Ideally a weldment - by this is meant the complete joint comprising the weld metal, heat affected zones (HAZ) and the adjacent parent metal - should have the same properties as the parent metal. There are, however, a number of problems associated with the welding of aluminium and its alloys that make it difficult to achieve this ideal. The features and defects that may contribute to the loss of properties comprise the following:
• Gas porosity.
• Oxide inclusions and oxide filming.
• Solidification (hot) cracking or hot tearing.
• Reduced strength in the weld and HAZ.
• Lack of fusion.
• Reduced corrosion resistance.
• Reduced electrical resistance.
This chapter deals with the first four of these problem areas, i. e. those of porosity, oxide film removal, hot cracking and a loss of strength. Before discussing these problems, however, there is a brief introduction as to how metals achieve their mechanical properties. Some of the terms used to describe specific parts of a welded joint are shown in Fig. 2.1.