Effects Of Microporosity And Inclusions On Mechanical Properties Of Aluminum Alloy

The effects of microporosity and inclusion size on mechanical properties of aluminum alloy were investigated by tensile and compression tests, hydrogen measurement, optical microscope and scanning electronic microscope (SEM). The distribution of microporosity and inclusion in alloy were determined. The relationship between the microporosity size and mechanical properties was obtained. The results show:Along the casting direction, the area fraction of microporosity decreases gradually from the shrink head to the position of3/4height, from0.116%to0.032%and while increases to0.0086%at the bottom of ingot. The porosity area fraction at the center of ingot is obviously greater than that at the edge of ingot in transverse section.The hydrogen content of ingot increases with increasing inflating hydrogen time and casting temperature, which lead to the decreasing of the mechanical properties. The hydrogen content increases from0.00202%to0.00498%with the increase of inflating hydrogen time from30s to20min, and the ulitimate tensile strength, yield strength and elongation decrease by20%,54%and41%respectively. With increasing of degassing time, the hydrogen content decreases, and while the mechanical properties increases.At different melting conditions, the changing rules of hydrogen content are similar obtained by solid and liquid hydrogen measurements. When the degassing time increases from lmin to20min, the solid hydrogen content decreases by66.4%, and while the liquid hydrogen only decreases by30%. So, the solid hydrogen content is not necessarily high when that is high in liquid state. The environment humidity and the dryness degree of the material and tools will affect the hydrogen content of ingot.The relatively large microporosity in6063alloy ingot has an irregular shape and mainly distributes at the interdendritic grain boundaries and triangle grain boundaries with the average size between50μm and100μm, the largest one is more than300μm. The small microporosity is round with the size between10μm and20μm.When the samples are tensile tested at100℃, the strain hardening exponent (n value) of large pore region at the top of the ingot(P1) is0.25, which is less than that of the small pore region at the bottom of the ingot(P4), which is0.37. When the samples are tensile tested at300℃, n1and n4are0.14and0.15, respetively. It is indicated that the size of pore has great influence on the n value when the alloy is tensile tested at a lower temperature. At300℃, the deviation between the ulitimate tensile strength of P1and P4is6MPa, however, the elongation differs by32%, which indicates that the pore size affects more strongly on the elongation than on the tensile strength. The fracture mode is greatly influenced by the microporosities with size larger than200μm, and while the microporosities with size less than100μm have little effect on the fracture mode. Thermal simulation compression test were taken to the alloy at different temperatures (25℃-500℃), strain rates (0.01s-1-10s-1) and different reductions (60%and80%). The flow stress decreases with rising of temperature, and while increases with increasing of strain rate. At the same temperature and strain rate, the flow stress of samples with small microporosities was larger than that of samples with large microporosities.The main inclusions having size of5μm-20μm in6063alloy are Al2O3, Mg2Si and AlFeSi, with different shapes of mesh, bulky flake and gathered blocks. The inclusions mainly distribute at the top (the area fraction is6.43%) and the bottom (area fraction is8.40%) of ingot, less in the center (the area fraction is3.42%) of ingot. From the results of tensile test, the inclusions have no obvious effects on the tensile properties. The observations of fractured surface show that the large inclusions (more than20μm) near sample surface easily become the source of crack, and while the small ones (less than lOμm) have almost no influence on the tensile fracture of the alloy.