Abstract
Partial segregation and shrinkage defects appear in local areas of a large counter-pressure ZL205A alloy shell with an abnormal figure during X-ray casting inspection. This paper takes these casting defects as the object of study, documents the locations where defects occur, and analyzes their impact on the mechanical properties of the castings. The causes of the defects were investigated through sampling observation, metallographic structure analysis, scanning electron microscopy (SEM) observation, and energy dispersive spectrometry (EDS) analysis. The results reveal that the casting defects are concentrated near the slot ingate, and the areas with shrinkage and segregation defects have varying degrees of alloy element enrichment. The formation of defective structures significantly reduces the mechanical properties of the material. Based on process analysis, the defects are primarily attributed to casting shrinkage stress, excessively high temperature fields, and insufficient refining. The mechanism of casting defect formation is discussed, and effective preventive measures are proposed to reduce thermal stress, avoid overheating of the structure, and promote uniform dispersion of the alloy.
1. Introduction
With the rapid development of aerospace and naval equipment, ZL205A aluminum alloy has received widespread attention and application in large load-bearing structural products due to its high strength and hardness, ease of machining, and excellent corrosion resistance. However, the wide solidification range of ZL205A aluminum alloy leads to a paste-like solidification mode, making the internal quality of castings susceptible to cooling conditions and prone to defects such as cracks, shrinkage, and segregation, which limit its practical applications.
2. Research Background and Significance
Compared to low-pressure casting technology, counter-pressure casting improves feeding ability by 4 to 5 times, significantly reducing the tendency for hot tearing during solidification, decreasing the formation of porosity in castings, and effectively enhancing casting quality and mechanical properties. Most military products are large, abnormal thin-walled shell components, which are typically produced using counter-pressure casting. A shell product was manufactured using ZL205A aluminum alloy through counter-pressure casting, featuring a complex structure with multiple layers, frames, and double-walled hollow tubes, with the largest casting dimension reaching 2,600 mm. Despite careful design considerations for the pouring system, melting process improvements, and cooling system optimization to ensure sequential solidification during the casting process, serious shrinkage and segregation defects were found in local areas during X-ray inspection of the casting blank.
3. Experimental Methods and Processes
Defects such as banding segregation and shrinkage were found in local areas of the ZL205A shell casting during X-ray inspection. Statistics showed that casting defects mostly occurred near the slot ingate, thick convex areas, and double-layer sand core skin areas. The following research methods were employed:
- Microstructure Observation: Using a Leica DM 2700M metallographic microscope to observe the microstructure.
- SEM Observation and EDS Analysis: Employing a TESCAN Mira 3 LMH SEM to observe the microstructure and fracture morphology of the alloy, with an accompanying EDS for elemental analysis.
- Mechanical Testing: Conducting tensile tests using a DNS-200 electronic material testing machine at a tensile rate of 0.2 mm/min, measuring three times and averaging the results.
Table 1. Mechanical Properties of Different Samples
| Sample Location | Tensile Strength (MPa) | Yield Strength (MPa) | Elongation (%) |
|---|---|---|---|
| Normal Area | 422 | 385 | 4.2 |
| Shrinkage Area | 356 | 343 | 0.5 |
| Segregation Area | 334 | 307 | 1.0 |
4. Results and Analysis
4.1 Fracture Morphology Observation
The tensile fracture morphologies of normal, shrinkage, and segregation samples. The normal tissue fracture exhibits many characteristics of ductile and quasi-cleavage fracture, with differently sized dimples distributed in the fracture tissue, macroscopically presenting many river patterns and tear ridges. The fractures of the shrinkage and segregation samples show obvious cleavage fracture characteristics, with almost no dimples present. The fracture tissue is distributed with many cleavage steps, and a large number of similar crystal structures are distributed in the fracture. A small number of river patterns extend along the stress direction, and the confluence direction of tributary cleavage steps represents the direction of fracture propagation. The segregation defect tissue tensile fracture exhibits obvious intergranular fracture, which is a typical characteristic of brittle fracture, with obvious overheating and melting phenomena at the grain edges.
