 After welding , a number of distinct regions can be identified in the weld area . The weld itself is called the fusion zone — more specifically , it is where the filler metal was laid during the welding process . The properties of the fusion zone depend primarily on the filler metal used , and its compatibility with the base materials . It is surrounded by the heat @-@ affected zone , the area that had its microstructure and properties altered by the weld . These properties depend on the base material 's behavior when subjected to heat . The metal in this area is often weaker than both the base material and the fusion zone , and is also where residual stresses are found . 
