The process of gas tungsten arc welding (GTAW) is applied with AlSi12 filler metal to weld 5052 aluminum alloy and Ti6Al4V titanium alloy in lap configuration. The welded lap joints showed dual features of welding and brazing on the aluminum and titanium parts of the joint, respectively. The continuous reaction layer established in the brazed area and its chemical composition by EDS analysis revealed that the intermetallic compounds (IMCs) layer of Ti(Al, Si)3 phase formed and aluminum was partially substituted by silicon. The thickness of reaction layer differed from 1μm to 3.5 μm with increase in welding heat input accordingly. Al, Ti and Si elements in the weld region and IMCs layer were recognized by analysis of EPMA. The maximum tensile shear strength attained at the optimized welding heat input was 376 N/mm and the joint fractured from 5052 aluminum alloy base metal. Furthermore, the microhardness was measured, and the hardness was higher at the Al/Ti interface than in the weld seam and Ti6Al4V alloy base metal because of the development of IMCs.