Thermal welding is a common joining technique for polymers. In this work we study the effect of various process parameters on the strength and ductility of a symmetric thermally welded joint through molecular dynamics (MD) simulations on carefully prepared and equilibrated macromolecular ensembles. Interdiffusion of mostly chain ends across the interface and formation of entanglements with chains on the other side constitutes the most important mechanism which determines the strength and ductility of the joint. At high temperatures, the entanglement distribution at the interface can become almost indistinguishable from the bulk rather quickly and without motions of the entire chains. The temperature at which the welding is performed and the welding time are the most important process parameters that control the number of entanglements formed across the interface, the interface width, the mechanical properties and mode of failure of the joint. Pressure and quenching rate have marginal effects on the ultimate properties of a thermally welded joint. Our results also indicate that the interface thickness of the welded joint varies linearly with the welding time. The toughness of the welded joint, for chain lengths more than the entanglement length, varies linearly with it. The toughness also scales as the one-fourth power of the time for which the polymers are held at the welding temperature.