Effect of Solution and Aging Treatment on the Microstructure and Properties of LAZ931 Mg-Li Alloy by Friction Stir Processing

Abstract

Heat treatment processes play a pivotal role in optimizing the microstructure and mechanical properties of Mg-Li alloys, thereby enhancing their performance and expanding their potential applications in structural and lightweight engineering fields. In this study, the influence of solution and aging treatments on the microstructure, phase transformation, and microhardness of friction-stir-processed (FSPed) LAZ931 Mg-Li alloy was investigated to obtain the optimal solution treatment temperature and time. An optimal solution treatment at 460 °C for 0.5 h under an Ar gas atmosphere facilitated complete α-phase dissolution with subsequent aging at 125 °C, triggering precipitation-mediated hardening. An X-ray diffraction (XRD) analysis identified a new MgLi₂Al phase in the stirring zone (SZ) in addition to the α, β, and AlLi phases. Aging kinetics at 125 °C showed that SZ hardness increased to 110.5 HV after solution treatment, which was 5.3% higher than the base metal (BM). After 3 h of aging, microhardness peaked at 86.5 HV before decreasing due to the decomposition of the metastable MgLi₂Al phase into the stable AlLi phase. The microhardness stabilized at around 78 HV, which was 16.2% higher than that of the original SZ. These experimental results provide a fundamental understanding of property structure for meeting the growing demand for lightweight materials and improving material properties.

1. Introduction

Advancements in science and technology have made lightweight design essential in aerospace, military, and automotive industries for cost reduction, performance improvement, and energy efficiency [1,2,3]. Mg-Li alloys, characterized by the properties of low density, high specific strength, and superior electromagnetic shielding, are increasingly preferred for lightweight design in high-load weight reduction applications [4,5,6,7]. The hexagonal close-packed (HCP) crystal structure of Mg alloys limits their plasticity and ductility at room temperature, while the addition of Li reduces the density and improves plasticity and mechanical properties by transforming the crystal structure and introducing the body-centered cubic (BCC) β-Li phase. With superior specific strength and stiffness, Mg-Li alloys show great potential in aerospace, military, automotive, and medical fields as ultra-light materials [8,9,10]. Enhancing the strength of Mg-Li alloys is critical for improving their performance as their lower absolute strength compared to Al and high-strength Mg alloys restricts their use in high-load applications. Zhang et al. successfully fabricated a nanocrystalline-strengthened β-phase Mg-Li alloy by further rolling the homogenized as-cast Mg-14Li-0.5Ni alloy at room temperature. The resulting alloy exhibited a 150% increase in strength compared to the as-cast material [11]. Pahlavani et al. investigated the annealing treatment of LZ71 and LZ97 alloy plates at various temperatures, assessing the fracture characteristics and behavior of both alloys [12]. Li et al. reported that the flow stress curve of LZ91 Mg-Li alloy displayed typical dynamic recrystallization behavior during uniaxial hot tensile tests conducted at varying temperatures and strain rates [13]. These studies have made significant contributions to the understanding of the microstructure and properties of Mg-Li alloys. However, the heat treatment processes employed in these investigations were relatively straightforward, and the optimization of microstructure uniformity was limited, which constrained the overall enhancement of alloy properties.

FSP, derived from friction stir welding (FSW), effectively refines the grain structure of Mg-Li alloys by inducing intense stirring that causes significant plastic deformation and grain fragmentation, promoting dynamic recrystallization and improving the mechanical properties. Studies have highlighted that FSP can lead to microstructural inhomogeneity, particularly between the advancing side (AS) and retreating side (RS) of the stir zone (SZ), due to variations in stirring intensity and thermal gradients [14]. Recent studies have demonstrated that FSP, among various processing techniques, effectively refines the microstructure, improves mechanical properties, and enhances the overall performance of Mg-Li alloys [15,16]. To address these issues, the present study integrated FSP with solution treatment and artificial aging. This combined approach not only significantly refined the grain structure and improved microstructural uniformity but also enhanced mechanical properties. The proposed integrated processing route offers a novel strategy and provides a theoretical foundation for the high-performance application of Mg-Li alloys.

