalanced thermal contraction
- Unequal distribution of liquid film
- Abrupt drops in liquid pressure
- Non-uniform filling of the molds
Formation of Hot Shortness in Die Casting
Abrupt thermal contraction enables the rapidly solidifying liquid to form a thin surface film. Oxide films entrapped in the system continue to thicken, starting to act as a stress concentrator. They eventually become semisolid clusters at the grain boundary.
Sliding of the clusters causes the liquid film to break down or rupture. Rapid temperature changes in the solidifying metal eventually initiate the semisolid film to fall apart. And the diffusion leads the die cast to crack at certain points across the film cluster.
Effects of Hot Shortness
Some of the adverse effects associated with this defect:
- Weakness –These internal cracks mess with the expected structural strength. Parts become weak to handle minimal workloads. They can even become completely useless based on the geometry.
- Brittleness –Considerable brittleness is the most concerning effect of hot shortness. The aluminum is unlikely to bear loads or impacts. Instead, die-casts may break down into pieces like glass instantly.
- More Issues –Interior cracks are serious problems in die casting. They often lead to problems within the die-cast parts in favorable conditions. So, larger cracks to worsen the condition are also possible.
Avoiding Die-Cast Hot Shortness
Total elimination of hot shortness is quite unlikely as many complexities lie within the mechanism. Still, it’s easy to minimize the defect to a negligible level with proper precautions. These would include –
- Correct Alloy Composition– Some compounds take more time to solidify. And there are some other compounds to slow the solidification. These compounds must have a balanced proportion to ensure a uniform loss of heat.
- Proper Temperature Range– Defects featuring the term ‘hot’ refer to involvement in high temps, which occurs due to pressing thermal stress from exceeding heat. Maintaining an accurate and uniform temp control can effectively alleviate the issue.
- Uniform Feeding of Liquid– Disruptive insertion of molten metal liquid has been a great cause of initiating hot-shortness. It’s necessary to provide a consistent system to allow steadfast feeding through the gate across the mold.
Sunrise Metal is one of the leading manufacturers of aluminum alloy castings. It features all the top-notch facilities under skilled experts to produce flawless die-cast parts. You’re welcome to contact us, letting experienced professionals provide suggestions regarding your projects.