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Currently one major barrier for the design of cast high-strength and high-toughness Al alloys is hot tearing. In this work, a strategy of combination of squeeze casting and microalloying (Ca/Ni eutectic elements) in Al-Cu-Mn based alloys was employed to inherit the excellent mechanical properties of Al-Cu alloys whilst simultaneously reducing their tendency to hot tearing. The developed alloys exhibit comparable castability (fluidity and hot tear resistance) to A356, which is widely available commercially. However, their comprehensive mechanical properties far exceed those of A356. Trace Ca/Ni additions markedly reduce the grain size of the alloy and simultaneously increase the fraction of intergranular low melting point eutectic liquid phases, which is beneficial to the improvement of liquid feeding. The fine equiaxed grains have excellent thermal shrinkage coordination, while the high volume fraction eutectic liquid phase delays the development of grain cohesive skeleton. Accordingly, the localized shrinkage stress/strain at the hot spot is released timely, thus inhibiting the emergence and propagation of hot cracks in the brittle solidification interval. The diminished hot tearing susceptibility is attributed to the reduced load onset temperature and load values in the hot spot region. A new intergranular bridging mechanism is discovered in low-Ca alloys: based on compositional segregation-induced nucleation of an intergranular bridging skeleton, which strengthens the intergranular adhesive force and effectively reduces the hot tearing tendency of the alloys. The alloys with high-Ca/Ni additions, however, reduce the hot tearing tendency by healing cracks through eutectic liquid phase backfill. The results of the hot tearing experiments of the developed alloys are analyzed with the current hot tearing evaluation criteria. The results demonstrate that the predictions of the Kou's criterion provide guidelines for the design of alloys that are resistant to hot tearing. • Trace amounts of Ca and Ni elements dramatically reduce the hot tearing tendency of the matrix alloy. • The Ca/Ni rich eutectic phase retards the formation of brittle dendritic skeleton and inhibits the initiation and extension of hot cracks. • Eutectic colonies induce the development of bridges between grains and enhance the cohesion of grain boundaries. [ABSTRACT FROM AUTHOR]

Hot tear susceptibility (HTS) of Al–Si alloys in the 0–5–3·0 wt-%Si has been determined using a constrained rod casting test and a quantitative index. Hot tear susceptibility was seen to decrease with increasing silicon content. Hot tear severity varied directly with solidification shrinkageβ, the non-equilibrium freezing range ΔT′, dendrite arm spacing and the effective grain size deff. The highest HTS was seen in the Al–0·5%Si alloy with the largest ΔT′, the highest β, the largest dendrite arm spacing, the lowest amount of eutectic phase feu, and, hence, the longest local freezing time tf. Correlation was observed between HTS and the theoretical hot tear criteria Clyne–Davies CSC, Feurer and Campbell CSCb indices, with Campbell's criterion showing a very strong correlation as HTSc≈0·03CSCb + 6 with R2=0·93, where HTSc is HTS of rod C. [ABSTRACT FROM AUTHOR]

The metallurgy of cast aluminum alloys has always been considered to be different from that of wrought alloys. Metallurgists have been taught that pores are intrinsic in cast aluminum alloys and that mechanical properties in cast aluminum alloys are controlled by dendrite arm spacing, the presence of Fe-bearing particles, and the size of Si particles in Al–Si alloys, which fracture and debond during deformation, leading to premature failure. Whether these effects are intrinsic or extrinsic, i.e., mere correlations due to the structural quality of castings, is discussed in detail. Ideal properties are discussed, based on findings presented mostly in physics literature. Pores and hot tears in aluminum castings are extrinsic. Moreover, the effect of dendrite arm spacing on elongation, precipitation, and subsequent fracture of β–Al5FeSi platelets, and finally Si particle fracture and debonding are all extrinsic. A fundamental change in how we approach the metallurgy of cast aluminum alloys is necessary. [ABSTRACT FROM AUTHOR]

The effects of Ni on the phases, microstructural evolution, hot tear susceptibility and mechanical properties of Al–6Zn-1.4Mg-1.2Cu alloys fabricated by gravity casting were investigated using X-ray diffraction (XRD), scanning electronic microscopy (SEM), optical microscopy, the N-Tec Hot tear and tensile test. The calculation of phase diagrams (CALPHAD) modelling was also conducted to understand the phase formation of the experimental alloys. The results showed that the secondary phases (including η-MgZn 2 , S–Al 2 CuMg and Al 3 Ni) are mainly distributed at grain boundary, while round S–Al 2 CuMg particles are also found in α-Al matrix. Ni only exists as Al 3 Ni with different morphologies and its number density ascends with the increase of Ni content in alloys, which also significantly improves the hot tear resistance. As a result, the increased Ni in the alloys remarkably improves the ultimate tensile and yield strength, while the elongation increases when Ni < 0.6 wt% followed by decreases when Ni > 0.6 wt%. • Ni in Al-Zn-Mg-Cu alloys exists as Al 3 Ni only and no effect on the phase formation of η-MgZn 2 and S–Al 2 CuMg. • Ni in Al-Zn-Mg-Cu alloys decreases T V and CSC*, thus improving the hot tear resistance. • When Ni < 0.6 wt.% , Ni improves the strength and elongation of Al–Zn–Mg–Cu–Ni alloys. • When Ni > 0.6 wt.%, the strength is increased but the elongation is decreased in Al–Zn–Mg–Cu–Ni alloys. [ABSTRACT FROM AUTHOR]