The Effect of Mg on the Microstructure and Mechanical Behavior of Al-Si-Mg Casting Alloys C.H. CACERES, C.J. DAVIDSON, J.R. GRIFFITHS, and Q.G. WANG The microstructure and tensile behavior of two Al-7 pct Si-Mg casting alloys, with magnesium contents of 0.4 and 0.7 pct, have been studied. Different microstructures were produced by varying the solidification rate and by modification with strontium. An extraction technique was used to determine the maximum size of the eutectic silicon flakes and particles. The eutectic Si particles in the unmodified alloys and, to a lesser extent, in the Sr-modified alloys are larger in the alloys with higher Mg content. Large Fe-rich -phase (Al 9 FeMg 3 Si 5 ) particles are formed in the 0.7 pct Mg alloys together with some smaller -phase (Al 5 FeSi) plates; in contrast, only -phase plates are observed in the 0.4 pct Mg alloys. The yield stress increases with the Mg content, although, at 0.7 pct Mg, it is less than expected, possibly because some of the Mg is lost to -phase intermetallics. The tensile ductility is less in the higher Mg alloys, especially in the Sr-modified alloys, compared with the lower Mg alloys. The loss of ductility of the unmodified alloy seems to be caused by the larger Si particles, while the presence of large -phase intermetallic particles accounts for the loss in ductility of the Sr-modified alloy. I. INTRODUCTION structure, [4,22–25] the size and morphology of the Si parti- cles, [23–27] and the solution treatment and aging condi- MAGNESIUM is added to Al-7 pct Si-Mg casting tion. [27,28,29] Plastic deformation results in the cracking of Si alloys to induce age hardening through precipitation of Mg- particles [4,23,24,30] and Fe-rich intermetallics. [17,18] When the Si particles (in some wrought alloys, these precipitates have dendrite cell size is small, the cracking is concentrated in recently been identified as having compositions close to the eutectic along the grain boundaries so that fracture is Mg 5 Si 6 . [1,2] The Mg content of the commercial alloys 356 intergranular. [24,31–33] On the other hand, when the cell size and 357 (equivalent Australian designations are 601 and is large, the final fracture occurs by microcrack nucleation 603) ranges from 0.3 to 0.4 pct and from 0.45 to 0.7 pct, and growth along the dendrite cell boundaries. [4,24,32,33] Frac- respectively. In general terms, it has been reported that a ture can then be termed transgranular. higher Mg content increases the yield stress while decreasing The results presented in this work complement previous the ductility [3–8] and the fracture toughness. [9] Besides its studies on the Al-7 pct Si-0.4 pct Mg alloy [18,19,24,25,33,34] major effect on the age-hardening potential, Mg depresses aimed at isolating the effects of the dendrite arm spacing the eutectic temperature and makes the eutectic Si structure from those of particle size and morphology. The present more heterogeneous. [10] It also seems to interfere with modi- experiments were intended to assess the effect of magnesium fication of the eutectic structure when using Sr additions, content on the eutectic Si microstructure, Fe-rich intermetal- making the eutectic structure coarser and less uniform. The lic phases, and mechanical behavior. Two levels of magne- Mg content also affects the types and total volume fraction sium were considered: 0.4 and 0.7 wt pct. of Fe-bearing phases, [11,12] which are known to have a detri- mental effect on the tensile properties, [13–16] especially in Be-free alloys. [17,18] The strain-hardening rate at low strains II. EXPERIMENTAL METHODS increases with the Mg content, increasing the rate of load shedding onto the Si particles, [19] and it has been suggested A. Materials and Casting Procedures that this may lower the ductility of high-Mg unmodified The study was carried out using commercial ingots of alloys. [20] unmodified alloys 356 and 357 containing approximately For a given Mg and Fe content, and ignoring possible 0.4 pct Mg and 0.7 pct Mg, respectively. The 0.4 pct Mg deleterious effects of porosity, [21] the tensile ductility and ingots were from the same batch used in earlier work. [24] strength of these alloys depend on the scale of the dendritic Plates measuring 27  115  180 mm were cast first with the unmodified material and then after chemical modifica- tion with strontium (as discussed below). The chemical com- C.H. CACERES, Senior Lecturer, is with the CRC for Alloy and Solidifi- positions of the different alloys, as obtained by inductively cation Technology (CAST), Department of Mining, Minerals and Materials coupled plasma–atomic emission spectroscopy, are given in Engineering, The University of Queensland, Brisbane, Australia 4072. C.J. Table I, in which we also define the alloy nomenclature DAVIDSON, Principal Research Scientist, and J.R. GRIFFITHS, Senior (“um” denotes “unmodified” and “Sr” denotes “Sr Principal Research Scientist, are with CSIRO Manufacturing Science and Technology, Kenmore, Australia 4069. Q.G. WANG, formerly Research modified”). Student, CRC for Alloy and Solidification Technology (CAST), Department A more detailed description of the casting procedure, of Mining, Minerals and Materials Engineering, The University of Queens- heat treatments, tensile testing, and metallography is given land, is Research Associate, Metal Processing Institute, SPI, Worcester, elsewhere. [24] Briefly, the plates were cast in a resin-bonded MA 01609. Manuscript submitted July 30, 1998. silica sand mold and large tapered cast-iron chills were METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 30A, OCTOBER 1999—2611