In recent years, considerable progress has been achieved in optimizing the mechanical properties of Mg-Li alloys through the implementation of diverse processing methodologies. FSP has emerged as a highly effective technique for Mg-Li alloys, demonstrating exceptional potential in microstructural refinement, homogenization, and the consequent enhancement of mechanical properties, including tensile strength, ductility, and fatigue resistance, ultimately leading to superior overall performance [17,18]. Li et al. [19,20] reported that FSP facilitates the precipitation of AlLi and Li₂MgAl phases, thereby improving the ultimate tensile strength (UTS) of LA103Z alloy. Concurrently, Song et al. [21] investigated the strengthening mechanisms in different FSP regions, revealing that FSP substantially reduces dynamic recrystallization and refines grain size through severe mechanical crushing. Zhu et al. [22] further demonstrated that FSP effectively refines and stabilizes the grain structure of Mg-9Li-5Al-4Zn alloy under varying thermal conditions. Complementary to FSP, heat treatment processes including solution and aging treatments have emerged as crucial approaches for property optimization. Zheng et al. [23] demonstrated that the solution treatment of as-cast Mg-11wt%Li-12wt%Zn alloy significantly enhances both corrosion resistance and mechanical strength. Wang et al. [24] reported synergistic improvements in the UTS, electromagnetic shielding effectiveness, and mechanical properties of Mg-9Li-3Al-1Zn alloy through combined solution treatment, rolling, and aging processes. Su et al. [25] developed a novel Mg-Li alloy system that exhibits exceptional mechanical stability following aging treatment, demonstrating significant potential for advanced structural applications. Peng et al. [26] demonstrated that a solution treatment within the optimal temperature range of 350–430 °C significantly enhances Mg-Li alloy strength, and the subsequent aging treatment facilitates the precipitation of strengthening phases. Ji et al. [27] identified the formation and aggregation of Al-Li phases during aging treatment as the primary factors influencing the mechanical properties of aged alloys. Consequently, the integration of FSP with thermal treatments represents a promising approach for optimizing the performance of Mg-Li alloys, offering the potential for superior mechanical properties and a refined, homogeneous microstructure, making them suitable for advanced engineering applications.

Mg-Li alloys, recognized as advanced lightweight structural materials, exhibit considerable potential for a wide range of engineering applications. However, the enhancement of their mechanical strength and microstructural uniformity remains a major scientific challenge. The combination of FSP with optimized T6 heat treatment parameters presents a promising strategy to overcome these limitations, thereby enabling the broader use of these alloys in high-performance applications. The analysis of the FSPed LAZ931 Mg-Li alloy has primarily been conducted only in China. The present study aimed to systematically explore the synergistic effects of FSP and T6 heat treatment on the mechanical properties and microstructural evolution of Mg-Li alloys, offering a theoretical foundation for improving the performance of Mg-Li light alloys.

2. Materials and Methods

2.1. FSPed LAZ931 Mg-Li Alloy and T6 Heat Treatment

The experimental material employed in this study consisted of hot-rolled LAZ931 Mg-Li alloy sheets with a nominal thickness of 10 mm. The chemical composition of the alloy is presented in

Table 1. Chemical composition of the LAZ931 Mg-Li alloy.

Alloy Composition	Li	Al	Zn	Mn	Si	Mg
Content (wt.%)	8.90	3.15	0.90	0.02	0.02	Bal.

