A tube welding fixture (10) for holding and axially aligning cylindrical tubing pieces (22, 24) in an end-to-end butted relation, in preparation for welding and assembly. The fixture has clamping surfaces (27, 28) which fit tightly around the outer walls of the tubing pieces and compress each piece between a clamping yoke (16) and a clamping base member (12). The clamping yoke (16) is removably attached to the clamping base member (11) for insertion and removal of the tubing piece (22, 24). The fixture (10) is constructed from heat conducting material which acts as a heat shield and sink, and blocks excess welding heat from conducting down the tubing piece (22, 24) and damaging adjacent components.