Pengaruh Kecepatan Putar Tool pada Proses Friction Stir Welding terhadap Sifat Mekanik dan Mikrostruktur Sambungan Aluminium 6061 dengan Tembaga Murni = The Effect of Rotational Speed on Friction Stir Welding Process against Mechanical Properties and Microstructure of Aluminium 6061 to Pure Copper Joints