Semisolid joining of aluminum A356 alloy by partial remelting and mechanical stirring

A method to reach the globular weld structure of A356 aluminum alloy using stirring the localized semisolid zone during butt-joining is developed. Since the heat conductivity of this alloy is very high, the accurate controlling of temperature during joining must be considered. A gas heating system was used to heat up the nitrogen gas up to the required temperature. A dried and free oxygen gas was prepared when a stream of nitrogen gas could pass closely around a hot element. Hot and pure nitrogen gas flow through a precise ceramic nozzle was used to create a localized semisolid pool. At this stage a fine stirrer was introduced into the weld seam in order to mix the two sides into a single uniform joint. Substrates were moved in direction of joint line by a small trolley to avoid the deviation of nozzle from the joint line and its distance and angle from the substrate. A fixture system was used to hold two substrates together on the trolley. A narrow hot plate was located on the trolley to heat up the joint line due to high heat conductivity of aluminum. Effect of gas temperature was investigated on the microstructure and mechanical properties of weld seam. Results showed that increase in temperature promoted the final welding properties, and also at liquid fractions less than 50% joining was not fully practical. The best mechanical properties were achieved with liquid fraction of about 70%.