The gas content of ordinary die casting is very high, and as a result, sufficient welding strength cannot be achieved due to blowhole (BH) formation when such casts are welded. Under this circumstance, it is necessary to find suitable welding conditions so that BH can be suppressed even when a casting contains a certain amount of gas. This paper discusses attempts to see if it is possible to suppress BH and thereby ensure the reliability of welding strength by controlling heat input. We have found out that when the gas content is below 3cm³/100gAl, base metal rapture occurs independent of the welding conditions. For gas contents in the range of 3 to 5cm³/100gAl and 5 to 7cm³/100gAl, the heat input should be less than 66J/mm² and 52J/mm², respectively. However, it is very difficult to achieve base metal rapture when the gas content in the casting exceeds 7cm³/100gAl. Moreover, in order to ensure the reliability of welding strength, the control value of the BH density has been found to be 16% when welded under controlled heat input.

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  Both ferritic ductile cast irons with the Mg/P compositional ratios of 0.6 and 2.2 were tensile tested at 673K. Specimens with the Mg/P ratio of 2.2 showed lower tensile elongation of 5% at 673K, which meant the embrittlement was caused by intergranular fracture. In contrast, impact tested specimens showed the lowest absorbed energy in the temperature range of 973 to 1073K, which corresponded to the embrittlement at 673K in tensile tests. It was found that temperatures showing embrittlement increased as the strain rate increased due to differences in experimental methods.

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  The characteristics of hydrogen embrittlement for two kinds of ADI in the absence of retained austenite (austempered ductile iron precipitated ferrite around graphite or α-ADI, and annealed at 673K for conventional austempered ductile iron or ADI-AN) were examined. Specimens were tested for embrittlement by dropping a water-based solution of 0.365%HCl at 0.013ml/s intervals using a cantilever type apparatus. After an extended period of time under sustained high bending stress, α-ADI and ADI-AN fractured with creep deflection. ADI having retained austenite was abruptly fractured without creep deflection in HCl solution. This suggests that the hydrogen embrittlement for ADI may be caused by the retained austenite in ADI.

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