This article describes the failure of the fixed core of a certain die-casting mold, including ablation, cracks, and aluminum adhesion, and analyzes the causes of their formation. The mold repair process includes marking, welding, CNC milling, and electrical discharge machining, and the quality of the repaired mold meets the technical requirements. At the same time, measures to improve the service life of die-casting molds were discussed, including reasonable design of mold structure, correct selection of mold materials, optimization of heat treatment process, appropriate usage environment, and standardized maintenance and repair.
Due to long-term exposure to high temperature, high pressure, and high-speed working environments, die-casting molds generate periodic stress and strain on their surface. After a certain period of use, they may experience damage such as cracking, burning, and cracking, leading to the inability of the molds to continue use. Only after repair can they be put into production. Therefore, in the process of using die casting molds, mold repair is very important, which can ensure the production of qualified products and extend the service life of the molds.
one
Failure forms and causes
Figure 1 shows the die casting die core of a certain aluminum alloy cylinder body, which is made of H13 steel. The failure occurred during production, and the failure form and cause analysis are as follows.
1.1 Ablation
Two approximately 1mm deep ablations occurred near the transverse runner and gate sleeve, and 12 ablations occurred at the outer convex edge and inner concave root of the mold core. The ablations reduced the volume of the mold, and the burned part must be repaired to obtain a die cast with qualified dimensions. The main reason for the occurrence of ablation phenomenon is the repeated high-speed erosion of the mold by aluminum liquid at about 670 ℃ under high pressure. Due to heat concentration and high temperature at the corners of the mold, the steel material is burned off after a long time.
1.2 Cracks
Small mesh cracks appeared on the forming surface of the mold directly opposite the inner runner, which was caused by the strong impact of the molten metal, resulting in significant compressive stress. At the same time, after the high-temperature mold was opened, the spray coating was subjected to quenching, and thermal stress was generated under the alternating action of cold and hot. Under the action of composite internal stress, small cracks gradually formed. As time goes on, as the cracks grow to a certain extent, the mold may fracture.
1.3 Adhesive aluminum
There is one aluminum sticking point at the root of the inner runner, and high-temperature and high-pressure aluminum liquid enters the steel matrix along the crack. Due to the high affinity between iron and aluminum at high temperatures, they are bonded together.
1.4 Design Changes
During production, there may be insufficient pouring of products at the end of the transverse sprue. The reason is that the size of the two inner sprues near the vertical sprue is too large, resulting in a high flow rate of the molten metal at this location and a low flow rate of the inner sprue at the end. The molten metal solidifies before the end flow fills the mold cavity, so it is necessary to reduce the size of the first inner sprue.
two
Mold repair process
2.1 Marking
Use a white marker to circle the area with larger ablation size and write "B" next to it, indicating the need for repair. Next to the smaller ablation, write "H" to indicate that this is a hole and does not require repair temporarily. Use arrows to indicate the design changes and write "design changes" that need to be repaired. Write "meat removal" in the area where aluminum is stuck, indicating the need for processing to remove the material.
2.2 Welding
Argon arc welding is a welding method that uses argon gas as the shielding gas. It has a small heat input and has a small thermal impact on the base material, making it less prone to defects such as cracks, undercuts, and pores. The welding quality is easily guaranteed. 9188GS welding machine is used in production, and AST0506 welding rod is used. Its composition is similar to that of mold steel H13, and it has good tempering resistance and heat fatigue resistance, as shown in Figure 2. Before welding, it is necessary to thoroughly remove cracks, oxides, and oil stains from the welding surface and dry it. The purpose of welding preheating is to reduce the tendency of molds to crack due to high welding temperatures, with a preheating temperature of 300-400 ℃. Welding adopts the principle of low heat input, high-frequency pulse current, and multiple small amounts of molten droplets, with a thickness of 4-5 mm for the overlay layer. After welding, it is necessary to undergo tempering to achieve the required hardness. The tempering temperature is 680-730 ℃, which should be 5-10 ℃ lower than the original tempering temperature to avoid reducing the hardness of the base material while maintaining appropriate toughness.
2.3 CNC milling
The repaired welding area needs to be mechanically processed to remove excess materials. The fixed mold core model file is opened in UG NX software for rough machining and precision machining CNC programming. After processing, program files are generated and transmitted to the machine tool. Lift the fixed mold core onto the machine tool workbench, mark the table and align it, then press and fix it tightly. Accurately align the tool according to the program requirements, set the machining coordinate system G54, call up the rough machining program, set the machining parameters, and start the machining. Call up the precision machining program, set machining parameters, and start machining, as shown in Figure 3.
