How to optimize the formation and prevention of hot cracks in LPDC castings, including thick walled corners and joint bars?

How to optimize the formation and prevention of hot cracks in LPDC castings, including thick walled corners and joint bars?
Hot cracking refers to the tearing of grain boundaries in the later stage of solidification, which is a high-risk defect, characterized by pressure leakage and stress fracture; Many people simply adjust the process, and the root cause is the sharp corners of the structure, sudden changes in wall thickness, poor shrinkage and solidification stress.
1、 Quantitative standards for structural fillet and wall thickness transition
The rounded corner R between the casting rib plate and the body is ≥ 3mm; High risk location R ≥ 5mm; Sharp angle R<2mm, concentrated solidification shrinkage stress, and high incidence of hot cracks.
Control of wall thickness mutation ratio: the ratio of adjacent wall thicknesses should not exceed 1.5 times; The wall thickness suddenly changes by more than twice, the shrinkage rate is inconsistent, and the grain boundaries are prone to tearing.
The width of thick reinforcement bars cannot exceed 1.2 times the wall thickness of the connected body. If the reinforcement bar is too thick, the reinforcement part will become an independent hot joint, and the joint position will first shrink and crack.
Add a release groove or process gap in the middle of the long straight reinforcement to release solidification shrinkage stress and avoid long-distance continuous shrinkage and cracking.
During the DFM review of Xinfeng Machinery, priority should be given to identifying T-joints, cross bars, and corner positions, and a fillet modification plan should be provided in advance to reduce the risk of later process fires.
2、 Process and mold matching prevention and control plan
Moderate cooling of molds in areas with high incidence of hot cracking, accelerating surface crust formation, enhancing surface strength, and resisting shrinkage tensile stress; But excessive cooling should not cause cold insulation.
The layout of the gate is close to the risk zone of hot cracking, ensuring that shrinkage continues until the end of solidification and reducing the tendency for looseness and cracking.
The temperature of the aluminum liquid should not be too high. High temperature grains should become coarse, and there should be an increase in low melting point phases at grain boundaries, which will increase the sensitivity to thermal cracking; A356 hot crack sensitivity range is above 730 ℃.
Shorten the holding time appropriately to avoid excessive solidification pressure stress combined with shrinkage stress; Do not blindly increase the holding pressure.
3、 Differences in hot crack control for overseas customers
Southeast Asian customers prioritize process fine-tuning, increasing rounded corners by on-site fitter polishing, and allowing infiltration to cover minor cracks.
Hot cracks in North American pressure parts are zero tolerance defects. During the DFM stage, solidification stress simulation is conducted, and the risk location must be structurally optimized. Infiltration remediation is not accepted.
European customers strictly control sudden changes in wall thickness, and the minimum requirement for rounded corners is directly marked on the casting drawings. It is not allowed to perform on-site grinding and modification after trial molding.
Middle Eastern thick walled engineering machinery castings have large cross-sectional dimensions and a large amount of shrinkage. Large rounded corners and dispersed gates are widely used to reduce stress concentration.
4、 Hot crack formation chain
Sharp corner/sudden change in wall thickness → concentration of solidification shrinkage stress → incomplete solidification of low melting point phase at grain boundaries → formation of hot cracks by pulling apart the grain boundaries;
Insufficient local shrinkage → accompanied by looseness, cracks extend along the location of looseness;
Aluminum liquid temperature is too high → low melting point at grain boundaries, more eutectic, narrower plastic window, and more prone to tearing.
5、 Long tail traffic layout ideas
Hot cracking, solidification stress, casting corner fillet, aimed at structural, simulation, and process engineers.
keywords LPDC casting hot tear prevention、hot crack at rib junction low pressure casting；
Problem class: Minimum radius to avoid hot tea aluminum casting;
Comparison category: Hot Tea vs Cold Crack Aluminum Casting.
Suitable for DFM casting structure optimization special page.
6、 Common misconceptions in the industry
Misconception 1: Hot cracking relies on increasing the holding pressure for compression, which will only increase the solidification stress and make the crack more severe.
Misconception 2: Cracks are caused by unclean aluminum liquid. The root cause of most hot cracks is structural stress concentration, and impurities only amplify the risk.
Misconception 3: Just polish the rounded corners a little bit, and if the R is too small, stress concentration will not fundamentally improve.
FAQ
Q: What is the minimum recommended rounded corner for rib board handover?
A：R≥3mm， High risk location R ≥ 5mm.
Q: What is the upper limit of the ratio of adjacent wall thickness?
A: Not more than 1.5 times.
Q: A356 high temperature induced thermal cracking range?
A: At temperatures above 730 ℃, there is an increase in low melting point phases at grain boundaries.
Q: What is the difference between hot cracking and cold cracking?
A: Grain boundary tearing at the end of hot crack solidification; Cold cracking leads to stress cracking after cooling.
Q: Long bar crack prevention measures?
A: Increase process gaps to release shrinkage stress.
Q: North American Hot Crack Policy for Pressure Components?
A: Zero tolerance, prioritize structural simulation optimization, and do not rely on infiltration.
Q: How to adjust the mold for hot crack location?
A: Moderate cooling to enhance surface strength.
Q: What is the effect of gating on hot cracking?
A: Shrinkage nearby, reduce looseness, and lower the risk of crack propagation.