Joining of Ni3Al Microcrystalline Foils by SHS Reaction

An attempt of obtaining Ni3Al/FeAl/Ni3Al joints via sintering of Ni3Al foil with a reactive spacer is described. The method is based on sintering technically pure iron and aluminium powders or Fe and Al P/M (Powder Metallurgy) compacts. For sintering the Ni3Al foils in two material conditions were selected – immediately after rolling and after soaking at the temperature of 1000 C for up to two hours. A presintering (stage one) was carried out at the temperature of 620◦C under cyclic variable load which initiated volumetric reaction SHS (Self-Propagating High Temperature Synthesis). The second stage (basic sintering) relied on free soaking of obtained joints at the temperature of 1200◦C in a protective argon atmosphere. This stage was carried out in two variants: the first variant – with fast heating of the Ni3Al/FeAl/Ni3Al system and maintaining in the temperature for 15 minutes and next slow cooling in air, and the second one heating and cooling of the Ni3Al/FeAl/Ni3Al system in a furnace (total time – 1hour). The following parameters of obtained samples were analysed: metallographic analysis using scanning microscope, grain size, chemical composition (point and linear analysis), and microhardness. In the first stage (presintering) an occurence of the SHS reaction was confirmed. Metallography studies revealed a zonal structure of the samples. Depending on applied variants of heat treatment (in the basic sintering), from two up to eight transient zones with different chemical composition appear in the joint structure. The first variant including rapid heating and cooling during basic sintering causes increase in hardness in transient zones (up to the level of 360 HV), occurrence of a hard (473±56 HV), non-visible in microscopic observation FeAl zone, and an appearance of local cracks on boundaries between the Ni3Al phase and a zone directly adjacent to it. For the second variant of the basic sintering microhardness in Ni3Al and transient zones is comparable while for FeAl grains amounts to 320±8 HV. First recognizing tests on disruption of obtained joints were done. They confirmed high quality of joints produced in the variant II. Additionally, a successful attempt to bond Ni3Al strips with a Fe-Al reactive spacers using high-current pulses was performed.