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An investigation has been made of the diffusion welding of 2017 aluminum alloy which is very difficult to join by the conventional fusion-welding technique. In the present investigation, the diffusion welding has been carried out in the temperature range above the solidus line where solid and liquid phases coexist, though it is a kind of solid-state welding. The purpose for this is to aid the intimate contact and the disruption of the tenacious oxide film at the bond interface by the formation of the liquid phase. As a result, the joint strength increased largely with raising the welding temperature above the solidus line, and became much higher than that of the joint welded below the solidus line. The maximum tensile strength in the as-welded state, which was obtained at the welding temperature of 853K, was 270MPa. It Was observed with high-temperature-optical microscope that the liquid phase formed preferentially at the bond interface as well as at the grain boundary in the range between the solidus and liquidus lines. These results indicate that the liquid phase forming preferentially at the bond interface promoted effectively the bond process of diffusion welding. The maximum tensile strength was obtained when the volume fraction of the liquid phase was 2〜3%. However, when the volume fraction of the liquid phase exceeded 3%, the joint strength decreased remarkably with the increase in the volume fraction of the liquid phase. In the joint welded with the liquid phase more than 3%, many porosities were observed at grain boundaries, and the degree of welding deformation, which was estimated from the increase in the cross-sectional area at the bond interface, became much higher than that of the joint having the maximum strength. The formation of these porosities is considered to be responsible for the decrease in the joint strength. The formation mechanism of the porosity is explained as follows: In the range between the solidus and liquidus lines, the liquid phase formed preferentially at the grain boundary, and so the strength of the grain boundary decreased rapidly with the increase in the liquid phase. Therefore, when the fraction of the liquid phase exceeded a critical value (*3%), the welding deformation of base metal caused cracks at grain boundaries. The crack which was not filled with the liquid phase is considered to remain as a porosity at the grain boundary. The tensile strength of the joint welded at 853K was increased to 400MPa by a post weld heat treatment consisting of ageing for 10^3ks at room temperature subsequent to a water quenching from 773K.