In recent years, the aerospace industry has increasingly adopted high-strength ZL205A aluminum alloy for structural castings, especially for large axisymmetric bodies. However, one of the most challenging sand foundry defects encountered in production is macrosegregation. This defect not only degrades mechanical properties but also raises safety concerns. My work focuses on understanding the formation mechanism of segregation defects in large ZL205A axisymmetric-body castings and on developing practical casting and grain refinement strategies to mitigate them. Through extensive experimentation, including scanning electron microscopy, energy dispersive spectroscopy, differential scanning calorimetry, and finite element simulation, I have identified the main features of segregation, quantified its effect on static mechanical properties, and proposed effective countermeasures. The results show that the combination of low-pressure casting, improved sand core collapsibility, and optimized Al-Ti-B grain refinement can significantly reduce the severity and frequency of sand foundry defects, particularly segregation, in large ZL205A castings.
1. Introduction
ZL205A is an Al-Cu-based high-strength casting alloy developed in China, with tensile strength exceeding 490 MPa in the T6 condition. It has been widely used for aerospace structural components such as missile shells, engine frames, and helicopter parts. Despite its excellent strength, the alloy is prone to macrosegregation during solidification because of its wide freezing range and complex multi-component chemistry. In sand casting production, segregation appears as fish-bone or banded patterns on X-ray films, typically located at the junction between the casting body and the slit gate. When the defect is severe, castings are rejected, leading to significant economic loss and schedule delays.
Sand foundry defects in aluminum alloys have been studied for decades, but systematic research on segregation in large ZL205A castings is still incomplete. Most existing studies focused on laboratory-scale specimens or simple geometries, while large axisymmetric bodies present unique thermal and mechanical boundary conditions. My research aims to bridge this gap by investigating segregation defects from three aspects: (1) characterization and mechanism, (2) influence of pouring process, and (3) influence of grain refinement. Throughout this thesis, I emphasize the role of sand foundry defects and how casting parameters can be adjusted to prevent them.
2. Materials and Experimental Procedures
2.1 Alloy Composition and Melting
The ZL205A alloy used in this study was prepared from high-purity aluminum (99.99%), Al-50Cu, Al-10Mn, Al-4Ti, Al-4V, Al-2Zr master alloys, and metallic cadmium. The nominal chemical composition is given in Table 1.
| Element | Cu | Mn | Ti | Zr | Cd | B | V | Al |
|---|---|---|---|---|---|---|---|---|
| Range | 4.6-5.3 | 0.3-0.5 | 0.15-0.35 | 0.05-0.20 | 0.05-0.25 | 0.0005-0.060 | 0.05-0.30 | Balance |
Melting was conducted in a 75 kW resistance furnace with a 500 kg cast iron crucible. A typical melt practice included the following steps: preheating the crucible to 300–500°C, adding AlMn10A, AlTi5A, AlZr4A, AlV4A, and returning scrap, then melting completely. After reaching 690–710°C, AlCu50A and Cd wrapped in aluminum foil were added and stirred. The melt was then superheated to 740–760°C, and AlTi4BA grain refiner was added. Degassing was performed with C2Cl6 tablets and rotary argon degassing. The melt was held for 15–20 minutes before pouring.
2.2 Casting of Large Axisymmetric Bodies
The component investigated is a cylinder with a diameter of 1400 mm, height of 500 mm, and wall thickness of 30 mm. Both gravity casting and low-pressure casting were evaluated. The mold was made of resin-bonded sand. For gravity casting, the pouring temperature was 715°C and pouring time was 27 seconds. For low-pressure casting, the parameters were: pressurization for filling pressure 25 kPa, filling pressure 40 kPa, holding pressure 45 kPa, holding time 300 seconds, and pouring temperature 715°C.
In the low-pressure process, the metal is pushed upward by gas pressure, providing smooth filling and effective feeding. This technology is especially beneficial for large thin-walled castings because it reduces turbulence and oxide inclusions, which are common sand foundry defects.
2.3 Specimen Preparation and Testing
After T5 heat treatment (solutionizing at 538°C for 10–18 h, water quench, then aging at 155°C for 8–10 h), the castings were inspected by X-ray radiography. Regions with segregation were marked and extracted for further analysis. Tensile specimens were machined from both sound and defective areas. The tensile tests were performed on an Instron 5582 universal testing machine at a strain rate of 1 mm/min. Differential scanning calorimetry (DSC) was carried out on a NETZSCH DSC 204F1 instrument with a heating rate of 10°C/min in an argon atmosphere. Microstructural characterization was performed using a Leica DM2500M optical microscope and a JEM-7001F field-emission scanning electron microscope equipped with an EDAX energy dispersive spectrometer.
