Key Engineering Materials Vols. 345-346

Abstract: Plasma-sprayed HA coatings (HACs) on Ti-6Al-4V substrates with post-heat treatments were employed to improve the microstructural homogeneity, bonding strength and reliability of the HACs. A defect-healing effect can be recognized to diminish coating defects with the hydrothermal treatment, and 150°C hydrothermally-treated HACs shows a significant improved bonding strength than 600°C vacuum heating HACs because of its dense structure from the defect-healing effect. Low-temperature hydrothermal treatment demonstrates a more superior crystallization effect than vacuum heat treatment. Based on the statistical calculation by the Weibull distribution function, hydrothermally-treated HACs show a wear-out failure with a higher Weibull modulus than vacuum heating HACs. Hydrothermally-treated HACs possessed better reliability can be attributed to the suppression of defect content (about 2.6-3.2 volume %) and its smaller strength data fluctuation.

Abstract: It is well-known that the vibration significantly affect the life of solder joint. In this paper, the effects of the vibration on the failure probability of the solder joint are studied by using the failure probability models such as the First Order Reliability Method (FORM) and the Second Order Reliability Method (SORM). The accuracies of the results are estimated by a help of the Monte Carlo Simulation (MCS). The reliability of the lead and the lead-free solder joint was also evaluated. The reliability of lead-free solder joint is found to be higher than that of lead solder joint.

Abstract: Negative SU-8 photoresist processed at several levels of lower temperature than conventional approach was investigated, and its low-temperature processing has been characterized in terms of delamination. According to two phases of statistical design of experiment (DOE), initially statistically significant variables of SU-8 photoresist processing were selected, and the selected variables were further investigated to find their effects on delamination of SU-8 microstructure. A three leveled factorial designed experiments were performed followed by a 2(6-1) fractional factorial designed as a screening experiment. Characterizing low-temperature SU-8 photoresist process, 27 samples were fabricated and the degree of delamination was measured. In addition, nine additional samples were fabricated for the purpose of verification. Employing a neural network modeling technique, a process model is established, and response surfaces are generated to investigate the degree of delamination associated with three process parameters: post exposure bake (PEB) temperature, PEB time, and exposure energy. From the response surfaces generated, two significant parameters associated with delamination are identified, and their effects on delamination were analyzed. The higher the post exposure bake (PEB) temperature at a fixed PEB time and the more delamination occurred. In addition, the higher the dose of exposure energy, the lower the temperature at which the delamination begins and the larger the degree of delamination. The results identified acceptable ranges of the three process variables to avoid the delamination of SU-8 film, which in turn might lead to potential defects in MEMS device fabrication.

Abstract: In this paper, Moiré interferometry technique is used to measure the in-situ displacement evolution of lead-free solder joints under high density (104A/cm2). An electromigration constitutive model is developed to simulate deformation of lead-free solder joint under current stressing. The simulation predicts Moiré interferometry measurements in both spatial distribution and time history evolution, which indicates that the model is reasonably good for predicting the mechanical behavior of lead-free solder joints under electric current stressing.

Abstract: A high carbon steel joint, S70C (0.72wt%C) was successfully friction stir welded without any postheat treatments. There are two methods for obtaining proper joint properties. The first method is to decrease the peak temperature to below A1, and the other method to decrease the cooling rate to less than the lower critical cooling rate. As a result, the ultimate tensile strength of 1214 MPa, 0.2 % proof strength of 700 MPa and elongation of 40% were obtained for a joint.

Abstract: To investigate mechanical fitness of abutment/fixture screw interface in the internal dental implant system, internal connection type implants from AVANA(Osstem®), Bioplant(Cowell-Medi®), Dio(DIO®), Neoplant(Neobiotech®), systems were used in this study. Each group was acquired 2 fixtures at random. Two piece type abutment and one piece type abutment for use with each implant system were acquired. Screw was respectively used to hold a two piece type abutment to a implant fixture. All samples were cross-sectioned with grinder-polisher unit and all specimens were analyzed the interface phenomena between implant fixture/abutment/screw contact interfaces by using field emission scanning electron microscope(FE-SEM). Implant fixture/abutment/screw connection interfaces of internal connection systems made in Korea were in good condition. The results of the above study showed that materials and mechanical properties and quality of milling differed depending on their manufacturing companies.

Abstract: In this study the effect of a free drop impact on the bolted joint of the transport package of radioactive material was studied under various directions. A drop test of a type A or a type IP transport package of radioactive material should be conducted under normal transport conditions. After a drop test the radioactive content of the transport package should not be loss or dispersal. A bolt tension during a drop impact under various directions was measured using a force sensor. And the torque to loosen a bolt was measured after the tests. A bolt tension measured by the tests was compared with the result of a finite element analysis. The effect of the drop direction was examined. The vertical drop directions shows larger bolt tension than horizontal drop. For the torque to loosen a bolt, a horizontal drop condition shows larger value than a vertical drop direction.

Abstract: Based on the results of model tests, the deformation and facture mechanisms of brittle rockmass containing coplanar close intermittent joints under direct shear condition were analysed , then the incipient crack strength and communicating destruction criterion of such rockmass were proposed in this paper. It was shown that the computing results suited the real measuring values well by comparing them, which indicated the criterions was reasonable ,reliable and has both the theory and construction significances.

Abstract: Considering the exhaust valve spindle consists of a valve head and a stem comprising the body, the use of different materials, that is, Nimonic 80A for the head and SNCrW for the stem, can reduce the manufacturing cost dramatically. The inertia welding was conducted to make the large exhaust valve for low speed marine diesel engines, superalloy Nimonic 80A for valve head of 540mm diameter and high alloy SNCrW for valve stem of 115mm diameter. Due to different properties of material like thermal conductivity and flow stress on the two sides of the weld interface, modeling is crucial in determining the optimal weld geometry and parameters. FE simulation was performed by the commercial code DEFORM-2D. A Good agreement between the predicted and actual welded shape was observed. It was expected that simulation will significantly reduce the number of experimental trials needed to determine the weld parameters, especially for welds of very expensive materials or large shaft. A variety of tests, including microstructure observation, tensile, hardness and fatigue test, are conducted to evaluate the quality of welded joints.

Abstract: In an effort to develop the numerical modeling method to optimize the friction stir welding process for quality products, the plastic deformation and thermal histories of friction stir welded AA6111-T4 sheets were numerically simulated. Material and heat flow during friction stir welding were calculated considering the momentum and energy balance equations under the steady state condition. The results agreed well with the temperature measurements, suggesting that the method can be applicable to optimize the friction stir welding process, if proper mechanical and thermal properties of materials are available.