Copper-to-copper welding presents several complex technical challenges, primarily due to the unique properties of copper as a material. One of the main issues is copper's high thermal conductivity. The purpose of this study is to determine the mechanical properties, such as tensile strength, hardness, and thermal conductivity, of welded metal products produced using the Gas Tungsten Arc Welding (GTAW) Technology. The filler material used is ERCuNi 90/10 rods. The welding method involves variations in welding heat input, specifically 1.09 kJ/mm, 1.13 kJ/mm, and 1.2 kJ/mm. The results of the study show that welding heat input affects the mechanical properties and thermal conductivity of copper. The highest tensile strength of 180 MPa at 1.2 kJ/mm is due to the higher heat input, which improves weld penetration and strengthens the metallurgical bond, enhancing the load-bearing capacity of the welded joint. The highest hardness of 132.12 HV or 1295 MPa is found in the weld metal (WM) due to microstructural transformation during solidification. The use of ERCuNi 90/10 filler contributes to the formation of a harder dendritic structure compared to the heat-affected zone (HAZ) and base metal. Meanwhile, the highest thermal conductivity of 206.72 W/mK occurs at 1.09 kJ/mm because the lower heat input reduces the mixing of filler metal with pure copper, preserving copper’s thermal properties better than at higher heat input. At higher heat input, increased nickel dilution from the filler reduces thermal conductivity, as nickel has lower thermal conductivity than pure copper