Elsevier

Materials & Design

Volume 31, Issue 8, September 2010, Pages 4007-4015
Materials & Design

Technical Report
Effect of trace additions of Sn on microstructure and mechanical properties of Al–Cu–Mg alloys

https://doi.org/10.1016/j.matdes.2010.03.012Get rights and content

Abstract

The work is aimed at investigating the influence of trace additions of Tin (Sn) on the microstructure, mechanical properties and age-hardening behavior of Al–6.2%Cu–0.6%Mg alloy system. Al–6.2%Cu–0.6%Mg alloys containing varying weight percentages (from 0 to 0.1 wt.%) of Sn were prepared by casting technique. The mechanical properties and microstructure of these alloys were investigated in the as-cast as well as different heat treated conditions. The composition of the different phases present in the microstructure was determined by energy dispersive X-ray (EDS) analysis. The average grain size of the annealed alloy was found to be maximum with trace content of 0.06 wt.% Sn. The hardness and strength of the alloy increased but the ductility reduced with increase in Sn content up to 0.06 wt.%. Precipitation hardening behavior of the alloys was investigated by analyzing the aging time required to attain the peak hardness value. Addition of trace percentage of Sn was observed to have no significant influence on the peak ageing time of the investigated alloy system.

Introduction

Interest in precipitation strengthened aluminum alloys as potential materials for structural use in aircraft and space applications is mainly due to their high strength to weight ratio. The 2XXX series Al–Cu–Mg alloys of 2124, 2219 and 2618 are extensively being used in aerospace structures demanding good heat resistance properties up to 150 °C [1], [2], [3], [4]. The present research trend to develop increased strength of these materials along with reasonable toughness and low density is by the addition of trace elements (microalloying) like Sn, In, Cd, and Ag [5], [6], [7], [8], [9], [10], [11] into the alloy matrix. The mechanical properties of these alloys are influenced by factors like alloy composition, strain history and the microstructure resulting from the thermo-mechanical treatment imparted before the final use. A clear understanding of the process parameters related to casting, secondary processing and hardening heat treatments is necessary for obtaining the required shapes and sizes with desired mechanical properties. These alloys are generally used after deformation processing followed by a sequence of heat treatments.

The final strength of the heat treatable aluminum alloys is decided by a sequence of solutionising at elevated temperature, quenching to room temperature followed by age hardening at some particular intermediate temperature. The strength and ductility achieved by the alloy depend on the composition, aging time and precipitation temperature of the hardening process. During the age hardening treatment, brittle second phase particles precipitate uniformly in the alloy matrix thereby increasing the strength of these materials. In Al–Cu–Mg alloys, the equilibrium precipitate phase is CuAl2 (θ), which is the main hardening phase of the system [12], [13]. This CuAl2 (θ) phase exhibits body centered tetragonal (BCT) crystal structure with lattice parameters of a = b = 0.607 nm and c = 0.487 nm [13] and has a microhardness value (HV) of 400–600 kgf/mm2 [14]. The first stage of precipitation reaction is the formation of clusters called Guiner–Priston zones (GP zones) which are generally mono-atomic layers of Cu on (0 0 1) planes of Al lattice. With increase in the ageing time, copper atoms diffuse to the GP zones and form additional layers onto the GP zones. The sequence of formation of the precipitates in the matrix can be represented as:GP zonesθ(thin discs, fully coherent with the matrix)θ(disc shaped and semi-coherent with the matrixθ(CuAl2,which is spherical and incoherent at the precipitate–matrix interface)

Studies have been carried out regarding the effect of some traditional alloying elements like Cu, Mg, Zn, Si and Ag and different heat treatments on mechanical properties of several commercial aluminum alloys. The strengthening process of non-precipitation hardening aluminum alloys is achieved mainly due to elements in solid solution and grain refinement. Ryen et al. [15] observed that both Mg and Mn when added in solid solution result nearly linear concentration dependence of the strength at a given strain value, whereas Mn was found to cause higher strengthening effect per atom than Mg.

