Mechanism of thermal stress cracking in die-casting molds, crack initiation, propagation, and prevention measures
Thermal fatigue cracking is the main reason for the scrapping of die casting molds. Many people only know it as "thermal cracking" but do not understand the logic of the formation of alternating stress. The failure path of Xinfeng mechanical disassembly is unknown.
1、 Complete cycle of thermal stress generation
Every injection, the high-temperature aluminum liquid rapidly heats the surface of the mold cavity; The rapid cooling of the release agent spray causes the surface to shrink rapidly. The temperature difference between the surface and the mold substrate is huge, resulting in alternating tensile and compressive stresses. After repeated cycles, microcracks first form at the grain boundary position, which continue to extend and eventually penetrate to form visible cracks. Alloy wheel mold has a larger fluctuation range in the temperature of the wheel cavity, making it a high-risk category for thermal fatigue.
2、 Priority location for crack initiation
Rounded corners, sprue erosion zones, locations of sudden wall thickness changes, weld repair areas, coating edges, and thin-walled areas near the mold cavity in the waterway. The stress concentration factor at sharp corners is multiplied, and even small temperature fluctuations will preferentially initiate cracking. Many low-priced molds omit rounded corners, which looks good in the early stage, but cracks appear in 20000 to 30000 molds.
3、 The Influence of Steel and Heat Treatment on Crack Resistance
Ordinary H13 has many impurities, carbide segregation, and fast crack propagation speed; High purity hot work steels such as 8407 and W360 have uniform microstructure and significantly stronger resistance to thermal fatigue. Insufficient quenching and tempering, high residual internal stress, and accelerated crack propagation after the mold is installed. Cracks caused by heat treatment defects cannot be cured by coating or nitriding in the later stage.
4、 How to reduce thermal stress at the process end
Stable mold temperature is the core, and the offline mold temperature machine maintains the temperature range of the mold cavity to reduce the cold and hot temperature difference. Release agent should not be sprayed excessively in the same location, as local rapid cooling can induce cracks. Control the upper limit of aluminum liquid temperature to avoid over temperature impact. The thermal fatigue speed of molds produced continuously in three shifts is much higher than that of intermittent production.
5、 Disposal boundary after crack formation
Microcracks with a depth of less than 0.5mm can be removed, polished, or surface protected again; Depth>1mm, cracks extend to the substrate and quickly recur after polishing. It is not recommended to continue refurbishing. Low pressure casting has a gentle filling process, weak temperature shock, and a significantly lower risk of thermal fatigue cracking under the same steel compared to high-pressure die casting. Google's "die casting mold thermal fatigue cracking mechanism" is aimed at materials engineers.
FAQ
Are thermal fatigue cracks and weld cracking the same thing?
Different, weld cracking is caused by welding stress; Thermal fatigue is the alternating stress of tens of thousands of cold and hot cycles.
To what extent can increasing rounded corners improve thermal cracking?
Reasonable rounded corners can reduce stress concentration and increase the crack initiation rate by more than 30%.
Can the coating prevent the propagation of thermal fatigue cracks?
No, the coating can only isolate the aluminum liquid and cannot prevent the propagation of stress cracks inside the substrate.
Does the higher the mold temperature, the less thermal fatigue?
Maintaining stable mold temperature within the specified range is beneficial; Excessive mold temperature can exacerbate aluminum adhesion and erosion.
How far is the appropriate distance between the internal waterway of the mold and the mold cavity?
Normally 10-15mm, it is prone to cracking due to thermal shock when it is too close, and insufficient cooling when it is too far away.
6. Can production continue even if microcracks have already appeared?
In the short term, the inspection cycle must be shortened as cracks will continue to grow longer and deeper.
7. Why are the crack speeds at different positions of the same mold different?
The temperature fluctuations, stress concentration, and aluminum liquid erosion intensity are completely different in various places.
8. Are there many thermal fatigue cracks in low-pressure molds?
Far less than high-pressure die casting, there are rarely large-scale cracks.
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