Capillary shaping is one of the most attractive options for the fabrication of aluminum frame components of a light weight car body structure. In this study, the maximum pulling rate of a pure ...aluminum fabricated by this technique was investigated using heat transfer analysis. Numerical and theoretical methods were applied to consider various cooling conditions. The maximum pulling rate depends on h / b, where b is the thickness of a product and h is the heat transfer coefficient between the coolant air and product. The maximum pulling rate increases with increasing h / b and decreasing melt temperature. Critical cooling length, which contributes to increased pulling rate, is smaller than 100mm and decreases with increasing h / b.
Capillary shaping is an upward pulling solidification technique for obtaining aluminum alloy hollow products with high structural stiffness and high mechanical properties. Recently, hollow frames ...with inner ribs and bent geometry are increasingly desired for optimizing car body stiffness and design of lightweight car body structures. Capillary shaping is an attractive process for manufacturing these components. However, it is necessary to control thermal conditions during the pulling process for fabricating bent products with high thickness accuracy, since thermal conditions influence the thickness of products and vary at the bent section due to differences in the pulling rates at inner and outer positions. In this study, the thermally stable conditions during the capillary shaping of aluminum alloy bent tubes were investigated. It is found that bent tubes with high thickness accuracy can be fabricated without any automatic controls of cooling conditions when thermally stable conditions are maintained.
To ensure dimensional precision in various service environments, this study investigated the strain variation of JIS ADC12 aluminum alloy die castings by heat treatment and its relation with the ...strain (hereinafter, growth) arising from the precipitation of silicon, copper, and magnesium. Expansion strain exceeding 0.1% was produced in a JIS ADC12 aluminum alloy die casting by heat treatment. Simultaneously, the half-width angles of the X-ray diffraction (XRD) peaks of the aluminum phase in the die casting decreased. Silicon, copper, and magnesium concentrated phases appeared in the aluminum phase after heat treatment. Consequently, the decrease in the half-width angles can be regarded as a result of the improved crystallinity of the aluminum phase because of the relaxation of the lattice strain by the precipitation of silicon, copper, and magnesium. That is to say, the strain variation of the ADC12 alloy die casting by the heat treatment is attributable to the precipitation of silicon, copper, and magnesium from the supersaturated aluminum phase. To verify the above relation quantitatively, growth attributable to the precipitation of silicon, copper, and magnesium from the aluminum phase and the transformation of the precipitated metastable Cu-Al compounds was estimated theoretically and compared with the measured strain variation yielded by heat treatment of the ADC12 alloy die casting. Results confirmed that the strain variation of the ADC12 alloy die casting by heat treatment corresponded fairly well to the growth arising from the precipitation of silicon, copper, and magnesium out of the aluminum phase and the transformation of the precipitated metastable Cu-Al compounds. Results also show that silicon, copper and magnesium precipitated at an early stage of heat treatment and that the silicon precipitation contributed mostly to the growth. The precipitated metastable Cu-Al compound, θ", was found transformed to another metastable compound, θ', and stable compound, θ, sequentially, thereby giving some expansion and contraction growth.
The growth of aluminum alloy die castings can be eliminated with a suitable heat treatment. To elucidate the parameters affecting heat treatment conditions, we investigated the effects of alloying ...elements on the precipitation behavior of supersaturated silicon in die castings during heat treatment using JIS ADC12 alloy (hereafter referred to as ADC12) and Al-11%Si alloy, considering that the former contains several alloying elements while the later no other elements except for silicon. Most of the supersaturated silicon in ADC12 alloy die castings precipitated with a short time of heat treatment, resulting in a large number of fine silicon precipitates dispersed in the primary aluminum phase. However, the supersaturated silicon in Al-11%Si alloy die castings needed a long time of heat treatment to precipitate and resulted in fewer and larger silicon precipitates in the primary aluminum phase than that of ADC12 alloy die castings. The concentration of magnesium, copper and silicon etc. was analysed in the silicon particle near the interface with aluminum matrix in ADC12 alloy die castings using a three dimensional atom probe. These alloying elements of magnesium, copper, etc. are considered to have formed clusters suitable as the nucleuses of silicon precipitation during die casting or at the early stage of heat treatment thus promoting the precipitation of the supersaturated silicon during heat treatment. To confirm this assumption, we examined the growth behavior of the die castings of Al-11%Si alloy with an addition of magnesium. The growth of the die castings with magnesium addition were faster than that of Al-11%Si alloy die castings during heat treatment, and the promoting effect of magnesium was verified on the precipitation of the supersaturated silicon in ADC12 alloy die castings. Therefore, the presence and the content of alloying elements such as magnesium, copper, etc. should be considered when deciding the heat treatment conditions for removing the growth of the die castings of Al-Si system alloys.
The measurement of the residual stress in aluminum alloy die castings by x-ray method was investigated in order to apply this method for improving the analysis accuracy of the deformation of ...aluminum alloy die castings. The residual stress of one flat type experimental die castings and six kinds of practical die cast products were assessed by the x-ray method. As a result, the stress occurring in a deformed die castings was found to be measurable by the x-ray method. However, the residual stress in the as-cast die castings measured by the x-ray method showed a lower compressive value than that measured by the stress relaxation method in which the die castings was cut off and the residual stress was measured by strain gauges. The cause of the inconsistency between the x-ray method and stress relaxation method was estimated as the decrease in the lattice constants of aluminum measured by x-ray due to the solid solution of silicon and copper in aluminum. Therefore, the x-ray method can be used to measure the residual stress in die castings after being calibrated by the stress relaxation method. The residual stress on the surface of the die castings was found to become tensile stresses after heat treatment at 200℃ or 250℃.