3. Characterization of Segregation Defects
3.1 DSC Results
DSC curves of the sound material and the segregated material are compared in Table 2. The sound sample showed a single main endothermic peak with an onset at 640.8°C and a peak at 664.0°C, corresponding to the melting of the α-Al matrix. In contrast, the segregated sample exhibited a low-temperature endothermic peak starting at 544.6°C, which is attributed to the melting of the Al2Cu eutectic. The main peak onset shifted to 625.9°C, indicating that the presence of large amounts of low-melting-point phases reduces the effective solidus temperature.
| Sample | Onset 1 (°C) | Peak 1 (°C) | Onset 2 (°C) | Peak 2 (°C) | Enthalpy (J/g) |
|---|---|---|---|---|---|
| Sound | 640.8 | 664.0 | — | — | 252.97 |
| Segregated | 544.6 | 550.5 | 625.9 | 659.1 | 198.66 |
3.2 Microstructure and Composition
Optical microscopy revealed that both sound and segregated specimens contain white second phases along grain boundaries. Energy dispersive spectroscopy showed that the white phases are highly enriched in Cu, with an atomic ratio Cu:Al close to 1:2, confirming that they are Al2Cu. In the segregated specimens, the Al2Cu networks are much thicker and more continuous than in sound ones. X-ray radiography displays these regions as white fish-bone patterns because Al2Cu has a higher density than the α-Al matrix.
For quantitative comparison, I measured the fraction of the white phase using point counting. In the sound sample, the Al2Cu fraction was below the detection limit of radiography, whereas in the segregated sample it covered 20–40% of the fracture surface. This demonstrates that segregation is essentially a severe local accumulation of eutectic Al2Cu, which is one of the most harmful sand foundry defects for Al-Cu alloys.
3.3 Mechanical Property Degradation
Table 3 compares tensile properties of sound and segregated specimens. The sound flat specimens (6 mm thick) had an average ultimate tensile strength (Rm) of 435.3 MPa, yield strength (Rp0.2) of 340 MPa, and elongation (A) of 9.18%. The segregated specimens had an average Rm of 386.2 MPa, Rp0.2 of 340 MPa, and elongation of 3.86%. Therefore, segregation reduces tensile strength by about 11% and elongation by about 58%, while yield strength remains almost unaffected.
| Condition | Rm (MPa) | Rp0.2 (MPa) | A (%) |
|---|---|---|---|
| Sound | 435.3 ± 10 | 340 ± 15 | 9.18 ± 1.8 |
| Segregated | 386.2 ± 16 | 340 ± 18 | 3.86 ± 0.4 |
Fracture surface analysis showed that sound specimens failed in a ductile manner with fine dimples, whereas segregated specimens displayed brittle intergranular fracture. The thick Al2Cu network along grain boundaries acts as a crack path, causing premature failure. Microhardness measurements also showed lower hardness in segregated regions (140–160 HV) compared to sound regions (162–164 HV), although the difference was not huge.
3.4 Formation Mechanism of Segregation
The formation of segregation in ZL205A can be understood through the solidification process. During cooling, primary α-Al dendrites form first. The remaining liquid becomes enriched in Cu because the partition coefficient of Cu in aluminum is less than unity. If the dendrite network is strained by thermal contraction or mold resistance, cracks can form in the semi-solid mushy zone. The Cu-rich liquid then flows into these cracks and solidifies as coarse Al2Cu eutectic. This mechanism is supported by the fact that segregation defects are predominantly located at the junction between the casting and the slit gate, where both thermal stress and shrinkage stress are concentrated.
The tendency for segregation can be expressed in terms of the temperature gradient \(G\) and solidification velocity \(v\). For a binary alloy, the local solidification time \(t_f\) is given by
$$ t_f = \frac{\Delta T}{G v} $$
where \(\Delta T\) is the freezing range. A large freezing range (about 89°C for ZL205A) combined with a low temperature gradient promotes a wide mushy zone and encourages the formation of sand foundry defects such as segregation and hot tearing.
Another factor is the stress that develops during solidification. The total stress \(\sigma_{tot}\) at the hot spot can be written as
$$ \sigma_{tot} = \sigma_{th} + \sigma_{shr} + \sigma_{mold} $$
where \(\sigma_{th}\) is the thermal stress caused by differential cooling, \(\sigma_{shr}\) is the shrinkage stress, and \(\sigma_{mold}\) is the mechanical restraint from the sand mold. When \(\sigma_{tot}\) exceeds the intergranular bonding strength, the dendrite network separates and the intergranular liquid is sucked into the gap. This is the fundamental origin of fish-bone segregation.