Copper addition to Al–Mg–Si alloys resulted in change in the precipitation sequence, enhancement of precipitation hardening kinetics, grain refinement [16], [17], [18], [19], [20], [21] and increase in peak hardness values [13]. Decrease in Mg content from 5.0% to 3.5% caused reduction of grain boundary segregations of the S1-phase in Al–Mg–Li–Me alloy ingots after homogenizing heat treatment [22]. Taleff et al. [23] reported that the strength of the Al–Mg alloys increased with increase in Mg content beyond 5.5%. However, no significant effect on tensile ductility, strain-rate sensitivity or flow stress could be observed for the alloy having Mg in the range of 2.8–5.5%. Ternary additions of Mn and Zr reduced the ductility and strain-rate sensitivity resulting from the alteration in necking-controlled failure to cavitation-controlled failure mechanism.

Addition of Si to Al–Cu alloys increased the fluidity during casting and reduced the tendency of hot cracking [24]. Investigation of trace additions of Sn by Mohamed et al. revealed formation of Sn particles of β-Sn within the CuAl2 network and tiny Mg2Sn particles on the eutectic Si particles in Al–Si–Cu–Mg and Al–Si–Mg alloys [25]. Addition of 0.15 wt.% of Sn increased ductility and toughness but reduced the strength and hardness of the cast alloys. Addition of 0.05 wt.% of Sn resulted in achieving the best mechanical properties for Al–7 wt.%Si–0.35 wt.%Mg alloy.

Liu et al. reported acceleration in the age hardening process and increase of the peak hardness with better thermal stability for Al–Cu–Mg–Mn–Zr alloy with addition of trace amounts of Ag and at the same time having no effect on grain refinement or recrystallization [12]. The yield strength was found to increase with increase in Ag content at both room and elevated temperatures with concomitant reduction in ductility. The formation of fine uniform plate-like Ω precipitates on Al {1 1 1} planes resulted in higher strengthening effect than θ′ phase. The effect of Ag on age hardening and mechanical properties was further substantiated by studies on 2519 aluminum alloys by Li et al. [26] and Vietz et al. [27]. Ag was observed to introduce some brittle modes of fracture or reduce plasticity for the alloys studied. Addition of Ag on Al–4%Cu alloy slowed down the low-temperature ageing speed thereby increasing the peak ageing time.

Scandium addition up to 0.4 wt.% increased the strength of Al–Mg alloys [28]. The strengthening was due to the direct hardening by formation of dispersed Al3Sc particles in the matrix and the sub-structural hardening for preservation of non-recrystallized structure. Addition of Zr intensified the effect of Sc addition and stabilized the structure of these alloys. Kaiser et al. [29] investigated that trace addition of Sc improves yield strength to a greater extent than tensile strength of Al–6 wt.%Mg alloys, for the fine coherent Al3Sc precipitates are more responsive to the yield behavior. The beneficial strengthening effect of these alloys was found to be limited to 0.4 wt.% of Sc addition. However, Sc is very expensive that leads to significant increase in cost of industrial applications.

The elevated temperature strength of Al–Mg–Mn–Zr alloy was increased by addition of trace amount of Er due to the precipitation of secondary Al3Er particles in the alloy matrix [30]. Though addition of Er up to 0.4 wt.% resulted in grain refinement, no increase in strength was observed due to the formation of primary Al3Er at the grain boundaries. Microalloying of 357 cast aluminum alloy with 0.1 wt.% of In increased the peak hardness value, delayed the precipitation of GP zones and increased the volume fraction of β″ precipitates during artificial ageing [31]. Effect of Y and Cr on the recrystallization behavior of Al–Sc alloys was studied and the recrystallization temperature was found to depend on the Sc content [32]. The phase composition and mechanical properties of Al–Mg alloys were investigated for Ce and Y additions, subjected to heat treatment under various conditions [33].

Literature is available regarding the influence of alloying elements like silver, tin, indium, and scandium on the structure and mechanical properties of some commercial aluminum alloys. However, only few reports are available regarding the effect of trace additions of the alloying elements (microalloying) on the heat treatable aluminum alloys. The present work is therefore aimed at investigating the influence of trace additions of Sn (up to 0.1 wt.%) on the microstructure, mechanical properties and age-hardening behavior of Al–6.2%Cu–0.6%Mg alloy system in the as-cast as well as different heat treated conditions.