4.2 Microstructure Observation and Composition Analysis
4.2.1 Metallographic Structure Observation
4.2.2 SEM Observation and EDS Detection
The microstructures of normal samples, shrinkage samples, and segregation samples observed under SEM. The scanning results are consistent with the metallographic structures, but the SEM images provide additional details on the morphology and distribution of precipitated phases. It was found that the microstructure of the shrinkage defect has fine pores on the grain boundaries, while the segregation defect has white precipitated eutectic phases along the grain boundaries, showing a network-like precipitation with obvious grain boundary melting pores, which severely inhibits dislocation movement and results in significant stress concentration, leading to a sharp decline in the toughness of the matrix.
EDS analysis was conducted on characteristic points of normal areas, shrinkage defects, and segregation defects. The results showed that the main element detected in the matrix of normal areas is Al (α-Al phase), with trace amounts of Mn, Ti, and Cd. In the segregation defect area, the white region is mainly rich in Cu and Cd eutectic phases. The formation of these defects is related to the uneven distribution of alloy elements during solidification.
Table 2. EDS Analysis Results of Different Sample Locations
| Sample Location | Detected Elements | Main Precipitated Phase |
|---|---|---|
| Normal Area | Al, Mn, Ti, Cd (trace) | α-Al phase |
| Shrinkage Area | Al, Cd, Cu | Al-based alloy with enriched Cd and Cu |
| Segregation Area | Al, Cu, Mn, Cd, Ti | Al2Cu, Al4Cu2Mn, Al-Cd-Ti phases |
4.3 Defect Mechanism Analysis and Solutions
The solidification crystallization temperature range of ZL205A is 89 °C, resulting in a short feeding distance and a tendency to solidify in a paste-like manner, which easily leads to casting defects such as segregation and shrinkage. During the solidification process, the α-Al phase precipitates first from the liquid phase. As the temperature decreases, the α-Al phase forms a skeletal distribution along the grain boundaries. Alloy element Mn partially dissolves into the α-solid solution and partially forms the Al2Mn2Cu phase with Al and Cu, distributed in a dispersed particulate form. The dispersed particles can effectively hinder dislocation movement and thus increase the strength of the alloy. Ti is introduced into the alloy from both ZL205A and the Al-5Ti-B modifier, and Ti reacts with Al to form the Al3Ti phase, which has a low mismatch with the α-Al matrix and can serve as a nucleation core for α-Al, restricting grain growth. Cd mainly contributes to aging strengthening of the matrix, directly enhancing the tensile strength and yield strength of the alloy.
5. Conclusion
(1) The presence of casting defects such as shrinkage and segregation significantly reduces the mechanical properties of ZL205A shell castings. The tensile strength, yield strength, and elongation of normal samples are 422 MPa, 385 MPa, and 4.2%, respectively, while those of shrinkage samples are 356 MPa, 343 MPa, and 0.5%, and those of segregation samples are 334 MPa, 307 MPa, and 1.0%.
(2) The fracture morphology of normal tissue shows a large number of dimples indicative of ductile fracture, river patterns indicative of quasi-cleavage fracture, and tear ridges. The fracture of the shrinkage defect sample exhibits obvious cleavage fracture characteristics, consisting of numerous cleavage steps and a few river patterns. The segregation sample fracture exhibits obvious intergranular brittle fracture characteristics, with grain boundary melting phenomena around the grains, and a large number of segregation phases distributed between and within the grains.
(3) Microstructure photos reveal that shrinkage and segregation defects mainly occur at the grain boundaries and are distributed in a network-like manner. Shrinkage defects show obvious overheating and remelting phenomena, mainly due to grain contraction under the influence of thermal stress.
(4) Based on the technological parameters, the main reasons for shrinkage porosity and segregation in the ZL205A casing casting during differential pressure casting are the large number of gated runners with gaps, poor yield of sand cores, unreasonable chill structure, and high pouring temperature. These factors lead to the easy formation of a higher temperature field during the pouring of the ZL205A large casing, resulting in larger thermal stresses within the mold cavity, and subsequently causing casting defects such as segregation and shrinkage porosity.