Study On Microstructure And Mechanical Properties Of DC Cast Wrought Hypereutectic Al-Si Alloy With High Strength And Toughness

Hypereutectic Al-Si alloys have broad application prospects in automotive and aerospace industry due to their low thermal expansion coefficient, high wear resistance and thermal conductivity as well as good castability. Coarse Si particles and poor ductility are the main shortcoming for these alloys. In this paper, billet of direct chill (DC) cast phosphorous modified hypereutectic Al-Si alloy was produced, and the alloy was subsequently hot-deformed and heat treated. A new wrought hypereutectic Al-Si alloy exhibited good strength and toughness for room temperature and high temperature was developed, which could be used in the manufacture of high power engine. The main conclusions can be summarised as follows:A new Al-17.5Si-4.5Cu-1Zn-0.7Mg-0.5Ni hypereutectic Al-Si alloy containing trace element of Zr, V and Ti was designed. Extremely fine microstructure of billet of hypereutectic Al-Si alloy was produced by DC casting combined phosphorous modification. The as-cast microstructure of DC cast Al-17.5Si-4.5Cu-1Zn-0.7Mg-0.5Ni alloy consists of primary Si, α-Al dendrites, Al-Si eutectics, and intermetallic compound of Al2Cu, Al9(Fe,Cu)Ni and Al8FeMg3Si6. The primary Si particles were obviously refined by phosphorous modification. For example,the mean size of the primary Si particles at half radius and in central region of the billet of100mm in diameter was13μm and19μm, respectively. The refinement mechanisms of the primary Si particle are heterogeneous nucleation (AlP particle as substrate) and the relative high undercooling produced by DC casting.Based on the refinement of microstructure, we carried out hot deformation on the conventional cast hypereutectic Al-Si alloy. The hot compression deformation behavior of the DC cast Al-17.5Si-4.5Cu-1Zn-0.7Mg-0.5Ni alloy at temperature range of400℃～500℃and strain rate of0.001s-1～1s-1exhibits characteristic of steady-state rheological. The flow stress of the alloy decreases with the increase of deformation temperature, and increases with the increase of strain rate. In the range of experimental conditions investigated, the hot compression deformation of the alloy is a process of thermal activation, and the average thermal activation energy of the alloy is261.215kJ/mol. The flow stress of the alloy can be expressed as:After hot deformation, the a-Al dendrites in the as-cast state were replaced by equiaxed grain structure; the spheroidal Si particles and intermetallic compounds homogeneously distributed in the matrix. Dynamic recrystallization took place during hot deformation. The ultimate tensile strength of the deformed alloy T6-treated at ambient temperature was396MPa, and the elongation was1.28%; the ultimate tensile strength at300℃was110MPa, and the elongation was12.24%, which lay the foundation for the alloy to be used in the manufacture of high power engine. The fractography of the room temperature and high temperature tensile test specimens exhibit characteristic of cleavage fracture and ductile fracture. The fracture of the room temperature tensile test specimens originate from the crack of the primary Si particles, while the fracture of the high temperature tensile test specimens originate from the incohesion of the Si particles between the Al matrix and the crack of primary Si particles.During artificial aging, the precipitation sequences of the hot deformed alloy were0sequence and Q sequence, the precipitation process were:super saturated solid solutionâ†'GP zonesâ†'θ" metastable phaseâ†'θ’metastable phaseâ†'θ equilibrium phase; super saturated solid solutionâ†'GP zonesâ†'Q" metastable phaseâ†'’metastable phaseâ†'Q equilibrium phase. The θ’phases heterogeneous nucleated in the matrix, they were more likely to nucleate and growth at high energy site, such as dislocations and grain boundarys. While the Q"/Q’phases homogeneous nucleated in the matrix and uniformly distributed. The peak strengthening of the hot deformed alloy can be attributed to the dispersed distribution of the0" phase and Q" phase in the matrix. During artificial aging at150℃～180℃, the morphology and distribution variation of the Q" phase was neglectable, while that of the0sequence changed a lot. The overaging of the hot deformed alloy can be attributed to the transformation of the θ" phase to the0’phase as well as the coarsening of the0’phase.The strengthening and toughening mechanisms of the DC cast phosphorous modified Al-17.5Si-4.5Cu-1Zn-0.7Mg-0.5Ni deformed alloy can be attributed to structure refinement, hot deformation. Si particle spheroidal strengthening and toughening, and precipitation. surplus-phase strengthening.