Influencing factors and application explanation of life of aluminum die

Time：2020-02-17 13:12:04 / Popularity： / Source：

Aluminum die casting mold is a widely used mold at present. Because of long production cycle, large investment, and high manufacturing accuracy, cost is relatively high. Therefore, aluminum stamping is expected to have a longer service life. However, due to influence of a series of factors such as materials, machining, and use, mold often fails prematurely and is scrapped, causing great waste. Basic factors affecting service life of aluminum die and corresponding precautions are analyzed through selection, design, manufacture, and use of mold.

Materials

Impact of mold materials on mold life is reflected in correct selection of mold materials, whether materials are good and use is reasonable. Statistics show that due to improper material selection and heat treatment, early failure of mold accounts for about 70%. Aluminum die casting mold are hot-working molds, and their use conditions are extremely harsh. Melting point of aluminum is 580-740℃. When diecast, temperature of molten aluminum is controlled at 650-720℃. In the case of diecast without preheating mold, surface temperature of cavity is raised from room temperature to liquid temperature, surface of cavity will be subjected to great tensile stress. When mold top is opened, cavity surface is subjected to extreme compressive stress. After thousands of die-casting, defects such as cracks occurred on mold surface. Therefore, requirements for aluminum die materials are high.

Structural design

Mold design manual details problems that should be paid attention to in design of aluminum die casting mold. It should be emphasized that structural design of mold should avoid sharp corners and excessive cross-section changes as much as possible. Sharp rounded corners can cause stress concentrations up to 10 times average stress. It is easy to cause early failure of mold. In addition, it is necessary to pay attention to deformation and cracking of subsequent heat treatment due to unreasonable structural design. In order to prevent heat treatment deformation and cracking, cross-sectional size should be uniform, shape should be simple and symmetrical, blind hole should be opened as a through hole as much as possible, and process hole can be opened if necessary. Avoid geometric gaps in structural design, including knife marks, angles, grooves, holes, and abrupt cross-sections to reduce occurrence of part structure and heat treatment defects.

Machining

Incorrect machining is likely to cause stress concentration, insufficient finish, machining does not completely remove decarburized layer formed by rolling and forging, which may cause early failure of material. In addition, during processing of mold, thicker template cannot be used to ensure its thickness. Because thickness of steel plate is doubled, amount of bending deformation is reduced by 85%, and lamination can only serve as a superposition. Bending deformation of two plates with same thickness as veneer is four times that of veneer. When processing cooling water channel, special care should be taken to ensure concentricity during double-sided processing. If head corners are not concentric with each other, connected corners will crack during use. Surface of cooling system should be smooth, preferably without machining marks. With advancement of technology, it is necessary to pay attention to tracking and use of advanced machining technology, which is conducive to improvement of precision of mold parts and components, so as to prolong service life of mold.

Grinding and EDM

Grinding processing time may cause local overheating of metal surface, resulting in high surface residual stress and microstructural changes, etc., which may lead to generation of grinding cracks. In addition, improper pretreatment of original structure, carbide segregation, coarse grains, and insufficient tempering can cause grinding cracks. Therefore, in the case of ensuring material, pay attention to selecting an appropriate coolant to control grinding process cooling, control grinding speed and reduce occurrence of cracks.
EDM can form a white bright hardened layer of quenched martensite on the surface of die after quenching and tempering. Thickness of hardened layer is determined by current intensity and frequency during processing, deeper during roughing and shallower during finishing. Hardened layer will cause great stress on mold surface. If hardened layer is not removed or stress is eliminated, surface of mold is prone to electrical cracking, pitting and cracking during use. Elimination of hardened layer or destressing can be achieved by: ① removing hardened layer with whetstone or grinding: ② reducing stress below tempering temperature without reducing hardness, which can greatly reduce surface stress of cavity.