. The FSP was conducted using a gantry-type FSW system (FSW-LM-AM16-2D, Beijing Saifost Technology Co., Ltd., Beijing, China) at the Henan Province Key Laboratory of Advanced Light Alloys, Zhengzhou University, China. A threaded rotating tool, manufactured from SKD61 tool steel, was utilized for the FSP operation. The tool geometry was characterized by a shoulder diameter of 15 mm, and the top and root diameters of the stirring pin, as well as the total length of the pin, were 5.7 mm, 6.9 mm, and 4.8 mm, respectively [5]. To ensure adequate tool–workpiece interaction, the tool was tilted forward by 2.0° relative to the normal direction (ND) of the board, and a plunge depth of 0.1 mm was maintained during processing. The rotational speed and traverse velocity were maintained at 800 rpm and 200 mm/min, respectively. The FSP was conducted on alloy plates with dimensions of 200 mm × 150 mm × 10 mm. Prior to processing, the surface oxide layer was mechanically removed using abrasive paper, followed by ultrasonic cleaning in anhydrous ethanol. The FSPed LAZ931 Mg-Li alloy was subjected to solid-solution treatment in a tube furnace (NBD-T1700X, Henan NOBODY Material Technology Co., Ltd., Zhengzhou, China). The samples were loaded into the furnace and heated at a rate of 10 °C/min until reaching the target temperatures of 460 °C, 490 °C, and 520 °C, where they were held for 0.5 h at each temperature for solid-solution treatment. After the prescribed holding time, the samples were rapidly quenched in water at room temperature. Throughout the FSP operation and solid-solution treatment, a high-purity Ar gas was utilized as the protective atmosphere to minimize oxidation during the high-temperature treatment [28]. Following the quenching process, the samples were prepared for the subsequent artificial aging treatment in an electric constant-temperature blast drying oven (DHG-9036A, Tianjin Zhonghuan Electric Furnace Co., Ltd, Tianjin, China). A schematic representation of the FSP process and the designated sampling locations for the microstructural analysis is shown in Figure 1.

2.2. Performance Characterization

The XRD was performed through a Rigaku Ultima IV diffractometer under the following conditions: a tube voltage of 40 kV, Cu-Kα radiation source, a testing angle range of 20° to 90°, and a scanning speed of 8°/min. The specimens were sequentially polished in sequence with sandpapers of 180, 400, 500, 800, and 1000 grit, followed by cleaning with anhydrous ethanol. The polished samples were then further refined using W2.5 diamond slurry on a P-2b metallographic polishing machine. Subsequent to polishing, the samples were etched in a 4% nitric-acid–ethanol solution for approximately 5 s and then rinsed with anhydrous ethanol. Optical microstructural (OM) observations were conducted using an Axio Lab A1 metallographic microscope (ZEISS, Oberkochen, Germany). Microhardness testing was performed using a Vickers microhardness tester (VICKERS 402 MVD, Wilson, Shanghai, China) with a 100 g load applied for 10 s. A 6 × 6 grid of test points, spaced 0.5 mm apart, was used to calculate the average hardness of the BM.

3. Results and Discussion

This section systematically investigates the effects of FSP and subsequent heat treatments on the microstructure and mechanical properties of the LAZ931 Mg-Li alloy. A detailed analysis is presented regarding the microstructural modifications and mechanical property changes induced by FSP, the influence of solution treatment at varying temperatures, and the effects of artificial aging on the alloy.

3.1. Microstructural Analysis of the BM and SZ

The LAZ931 Mg-Li alloy, with a nominal Li content of 9.0 wt.%, exhibits a characteristic dual-phase microstructure consisting of both α-Mg and β-Li phases. Following the rolling process, microstructural characterization revealed that the α-Mg phase underwent significant deformation, resulting in the formation of elongated grains ranging from 250 to 300 μm in length, which exhibited pronounced alignment parallel to the rolling direction within the β-Li matrix. The β-Li phase maintained its equiaxed grain morphology with an average grain size of approximately 50 μm, while serving as the host for numerous finely dispersed second-phase precipitates. The optical microstructure of the cross-sectional sample following FSP is illustrated in Figure 2. The region with fine grains was found near the advancing side of the thermo-mechanical-affected zone (AS-TMAZ) and was designated as the fine-grained part (FGP), whereas the region with coarse grains near the retreating side of the thermo-mechanical-affected zone (RS-TMAZ) was designated as the coarse-grained part (CGP). As elucidated in Figure 2, the processed zone was devoid of pores and cracks, attesting to the achievement of a defect-free and dense microstructure through FSP. This observation suggests that the processing parameters, which incorporate an appropriate level of heat input, effectively diminished the occurrence of defects, particularly cracks that might originate from insufficient plastic deformation.

