Laser Welding of Rheocast Semi-Solid Metal (SSM) Processed Wrought Aluminium Alloy 7017

Near-net shape casting of wrought aluminium alloys has proven to be difficult due to a tendency towards hot tearing during cooling.  Actual shape rheocasting, or semi-solid metal (SSM) processing followed by high pressure die casting (HPDC), has recently been shown to be an effective alternative to conventional die casting, yielding near-net shape wrought aluminium alloy castings without the risk of hot tearing.  This casting process involves pouring the liquid metal into a processing cup, which is then transferred into a coil for induction stirring and simultaneous forced air cooling.   When the metal reaches the semi-solid casting temperature (in the dual-phase liquid + solid state), the metal is transferred to a high pressure die casting machine and cast to near-net shape.  This modifies the as-cast microstructure, yielding a more globular proeutectic phase, and results in mechanical properties in the T6 condition closely approaching those of wrought material in the same condition.  Very little information is currently available on the response of SSM-HPDC material to welding.  This project therefore investigated the influence of autogenous laser welding on the microstructure and mechanical properties of aluminium alloy 7017 after rheocasting.  Initial results indicate that it is possible to successfully laser weld this material without hot cracking.  The effect of laser welding on the rheocast microstructure in the heat-affected zone and weld metal will be shown, and the hardness and tensile properties of the resulting joints in the as-welded and heat treated (T6) condition will be tested and related to the microstructures achieved.