On the use of trace additions of Sn to enhance sintered 2xxx series Al powder alloys - ScienceDirect

Materials Science and Engineering: A

Volume 268, Issues 1–2, 15 August 1999, Pages 32-39

Materials Science and Engineering: A

https://doi.org/10.1016/S0921-5093(99)00126-4 Get rights and content

Abstract

The mechanical properties of a typical sintered aluminium alloy (Al–4.4Cu–0.8Si–0.5Mg) have been improved by the simultaneous use of trace additions of Sn, high sintering temperatures and modified heat treatments. Tin increases densification, but the Sn concentration is limited to ≤0.1wt% because incipient melting occurs during solution treatment at higher Sn levels. A sintering temperature of 620°C increases the liquid volume over that formed at the conventional 590°C sintering temperature. However, the higher sintering temperature results in the formation of an embrittling phase which can be eliminated if solution treatment is incorporated into the sintering cycle (a modified T5 heat treatment). These conditions produce a tensile strength of 375 MPa, an increase of nearly 20% over the unmodified alloy.

Introduction

It has recently been shown [1] that trace additions of Sn and, to a lesser extent, Pb, In, Sb and Bi activate the sintering of an Al–Cu–Mg alloy. These elements have high vacancy binding energies and high diffusivities and therefore diffuse into the Al matrix ahead of the copper and form trace element-vacancy clusters. This ties up the vacancies, limiting copper diffusion and hence delaying the transient aspect of the sintering cycle. Larger volumes of liquid therefore remain for longer times, providing the sintering enhancement. The vacancy-trace element clustering mechanism is similar to that which effects age hardening in wrought Al–Cu alloys [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12].

Here this trace element effect is applied to enhance the properties of a typical commercial alloy, Al–4.4Cu–0.8Si–0.5Mg. This alloy, essentially identical to the wrought 2014 alloy, was developed by Alcoa nearly 30 years ago [13], [14], [15] and has remained virtually unchanged since.

Section snippets

Experimental

Powder alloys were prepared by mixing elemental powders for 30 min in a Tubular powder mixer. The powder details are presented in Table 1. One weight percent stearic acid was mixed with the powder as a lubricant. Compaction of the specimens was performed using a hand operated Carver hydraulic press and a floating rectangular die. Specimens were pressed to a green density of 91% using a pressure of 220 MPa. These conditions produced specimens whose dimensions were ≈40×9×3.5 mm. Green density was

Results and discussion

As a first iteration, up to 0.25wt%Sn was added to the conventional Al–4.4Cu–0.8Si–0.5Mg alloy and processed using the standard sintering cycle. Although Sn additions enhance sintering (Fig. 2), there is a negligible effect on T6 properties (Fig. 3). A simple tin addition is therefore not efficacious. This is in contrast to tin’s effect on a pseudo-binary Al–4Cu–0.15Mg alloy [16], where sintered density, ageing response and T6 properties are all improved.

Precipitation in Al–Cu–Mg alloys with

Conclusion

Production of an Al–4.4Cu–0.8Si–0.5Mg alloy with nearly a 20% improvement in properties over a typical commercially available 2xxx alloy has been attained by the simultaneous addition of 0.1wt%Sn, increasing the sintering temperature from 590 to 620°C and incorporating the solution treatment into the sintering cycle (a modified T5 condition). The Sn and the high sintering temperature increase densification, while the modified T5 solution treatment limits the formation of embrittling phases.

Acknowledgements

The authors thank Comalco Aluminium and ACL Bearing Company for suppling some of the metal powders. This work was funded by the Australian Research Council.

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