Friction stir spot welding between a 1?5 mm 6061 Al-T6 sheet and a 1?5 mm Cu sheet was conducted. The effects of the following parameters on the lap shear weld strength were investigated: tool pin length, shoulder plunge depth, weld time and tool rotation speed. With rotation speeds of 2000 rev min 21 and a 2?60 mm tool pin length, weld strengths just over 2000 N were produced. Welds made at 1000 rev min 21 with 1?83 or 2?60 mm tool pin lengths produced weak welds, with strengths below 900 N. Stronger welds had a Cu ring extruded upward from the lower Cu sheet into the upper Al sheet, which promoted bonding and interlocking between the sheets. The interface between this Cu ring and the surrounding aluminium was free of Al/Cu intermetallics. The weld strength appeared driven by the aluminium layer thickness between the tip of the Cu ring and the keyhole with the strength increasing by 2833 N mm 21 of aluminium layer thickness.

High alloyed Si ferritic ductile irons can offer potential benefits because they combine high strength, ductility at room temperature, and low oxidation rate at high temperature. However, there is one known drawback and that is these cast irons have limited performance during thermal cycling due to a significant drop of ductility at warm temperatures. This decrease in ductility has been linked to poisoning ferrite grain boundaries by Mg. Therefore, thermodynamic simulations were used to identify altering additions which were able to meditate this negative effect by forming intermetallic phases with Mg. To verify thermodynamic predictions, three alloys were cast including a base and two high Si ductile irons with additions of P and Sb. High-temperature performance of these alloys was experimentally verified including tensile properties at warm temperatures (350-550°C), oxidation in air at temperatures (700-800°C), and thermal cycling between 300 and 800°C. SEM and TEM analyses confirmed that the studied additions reacted with Mg forming different compounds which could prevent poisoning ferrite grain boundaries and improve high-temperature performance of high Si ductile iron.

Silicon and molybdenum (SiMo) ductile iron is commonly used for exhaust manifolds because these components experience thermal cycling in oxidizing environment, which requires resistance to fatigue during transient thermomechanical loads. Previous studies have demonstrated that alloying elements, such as Al, to SiMo ductile iron reduces the amount of surface degradation during static high-temperature exposure. However, deterioration of sphericity of the graphite nodules and a decrease in ductility could affect the tendency of cracking during thermal cycling. In this article, the effect of Al alloying on static and transient thermomechanical behavior of SiMo ductile iron was investigated to optimize the amount of Al alloying. A thermodynamic approach was used to confirm the effect of the Al alloying on the phase transformations in two SiMo cast irons, alloyed by 1.8% Al and 3% Al. These two alloys were cast in a laboratory along with the baseline SiMo ductile iron. Several experimental methods were used to evaluate the dimensional stability, physical properties, static oxidation, and failure resistance during constrained thermal cycling testing to compare their high-temperature capability. Experimental results verified that Al alloying increases the temperature range and decreases volume change during eutectoid transformation, which together with enhancement of oxidation protection improved the dimensional stability. Thermocycling tests showed that the number of cycles to failure depends on the amount of Al alloying and the applied high-temperature exposure during each cycle. SEM/EDX, high-resolution TEM and µCT analysis were used to verify the mechanism resulting from the Al alloying protection. It was shown that an optimal level of Al alloying for balancing oxidation and thermal cracking resistance depends on thermomechanical conditions of application.

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