Aluminum alloy common micro-defects
Shrinkage and loose
Both belong to the void class defects, coagulation volume supplement is not sufficient, only the shape and location of occurrence is different
Casting is generally not the final solidification, the formation of tubular or twig-shaped hole, a larger area. Visual surface shrinkage visible, the internal shrinkage can be found at low magnification inspection.
Generally exist in the dendrite fine and scattered holes, resulting in the internal structure of the casting is not dense. Low magnification visible visual loose, known as macroscopic loose; under the microscope to see the loose become slightly loose. Microscopic loose often forked black holes distributed in the dendrite, the hole even when serious network. If after heat treatment, loose holes blunt edges, sleek appearance.
In the process of solidification, due to shrinkage blocked, the internal gradually larger internal stress, the party stress exceeds the strength limit of the casting cracks. High temperature cracks are called high-temperature cracks, the crack is tortuous, along the grain boundary or dendrite development, the fracture is often yellow or dark crack. Cracks in the low temperature zone called the low-temperature cracking, the general development of transcrystallization, crack fracture with metallic luster.
Heat treatment crack (divided into two kinds)
Due to the quench cooling is too intense, uneven cooling aluminum parts, causing a lot of stress, stress-induced cracking in the area. Cracks more rigid, most of the crystal, a small part of the transcrystallization, the fracture is relatively flat, are brittle fracture
As the quenching heating temperature exceeds the alloy solidus temperature line, causing the grain boundary and eutectic melting, the formation of along the crystal or along the dendritic distribution of the over-burned crack quenching, severe mesh formation, and coarse tissue.
Metallographic examination often found in a variety of inclusions, divided into the following three categories.
Aluminum oxide inclusions
The oxide scale of the aluminum scale casts into the castings during pouring into inclusions. The black flocs or bent filaments appear under the microscope and the outline is unclear.
Casting profiles of non-metallic inclusions
Casting Profiles - The main quartz sand (SiO2) is mixed with molten metal during casting to form non-metallic inclusions. Metallographic observation can be seen when the outline was square or polygonal block, often appear in relief.
Inclusion of flux
Molten metal flux added to the flux is not clean inclusions. Due to flux inclusions in the metallographic sample is easy to dissolve to form a hole, but still visible in the hole residual flux and the adhesion of metal particles and so on.
Aluminum alloy in the solidification process, due to various reasons, resulting in various parts of the casting chemical composition and organization of non-uniform phenomenon. Metallographic examination can be seen in the organization segregation distribution. Depending on the site of occurrence, segregation can be divided into the following categories.
1. Crystal segregation
Rapid spread of the crystallization process can not be fully caused by the internal composition of the grain heterogeneity. This is a common phenomenon, with no significant effect on mechanical properties.
2. Regional segregation
As the crystallization has led to differences in the composition of different regions. If the center is rich in low melting point components are said to be segregation; if the outer enrichment of low melting point composition is called negative segregation.
3. eutectic segregation
Aluminum alloy solidification process. If a sinkhole-like defect occurs, the liquid phase of the eutectic component fills therein and causes coarse eutectic domains to become eutectic segregation. This is a harmful form of organization that has an adverse effect on performance.
Aluminum alloy added titanium, zinc and other refractory elements to form high melting point flaky compounds. These compounds crystallize earlier during solidification. High density, easily sinking density segregation. This segregation of packed compounds has an adverse effect on the properties of the alloy.
Aluminum alloy casting process, due to liquid metal, mold or other reasons, the foreign gas into the casting to form a more than the size of the holes called pores. Stomata can appear on the surface or inside, the shape often pear-shaped, oval or round. The inner wall is smooth and shiny. The outline is clearly visible. Stomata larger, at low magnification and fracture inspection can also be found.
During the melting and pouring process, the aluminum alloy liquid absorbs a large amount of hydrogen. When the casting solidifies, the solubility of hydrogen decreases, and the alloy precipitates hydrogen gas to form tiny holes called pinholes. Under the microscope, the pinholes are round or branch-shaped holes distributed between the dendrites. In Al-Si alloys, especially the higher eutectic alloys containing silicon, the hydrogen absorption tends to be serious, and the pinholes tend to be large in size, mostly dot-shaped and sometimes polygonal. Al-Cu and Al-Mg alloys tend to form microscopic loose, pinhole and this loose mixed as one, was polygonal continuous holes.
Solid solution strengthening phase incompletely dissolved
Aluminum alloy processing temperature is too low or not enough with insulation. So that after quenching the alloy structure more residual solid solution strengthening phase. Especially in the thick part of the workpiece is easy to form a thick reinforced phase is more likely to cause incomplete dissolution, often in the grain boundary and dendrite residual reinforcement phase, resulting in low mechanical properties of aluminum alloy.