Effects of lanthanide microalloying additions on the microstructure and mechanical properties of Al-Si-Cu-Mg cast alloys

Al-Si-Cu-Mg cast alloys are widely used in the automotive and aviation industry, especially in powertrain production due to their low density, good mechanical performance, machinability, and excellent castability. The powertrain components, which generally run at around ~200 ℃, need to show thermodynamically stable microstructure and durability. Advanced mechanical properties in Al alloys are related to the microstructure. To achieve improved mechanical properties, preliminary molten metal treatments, which are grain refinement and eutectic modification by additive elements, are applied for the Al-Si-based cast alloys. Although it is known that the addition of lanthanide elements refines the primary Al grains and modifies the eutectic structure, the contribution of lanthanides to the formation of thermally stable microstructure in Al-Si-based alloys is needed to be investigated. The main objective of this research is therefore to investigate the influence of lanthanide gadolinium (Gd) on the evolution of the microstructure of the primary Al-7Si-3Cu-0.3Mg alloy before and after T6 heat treatment and tensile properties of T6 heat treated primary Al-7Si-3Cu-0.3Mg alloy at room and high temperatures. A certain holding time is applied for the dissolution after the addition of alloying elements into the Al alloy bath. While the ideal holding time for the traditional additive elements, such as Cu, Ti, Sr, Na, etc., is well known, there is a lack of information on the addition of Gd in the casting practice. For the first step, an investigation on the ideal holding time of Gd addition into the Al-Si molten bath was necessary. The results showed that 45 minutes of holding time after the addition of Gd into the Al-7Si-0.3Mg alloy bath offers the ideal contact time, especially when the additional level is higher. The Gd level in the molten baths and microstructural properties remained constant with further holding times, which indicates no fading effect. The obtained results about the holding time can refresh the lack of knowledge and the fading effect of Gd in the Al-7Si-0.3Mg alloys. This also led to the casting practice of further investigations in this research. It was observed that Gd has no effect on grain refinement on the alloys solidified at low or high cooling rates. The eutectic structure remained constant after the addition of 0.1 wt. % Gd. It was observed that 0.3 wt. % Gd and further additions suppressed the eutectic plateau and Gd preferentially reacted with P forming GdP instead of AlP. The eutectic Si crystals that solidified at a low cooling rate were refined after the addition of 0.3 and 0.5 wt. % Gd while the morphology of eutectic Si was partially modified at a higher cooling rate. EBSD orientation maps exhibited how the crystallographic misorientation between primary and eutectic Al increases with increasing Gd content. Moreover, GdP was the nucleant for the GdAl2Si2 (only in Al-7Si-0.3Mg alloy) and GdAlSiCu intermetallics. Heat treatment in Al-7Si-3Cu-0.3Mg alloy caused the transformation from GdAl2Si2 to GdAlSiCu intermetallic, which has a plate-like brittle structure. This transformation consumed a certain amount of Cu that was supposed to be dissolved in Al matrix during the heat treatment. Moreover, it is observed that with the increasing Gd content, tensile properties gradually decreased in T6 heat-treated Al-7Si-3Cu-0.3Mg alloy at room and high temperatures. The fracture surface analysis showed that the brittle GdAlSiCu phase dominates the fracture mode at both testing temperatures.