<p>  The structure and hardness of aluminum alloy (JIS : AC4C) obtained by solidification after electric current melting by use of a spot welding machine were investigated focusing on structure formation through the loss of homogeneity and its properties. The molten area of the sample increased with increasing heat input and the columnar sample melted with further heat input. The center and outside areas of the samples exhibited a granular α-Al structure and fine eutectic structure, respectively. The fine structure with α-Al and eutectic phases of α-Al and Si formed in the whole area of the sample under a maximum heat input of 400J. The hardness of the outside area was generally higher than that of the center area, except when the heat input was maximum. It is found that the change in hardness with heat input was caused by the change in the area of the eutectic phase of α-Al and Si. The hardness of the outside area of the sample solidified under an appropriate heat input, e. g., 270J was twice as high as that of the as-cast ingot. Furthermore, the outside area of the appropriately solidified sample further hardened with the T5-heat treatment and the hardness was 50% higher compared to the center area. Thus the outside of the appropriately solidified sample, achieved by suitable heat input, markedly enhanced its hardness, bringing about the potential for wear resistant improvement. </p>