The microstructural characteristics of the dual-phase Mg-Li alloy, consisting of α-Mg and β-Li constituent phases, are systematically illustrated in Figure 3a. Notably, the β-Li matrix phase contained discrete granular AlLi intermetallic precipitates, while the α-Mg phase matrix was characterized by the presence of lamellar secondary phase constituents. Figure 3b displays an enhanced magnification of the modified microstructure, unequivocally revealing that FSP achieved significant grain refinement, reducing the average grain dimensions to approximately 10–20 μm with the subsequent development of equiaxed grain morphology. This microstructural transformation suggests that intense mechanical deformation induced by the rotating tool promoted initial grain fragmentation, followed by thermally activated dynamic recrystallization mechanisms under coupled thermomechanical processing conditions. As depicted in Figure 3c, the transition zone exhibited heterogeneous microstructural features comprising residual coarse grains inherited from the BM alongside FGP generated through FSP-induced dynamic recrystallization. A marked microstructural gradient was observed between the AS and RS, with the AS displaying enhanced grain refinement efficiency. This asymmetry primarily originated from the differential thermo-mechanical processing conditions and non-uniform material flow patterns inherent to the FSP procedure. Figure 3d delineates the progressive microstructural transition extending from the AS-TMAZ interface to the fully processed FSP region. The distinct grain elongation patterns within the TMAZ provide direct microstructural evidence of vertical upward material transport phenomena within the SZ, as indicated by the red arrow. These flow dynamics not only induced significant grain boundary curvature but also facilitated crystallographic reorientation processes at interfacial regions through severe plastic deformation mechanisms.

Figure 4 comparatively illustrates the XRD profiles acquired from the SZ and BM. A detailed analysis demonstrated a systematic shift in the α-Mg phase diffraction peak toward higher angles in the BM, indicative of lattice contraction arising from the substitutional incorporation of Li atoms into the α-Mg matrix to form a supersaturated solid solution. Conversely, the β-Li phase exhibited a distinct peak shift toward lower angles, consistent with lattice expansion caused by the dissolution of larger Mg atoms within the β-Li matrix, thereby forming a β-Li solid solution. The absence of the Zn-related diffraction peak in Figure 4 was mainly attributed to the low Zn content and its dominant solid-solution existence in the matrix. After FSP, a MgLi₂Al diffraction peak emerged at about 37°, along with α, β, and AlLi phases. The AlLi phase peak intensity decreased notably compared to that of the BM, suggesting thermal-induced decomposition into Al and Li atoms in the solid solution. Comparing the XRD peaks in the BM and SZ showed the (0002) α-Mg diffraction peak weakened in the SZ, while the (11$\stackrel{\_}{2}$0) and (10$\stackrel{\_}{1}$1) intensities increased. The (0002) α-phase content decrease resulted from the grain fragmentation, α-phase transformation, recrystallization, and slip-system activation. Notably, the preferred-orientation crystal plane (110) of the β-phase exhibited no substantial alterations.

A comparison of the microhardness values for the α and β phases in the BM is presented in Figure 5. The BM exhibited an average microhardness of 63.7 HV, with pronounced phase-specific heterogeneity: the α-phase demonstrated a markedly elevated hardness of 69.3 HV, while the β-phase registered a reduced value of 61.9 HV. This disparity originated from intrinsic crystallographic distinctions, wherein the α-phase, characterized by an HCP lattice configuration and denser atomic packing, inherently resisted dislocation motion more effectively, thereby enhancing its strain-hardening capacity. Conversely, the β-phase, featuring a BCC structure with reduced atomic coordination and weaker interatomic bonding, facilitated greater plastic deformability. Post-process microstructural characterization via XRD and OM within the SZ elucidated synergistic strengthening mechanisms induced by FSP. These include (i) pronounced grain refinement via dynamic recrystallization; (ii) solute-mediated solid-solution strengthening from lattice-distorting alloying elements; and (iii) precipitation hardening through thermally activated intermetallic phase formation. Collectively, these microstructural modifications accounted for the enhanced mechanical performance observed in FSPed specimens.

