FACTORS AFFECTING RAFT FORMATION IN ELECTRON-BEAM BUTTONS

The electron beam button melting (EBBM) test is used to determine the cleanness of alloys such as nickel-base superalloys and steels used fo r critical engineering applications. The oxide particles are concentra ted into an area on the top surface of the button referred to as the r aft. The factors affecting the formation of inclusion rafts in electro n beam buttons have been investigated. Comparisons with the fluid now in weld pools have led to the proposal that the direction of the surfa ce flows in the molten EBBM pool may have a decisive effect on raft fo rmation. Mathematical modelling of the fluid flow in the molten pool f ormed in EBBM indicated that thermocapillary (Marangoni) forces may be dominant and this viewpoint has been confirmed by EBBM trials with IN 718 in which the surface flows were monitored by following the movemen t of Al2O3 particles. It is proposed that the direction of these surfa ce flows is determined by the sulphur content of the heat and that goo d rafts can be obtained with sulphur contents greater than 10 ppm.