材料工程基础课件：Chapter 4 The Joining of Metals.ppt

Chapter Five The Joining of Metals 1. Joining processes There are innumerable instances in which metals have to be joined to one another. In many cases joining is part of the production process, e.g. a component may be too complicated or too big to make in one piece and several simpler or smaller parts may have to be joined together. 4. Soldering and brazing In soldering and brazing, although the surfaces to be joined are heated the temperatures developed remain well below the melting point of either of the materials to be joined. However, local dimensional changes may cause distortion of the workpieces and result in residual stresses. If excessive heating is allowed to occur, then structural changes may take place in the materials. 5. Welding processes Welding processes may be divided into two main groups depending on whether or not pressure is used to bring the pieces to be joined together. In fusion welding without pressure, the faces of the two pieces of metal to be joined are heated and melted and extra molten metal from a filler rod may be added. The filler metal is of the same or similar composition to the metal being joined. 6. Fusion welding Fusion welding processes may be subdivided as shown in fig. 5.2. 7. Pressure Welding 8. Weld Defects Exercise 4: Explain the different types of welding processes Fig 5.6 Spot welding Fig 5.7 Seam welding In the electrical resistance welding methods, heat is generated by the resistance to a heavy current passing across the interface of the two components to be joined. The components are part of the low voltage electrical circuit, the areas to be welded are cleaned to remove surface impurity then press together before a current is passed; the pressure is maintained during the current flow and for a very short time afterwards. The heat developed during the passage of the current is enough to melt the metal in the area of the electrodes and thus a good strong weld is produced. * * Joining processes