Abstract: In order to reduce the occurrence of cracks in die-casting molds and extend the service life of die-casting molds, ordinary hot work mold steel and high molybdenum hot work mold steel were used to test the number of molds that produce turtle cracks. The mold number and depth of turtle cracks on the surface of two types of mold steel after nitriding were examined, and the residual stress during mold processing was analyzed. The results showed that the lifespan of molds with gas surface nitriding under the same product and production conditions was extended by 53% to 75% compared to molds without surface nitriding.
At present, the development of automobiles is driving double-digit growth in aluminum alloy die-casting parts every year. The characteristics of die-casting technology are high temperature, high pressure, and high speed. Due to the complex structure, precision, and pressure resistance requirements of automotive parts, die-casting molds often work in harsh environments of high temperature, high pressure, and high speed. Usually, they need to be scrapped after 80000 mold cycles, which affects the 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 the lifespan of die-casting molds, such as steel grade (composition of mold steel), heat treatment, mold processing technology, mold structure, surface treatment, and maintenance. The reasons for the scrapping of die-casting molds include hot cracking (aluminum alloy die-casting molds account for 60% to 70%), brittle penetration cracking (aluminum alloy die-casting molds account for 10% to 20%), corrosion, melting, deformation, etc. This project aims to extend the lifespan of molds by conducting crack tests on different steel grades, mold processing techniques, and surface treatments, and successfully applying them to the die-casting production of a certain support component.
1. Mechanism of crack formation and selection of steel grade
The formation of cracks is related to the thermal fatigue stress and residual stress of the mold itself. Die casting is a process of alternating cooling with mold release agent and heating with aluminum liquid, resulting in thermal fatigue stress on the surface of the mold cavity. When the accumulated thermal fatigue stress exceeds the thermal fatigue strength of the mold steel itself, small thermal cracks begin to appear on the surface of the mold cavity, which gradually widen and deepen in subsequent die-casting production, and collapse appears on the surface of the mold cavity after being connected into one piece. The processing technology and shape structure of the mold will generate residual stress concentration, which is prone to brittle penetration cracks. It will also continuously widen and deepen during die-casting production, eventually forming large cracks in the mold.
Due to the high requirements of die-casting molds for automotive parts, imported advanced mold steels with high thermal fatigue strength, such as 8418, DH31-EX, and DAC55 mold steels, were selected for the experiment. The composition data was taken as the median and compared with the commonly used mold steel H13, as shown in Table 1. It can be seen that the first three types of mold steel share a common feature of high Mo content. Adding a small amount of molybdenum to steel can improve its strength, especially high-temperature strength and toughness, enhance its corrosion resistance in acidic and alkaline solutions and liquid metals, as well as its wear resistance, good hardenability, weldability, and heat resistance.
2. Equipment selection and testing methods
The cycle of the die-casting process is as follows: open the mold, remove the die-casting part, spray water-soluble release agent into the mold cavity, blow dry the mold cavity, close the mold, and die-casting. Therefore, die-casting is a process of alternating cooling (spraying water-soluble release agent) and heating (die-casting) cycles. According to the working principle of die-casting, corresponding experimental instruments have been developed, as shown in Figure 1.
The test object is a mold steel sample with a diameter of 80 mm. A 5 mm diameter hole is drilled on the back of the sample to install a thermocouple. The front end of the temperature measurement hole is 20 mm away from the heating end face. The high-frequency instrument is connected to a heating plate with a diameter of 35 mm. The cooler uses a spray gun connected to tap water and compressed air. After the sample is rapidly heated to 600 ℃ by high frequency wave, it is cooled to 150 ℃ by spray, recorded as one mode, and repeated circulation.
3. Analysis of experimental results
Firstly, a comparison was made between the ordinary mold steel H13 and the mold steel DAC55 with high Mo content, and the number of molds with different hardness exhibiting surface turtle cracks was obtained, as shown in Table 2. It can be seen that the higher the hardness, the higher the thermal fatigue resistance, and the larger the number of thermal cracking modes. Under conditions of similar hardness, DAC55 steel with high Mo content exhibits a 66% higher number of surface cracks compared to H13.
