The influence of Fe and Cr on the microstructure of cast Al-Si-Mg alloys
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I.
INTRODUCTION
C A S T aluminum alloys of the type A356 (A1-7 pct Si0.3 Mg) have widespread applications, especially in the automotive and aircraft industries. Increasing demands on such material properties as tensile strength, ductility, and corrosion behavior have pointed to the need for close microstructural control through tighter specification of composition, casting practice, and subsequent heat treatment. One of the most important parameters in determining the properties of these alloys is the rate of solidification. This determines the coarseness of the microstructure and the amount of porosity. Increased solidification rates result in finer microstructures and improved mechanical properties. 1,2 Iron is the most deleterious impurity element in cast aluminum alloys since, as shown in Figure 1 for the ternary A1-Fe-Si system, intermetallic phases such as a (FezSiA18) and/3 (FeSiA15) form during solidification. Of these intermetallic phases /3 in particular is considered to be detrimental for the ductility of the material. It has been shown previously that the size and amount of iron-containing phases is strongly influenced by solidification rate 5'6'7 and that alloying elements such as Mn, Cr, Ni, and Co can change the morphology of the intermetallic phases or enhance the precipitation of phases which are less harmful than/3.6,8 Mn is the most common alloying addition which is used to modify the morphology and type of intermetallic phases in cast aluminum alloys. It has been reported that additions of Cr can have a similar effect, 3 but the microstructural details are not clear. In the present investigation the influence of Cr additions on microstructure has been studied as a function of solidification rate in AI-7 pct Si-0.3 pct Mg alloys containing 0.2 pct and 0.5 pct Fe. These iron contents are typical of alloys prepared from primary aluminum and remelt aluminum ingots, respectively.
G. GUSTAFSSON, formerly with Chalmers University of Technology, Grteborg, Sweden, is now with Laboratory for Metallic Materials AB Volvo, G~iteborg, Sweden. T. THORVALDSSON is Physical Metallurgist at R and D Center of AB Sandvik Steel, Sandviken, Sweden. G.L. DUNLOP is with the Department of Physics, Chalmers University of Technology, GSteborg, Sweden. Manuscript submitted September 12, 1984. METALLURGICALTRANSACTIONS A
L
"c 800 700 r
/
/
600 500
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S i wl
t% Fe
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AI Fig. 1 - - Liquidus surfaces in the Al-rich comer of the AI-Fe-Si equilibrium phase diagram. Based on data from Mondolfo3 and Rivlin and Raynor. 4
The microstructural investigation was carried out by a combination of optical metallography, electron probe microanalysis (EPMA), scanning electron microscopy (SEM), and analytical transmission electron microscopy (TEM/STEM/EDX). The detailed microstructure of cast aluminum alloys as elucidated by transmission electron microscopy (TEM) has received only a little attention previously,9'1~and therefore some emphasis has been placed on this aspect in the present work.
II.
EXPERIMENTAL
The investigation w
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