Intermetallic barrier layer formation
Intermetallic layer formation: brittle intermetallic phases (e.g., Fe2Al5) form directly in the joint zone when aluminum and steel briefly melt during welding. This hard, crack-prone layer separates the two materials and breaks immediately under load.
Physico-Chemical Mechanism of Action
At the interface between liquid aluminum and solid steel, iron and aluminum atoms diffuse into each other. During cooling, brittle intermetallic phases such as FeAl3 or Fe2Al5 form. These phases have a highly ordered crystal structure without slip planes and reach hardnesses up to 1000 HV. The layer thickness grows exponentially with increasing temperature and contact time. Rapid cooling freezes a thin but hard layer.
Metallurgical Relevance & Welding Risks
The brittle layer has almost no ductility. Under shrinkage stresses during cooling, cracks open up along the layer immediately. The joint fails even under low loads. Above a layer thickness of about 10 µm, the entire weld becomes unusable. The different thermal expansion of aluminum and steel creates additional stresses. Porosity can form if oxides or moisture are trapped in the joint area.
Shop Practice & Action Guidelines
The formation of the intermetallic layer can only be minimized, not completely prevented. Keep the molten pool contact time under 0.5 seconds. Limit heat input by using a short arc, high travel speed, and low current. The goal is an intermetallic layer thickness below 5 µm. Use transition pieces made of explosion-clad aluminum-steel, or use friction welding or laser beam welding with precise energy control. Clean the joint surfaces thoroughly and remove oxide layers. Avoid preheating the steel, as heat accelerates diffusion.