Al-Zn-Mg-Cu high-strength aluminum alloy is widely used in aerospace, transportation and other fields due to its excellent properties. However, due to wide solid-liquid temperature range of this alloy and poor slurry fluidity, it is prone to thermal cracking during casting process, which often results in low yield and limits its application scope. Rheological die-casting technology has characteristics of uniform and fine structure, nearly spherical shape, not easy to entrain air during mold filling, small solidification shrinkage and high-pressure feeding. It has a small tendency to produce hot cracks and holes, which is beneficial to control of hot crack defects during deformed aluminum alloy casting process. Therefore, understanding hot cracking mechanism during rheological die casting process of Al-Zn-Mg-Cu alloy and effectively controlling occurrence of hot cracks, holes and other defects by adjusting process parameters is the key to realizing casting of high-strength aluminum alloy parts and their large-scale application. Generation of hot cracking defects is closely related to holes. In rheological die casting, holes are often mainly caused by shrinkage cavities. Although a lot of research has been done on mechanism and influencing factors of hot cracking, there are few research reports on tendency of hot cracking during rheological die casting of Al-Zn-Mg-Cu alloys. In order to explore hot cracking tendency of rheological die-casting Al-Zn-Mg-Cu high-strength aluminum alloy, a semi-quantitative hot cracking evaluation method was adopted to analyze fracture structure and microscopic morphology of holes in hot cracked specimens, and effect of boosting specific pressure on tendency of hot cracking was systematically studied.
Graphical results
Use a resistance furnace to melt Al-Zn-Mg-Cu alloy raw material to 720℃, keep it warm for 30 minutes, pass in argon gas for refining for about 10 minutes, degas and remove slag, then let it stand, then transfer slurry in the resistance furnace to EMS-05SM temperature-controlled electromagnetic stirring furnace, feed argon gas again (about 5 minutes) for degassing. When melt cools to 640℃, electromagnetic stirring starts immediately, stirring power is fixed at 3kW and stirring frequency is constant at 20Hz. During electromagnetic stirring period, a K-type thermocouple was used to monitor slurry temperature. When temperature dropped to 630℃, a semi-solid slurry was obtained for die casting.

Table 1 Chemical composition of Al-Zn-Mg-Cu alloy for testing (%)

 

Figure 1 Schematic diagram of die castings and dimensions of hot cracking evaluation specimens

Table 2 Number and probability of hot cracking of samples

 

Figure 2 Effect of boosting specific pressure on hot cracking tendency coefficient

 

Figure 3 Typical distribution locations of sample fracture behavior
Figure 4 shows hot crack fracture morphology of Al-Zn-Mg-Cu alloy samples die-cast under different pressurization ratios. It can be seen that fracture surface of sample has shrinkage defects distributed, and cross-section flatness is not high. As supercharged specific pressure increases, shrinkage holes gradually expand outward from center of sample, and size of shrinkage holes changes significantly. When specific pressure is less than 81MPa, large shrinkage holes are distributed in the center area of fracture surface. At 81MPa, center area of fracture surface shows shrinkage porosity, and shrinkage holes are mainly distributed in radial 0.5R area of fracture cross section; when it is greater than 81MPa, center area of fracture surface shows shrinkage porosity. Existence of tissue looseness can no longer be clearly seen in the area, and size of shrinkage holes has been further reduced. In addition, in all fracture edge areas, it can be seen that there is a relatively flat cross-section area, structure is also relatively uniform and dense.

 

Figure 4 Fracture morphology of die-cast specimens under different pressurization ratios

 

Figure 5 Sampling position of the hot cracked fracture section of the die-casting sample

 

Figure 6 Microstructure morphology of center region of fracture surface of Al-Zn-Mg-Cu rheological die-casting sample

 

Figure 7 High-magnification image of particles on fracture surface of die-casting sample and EDS results
(a) Broken tail (b) Point 1EDS (c) Striped precipitated phase (d) Point 2EDS
Figure 8 shows microstructure where radial direction of fracture is about 1.0R (edge area). It can be seen that there are a large number of dimples on fracture edges of die castings under different pressurization ratios. Macroscopically, fracture surface of Al-Zn-Mg-Cu alloy rheological die-casting sample shows brittle fracture characteristics, and plastic deformation also exists in local areas, manifesting as dimples. This fracture form can be judged to be a micropore aggregation type fracture, which is a ductile fracture and manifests as a macroscopic brittle micropore type fracture. This type of fracture usually occurs when a cracked sample of a high-strength material is stretched at room temperature and material's toughness is insufficient when crack propagates. Microstructure morphology is small and evenly distributed equiaxed micropores. Figure 9 is a high-magnification microscopic morphology picture of dimples. The largest dimple diameter can reach more than 28 μm, cracks and precipitated phases can be observed at the bottom of dimple.

 

Figure 8 Morphology of fracture edge area of Al-Zn-Mg-Cu rheological die-casting sample
(a)67MPa (b)74MPa (c)81MPa (d)87MPa (e)94MPa

 

Figure 9 Microscopic morphology of dimples

 

Figure 10 Schematic diagram of fracture stages of Al-Zn-Mg-Cu alloy rheological die castings
Fracture of Al-Zn-Mg-Cu alloy rheological die castings is divided into three stages (see Figure 10): Stage I is in the center area of sample, which is also main location of crack source, showing brittle fracture characteristics. Its formation is mainly affected by insufficient feeding in the center of sample, especially when pressurization specific pressure is small, large shrinkage holes are formed and lead to formation and expansion of crack sources; Stage II is a combination of brittle-ductile fracture, this area is transition stage of fracture; Stage III is ductile fracture stage. This area is close to surface of sample, grain structure is fine and uniform, and it has better toughness, so dimples appear on fracture surface. This stage is also final stage of fracture and should be instantaneous fracture when reaching critical value. Bonding strength of grain boundaries is main factor affecting crack expansion. Precipitated Fe-rich phase will weaken grain boundaries, promote crack expansion, and increase possibility of hot cracking.

 

Figure 11 Macroscopic morphology of cross-section of Al-Zn-Mg-Cu alloy rheological die-casting specimen at different boosting specific pressures

 

Figure 12 Porosity statistics of Al-Zn-Mg-Cu alloy rheological die castings under different pressurization ratios

 

Figure 13 Change curves of number of holes, shape factor and equivalent diameter of Al-Zn-Mg-Cu alloy rheological die casting samples under different boosting pressures

 

Figure 14 Microstructure of Al-Zn-Mg-Cu alloy casting structure shrinkage cavity location
In conclusion
Increasing boosting specific pressure can effectively compensate for shrinkage at the end of solidification, reduce occurrence of hole defects, and improve the overall density of castings. Continuously increasing boosting specific pressure will reduce porosity reduction effect of castings. While supercharging, it will give molten metal an impact force. When supercharging specific pressure is too high, it may cause cracking of solidifying tissue and aggravate tendency of hot cracking. Diameter of parallel section area is smaller, and holes reduce effective bearing area for stress, making it easier for thermal cracks to occur in this area. Area near gate tends to have a longer solidification time and has a better feeding effect, so there are fewer holes. However, at the same time, due to poor performance of incompletely solidified structure, excessive pressure increase is more likely to cause stress in the area near gate to exceed critical value and cause cracking, which increases tendency of hot cracking.