Optimizing the tensile properties of Al–Si–Cu–Mg 319-type alloys: Role of solution heat treatment Y. Han a , A.M. Samuel a , H.W. Doty b , S. Valtierra c , F.H. Samuel a,⇑ a Université du Québec a Chicoutimi, Québec, Canada b General Motors, Materials Engineering, 823 Joslyn Avenue, Pontiac, MI 48340, USA c Corporativo Nemak, S.A. DE C.V., P.O. Box 100, Garza Garcia, N.L. 66221, Mexico article info Article history: Received 19 September 2013 Accepted 24 January 2014 Available online 3 February 2014 Keywords: Aluminum alloys Sr modification Sr–Mg interaction Incipient melting Tensile properties abstract The work presented in this study was carried out on Al–Si–Cu–Mg 319-type alloys to investigate the role of solution heat treatment on the dissolution of copper-containing phases (CuAl 2 and Al 5 Mg 8 Cu 2 Si 6 ) in 319-type alloys containing different Mg levels, to determine the optimum solution heat treatment with respect to the occurrence of incipient melting, in relation to the alloy properties. Two series of alloys were investigated: a series of experimental Al–7 wt% Si–3.5 wt% Cu alloys containing 0, 0.3, and 0.6 wt% Mg levels. The second series was based on industrial B319 alloy. The present results show that optimum combination of Mg and Sr in this study is 0.3 wt% Mg with 150 ppm Sr, viz. for the Y4S alloy. The corre- sponding tensile properties in the as-cast condition are 260 MPa (YS), 326 MPa (UTS), and 1.50% (%El), compared to 145 MPa (YS), 232 MPa (UTS), and 2.4% (%El) for the base alloy with no Mg. At 520 °C solu- tion temperature, incipient melting of Al 5 Mg 8 Cu 2 Si 6 phase and undissolved block-like Al 2 Cu takes place. At the same time, the Si particles become rounder. Therefore, the tensile properties of Mg-containing alloys are controlled by the combined effects of dissolution of Al 2 Cu, incipient melting of Al 5 Mg 8 Cu 2 Si 6 phase and Al 2 Cu phase, as well as the Si particle characteristics. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction The purpose of solution heat-treatment of 319-type alloys is to put the maximum amount of hardening solutes, such as Cu and Mg, into solid solution in the aluminum matrix. The solution tem- perature must, therefore, be as close to the eutectic temperature as possible, while at the same time, remaining at a safe level below the maximum in order to avoid overheating and partial melting of the copper phases [1]. In Al–Si–Cu alloys, with the segregation of the alloying elements, the composition may exceed the critical composition locally and, if the temperature exceeds the copper eutectic temperature, incipient melting of the Al 2 Cu phase will occur at the grain boundaries, and result in the formation of shrinkage cavities, thereby lowering the alloy soundness and mechanical properties. In Al–Si–Cu alloys, with the segregation of the alloying elements, the composition may exceed the critical composition locally, in which case, incipient melting will occur [2,3]. Gauthier et al. [4,5] showed that the best combination of tensile strength and ductility for a 319.2 alloy was obtained when the as-cast material was solution heat-treated at 515 °C for 8-16 h, followed by quenching in warm water at 60 °C and artificial aging. Peak-aging was attained after 24 h at 155 °C or 5 h at 180 °C; and the associated tensile properties were 253 MPa (YS), 403 MPa (UTS) and 1.2% elongation. De la Sablonnière and Samuel [6] observed that a two-stage solution heat-treatment consisting of 12 h at 510 °C (to dissolve the copper-containing phases), followed by 12 h at 540 °C (to dissolve Al 5 FeSi needles and so increase the area of the Al matrix) was a very effective heat-treatment for low Mg-containing and low intermetallic-containing 319 alloys. The two-stage solution heat-treatment process has a twofold purpose in that it aims at (a) the dissolution of copper-containing phases, mainly Al 2 Cu and Al 5 Mg 8 Cu 2 Si 6 at 510 °C, and (b) spheroidization of the eutectic silicon particles and dissolution of b-Al 5 FeSi at higher temperatures [7,8]. Optimization of heat-treatment parameters may lead to the best combination of ductility, strength and hardness for a given cast component. One of the most common tempers used in the metal- casting industry for Al–Si–Cu alloys is the T6 heat treatment. It is generally accepted that this treatment allows for full enhancement of the mechanical properties in terms of higher strength and hard- ness values, attributed to a large amount of precipitation strength- ening which occurs during the aging process. Ibrahim et al. [9] http://dx.doi.org/10.1016/j.matdes.2014.01.060 0261-3069/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +1 418 545 5406; fax: +1 418 545 5012. E-mail address: fhsamuel@uqac.ca (F.H. Samuel). Materials and Design 58 (2014) 426–438 Contents lists available at ScienceDirect Materials and Design journal homepage: www.elsevier.com/locate/matdes