Characterization of ZnAl cast alloys with Li addition

Highlights

• High temperature soldering materials ZnAl with Li were designed and characterized.

• Precipitates from Al–Li system, was observed and confirmed using TEM and XRD.

• Addition Li to eutectic ZnAl caused increased the mechanical properties.

• Li content caused increased electrical resistivity, microhardness and CTE.

Abstract

The addition of Li to eutectic ZnAl alloy caused improved mechanical and thermal properties, with potential application in new design materials used in the automotive and aerospace industries. The alloys with 0.05, 0.1, 0.2, 0.5 and 1.0 Li (wt.%) content were studied using calorimetry, to determine the melting temperatures. Thermal linear expansion and electrical resistivity measurements were performed at temperatures of − 50 °C to 300 °C and 30 °C to 300 °C. The microstructure of the specimens was analyzed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Chemical microanalysis was performed using energy-dispersive X-ray spectroscopy (EDS) on SEM and TEM. The possibility of the occurrence of Al–Li, Li–Zn and Al–Li–Zn precipitates was studied by X-ray diffraction (XRDs) and selected area electron diffraction (SAED). The addition of Li to eutectic ZnAl alloy increased the electrical resistivity and the coefficient of thermal expansion; however, the melting point did not change. The mechanical properties, strain and microhardness increased with Li content in alloys.

Introduction

Higher performance and improved properties of soldered connections could improve such joints for ever more demanding applications. The high mechanical properties and improved corrosion resistance obtained by the addition of alloying elements may extend the use of Al–Zn alloys. The superplastic deformation during static annealing of a Zn–22Al (at.%) alloy was determined [1] by analysis of the kinetics of grain growth. Senkov and Myshlyaev [1] found that, in order to enhance the grain growth rate, a higher strain rate is required. The rate of grain growth is controlled by solute diffusion along the grain boundaries. During the soldering process using ZnAl on Cu substrate [2], [3] or Cu/Zn-Al/Cu [4], [5], intermetallic compounds (IMCs) from the Cu–Zn system formed at the interface. The addition of Ag to the solders [6] could block the growth of IMC layers with Cu [7] and Mg [8] at the interface, because Ag forms with Zn IMC precipitates in solder, inhibiting diffusion of Zn to the interface. For soldering on an Al substrate, different characteristics are observed, as the solder may dissolve Al pads and not create the IMCs at interface [3], [6]. Possible applications of alloys from the Al–Zn system are solder for connecting Al/Cu [2], [5], Al/Mg [8], and Al/steel [9], [10], but characterization of the thermal and mechanical properties of the cast alloys is required for the interpretation of phenomena occurring at the interface. Investigations into the corrosion resistance of Zn15Al (5 wt.%) [11], [12] show that ZnO is the product of the reaction, and that samples after homogenization have higher corrosion resistance. The preliminary study [13] shows that the addition of Li to an Al–Zn alloy leads to increased corrosion resistance and improved mechanical properties. The additions of Ag [6], Cu [14], In [15] and Na [16] to the solder limit the growth of phases at the interface, and increase the mechanical, electrical and thermal properties of cast alloys, thereby improving soldered connections. The Ag content added to eutectic ZnAl alloys causes the formation of IMCs AgZn₃, which reduce the thickness of the intermetallic layers at the interface and increase mechanical properties [6]. Similar to added Ag, the addition of Cu and Na caused increased mechanical properties by the formation of precipitates from the Cu–Zn system, and NaZn₁₃ in cast alloys [14], [16]. The Li forms several phases with Al and Zn, corresponding to the binary phase diagram of Al–Li [17] as AlLi, Al₂Li₃ and Al₄Li₉, and Li–Zn [18] as LiZn, Li₂Zn₃, LiZn₂, Li₂Zn₅ and LiZn₄. Three IMCs (τ₁ — Li₃ZnAl₅, τ₂ — Li₂₆Al₆(Zn_1 − xAl_x)₄₉, τ₃ — LiZn₃Al) also formed in the ternary Al–Li–Zn system [19], [20], and these can further improve the mechanical properties of cast alloys, and block the growth of the IMC layer at the interface. The Li content in Al–Li–Mg–Cu–Zr alloys [21] caused the creation of a δ’ - Al₃Li phase during the aging process, and this phase strengthens the precipitation of δ’. This is because the δ′ phase has the lowest nucleation activation energy, and Li, with higher vacancy-bond energy, contributes to the different precipitation response in the alloy at temperatures of 120, 160 and 200 °C [21]. In the literature data there is no information about the effect of Li addition to ZnAl alloys on mechanical and thermal properties, only liquid properties as density, surface tension and viscosity were experimental designed [22].

Therefore, in this study the following properties of ZnAl eutectic cast alloys with Li addition of 0.05, 0.1, 0.2, 0.5 and 1.0 (wt.%) were investigated: the influence of Li on the melting temperature, the coefficient of thermal linear expansion, the electrical resistivity, the microstructure, and mechanical properties.