Cold cracks refer to the separation or rupture in die casting due to dynamic or mechanical overload. This defect primarily occurs in cold chamber die casting when the liquid metal alloy within the mold collapses. The buildup of tensile stress during room-temperature cooling triggers the problem.
A cold crack in casting differs considerably from warm, solidification, or hot cracks. This article briefly outlined the characteristics, causes, effects, and prevention of cold cracks.
Characteristics of Die-casting Cold Cracks
Prime characteristics to identify die-casts with a cold crack include –
- A straight or somewhat broken line of cracks with uniform width.
- Cleavages to separate the material compound are clearly visible.
- Surfaces feature a mild oxidation color or a pure metallic luster.
- Considerably smooth trend in cracking along the grain boundary.
- An entirely visible rupture or fracture delimits the sharp edges.
- Eventual propagation of cracks through the entire cross-section.
- Noteworthy transgranular growth in macroscopic morphology.
- Sharp tips without decarburization or oxidization on either side.
Causes of Cold Cracks in Die Casts
The development of tensile stress at certain points is the primary cause of this issue. But the reasons to initiate the stress buildup include –
- Defective Casting Structure –Non-uniform wall thickness leads to obstruction in rigidity to trigger thermal stresses. Temporary shrinkage will occur to exceed the tensile stress, initiating cold cracks.
- Inefficient Riser System –Improper riser settings force the gate to solidify before the molten alloy or metal. Its inward shrinkage sparks a considerable increase in tensile stress, ultimately causing the defect.
- Improper Cooling Condition –Distinction in cooling speeds across the die slows down the solidification in some parts. The aggravated speed results in excessive thermal stresses to develop a cold crack.
- Poor Chemical Composition –High carbon content in metal alloys, especially in steel, often leads to this defect. Brittle elements, Phosphorus (P), and anti-graphitization particles aggravate cold cracks.
- Poor Parameter Control –Removing the cast too early or excessive drop in temperature during sand dropping compromises the impact resistance. And the casting eventually get ruptures.
Effects of Die Cast Cold Cracks
Dealing with the defect isn’t easy as it comes with some of the most disadvantageous setbacks –
- Damage in structural integrity due to the continuity of cracks.
- Loss in aesthetics with visible outer fractures of certain colors.
- Reduced mechanical strength from the separated portions.
- High increase in production cost over remelting and remaking.
- Future worsening of the defect triggers additional problems.
Preventing Cold Cracks in Die Casting
It’s possible to avoid forming such cracks on casts as the affected ones are unusable. The following countermeasures can help limit the extent of the problem to zero –
- Uniform Wall Thickness –Maintaining an even thickness throughout the casting wall prevents many undesired defects, including cold cracks. Therefore, sharp edges to feature a thick transition should have a detailed design to minimize the effect.
- Consistent Cooling Speed –An even cooling rate across the casting also suppresses unwanted issues like a cold crack. A good gate system with satisfactory wall thickness can ensure controlled temperature during cooling.
- Proper Chemical Composition –Keeping the chemical percentages to a perfect range is difficult due to its direct effect on properties. Still, avoiding elements initiating excessive stress, brittleness, or shrinkage requires consideration from the beginning.
- Casting Measure Inspection –Additional measurements like shrinkage constraint, sharp transition, recommended temp range, etc., need an inspection. Keeping these production measures within the preferred limits during casting can eliminate cracks.
Every step during production should be closely monitored to control cold cracks. From the design to the casts’ solidifying, maintaining the maximum standard and accuracy is the key to confronting the defect.