4. Effect of Casting Process on Segregation
4.1 Gating System Design
For the large axisymmetric body, a vertical slit-gate system with a central sprue was selected. The size of the slit gate was optimized using a three-factor, three-level orthogonal array. The factors were slit width δ, slit length b, and runner diameter d. The levels are listed in Table 4.
| Factor | Level 1 | Level 2 | Level 3 |
|---|---|---|---|
| Slit width δ (mm) | 15 | 25 | 35 |
| Slit length b (mm) | 25 | 30 | 35 |
| Runner diameter d (mm) | 60 | 80 | 100 |
Finite element simulations using ProCAST were performed to evaluate filling and solidification. The best combination was selected based on complete filling within 3 s and a low peak temperature difference during solidification. The optimal parameters were δ = 25 mm, b = 35 mm, and d = 80 mm. These parameters were subsequently used for all pouring experiments.
4.2 Gravity versus Low-Pressure Casting
To compare the effect of pouring process on sand foundry defects, I simulated both gravity and low-pressure casting using finite element software. Figure 1 shows the simulated temperature fields during solidification. In gravity casting, the temperature gradient is non-uniform, and the last solidifying region appears near the slit-gate root. In low-pressure casting, a more uniform temperature distribution is achieved, and the last solidifying region is shifted into the feeding system.
The peak temperature differences are summarized in Table 5. Low-pressure casting reduced the maximum temperature difference in the longitudinal direction from 186°C to 64°C, and in the radial direction from 312°C to 108°C. The overall peak temperature difference dropped by 62%, from 324°C to 126°C. This reduction is critical because a lower temperature gradient minimizes the width of the mushy zone and reduces the driving force for intergranular liquid flow.
| Process | Total peak ΔT (°C) | Horizontal peak ΔT (°C) | Vertical peak ΔT (°C) |
|---|---|---|---|
| Gravity | 324 | 312 | 186 |
| Low-pressure | 126 | 108 | 64 |
I then conducted actual pouring experiments. Three batches of castings were produced under each process. X-ray inspection results are presented in Table 6. A total of 125 segregation defects were found in the three gravity-cast batches, whereas only 23 defects were found in the three low-pressure-cast batches. This demonstrates that low-pressure casting dramatically reduces the occurrence of sand foundry defects in large ZL205A castings.
| Process | Shrinkage porosity | Gas porosity | Linear segregation | Plate-like segregation | Fish-bone segregation | Total |
|---|---|---|---|---|---|---|
| Gravity | 42 | 24 | 45 | 8 | 6 | 125 |
| Low-pressure | 14 | 5 | 3 | 0 | 1 | 23 |
4.3 Sand Core Collapsibility
Another important factor affecting segregation is the resistance of the sand core to the solidification shrinkage of the casting. In the original process, the sand core was solid, providing a strong restraint. I modified the core design by making it hollow (100 mm wall thickness) and dividing it into four segments with 6–8 mm gaps between them. This design improves the collapsibility of the core, allowing it to deform and thus release some of the mechanical stress.
The effect of the improved core was verified in both gravity and low-pressure casting. With the modified core, the total number of segregation defects in gravity casting decreased from 125 to 63, and in low-pressure casting from 23 to 10. This confirms that reducing mold restraint is an effective way to suppress hot cracking and subsequent segregation. The contribution of core collapsibility to the relaxation of stress can be estimated by considering the deformation capacity \(\varepsilon_{core}\) of the core. If the core can accommodate the linear contraction \(\alpha_s \Delta T\) of the casting, the stress is reduced. The required gap \(g\) is
$$ g \geq \alpha_s \Delta T L $$
where \(L\) is the characteristic length of the casting. For aluminum alloys, \(\alpha_s \approx 2.4 \times 10^{-5} \, \text{K}^{-1}\), and for a 500 mm long section with 100°C temperature drop, \(g \geq 1.2\) mm. The 6–8 mm gaps were therefore more than sufficient.
5. Effect of Grain Refinement on Segregation
5.1 Optimization of Al-Ti-B Addition
Grain refinement is known to improve feeding and hot-tearing resistance. In the traditional process, Al-5Ti-B master alloy was added in one batch. However, due to the long holding time in sand casting production, Ti-rich particles settle and the refining effect degrades. I optimized the process by adding two-thirds of the Al-4Ti-B master alloy during melting and the remaining one-third 30 minutes before pouring. This multiple-step addition maintains a high number of effective nucleant particles in the melt.
