| The SEM, OM, FESEM, XRD and DSC were used, in combination with multicomponent phase diagram analysis and phase extraction technology, to reveal the rules of microstructure evolution of Al-Si-Cu-Ni-Mg piston alloys and strengthening mechanism at room and elevated temperatures. The optimal alloy design was carried out and some progresses were obtained.For a clear understanding of the roles of different Ni-rich phases, the different contributions of Ni-rich phases to elevated-temperature strength were studied. It was found theε-Al3Ni was prone to exhibit block-like morphology,δ-Al3CuNi phase tended to form strip-like morphology, andγ-Al7Cu4Ni phase often had skeleton-like morphology. Four alloys were designed and their strengths were tested. The volume fractions and morphology characterizations ofε-Al3Ni,δ-A13CuNi andγ-Al7Cu44Ni phases were calculated. The strengthening differences of these Ni-rich phases were analyzed:the strip-like morphology would enableδ-Al3CuNi phase to have the highest volume utilization and highest elevated-temperature strength contribution rate; The strip-like intermetallics distributed in grain boundaries was best to increase the elevated-temperature strength of Al-Si piston alloys, followed by skeleton-like morphology and block-like morphology phases in sequence.0.5 wt.% Cr and 0.8 wt.% Fe were added into optimized Al-Si-Cu-Ni-Mg piston alloy, a-Al(Fe,Cr)Si phase was formed, acting as supportive strengthening phase, and integrated organically with the Ni-rich phases acting as main strengthening phase. So a-Al grains were encircled by thermal-stable intermetallics, and closed and semi-closed net-like eutectic colonies were formed, which can prevent the slide of grain boundaries. The UTS at 350℃was improved by 26% finally.The influences ofα-Al morphology on the second phases and mechanical properties were explored. The unrefinedα-Al dendrites exhibit long column morphology and its second arms can disperse the intermetallics; the refined a-Al exhibit near equiaxed morphology and worsen dispersion of intermetallics. After the refinement, the room-temperature strength increased, but the elevated-temperature strength decreased. The different influences ofα-Al refinement on the room and elevated temperature strength are due to the different fracture mechanisms. The increase of temperature results in the decrease of strength and increase of plastic ofα-Al, which leads to a change from fragile fracture and transgranular fracture to ductile and intergranular fracture. The main factor influencing the strength changed after the fracture mechanism change. At room temperature,α-Al still bear some stress, so the developed long column dendrites are favorable for crack propagation, leading to the decrease of room-temperature strength. Theα-Al morphology weighs stronger than intermetallics morphology on strength; at elevated temperature, the load is almost shouldered by the thermal-stable strengthening phases, and the intermetallics morphology weighs stronger thanα-Al morphology on strength;Further conclusion can be deduced that the micronization of (α-Al+Si) eutectic colony can improve the mechanical properties of alloys. Taking the volume stability into account and keeping fine primary Si, the complex modification is the most suitable and important way. The developed dual melts complex modification process (by P and Sr) can avoid the mutual poisoning effect of P and Sr, and fine microstructure can be obtained. The primary Si, eutectic Si andα-Al are well refined, so the microstructure is filled with coralloid eutectic Si, regular fine primary Si and fineα-Al. Such microstructure would be helpful for the dispersion of intermetallics and will improve the comprehensive mechanical properties. |
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