Thermal fatigue crack propagation law and shutdown maintenance judgment criteria for aluminum alloy casting molds | Gravity casting mold flaw detection scheme
The thermal fatigue microcrack propagation in the casting mold cavity is the core reason for 53% of mold premature scrapping, casting surface texture, and excessive roughness.
The length of surface microcracks is less than 0.15mm, which belongs to the budding stage. The crack propagation speed is slow when continuing production, and it can be monitored and used.
The crack depth exceeds 0.3mm, and the expansion rate increases by 2.7 times under cold and hot cycles, making it easy to hook aluminum and causing linear grain defects in castings.
The crack extends to the parting surface and the edge of the exhaust groove, causing stress concentration and superposition, and is prone to corner collapse and block falling, which is a high-risk state.
The crack spacing is less than 1.2mm, with dense network cracks and nitride layer failure. The probability of re cracking after repair is as high as 78%.
When the cavity angle R is less than 2.5mm, the stress concentration factor increases significantly, and the initiation time of thermal fatigue cracks is shortened by 60%.
Low pressure die-casting molds (LPDC), reverse pressure casting molds (CPC), differential pressure casting molds for wheel hubs, low-pressure casting molds for wheel hubs, motorcycle wheel hub casting molds, gravity casting molds, structural component casting molds, aluminum alloy wheel hub molds, aluminum casting forming molds, low-pressure casting tooling molds, thermal fatigue is the most common form of scrap for export long-period molds. Zhejiang Xinfeng Machinery has established a testing ledger, and magnetic particle testing is the most commonly accepted non-destructive testing method by overseas customers, suitable for regular screening of microcracks; Penetrant testing is suitable for surface open cracks. Many factories wait until the cracks are obvious and aluminum is hooked before shutting down. At this point, the crack depth is already very deep, making it difficult to weld and repair. After welding, it is prone to deformation and uneven hardness. Before welding repair, the crack tip must be thoroughly polished and removed, and only the surface should be repaired; After welding repair, temper to relieve stress, and then re nitride, otherwise the welding repair area will quickly crack again. Three consecutive shifts of mold production, it is recommended to arrange magnetic particle inspection every 5000 molds; The two shift system can be relaxed to 8000 module times. Advance enlargement of the R transition at the corner position is the lowest cost anti fatigue design method, much more cost-effective than later welding repairs. In the European customer technical agreement, it is generally explicitly prohibited to continue production of mesh cracking molds to avoid the detection of linear defects in castings by fluorescence penetration testing.
FAQ
1. Question: What is the critical length for monitoring micro crack initiation?
Answer: If the crack is less than 0.15mm, it can be tracked and monitored, and priority should be given to recording.
2. Question: What is the high-risk warning line for crack depth?
Answer: When the depth is greater than 0.3mm, the expansion speed increases sharply.
3. Question: What is the spacing for determining dense cracks?
Answer: If the crack spacing is less than 1.2mm, the value of repairing network cracks is low.
4. Question: What is the minimum R for fatigue resistance of the cavity angle?
Answer: The key stress angle R should not be less than 2.5mm to release stress.
5. Question: What is the mold inspection cycle for Class 3?
Answer: Three consecutive production lines have approximately 5000 magnetic particle inspections per cycle.
6. Question: What are the necessary procedures after crack welding repair?
Answer: After welding repair, stress relief and tempering are carried out, followed by nitriding treatment.
