Materials Science Forum Vols. 654-656

Abstract: Many investigations about superalloys and coatings have been done in the laboratory, but evaluating the degradation condition of hot section components during service is still important not only for repair and reuse but also for outage prevention. Time dependent degradation for second stage blades of gas turbine was investigated. The degradation analysis for used blades was divided into microstructure changes by position of the blade and mechanical tests of high temperature tensile test. In the microstructure analysis, the rafting and coarsening of γ', MC decomposition and TCP phase formation occurred and progressed with increasing service time, and especially the leading and trailing edge of top layer should be a check points for used blade. High temperature tensile results of 25,000 and 52,000 hrs used blades were also compared with serviced time and position in each blade.

Abstract: Diamond composite materials are classified as superhard and exhibit exceptional abrasive resistance. Cemented tungsten carbide tools with a thick coating of diamond composite material (PCD) are finding increased usage in materials cutting operations in manufacturing, mining, minerals, gas and petroleum exploration and civil construction industries. Two major advantages derived from these coated tools are: (a) increased wear resistance and hence increased life-span of these tools and (b) their proven ability to handle “difficult-to-machine” materials as well as high-strength, extremely abrasive materials such as quartz-rich rocks, granites and basalts. In this research, the variability of the wear resistance of PCD coated tungsten carbide is correlated with microstructural variations. A detailed study of the microstructure and distribution of phases was performed using SEM, cathodoluminescence (CL) imaging, direct x-ray imaging, Raman spectroscopy as well as residual stress measurements using neutron diffraction.

Abstract: This paper presents a feasible machining test to measure, compare and predict the machinability of different titanium alloys. A drilling test was developed and investigated on the two most common grades of titanium, commercial purity and Ti6Al4V. The experiments and analysis revealed that tool wear followed a characteristic pattern for all machining conditions investigated. When machining Ti6Al4V, tool life was shorter and cutting forces higher compared with commercial purity Ti. Paradoxically, despite the more difficult machining, Ti6Al4V samples had better surface integrity than commercial purity samples. A procedure was developed that could be incorporated into a real-time process monitoring device to warn of imminent tool failure.

Abstract: In this work the static and dynamic tensile properties of vanadium alloy V-5Cr-5Ti were investigated at strain rates ranged from 3.3x10-5/s to 1.2x102/s. The material microstructures were analyzed using optical microscope, SEM, TEM, XRD and EDS. Results show that the yield strength a increases with strain rate. The brittle-ductile transition strain-rate is about 101/s to102/s. At room temperature the tensile fracture at a low strain rate occurs via mixed modes of microvoid aggregating and transgranular cracking; at high strain rate the fracture occurs via a brittle mode. The analysis by TEM, XRD and EDX shows the existence of lath martensite, and on there exits precipitate phase of Ti(O,C, N) in grain boundaries.

Abstract: This paper presents a detailed failure analysis on a rivet, which broke into two parts from the joint place between head and shank under a load of seven tons during service, by the combination of optical microscopy, SEM and EDS. The metallographic examination shows some micro-inclusions in the sample and many micro-cracks along the grain boundaries. SEM and EDS results show that the surface of the rivet was plated with a zinc coating and it was eroded severely. The fracture surface exhibits the characteristic of quasi-cleavage fracture, indicating an intergranular brittle fracture mode. Some micro-pores were found on the facets of intergranular cracked grains. Most importantly, the Crow-toe Pattern, one of river patterns or hairline seams, which is a typical microstructure of hydrogen embrittlement, was observed on the fracture. Based on above analysis, it is concluded that the failure of the rivet is caused by the hydrogen embrittlement. The hydrogen, most likely, comes from the zinc plating process. Due to the aggregation and diffusion of hydrogen into the matrix, the brittle cracking happens in the regions with stress concentration because of the volume expansion effect under the external force.

Abstract: The high-speed fracture phenomena of glass bottles by using underwater shock wave technique for recycling the glass containers were discussed. The proposed technique is an efficient method for the crushing of glass bottles. In this study, the influence of various explosive conditions, that is, the explosive shape, set position, amount, the bottle size and the contents of bottle, on the cullet size, which is the crushed glass fragments, was studied. Furthermore, the behaviors of underwater shock wave were observed by high-framing photograph and the high-speed fracture mechanism was clarified.

Abstract: Recently the demand for demolition of unnecessary cylindrical silo structure is increasing due to the deterioration and malfunctional conditions and the issue of demolition is in the limelight. To minimize environmental hazards caused during the process of demolition, the explosive demolition method has been applied increasingly. The concrete silo structure is often built by the slip-form method which is a continuous concrete casting operation. As a result, the structure does not have expansion joints or other structural weaknesses that can be used to assist in the demolition process. This study presents the pre-weakening of explosive demolition of the cylindrical concrete silo structure by overturning method. Pre-weakening for the explosive demolition of the structure is usually conducted based on the field experience without technical guideline for the pre-weakening procedure. Hence, there always exist safety-related concerns. To demolish the structure by blast effectively and safely, rational approach to the pre-weakening and evaluation method of the load carrying capacity of the pre-weakened structure should be established. To do this, mechanical properties of concrete are estimated by experiment prior to structural analysis. Based on the result of investigation, design or operation methods of pre-weakening for the blast demolition are discussed.

Abstract: The purpose of this research is the development of a high strength α-β brass (Cu-40Zn) with additions of elements of small solid solubility in brass. Cu-40Zn with 0.6 wt.% Tin (Sn), 0.73 wt.% Chromium (Cr) and 0.51 wt.% Iron (Fe) were prepared by casting (Cu-40Zn-CrFeSn). The yield stress (YS) and ultimate tensile stress (UTS) of extruded Cu-40Zn-CrFeSn was 291 MPa and 601 MPa, 23 % and 36 % higher than that of extruded binary Cu-40Zn alloy. Vickers micro hardness of 158 Hv was higher than that of extruded Cu-40Zn alloy (131 Hv). In addition, the elongation of extruded Cu-40Zn-CrFeSn was 35 %. The strengthening mechanisms of these alloys were considered as follows; one was a solid solution strengthening of Cr, Fe and Sn additives which were identified by SEM-EDS. The other was increasing of the area ratio of β-phase in Cu-40Zn-CrFeSn, compared to that of Cu-40Zn.

Abstract: Knowledge of the mechanical properties of parts processed by Fused Deposition Modelling (FDM) rapid prototyping process is essential for engineering applications of such parts as the mechanical strength of parts depends heavily on the FDM process parameters selected during part fabrication. Little knowledge is available for the Polycarbonate (PC) material used in the FDM systems. This paper presents results of the experimental work on the effect of the FDM process parameters such as air gap, raster width, and raster angle on the tensile properties of PC. Results show that FDM made parts have tensile strength in the range of 70 to 75 % of the moulded and extruded PC parts. The results will be valuable for different functional applications of FDM produced parts and assemblies.

Abstract: Semi-automatic and automatic pulsed gas metal arc welding (GMAW) of aluminium alloy 5083 with ER5356 filler wire causes considerable softening in the weld. The tensile strength of dressed automatic welds approaches that of the base metal, but the stress concentration caused by the weld toe in undressed semi-automatic welds reduced the tensile strength significantly. Fully automatic welds displayed improved fatigue properties compared to semi-automatic welds.