2.4 Electric discharge machining
If there are narrow grooves in the area that needs to be processed, electric discharge machining should be used. Graphite electrodes should be designed and processed, and the electrodes should be installed and fixed on the electric discharge machining machine. The fixed mold core should be fixed on the machine tool, and reasonable discharge parameters should be set for electric discharge machining, as shown in Figure 4.
three
Measures to improve the service life of die-casting molds
3.1 Mold structure design
The wall thickness of die-casting parts should meet the normal and minimum wall thickness requirements of the alloy. While ensuring strength and stiffness, it is advisable to design thin-walled parts with uniform wall thickness to prevent shrinkage and porosity. Reasonably selecting the size of the casting fillet based on the connection method between the two walls of the casting is beneficial for metal liquid filling, exhaust, and mold strength. The demolding angle should be appropriately selected according to the type of alloy and the characteristics of the surface, in order to facilitate the smooth demolding of the casting.
Using software such as ANSYS to analyze the strength and stiffness of the mold, ensuring that the mold is not damaged or deformed. The pouring system should minimize the impact on the core as much as possible. Thickening the gate can reduce mold sintering and lower the impact speed of the molten metal. Correctly select the tolerance fit and surface roughness of each component. Maintain thermal balance of the mold. The distance between the cooling water channel and the surface and corners must be sufficiently large. Try to use insert structures for easy maintenance and replacement. Choose a large angle R under possible conditions to avoid stress concentration.
3.2 Selection of Mold Materials
The service life of die-casting molds is closely related to the material. The chemical composition, metallographic structure, and mechanical properties of the mold material should be confirmed, and if necessary, rechecked and inspected to ensure that the material quality meets the requirements.
Choose hot work mold steel that is resistant to heat fatigue and has good thermal stability. It is recommended to use 8407 or refined H13, and the service life of aluminum die-casting molds can reach 700000 to 100000 times. E38K is suitable for temperatures below 700 ℃, and the service life of aluminum die-casting molds can reach 200000 to 400000 times. 2367 is suitable for temperatures below 700 ℃, and the service life of aluminum die-casting molds can reach 400000 to 600000 times. In production, the correct selection of mold materials should be based on the type of processing materials, product characteristics, and production batch, in order to improve economic benefits.
3.3 Heat treatment of molds
Heat treatment changes the microstructure of the workpiece, thereby altering its strength, hardness, toughness, wear resistance, and other performance indicators. The correctness of heat treatment directly affects the service life of the mold. Therefore, it is necessary to reasonably control the quenching temperature and time, cooling rate, and tempering temperature. The designed hardness of the mold core is set to HRC44~46, and the heat treatment process is graded heating at 480, 700, and 850 ℃+vacuum oil quenching at 1050 ℃+secondary tempering at 600 ℃. During mass production, the mold core undergoes another tempering process. Due to wire cutting, welding repair, and discharge causing damage to the structure of the mold core, it is necessary to adjust the medium back. At the same time, surface strengthening technology can be used to improve the service life, commonly used techniques include ferrite nitrogen carbon co infiltration technology and PVD coating technology.
3.4 Mold usage environment
Reasonably select the preheating temperature and working temperature of the die casting mold based on the type of alloy, casting wall thickness, and structural complexity. The preheating temperature of aluminum alloy die-casting molds is 150-180 ℃, and the working temperature is 180-240 ℃. Choosing a higher temperature appropriately can significantly improve the lifespan of the mold. Ensure that the mold is properly cooled, and the temperature of the cooling water should be maintained at 40-50 ℃. If the machine is temporarily shut down, try to close the mold and reduce the amount of cooling water to avoid thermal shock to the mold when it is restarted. Under the premise of ensuring good molding, use a lower pouring temperature.
3.5 Mold maintenance and upkeep
Standardized maintenance can keep the mold in good condition. After the new mold trial, perform stress relief tempering. When the new mold is used for 1/6-1/8 of its design life, that is, 10000 aluminum die casting molds, 5000 magnesium and zinc die casting molds, and 800 copper die casting molds, the mold cavity should be tempered at 450-480 ℃, polished and nitrided to eliminate internal stress and slight cracks on the surface of the cavity. Perform the same maintenance every 12000 to 15000 cycles in the future. After using 50000 molds, maintenance can be carried out every 25000 to 30000 molds. The above method can significantly slow down the speed and time of cracking caused by thermal stress.
When there are failure defects in the mold that affect production and use, arc welding can be used for repair, and it can be repaired multiple times. The repaired mold has significantly improved its service life, ensuring normal production while reducing production costs.
four
conclusion
This die-casting mold has failure defects such as ablation, cracks, and aluminum sticking in the fixed core, and there is one design change. Therefore, mold repair treatment is required. After marking, welding, CNC milling, and electrical discharge machining, the quality of the repaired mold meets the technical requirements. The measures to improve the service life of die casting molds include reasonable design of mold structure, correct selection of mold materials, optimization of heat treatment process, appropriate use environment, and standardized maintenance and repair.
Author:
Wang Guilin
Changzhou College of Information Technology
This article is from the Journal of Special Casting and Nonferrous Alloys, Volume 42, Issue 3, 2022
