在镁转印箱外壳中穿

One OE supplier estimates that two million transfer case housings have been made from magnesium. In the aftermarket, these parts have been working their way into传播shops at an ever-increasing rate. The topics I am going to discuss here are: why magnesium transfer case housings wear so dramatically while their similar aluminum counterparts do not, and how this affects repair choices.

Transfer cases with magnesium housings include but are not limited to: the Ford/BorgWarner4405.和4411, and the 1999 and newer GM/New Venture 136, 149, 236, 246, 261 and 263 models. The type of magnesium alloy dealt with here is AZ91D, which is commonly used in these and other automotive applications. Wear in the Ford/BorgWarner (BW), typically causes engagement issues and rear seal leaks, while the GM/New Venture (NV) units show up with pinhole leaks in the housing and a loss of fluid. Magnesium transfer cases can be identified by the letters “Mg” inside a triangle cast on the outside of the housing.

First, a little background about magnesium. It is the eighth most abundant element in the earth's crust and the lightest structural material. It is 75% lighter than steel and 33% lighter than aluminum. Its light weight and reasonable strength make it a popular material for automotive engineers trying to trim weight from vehicles. In the BorgWarner transfer case application, 10 pounds were saved by using magnesium vs. aluminum. A common perception is that magnesium corrodes easily. However, the bare material actually rates better at corrosion resistance than aluminum. Where magnesium has a problem is when it is close to other metals and salt. The result is “galvanic” corrosion, which can rapidly corrode magnesium.

If you have worked on many of these units from areas with salt-treated roads, you’ve surely seen the external case material corroded away. To deal with galvanic corrosion, magnesium transfer cases use separators and special coatings on bolts, and even have special contours so that road debris (salt) is less likely to compact into small spaces. One other interesting fact about magnesium: it is a metal that can burn. When burning, it does not require oxygen to support combustion and water can actually feed the reaction. Class D fire extinguishers and smothering with sand are recommended to put out fires, so before machining or welding, take necessary precautions and be aware of what you are dealing with.