Abstract: This article conducts a failure analysis on service life of aluminum alloy die-casting molds. Extend service life of mold and mitigate early failure of mold through analysis. Make aluminum alloy die-casting mold exert its maximum effect during service period.
Introduction: Aluminum alloy die-casting molds are molds that work under harsh conditions of high temperature, high speed, high pressure, and repeated impact of liquid aluminum alloy on mold. Since such molds usually have high manufacturing costs, long cycle times, and short mold service life, premature mold failure will seriously affect product quality and production efficiency. Below I will analyze failure modes of nearly 400 molds in our company.

 

Analysis of failure causes of aluminum alloy die-casting molds
Main failure forms of our molds are: erosion and erosion failure, thermal fatigue cracking failure, and fracture failure.
There are many reasons for failure of aluminum alloy die-casting molds, both external and internal. External factors are: whether temperature of aluminum liquid is normal, whether mold is preheated, amount of release agent sprayed, whether size of die-casting machine matches, die-casting pressure is too high, soup mouth speed is too fast, whether opening of cooling water is synchronized with die-casting production or whether waterway opening is reasonable, whether mold structure design is reasonable, size of casting, wall thickness, structural shape, etc.      Internal factors are: selection of mold steel, heat treatment process, internal processing stress in mold manufacturing process, etc. By analyzing mold failure modes, prevention can be made in advance to subsequently improve service life, product quality and production efficiency of mold.

1.1 Mold thermal fatigue cracks are caused by repeated cooling and heating of mold during die-casting production. They are usually formed on the surface of mold cavity or at internal thermal stress concentration. After crack is formed, stress is redistributed. When crack develops to a certain length, stress relaxation occurs due to plastic strain and crack stops expanding. As number of cycles increases, some small cavities appear near crack tip and gradually form micro-cracks, which merge with main cracks that have begun to form. Cracks continue to expand, and finally cracks are interconnected to form network-like cracks, which leads to mold failure.
1.2 Mold hot cracking is mainly caused by thermal stress fatigue of material, that is, a continuous, local, and permanent structural change in material due to repeated or pulsating strain under action of thermal stress. The most important material property corresponding to thermal cracking is material's thermal fatigue resistance.
1.3 So far, standards for measuring thermal fatigue resistance of materials have not been unified. Therefore, how to use conventional mechanical properties to reflect thermal fatigue resistance becomes more important. Some data show that the most important mechanical property related to thermal fatigue resistance is strength of material. Another view is more inclined to toughness of material. From analysis of thermal fatigue test results, there are fatigue striations in thermal fatigue cracks of aluminum die-casting molds, which are dominated by elastic strain. Moreover, service life of most molds is more than 100,000---200,000 times, and they have characteristics of high cycle fatigue. According to fatigue strength theory, materials with high strength are better able to adapt to long-term elastic strains. In this case, thermal fatigue resistance is more closely related to strength. However, if working temperature of cavity surface is high (such as die-casting copper and iron-based alloys, etc.) or other factors cause mold to generate excessive thermal stress, causing strain amplitude to greatly exceed elastic strain range of material, strain will be dominated by plastic strain and have low-cycle fatigue characteristics. At this time, materials with high toughness have greater advantages. Therefore, when assessing thermal fatigue performance of materials, attention should be paid to distinguishing fatigue forms of materials, and based on this, appropriate strength and toughness matching of mold is determined.

2.1 Fracture failure occurs when die-casting mold will develop cracks at the weakest point under action of injection force, especially if scribing marks or electrical machining marks on molding surface of die-casting mold have not been polished, or clear corners of molding, fine cracks will appear first. When there is a brittle phase at grain boundary or grains are coarse, it is easy to break. In brittle fracture, crack expands very quickly, which is a very dangerous factor for chipping failure of die-casting mold. For this reason, on the one hand, all scratches and electrical machining marks on mold surface must be polished, even if they are in gating system, they must be polished. In addition, die-casting mold material used is required to have high strength, good plasticity, impact toughness and fracture toughness.

3.1 Commonly used die-casting alloys include zinc alloy, aluminum alloy, magnesium alloy and copper alloy, as well as pure aluminum die-casting. Zn, Al, and Mg are more active metal elements. They have good affinity with die-casting mold materials, especially Al, which is easy to bite mold. When hardness of die-casting mold is higher, corrosion resistance is better. On the contrary, if there are soft spots on molding surface, corrosion resistance is unfavorable. However, in actual production, erosion is only a local part of die-casting mold. For example, parts where gate is directly washed away (core, cavity) are prone to corrosion, and aluminum alloy mold sticking is prone to occur in softer hardness areas. When aluminum alloy die-casting mold is in operation, it is washed away by high temperature and high pressure of molten aluminum liquid, repeated action of alternating stress causes corrosion and erosion.
3.2 Erosion and erosion are caused by mechanical and chemical reactions
This is due to combined effect of mechanical and chemical corrosion. Molten aluminum alloy is injected into mold cavity at high speed, causing mechanical abrasion of cavity surface. At the same time, metallic aluminum and mold materials generate brittle iron-aluminum compounds, which become a new source of hot cracks. In addition, aluminum filling into crack creates a mechanical interaction with crack wall, and superimposes with thermal stress, which intensifies tensile stress at crack tip, thereby accelerating crack expansion. Improving high-temperature strength and chemical stability of materials is conducive to enhancing corrosion resistance of materials.

 

Mold service life is a comprehensive reflection of various indicators such as mold material performance, mold design, processing and heat treatment technology, mold use and maintenance, etc. within a certain period of time. Therefore, studying life characteristics of molds and analyzing failure mechanisms of molds have attracted more and more attention in the field of mold industry technology. Research on die-casting mold failure and improving die-casting mold life is of great significance to die-casting production. At present, service life of aluminum die-casting molds in my country is generally low, about 50,000-100,000 molds, while it is 100,000-200,000 molds in Japan and 100,000-250,000 molds in Germany. Many die-casting plants still use traditional 3Cr2W8V mold steel. As a result, mold will be scrapped prematurely, production cost of die castings will increase, and labor productivity will decrease. Top priority should be to accelerate promotion of high-quality mold materials such as H13 steel, further strengthen research and development of aluminum die-casting mold materials, develop high-quality aluminum die-casting mold steel suitable for my country's national conditions based on digestion and absorption of foreign high-quality mold materials.