Heat treatment

Improper heat treatment is an important factor leading to early failure of mold. Heat treatment deformation is mainly caused by thermal stress and tissue stress. When stress exceeds yield strength, material undergoes plastic deformation. When stress exceeds strength limit, it causes quench cracking of part. Attention should be paid to following points during heat treatment of aluminum die casting mold:
(1) When forging is not cooled to room temperature, spheroidizing annealing is performed.
(2) After roughing and before finishing, add quenching and tempering treatment. To prevent hardness from being too high, which makes machining difficult, hardness should be limited to 25-32HRC. Before finishing, stress relief tempering shall be arranged.
(3) Pay attention to heating temperature and holding time of steel during quenching to prevent austenite from coarsening. When tempering, heat is maintained at 20mm/h, number of tempering is generally 3 times. In the case of chlorine penetration, third tempering can be omitted.
(4) Attention should be paid to decarburization and carburization of cavity surface during heat treatment.
(5) When nitriding, pay attention that there should be no oil stain on nitriding surface. After cleaning surface, it is not allowed to touch it by hand directly. Wear gloves to prevent nitrided surface from being stained with oil, which will cause uneven nitrided layers.
(6) Between two heat treatment processes, when temperature of previous one is reduced to touch of hand, next one is performed, and it cannot be cooled to room temperature.
(7) Static quenching method, salt furnace quenching, and special fixture quenching are used to reduce heat treatment deformation.
(8) Tracking and using advanced heat treatment equipment, processes, such as vacuum furnace and flow particle furnace heating, can improve surface finish of mold, help to control heat treatment deformation, extend life of parts.

Production operations

When determining injection speed of aluminum die casting mold, speed must not be too high, too high speed promotes mold corrosion and increased deposits on cavity and core: but too low will easily cause casting defects. Therefore, for aluminum die casting mold, minimum injection speed is 18m/s, maximum injection speed should not exceed 53m/s, and average injection speed is 43m/s. During use of mold, casting process must be strictly controlled. As far as process permits, try to reduce casting temperature, injection speed of molten aluminum, and increase preheating temperature of mold. Preheating temperature of aluminum die-casting mold is increased from 100-130℃ to 180-200℃, life of mold can be greatly improved. At the same time, improper adjustment and use of machine tool is avoided, which causes early failure of mold.

Mold maintenance

1. Clear mold cavity deposits in time. After mold is used for a period of time, there will be deposits on cavity and core. These deposits are a combination of mold release, coolant impurities, a small amount of die-casting metal at high temperature and pressure. Some deposits are hard, adhere strongly to surface of core and cavity, making it difficult to remove. When removing sediment, it can be removed by grinding or mechanical methods, taking care not to hurt other profiles and cause dimensional changes.
2. Frequent maintenance keeps mold in good working condition. After new mold is tested, pay attention to stress relief tempering without cooling mold to room temperature. When new mold is used for 1/6-1/8 of design life, that is 10,000 shots, mold cavity and mold frame should be tempered at 450-480℃, cavity should be polished and chlorinated to eliminate internal stress and slight cracks on cavity surface. Same maintenance will be performed every 12000-15000 molds later. When mold uses 50,000 shots, maintenance can be performed every 25,000-30000 molds. Through maintenance, speed and time of cracks due to thermal stress can be significantly reduced, life of mold and quality of casting can be effectively improved.
3. Repair mold defects in time. In the case of severe erosion and cracking, surface of mold can be nitrided to improve hardness and wear resistance of mold surface. When nitriding, pay attention to bonding strength of substrate and nitriding layer. Thickness of chloride layer should not exceed 0.15mm. An excessively thick nitriding layer is liable to fall off at parting surface and sharp corners. Welding can also be used to repair surface defects of mold. When welding, pay attention to matching of composition of electrode and welding part, cleaning of welding surface and drying of electrode, trimming and finishing of cavity after welding.
There are many factors that affect service life of aluminum stampers, involving multiple aspects, as a manufacturing industry. Mush correctly design mold structure, accurately manufacture mold parts, formulate reasonable heat treatment processes, use molds reasonably according to process specifications, repair and maintain in time. It can give full play to performance of mold materials, effectively improve quality and service life of aluminum die.

Gud Mould Industry Co., Ltd