Prediction of Casting Fatigue Life

Aluminum alloys have the characteristics of high specific strength, corrosion resistance, and low cost, and are widely used in the aerospace and automotive industries. However, traditional casting processes (sand mold and gravity) have shrinkage defects in cast aluminum alloys, which seriously affect the mechanical properties, especially fatigue performance. Therefore, it is necessary to improve the casting process to reduce shrinkage defects. Differential pressure casting is a reverse gravity casting technology that can control the filling speed and continue to pressurize the aluminum liquid after filling, allowing the casting to solidify under greater pressure, improving the filling capacity of the aluminum liquid, reducing the formation of casting defects such as shrinkage porosity, and obtaining finer dendrites. It is suitable for various thin-walled aluminum alloy castings with complex structures and strict performance requirements, such as automotive steering knuckles, control arms, brackets, etc.

The main factor affecting the mechanical properties of cast aluminum alloys without defects is the size of the secondary dendrite spacing (SDAS). At present, there are many reports on the relationship between secondary dendrite spacing and mechanical properties, and there are quantitative formulas for predicting mechanical properties through secondary dendrite spacing. However, there are few reports on the quantitative research of secondary dendrite spacing on fatigue performance. This study produced castings with fewer defects using differential pressure casting technology. Samples were taken from different areas of the castings for material fatigue testing, and the secondary dendrite spacing at the sampling site was measured to establish a quantitative relationship between the secondary dendrite spacing and the fatigue performance of A356 aluminum alloy produced by differential pressure casting.

Aluminum ingots are melted in a melting furnace and then poured into a crucible furnace. The hydrogen content is controlled by a rotor degasser and high-purity argon gas, and Al-5Ti-B and Al-10Sr intermediate alloys are added for refinement and modification treatment. Die casting process: First, the mold cavity and furnace cavity are pressurized simultaneously to establish system pressure. Then, the mold cavity maintains pressure, while the furnace cavity continues to pressurize to create a pressure difference, causing the aluminum liquid to enter the mold through the lifting pipe. Then, the aluminum liquid fills and continues to pressurize in the mold. Then, the mold cavity releases pressure, and the furnace cavity maintains pressure for a period of time until the casting is formed. Finally, the furnace cavity releases pressure and the casting cools down.

Samples were taken from different areas and wall thickness positions of the casting for fatigue specimen preparation. The size schematic diagram is shown in Figure 2, which meets the ASTM-E466 standard. The fatigue specimen is processed by a CNC lathe, and then polished with sandpaper and diamond particles to achieve a surface roughness Ra of less than 0.1 μ m. The material fatigue test was conducted on the INSTRON-8801 electro-hydraulic servo fatigue testing machine at room temperature, with a stress ratio of R-1 and a frequency of 40 Hz. The fracture surface was analyzed using a ZEISS EVOMA15 scanning electron microscope. Samples were taken near the fracture surface of the fatigue specimen for metallographic observation. After sanding and mechanical polishing, they were corroded with HF solution with a volume fraction of 0.5% using ZEISS Axio Vert A1 type metallographic microscope was used for microstructure characterization, and then the secondary dendrite spacing was measured using the microscope’s built-in software Pro image. SDAS=d/n, where d is the length from the center of the measured dendrite to the center of the last dendrite; N is the number of dendrite arms. When measuring the secondary dendrite spacing, observe each sample for 10 fields of view and calculate the average and standard deviation.

The metallographic structures of differential pressure castings with different secondary dendrite spacings were measured to be (19 ± 1) μ m and (40 ± 2) μ m, respectively.
From the low magnification morphology, it is evident that the microstructure with smaller secondary dendrite spacing is finer. From the high magnification morphology, a typical hypoeutectic Al Si alloy microstructure can be observed, with the light gray area representing the primary α – Al matrix and the dark gray area representing eutectic Si particles. In addition, eutectic Si particles with smaller secondary dendrite spacing are more dispersed than those with larger secondary dendrite spacing.

(1) Due to differences in cooling rates, the secondary dendrite spacing in different areas of differential pressure cast A356 aluminum alloy castings is uneven. Secondary branch The distribution range of crystal spacing is between 19~40 μ m, and the eutectic Si particles in samples with smaller secondary dendrite spacing are more dispersed.
(2) A quantitative relationship between fatigue life, stress, and secondary dendrite spacing of differential pressure cast A356 aluminum alloy was established through experiments, and S-N curves corresponding to different secondary dendrite spacings were obtained. Under the same stress, the fatigue life decreases with the increase of secondary dendrite spacing, and the effect of secondary dendrite spacing on fatigue life can be understood as diffusion strengthening.
(3) Through experimental verification, the maximum absolute logarithmic relative error of fatigue life prediction results is below 2%. This quantitative relationship can effectively predict the fatigue performance of differential pressure castings with different secondary dendrite spacings in different regions.

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