Elsevier

Journal of Alloys and Compounds

Volume 551, 25 February 2013, Pages 596-606
Journal of Alloys and Compounds

Phase formation in as-solidified and heat-treated Al–Si–Cu–Mg–Ni alloys: Thermodynamic assessment and experimental investigation for alloy design

https://doi.org/10.1016/j.jallcom.2012.10.182Get rights and content

Abstract

Thermodynamic simulations based on the CALPHAD method have been carried out to assess the phase formation in Al–7Si–(0–1)Ni–0.5Cu–0.35Mg alloys (in wt.%) under equilibrium and non-equilibrium (Scheil cooling) conditions. Calculations showed that the T-Al9FeNi, γ-Al7Cu4Ni, δ-Al3CuNi and ε-Al3Ni phases are formed at different Ni levels. By analyzing the calculated isothermal sections of the phase diagrams it was revealed that the Ni:Cu and Ni:Fe ratios control precipitation in this alloy system. In order to verify the simulation results, microstructural investigations in as-cast, solution treated and aged conditions were carried out using electron probe microanalysis (EPMA), scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). Furthermore, cooling curve analysis (CCA) was also performed to determine the freezing range of the new alloys and porosity formation during solidification. Hardness measurements of the overaged samples showed that in this alloy system the δ-Al3CuNi phase has a greater influence on the overall strength of the alloys compared to the other Ni-bearing precipitates.

Highlights

► Phase formation in Al–Si–Ni–Cu–Mg–Fe system have been investigated. ► T-Al9FeNi, γ-Al7Cu4Ni, δ-Al3CuNi and ε-Al3Ni are formed at different Ni levels. ► Thermally stable Ni-bearing precipitates improved the overaged hardness. ► It was found that Ni:Cu and Ni:Fe ratios control the precipitation. ► δ-Al3CuNi phase has more contribution to strength compare to other precipitates.

Introduction

Al–Si casting alloys with Cu and Mg additions, such as A356 (Al7Si0.3Mg) and A319 (Al7Si4Cu), are widely used in automotive engine applications [1], [2], [3], [4], [5]. The silicon provides good fluidity and a broad range of high strengths at room temperature can be obtained by age hardening [6], [7]. However, the major disadvantages of these age-hardened alloys are poor thermal and metallurgical stability. These alloys lose their strength at high temperatures due to Ostwald ripening [5], [8], [9]. Over the last 15 years, research has been carried out to develop new aluminum alloys with improved high temperature performance. This new generation of alloys rely on the presence of thermally stable intermetallic precipitates which retain their strength when the age-hardening precipitates have lost their strengthening effect [5], [10]. Different alloying elements such as Cu, Ni, Mn, Cr, Zr and most recently Sc and Er have been added to Al alloys [5], [11], [12]. Such alloying additions form thermally stable dispersoids within the microstructure that reinforce the Al matrix and grain boundaries at high temperatures.

By adding Cu to the A356 alloy, Feikus [13] developed an Al–7Si–0.3Mg–0.5Cu alloy which showed substantial improvement in mechanical properties at 250 °C. Copper additions enhanced the age-hardening response via the introduction of two new age-hardening precipitates, Q-Al5Mg8Cu2Si6 and θ-Al2Cu into the microstructure [14]. Based on the improvement of the Al-7Si–0.3Mg–0.5Cu alloy with Ni additions, Heusler et al. [15] presented a new alloy for Diesel engine applications in which the Cu, Mg and Fe levels were adjusted to give an Al–7Si–0.4Mg–0.4Cu–0.5Ni–0.4Fe alloy. This alloy exhibited a 20% increase in fatigue strength and positive effects on tensile strength. This improvement was attributed to the presence of Ni-bearing intermetallics which formed due to the large driving force for compound formation between Al and Ni (size factor, electronegativity) and low solubility of Ni in aluminum (max  0.05% at the eutectic temperature) [16]. According to the phase diagrams, Ni can combine with Al, Cu and Fe forming the following intermetallics which could be in equilibrium with other phases in this alloy system [17]: ε-Al3Ni, δ-Al3CuNi, γ-Al7Cu4Ni, T-Al9FeNi. These intermetallics are thermally stable and impart high strength at elevated temperatures [18]. However, the brittleness of Ni-bearing intermetallics lowers the ductility, hence high temperature heat-treatment is usually conducted to increase ductility. Ni is also known as a Fe corrector. Through a peritectic reaction the brittle plate-like β-Al5FeSi intermetallics transform into T-Al9FeNi phase [19], [20].

