**3. Conclusion**

Some welding process can be employed to weld aluminum alloys. In this chapter the fundamental characteristics of the most common welding processes have been presented, such as: shielded metal arc welding (SMAW), gas metal arc welding (GMAW), gas tungsten arc welding (GTAW), friction stir welding (FSW), and a new welding technique named modified indirect electric arc (MIEA). Special attention has presented on welding of 6061-T6 aluminum alloy welded by MIEA and FSW. In the case of MIEA welds important microstructural characteristics in terms of morphology and grain size has been observed with respect to those obtained by a multi-pass welding process (GMAW). It means that when

Welding of Aluminum Alloys 85

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MIEA is used, the solidification process tends to promote a heterogeneous nucleation, thus an auto-refinement of the grain size is promoted. However, when multi-pass welding process is employed (GMAW) columnar-epitaxial solidification prevails causing an increase in terms of grain size. On the other hand, the grain structure in the fusion zone produced by FSW has the better characteristics in terms of grain size (∼10 μm).

A few mechanical properties after a welding process of 6061-T6 aluminum alloy have been presented. It is observed that quasi-static mechanical properties decrease in a dramatic manner in MIEA as well as in FSW, this aspect is totally related to the micro-structural transformation in the heat affected zone of very fine needle shape β'' precipitates to coarse bar shape β' precipitates produced by the thermal effect during the welding process (thermodynamic instability). This micro-structural transformation has been quantified by means of a micro-hardness map from which is possible to observe the soft zone formation where the failures are presented after a monotonic load (tension load).

Fatigue crack growth behaviors in weld metal, HAZ and base metal of 6061-T6 welded joints obtained by MIEA were quantified. It was observed that the worst crack growth conditions are presented in the fusion zone (weld metal), which are related to brittle microstructure characteristics due to abundant presence of eutectic Si. A comparison between weld metal for FSW and the MIEA indicates that fatigue crack growth rate in the MIEA is higher than that in FSW; it means that for a critical crack length, the ΔK represents a 57% of the base material, whereas in the case of FSW it reaches a 79%. In addition, it was observed that the fatigue crack growth rate in the HAZ tends to be similar in both welding processes.
