Which material condition typically requires preheating before welding?

• A. Alloy steels or steels with high hardenability
• B. Very thin sections
• C. Mild steel with low hardenability
• D. Aluminum alloys

Answer: (D)

Preheating before welding is used to slow the cooling of the weld and heat-affected zone, reducing residual stresses and the risk of cracking or distortion. Aluminum alloys, in particular, benefit from a moderate preheat because they conduct heat away quickly and have an oxide surface that can trap porosity and promote cracking if cooled too fast. By warming the joint to a light temperature, you slow solidification and improve ductility and weld integrity, especially in thicker sections or with alloys prone to cracking. The exact preheat depends on alloy and thickness, but a modest preheat is commonly used for aluminum welds to help prevent defects.