Section snippets

Experimental procedures

The starting materials used for the preparation of the alloys were commercially pure aluminum ingot (99.9% pure), electrical grade copper, magnesium ingot (99.9% pure) and high purity Tin powder (99.99% pure). Al–33 wt.% Cu master alloy and Al–50 wt.% Sn master alloy were first prepared by melting commercially pure Al with Cu and Sn separately. Known quantity of Al–Cu master alloy and 0.6 wt.% of Mg ingot were then melted with Al ingots in a graphite–clay crucible heated by an electric resistance

As-cast alloys

Fig. 1 shows the optical micrographs of the cast Al–Cu–Mg alloys containing 0, 0.06 and 0.1 wt.% of Sn. Low magnification reveals dendrite structures in cast condition. Fig. 2 shows the high magnification SEM micrographs of cast Al–Cu–Mg alloys containing 0 and 0.06 wt.% of Sn under back scattered electron imaging. High magnification observation reveals two different phases in the grain boundary areas of the alloy matrix: a white phase (Phase-A), uniformly segregated along the grain boundaries

Conclusions

The microstructure, mechanical properties and age-hardening behavior of Al–6.2Cu–0.6 Mg alloys containing trace additions (varying from 0 to 0.1 wt.%) of Sn were investigated. The significant results and conclusions of the present work are as follows:

  • (1)

    The cast Al–Cu–Mg alloys revealed dendrite structures. Two types of second phases were observed at the grain boundary regions of the alloy matrix. One of them existed with script morphology. The other phase was identified as “θ” phase of CuAl2.

  • (2)

    The

Acknowledgments

The authors are thankful to Mr. Rituraj Saikia and Mr. Sanjib Sarma, Department of Mechanical Engineering, Indian Institute of Technology Guwahati, for their useful assistance during conduction of tensile tests.

References (46)

  • Xiao Yan Liu et al.

    Microstructure and mechanical properties of Al–Cu–Mg–Mn–Zr alloy with trace amounts of Ag

    Mater Sci Eng A

    (2009)
  • S.P. Wen et al.

    The effect of erbium on the microstructure and mechanical properties of Al–Mg–Mn–Zr alloy

    Mater Sci Eng A

    (2009)
  • L. Reich et al.

    Evolution of Ω phase in an Al–Cu–Mg–Ag alloy – a three dimensional atom probe study

    Acta Mater

    (1998)
  • Vesna Maksimovic et al.

    The effect of microalloying with silicon and germanium on microstructure and hardness of a commercial aluminium alloy

    J Serb Chem Soc

    (2003)
  • W.F. Miao et al.

    Effects of Cu content and presaging on precipitation characteristics in aluminium alloy 6022

    Metall Mater Trans A

    (2000)
  • V.I. Elagin

    Ways of developing high-strength and high-temperature structural aluminium alloys in the 21st century

    Metal Sci Heat Treat

    (2007)
  • Øyvind Ryen et al.

    Strengthening mechanisms in solid solution aluminium alloys

    Metall Mater Trans A

    (2006)
  • Tamizifar M, Lorimer GW. Aluminum alloys: their physical and mechanical properties. In: Arnberg L, Lohne O, Nes E, Ryum...
  • D.K. Chatterjee et al.

    A study of the effect of magnesium loss and of the addition of copper on the ageing of aluminum–magnesium–silicon alloys

    J Inst Metals

    (1973)
  • R.J. Livak

    Effects of copper and chromium on the ageing response of dilute Al–Mg–S alloys

    Metall Trans A

    (1982)
  • H. Suzuki et al.

    A consideration on two-step aging in Al–Mg–Si alloy

    J Jpn Inst Light Metals

    (1980)
  • Sakurai T, Eto T. Aluminum alloys: their physical and mechanical properties. In: Arnberg L, Lohne O, Nes E, Ryum N,...
  • D.J. Chakrabarti et al.

    Automotive alloys II

    (1998)
  • Cited by (63)

    View all citing articles on Scopus
    View full text