Ultrasonic welding (UW) has been widely used for joining dissimilar metals, such as Al-Mg,[
1‐
3] Al-Steel,[
4‐
6] Al-Cu,[
7] and Al-Ti[
8,
9] because its low welding temperature and low energy input avoid the formation of a thick and brittle intermetallic layer at the weld interface. Although the energy input of UW is quite low, brittle intermetallic compounds (IMC) have still often been observed at weld interfaces.[
1,
4,
10‐
12] Once the coverage and thickness of the IMC layer exceeds a certain level, it becomes detrimental to the mechanical properties of the welded interface.[
13] Thus, many studies have investigated IMC formation and growth on dissimilar metal interfaces.[
1,
3,
10,
12,
14] However, elemental segregation at dissimilar metal interfaces has received less attention, although this could also have an important influence on a weld’s mechanical properties and on IMC growth. Fuji
et al.[
13] have studied segregation for friction welded titanium-to-aluminum interfaces using transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) elemental analysis. They observed an Al
3Ti IMC layer formed on the weld interface as well as more than 20 at. pct Si segregated to the interface between the Al
3Ti intermetallic phase particles and the titanium base metal. The aluminum component of the weld contained silicon and the authors proposed that the Si segregation at the interface retarded the growth of the Al
3Ti intermetallic layer by acting as a barrier layer for diffusion.[
13] In addition, the Al-Ti weld with Si segregation showed better mechanical properties (bend angle)[
13] for an identical heat treatment time compared with Al-Ti welds without Si segregation. However, in Fuji
et al.’s study[
13] (year: 1995), Si segregation was only detected at Al-Ti weld interfaces after heat treatment (873 K, 1 hour and 873 K, 0.1 hour), and no interface Si segregation was observed in the as-welded Al-Ti condition. Therefore, it is still unknown whether silicon or other important alloying elements in aluminum alloys (
e.g., Cu, Mg) and titanium alloys (
e.g., V) can segregate to the weld interfaces in the short process times associated with solid-state welding processes (
e.g., friction welding, friction stir welding, and UW). Furthermore, the dispersion of the surface oxide layers on Al and Ti base metals during UW is also of great interest to the quality of the weld and has not been studied in detail. In this paper, we apply scanning transmission electron microscope (STEM) imaging and nanoscale EDS elemental mapping to investigate the distribution of all alloying elements and oxides at the as-welded UW aluminum-titanium interfaces, providing higher resolution microstructural characterization than has been previously reported.