Effect of pouring and cooling temperatures on microstructures and mechanical properties of as-cast and T6 treated A356 alloy

The A356 castings were fabricated using a well-developed temperature controlled permanent mold.To improve the strength and hardness of cast A356,the microstructures and mechanical properties of as-cast and T6 heat treated A356 alloy with various mold and pouring temperatures were studied.The results reveal that the undercooling is closely related to the mold and pouring temperatures.As the mold/pouring temperature changed from 258°C/680°C and 270°C/680°C to 288°C/650°C,the in-situ undercooling is 12°C,17°C and 11°C,respectively.It is observed that the Si phase changes from long continuous flake to discontinuous globular-fibrous morphology after T6 heat treatment as the mold and pouring temperature is 270°C/680°C,and the T6 heat treated specimens exhibit better mechanical properties in comparison to those as-cast ones with an increase of 162%and 102%in yield strength and elongation,which are 34.6%and 190%higher than the ASTM B108-03 a standard,respectively.As a result,the tensile fracture morphology of the as-cast A356 alloy shows quasi-cleavage fracture and the T6 heat treated A356 alloy shows ductile fracture.

Characterization and kinetic modeling of secondary phases in squeeze cast Al alloy A380 by DSC thermal analysis

Thermal analyses on squeeze cast aluminum alloy A380(SC A380) solidified under 90MPa were carried out to study the microstructure development of the alloy, in which a differential scanning calorimeter(DSC) was employed. During the DSC runs, heating and cooling rates of 1, 3, 10, and 20 °C·min^(-1) were applied to investigate the heating and cooling effects on dissolution of secondary eutectic phases and microstructure evolution. Various reactions corresponding to troughs and peaks of the DSC curves were identified as corresponding to phase transformations taking place during dissolution or precipitation suggested by the principles of thermodynamics and kinetics. The comparison of the identified characteristic temperatures in the measured heating and cooling curves are generally in good agreement with the computed equilibrium temperatures. The microstructure analyses by scanning electron microscopy(SEM) with energy dispersive X-ray spectroscopy(EDS) indicate that the distribution and morphology of secondary phases present in the microstructure of the annealed sample are similar to the as-cast A380, i.e., strip β(Si), buck bone like or dot distributed θ(Al_2Cu), β(Al_5Fe Si) and Al_(15)(FeMn)_3Si_2. Two kinetic methods are employed to calculate the activation energies of the three common troughs and three common peaks in DSC curves of SC A380. The activation energies of the identified reaction θ_(CuAl_2) = α(Al)+β(Si) is 188.7 and 187.1 k J?mol^(-1) when the activation energies of reaction α(Al)+β(Si)→θCu Al_2 is^(-1)22.7 and^(-1)21.8 k J?mol^(-1), by the Kissinger and Starink methods, respectively.

Tensile ductility improvement of AlSi9Cu1 alloy by chemical composition optimization

Diesel engines, characterized by higher breakout pressure and compression ratio in comparison with gasoline engines, require particularly elevated tensile properties for their engine parts. In order to maintain both high strength and high ductility in the cylinder head, i.e., to obtain higher percent elongation without further reducing the tensile strength, Al Si9Cu1 alloy was used to prepare the cylinder head in an aluminum diesel engine. At the same time, the effect of different modification elements, Na or Sr, and Fe content on the reduction of secondary dendrite arm spacing(SDAS) was discussed, and the design of T7 heat treatment parameters were analyzed in order to improve the tensile ductility. The result shows:(1) The SDAS is as small as 18±3 μm for the Sr modified alloy.(2) The percent elongation of the alloy with Sr modification increases by 66.7% and 42.9%, respectively, compared with the unmodified alloy and the alloy with Na modification.(3) Lower Fe content alloy(0.10%) gives good results in percent elongation compared to the alloy with higher Fe content(0.27%); in particular, after Sr modification and T7 heat treatment, the elongation of over 5% is obtained.

Nano-yttrium-containing precipitates of T6 heat-treated A356.2 alloy when trace yttrium(Y less than 0.100 wt%) added

To investigate the effect of yttrium(Y) on microstructure refinement and mechanical properties of aluminum alloy A356.2,the different trace contents of Y(0 wt%,0.025 wt%,0.050 wt%,0.075 wt%,or 0.100 wt%)were introduced into the liquid alloy.The alloys were fabricated in a preheated permanent mold,and subsequently treated by a T6 heat treatment.The results of tensile testing indicate that the yield strength(YS),the ultimate tensile strength(UTS) and the elongation(El) of the A356.2 alloy are improved by the Y additions.The YS dependence on grain size for the test alloys follows the Hall-Petch equation,which gives YS=-354.1+2875.2 d^(-1/2) with a correlation of R^(2)=0.83.As 0.050 wt%Y is added,the optimum values of the YS,UTS and El are achieved after T6 heat treatment.The secondary phases were identified by X-ray diffraction(XRD) which mainly consisted as Si,Mg2 Si and Al3 Y.The scanning electron microscope(SEM) and energy-dispersive spectrometer(EDS) analyses reveal the presence of the nano-sized Al3 Y particles on the surface of the Si phase.The A356.2 alloy with the Y addition is strengthened by the dendritic refinement,and the presence of the micron-and nano-sized Al3 Y precipitates.