The necessity of preserving resources and to reduce environmental pollution makes light weight concepts highly interesting for the transportation market, with light weight being essential for newly developed electric and hybrid vehicles. However, some components cannot be replaced only by aluminium, but need to be combined with steel in order to achieve the desired mechanical characteristics. Therefore, there is great interest in developing processes to manufacture aluminium/steel hybrid structures that present a good bond.In the present work a range of processing conditions for improving the bond strength between S355J2H steel inserts and AlSi7Mg casting alloy were investigated. Before casting, different chemical, thermal and mechanical treatments were applied to the steel insert: As-received condition, preheating, shot blasting, pickling, hot dip aluminizing, hot dip galvanizing, zinc coating and nickel/copper plating. The steel/AlSi7Mg interfaces were characterized by optical microscopy (LOM), scanning electron microscopy (SEM) and x-ray diffraction (XRD). Special attention was paid to the presence of defects, formation of oxides and/or intermetallic phases in the reaction zone. The interface shear strength has been assessed by the push-out test, and the results have been correlated with microstructural observations at the interfaces. Combinations of different insert treatments were also investigated.
Sulfur, an element that belongs to group 16 (chalcogens) of the periodic table, is an excellent promoter of nucleation substrates for graphite in cast iron. In ductile iron, sulfur favors a higher nodule count, which inhibits the risk of carbides and of microporosity. It is reasonable to expect that other elements from group 16, such as selenium or tellurium, play similar roles in the nucleation of graphite. The objective of this paper was to investigate the effect of selenium on the process of graphite formation. Thermal analysis cups were poured to evaluate the nodule count and size distribution. Some of the cups were not inoculated, while others were inoculated with a Ce-bearing inoculant, or with the Ce inoculant and additions of Se. Cross-shaped castings were also poured to quantify the microporosity regions by tomography. It appears that selenium additions modify the number and size of graphite particles, as well as the volume of microshrinkage. Direct correlations between these three parameters were found. Advanced Extensive Field Emission Gun Scanning Electron Microscope (FEG-SEM) techniques were used to identify the nature of the main nucleation compounds. Selenides, combined with Mg and rare earths, were observed to serve as nuclei for graphite. Their presence was justified by thermodynamics calculations.
The tensile strength of near-eutectic grey iron can be increased from 230-300 to 300-345 MPa, without a significant increase in hardness, through 0.3-0.4%Ti addition to low sulphur (<0.01%S) iron. This is due to the combination of higher primary austenite/eutectic ratio and the precipitation of superfineinterdendritic-graphite (SIG), characterised by a fine (10-20 μm) and highly branched fibrous structure. To reveal the influence of the %Ti on graphite shape evolution during solidification and its relationship to the solid fraction, quenching experiments at successive solidification stages were carried out on hypoeutectic alloys with 0.18% and 0.32%Ti. The graphite shape factors were measured, and their evolution as a function of the titanium content and the solid fraction was analysed. SEM was used to evaluate the change in graphite shape during early solidification, as well as its nucleation and growth. The correlation between the oxygen in the melt and SIG formation was also explored. It was concluded that nucleation of graphite in SIG irons occurs on carbon rich regions at the austenite/liquid interface and, sometimes, on titanium carbides. Solidification velocity-undercooling curves were used to explain the transition from lamellar (type-A), to interdendritic (type-D), to SIG, and then to coral graphite.
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