1,720,977 research outputs found
An Investigation into the Surface Finish of Laterally Face Milled Cobalt-Chromium-Molybdenum Alloys
No full textThe surface finish of multiple machined dental alloy specimens has been analysed, evaluated and compared. The dental alloy used in this investigation was cobalt-chromium-molybdenum (CoCrMo). CoCrMo is a popular biomedical material commonly found in artificial joints and dental implants. Lateral side milling was used to machine the specimens while four different cemented tungsten carbide (WC) mills were used at varying rotational spindle speeds to provide a comprehensive look into how different configurations would affect the surface quality of this alloy. In dental applications a high quality surface finish of the implant is crucial to minimize plaque and food build up and therefore an in-depth investigation into the link between machining specifications and surface finish was needed. Cutting forces of each experiment were supplied with the milled specimens. Scanning electron microscopy of each specimen and mill in this study was then undertaken. Furthermore, micro hardness testing, and roughness testing were undertaken on the dental alloys to create a comprehensive results profile. It was observed that in general, roughness increased with increasing spindle speed, though multiple data sets were incomplete. Obtained cutting forces were shown to vary for each mill and spindle speed. The primary observation from this study was that surfaces that appeared to be smooth were microscopically rough with chip rewelding, mill vibrations, mill wear, and the minimum chip thickness mechanism dictating the extent of the surface roughness. Finally, a DLC mill coating was found to be not suitable for milling CoCrMo
Modes and progression of tool deterioration and their effects on cutting force during end milling of 718plus Ni-based superalloy using cemented WC-Co tools
Get PDFUnderstanding the detailed progression of cutting tool deterioration and how deterioration affects cutting force (F) during milling of difficult-to-cut Ni-based superalloys is important for the improvement of machinability of the alloys. It also serves to clarify whether and how an F-based method for monitoring tool deterioration is possible. This understanding is however far from sufficient, as is explained in this thesis after a comprehensive review of the literature. The aim of the present research is thus to determine and explain the modes and progression of tool deterioration and how cutting forces may vary due to the various deterioration features of the cutting tool edge
Effects of Increasing Laser Power on Microstructure Formed During Selective Laser Melting of Co-29Cr-6Mo Alloy
No full textFor a wider medical/dental and aerospace applications of 3D printed parts made using selective laser melting (SLM) Co-29Cr-6Mo alloy, how SLM parameters affect the quality and properties of parts need to be understood. In this PhD research, how laser power (P) affects the geometry of tracks was studied first. This track geometry relates well to a major defect, lack of fusion (LOF), which affects part quality. Through examining extensively the track profiles, the effect of P on the amount of LOF has been found and has been geometrically explained. Furthermore, we have identified an abnormal type of LOF caused by spatters and how melt penetration may reduce this type of LOF has also been shown. The second and major part of this thesis was to study how the tiny melt rapidly solidifies during SLM, as it is well known that solidification microstructure directly relates to properties of the parts. Cellular solidification without a planar layer has been observed and explained based on constitutional supercooling. Growth direction selection during epitaxial growth has been identified and explained considering local heat flux direction. Finally, cell size measurement and growth rate estimation have enabled the calculation of thermal quantities during SLM solidification
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Microstructure Evolution and Phase Transformation of Welded Metastable Beta-titanium Alloy (Ti-5Al-5V-5Mo-3Cr)
No full textTi-5Al-5V-5Mo-3Cr (Ti5553, in wt%) is a recently developed metastable β titanium alloy, specifically designed to replace Ti-10V-2Fe-3Al (VT22) in the manufacture of large airplane components. The studies of Ti5553 have drawn the attention of many researchers. Ti5553 shows reasonable weldability and can be welded autogenously (without filler metal). Most of the previous studies were directed at forged material behaviour. There is still a lack of understanding of the microstructure evolution and the phase transformation in regards both similar and dissimilar welded material.
The main part of this study investigates the microstructure evolution and phase transformation of Ti5553-Ti5553 similar weldment upon various post weld heat treatment (PWHT). The microstructure analysis was carried out by optical, scanning electron microscopy (SEM) and electron transmission microscopy (TEM). Microstructural results show that the weld zone in Ti5553 can retain the β phase in the as-welded condition. SEM micrographs reveal the different morphology and growth rate of the α precipitation at 500℃ and 600℃ ageing temperature. TEM results in the fusion zone (FZ) area of an as-welded (AW) specimen show the diffraction patterns of the ω phase. This ω phase was retained as athermal ω_a which formed during cooling from the welding process. All evidence proved that ω_a in the weld zone improved the precipitation rate. However, there was no evidence for an isothermal ω at a 500℃ ageing temperature.
The volume fraction and size of α precipitates have a major influence on hardness and tensile strength. At around 30mins of ageing time at either temperature, the α platelets reached an equilibrium of precipitation and hence had the highest volume fraction which resulted in the highest tensile strength. Increasing the ageing time resulted in the α laths coarsening, especially for samples aged at 600℃. The coarsened α laths caused a decrease in hardness. However, changes in tensile strength were not significant. Most of the fractures occurred in the FZ. Fractographic analysis showed dimple rupture via microvoid coalescence for all tensile tested pieces.