To assess the hardness variation across the transverse direction from the base material to the mixing zone, a line parallel to the upper surface, located 3 mm from the processing surface, was selected. Test points were taken every 1 mm along this direction. The hardness distribution along Line 1 (as shown in Figure 2) on the cross-section of FSPed specimens is presented in Figure 6. The distribution exhibited higher hardness values in the SZ compared to that in the BM region. The maximum hardness, approximately 78.8 HV, was observed in the FGP, attributed to significant grain refinement of both the α and β phases. The hardness in the CGP decreased to around 67.8 HV, although it remained higher than that of the BM region. This hardness improvement can be attributed to three factors: (1) the finer grain structure in the SZ, which promoted strengthening according to the Hall–Petch relationship; (2) the dissolution of Al atoms from the second phase into the β phase during FSP, leading to lattice distortion and enhanced dislocation pinning; and (3) the precipitation of the metastable MgLi₂Al phase during cooling, which impeded dislocation motion and further increased hardness [29]. The microhardness profile across the TMAZ exhibited a distinct spatial gradient correlated with proximity to the BM. Regions adjacent to the BM interface demonstrated comparable microhardness values to the parent BM (63.7 HV), whereas distal TMAZ domains displayed a progressive hardness elevation (up to 72.4 HV). This gradient evolution arose from diminishing thermomechanical processing effects with increasing distance from the SZ, where intensified plastic strain and dynamic recrystallization generated refined microstructures with elevated dislocation densities and precipitation strengthening.

3.2. Effect of Heat Treatment on the Microstructure and Properties of the SZ

A different solution treatment temperature and time can lead to significant changes in the microstructure and mechanical properties of the SZ [30,31,32]. The OM images of the FSPed LAZ931 after solid-solution treatment at different temperatures for 0.5 h are illustrated in Figure 7. Figure 7a demonstrates a significant reduction in granular second phases within the β phase, suggesting that the elevated temperature facilitated second-phase dissolution. For Figure 7b, the α phase volume was significantly reduced and spheroidized, with acicular α phase precipitates within the β phase matrix. This behavior may have resulted from the dissolution of the α phase into the β phase at 490 °C, followed by precipitation from the supersaturated β phase during cooling. Figure 7c presents the OM image of the solid-solution treatment at 520 °C for 0.5 h, revealing a finer acicular structure compared to that from the treatment at 490 °C. A comparison of the results in Figure 7 indicated that the structure treated at 460 °C exhibited the most uniform microstructure, with no acicular phase precipitation and relatively complete second-phase dissolution. Therefore, a solid-solution treatment at 460 °C yielded the optimal results.

The OM images were subsequently analyzed for solid-solution treatments at 460 °C for 0.5, 1.0, and 1.5 h, as shown in Figure 8. Figure 8a shows that the β phase exhibited a grain size of approximately 40–60 μm, with residual second-phase particles present. In Figure 8b, the partial dissolution of the α phase is observed, resulting in a significant reduction in its volume. An extended solid-solution treatment led to α phase coarsening and the aggregation of second-phase particles. Figure 8c demonstrates the further coarsening of the α phase as indicated by the red circle area, with grain sizes reaching 70–90 μm. Figure 8d reveals the precipitation of large second-phase particles along the grain boundaries. A prolonged solution time induced α phase coalescence, resulting in grain coarsening and overheating, as reported in Ref. [33].

The XRD spectra of the FSPed LAZ931 alloy after solution treatments at different temperatures and times are depicted in Figure 9. The results indicated that the alloy matrix mainly consisted of α-Mg and β-Li phases, with additional peaks corresponding to AlLi and MgLi₂Al phases. While most AlLi and MgLi₂Al phases dissolved into the matrix, the higher melting point of AlLi inhibited complete dissolution, leaving residual AlLi particles. Additionally, the MgLi₂Al phase may form metastable phases during quenching. Figure 9a illustrates that increasing the solution temperature reduced the intensity of the α-phase diffraction peaks, indicating partial dissolution of the α-phase into the β-matrix. The progressively smaller differences in peak intensities suggested recrystallization, leading to the formation of uniform, strain-free grains. Figure 9b illustrates that the phase composition remained unchanged with increasing insulation times, while the diffraction peaks sharpened due to α-Mg grain coarsening.

