# Multi-Objective Optimisation of Welding Parameters for AZ91D/AA6082 Rotary Friction Welded Joints
## 1 Introduction  
Recently, much attention has been focused on magnesium and aluminium alloys in many industrial sectors because of their unique properties. For their practical applications, joining technologies should also be developed in addition to considering issues such as alloy design, microstructure control, plastic forming, casting, and surface treatment [1]. Combining aluminium alloys with magnesium alloys contributes to reducing the weight of components and increasing production efficiency by replacing aluminium alloys with magnesium alloys wherever possible. Joining the mentioned metals is very difficult using conventional welding methods [2]. During standard joining, due to the melting of materials and a large amount of heat supplied, significant grain growth occurs in the heat-affected zone (HAZ), and many brittle intermetallic metal compounds (IMCs) are formed [3]. The aluminium-magnesium equilibrium system has partial solubility of magnesium and aluminium. However, in the range of several percent by weight and the remaining compositions, the IMCs are made [4, 5]. The magnesium alloy was efficientlyjoined with the aluminium alloy by laser [6; 7] and diffusion welding methods [8; 9]. Various Mg/Al joints are made primarily by friction stir welding (FSW) to take advantage of the lower ability to form an intermetallic phase in the case of solid-state welding [10; 11; 12]. Authors in the following papers [10; 13; 14] observed the presence of precipitates of the intermetallic phase, which caused a substantial increase in hardness at the weld zone. The defect-free joints were observed for various rotational speeds in the study [15]. Moreover, no intermetallic phases or cracks were found in the joint area. The maximum value of 132 MPa tensile strength and extremely low elongation of 2% was achieved for a tool rotation speed of 1000 rpm in reference [16]. The literature results indicate the possibility of producing sound Mg/Al welded joints with different mechanical properties using solid-state friction welding methods [17; 18]. Recently, a relatively common solid-state welding method is rotary friction welding (FRW) [19]. Friction welding of metals can be performed using an inertia friction welding (IFW) or direct-drive rotational friction welding (DD-FRW) process. In the IFW, the rotating part is connected to a flywheel, and energy is supplied to the joint through the loss of kinetic rotational energy. The DD-FRW method directly converts mechanical energy into thermal energy on mutually rubbing surfaces in the contact area [20]. The friction welding process can be divided into two periods: friction and upsetting. The key parameters of the FRW process are friction time (FT), friction force (FF), upsetting force (UF) and rotational speed (RS). The parameters determine the temperature in the contact area and the temperature gradient in the weld zone. Friction time is necessary to heat the elements to the required temperature. The key process parameter during the upsetting period is the upsetting force (UF). The UF parameter is applied to heated metals. The upsetting force brings the crystals of the rubbing surfaces closer to the distance of the spatial mesh [21]. Moreover, the upsetting parameter facilitates the diffusion process and leads to the consolidation of the joint. The optimisation of welding parameters is necessary for the proper course of thermal phenomena and its effects on the mechanical properties and microstructure of the joint [20]. Many works have presented the optimisation of welding parameters for dissimilar metals using statistical methods, neural networks, response surface methods, factorial design, genetic algorithms, or hybrid artificial intelligence methods. A factorial design for selecting the process parameters and predicting the bead width, height and penetration in the joint was investigated [22]. The optimal weld pool geometry in the tungsten insert gas (TIG) welding of the stainless steel was studied [23].