ORIGINAL ARTICLE Weldability, microstructure, and residual stress in Al/Cu and Cu/Al friction stir spot weld joints with Zn interlayer Adel Boucherit 1 & Said Abdi 1 & Mouloud Aissani 2 & Brahim Mehdi 3 & Khadidja Abib 3 & Riad Badji 2 Received: 16 June 2020 /Accepted: 29 September 2020 # Springer-Verlag London Ltd., part of Springer Nature 2020 Abstract In this work, the effect of lap joint configuration and Zn interlayer addition on the microstructure, the residual stress state, and the quality of Al/Cu (configuration #1) and Cu/Al (configuration #2) friction stir spot welds (FSSW) was investigated. The study revealed the close dependency of the weld joint quality on the pin length and Zn addition. The higher the pin plunge depth is, the greater the obtained tensile shear strength. The Zn addition reduced sensibly the thickness of Al 2 Cu layer (from 10 to 2 μm) and favored the formation of the Al 4.2 Cu 3.2 Zn 0.7 precipitate that hindered the formation of detrimental Al 4 Cu 9 compounds. Material flow analysis revealed the presence of an intermixing zone containing thin continuous intermetallic layer (approximately 2.07 μm) at the weld interface of configuration #1. Meanwhile, the Cu material was covered by the Zn layer, which resulted in a hardness increase (228 HV) at the stirred zone. In addition, a significant increase of the tensile shear strength from 1650 to 3600 N was noticed (an improvement rate of ≈ 118%). Conversely, in configuration #2, the Zn foil was squeezed out of the spot weld interface resulting in the absence of material intermixing, discontinuous intermetallic layer, low hardness level (140 HV), and weak improvement rate of the shear strength (≈ 53%). The Zn interlayer addition resulted in a quasi-symmetric distribution of the residual stresses and shifted their nature from tensile stresses (+ 60 MPa) to compressive ones (− 10 MPa). Keywords FSSW . Aluminum . Copper . Zinc interlayer . Tensile shear . Microstructure . Intermetallic compounds . Residual stress 1 Introduction The dissimilar joining of aluminum (Al) to copper (Cu) is increasing more and more in many industrial fields such as electric power, aerospace, and ship-building owing to the out- standing mechanical and thermal properties, as well as the good corrosion resistance of these materials [1–5]. It has been suggested that Cu should be replaced by Al due to its cost- effectiveness and considerable mass reduction [6]. Besides, joining both materials using conventional fusion welding is fairly challenging because of the formation of brittle interme- tallic compounds (IMCs) and the solidification defects such as porosities and chemical segregations. This compromises the mechanical properties of the weld joint mainly if the IMCs’ fraction within the microstructure is high and their average size exceeds 2.5 μm[7]. From a technological viewpoint, fusion processes are not suitable for such materials, as they generate solidification defects that decrease the weld joint per- formances. However, solid-state joining processes are known to reduce the IMCs’ formation within the microstructure. The literature reports some papers that deal with the joining of Al to Cu using FSSW. Recently, Pandey et al. [8] investigated the influence of welding parameters such as tool rotation speed, pin tool profile, welding speed, and tool offset position on FSW of aluminum alloys. They indicated that both the tool rotational speed increase and the threaded pin tool result in high heat generation and enhancement of material flow. This improves the tensile strength and the ductility of the weld joint [8]. On the other hand, Gaohui et al. [9] studied the effect of dwell time parameter and concluded that the tensile shear * Riad Badji riadbadji@gmail.com; r.badji@crti.dz 1 Laboratoire des Sciences et de Génie des Matériaux, LSGM, Université des Sciences et de la Technologie Houari Boumediene, USTHB, Algiers, Algeria 2 Research Center in Industrial Technologies CRTI, P.O. Box 64, Cheraga, 16014 Algiers, Algeria 3 Laboratoire de Physique des Matériaux, Faculté de Physique, Université des Sciences et de la Technologie Houari Boumediene USTHB, B.P. 32, El-Alia, Bab-Ezzouar, DZ-16111 Algiers, Algeria The International Journal of Advanced Manufacturing Technology https://doi.org/10.1007/s00170-020-06202-z