Materials Science Forum Vols. 532-533

Abstract: Interference-fit fasteners are used in large numbers throughout aircraft structures. Conventional installation methods are driving fasteners into aperture using pneumatic or hydraulic tooling, installation damage always occurs using these methods. This paper introduces a new method of driving interference-fit fastener using stress wave, reports on a comparison of driving quality between stress wave driving and hydraulic force driving. The experiments completed by authors indicate that stress-wave method can drive the fasteners with larger interference which conventional installation method can not implement; stress wave driving method provides high installation quality and repeatable results compared with the conventional driving method; and stress wave driving method is convenient to be used. The experiments prove that the protuberant material produced by stress wave method is less than that of conventional installation methods, this is advantaged to improve the fatigue strength of joint.

Abstract: The introduction of ceramic particulate into metallic powder will unavoidably lower the compressibility and formability of the mixed powder. In order to overcome these problems, in this study, warm compaction was introduced in the forming of an NbC particulate reinforced iron-base valve-guide cup, which is used in a combustion engine. Warm compaction was used not only because it can provide compacts with high green density but also it can increase the formability of the mixed powder. The part composed of an iron-base material which possesses 10wt%NbC with a relative density of 97.7%, a tensile strength of 815MPa, an elongation of 1.5%, a hardness of HRC33 and an impact toughness of 11J/cm2. Its working surface composed of an iron-base material which possesses 15wt% NbC with a high relative density of 98.2%, a tensile strength of 515MPa, a hardness of HRC 58 and a remarkable tribological behavior. The sintered part successfully passed a 500 hours bench test. No serious wear on the working surface can be observed after the test. Results indicated that the sintered part has excellent wear resistivity and the NbC particulate reinforced iron-base composite is a suitable material for parts that work under severe wear condition.

Abstract: Based on the two-stage forming technology, the casting AZ31 Magnesium alloy bar was forged into cylindrical straight inner gear between the temperature 250°C-400°C. At 250°C, the teeth of the inner gear are almost formed. But there are some cyclic cracks on the surface of the sample. When improving the temperature above 300°C, the surface quality of the sample has greatly improved. According to the result of this experiment, the best temperature range for forging AZ31 magnesium gear is 280°C to 380°C.The forming load gradually reduced with the temperature improved. At 250°C, the forming load is 93t. At the 400°C, the forming load reduces to 80t.The initial grain size of AZ31 magnesium alloy bar is 22μm. The microstructure evolution during the warm deformation was observed by optical microscopy (OM). It is demonstrated that the grain refinement happened during the deformation process.

Abstract: 3A21 metal sheet was shocked by Nd-glass laser in oblique angle under the function of pulse energy 42 J, pulse width 23 ns, pulse power 1.2×109 W, the angle between input beam and normal of sample is 30°. The center of the deformation of the sheet is 3 mm away from the geometric center. Residual stresses of crossing center side in length direction and the distribution of space in the positive and negative sides are measured by XRD, which are compressive stress 100 MPa. The diagonal length of the sheet is longer than the one of the crossing center side in length direction, the force that needs to form in the diagonal direction is bigger, so compressive stress is much bigger. The capstone of the square sheet is squeezed, the procession seriously increase, so compressive stress is the biggest one.

Abstract: It is necessary to drill micro holes in some hard brittle materials. Comparing with electro discharge machining, electrochemical machining and laser machining, the ultrasonic machining (USM) has unique advantages in hard brittle materials machining. The aim of this paper is to present experimental work that has been done in micro holes machining by micro USM. The main machining mechanisms of USM are discussed. It is pointed out that the contributions of each mechanism to material removal rate are different in different cases. By using micro USM with workpiece vibration and tool feeding in constant rate, the experiments of micro holes machining are carried out and the process laws of tool wear are studied. Some phenomena are found and the causation of causing these phenomena is analyzed. A Ø13μm micro hole is obtained.

Abstract: Si3N4- Si2N2O composites were fabricated with amorphous nano-sized silicon nitride powders by the hot press sintering(HPS). The Si2N2O phase was generated by an in-situ reaction 2Si3N4(s)+1.5O2(g)=3Si2N2O(s)+N2(g). The content of Si2N2O phase up to 60% was accepted when the sintering temperature was 1650°C and decreased whether the sintering temperature was increased or not, which indicated that the reaction was reversible. The mass loss, relative density and average grain size increased with raising of sintering temperature. The average grain size was less than 500nm if the sintering temperature was below 1700°C. The sintered body crystaled completely at 1600°C . The microstructure crystaled in 1600°C indicated that most of the grain size was in 150-250nm. The aspect ratio of some grains reached 1.5. The superplastic deep-drawing forming could be undertaken at 1550°C with a forming velocity of 0.2mm/min. The complex-shape gears could be formed by a sinter-forging technology when the sintering temperature was 1600°C and the superplastic forging temperature was 1550°C.

Abstract: Micro-flowing technique gained popular applications in microdevices of microelectromechanical systems (MEMS), and the performance of micro-devices is greatly determined by the properties of micro-flow. This paper studied the characteristics of different viscosity fluid flowing over microchannels with different diameters and lengths under low pressure driving, and the influence of scale effects on the flowing characteristics of low viscosity fluids was also examined. The experiments studied the flow rate–pressure characteristics of distilled water flowing over microchannels with diameter of 13 μm, 20 μm, and silicon oil flowing over microchannels with diameters of 50 μm, 100 μm. The results indicate that, when the diameter of micro-channel is more than 20 μm, the flowing characteristics of distilled water and silicon oil agrees well with conventional flow theory, and when the diameter of microchannels is 13μm, the flowing characteristics are related to the length of micro-channel. When the length is relatively shorter, the flowing characteristics are almost in agreement with the conventional flow theory. When the length reaches 100 mm, the flow rate is much higher than the values predicted by theoretical calculation when the length reaches 100 mm. It is obvious that scale effect arises when the length arrives to 100 mm and the velocity slippage results in the great increase of flow rate.