Edison Welding Institute (EWI) has developed materials joining technologies for the US Army’s Future Combat System (FCS) program under the direction of The Army Research Laboratory (ARL). In choosing structural materials for the FCS variants, the Army has to balance different factors such as producibility, performance, and maintainability. In different parts of combat vehicle structures, one encounters the need to weld dissimilar aluminum alloys. Previous efforts related to the Expeditionary Fighting Vehicle (formerly AAAV) have shown that friction stir welding can be successfully used in joining 2219/6061 to 2519. Recent efforts funded by ARL have led to successful friction stir joining of 5059/5083 to 2139/2195 in various gages and joint configurations. This presentation will focus on related processing/tool design challenges.