Introduction
Aluminium alloy castings have been used successfully in various branches of industry due to their excellent specifications and cost-effectiveness. With the development of modern industry and the expansion of the application area of aluminium alloy castings, people are paying more and more attention to the quality of aluminium alloy castings. However, all processes in the production of aluminium alloy castings may, at one time or another, produce defects. It is difficult to eliminate all aluminium alloy casting defects by conventional casting process. This paper will discuss the common defects in aluminium alloy castings, their causes and analysis and preventive measures.
Types of Defects in Aluminum Alloy Castings
1. Crack Defects
Hot tears or shrinkage cracks are the two main types of crack defects in aluminum alloy castings.
(1) Hot Tears. Hot tears occur due to thermal stresses that develop during the solidification and cooling of the aluminum alloy casting, usually on the surface. Hot tears can be produced by the thermal stress generated when the heat flow rate in the casting is too high, leading to cold and hot areas in the casting and shortening the solidification time of the casting; thermal stress generated by the different cooling rate of the local casting body, or thermal stress generated by the self-welding pattern of the casting.
(2) Shrinkage Cracks. Shrinkage cracks are mainly caused by local shrinkage of the aluminum alloy casting. When the internal casting stress exceeds the strength of the aluminum alloy casting material, the length of time and intensity of the cracks depend on the shrinkage area, the structure of the casting surface and the cast material properties. Common causes of shrinkage cracks include the wrong design of the casting, the over-thickness of the casting wall, the liquid shrinkage caused by the shrinkage of the metal and the die surface without cold compaction.
2. Porosity Defects
The most common type of porosity defect in aluminium alloy castings is gas porosity. It occurs when gas is released from the melt before filling, then getting trapped in the castings and forming bubbles. Gas porosity is mostly caused by the presence of hydrogen dissolved in the melt, and its main sources are the melting atmosphere, degassing, stirring and slag impurities. Porosity inside the cavity of a casting is a type of air hole, which is also caused by gas during the solidification process of metal.
3. Segregation Defects
Most aluminium alloy castings are prone to segregation defects caused by the segregation of molten metal elements. Segregation defects mainly occur in the joint of the cross section. For the same section, different metal components have different solidification temperatures, so the segregation of metal components leads to the difference in solidification time, resulting in different cooling speed and metallographic structure of different metal elements. The resulting segregation defects mainly include unorganized grain structure, loose organization and coarse carburized layer.
Analysis and Prevention Measures
1.Analysis
Assuming that a defect occurs in the aluminum alloy casting, it is necessary to carry out a specific analysis to find out the causes of the defect. The analysis is mainly conducted through theoretical analysis and experimental methods. Theoretical analysis mainly includes particle size analysis, structure analysis and computations about the casting; experimental methods mainly include light and electron microscopy (SEM), component analysis and thermal analysis.
2. Prevention Measures
(1) Optimization of Design Parameters. The design parameters of castings should be optimized to reduce the internal stress in castings and select the most suitable casting process and casting process parameter according to the design parameters.
(2) Improved Melting Process. The limiting factors in reducing gas porosity in aluminum alloy castings are gas solubility and gas release. Therefore, the vacuum degassing of the melt, purification of alloy raw materials, chemical solidification and slag flux improvement technology should be used to reduce gas pores and segregation.
(3) Improved Casting Process and Post-Casting Processing. The casting technique is used to improve the flow characteristics of the casting and reduce casting defects. The relevant post-casting processing measures should also be taken, such as Shot peening, annealing and die compaction treatment.
Conclusion
In summary, the most common defects in aluminium alloy castings are crack defects, porosity defects, and segregation defects. By theoretically analysing and experimentally testing the aluminum alloy castings, the causes of these defects can be analysed. Appropriate treatment measures should be taken in order to reduce or even eliminate these defects. Design optimization, improved melting and casting technologies, and post-casting processing are all important measures to produce defect-free aluminium alloy castings.