Simulation and casting trials showed that the optimized process reduced the average grain size from approximately 185 μm to 91 μm, a reduction of 50.8%. The improvement is clearly visible in both etched macro-specimens and optical micrographs. Table 7 summarizes the grain refinement results.
| Property | Conventional process | Optimized process | Improvement |
|---|---|---|---|
| Grain size (μm) | 185 | 91 | -50.8% |
| Flow length (mm) | 184 | 246 | +33.7% |
| UTS (as-cast, MPa) | 195 | 219 | +11% |
| YS (as-cast, MPa) | 83 | 94 | +11.7% |
| Elongation (as-cast, %) | 8 | 12 | +33% |
5.2 Flowability and Hot Tearing Resistance
The fluidity of the alloy was evaluated using a spiral test. The optimized grain-refined alloy exhibited a flow length of 246 mm compared to 184 mm for the conventional alloy, representing a 33.7% increase. This is beneficial because improved fluidity allows better feeding of the solidification shrinkage, reducing the tendency for the intergranular liquid to be drawn into the tensile regions that cause segregation.
Hot tearing susceptibility was evaluated using a hot-ring test. In this test, steel cores of different diameters are placed in the center of a disc-shaped cavity. The first ring width at which a crack appears is recorded. Table 8 shows that for the conventional alloy, hot tearing occurs at a ring width of 27.5 mm, while the optimized alloy does not crack even at 22.5 mm. The finer grain structure increases the contact area between grains and raises the intergranular bonding force, thereby suppressing hot cracking.
| Alloy condition | 27.5 mm ring | 25 mm ring | 22.5 mm ring |
|---|---|---|---|
| Conventional | Cracked | Cracked | Not cracked |
| Optimized | Not cracked | Not cracked | Not cracked |
The underlying mechanism can be explained by the Hall-Petch relationship:
$$ \sigma_s = \sigma_0 + K d^{-1/2} $$
where \(\sigma_s\) is the yield strength, \(\sigma_0\) is a friction stress, \(K\) is a constant, and \(d\) is the grain diameter. Finer grains provide more grain boundary area and thus strengthen the material. More importantly, during the final stage of solidification, a finer grain size promotes the formation of numerous dendrite bridges. These bridges connect adjacent grains and increase the resistance to separation. The time required for creep-like deformation of the bridges is longer, so the alloy can withstand more strain before cracking.
5.3 Effect on Large Casting Segregation
Using the optimized grain refinement process, I poured three additional batches under both gravity and low-pressure casting. The total number of segregation defects in the gravity-cast batches was 77, compared to 125 with the conventional grain refinement. For low-pressure casting, the number decreased from 23 to 16. This result confirms that grain refinement alone is effective in reducing sand foundry defects, and the combination of grain refinement with low-pressure casting gives the best overall quality.
It is important to note that the optimal casting practice is not simply the sum of individual improvements. The low-pressure casting provides a cleaner filling and a more favorable thermal field, while the improved core collapsibility reduces mechanical stress, and grain refinement strengthens the semi-solid network. Together, these measures address the root causes of segregation: excessive temperature gradient, high mechanical restraint, and weak intergranular cohesion.
6. Conclusions
From my research, the following conclusions can be drawn:
- The typical segregation defects in large ZL205A axisymmetric castings are fish-bone-like uneven patterns located at the junction between the casting body and the slit gate. The defects consist of coarse Al2Cu eutectic that forms along grain boundaries and cannot be fully dissolved by the T5 heat treatment.
- Segregation significantly reduces the tensile strength and elongation of ZL205A alloy. In my experiments, the segregated specimens showed an 11% reduction in ultimate tensile strength and a 58% reduction in elongation compared to sound specimens, while yield strength remained unchanged. The fracture mode changes from ductile microvoid coalescence to brittle intergranular fracture.
- The formation of segregation is caused by tensile and shear stresses in the semi-solid mushy zone, which leads to intergranular separation. The Cu-enriched residual liquid fills the cracks and solidifies as Al2Cu. A high temperature gradient and poor sand core collapsibility increase the severity of the defect.
- Low-pressure casting significantly reduces the temperature gradient and promotes a beneficial solidification sequence. The number of segregation defects was lowered from 125 to 23 in three batches of production. The combination of low-pressure casting with improved core collapsibility further reduced the defect count to 10.
- Grain refinement using a two-step addition of Al-Ti-B master alloy reduces the average grain size from 185 μm to 91 μm. The optimized refining process improves alloy fluidity by 33.7%, increases as-cast tensile strength by 11%, and substantially improves hot tearing resistance. The refined alloy exhibits stronger dendrite bridging, which delays intergranular separation and minimizes sand foundry defects in the final casting.
In summary, sand foundry defects in large ZL205A castings can be effectively controlled by optimizing the pouring method, increasing sand core collapsibility, and refining the grain structure. The findings of my thesis provide both scientific insight and practical guidelines for foundry engineers working with high-strength aluminum alloys. Further work is recommended to quantitatively model the competition between feeding flow and crack formation, and to evaluate the long-term fatigue performance of castings repaired by welding or by process optimization.