Common Sense I of Die Casting: Analysis of Damage Causes of Die Casting Die
Mold damage analysis: In die casting production, the most common forms of mold damage are cracks and cracks. Stress is the main cause of die damage. Thermal, mechanical, chemical and operational shocks are all sources of stress, including mechanical stress and thermal stress. The stress is generated from:
1. During mold processing and manufacturing
1) Quality problems of blank forging
Some molds have cracks after only a few hundred pieces are produced, and the cracks develop rapidly. It is possible that only the external dimensions are guaranteed during forging, and loose defects such as dendrite, carbide inclusion, shrinkage cavity and bubble in the steel are stretched along the processing method to form streamline, which has a great impact on the final quenching deformation, cracking, embrittlement and failure tendency in use.
2) The cutting stress generated during turning, milling, planing and other final processing can be eliminated through intermediate annealing.
3) Grinding stress and friction heat are generated during grinding of hardened steel, resulting in softening layer and decarburization layer, which reduces the thermal fatigue strength and easily leads to hot cracks and early cracks. After finishing grinding, H13 steel can be heated to 510 － 570 ℃, and the stress relief annealing can be carried out at a thickness of 25 mm for one hour.
4) EDM generates stress. A layer of white bright layer enriched with electrode elements and dielectric elements is generated on the surface of the mold, which is hard and brittle. This layer itself will have cracks and stress. During EDM, high frequency shall be used to minimize the white and bright layer. Polishing method must be used to remove the layer and tempering treatment must be carried out. The tempering shall be carried out at the third tempering temperature.
2. During mold processing
Improper heat treatment will lead to die cracking and premature scrapping. Especially, if only quenching and tempering are used, and then surface nitriding process is carried out, surface cracking and cracking will occur after thousands of die castings.
The stress generated during steel quenching is the result of superposition of thermal stress during cooling and structural stress during phase transformation. The quenching stress is the cause of deformation and cracking. The solid must be tempered to eliminate the stress.
3. During die casting production
1) Mold temperature
The mold should be preheated to a certain temperature before production, otherwise, when the high temperature liquid metal fills the mold, it will produce chilling, which will increase the temperature gradient of the inner and outer layers of the mold, form thermal stress, and make the mold surface crack, or even crack.
In the production process, the mold temperature keeps rising. When the mold temperature is overheated, it is easy to cause die sticking, and the failure of moving parts will lead to damage to the mold surface.
The cooling temperature control system shall be set to keep the mold working temperature within a certain range.
2) Filling mold
Liquid metal filling with high pressure and high speed will inevitably produce intense impact and scouring on the mold, resulting in mechanical stress and thermal stress. During the impact process, metal liquid, impurities and gases will also have complex chemical interactions with the mold surface, and accelerate corrosion and crack generation. When the liquid metal is wrapped with gas, it will expand first in the low pressure area of the mold cavity. When the gas pressure rises, it will produce inward blasting, pulling out the metal particles on the surface of the mold cavity, causing damage and cracks due to cavitation.
3) Mold opening
In the process of core pulling and mold opening, when some components are deformed, mechanical stress will also be generated.
4) Production process
In the production process of each die casting, due to the heat exchange between the mold and the liquid metal, the mold surface has a periodic temperature change, which causes periodic thermal expansion and contraction, and generates periodic thermal stress. For example, when pouring, the mold surface is subject to compressive stress due to temperature rise, and after the casting is ejected from the mold, the mold surface is subject to tensile stress due to temperature drop. When this alternating stress circulates repeatedly, the stress accumulated in the die becomes larger and larger. When the stress exceeds the fatigue limit of the material, cracks will occur on the die surface.
Common Sense II of Die Casting: Methods to Prevent Mold Damage
1. Good casting structure design
The wall thickness of the casting shall be as uniform as possible to avoid hot spots, so as to reduce the thermal fatigue caused by local heat concentration of the mold. The corners of castings shall have proper casting fillets to avoid stress generation due to sharp corners on the mold.
2. Reasonable mold structure design
1) Each element in the mold shall have sufficient stiffness and strength to withstand pressure without deformation. The die wall thickness must be sufficient to reduce deformation.
2) The gating system is designed to minimize the impact and erosion on the core.
3) The tolerance fit and surface roughness of each component shall be correctly selected.
4) Maintain mold heat balance.
3. Standardize the heat treatment process
The metallographic structure of the material can be changed through heat treatment to ensure the necessary strength, hardness, dimensional stability at high temperature, thermal fatigue resistance and material cutting performance.
Only the correct heat treatment process can obtain the best mold performance, and the performance of steel is controlled by quenching temperature and time, cooling rate and tempering temperature.
4. Die casting production process control
1) Temperature control: mold preheating temperature and working temperature; The pouring temperature of alloy shall be lower on the premise of ensuring good molding.
2) Reasonable die casting process: specific pressure and filling speed.
3) Adjust the clamping force of the machine to make the force on the die uniform. Attention shall be paid to cleaning the scraps on the mold surface to avoid uneven force on the mold surface and deformation caused by these superfluous objects during mold closing.
4) The alloy smelting shall be strictly controlled to reduce the gas in the liquid metal.
5. Mold maintenance
1) Periodic stress relief
2) Mold repair