Liquid film formation and cracking during friction stir welding

The influence of tool rotation speed and travel speed during friction stir welding is studied in the case of Al 2024 which contains low melting point phases. When a tool rotation speed of 1600 rev min(-1) is applied with a travel speed of 100 mm min(-1), S-phase (Al2CuMg) particles in the base material form melted films, promoting the formation of spalling defects comprising subsurface cracks along the trailing edge of the weld below the tool shoulder. Decreasing the tool rotation speed to 400 rev min(-1) prevented formation of the melted films by reducing the peak temperature in the stir zone, and avoided formation of spalling defects. When the travel speed is decreased to 32 mm min(-1) when using 1600 rev min(-1), dissolution of the S-phase occurs and fewer melted films remain in the stir zone. This implies that travel speeds are limited in alloys which contain low melting point phases since these promote cracking defects.