1. Contact corrosion of dissimilar metals

A variety of metals are used in shipbuilding, so contact with dissimilar metals is inevitable. When the aluminum alloy is directly connected to copper, brass, and bronze, its electrochemical corrosion can be stronger than that when the aluminum alloy is connected to steel, lead, and nickel, and the aluminum alloy in the bimetal (dissimilar metal) joint immersed in seawater Chemical corrosion is much stronger than in other media. For example, the corrosion of the copper alloy submarine valve cover installed on the aluminum alloy hull is particularly serious. Some watertight fillers contain corrosives such as lead and mercury or the joints are not close, which is also one of the causes of corrosion. Although some dissimilar metals are not immersed in seawater, they can also corrode. If the aluminum alloy plate under the steel bracket of the plate outfit is often corroded very seriously. The reason is that the steel bracket is not galvanized and other necessary protection is adopted. If the same steel bracket is galvanized or a rubber gasket is placed between the fittings, the corrosion of the aluminum alloy will be extremely slow.

The steel flange of the smoke pipe outlet installed on the side of the ship is in contact with the aluminum alloy plate of the hull. Due to the contact of the dissimilar metals in the seawater and the temperature of the exhaust pipe is relatively high, the corrosion of the aluminum alloy hull plate is more serious. The edges of openings are often cracked and perforated due to corrosion.

2. Contact corrosion between aluminum alloy and wood

When repairing ships, the contact corrosion between aluminum alloy and wood can often be seen. For example, the aluminum alloy angle material contacting the fender wood is very severely corroded due to the direct attack of wood fluid. Some corner aluminum is broken and the outer aluminum plate is perforated. Corrosion of corner aluminum is as loose as bagasse.

3. Six crafts

01. Low pressure casting

At present, 90% of factories in China's mainstream manufacturing industry of aluminum alloy wheels are using this process for production. The products produced by this process are mainly sold to OEM and overseas retail markets.

The low-pressure casting process is currently quite mature in mainland China, with the advantages of moderate equipment cost, high production efficiency, high material utilization rate, and suitable for less manual production management. This process has been approved by all OEMs. The aluminum alloy wheels manufactured by low-pressure casting process, because the spokes are finally cooled and solidified, the spokes of some specially shaped wheels are prone to quality problems such as shrinkage, and the rim part has better strength due to the earliest crystallization.

02, gravity casting
The most traditional casting process, currently about 15% of domestic enterprises still retain the gravity casting production process, the products are mainly aimed at the domestic and overseas retail markets. The advantages of this process are low equipment cost, cheap molds, and good spoke quality. The disadvantages are low metal utilization, low production efficiency, and low cost performance. Most OEMs have now rejected wheels produced by this process. Aluminum wheels The manufacturing industry is gradually being marginalized.

03. Forging spinning process with high-end forming technology
At present, only about 10% of domestic companies use this process for production, and the products are mainly supplied to the domestic and foreign buses, trucks and high-end car markets. The advantage is that the product has good internal quality and the product strength is high quality and light; the disadvantage is that the equipment cost is high. The spoke shape of the product manufactured by the traditional forging process can only be completed by milling, the metal utilization rate is low and the production efficiency is not high. Forging is divided into traditional forging and rolling forging, which can directly forge the shape of the spokes.

04. Casting spinning is divided into two processes: "low pressure casting + spinning" and "gravity casting + spinning".
At present, Korean car companies have a tendency to fully adopt "low pressure casting + spinning" for the molding process of aluminum wheels. Other car systems also have some products that have demand for this process. This process is the mainstream process in the casting and spinning process and is an OEM product. Most of the companies use this process; at the same time, a small number of companies doing overseas retail market use the "gravity casting + spinning" process. In theory, this process is feasible, it really puts The advantages of gravity casting and spinning are combined, and the product quality is improved. However, because the blank is produced by gravity casting, the economics of this process are very bad and the production cost is very high. In short, the cast spin product, because the spoke part is cast, it has the same advantages and disadvantages as the product produced by the casting process; and the rim part is spun out, so the air tightness is better. The weight of the product can be reduced in theory, but the effect is not obvious in practical application.

05. Liquid die forging Currently, only two companies in the mainland are using this process for production. The equipment and technology are from South Korea.
In fact, this process is not forging, and should be classified as high-pressure casting. It is to pour a certain amount of aluminum alloy liquid directly into the mold and continue to apply mechanical static pressure to crystallize and solidify the metal under pressure and force to eliminate the formation of solidification shrinkage. The shrinkage cavity is loosened to obtain a liquid die forged aluminum alloy wheel blank without casting defects. Compared with the forging process, the advantage is that the equipment investment is low, the rim can be directly formed, and the mechanical properties are generally higher than that of ordinary castings, and it is close to or even reaches the level of forgings. At the same time, it does not have the anisotropy usually present in forgings. The disadvantage is that liquid die forging products are the same as traditional forging products, which require milling to complete the shape of the spokes.

06. Differential pressure casting At present, only a few enterprises in the mainland are using this process.

Differential pressure casting is a casting method derived from low-pressure casting. The difference from low-pressure casting is that the casting mold is set in a sealed cover. The casting mold is under the gas pressure in the sealed cover. The pressure of the gas in the holding furnace during molding is greater than the pressure of the gas in the sealing cover. Since the casting is crystallized and solidified under the effect of higher pressure, it can ensure that the casting is more dense than the low-pressure casting product and the yield is high. The disadvantage is higher equipment cost and maintenance costs.

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