Defects and rejects of magnesium alloy ingot

Defects and rejects of magnesium alloy ingot

In magnesium alloy ingot, the common defects and waste products include cracks, flux slag inclusion, oxidation inclusion, metal intermediate compounds, pores and cold insulation (layer formation), etc. In addition, there are feathered (fan-shaped) defects, which can be indicated by macroscopic test pieces or break openings, and which may cause rupture of the ingot during rolling and free forging. In principle, fan-shaped crystals are not waste.

1 crack

Obvious cracks in the ingot shall be treated as waste products, and hair-like cracks exceeding the amount of milling on the ingot surface shall also be discarded.

2 cold

Cold insulation is caused by slow casting speed, low casting temperature, unstable control of metal liquid level in the crystallization tank, improper selection of casting funnel, unreasonable taper of the crystallization tank and oblique positioning of the crystallization tank.

The prevention method is to adopt automatic control of liquid level and increase casting speed and casting temperature appropriately. If hot crack is caused by increasing casting speed and casting temperature, the height of crystallization tank can be raised appropriately, and the ingot without cold barrier and crack can be obtained. Or increase the taper of the crystal groove to reduce the heat conduction of the ingot skin to prevent cold insulation.

3 Banded pores

The defect is composed of scattered micropores. It often appears on the large surface of MB3 alloy flat ingot and is sometimes associated with cold insulation. This defect is difficult to show. After milling the surface of the ingot, it appears in the form of bright spots where there are banded pores, and there need to be a series of cracks and holes. At these sites, it is not easy to oxidize to color and therefore will not be conducive to anodic oxidation protection.

Banded pores are the result of the development of volume crystallization, and tend to occur when the melt in the liquid cavity is stirred or when the metal in some areas is insufficient. Banded pores generally occur only in fine grain ingot castings. In the study of its microstructure, it is found that a large number of banded pores are generated at the grain boundary. It reduces the mechanical properties of the ingot and may cause the ingot crack.

4 flux slag inclusion

Flux inclusion is the most dangerous hidden danger in magnesium alloy, because it may be the source of fracture. The slag inclusion of flux is due to the unreasonable smelting and refining process.

Improper selection of flux and melt overheating. When the process is reasonable, refining and coating agent selection, flux slag inclusion waste generally not more than a few percent.

5 Oxidized inclusion

This defect is actually a thin film of magnesium oxide, with magnesium oxide and magnesium sulfide part of the metal intermediate compound mixed, it is spherical on the test fragment mouth. This defect is due to imperfect or incorrect workmanship.

Intermediate metallic compounds

The formation mechanism of metallic intermediate compound is similar to that of aluminum alloy. The phase composition of metallic intermediate compounds varies from alloy to alloy.