Evolution of Primary and Eutectic Si Phase and Mechanical Properties of Al2O3/Al-20Si Composites under High Pressure

To further improve the mechanical properties of Al-Si alloys. The phase, microstructure and mechanical properties of Al2O3/Al-20Si composites under different pressures were studied. The results show that the phase of Al2O3/Al-20Si composites are composed of α-Al phase, β-Si phase and Al2O3. Under the condition of hot-pressing sintering (0.02 GPa), a large number of Si phases with irregular shape and sharp angle are distributed on the α-Al matrix. Under high pressure solidification, the growth of primary Si phase is inhibited and the eutectic Si is spheroidized obviously. The microhardness of Al2O3/Al-20Si composite increases from 102.5 HV0.05 at 0.02 GPa to 156.4 HV0.05 at 4 GPa, which increases by 52.6%. The compressive strength increased from 381.5 MPa at 0.02 GPa to 469.1 MPa at 4 GPa, increasing by 23%. With the increase of solidification pressure, the fracture mechanism changes from cleavage fracture to quasi cleavage fracture.

Dynamic Compression Induced Solidification

This study presents a method for the determination of the dynamic pressure-dependent solidification of polycarbonate (PC) during flow using high pressure capillary rheometer (HPC) measurements. In addition, the pressure-dependent solidification was determined by isothermal pressure-volume-temperature (pvT) measurements under static conditions without shear. Independent of the compression velocity, a linear increase of the solidification pressure with temperature could be determined. Furthermore, the results indicate that the relaxation time at a constant temperature and compression rate can increase to such an extent that the material can no longer follow within the time scale specified by the compression rate. Consequently, the flow through the capillary stops at a specific pressure, with higher compression rates resulting in lower solidification pressures. Consequently, in regard to HPC measurements, it could be shown that the evaluation of the pressure via a pressure hole can lead to measurement errors in the limit range. Since the filling process in injection molding usually takes place under such transient conditions, the results are likely to be relevant for modelling the flow processes of thin-walled and microstructures with high aspect ratios.

Effects of Solidification Pressure and Heat Treatment on the Microstructure and Micro-Hardness of AlSi9CuMg Alloy

The effect of high pressure (135 MPa) and the following heat treatment on the microstructure and micro-hardness of the squeezing cast AlSi9CuMg alloy is investigated, using optical microscopy (OM), Vickers tester, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results indicate that the application of high pressure can increase under-cooling and the cooling rate during solidification and cause the refinement of the microstructure. The enhanced melt flow resulting from high pressure can also break the dendrite to form the spherical and elliptical primary α (Al) grains during the early stage of solidification. The winter-sweet flower-shaped primary α (Al) phases can also be formed through plastic deformation caused by the flow of the partially solidified melt. The ageing treatment results showed that a maximum (peak) micro-hardness value was obtained for each of the three ageing temperatures at different ageing times, and the highest peak value was achieved at 175 °C for 480 min. The micro-hardness change of the sample under different ageing processes was attributed to the variation of type, density, and size of the precipitates.

Effect of the Pressure on Microstructure of A380 Aluminum Alloy Prepared by Squeeze Casting

Effects of the solidification pressure on microstructure and mechanical properties of A380 alloy have been experimentally investigated. The obtained results show that the microstructures of the A380 aluminum alloy is fined, the porosity is decreased and the mechanical properties is improved remarkably with the increase of the solidification pressure. When the squeeze pressure increases from 0 MPa to 75MPa, the size of dendrite arm space decreases from 914 μm to 313 μm, reduced by 66%; the eutectic tissue volume fraction increases from 22.5% to 41.36%; the porosity decreases from 4.91% to 1.23%; the secondary dendrite spacing decreases from 39 μm to 18 μm; the size of needle-like β-Al5FeSi phase decreases significantly, and a few Chinese script α-Fe phases was observed in the grain boundary.

Microstructure and Compression Deformation Behavior in the Quasicrystal-Reinforced Mg-6Zn-2Y Alloy Solidified under Super-High Pressure at Room-Temperature

The microstructures of the Mg-6Zn-2Y alloy solidified under high pressures were investigated using scanning electronic microscopy (SEM) and X-ray diffraction (XRD). The room-temperature compression behavior was analyzed through experiments, showing that the microstructures of the alloys are consisted of α-Mg and quasicrystal I-Mg3Zn6Y phases. With solidification pressure increasing, the microstructures were refined, and the morphologies of the inter-dendritic secondary phase were improved from continuous networks into long-island and granule. The compression strength, yielding strength and compressibility were increased significantly corresponding with solidification pressure, from 259.02 MPa, 230.39 MPa and 18.3% under ambient pressure to 361.43 MPa, 272.25 MPa and 33.1% under high pressure of 6 GPa. The cleavage planes are flat, and the cleavage steps are straight under ambient pressure. However, the cleavage planes are small and rough under 4-6 GPa; tearing dimples occur in the tearing area, indicating that the degree of cleavage fracture decreases under high pressure.