Why Do Hot Cracks Form in Aluminum Welds?

Hot cracking in aluminum-alloy welding mainly includes solidification cracking in the weld metal and liquation cracking in the heat-affected zone. Both occur at high temperature when a weak liquid film cannot accommodate the tensile strain generated during cooling.

Why Aluminum Alloys Are Susceptible

Aluminum expands roughly twice as much as steel for the same temperature change. During solidification, low-melting eutectics can remain along grain boundaries while the surrounding metal contracts. Alloys with a wide freezing range are especially vulnerable because the continuous liquid film persists longer and has little ability to carry strain.

Crack susceptibility depends on base-metal composition, filler metal, joint restraint, and welding parameters. Pure aluminum is the least susceptible. Al-Cu alloys are among the most susceptible, and duralumin Al-Cu-Mg and high-strength Al-Zn-Cu-Mg alloys are generally unsuitable for conventional fusion welding.

How Filler Metal Changes Cracking

A filler can alter weld composition, freezing range, and eutectic quantity. An Al-Si wire containing about 5% Si, such as 4A01 in the source, has good crack-healing ability, although its weld strength and ductility are comparatively low. Small additions of Ti, Zr, V, or B can refine grains and reduce cracking by distributing strain over more grain boundaries.

How to Reduce Hot Cracking

• Select a compatible base-metal and filler-metal combination that minimizes the vulnerable freezing range.
• Use a concentrated heat source and avoid unnecessarily high current.
• Control travel speed. Excessive speed combined with high current can increase crack sensitivity.
• Reduce joint restraint through appropriate groove, tack, and welding-sequence design.
• Avoid intermittent welding where repeated starts and stops create severe local thermal cycles.

No single parameter eliminates hot cracking. Composition control, grain refinement, heat-input control, and restraint reduction must work together.