Al–Si alloys with a substantial amount of Ni are used extensively for engine pistons in which high strength at elevated temperatures is required [21], [22], [23], [24]. The Si, Fe, Cu and Ni contents of these alloys are relatively high, ranging from 10 to 25 wt.%, 0.4 to 1.5 wt.%, 0.5 to 6 wt.% and 0.5 to 4 wt.% respectively [17], [18], [21], [22], [24]. The phase equilibria of the Al–Si–Mg–Cu–Fe–Ni system in the range of piston alloys have recently been studied by different investigators [17], [18]. Nevertheless, the compositional range of the new engine alloys is outside those of the piston alloys. Thus, the development and optimization of these alloys require the reassessment of the alloy system with much lower amounts of Si, Cu, Mg, Ni and Fe, wherein, thermodynamic calculations can be a powerful tool. Rigorous thermodynamic analysis of this alloy system requires the calculation of the multicomponent Al–Si–Cu–Mg–Ni–Fe phase diagram; this is a complicated task but has great practical significance. In the present work, the influence of Ni on the microstructure and precipitation of Al–7Si–0.3Mg–0.5Cu alloys with 0.1–1.0%Ni was investigated. Thermodynamic calculations were carried out via ThermoCalc software using the TTAL7 database [25] and combined with experimental data to generate information for alloy design.

Section snippets

Experimental procedures

Commercial grade A356 alloy supplied by Rio Tinto Alcan Inc. was melted in a silicon carbide crucible using a high frequency NORAX (Canada) induction furnace. Ni and Cu were added to the melt from a set of master alloys (Al–20 wt.%Ni and Al–33 wt.%Cu) at 760 and 730 °C, respectively, holding for 15 min to dissolve at the specified temperatures. After 3 min of degassing with high purity argon gas, the alloys were cast into a pre-heated (400 °C) permanent mold at 730 °C to obtain thin-plate castings

Thermodynamic calculations

Fig. 1a shows a calculated isopleth of the Al–Si–Cu–Mg–Fe–Ni system (Al–Si with fixed values of 1%Ni, 0.5%Cu, 0.35%Mg, and 0.1%Fe). The broken line represents the Al–7Si–0.5Cu–0.35Mg–0.1Fe alloy with 1%Ni. Since the Fe level is important in these alloys, Fe was included in all calculations. Fe in Al alloys is generally considered to be an impurity which may form brittle phases and, consequently, the Fe level is kept as low as possible to minimize any adverse effects on mechanical properties.

Conclusions

Thermodynamic modeling and experimental investigation have been carried out in combination to study the effects of Ni additions on the microstructure and phase formation of A356 + 0.5%Cu alloy.

  • 1.

    Four different Ni bearing phases are formed as a result of Ni addition: T-Al9FeNi, γ-Al7Cu4Ni, δ-Al3CuNi and ε-Al3Ni.

  • 2.

    EPMA results revealed wide range of existence for these intermetallics.

  • 3.

    The analysis of phase field distribution showed that the Ni:Cu and Ni:Fe ratios determine which precipitate will form

Acknowledgments

This work was supported by the NSERC (Natural Sciences and Engineering Research Council) of Canada Strategic project grant with Rio Tinto Alcan (RTA). The financial support of REGAL (FQRNT Regroupement Aluminum Research Center) is also acknowledged. The authors thank Pierre Vermette of McGill University for his assistance in experimental work. Amir Rezaei Farkoosh would like to gratefully acknowledge financial support from McGill University through the MEDA (McGill Engineering Doctoral Award)

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