Hardness distributions are measured by a Vickers micro-hardness tester along the cross-sections of the welded specimen at the upper, center and lower parts across a welding surface. Fig. 6 shows the hardness distribution along a weldment section across a welded surface of 6-mm-thick aluminum alloy and stainless steel specimens. A decrease in hardness of the aluminum specimen adjacent to the welded surface is noted, while that on the stainless steel side is observed in the area 2 mm away from the welded surface and an increase in hardness is noted adjacent to the welded surface. The increment is largest along the central part of the section as compared with the upper and lower parts, which are close to a clamping vice, due to heat conduction.