Volume 61, Issue 1

Large gas engines play a key role in energy supply and transport. Due to emission regulations and high fuel prices, engines achieving both high efficiency and low emissions are required. Development efforts to that effect have resulted in combustion methods relying on lean air-fuel mixtures as well as high ignition pressures. The conditions in the combustion chamber cause high wear on the spark plug electrodes. Conventional electrodes have a limited lifetime and they are expensive due to the use of precious metal alloys. Compared with conventional materials, alternative ceramic electrode materials offer advantages in terms of high thermal stability, high oxidation and corrosion resistance, as well as lower material prices. Disadvantages include insufficient electrical conductivity, brittleness and the difficulty of joining them with metallic materials. In this paper wear and oxidation behavior of selected materials are evaluated, using ceramographic methods. Microsections of the joining zone are used to analyse the change in the microstructure resulting from the bonding process. The results show the strengths, weaknesses and limitations of the selected ceramics as electrode materials, based on ceramographic methods, which helped essentially to understand the oxidation behavior, the bonding process and wear.

A clear characterization of layers of scale on hot-rolled steels does not only require a careful preparation of the samples but also a delineation of the individual scale phases. In this work, it could be shown that etching using dilute hydrochloric acid improved the distinction of the individual oxide phases during the light microscope examination of layers of scale. Furthermore, metallurgical analyses provide valuable information on the quantitative composition as well as on the qualitative element distribution in the layer of scale thus also facilitating the distinction of the scale phases.

Transformations in microstructural characteristics and mechanical attributes of friction stir welded 3 mm thick Ti–6Al–4V alloy plates was investigated by employing distinctive tool rotational and traverse speeds. Impact of these parameters on microstructural transitions, generation of flaws, hardness, and tensile properties of the joints were analyzed. Increase in rotational speed from 1200 rpm to 1600 rpm have contributed for escalation in temperature, even above the β transus temperature. Large sized lamellar alpha grains was found to be transformed into finely refined lamellar alpha + altered beta grains in uppermost portion of nugget zone of joints fabricated at 1600 rpm and 50 mm/min combinations. This transformation have occurred due to the impact of the thermal cycles and stirring mechanism. These joints were found to be free from flaws including volumetric related defects, kissing bond, tunnel flaws. Majority of the fabricated joints possessed lowest value of mechanical properties in their heat affected zone and exhibited fracture in this zone. Properties of Ti–6Al–4V alloy joints were evaluated with respect to pseudo index of heat and it was observed that rotational speed of the tool is a dominant parameter in impacting both the mechanical attributes and microstructural transformations of the joints.