Key Engineering Materials Vol. 830

Abstract: Compared with conventional metal materials, composite materials can sustain more loads under lower weight and lower space, which is more suitable to use in high strength environment for structure parts application in aerospace, military, transportation and leisure. The enormous aeronautical components and structures of composite materials require assembly and machining. Drilling is the most important making-hole process in the final assembly. However, conventional drill cannot avoid delamination in drilling composite materials effectively depending on tool geometry only. Delamination caused by drilling thrust has been showed as one of the most problematic defects after drilling composite laminates. Drilling of composite plates using a new step core-ball drill (NSCBD), which is a special drill to improve the chip flow and reduces the thrust force (delamination) at the exit of hole, is studied and compared to the traditional step core-ball drill (TSCBD). The thrust force of new step core-ball drill in drilling composite materials was investigated in this study. The theoretical analysis and experimental results show that the NSCBD can improve the thrust force (delamination) and efficiency than TSCBD in drilling composite materials.

Abstract: In this report we report a simplest way to synthesis inorganic Tungsten disulfide (WS₂) nanorods. In this research work we used Tungsten trioxide (WO₃) to produce tungsten disulfide with hydrogen gas and sulfur gas to synthesis WS₂ nanorods at ambient temperature. This synthesis was done by two steps. The first step is oxide reduction and the second step is Sulfuration. And we have analyzed the changes in the nanorod structure when the reaction time is increased and when the temperature is changed at constant gas flow. The synthesized nanorods are analyzed by SEM, EDS and XRD. We report that we have successfully synthesized WS₂ nanorods with the dimension of 100 to 300 nm in diameter and few micrometers in length. And we also report the changes in the structural morphology when the temperature was increased. When the temperature was increased to 1000oC the structure become very ranom.

Abstract: Investigation into behaviors of aluminum alloy to be metal formed at the room temperature is conducted in this study. An index is used to evaluate the sensitivity of temperature, that is, index of relative normalized temperature rise to steel called normalized temperature rise index per steel which helps researchers to obtain some insight on new materials based on experiences of steel forging. An investigation to an aluminum alloy shows that the index is quite high, implying that temperature effect as well as rate-dependence effect on the forming processes of aluminum alloy at the room temperature cannot be neglected. Some details of thermomechanical predictions of a relatively high-speed automatic multi-stage forging process of a yoke with highly deformed region are given to reveal the importance of temperature and/or strain rate even in cold forging of aluminum alloy parts with high speed and high strain.

Abstract: A phenomenological model presented by the authors in the previous study, which is a kind of two-region exponential function model, is used to describe flow stress behaviors of bearing steel, STB2. In this model, flow stress is calculated using two separated equations for hardening and softening regions. Peak stress, peak strain, hardening coefficient, steady state stress and softening coefficient are the required parameters for the model. These parameters are then either interpolated using linear regression or used to find some fitted functions of strain rate and temperature to identify the flow stress. The former is called the piecewise bi-linear function model (PLF model) while the latter the closed-form function model (CFF model). It has been shown that the flow stress curves of STB2 steel obtained by these two models are in good agreement with experimental results.

Abstract: The material constants of a phenomenological model for predicting the grain size evolution during hot deformation are determined using first and second stages of a hot forging process. The constants including initial grain size exponent, strain exponent, strain rate exponent and dynamic recrystallization activation energy for SCR420HB bearing steel were obtained through finite element optimization technique introduced by the same authors in literature. In the optimization, the error between the experimental and predicted grain sizes at some sample points were minimized by adjusting the material constants as the design variables. The predicted grain size after the third stage using the optimized material constants was compared with the experimental values to validate the obtained material constants. The comparison showed that the model predicts the grain size with tolerable error when the obtained material constants are applied.

Abstract: To evaluate the fatigue behaviors of AZ61 magnesium alloy with different weight percentages (0, 1 and 2) of silicon carbide particles (SiCp) were fabricated through gravity casting method. In addition, stress-controlled low-cycle fatigue test of SiCp reinforced magnesium alloys AZ61 were performed in ambient atmosphere at room temperature using ASTM 606 standard specimens. Fatigue measurement results proved, that the fatigue life of SiCp reinforced metal matrix composites (MMCs) decreased with increasing SiCp content. However, the results of the cyclic ductility decreased owing to the presence of significant amount of SiCp, which induces the brittleness of fatigue properties. This is probably occurring because of increasing the SiCp content in the matrix causes highly localized plastic strain. In addition, a high concentration of stress results around the reinforcements particles regions initiate the crack leading to rapid failure of MMCs. Therefore, the SiCp did not act as a stress reliever and it behaves in a brittle manner for the crack propagation through the particles.