HighlightsLow pressure infiltration is suitable to produce MMSFs with hollow iron spheres.The MMSFs showed plastic yielding and long, slowly ascending plateau region.The matrix and the heat treatment strongly influence the properties of the MMSFs.The full-scale FEM model gives excellent agreement compared to the measured values. Highlights (for review) *Manuscript Click here to view linked References AbstractAluminium alloy syntactic foams reinforced with iron hollow spheres were produced by low pressure, liquid phase inert gas infiltration technique. Four Al alloys (Al99.5, AlSi12, AlMgSi1 and AlCu5) and Globomet grade iron hollow spheres were used as matrix and reinforcing material, respectively. The produced composite blocks were characterised according to the ruling standard for compression of cellular materials in order to ensure full comparability. The compressive test results showed plastic yielding and a long, slowly ascending plateau region that ensures large energy absorption capability. The proper selection of the matrix material and the applied heat treatment allows for a wide range of tailoring of the mechanical properties. For design purposes, the full-scale finite element method (FEM) model of the investigated foams was created and tested on Al99.5 matrix foams. The FEM results showed very good agreement with the measured values (typically within 5% in the characteristic properties and within 10% for the whole compression curve).

Hybrid metal matrix syntactic foams (hybrid MMSFs) are particle reinforced composites in which the reinforcement is the combination of more than one grade of hollow spheres. The difference between the spheres can be in their chemical composition, dimension, physical properties etc. In this study AlSi12 matrix hybrid MMSFs with monomodal Globocer (Al2O3 and SiO2 based ceramic) and Globomet (pure Fe) reinforcements were produced by pressure infiltration. The investigation parameters were the ratio of the hollow sphere grades and the aspect ratio of the specimens. Microstructural investigations showed almost perfect infiltration and favourable interface layer, while quasi-static compression tests showed that the composition of the reinforcement and the aspect ratio of the specimens have determinative effect on the characteristic properties (compressive and flow strength, fracture strain, stiffness and absorbed energy). This nature of the MMSFs ensures the possibility to tailor their properties in order to optimise them for a given application.

Iron hollow sphere filled aluminum matrix syntactic foams (AMSFs) were produced by low pressure, inert gas assisted infiltration. The microstructure of the produced AMSFs was investigated by light and electron microscopy, extended by energy dispersive X-ray spectroscopy and electron back-scattered diffraction. The investigations revealed almost perfect infiltration and a slight gradient in the grain size of the matrix. A very thin interface layer that ensures good bonding between the hollow spheres and the matrix was also observed. Compression tests were performed on cylindrical specimens to explore the characteristic mechanical properties of the AMSFs. Compared to other (conventional) metallic foams, the investigated AMSFs proved to have outstanding mechanical properties (yield strength, plateau strength, etc.) and energy absorbing capability.

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In duplex stainless steels the ideally 1:1 ratio of austenite-to-ferrite phases ensures the outstanding mechanical and corrosion properties compared to other, conventional stainless steel grades. However, this phase balance can be easily shifted to a mostly austenitic or mostly ferritic microstructures, depending on the welding process and heat input. In order to determine the phase ratio, several methods are available to use, such as Feritscope measurements, ASTM E562 manual point count method (on metallographic images) or quantitative image analysis. From these methods, Feritscope measurements cannot be applied to determine phase quantification in the narrow heat affected zone of duplex stainless steel welds – because of the very limited heat input. The manual point count method is very dependent of the assessor and cannot be automated. In this paper a histogram-based image analyzing process was developed, using Beraha's etchant solution. The results were compared to Feritscope measurements and a very good correlation (R2 = 0.9995) was found. This method will give the ability to easily and automatically measure phase ratio in weld metal, heat affected zone or in subsurface regions of multi-pass welds.

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