Mechanical Properties of Friction-Stir Welded Titanium Joint

摘要

The Commercial purity titanium (cp-Ti) plates with a 2 mm thickness, were butt-welded by friction-stir welding (FSW). Using a W alloy tool, FSW was performed at 200rpm and 50-300 mm/min. The mechanical properties of the joints were evaluated using tensile tests and Vickers micro-hardness tests. The microstructure was observed by optical microscopy (OM), field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). An electron back-scattering pattern (EBSP) technique was also used in the FE-SEM to clarify the crystallographic features of the microstructure. The tensile strength of joints significantly increases with the increasing welding speed because the grain size decrease and the dislocation density increases. The tensile strength of the specimen welded at 200 mm/min exceeds that of the base metal, and therefore the specimen fractured in the base metal during the tensile tests. However, the defects were formed in the specimen at 300 mm/min leading to decrease in tensile strength. The friction-stir welded cp-Ti microstructure consists of recrystallized grains with many dislocations, and the dislocation density increased with the increasing welding speed (decreasing heat input). For pure Al or IF steel, few dislocations are observed because the recovery occurs during cooling. Accordingly, the change in strength of the stir zone by varying heat inputs is greater for cp-Ti than for pure Al or IF steel.