The effect of different heat treatment approaches on the mechanical properties of a low-density Thermosilit ZA27 syntactic foam

• Thermosilit particles , known for their high melting points, are emerging as a new lightweight filler crucial for producing light metal syntactic foams . • Several porous Thermosilit-ZA27 syntactic foam samples (MSF) samples were successfully fabricated using the counter-gravity infiltration casting method achieving a density of 2 g/cm 3 . • Complete heat-treated (CHT) significantly improved the plastic mechanical properties and promoted more uniform deformation due to increased ductility. In contrast, partial heat treatment (PHT) led to a sharp increase in elastic properties but resulted in brittleness deformation . Metal matrix syntactic foam (MSF) has garnered significant attention due to its high strength-to-density ratio. While hollow particles are well-studied in this field, porous fillers remain underexplored. This study introduces Thermosilit, an innovative, low-cost porous filler. Thermosilit is ultralight, with bulk and particle densities of 0.101 g/cm³ and 0.195 g/cm³, respectively, and was compared to expanded Perlite. Results show that Thermosilit is similar to Perlite in density and strength, with minor differences. This research used 1-1.2 mm Thermosilit particles and ZA27 alloy to create a novel foam through counter-gravity infiltration. Twelve MSF samples were produced: three as-cast (AC), four with complete heat treatment (CHT), and four with partial heat treatment (PHT). The average foam density was 2.00 g/cm³, a 57% reduction compared to solid ZA27. Compression tests were performed to assess mechanical properties and the impact of different heat treatments. All samples exhibited typical stress-strain behavior, including elastic, plateau, and densification phases. AC and PHT samples showed a sharp stress drop after peak stress, with PHT samples displaying greater fluctuation. In contrast, CHT samples demonstrated steadily increasing stress due to aging, significantly enhancing energy absorption, plateau stress, and energy efficiency. Furthermore, scanning electron microscopy (SEM) was employed to examine the microstructure of the syntactic foams in all groups. The analysis showed that heat-treated and aged foam |(CHT) modifies the ZA27 alloy matrix from a multiphase dendritic structure to a more spheroidized morphology, leading to improved ductility. PHT samples showed increased 1% offset yield strength but reduced plasticity, while CHT improved ductility by altering the microstructure into a multiphase system. Optical analysis revealed brittle deformation in AC and PHT samples, while CHT samples exhibited a more ductile, layer-by-layer deformation mechanism.