Cracking and failure in aluminum alloy die-casting molds not only impact mold production quality and efficiency but also increase production costs. In reality, the causes of cracking and failure in aluminum alloy die-casting molds are diverse. These include repeated impacts from liquid metal, hot-corrosive production conditions, mold material, and electrical discharge machining (EDM). This article specifically analyzes cracking and failure in aluminum alloy die-casting molds.

Aluminum alloy die-casting molds operate under conditions that require them to withstand high temperatures and high pressures. Furthermore, when in operation, aluminum alloy die-casting molds must withstand repeated impacts from liquid metal. In actual production, aluminum alloy die-casting molds suffer from high production costs, long production cycles, and premature failure, which significantly shortens their service life. If cracking and failure in aluminum alloy die-casting molds are not effectively addressed, they will directly impact the economic profitability of the manufacturer. Consequently, an increasing number of aluminum alloy die-casting manufacturers are increasing their focus on cracking and failure analysis. The following article uses H13 hot work die steel as an example, analyzing the causes of cracking and failure in aluminum alloy die-casting molds from the perspectives of aluminum alloy mold material, microstructure, and EDM machining, combined with actual production processes. It also proposes corresponding solutions.

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Analysis of Causes of Cracked Failure in Aluminum Alloy Die-Casting Molds

1.1 Chemical Composition

H13 hot work die steel primarily contains the elements C, Si, Mn, Mo, Cr, and V. Chemically, H13 hot work die steel is a low-Si, high-Mo type of hot work die steel. During production, manufacturers will appropriately reduce or increase the Si content in the steel based on actual needs. Reducing the Si content effectively reduces segregation, further refines the austenite grains, and improves the steel's strength and toughness. Increasing the Mo content, on the other hand, improves the steel's hardenability, tempering resistance, and thermal conductivity. It also effectively prevents the precipitation of grain boundary carbides and the transformation of bainite. Practice has shown that low-Si, high-Mo steel reduces the incidence of undercooling during solidification, effectively preventing problems such as dendrites, cellular columnar crystals, and dendritic segregation. Mo and V combine to form alloy carbides, such as VC, MoC, and Mo2C. Under suitable high-temperature conditions, these alloy carbides precipitate in a fine, dispersed state, significantly enhancing the hot hardness of the high-temperature material. Although H13 hot work die steel exhibits strong crack resistance based on its chemical composition, we have observed premature cracking failure in H13 hot work die steel during actual operation. To better analyze the causes of these cracking failures, further analysis is necessary, combining the microstructure of H13 hot work die steel.

1.2 Microstructure

To comprehensively examine the microstructure of aluminum alloy die-casting molds, this section examines both unused and used mold materials after vacuum quenching and tempering to identify the causes of these cracking failures.

1.2.1 Mold Materials Not Used After Vacuum Quenching and Tempering

Practical experience revealed that the matrix of the research object exhibited an uneven microstructure after heat treatment. Observation of the object under a low-power microscope revealed a large number of granular carbides precipitated within the matrix, exhibiting segregation. In other words, these carbides were larger in volume than those in the normal microstructure. The excessive precipitation of carbides and alloy carbides resulted in the loss of a significant amount of surrounding carbon and alloying elements from the mold material. Under normal circumstances, segregated carbides during quenching and tempering would not dissolve easily. However, due to their lack of carbon and alloying elements, they readily transformed into martensite upon high-temperature heating, reducing martensite yield and significantly reducing the strength and toughness of the steel, making it susceptible to fracture. Observation of the steel in the annealed state before vacuum quenching using a low-power metallographic microscope revealed similar segregation within the steel matrix. This phenomenon indicates a lack of uniformity in the raw steel material. If segregation is not effectively addressed, the risk of mold cracking and failure increases, ultimately impacting the mold's service life.

1.2.2 Mold Material After Use

In actual production, H13 hot-working steel molds typically develop varying degrees of cracking on their surfaces after approximately 30,000 cycles. These cracks include intergranular fractures and pits formed at the intersection of multiple cracks after spalling. This phenomenon is often caused by metallurgical defects in the raw materials.

1.3 Electrospark Machining

Electrospark machining is a commonly used machining method for aluminum alloy die-casting molds. Compared to other machining methods, this method offers advantages such as high machining precision, high automation levels, and ease of machining irregularly shaped parts. However, the sparks released during machining are characterized by high temperature and high pressure, and the working fluid's temperature drops rapidly when idle, resulting in the steel surface being divided into a remelted zone and a heat-affected zone.

