Orientation relationships and morphology of S phase in friction stir welded Al–Cu–Mg alloy

Abstract

It is generally believed that S phase has lath-morphology. Both lath and rod-shaped S phase were observed in the present study and a closer observation of the selected area diffraction patterns (SADPs) revealed an interesting difference in their orientation relationship with the matrix. A modified orientation relationship has been recently discovered and related to the S phase morpholgy. Although at this time, the effect of the orientation relationship and morphology of S phase on the mechanical properties of Al‐Cu‐Mg alloys is not understood, it is worthwhile to investigate this observation for increasing the understanding of S phase precipitation and growth. Although there have been several investigations relating to the microstructure of friction stir welds in Al‐Cu‐Mg alloys [1‐4], precipitation and growth of S phase in these welds have not been adequately addressed. Two friction stir welds produced on 1-mm thick 2024T3 commercial aluminum alloy (Al‐4.4Cu‐1.5Mg‐0.5Mn) using different heat inputs were studied using transmission electron microscopy (TEM) operating at 200 kV. The process conditions of these welds are shown in Table 1. The ratio of tool rotation speed and the traverse rate gives the heat index N/v. The tool used for friction stir welding consisted of a shoulder with a diameter of 7 mm and a 1 mm long pin having a diameter of 2.5 mm. Based on the process parameters and the tool dimensions, it can be calculated that the heat input per unit length for the low heat input weld and high heat input weld were 99.8 and 269.8 kJ/mm, respectively. This suggests that the high heat input weld experienced significantly higher peak temperature as compared to the low heat input weld. The location in the heat affected zone (HAZ) just outside the thermomechanically affected zone exhibited coarse S phase, which had precipitated due to the heat of friction stir welding. Figure 1 shows the bright field (BF) images and SADPs from these regions for both welds. The majority of S phase in the low heat input weld (1) exhibited a rod morphology, while the S phase in the high heat input weld (2) exhibited a lath morphology. It should be noted that in the [001] SADP for the low heat input weld, both {112}S and {131}S reflections were strong, while in case of the high heat input welds, only the {131}S reflections were strong.