At present, development of automobiles has rapidly increased demand for aluminum alloy die castings. Due to complex and precise structure of automobile parts and pressure-resistant requirements, die-casting molds often work in harsh environments of high temperature, high pressure and high speed. They are usually scrapped after 80,000 molds are produced, which affects quality, efficiency and cost of aluminum alloy die-casting production. Therefore, manufacturing complex, precise and long-life die-casting molds has become an urgent requirement for manufacturers. There are many factors that affect life of die-casting molds, such as steel type, heat treatment, mold processing technology, mold structure, surface treatment and maintenance, etc. Reasons for scrapping of die-casting molds include thermal cracks (60% to 70% of aluminum alloy die-casting molds), brittle penetration cracks (10% to 20% of aluminum alloy die-casting molds), sticking corrosion, corrosion, deformation, etc. In order to extend life of die-casting mold, different steel types, mold processing techniques and surface treatment techniques were tested specifically for cracks, and were successfully used in die-casting production of a certain bracket part.

Formation of cracks is related to thermal fatigue stress suffered by mold and its own residual stress. Die casting is a process of alternating cycles of cooling by spraying release agent and heating of molten aluminum, so thermal fatigue stress will be generated on the surface of mold cavity. When thermal fatigue stress accumulation exceeds thermal fatigue resistance of mold steel itself, small thermal cracks will begin to appear on the surface of mold cavity, which will continue to widen and deepen during subsequent die-casting production, and cavity surface will collapse after being connected into one piece. Mold processing technology and shape structure will produce residual stress concentration, which is prone to brittle penetration cracks. They will also continue to widen and deepen during die-casting production, and finally form large cracks in mold.
Due to high requirements for die-casting molds for automotive parts, imported mold steels with high thermal fatigue resistance, such as 8418, DH31-EX and DAC55 mold steel, were selected in test. Composition data were taken as median value and compared with commonly used mold steel H13, see Table 1. It can be seen that the first three types of mold steel have a common feature of high Mo content. Adding a small amount of Mo to steel can improve strength of steel, especially high temperature strength and toughness, improve corrosion resistance of steel in acid, alkali solutions and liquid metal, as well as wear resistance, hardenability, weldability, heat resistance, etc. of steel.

Table 1 4 types of mold steel composition (%)

 

Figure 1 Schematic diagram of test equipment

Table 2 Modes of surface cracks in two steel materials with different hardness

 

Figure 2 Hardness value diagram of two surface treatment processes
It can be seen that the higher hardness of mold steel, the higher thermal fatigue resistance, and the higher mode number of surface cracks. Because the higher the overall hardness of mold steel, the lower internal toughness, and large cracks are prone to occur at locations where residual stress in mold is concentrated. To this end, another surface treatment process was tested to increase surface hardness to improve thermal fatigue strength of mold steel while ensuring internal toughness of mold steel, see Figure 2. As can be seen from Figure 2, hardness of salt bath nitriding will be greater than that of gas nitriding. Theoretically, mode of surface cracks in salt bath nitriding will be greater than that of gas nitriding. Untreated samples, salt bath nitrided samples and gas nitrided samples were used to conduct alternating hot and cold cycle tests. Material was H13 steel, as shown in Table 2. It can be seen that maximum mode number for surface cracks to appear is 3600, so mode number of test is set to 5000. Test results are shown in Figure 3 and Table 3. It is found that average depth and maximum depth of cracks in sample without surface treatment are the largest; average depth and maximum depth of cracks in gas nitrided sample are the smallest. Table 3 shows that surface cracks appear earliest on salt bath nitrided samples, while surface cracks appear latest on gas nitrided samples. Mode of surface cracks in salt bath nitriding is smaller than that in gas nitriding. At the same time, average depth and maximum depth of surface cracks are also deeper than those in gas nitriding. Reason is that a 0.01mm white layer appears on salt bath nitrided surface, causing stress concentration.

 

Figure 3 Average depth and depth of cracks for three types of mold steel surface conditions
maximum depth

Table 3 Modes of surface cracks in H13 steel under different surface conditions

 

Figure 4 Residual stress generated by cutting of H13 mold steel
Residual stress produced by electric discharge machining of H13 mold steel was studied, and obtained residual stress is shown in Figure 5. Electrical discharge machining will produce a white layer in cavity. Residual stress value is relatively large and depth of white layer is only 0.01mm. It can be removed by polishing with abrasive sandpaper and oilstone. In order to explore reducing mold failure caused by cracks, a bracket part mold was selected for comparative testing. Bracket structure is shown in Figure 6. Basic size of bracket is 155mm*150mm*86mm, and casting mass is 0.85kg.

 

Figure 5 Residual stress generated by electrical discharge machining of H13 mold steel

 

Figure 6 Bracket parts

Table 4 Comparative analysis of mold life of three types of mold steel with different surface treatments

In order to reduce occurrence of cracks in die-casting molds and extend service life of die-casting molds, it was found that thermal fatigue resistance of mold steel with high Mo content is significantly higher than that of ordinary mold steel. Heat treatment hardness of mold steel is high, and the higher thermal fatigue resistance, the greater number of thermal cracks. In order to take into account internal toughness of mold and thermal fatigue resistance of cavity surface, appropriately reducing heat treatment hardness of mold steel and performing surface treatment can effectively increase service life of mold. In addition, stress relief annealing must be performed after mold is made.