The so-called remelting zone refers to the surface metal melted by the high temperatures released during discharge. Because the melt is not completely ejected, the retained melt solidifies as the working fluid cools. The remelting zone is often located in the uppermost layer of the steel surface. Compared to the remelting zone, the heat-affected zone (HAZ) does not melt after being subjected to high temperatures; instead, the metallographic structure changes accordingly. Through extensive experience, we have found that the hot mold process also increases the risk of mold cracking and failure within the remelting and HAZ zones. Although the metallographic structure of aluminum alloy die-casting molds processed by EDM remains unchanged after being baked in a gas furnace, slight cracks may appear in the remelting zone. These microcracks extend into the HAZ, further increasing the severity of mold cracking and failure.

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Preventative Measures for Cracking and Failure of Aluminum Alloy Die-Casting Dies

2.1 Increase the Number of Metallographic Samplings for Aluminum Alloy Die-Casting Die Raw Materials. In actual production, many manufacturers, in order to expedite production, fail to conduct sufficient metallographic testing on purchased raw materials. To reduce cracking and failure in aluminum alloy die-casting molds, manufacturers need to increase the number of metallographic testing samples of raw materials in the annealed and vacuum quenched and tempered states, thereby ensuring that the mold materials meet actual production requirements to the greatest extent possible. When sampling mold materials, manufacturers must scientifically and rationally select sampling locations. When sampling aluminum alloy die-casting molds, manufacturers typically sample the gate to ensure the integrity of the core and accurately test the mold raw material quality.

2.2 Effectively Address the Thermal Remelting Zone Caused by EDM

The thermal remelting zone caused by EDM is hard and brittle, making it highly susceptible to microcracks during processing. The probability of microcracks is particularly high during the flame-baking process. To prevent cracking and failure in aluminum alloy die-casting molds, it's necessary to scientifically and rationally avoid the formation of remelting zones. After EDM, the remelting zones must be promptly removed and the mold tempered to significantly eliminate residual stress in the affected layer.

2.3 Effectively Preventing Early Cracking

Premature cracking and failure in aluminum alloy die-casting molds is often caused by excessively high forging temperatures during the blank production process. This type of cracking is an irreparable defect. Therefore, during the blank production process, manufacturers must strictly control the forging temperature. During the quenching heating stage, the heating time must be scientifically and rationally scheduled to effectively control the heating temperature and prevent decarburization. During the quenching cooling stage, the cooling time must be effectively controlled, striving to complete the quenching and cooling process in the shortest possible time. Designers should ensure that cooling channels are sufficiently spaced from mold surfaces and corners to ensure smooth cooling.

2.4 Scientifically and Appropriately Heat Treat the Mold

The quality of the die-casting mold raw materials significantly impacts the lifespan of aluminum alloy die-casting molds. Therefore, personnel must select appropriate die-casting mold raw materials based on actual needs. After the aluminum alloy die-casting mold raw materials are finalized, they must be heat treated promptly. Furthermore, stress relief procedures must be implemented during the production phase to prevent stress concentration and ensure proper radius control. Generally, after approximately 10,000 uses, aluminum alloy die-casting molds require timely tempering to relieve stress and effectively prevent stress concentration that can lead to cracking and failure. To maximize the overall lifespan of the mold, multiple tempering stress relief methods can be employed.

2.5 Scientifically and Appropriately Control the Temperature During the Aluminum Alloy Die-casting Mold Production Process

The production process of aluminum alloy die-casting molds involves high temperatures and high pressures. Therefore, scientifically and appropriately controlling the temperature during production is crucial. During production, manufacturers can use appropriate thermometers to measure the maximum temperature during the die-casting process and implement effective control measures to keep the temperature below 650°C.

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Conclusion

In summary, with the rapid economic development of my country, the number and scale of aluminum alloy die-casting mold manufacturers have expanded rapidly. However, due to the high production costs of aluminum alloy die-casting molds, failure to effectively ensure quality directly impacts the production efficiency and market competitiveness of manufacturers. Cracking and failure are common quality issues in aluminum alloy die-casting molds during production. This has not only attracted widespread attention from manufacturers but also has led to widespread use by consumers. Therefore, it is crucial to strengthen research on cracking and failure in aluminum alloy die-casting molds. This article analyzes the causes of cracking and failure in aluminum alloy die-casting molds and proposes preventative measures. We hope this will provide valuable reference for relevant practitioners and promote the sustainable, rapid, and healthy development of my country's aluminum alloy die-casting mold industry.