Abstract: The present invention provides a structural assembly comprising a first workpiece and a second workpiece. The second workpiece at least partially underlies the first workpiece so as to define an interface therebetween. The assembly includes a friction stir weld joint joining the first and second workpieces. The assembly also includes an interface layer positioned between the first and second workpieces. The interface layer comprises a material having a melting temperature lower than the solidus temperatures of the first and second workpieces. The interface layer at least partially fills the interface proximate to the friction stir weld joint to thereby increase the strength and fracture toughness of the weld joint, as well as other mechanical and chemical properties, including resistance to crack growth and corrosion resistance.