Al-Si alloy is widely used to manufacture parts in automobiles, rail transit, aerospace and other fields due to its good casting properties and mechanical properties. In its service environment, it may withstand cyclic loads, so fatigue performance of this type of aluminum alloy castings is very important. However, fatigue performance of aluminum alloy castings is affected by many factors. GERBE S et al. found that the larger secondary dendrite arm spacing (SDAS) value of cast aluminum alloy structure, the lower fatigue performance. Since aluminum alloys inevitably have casting defects (such as shrinkage cavities, pores, oxide films, etc.), there are many studies on various properties of aluminum alloys. It is generally believed that the larger size of casting defects, the worse fatigue performance of casting.
Casting defects in aluminum alloys are generally eliminated by optimizing casting process. Aluminum alloy semi-solid forming technology has high material utilization rate and high forming efficiency, can produce high-performance and complex-shaped parts, which can effectively reduce production costs. At the same time, semi-solid die castings have good density, uniform structure, no coarse dendrite structure found in ordinary castings, and have good mechanical properties. This topic uses conventional die-casting and semi-solid die-casting methods to prepare ZL114A aluminum alloy fatigue specimens, and compares fatigue properties of castings formed by different processes through high-cycle fatigue tests. Observe and analyze fracture surface of fatigue specimens to explore relationship between forming methods and defects and fatigue properties of die-cast specimens, providing a reference for research on die-casting process of high-fatigue performance aluminum alloy parts.
Graphical results

Table 1 Chemical composition of ZL114A alloy (%)
First, melt about 8kg of ZL114A aluminum ingot in a graphite crucible in an electromagnetic stirring furnace, raise temperature to 700℃, and keep it warm for 30 minutes. Then pass argon gas into melt for refining for 10 minutes. After degassing and removing slag, let it stand and pour die-casting sample at 670℃. Electromagnetic stirring was performed at 605℃ with a frequency of 5Hz, a time of 60s, a power of 3kW to prepare a semi-solid slurry and die-cast it. Die-casting sample is a round sample, and specific dimensions are shown in Figure 1. Prepared samples were machined in accordance with requirements of GB/T3075-2008, surface of samples was polished to Ra0.2, then numbered numerically, and density of samples was measured using drainage method. PLG-100 high-frequency resonance fatigue machine was used to conduct axial fatigue test, and stress ratio R=0.1. Set 4 stress levels (stress levels), namely 70, 80, 90, and 100 MPa, take 6 samples for each stress level. VHX-1000 ultra-depth-of-field microscope and Inspect F50 scanning electron microscope were used to analyze fracture surface of fatigue sample. After polishing and etching a section near fracture surface of fatigue sample, metallographic structure was observed and photographed. Perimeter and area of defective area are measured through a super-depth-of-field microscope to calculate average equivalent diameter of defective area. Calculation formula is:

 

 

Figure 1 Dimensions of die-cast specimens for fatigue testing

 

Figure 2 Fatigue test results of liquid die-casting specimens

 

Figure 3 Fatigue test results of semi-solid die-cast specimens

 

Figure 4 Relationship between fatigue specimen density and average equivalent diameter of crack source defects

 

Figure 5 Relationship between density and fatigue life of liquid die-casting specimens

 

Figure 6 Relationship between density and fatigue life of semi-solid die-casting specimens

Table 2 Regression analysis results of density and fatigue life of semi-solid die castings under various stresses
Typical macro fatigue fracture morphology of the two batches of samples is shown in Figure 9. Cross-sectional defects near fracture are shown in Figure 10. As can be seen from Figure 9a, in addition to central shrinkage area, a large number of pores appear in crack expansion area and instantaneous fracture area on fracture surface of liquid die-casting sample. It can also be seen from Figure 10a that there are a large number of pores dispersedly distributed in section near fracture, and there are also larger shrinkage pores. As can be seen from Figure 9b, fracture morphology of semi-solid die casting has very few holes except for large hole in the middle, and they are close to the center. Figure 10b also reflects pattern that holes are concentrated in the middle. It is worth noting that compared with large middle hole in fracture topography in Figure 9b, pores on fracture surface are much smaller. Therefore, it is speculated that holes may be interconnected or located very close in three dimensions, so that crack source hole merges with surrounding small holes in initial expansion stage and finally forms a large hole crack source.

 

Figure 7 Average equivalent diameter and fatigue life of crack source defects in liquid die-casting specimens

 

Figure 8 Average equivalent diameter and fatigue life of crack source defects in semi-solid die-casting specimens

 

Figure 9 Typical fracture morphology of fatigue specimens with different processes

 

Figure 10 Defects in the near-fracture section of fatigue specimens with different processes

 

Figure 11 Near-fracture microstructure morphology of fatigue specimens with different processes
(1) Compared with liquid die-casting, fatigue performance of semi-solid die-cast ZL114A aluminum alloy specimens is generally higher and more stable. Semi-solid die-cast ZL114A aluminum alloy sample has good fatigue performance when density is greater than 2.62g/cm3. Under 70MPa stress, fatigue life can reach 107 times.
(2) Fatigue life of semi-solid die-cast specimens is proportional to density. The higher density of specimen, the higher fatigue life of specimen. Fatigue life of liquid die-casting samples is mainly affected by hole size. The larger hole size, the lower fatigue life.
(3) There are holes in the entire cross section of liquid die-casting sample, and cracks originate from large shrinkage holes. Holes in the semi-solid die-casting sample are concentrated near center, crack initiation source tends to be combined effect of shrinkage and porosity in the center.