Friction stir welding of 2219 aluminum: Behavior of θ (Al2Cu) particles

TLDR: In this article, an experimental study was conducted to determine if the maximum temperature in the workpiece can reach the lower bound of the melting temperature range and trigger liquation during friction stir welding (FSW) of aluminum alloys as some computer simulation has suggested.

Abstract: An experimental study was conducted to determine if the maximum temperature in the workpiece can reach the lower bound of the melting temperature range and trigger liquation during friction stir welding (FSW) of aluminum alloys as some computer simulation has suggested. Alloy 2219, which is essentially a binary Al-Cu alloy, was selected as the material for study because of its clear lower bound of the melting temperature range, that is, the eutectic temperature 548°C. In addition to FSW, gas metal arc welding (GMAW) of Alloy 2219 was also conducted to provide a benchmark for checking liquation in FSW of Alloy 2219. The microstructure of the resultant welds was examined by both optical and scanning electron microscopy. It was found that in GMAW of Alloy 2219, θ (Al 2 Cu) particles acted as in-situ microsensors, clearly indicating the onset of liquation by reacting with the surrounding aluminum matrix and forming distinct composite-like eutectic particles upon reaching the eutectic temperature. In FSW, on the other hand, no evidence of θ-induced liquation was found as the welds contained θ particles alone and no eutectic particles, suggesting that the eutectic temperature was not reached during FSW. However, in most friction stir welds large θ particles were observed, some exceeding 100 μm and even 1 mm in length as compared to the normal θ particles of only about 10-15 μm in length in both the base metal and the weld, that is, the stir zone or nugget. The large θ particles appeared to have formed during FSW from agglomeration of fractured θ particles and the smaller ones of the θ particles in the workpiece. No apparent correlation between the extent of agglomeration and the welding condition was found.