From Table 1, it can be seen that the higher the hardness of the mold steel, the higher its resistance to thermal fatigue, and the higher the number of molds with surface cracks. Because the higher the hardness of mold steel, the lower its toughness, and large cracks are prone to occur at the location of residual stress concentration in the mold. To this end, another surface treatment process was tested to improve the thermal fatigue resistance of the mold steel by increasing the surface hardness while ensuring the internal toughness of the mold steel, as shown in Figure 2.
From Figure 2, it can be seen that the hardness of salt bath nitriding is greater than that of gas nitriding, and theoretically, the number of surface cracks in salt bath nitriding will be greater than that of gas nitriding. Cold hot alternating cycle tests were conducted on untreated samples, salt bath nitrided samples, and gas nitrided samples, respectively. The material used was H13 steel. According to Table 2, the maximum mold number at which surface turtle cracks appeared was 3600, so the mold number for the test was set to 5000. The experimental results are shown in Figure 3 and Table 3. It was found that the average depth and maximum depth of turtle cracks in the sample without surface treatment were the highest; The average depth and maximum depth of cracks in the gas nitriding sample are the smallest. Table 3 shows that surface cracks appeared earliest in the salt bath nitrided sample; The latest appearance of surface turtle cracks in gas nitriding samples. The number of surface cracks in salt bath nitriding is smaller than that in gas nitriding; The average depth and maximum depth of surface cracks are also deeper than gas nitriding. The reason is that a 0.01 mm white layer appears on the surface of the salt bath nitriding, resulting in stress concentration.
In addition to studying steel and surface treatment, this project also investigates the residual stresses generated by mold processing technology. The residual stress values obtained through X-ray detection for commonly used high-speed cutting processes in cavity machining are shown in Figure 4. From the data comparison, it can be seen that the cutting speed should not be too high. This requires the removal of residual stress during subsequent annealing treatment, otherwise it may easily cause mold cracking in die-casting production.
The residual stress generated by the discharge machining of the mold was studied, and the residual stress values obtained are shown in Figure 5. The reason is that electrical discharge machining will produce a white layer in the mold cavity, and the residual stress value is relatively large. The depth of the white layer is only 0.11 mm, which can be removed by hand polishing with sandpaper and oilstone.
To explore the reduction of mold failure caused by cracks, a bracket component mold was selected for comparative testing. The product structure is shown in Figure 6. The basic dimensions of the product are 155 mm × 150 mm × 86 mm, and the casting mass is 0.85 kg.
According to the previous experimental data, the same product is made with three sets of molds made of three types of mold steel, using the same mold manufacturing process. Due to the fact that the 8418 manufacturer does not agree that gas nitriding can reduce surface cracks on molds, only DH31-EX and DAC55 steel were subjected to gas nitriding after passing the dimensional inspection of the trial mold products. Through actual production data, methods to reduce mold failure caused by cracks were explored. After one year of production, the lifespan of molds treated with gas nitriding was significantly improved, as shown in Table 4.
4. Conclusion
To reduce the occurrence of cracks in die-casting molds and extend the service life of die-casting molds, attention should be paid to the following aspects: the thermal fatigue strength of mold steel with high Mo content is significantly higher than that of ordinary mold steel. The higher the hardness of mold steel after heat treatment and the higher its resistance to thermal fatigue, the larger the number of molds with hot cracking. To balance the internal toughness of the mold and the thermal fatigue resistance of the cavity surface, reducing the heat treatment hardness of the mold steel appropriately and performing surface treatment can effectively improve the service life of the mold. After the mold production is completed, stress relief annealing must be carried out.
This article is from: "Special Casting and Nonferrous Alloys" magazine, Volume 40, Issue 08, 2020
