Friction stir welding (FSW) is an energy-efficient, solid-state joining technique with a wide range of applications. It was particularly effective for combining incompatible metals, such as copper (Cu) and Aluminium (Al). The purpose of this research was to design and build tools for FSW in order to ascertain the best welding and rotational speeds for Al–Cu butt joint welding. The study investigates the effect of process parameters such as tilt angle, pin offset, tool design, welding speed, and rotational speed on the quality of FSW joint. Mechanical properties are assessed using tensile, hardness, and impact testing. A nearly flawless joints was observed at a rotational speed of 1100 rpm. Furthermore, multi-response optimizations are carried out using a Taguchi L9 orthogonal array to ascertain the ideal set of FSW process parameters for optimized tensile strength and hardness. The optimal mechanical properties, encompassing ultimate tensile strength (132.609 N/mm2), hardness, and impact strength, were attained with a cylindrical tool pin profile at a rotational speed of 1100 rpm and a traverse speed of 50 mm/min. Mechanical characteristics were decreased due to excessive or insufficient tool rotational speed to traverse speed ratios leading to overheating or insufficient material stirring. These results provide valuable insights into the relationship between process parameters and joint quality, and observed the rotational speed was having a major impact on the joint properties.