_Article_  
Effects of Surface Roughness and Force of Electrode  
on Resistance Spot Weldability of Aluminum  
6061 Alloy  
[PERSON] 1,2, [PERSON] 1, [PERSON] 1,@, [PERSON] 2 and [PERSON] 1,*@  
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joining of an aluminum alloy. Until now, research on joining of aluminum alloys has mainly focused on friction stir welding (FSW) and mechanical joining methods. Some studies were also carried out on the dissimilar joining of an aluminum alloy and steel [2]. RSW is one of the most commonly used joining processes in the automotive industry and has advantages of being cheap, fast, and easy to automate. However, performing RSW on aluminum alloys for car bodies is difficult because of the following reasons. Aluminum alloys have lower bulk resistance and higher thermal and electrical conductivities than steel; therefore, compared to steel, aluminum alloys need two- or three-time higher welding currents. In addition, aluminum readily reacts with oxygen in the atmosphere forming an oxide layer on the surface of the alloy, which hampers the growth of uniform nuggets in the weld. The oxide layer induces high contact resistance on the surface of the aluminum alloy, generating a large amount of heat, which may cause contamination of the electrode tip and deterioration of the welding quality. Various studies have been carried out to obtain a reliable weld via uniform decomposition of the oxide layer on the aluminum alloy surface during the RSW process. [PERSON] and [PERSON] have reported that high electrode forces are required for aluminum alloys to remove the aluminum oxide layer during RSW [3]. In addition, many researchers have studied the effect of aluminum sheet surface conditions on the resistance spot weldability. For instance, [PERSON] et al. have studied the effect of the surface condition on the resistance spot weldability of the aluminum 5252 alloy [4] by comparing the weldability of the as-received specimens with that of the specimens pickled in NaOH and oxalic acid. They have reported that excellent weldability can be achieved by removing the oxide layer through pickling. Similarly, [PERSON] et al. have investigated the effects of sheet surface conditions on the electrode life during RSW on a 5A02 aluminum sheet [5]. A comparative study of the electrode lifetime performed using three surface treatment conditions, i.e., chemical cleaning, degreasing, and electric-arc cleaning, indicated that degreasing is the most practical method and may be suitable for automotive. Moreover, [PERSON] et al. have studied the effects of surface conditions on the resistance spot weldability of aluminum 5754 and 6111 alloys [6]. They recommend a 'full cleaning' process to prevent sticking and also report that the usable welding range can be extended using a closure sheet with electro-discharge texture (EDT). Studies have been conducted to improve the resistance spot weldability of aluminum alloys by modifying the surface conditions of the electrode tip rather than the surface conditions of the aluminum alloy sheets. For instance, [PERSON] et al. have investigated the weldability of aluminum 6111 and 5754 alloys using electrodes with a multiring design on the surface [7]. This multiring electrode deforms the aluminum sheet surface upon contact and destroys the oxide layer on the aluminum surface, leading to a stable aluminum spot welding process. In addition, [PERSON] et al. have investigated the resistance spot weldability and fatigue behavior of an aluminum 5754 brought sheet and an Aural 2 die casting sheet using a multiring domed electrode [8]. They have found that the nugget size, which affects the tensile and bending strength, dominates the fatigue life. Moreover, [PERSON] et al. have studied the effects of grinding, polishing, milling, and sandblasting on the electrode lifetime during RSW of aluminum 5182 alloy [9]. [PERSON] et al. have investigated the effects of electrode face radius and force on the resistance spot weldability of aluminum 6061 alloy [10].