Characterization of ZnAl cast alloys with Li addition
Graphical abstract
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(Zn1 − xAlx)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.
Section snippets
Experimental
High purity metals of Al, Zn (99.999%), and Li (99.5%) were used for alloy preparation. As was shown in [6], [14], [16], the alloys were prepared in a glove box filled with high purity Ar (99.9999%), and a high temperature cleaner for reducing N2. Samples were melted in Mo crucibles in a resistance furnace, and then cast in heated (~ 400 °C) graphite molds (with casting dimensions Ø 12 mm × 140 mm), subjected microstructure observation and mechanical testing, or drawn into quartz capillaries (with
Calorimetry measurements
The melting temperature of cast alloys of ZnAl with Li addition (0.5 and 1.0 wt.%) were obtained using DSC measurements (as presented in Fig. 1). These temperatures are shown in Table 1. Generally, two peaks were observed on the calorimetry curve for each alloy. The first peak corresponds to the reaction as a solid solution from the α to the α’- phases above monotectoid temperature (277 °C) [23], in our case 281 °C. The second peak corresponds to melting. The first peak, with increasing Li
Conclusions
The influence of the addition of Li to eutectic ZnAl on thermal and mechanical properties was studied. Increasing Li content in ZnAl–Li alloys caused increasing CTE and electrical resistivity, which correlate with increasing grain boundaries caused by the formation of IMC precipitates. In the temperature dependency vs. electrical resistivity tests, the change of slope correlated with changes in the solid state from α to α’, similar to ZnAl alloys. For the ZnAl1·0Li alloy, a higher electrical
Acknowledgement
This work was financed by the Ministry of Science and High Education of Poland grant IP2014 011473 “Effect of addition of Na, Li and Si to eutectic ZnAl alloys on phenomena occurring at the interface of soldered joints”, in the years 2015–2017.
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