Analisis Tegangan Sisa dan Konduktivitas Listrik pada Hasil Pengelasan Material Aluminium 6061 dan Tembaga Murni Menggunakan Proses Friction Stir Welding = Analysis of Residual Stress and Electrical Conductivity on Welding Results of 6061 Aluminium and Pure Copper Materials Using the Friction Stir Welding Process