Papers by Keyword: Thermal Cracks

Abstract: It is well-known that a series of cracks sometimes gets initiated perpendicular to the cutting edges on the rake faces of brittle cutting tools made of materials such as cemented carbide, ceramics, and cermet under high-speed intermittent cutting. The tools used in intermittent cutting processes are exposed to elevated temperatures during cutting and then cool quickly during the noncutting time. Previous studies have suggested that such repeated thermal shocks generate thermal stress in the tool and that the thermal cracks are then propagated by thermal fatigue. Recently, high-speed machining techniques have attracted the attention of researchers. To apply new cutting tool materials to this machining process, it is important to evaluate their thermal shock fatigue resistances. During high-speed intermittent cutting, the frequency of thermal shocks becomes high and the action area of the thermal shocks is limited to the rake face of the tool. Therefore, conventional thermal shock resistance evaluation methods are unsuitable for this case. Consequently, the authors have developed a new experimental evaluation method using a CO₂ laser beam. In this study, we irradiated cemented carbide and TiN cermet cutting tools with the CO₂ pulse laser beam and gauged the effectiveness of the proposed thermal shock fatigue resistance evaluation method. The results show a correlation between the thermal shock due to the CO₂ pulse laser beam and those due to the intermittent cutting experiments.

Abstract: Aluminum-matrix-nanoparticle-composites were produced by ball milling of micro scale aluminum powder in air atmosphere with subsequent consolidation by hot extrusion and also additional hot swaging. They were investigated in this condition after step by step isochronal annealing with successive increasing annealing temperature and quenching into water to room temperature. The material was investigated by amplitude dependent damping, hardness and density measurements, all at room temperature. For all measured amplitude dependent internal friction (ADIF) curves the damping increases with increasing strain amplitude. After some annealing treatments a knee occurs in the medium strain amplitude region of these curves. Moreover between annealing temperatures from 360°C to 480°C the strain dependent damping becomes a maximum, i.e. a peak in the ADIF curves occurs. Other ADIF curves of quenched and fatigued material show characteristic peaks that can be attributed to individual single cracks. It is shown that all these effects are due to the formation, opening and compression of cracks present in the sample or created by thermally exerted stresses.

Abstract: The influence of thermal cycling between - 196 °C and 200 °C and equivalent heat treatment at 200 °C on the amplitude dependence of internal friction at room temperature has been studied in as cast Cu – Al - Mn shape memory alloys with different chemical compositions. Using X-ray diffraction one composition was found to be austenitic and two others martensitic with two martensite types (2H and 18R) at room temperature. All specimens were thermally cycled for 100 times. During one thermal cycle the specimen underwent altogether two phase transformations one in each direction. Thermal cycling causes microstructural changes in the specimens due to atomic reordering, thermal stresses, which are generated in the martensitic state due to the anisotropy of thermal expansion, or due to the nucleation and propagation of interphase cracks in parent phase. During repeated thermal cycling the transition peaks obtained in mechanical spectroscopy became narrower due to an enduring change of the microstructure and annealing effect at 200 °C. To compare between the effects of thermal cycling and heat treatment one martensitic specimen was annealed at 200 °C. For selected cycle numbers and heat treatment times the amplitude dependence of damping was measured at room temperature. The influence of thermal cycling of martensitic specimens on the damping level was found to be similar to the influence of heat treatment at 200 °C. It is most likely that the highest heat treatment temperature is more important for the amplitude dependence of damping than the temperature change during thermal cycling. Cracks due to thermal cycling were found in all cycled specimens. They have no significant effect on the amplitude dependence of damping of the martensitic samples, whereas some small influence could be observed in austenitic samples at room temperature.