Investigation on AW dissimilar welding Ti5553-Ti64 and Ti5553-CPTi suggested that Ti5553 is weldable to the most common titanium alloys (Ti64 & CPTi) and with reasonable tensile strength. All fractures occurred at the low-strength area. Hardness profiles indicated higher hardness in the FZ. Electron probe micro-analysis (EPMA) was employed to investigate material flow in the melt pool
Modes and Progression of Tool Deterioration and Their Effects on Cutting Force During End Milling of 718Plus Ni-based Superalloy Using Cemented WC-CO Tools
No full textUnderstanding the detailed progression of cutting tool deterioration and how deterioration affects cutting force (F) during milling of difficult-to-cut Ni-based superalloys is important for the improvement of machinability of the alloys. It also serves to clarify whether and how an F-based method for monitoring tool deterioration is possible. This understanding is however far from sufficient, as is explained in this thesis after a comprehensive review of the literature. The aim of the present research is thus to determine and explain the modes and progression of tool deterioration and how cutting forces may vary due to the various deterioration features of the cutting tool edge.
Experimentally, the study started by using a typical milling condition with both uncoated and coated cemented carbide (WC-Co) tools. Milling was conducted in either dry or wet conditions. After each pass of a selected distance, the tool was examined in detail in the same manner. Thus, tool deterioration could be monitored more closely and failure mechanisms could be identified and explained. Following on the study on determining the modes of tool deterioration, the progress of deterioration and cutting forces during milling were carefully monitored. Through analysing the monitored tool deterioration features and measured force data, how edge wear, chipping and breakage in cutting edge and beyond the edge contribute to the variation of cutting forces could be studied and better understood. Furthermore, experiments have also been conducted using workpiece in a hardened state.
It has been observed that the commonly recognised build-up layer in the initial stage does not significantly affect the tool deterioration process. Instead, from the beginning of milling, cutting forces/stresses could cause small chipping locally in the initially sharp cutting edge. Fracturing locally with cracks propagating outside the cutting edge along the flank face in the subsurface region could also take place and was consistent with the direction of the cutting force. There was an initial period of time during which a number of microcracks had initiated in and near the cutting edge on the rake face side. These cracks soon propagated resulting in extensively fracturing and blunting of the tool. Coating of the tools had provided little protection as in the cutting edge area the coating had broken away soon after milling started. The major tool failure mode was Co binder material having heavily deformed to fracture, separating the WC grains. Loss of strength in binder material at cutting temperatures is also discussed. As would be expected, the general trend of how F increased as the number of pass (Npass) increased agreed with the general trend of increasing flank wear (VB) as Npass increased. However, the F-VBmax plot has shown a rather poor F-VBmax relationship. This was the result of the different modes of tool deterioration affecting VBmax differently, but VBmax did not represent fully the true cutting edge of the deteriorating tool insert. Chipping and breakage of the inserts confined in the cutting area, resulting in the significant blunting of the edge area, causing a high rate of F increase as VBmax increased and completely deteriorated 6 minutes within of milling time. Fracturing along the face of thin pieces effectively increased VBmax without increasing the cutting edge area and without further blunting the edge, thus no increase in F was required. The high rate, meaning high ∆F/∆VBmax, results from the effect of the edge deterioration/blunting on the reducing the effective rake angle and thus increasing F is suggested and discussed. The use of coolant has not been found to affect tool deterioration/life and cutting force. Explanation for this will be given considering the deformation zone for which coolant does not have an effect. An increase in feed rate has reduced the tool life and the mode of deterioration has become more edge chipping/fracturing dominant, leading to a better F-VBmax relationship.
Finally, it has been observed that the rate of tool deterioration is not higher when the hardened workpiece material is used. The modes and progression of deterioration of tools using hardened workpiece were determined to be comparable to those when annealed workpiece was used. Furthermore, the trends of increase in cutting force as milling pass increases have been observed to be similar for both workpiece material conditions. Interrupt milling experiments followed by hardness mapping has indicted that the workpiece hardened state has not affected the deformation area significantly, although increase in hardness in a similar amount in the severe deformed region has been found for both cases. It is suggested that temperature increases in the narrow deformation zone to be similar for both workpiece conditions and at high temperatures hardening mechanisms do not operate, and thus cutting force values do not differ significantly. Furthermore, the modes and rate of tool deterioration on the hardened workpiece was comparable to the annealed workpiece
Effects of Increasing Laser Power on Microstructure Formed During Selective Laser Melting of Co-29Cr-6Mo Alloy
No full textFor a wider medical/dental and aerospace applications of 3D printed parts made using selective laser melting (SLM) Co-29Cr-6Mo alloy, how SLM parameters affect the quality and properties of parts need to be understood. In this PhD research, how laser power (P) affects the geometry of tracks was studied first. This track geometry relates well to a major defect, lack of fusion (LOF), which affects part quality. Through examining extensively the track profiles, the effect of P on the amount of LOF has been found and has been geometrically explained. Furthermore, we have identified an abnormal type of LOF caused by spatters and how melt penetration may reduce this type of LOF has also been shown. The second and major part of this thesis was to study how the tiny melt rapidly solidifies during SLM, as it is well known that solidification microstructure directly relates to properties of the parts. Cellular solidification without a planar layer has been observed and explained based on constitutional supercooling. Growth direction selection during epitaxial growth has been identified and explained considering local heat flux direction. Finally, cell size measurement and growth rate estimation have enabled the calculation of thermal quantities during SLM solidification
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
- …
