Die-casting molds work under high temperatures and high pressures, have high requirements for thermal fatigue strength and corrosion resistance. Due to high production cost and long cycle of die-casting molds, service life of mold is very important. Main failure modes of molds include: plastic deformation, thermal fatigue, corrosion, erosion and damage, etc. In actual production process, actual service life of mold is affected by a variety of factors, such as material selection, heat treatment process, process conditions, surface strengthening treatment, structural design, mold processing technology, maintenance, etc.
Thermal fatigue is main early failure form of die-casting molds. A statistical analysis of current causes of die-casting mold failure found that mold failure due to thermal fatigue accounts for approximately 70% of all mold failure forms. Therefore, this paper mainly conducts research and analysis on thermal fatigue failure caused by thermal stress accumulation during production process of die-casting molds, aiming to provide a reference for its failure control. Magma software was used to perform numerical simulation analysis on thermal stress field of a large powertrain component mold. Through thermal fatigue life calculation, actual service life of part distance between mold and gate can be estimated, design optimization measures can be proposed for areas with lower service life, thereby improving service life of the entire mold.
Graphical results
According to different analysis objects, thermal stress simulation models mainly include thermoelastic model, thermoviscosity model, thermoelastoplastic model and thermoelastoplastic model. Among them, thermoelastoplastic model is the most widely used. Thermoelastoplastic model does not consider viscosity effect of material and assumes that deformation of material before yielding is elastic deformation and deformation after yielding is plastic deformation. Elastic modulus and yield stress of material are only considered as a function of temperature in calculation process, and when material is close to liquidus temperature, corresponding elastic modulus and yield stress are both 0. Die-casting mold is H13. Analysis is carried out based on melting point and operating temperature of H13 steel. Thermal stress analysis calculation uses thermoelastic model.

 

Figure 1 Take point A/B, point C/D and point E/F at different locations from gate.

 

Figure 2 Single cycle mold surface temperature changes in area A, area C, and area E
It can be seen from simulation results that maximum compressive stress and tensile stress on mold surface are quite different. This is mainly because during mold closing period, fixed mold surface of mold is not only restrained by internal thermal expansion, but also by clamping restraint of movable mold. After mold is opened, fixed mold surface is only restrained by shrinkage. Figure 4 is mold surface strain change curve of areas A, C and E during one die-casting cycle. It can be seen that maximum strains in the three areas of mold are 0.034, 0.025 and 0.022mm respectively. At the same time, it was found that maximum strain on mold surface occurs when mold is just opened. Because fixed mold is no longer constrained by movable mold clamping force after mold is opened, mold surface changes from compressive stress to tensile stress. When mold deforms outside yield zone, elastic deformation will occur, so strain amount will decrease later in cycle.

 

Figure 3 Stress change curves of surface in areas A, C, and E of mold during multiple cycles

 

Figure 4 Single-cycle surface strain value changes in areas A, C, and E of mold

 

Figure 5 H13 mold steel material strain-life relationship curve
Partial cutting of mold and insert processing. Local insert cutting can make local mold cooling system processing process simpler. Distance between cooling point and mold forming surface can be designed to be less than 5mm, which has a better cooling effect on local high-temperature area of mold. Insert structure is relatively simple, which is conducive to obtaining better physical properties during heat treatment process and reducing process difficulty of surface strengthening treatment; local inserts can be made of materials with better physical properties without having a great impact on the overall production cost of mold; premature aging and failure of inserts facilitates re-production and replacement, extending the overall service life of mold. Partial insert design of gearbox housing mold studied is shown in Figure 6. An independent high-pressure point cooling design is partially adopted. Point cooling design is shown in Figure 7.

 

Figure 6 Gearbox housing mold insert design

 

Figure 7 Insert cooling duct design

 

Figure 8 Cracked state of insert blank surface

 

Figure 9 Cracks on the surface of the insert
In conclusion
Based on numerical simulation results of mold temperature field, stress field on mold surface is simulated and analyzed. Mold surface is periodically affected by tensile and compressive stress during die-casting production process, leading to thermal fatigue. Accuracy of simulation results was verified through calculation of thermal fatigue life, service life estimation of front and middle stages of mold filling, finally through actual production verification and mold life statistics.