Aluminum alloy common micro defects

Shrinkage and looseness

The same kind of cavity type defect, the volumetric filling in solidification is not sufficient, only the shape and the occurrence site are different.

shrinkage cavity

Generally, the solidification at the end of the casting is not good, forming a tubular or branch-like hole with a large area. Surface shrinkage holes are visible visually, and internal shrinkage cavities can be found at low magnification inspections.

loose

Generally, there are small and scattered pores between the dendrites, resulting in the intimacy of the internal structure of the casting. The looseness visible in the low-spectrum inspection is called macroscopic looseness; the looseness that can be seen under the microscope becomes microscopic looseness. Microscopic looseness is often distributed in the dendrites between the dendrites. In severe cases, the pores are connected into a network. If heat treated, the edge of the loose hole becomes dull and the shape is smooth.

crack

Casting crack

During the solidification process, due to the shrinkage of the shrinkage, a large internal stress is gradually formed inside, and the casting stress is generated when the party stress exceeds the strength limit. Cracks generated at high temperatures are called high temperature cracks, cracks are zigzag, and develop along grain boundaries or dendrites. Crack fractures are often yellow or dark black. Cracks generated in the low temperature region are called low temperature cracks, generally undergoing grain penetration, and the crack fractures have metallic luster.

Heat treatment crack

Divided into 2 kinds

Quenching crack

Due to the intense quenching of the quenching, the aluminum alloy parts are not uniformly cooled, causing a large internal stress and causing cracking in the stress concentration area. The crack is relatively straight, most of which are along the crystal, a little part of the crystal, the fracture is relatively flat, belonging to brittle fracture

The over-fire cracking causes the grain boundary and the eutectic to melt due to the quenching heating temperature exceeding the solid-phase temperature line of the alloy, and forms a super-burning crack along the crystal or along the dendrites when the quenching water is cooled, forming a network in a serious condition, and the structure is coarse. .

Inclusion

In the metallographic examination, various inclusions are often found. They are classified into the following three categories.

1. Aluminum oxide inclusions

The oxide scale of aluminum enters the mold at the time of pouring to form oxide inclusions in the casting, and is black with a floc or curved filament under the microscope, and the outline is unclear.

2. Non-metallic inclusions of cast profiles

Casting Profile—The main quartz sand (SiO2) is mixed with molten metal during casting to form non-metallic inclusions. When the metallographic observation, the outline is a square or polygonal block, often appearing in a relief shape.

3. Flux inclusions

The flux added during the smelting of the molten metal is not clean and causes flux inclusions. Since the flux inclusions are easily dissolved in the metallographic preparation to form holes, residual flux and adhered metal particles are still visible in the hole.

Segregation

During the solidification process of aluminum alloy, due to various reasons, the chemical composition and microstructure of various parts of the casting are not uniform. The distribution of the segregation of each tissue can be seen during the metallographic examination. Segregation can be classified into the following types depending on the location of occurrence, and the like.

Intragranular segregation

When the crystallization is fast, the diffusion process cannot be sufficiently performed, resulting in uneven composition of the internal crystal grains. This is a common phenomenon and has no significant effect on mechanical properties.

2. Regional segregation

Differences in composition of different regions due to different crystallization sequences. If the center is enriched in low-melting components, it is called positive segregation; if the outer layer is enriched in low-melting components, it is called negative segregation.

3. Eutectic segregation

During the solidification of the aluminum alloy. If a shrinkage-pore defect occurs, the liquid phase of the eutectic composition fills in, causing a coarse eutectic structure region to become eutectic segregation. This is a harmful form of tissue that has a negative impact on performance.

4. Density segregation

A refractory element such as titanium or zinc added to the aluminum alloy forms a high melting point flake compound. These compounds crystallize earlier during solidification. The density is large, and it is easy to sink to form density segregation. This segregation of compounded compounds has an adverse effect on the properties of the alloy.

Stomata

During the casting process of aluminum alloy, foreign gases are mixed into the casting due to liquid metal, mold or other reasons to form pores of a larger size. The pores can appear on the surface or inside, and the shape is often pear-shaped, elliptical or round. The inner wall is smooth and shiny. The outline is clearly visible. When the pores are large, they can be found at low magnification and fracture inspection.

Pinhole

During the smelting and casting 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 to form small pores called pinholes. Under the microscope, the pinholes are round or branched and are distributed between the dendrites. Among the Al-Si alloys, especially those having a high silicon-containing eutectic alloy, the hydrogen absorption tendency is severe, and the pinhole tends to be large, mostly in the form of dots, and sometimes in a polygonal shape. Micro-porosity is easily formed in Al-Cu and Al-Mg alloys, and pinholes are mixed with such looseness, and are polygonal holes continuous with each other.

Solid solution strengthening phase is not completely dissolved

When the aluminum alloy is treated, the temperature is too low or the insulation is insufficient. There is a large amount of residual solid solution strengthening phase in the alloy structure after quenching. Especially in the thick part of the workpiece, it is easy to form a coarse strengthening phase, which is more likely to cause incomplete dissolution. Frequently in the grain boundary and dendrites, the residual strengthening phase causes the mechanical properties of the aluminum alloy to be low.