classified  into  two  groups:  consumable  and  nonconsu- mable. Consumable electrodes not only provide a path for the current but they also supply fuller metal to the joint.  An  example  is  the  electrode  used  in  shielded metal-arc  welding.  Nonconsumable  electrodes  are  only used as a conductor for the electrical current, such as in gas tungsten arc welding. The filler metal for gas tung- sten arc welding is a hand fed consumable welding rod. Additional  information  about  filler  rods  and  elec- trodes is covered in other chapters of this TRAMAN that deal  with  specific  welding  processes. FLUXES Before  performing  any  welding  process,  you  must ensure the base metal is clean. No matter how much the base metal is physically cleaned, it still contains impu- rities. These impurities, called oxides, result from oxy- gen combining with the metal and other contaminants in the base metal. Unless these oxides are removed by using a proper  flux, a faulty weld may result. The term flux refers to a material used to dissolve oxides and release trapped gases and slag (impurities) from the base metal; thus the flux can be thought of as a cleaning agent. In performing this function, the flux allows the filler metal and the base metal to be fused. Different types of fluxes are used with different types of metals; therefore, you should choose a flux formulated for a specific base metal. Beyond that, you can select a flux based on the expected soldering, braz- ing, or welding temperature; for example, when brazing, you should select a flux that becomes liquid at the correct brazing temperature. When it melts, you will know it is time to add the filler metal. The ideal flux has the right fluidity at the welding temperature and thus blankets  the  molten  metal  from  oxidation. Fluxes are available in many different forms. There are fluxes for oxyfuel gas applications, such as brazing and soldering. These fluxes usually come in the form of a paste, powder, or liquid. Powders can be sprinkled on the base metal, or the fuller rod can be heated and dipped into  the  powder.  Liquid  and  paste  fluxes  can  be  applied to the filler rod and to the base metal with a brush. For shielded metal arc welding, the flux is on the electrode. In this case, the flux combines with impurities in the base metal, floating them away in the form of a heavy slag which shields the weld from the atmosphere. You should realize that no single flux is satisfactory for  universal  use;  however,  there  are  a  lot  of  good general-purpose fluxes for use with common metals. In general, a good flux has the following characteristics: It is fluid and active at the melting point of the fuller metal. It remains stable and does not change to a vapor rapidly within the temperature range of the weld- ing  procedure. It dissolves all oxides and removes them from the joint  surfaces. It adheres to the metal surfaces while they are being heated and does not ball up or blow away. It does not cause a glare that makes it difficult to see the progress of welding or brazing. It is easy to remove after the joint is welded. It is available in an easily applied form. CAUTION Nearly all fluxes give off fumes that may be  toxic.  Use  ONLY  in  well-ventilated  spaces. It is also good to remember that  ALL welding operations require adequate ventilation whether a flux is used or not. WELD JOINTS The weld joint is where two or more metal parts are joined by welding. The five basic types of weld joints are the butt, corner, tee, lap, and edge, as shown in figure 3-6. Figure  3-6.—Basic  weld  joints. 3-5