Effects Of Hydrogen And Inclusions On The Microstructures And Mechanical Properties Of Aluminum Alloys

In present work, the numerical simulation, the tensile testing, thermal compression testing, fatigue testing, hydrogen measurement, optical microscopy, scanning electron microscopy and energy dispersive spectrum analysis were used to investigate the effects of casting parameters and alloying elements on the content of hydrogen and inclusions in two kind of aluminum alloys, the relationship between hydrogen content and inclusions content, and effects of microporosity size and inclusion size on mechanical properties, thermal flow stress behavior and fatigue properties.The results of numerical simulation show that the initial hydrogen content that is lower than0.1ml/100g has influence on microporosity affected zone, the deviation of areal fraction of microporosity between the first and after solidified regions increases when the cooling speed is slower. The lower initial hydrogen content, the faster cooling speed, and the slower cooling speed at the upper surface are beneficial to reduce the hydrogen content in ingot. The movement of inclusions during casting process is the coeffect of the adhesion force by melt, floating force or gravity, when the inclusion density is greater than the melt density, the inclusions will sink, otherwise they will float, and they will distribute randomly when the densities between inclusion and melt are close. The floating or sinking extent of inclusions increases with increasing the equivalent diameter of inclusions. When the equivalent diameter or the shape factor of inclusions are smaller than5μm or0.1, respectively, the inclusions will be mainly affected by the melt adhension force and distribute randomly in ingot after solidification.The hydrogen content in Al melt increases and the density of RPT sample decreases with increasing the environmental humidity and the melt temperature. With the increase in melt weight, the density of RPT sample increases, and while hydrogen content in Al melt decreases. With the increase in refining time, the hydrogen content in Al melt decreases gradually, the hydrogen content decreases. At the same melting conditions, the hydrogen content decreases with increasing cooling speed.The areal fraction of microposity in1050alloy ingot decreases gradually from the top (0.116%) to3/4height (0.032%), while increases at the bottom (0.086%). At the same cross-section, the areal fraction of microporosity at the edge is obviously less than that at the center. The large pores have irregular shape and and the average size between50μm and100μm, and mainly distribute at interdendritic boundaries and triangle grain boundaries in6063alloy ingot. The small pores have round shape and the average size between10μm and20μm, and distribute inside grains and grain boundaries.The ultimate tensile strength, yield strength and elongation decrease with increasing the area fraction of microporosity in6063alloy. According to the fracture surface observations, the pores with size larger than200μm are the main factor leading to the fracture, while the pores with size less than100μm has a little effect on the fracture.When the alloy were thermal compression tested at25℃-500℃and at strain rates of O.Ols-1～10s-1, the flow stress decreases with increasing temperature, and increases with increasing the strain rate. The flow stress in sample having small pore is higher than that in sample having large pore when deforming at same conditions.The constitutive equation in sample having small pore is: ε=1.09677×1010[sinh(0.0091σ)]6.52738exp[-111.9083×103/(RT)] The constitutive equation in sample having large pore is: ε=5.38×105[sinh(0.029038σ)]3.44613exp[-96.1032×103/(RT)]The results of fatigue test and fracture surface observations show that the crack mainly origins at the pore near the surface. The large pores with size of50μm-200μm in sample of water-cooled copper mold cast ingot seriously affect the fatigue properties, while the small pores with size smaller than30μm have minor effect on fatigue properties.The main inclusions are Al2O3and MgO in1050alloy and6063alloy, and usually distribute along with pores. The increase in inclusion content will cause a corresponding increase in hydrogen content. When the inclusion content in Al melt increases, the inclusions tend to accumulate and form a porous structure.Both traditional quantitative metallographic method and hot alkaline deep etching method can evaluate the areal fraction of the oxide inclusions in aluminum alloy. But the area fraction obtained by quantitative metallographic method includes oxides and AlFeSi phase, which requires a high quality of sample surface. The areal fraction obtained by the hot alkaline deep etching method is oxide inclusions, and the sample preparation is simple. The hot alkaline deep etching method can provide a convenient and effective method for industry to inclusion detection.The inclusions in6063alloy mainly distribute at the top and bottom of the ingot. The results of tensile tests and fracture surface observations show that the small size inclusion has minor effects on strain hardening exponent and the tensile properties of the alloy. The inclusions located at the surface of tensile sample and its size larger than20μm are easy to become a fracture source, and while the inclusions with size less than10μm have little effect on tensile fracture behavior.