PSI - Issue 13

Lenka Kuchariková et al. / Procedia Structural Integrity 13 (2018) 1577–1582 Lenka Kuchariková/ Structural Integrity Procedia 00 (2018) 000 – 000

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from at least six specimens. The Vickers microhardness in the as-cast state was measured in HTW Dresden, Germany, using an MHT-1 microhardness tester: 1 g load for 10 s (HV 0.01) at the room temperature. Twenty measurements were taken per each sample and the median microhardness was determined. The fracture surfaces were analysed by a VEGA LMU II scanning electron microscope (SEM) and by energy dispersive X-ray spectroscopy (EDX), to evaluate the fracture mode and the nature of the microstructural components. The metallographic samples were prepared by standard procedure: wet ground on SiC papers, DP polished with 3 μm diamond pastes, followed by Struers Op-S and etched by 0.5 % HF. Some samples were deep-etched for 15 or 30 s in HCl solution in order to reveal the 3D morphology of the Si phase and the secondary phases.

3. Results and discussions

The mechanical properties of the primary A226 cast alloy, according to standards are: tensile strength (240 ÷ 310 MPa), offset 0.2 % yield stress (140 ÷ 240 MPa); however the low ductility limits (0.5 ÷ 3 %) and hardness HBW 80 ÷ 120. The results show that the secondary material has comparable mechanical properties to the primary alloy and the better material properties after the heat treatment, confirmed by changes in microstructure (Table 1).

Table 1. Results of the tests of the experimental material basic mechanical properties. Material UTS (MPa) HBW 2.5/62.5/15 E (J) AS 211 98 3 T6 311 139 3

The Vickers microhardness test results confirmed that particles of eutectic Si (1166 HV0.01) and the Fe-rich needles phases (829 HV0.01) have the higher hardness and thus almost the zero values of plastic properties. The matrix of experimental alloy is characterized by the high plasticity, because HV 0.01 was only 86. The other phases' Vickers microhardness values were: 275 HV0.01 for Al 15 (FeMn) 3 Si 2, 309 HV 0.01 for Al-Al 2 Cu-Si 149 HV 0.01 for Al 2 Cu. The volume fraction, chemical composition and morphology of intermetallic compounds exert significant influence on technological and mechanical properties of alloys used especially in the transportation industry, Belan (2015). Structure of the secondary hypoeutectic A226 cast alloy (in both states) consists of the α -phase, eutectic (mechanical mixture of Si in α -phase) and intermetallic Fe- and Cu-rich phases (Figs. 1, 2 ). The α -phase precipitates from the liquid as the primary phase in the form of dendrites and is nominally comprised of Al and Si. Experimental alloy was not modified and so the eutectic Si particles are in the as-cast state in a form of platelets (Fig. 1b), which on scratch pattern are in a form of needles (Fig. 1a). The need to influence the morphology of intermetallic phases and Si particles, led to using the heat-treatment to experimental alloys, what caused that the Si phase's morphology was changed (Fig. 1c, d). The Si particles were observed in a form of fragmented and smaller particles with rounded edges. The deep-etching confirmed changes in morphology. The Si particles were observed in a form of very small fragmented platelets and in some places in a form of sticks (Fig. 1d). The Si sticks are formed as a result of the solution heat treatment, Kuchariková et al. (2016b). Different intermetallic phases (Fe-rich and Cu-rich) are concentrated mainly in the interdendritic spaces in both states of experimental material. These results correlate with observation of Kim et al. (2006). The Fe-rich phases in a skeleton-like form and the Cu-rich phases as the ternary eutectic phases, were observed mostly in microstructure of experimental material (Fig. 1a,b) despite the fact that intermetallic phases in such types of aluminium materials are mostly Al 2 Cu, Mg 2 Si, α -Al 15 (Fe,Mn) 3 Si 2 and β -Al 5 FeSi, Stadler et al. (2013). The strengthening phases like θ(Al 2 Cu), β(Mg 2 Si) improve mechanical properties of aluminium alloys, but phases Al 15 (Fe,Mn) 3 Si 2 and especially β -Al 5 FeSi, decrease these properties (increase crack initiation, decrease the ductility and also lead to formation of the excessive shrinkage porosity defects in castings), Kral (2005) and Rios et al. (2003). Phases in form of needles were observed only in a small volume thanks to the ratio of Fe and Mn (2 : 1), which was also examined by Kim et al. (2006) and Seifedine et al. (2008). The formation of this phase (Al 5 FeSi) was reduced by the "neutralizer" Mn and the Al 15 (Fe,Mn) 3 Si 2 phase was formed in the skeleton-like form, as mentioned by Kim et al. (2006). Application of the heat treatment caused that the Fe-rich phases were fragmented into smaller particles of the skeleton-like form and the Cu-rich phases were dissolved and fragmented into smaller rounded particles, sometimes not even visible on metallographic samples (Fig. 1c, d).