Welding of aluminum-steel dissimilar materials

Aluminum alloys have excellent properties like low density, good corrosion resistance, and high electrical and thermal conductivity. Using aluminum alloys instead of steel can greatly reduce the weight of welded structures. Steel, on the other hand, has good weldability and mechanical properties, so aluminum-steel welded structures are widely used in industries like automotive and shipbuilding.

Aluminum melts at 660°C, which is 700-900°C lower than steel. During welding, the lower-melting aluminum melts first, while the steel is still solid. Because of the significant difference in density between aluminum and steel, the aluminum in the molten pool floats on top of the steel, which can cause the weld composition to be uneven after cooling. The large difference in linear expansion coefficients between aluminum and steel also leads to high residual stress in the weld joint, which can cause weld cracks.

To achieve a reliable aluminum-steel connection, it’s necessary to overcome the barrier effect of the oxide layer on aluminum and aluminum alloys and to prevent or reduce the formation of brittle intermetallic compounds at the aluminum-steel interface. The current welding methods are:

1. Pressure Welding

Pressure welding is a method of welding where pressure is applied to the workpieces (with or without heating) during the welding process to complete the weld. When welding aluminum and steel, commonly used methods include explosive welding, magnetic pulse welding, friction stir welding, and diffusion welding.

2. Fusion Brazing

Fusion brazing of aluminum-steel dissimilar materials combines features of both fusion welding and brazing. During the welding process, the aluminum alloy and filler metal melt and, after solidifying, bond together to form the fusion weld joint; the steel does not melt. The molten filler metal is drawn into and fills the gaps between the solid steel parts by capillary action. The liquid filler metal diffuses and dissolves with the steel and, once solidified, forms a strong brazed joint, allowing the connection of aluminum alloy and steel dissimilar metals.

3. Brazing

Brazing of aluminum-steel dissimilar materials involves placing the filler metal in the gap of the joint, heating it until it melts, while the base materials do not melt. The liquid filler metal then seeps into the gap of the solid workpieces, and after cooling and solidifying, it forms a strong connection.

Among the three welding methods for aluminum-steel dissimilar materials, pressure welding and brazing can achieve aluminum-steel joining, but they have certain limitations regarding the size and shape of the workpieces, and the production efficiency is low. Fusion brazing, on the other hand, can control the formation of brittle intermetallic compounds between aluminum and steel through the filler metal. In particular, laser wire-fed fusion brazing features low heat input, fast welding speed, and ease of automation, allowing for high-quality and efficient fusion brazed joints with a broad application prospect.