Microstructure and mechanical properties of friction stir welded duplex Mg–Li alloy LZ91

Abstract

Superlight duplex Mg–Li alloy LZ91 with a thickness of 3 mm was successfully joined by friction stir welding utilizing a tungsten carbide tool. The microstructures in the stir zone showed refined grains consisting of an equiaxed Mg-rich α phase and a Li-rich β phase. The grains in the Mg-rich α phase showed randomized crystal orientations after dynamic recrystallization. Due to the grain refinement and the random orientation of the Mg-rich α phase, the mechanical properties of the stir zone were optimized, and the hardness of the stir zones were higher than that of the base metal. At the same time, the heat affected zone showed no decrease in its strength. Digital image correlation analyses revealed that the strain in the stir zone and base metal were respectively 0.05 and 0.6 before the samples fractured during the joint tensile tests. Therefore, the joints demonstrated 100% tensile efficiency compared with the base metal.

Introduction

Due to the intrinsic hexagonal closed-packed (hcp) lattice structure of Mg and many of its common alloys, these materials often have poor ductility and unfavorable formability at room temperature due to the lack of an available slip system [1]. Recent studies have revealed that the addition of lithium can optimize the ductility and formability of Mg alloys [2]. When more than 5 wt% Li (equivalent to Mg-17 at% Li) is added, the newly introduced Li-rich bcc phase (β phase in Mg–Li systems) provides more available slip systems which significantly improves the formability and ductility of the alloys [2,3]. Lithium addition has also been reported to decrease the critical resolved shear stress (CRSS) of the non-basal slip systems (especially <a> prismatic, <a> pyramidal, and <c+a> pyramidal) in the initial Mg-rich α phase (hcp structure) by decreasing the c/a ratio [2]. These two advantages improve the room-temperature ductility of two-phase Mg–Li alloys [3]. Moreover, because of the ultra-low density (typically less than 1.6 g/cm³) of Li-added Mg alloys, they are widely considered to be ideal structural materials for replacing Al alloys and steels in the manufacturing of high-speed transportation vehicles in order to increase the energy efficiency [4]. To expand the industrial applications of Mg–Li alloys, it is necessary to develop an effective and environmentally-friendly joining method. Only a few studies have reported the application of fusion welding to join Mg–Li alloys [5,6], and most of the joints obtained by fusion welding typically show lower strengths compared with the base metal (BM) due to grain coarsening in the heat affected zone (HAZ). Moreover, the addition of Li to Mg alloys significantly increases the chemical reactivity. To prevent ignition, a large amount of argon gas is needed during the fusion welding of Mg–Li alloys. Therefore, an environmentally-friendly and economically practical welding method other than conventional fusion welding is needed to expand the industrial applications of Mg–Li alloys.

Friction stir welding (FSW), a novel solid-state joining technique developed by TWI is considered an ideal welding method for light metals [7]. FSW does not require melting and can be used to obtain high-strength joints without defects caused by conventional fusion welding. Researchers have successfully used FSW on light metals such as Al alloys, Mg alloys, and Ti alloys [[8], [9], [10], [11]], but few studies have reported its use for duplex Mg–Li alloys.

In this study, FSW was performed using a steel tool (SKD61, equal to H-13) and a cemented carbide tool (WC) on 3-mm thick samples of a duplex Mg–Li alloy (LZ91). Three tool rotation speeds were finallyused to evaluate the effect of the heat input on the microstructure and mechanical properties.