The microhardness of the FSPed LAZ931 alloy after solution treatments at different temperatures and times is depicted in Figure 10. As shown in Figure 10a, the results display that the hardness of the FSP samples after the solution treatment was 110.5 HV, representing a 64.7% increase compared to that of the untreated FSP samples. The solution treatment promoted the dissolution of the second phase, with atoms such as Al and Zn dissolving into the matrix, causing lattice distortion that hindered dislocation motion. As a result, the hardness of the solution-treated samples was higher than that of the untreated samples. The hardness variation was not obvious with increasing solution treatment temperatures, indicating that temperature elevation had a minimal effect on solid-solution strengthening. Figure 10b shows a gradual hardness decrease with prolonged solution treatment time, attributed to overheating, which promoted matrix and second phase aggregation and grain coarsening, thus weakening precipitation strengthening and grain refinement effects [34].

3.3. Effect of Artificial Aging on the Microstructure and Properties of the SZ

During artificial aging, different aging times resulted in significant changes in both the microstructural morphology of the precipitates and the hardness [35,36,37]. To investigate the effect of precipitate microstructural changes during aging on hardness, the microstructures of samples solution-treated at 460 °C for 0.5h were examined after aging at 125 °C for 3 and 7 h, as shown in Figure 11. Compared with the result of the samples after solid-solution treatment, the grain size and the morphological characteristics of the samples depicted in Figure 11a,b exhibited no significant alterations. Figure 11c displays the high-magnification microstructure after aging at 125 °C for 3 h, showing fine granular precipitates within the β phase grains, at the α/β phase boundary, and along the β phase grain boundaries. The OM image in Figure 11d after 7 h of aging shows that the matrix α phase morphology remained slightly changed with an increased aging time. Small acicular α phase precipitates formed within the β phase, and continuous network structures precipitate at the β phase grain boundaries.

In order to evaluate the impact of solid-solution treatment and artificial aging on the microhardness of the SZ, Figure 12 illustrates the hardness variation of FSPed LAZ931 alloy after solid-solution treatment at 460 °C for 0.5 h and subsequent artificial aging at 125 °C. The results indicated that the hardness of the SZ after solid-solution treatment and artificial aging was higher than that of the untreated SZ. The hardness declined sharply within the first hour of aging. Subsequently, the hardness increased initially and then decreased with prolonged aging, and the hardness of the SZ after artificial aging was lower than that after solution treatment. The hardness attained a maximum of 86.5 HV after 3 h of artificial aging, representing a 28.9% increase compared with that of the solution treating. The hardness stabilized at approximately 78.0 HV with extended aging, exhibiting a 16.2% improvement over that of pre-solution treating. As reported in Ref. [38], Mg-Li alloys precipitate the metastable MgLi₂Al phase at the initial stage of artificial aging, which decomposes into the soft and stable AlLi phase with prolonged aging. The aggregation and growth of the AlLi phase subsequently induce over-aging softening.

4. Conclusions

This study investigated the effects of different solution treatment and artificial aging processes on the microstructure, phase structure, and mechanical properties of FSPed LAZ931 Mg-Li alloy. The conclusions are as follows:

(1)

The hot-rolled LAZ931 Mg-Li alloy manifests a dual-phase microstructure comprising α-Mg and β-Li matrices with dispersed AlLi intermetallic particulates. The BM exhibits an average microhardness of 63.7 HV, with phase-specific values of 69.3 HV (α-phase) and 61.9 HV (β-phase), correlating with their respective crystallographic packing densities. FSP enhances the composite hardness to 67.1 HV through synergistic grain refinement (10–20 μm equiaxed grains) and strain-induced strengthening mechanisms.

(2)

Isothermal solution treatment at 460 °C for 0.5 h achieves near-complete dissolution of secondary phases, yielding a homogenized microstructure with a hardness of 110.5 HV. Peak hardness (112 HV) occurs at 490 °C, concomitant with nanoscale precipitate formation. Elevated temperatures induce deleterious grain coarsening and incipient melting, resulting in mechanical degradation. Microstructural optimization is attained at 460 °C, balancing dissolution kinetics and precipitate suppression.

(3)

Artificial aging at 125 °C demonstrates suboptimal strengthening efficacy, with aged hardness values consistently below solution-treated benchmarks. A transient hardness maximum of 86.5 HV is achieved after 3 h, followed by progressive softening attributable to over-aging phenomena—specifically, the transformation of metastable zones into coarse equilibrium precipitates with reduced strengthening contributions.