Study of a 6061 Weld Heat Affected Zone Made with GMAW

Al-Mg-Si alloys are extensively studied since decades because of their large utilization, especially, in the transportation industry. Indeed, these aluminum alloys have interesting properties such as good corrosion resistance, high mechanical strength vs. weight ratio, low density and good weldability. These characteristics make them suitable for structural applications. The main attribute of these alloys come from their ability to be strengthening by precipitation hardening. Formation of precipitates occurs during natural or artificial aging. So parameters of heat treatments, i.e. time and temperature, determine the precipitation state and thus the hardening level.

Hardening precipitates are metastable, i.e, heat sensitive. So, precipitation state can be altered by any heat input during industrial transformations and, especially, during welding.  So, it is important to understand the phenomenon occurring during welding to evaluate the mechanical strength after joining. In this study, the heat affected zone (HAZ) of a weld made with a gas metal arc welding process (GMAW) called “Cold Metal Transfer” were analyzed.

The parent metal used is 6061 aluminum alloy. Microstructure and main precipitation sequence of this alloy were previously studied and are now accepted. However studied samples were isothermally heat treated and yet they cannot be directly compared to a sample extracted from a weld. Indeed, heating and cooling rate are different and HAZ is non isothermally heated. Thus, this study proposes a characterization of the different precipitation states present through the HAZ of a GMAW weld by TEM investigation. Moreover, DSC was used to determine parameters of the precipitation present in the HAZ, such as heating and cooling rates and maximum temperatures.

These results show evolutions of the microstructure and of the precipitate distribution. Secondary phases are also present. These results are discussed and compared regarding other TEM and DSC studies and other works characterizing 6061 welds made with different processes.