Defect characteristics and detection methods
Slag inclusions with gray and gray black fracture are often found in the parts or corners with poor mold ventilation, and they are flocculent or black block on the X-ray film. Oxide inclusions in castings can be found by X-ray or fracture inspection. It can be found by fluorescence inspection, sand blowing or oxidation treatment after heat treatment, alkali cleaning, acid pickling and anodizing.
Causes of formation
- Oxidation slag inclusion (primary slag inclusion) produced in smelting process: ① Impure charge with oxide. ② Poor quality of flux used, poor degassing, slag removal and deoxidation effect. ③ Improper operation in smelting process causes serious oxidation of alloy liquid.
- Oxidation slag (secondary oxidation) produced by alloy liquid during pouring: ① The unreasonable design of pouring system makes the molten alloy produce vortex, turbulence, splash and other phenomena, which cause serious oxidation. ② Too high pouring temperature and improper pouring operation result in re oxidation of alloy liquid.
Prevention methods and remedial measures
- Strictly control the metallurgical quality and surface state of the furnace charge. It is strictly prohibited to use the furnace charge with oil stain and corrosion scar. The use proportion of the low-grade and poor quality return furnace charge shall be strictly controlled.
- Select the refining agent or refining tool with good refining effect to fully refine the alloy liquid. Before pouring, make sure that the alloy liquid has enough standing time to float the oxide and other slag. Before pouring, clean the oxide skin and slag on the liquid surface.
- Select the flux with good refining effect suitable for the alloy brand
- If necessary, 0.002% beryllium can be added to the high magnesium aluminum magnesium alloy to reduce the oxidation of the alloy liquid
- Improve the design of the pouring system, improve the skimming and slag collecting functions of the cross sprue, or place the filter (net) and slag collecting bag in the pouring system
- Change the position of the casting in the mold and adopt an open gating system to prevent the inner gate from flushing against the cavity wall or core
- Adopt continuous and uniform liquid flow, pour the mold slowly and evenly, keep the sprue cup full of alloy liquid all the time, forbid pouring to interrupt the flow, try to keep the distance between the sprue nozzle and the sprue cup ≤ 50 mm, to prevent the air in the open air or exhaust fan from blowing through the alloy liquid flow during pouring; before pouring, remove the oxide skin in the ladle
- When pouring high magnesium aluminum alloy castings, if necessary, the mold cavity and pouring system can be sprinkled with anti-oxidation protective agent (50% sulfur powder and 50% boric acid powder respectively or 100% sulfur powder)
- Smelting tools shall be kept clean without oxide scale
- The molding sand and core sand shall not be mixed with organic substances that are easy to oxidize and produce gas, such as pulverized coal. The humidity, compactness and uniformity of molding sand shall be strictly controlled. The storage time of sand mold or core shall not exceed 4H. If it exceeds 4h, the re baking device shall be opened. The core shall be dried thoroughly before being loaded into the sand mold. The placed cold iron shall be blown with sand, without oxidation spots, and dried before being loaded into the sand mold
- Inclined pouring is adopted, and it is forbidden to subcontract the alloy liquid