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A B S T R A C T This study investigates the fracture behaviour of refill friction spot welding welds under shear tensile loading. Overlap joints of 6181-T4 aluminium were produced in 1.7-mm sheets by varying the rotational speed from 1900 to 2900 rpm and the welding time from 2 to 3.4 s while keeping the plunge depth constant at 1.75 mm. After shear tensile tests, the samples were analysed using optical microscopy and scanning electron microscopy. The strength of the weld and its ductile/brittle behaviour are associated with the nucleation, growth and propagation of two types of cracks: circumferential cracks and annular cracks. Welds produced with longer welding times (≥3 s) and slower rotational speeds (1900 rpm) had higher strengths, low scattering and high energy absorption prior to failure, while welds produced with short welding times (2 to 2.4 s) resulted in poor joints, especially when they also used high rotational speeds.

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In the present work the effects of different plasma nitriding processes on the high speed steel AISI M2 were investigated, comparing the wear performance and the microstructure of the steel. Specimens were nitrided at temperatures between 450 and 550°C for between 18 min and 2 h using a gas composition of 5 or 25%N2, balance H2. The wear test was carried out using an Amsler machine and a wheel-fiat type test. X-ray diffraction analyses, chemical composition depth profiles by glow discharge optical spectroscopy, microhardness tests, and optical and scanning electron microscopy of the layers were performed. The specimens nitrided at 550°C showed spalling in wear tests indicating a brittle surface, while the specimens nitrided for 1 h at 450 and 500°C (25%N2) demonstrated reduced friction coefficients and adhesive wear.

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