1. Contact corrosion of dissimilar metals
A variety of metals are used in boat building, so it is inevitable that you will come into contact with different kinds of metals. When the aluminum alloy is directly connected with copper, brass, and bronze, its electrochemical corrosion can be stronger than that of the aluminum alloy at the bimetal (dissimilar metal) joint immersed in seawater when the aluminum alloy is connected with steel, lead, and nickel.
Much stronger in other mediums. For example, corrosion is particularly severe on copper alloy submarine bonnets mounted on aluminum alloy hulls. Some watertight packings contain corrosive substances such as lead and mercury or the joints are not tight, which is also one of the reasons for corrosion.
Some exotic metals will corrode even though they are not submerged in seawater. If the aluminum alloy plate under the plate-mounted steel bracket is often severely corroded. The reason is that the steel brackets are not galvanized and other necessary protective measures are taken.
If the same steel brackets were galvanized or rubber gaskets were placed between the fittings, the aluminum alloys would corrode extremely slowly. The steel flange of the exhaust 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 dissimilar metals in seawater, the temperature of the exhaust pipe is high, and the corrosion of the aluminum alloy hull plate is relatively serious.
The edges of the opening are often cracked and perforated by corrosion. 2. Contact corrosion between aluminum alloy and wood Contact corrosion between aluminum alloys and wood can often be seen when repairing ships.
For example, the aluminum alloy angle material in contact with the fender wood is very severely corroded due to the direct erosion of the wood liquid. Some corner aluminum is broken, and the outer aluminum plate is perforated. Angle aluminum corrosion loose as bagasse.
3. Six major crafts 01. Low pressure casting At present, 90% of the factories in the mainstream manufacturing industry of aluminum alloy wheels in my country adopt this process for production.
Products produced by this process are mainly sold to OEM and overseas retail markets. The low-pressure casting process is quite mature in mainland China. It has 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. Aluminum alloy wheels manufactured by low-pressure casting process, due to the final cooling and solidification of the spokes, some special-shaped spokes are prone to quality problems such as shrinkage and porosity, while the rim part has better strength due to the earliest crystallization. 02.
Gravity casting The most traditional casting process, about 15% of domestic enterprises still retain the gravity casting production process, and the products are mainly for domestic and foreign retail markets. The advantages of this process are low equipment cost, cheap molds and good quality spokes. The disadvantages are low metal utilization rate, low production efficiency and low cost performance.
Most OEMs have now rejected wheels produced using this process. The aluminum wheel manufacturing industry is gradually being marginalized. 03.
Forging process of high-end forming technology At present, only about 10% of domestic enterprises adopt this process for production, and the products are mainly supplied to domestic and foreign passenger cars, trucks and high-end car markets. The advantage is that the product has good internal quality, high strength and light weight; 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 spoke. 04. Casting and spinning are 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 forming process of aluminum wheels. Other automotive systems also have products that require this process. This process is the mainstream process in the casting spinning process, and it is an OEM product.
Most companies use this process; at the same time, a few companies in the overseas retail market use the "gravity casting + spinning" process. Theoretically speaking, this process is feasible, and it really combines the advantages of gravity casting and spinning to improve product quality. However, since blanks are produced by gravity casting, the economics of this process are poor and production costs are high.
In short, the cast-spin product, because the spoke part is cast, has the same advantages and disadvantages as the product produced by the casting process; the rim part is rotated out, and the airtightness is better. In theory, the weight of the product can be reduced, but the effect is not obvious in practical application. 05.
Liquid die forging At present, there are only two companies in mainland China using this process for production. Equipment and technology are from South Korea. In fact, this process is not forging, it 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 make the metal crystallize and solidify under the action of pressure and force, and eliminate the formation of solidification shrinkage cavity. The shrinkage cavity is loosened to obtain a liquid die forging aluminum alloy wheel hub 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 ordinary castings, approaching or even reaching the level of forgings.
At the same time, it does not have the anisotropy that usually exists in forgings. 06. Differential pressure casting At present, only a few enterprises in mainland China adopt 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 the sealing cover. The casting mold is under air pressure inside the sealing cap.
During molding, the pressure of the gas in the holding furnace is greater than the pressure of the gas in the sealing cover. Since the casting is crystallized and solidified under the action of higher pressure, it can ensure that the casting is denser than the low-pressure casting product, and the yield is high. The disadvantage is that the equipment cost and maintenance cost are high.
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