**1. Introduction**

Al-Cu bilayer sheet offers an attractive combination of high thermal and electrical conductivity and good corrosion resistance. This combination of metals is also comparatively cheaper than the standalone Cu sheet. Owing to these salient properties, the Al-Cu metal has found a host of applications in aerospace, chemical, transport, electronic, and power industries [1]. The most common applications include electrical connectors, power supply module, power LED, heat sinks, electromagnetic shielding, solder float, and radiators. Another interesting application of Al-Cu sheet is that it can be employed as a fire-resistant material [2]. Numerous conventional joining techniques such as brazing and laser welding are applied to join Al and Cu but it is challenging because of the difference in the physical and chemical properties of the metals and the tendency to form brittle intermetallic compounds (IMCs) during the formation of welded joints [3]. These IMCs can impair the mechanical and electrical properties of the joint [4].

The temperature of metals in solid-state joining, on the other hand, does not approach melting point and this fact minimizes the undesired formation of intermetallics in dissimilar joints. Friction stir

welding (FSW) is relatively an innovative solid-state welding technique, whereby joining is realized by the stirring action of a pin-tool without applying any significant external heating. This aspect renders FSW a very competitive process for joining the dissimilar metals, as described in the literature [5–8]. Besides experimental analyses, numerical modeling has also significant contribution in establishing the fundamentals of FSW process. Luo et al. [9] worked on numerical modeling of AA2A14-T6 to visualize the material flow during the FSW process. They found that either high welding speed or low rotational speed could cause welding defects such as holes and cracks. Dialami et al. [10] performed numerical modeling to study the microstructure evolution of AZ31Mg alloy during FSW and established relation among grain size, strain rate, micro-hardness, and temperature.

Some researchers [11,12] have attempted to produce dissimilar joints of Al with other metals. Sharma et al. [13] utilized five different pin profiles including cylindrical, taper, cylindrical cam, taper cam, and square to produce butt joint between AA5754 and commercially pure Cu. They reported that square-pin offered better mechanical properties because it facilitated mixing at the nugget zone. Akbari et al. [14] examined the trend of mechanical properties with respect to the material position in dissimilar 7070 Al/Cu lap joint and showed that better welded joint quality achieved when Al was placed on the top of Cu. Bisadi et al. [15] studied the effect of FSW parameters on the microstructure and mechanical properties of Al 5083 and commercial Cu FSW lap joint and reported that very low and very high welding temperature can lead to several joint defects like channel and voids defects. Celik et al. [16] performed a similar investigation on Al/Cu butt joint and suggested that higher tensile strength is attributed to dispersion strengthening of fine Cu particles distributed over the Al material in the stir zone.

Karimi et al. [17] investigated the effect of tool material on the metallurgical and mechanical properties of dissimilar Al/Cu butt joints. They found that the tool with low thermal conductivity produced better welded joints. Çevik et al. [18] employed uncoated and TiN-coated X210Cr12 steel tools in order to fabricate 7075-T651 Al butt joints revealing that the uncoated tool produced favored results. Bozkurt et al. [19], on the other hand, observed opposite findings while butt welding of AA2124-T4 alloy with uncoated and CrN and AlTiN-coated HSS tools.

Although some scholars [14,15,20–23] have performed work on different aspects of FSW of Al/Cu lap joint, a common agreement has not yet arrived in certain respects. As an example, the effect of tool material has not been agreed upon. Furthermore, nature of the influence of various factors is associated with the type of weld (say butt or lap). Therefore, more investigations are required to acquire a thorough understanding on FSW of Al/Cu lap joints. Moreover, most studies furnish knowledge on one or two aspects of the process. A comprehensive study undertaking a range of important aspects/factors can provide useful insights to the user for successful welding. The present study is an attempt in this direction wherein the effects of a number of factors namely tool geometry, tool material, process parameters, stacking sequence, and heat sink are taken into account.

Two types of tool materials namely HSCo and WC; two thicknesses of materials (1.65 mm and 3 mm for Cu and 2.15 mm and 4 mm for Al); and two types of tool geometries namely round and square are employed. To successfully produce Al/Cu lap joints at varying of these variables, the welding speed and rotational speed are altered over a range. It is observed that the joining results (i.e., lap shear strength and defects) vary as either of the tool material, plate thickness, or tool geometry is changed. Further, the favorable range of feed and speed also experiences a change with a change in the rest of the conditions. The presented results can serve as a guideline to produce sound FSW Al/Cu lap joints.
