## 1 Introduction  
Friction welding is a joining method that enables the relatively quick and cheap joining of various metals. The physical process of friction welding involves using mechanical energy and converting it into heat, as a result of friction on the contact surface. Friction welding is a solid-state welding method for different metals [1]. The material to be welded is heated to a temperature below its melting point. After stopping the spindle rotation, the joint is pressed and plasticized. As a result of the applied upsetting force, the material is removed to the outside where a flash weld is formed [2]. The friction welding process is a heat-activated process that causes the deformation of the welded material and a large temperature gradient in the heat-affected zone (HAZ). Moreover, large effective stresses are generated in the joint and the microstructural changes as a result of high temperatures at the interface [3]. The basic welding parameters in the direct-drive friction welding process are the rotational speed (RS), friction pressure (FP), upsetting pressure (UP), friction time (FT) and upsetting time (UT). The rotational speed is related to the material to be welded and the diameter of the specimens. The rotational speed also determines the heating rate at the interface. Increasing the rotational speed might lead to the softening of the material, a greater extent of recrystallisation, or even increased intermetallic formation. The maximum temperature in the joint during frictional heating depends not only on the pressure but also on the temperature gradient, which depends on the rotational speed in particular [2]. In rotary friction welding, heat is generated by friction, which results in the significant plasticization of the material. The friction pressure as well as subsequent upsetting during rotary friction welding cause changes in the microstructure of the joint and its surroundings, which can be compared with the changes caused by hot plastic working. A high temperature in some situations, especially when the process lasts longer, enables diffusion across the interface, resulting in a change in the chemical composition in the joint area. The parameters are selected depending on the type of materials to be welded. The friction pressure affects the heating rate of the HAZ. The greater the friction pressure, the higher the rate of plastic deformation in the welded materials and the higher the heating rate in the contact area. The friction time is set in close correlation with the friction pressure and rotational speed. This should ensure an optimal heating speed and the minimal shortening of the welded elements. The friction time is largely influenced by the thermal conductivity of the welded material. During the friction process, the greatest amount of heat is released around the circumference of the friction surface, where the metal achieves the greatest plasticity. The center of the friction surface is heated due to heat conduction. The larger the diameter and the lower the thermal conductivity of the metal, the lower the temperature at the center of the friction surface. The friction time should be selected so that the temperature is equalized over the entire friction surface. The upsetting pressure and time are parameters that mainly affect the quality of the joining materials. In rotary friction welding, the control of the peak and equilibrium torque levels are considered to be crucial for a sound weld. The axial pressure applied during friction welding will affect the rate of shortening (upset), which is usually constant during the equilibrium torque period. Moreover, the 'upset' can be linearly proportional to the axial pressure [3]. Many works have been dedicated to the thermomechanical processes that occur during friction welding. These thermal phenomena have most often been solved using a finite element method (FEM). Thermo-physical data and the material constants of plastic deformation are necessary for the modelling of the welding process.