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Studies were made on the effects of nitrogen, oxygen and hydrogen gases added to argon gas on the weldability and properties of MIG-welds in commercially pure aluminum and its alloys. It was concluded that the addition of nitrogen up to 10% was effective in practical applications for increasing the depth of penetration and tensile strength respectively, and refining the structure in welds. In commercially pure aluminum, it decreased the porosities of welds with little formation of scale on bead surface; while, in Al-Mg alloys, it formed brown scale on bead surface and the porosities were not always decreased. The addition of oxygen up to 5% was considered to be advantageous in practical applications, because it increased the depth of penetration and decreased the porosities of welds. However, its addition over 5% was harmful because of the occurrence of puckering and the formation of excessive oxide film on bead surface. The addition of hydrogen was harmful, for it remarkably increased the porosities, resulting in the loss of the mechanical properties of welds.