A hot crack (also referred to as warm crack) is a solidification crack occurring towards the end of solidification under the effect of tensile stress caused by shrinkage.
Hot cracks generally only appear during the later stages of the solidification process when only a low amount of residual melt is present between the solidified casting grains (crystallites) and at the same time shrinkage-related stresses act upon the residual melt zones. These types of stresses may occur if casting sections impair their mutual shrinkage associated with solidification of the casting or in case that cores or mold parts prevent free shrinkage.
A characteristic feature of hot cracks is that their patterns are often branched and furcated Typical patterns on the warm crack rupture surface are the dendritic structures in the opened warm crack shoulders and the rupture surfaces. The dendrite tips are easily visible since it is typical for the surface to crystallize freely out of the molten metal.
A schematic model of this warm crack process according to J. Campbell is illustrated in Figure 1 and shows the importance of the amount of residual melt present and its flowability with regard to occurrence and prevention of warm cracks.
Consequently, alloys with relatively short solidification intervals and great amounts of residual eutectic present, which is capable of sealing gaps or incipient cracking due to its good flowability, display favorable warm crack resistance. In contras to that, alloys with a wide solidification interval generally have a greater tendency towards warm cracking (e.g. Al Cu4Ti or Al-Mg alloys).
Formation of warm cracks always takes place prior to the metal reaching its solidus temperature. In case of inadequate post-feeding of solidifying melt (i.e. risers in sand casting or ingotcasting dimensioned too small or final pressure in die casting too low) warm crack formation commences at slightly higher temperatures during the solidification interval. If sufficient post and seal feeding is ensured, hot cracks only occur shortly before reaching the solidus temperature.
Moreover, occurrence of warm cracks always is a matter of component design as well (s. Favorable casting design, Design of castings). Aside from factors such as chemical composition and solidification morphology (solidification type) warm cracks may only occur if tensions occur after reaching the coherent condition (after formation of a dendrite network at the dendrite coherence point). It is particularly the sections with increased material accumulation (e.g. thick walls) and delayed solidification that are the preferred starting points for warm cracking.
Accordingly, it is possible to avoid cracking by re-designing the casting and through cooling the casting mold. Low melting temperatures and additives for grain refinements are additional process-related and metallurgic countermeasures. In die casting, early demolding of the cast part or a change in mold temperature and local cooling may help prevent or minimize initiation of warm cracking.