Rare earth RE elements addition can improve

Rare earth (RE) elements addition can improve the specific strength and modulus of Mg alloys. Among them, Mg–Nd–Zn alloys can generate excellent mechanical performances with only a small amount of Nd and Zn additions [19–23]. In this Bindarit work, a common commercial Al alloy 6061 and a potential Mg–Nd–Zn–Zr alloy were chosen for friction stir lap linear welding (FLSW). Various welding parameters were performed to optimize the rotation and travel speed. Meanwhile, the temperature fields were monitored to study the effect of welding parameters on system heat input. The optimized weld was then subjected to short time solution to achieve better tensile shear properties.

Experimental procedures
6061 Al alloy and NZ30K Mg alloy sheets with same thicknesses of 3 mm were FSLWed. The chemical compositions of the parent materials are listed in Table 1. All welds were made in lap joint configuration of 6061Al alloy on top and NZ30K on the bottom as shown in Fig. 1. The cylindrical threaded welding tool was made from H13 tool steel and the shoulder diameter, pin diameter, and pin length were 15 mm, 4 mm and 3.8 mm, respectively. The tool tilt angle was 2.5° and the shoulder plunge depth was kept 0.8 mm during welding procedure. The right hand threaded tool used in this research was set to rotate clockwise. The rotation speed of 600, 900, 1200 and 1500 rpm and travel speed of 60, 90 and 120 mm/min were performed.
The temperature field during the FSLW was measured at the overlapping part as shown in Fig. 1. During every single run, eight K-type thermocouples (1 mm in diameter) were inserted into eight small holes (1.2 mm in diameter) drilled in two sides of the two layers for different depths respectively. The graphic scheme of the placements and depths distribution of drilled holes is shown in Fig. 2. Two layers named top layer and bottom layer were measured for all the parameters. In the process, only one layer was measured each pass to avoid the deviation from excessive lack of parent materials. As for the temperature of the center, the average value of the two center holes shown in Fig. 2 was adopted. The thermocouples were linked to a system controlled by the PC, which enabled a temperature sampling frequency of 100 Hz.
The FSLW joints were cross-sectioned perpendicular to the welding direction for microstructure observation. The samples were polished with diamond polishing agent and etched by three steps [21,22]. Firstly, the specimens were etched in a solution of 10 ml acetic acid, 10 ml distilled water, 4.2 g picric acid in 100 ml alcohol for 5 s to reveal the microstructure of NZ30K alloy. Secondly, the specimens were etched in a solution of 20 g NaOH in 100 ml distilled water for 40 s to reveal the grain boundary of Al alloy. The last step was to dye the specimens by the solution consisting of 4 g KMnO4 and 2 g NaOH in distilled water for 10 s. The macrostructure and microstructure were obtained with an optical microscope (OM) (Zeiss Axio Observer A1). The details of the joints microstructure, including the element distribution within the weld zone were analyzed using a FEI company produced low vacuum scanning electron microscopy (LV-SEM) equipped with an energy-dispersive X-ray spectroscopy (EDS) analysis system. XRD (RigakuUltima IV) was used to examine the phases of the joints with the analysis angles ranged from 20° to 100°. Tensile shear test was carried out on Zwick Z100 machine according to ASTM D 3164 [24]. The length and width of the specimens were 90 mm and 20 mm, respectively. To ensure tensile loading direction parallel to the weld region, a supporting plate was used. And the displacement rate was 1 mm/min.

Results and discussions

In this work, some parameters were adopted to the friction stir linear welding of magnesium alloy NZ30K and aluminium alloy 6061, and the following conclusions are given: