The term white metal denotes castings that typically fall into three general classes according to their composition: zinc, aluminum, and magnesium. Lead and  tin alloy castings differ because they require soldering rather than welding.

The weight of the metal identifies the type of alloy. Of the three, magnesium alloy is lightest, followed by aluminum alloy. Zinc alloy, nearly as heavy as iron, is heaviest. The commonly used aluminum and magnesium alloys melt at 585 to 645°C (1080 to 1195°F). The zinc alloys melt at 385°C (725°F).

Welding Zinc Alloy Die Castings

Total production of zinc alloy die castings is much greater than aluminum or magnesium and therefore zinc castings are more commonly encountered. Arc welding is not practical for repairing broken zinc castings because the arc temperature is so much higher than the melting point of zinc. Special techniques must be used with oxyfuel gas welding. The temperature of a neutral torch flame is about 3200°C (5800°F), although when using a considerable exces!; of acetylene, as this type of welding requires, the temperature of the flame is somewhat lower.

Torch Tip- Since the oxyacetylene flame is much hotter than necessary, welding this alloy requires a very small welding tip, about the size of a No. 72 drill bit.

Applying Heat- The excess acetylene flame should burn yellow but should not coat the metal with soot. The welding rod required is an alloy that will flow smoothly at the right temperature.

Since the melting temperature of the alloy is relatively low, too much heat will ruin the casting.

Joint Preparation- Preparation includes forming a V in the crack, or if broken all the way through, grinding or filing the edges to an angle of about 45″, and then lining up the parts on a carbon block. Chromium- or nickel-plated parts require the removal of plating from the weld site. Welder’s clay, used as a support under the weld, prevents the metal from flowing away or sagging.

Applying heat to the casting will cause the metal to flow. Turn the flame parallel to the surface, and maintain heat with the side of the flame. Heat the welding rod to the melting point, and touch the rod to the joint; the rod should flow into the V with complete fusion. Repeat this operation until the break fills completely. Unless the rod penetrates into the weld and breaks the surface tension, the rod will lie on the surface without fusing.

Puddling is only necessary when the operator has piled the rod on top of the weld instead of fusing it to the base metal. Heat the base metal and rod to a flowing temperature, and using a bronze rod as a puddler, work the rod into the base metal. Welding these alloys requires very careful manipulation of the torch and patience from the welder.

Determine whether the metal is weldable by attempting a weld on a small part of the break where little harm will be done.

Welding Magnesium Die Castings

Magnesium die castings are the industry’s lightest structural metal, weighing approximately two-thirds as much as aluminum and less than one-quarter as much as iron or steel. The low-weight characteristic is due to the high magnesium content of the alloys, which is usually between 90% and 98%.

Oxyacetylene welds on magnesium alloys require a rod and a special flux. Any flux left in the weld will promote corrosion. After welding, the part requires a thorough rinse with hot water and treatment with a chrome pickle solution. It is then ready for painting. See MAGNESIUM ALLOYS, Weldability.

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