Formation of Iron-Rich Intermetallic Phases in Al-7Si-Mg: Influence of Cooling Rate and Strontium Modification
- PDF / 2,487,638 Bytes
- 18 Pages / 593.972 x 792 pts Page_size
- 9 Downloads / 185 Views
-MG alloys have good castability, corrosion resistance and high-specific strength, particularly in the heat-treated condition.[1] Al-7Si-Mg castings are used in automotive and aerospace industries and are now starting to become applied in fatigue-critical applications.[2–4] A deep understanding of microstructure and defect formation during solidification is important to produce reliable castings for fatigue-critical applications.[3] Semi-solid metal (SSM) castings typically have low gas entrapment and shrinkage porosity defects compared to high pressure die casting (HPDC).[5] Therefore, aluminum SSM castings can be heat treated to T6 condition to increase the mechanical properties with low blistering occurrence.[5] Typically, initiation of fatigue cracks occurs at defects that are near or at the casting surface such as pores and oxides.[2,3,6] Eutectic silicon, intermetallic phases, magnesium content and cooling rate also influence the fatigue life of
JORGE SANTOS, ANDERS E.W. JARFORS and ARNE K. DAHLE are with the Department of Materials and Manufacturing, School of Engineering, Jo¨nko¨ping University, P.O. Box 1026, 55111 Jo¨nko¨ping, Sweden. Contact e-mail: [email protected] Manuscript submitted January 29, 2019. Article published online July 2, 2019 4148—VOLUME 50A, SEPTEMBER 2019
Al-7Si-Mg castings.[2,7,8] Strontium modification is widely used to modify eutectic silicon and consequently improve the mechanical properties of Al-Si alloys.[2] Al-Si alloys typically contain iron which decreases the ductility and fatigue life of the castings by the formation of iron-rich intermetallic phases.[9] The size, type and volume fraction of iron-rich intermetallic phases formed during solidification of Al-7Si-Mg alloys depend on the composition of the alloy and solidification conditions.[10,11] Intermetallic phases cannot always be identified by their morphology.[12] The plate-like d-Al3FeSi2 phase may be misidentified as b-Al5FeSi phase, particularly in modified Al-Si alloys.[13,14] The iron-rich d phase is termed as d-Al3FeSi2[12–15] or d- Al4FeSi2[16,17] in the literature. In this study, this phase will be described as d-Al3FeSi2. In a strontium modified Al-11.7Si alloy, manganese addition did not result in formation of the script-like a-Al14Fe3Si2 phase and instead plate-like intermetallic phases containing manganese were plentiful.[13] Yu et al.[16] found that the intermetallic phases have a Chinese script morphology in an unmodified cast Al-10Si-0.3Fe alloy, while intermetallic phases with a needle-like morphology were dominant in the strontium modified alloy. Timpel et al.[18] found script-like iron-rich a- Al14Fe3Si2 phase in an unmodified Al-10Si-0.1Fe alloy, while the strontium-modified alloy contained a- Al14Fe3Si2 phase and a thin platelet d-Al3FeSi2 phase. High silicon contents, cooling rates and strontium modification favors the
METALLURGICAL AND MATERIALS TRANSACTIONS A
formation of thin platelet d-Al3FeSi2 phase[15,16,19,20] at the expense of the platelet b-Al5FeSi phase in cast Al-Si alloys. Strontium addition promot
Data Loading...
