Joining high current wires outdoors
60 A through 18 AWG copper is going to raise the temperature of the wire to much greater than 60°C. Calculating the steady-state temperature of an insulated wire depends on the ambient temperatures, how it is installed, and the materials it is surrounded with.

According to Table 310.15(B)(17) of the 2017 NEC, 18 AWG copper insulated with PVC installed in free air at 30°C will reach a steady-state temperature of 90°C with current of 18 A.

By the way, Teflon insulation would not be necessary if your goal is 60°C. Common TFFN building wire (PVC insulation with a nylon skin) is designed for a maximum temperature rating of 90°C. All standard electrical splices will be rated for at least 60°C, there should not be any need for special materials.

I would expect about 15 A would result in a temperature of 60°C on the copper surface of 18 AWG wire. Since the DC resistance per foot at 60°C of 18 AWG is 7.38 ohms / 1000 ft, a voltage from a perfect voltage source of only 5.5 V would result in 15 A.