Call number: LD2668 .T4 1965 M67 / Master of Science

Microdefects in wafers sliced from selected positions along Czochralski (CZ)-grown, silicon single crystal ingots were investigated by means of transmission electron microscopy (TEM). Specimens taken from the central regions of these wafers, previously subjected to specific thermal treatments, were prepared either by ultrasonic cutting and jet thinning or by an anisotropic thinning method. Ultrasonic cutting was found to generate microdefects in the thin surface regions of the TEM specimen discs. The density of ultrasonically generated defects (USD's) was found to vary directly with the ultrasonic energy input from the cutter. Ultrasonic waves transmitted through abrasive slurry into the discs, causing lattice vibrations, are believed to be responsible for the microdefect generation. Anisotropic thinning for the preparation of TEM specimens was carried out in an agitated bath of KOH-Isopropyl Alcohol (IPA)-H₂O at 80°C and 60°C. A great number of high-surface-quality, self-supporting thin films can be produced with large (about 30 mils square) electron-transparent areas. Edges of the thin films are in <110> directions and can be used as quick reference for defect orientation during electron microscopy. Specimens from heat-treated wafers disclosed the presence of precipitates measuring some 100-1500 nm on one side, surrounded by prismatic dislocations punched out in <110> directions in the crystal. The precipitates appear to be thin platelets (less than 40 Å in thickness), lying on {100} planes and are viewed either as flat squares or rectangles, or as edge-on rods inclined 45° to the <110> directions. The edges of the platelets are in <110> directions. Prismatic punched-out dislocation loops are formed in rows, the axes of which are in <110> directions. A row of loops seen edge-on is similar in size if its axis is in the surface <110> directions. When loop axes are in the oblique <110> directions from the surface, they appear as closed rhombus loops with line senses in <112> directions. Their size increases with distance from the precipitate. The observed dislocation loops were found to be of interstitial type with a Burger's vector of a/2 <110>. The total defect density (precipitates and dislocation loops) of a specimen depend strongly on the thermal history of the wafer and on the wafer position in the ingot.

The vision of this thesis is to improve the machinability of workpiece steels. Workpiece material frequently transfers to the cutting tools during machining, and the transfer layers then forming on the tools may give both good and bad effects on machining performance and tool life. The objective of this work is to understand the effects of alloying element additions to workpiece steels on material transfer and the roles of the formed transfer layers on friction characteristics and wear of tools. To isolate and study the influence of the individual alloying elements, model steels are specifically designed. These steels include one reference with C as the only alloying element and others alloyed also with single additions or combined additions of 1 mass% Si, Mn, Cr and Al. The experiments are performed using both a sliding test, simulating the material transfer in milling, and a turning test. In a sliding contact, the mode of transfer is strongly dependent on the normal load and sliding speed. Material transfer initiates extremely fast, in less than 0.025 s, and characteristic transfer layers develop during the first few seconds. The different steel compositions result in the formation of different types of oxides in the transfer layers. At the workpiece/tool interface where the conditions involve high temperature, high pressure and low oxygen supply, easily oxidized alloying elements in the steel are preferentially transferred, enriched and form a stable oxide on the tool surface. The degree of enrichment of the alloying elements in the oxides is strongly related to their tendencies to become oxidized. The difference in melting temperature of the oxides, and thus the tendency to soften during sliding, explains the difference in the resulting friction coefficient. The widest differences in friction coefficients are found between the Si and Al additions. A Si containing oxide shows the lowest friction and an Al containing oxide the highest. The damage mechanism of coated tools is chiefly influenced by the form and shear strength of the transferred material. Absence of transfer layer or non-continuous transferred material leads to continuous wear of the coating. Contrastingly, continuous transfer layers protect it from wear. However, transfer layers with very high shear strength result in high friction heat and a large amount of steel transfer. This leads to rapid coating cracking or adhesive wear.

A dissertation submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Masters of Science in Engineering. Johannesburg, 2017 / This dissertation presents the zinc recycling of tool grade cemented tungsten scrap material generated during commercial production at Pilot Tools Pty Ltd (South Africa), production of powders and alloys from the zinc recycled materials, and evaluation of the properties of the recycled and un-recycled powders and alloys. Tool grade cemented tungsten carbide inserts were subjected to the zinc recycled process under controlled conditions. Tungsten carbide, cubic (TiC, TaC, NbC, TiCN) carbides and Co were recovered from the recycled scrap material. Two recycled alloys, R and RA, and two un-recycled alloys NS and N were produced following the conventional powder metallurgy route. Alloy R was made from 100 % zinc recycled powder with stoichiometric adjustment of C only, and alloy RA was made from 100% zinc recycled powder with stoichiometric adjustment of C and Co. Alloy N was produced under the same conditions as the recycled alloys R and RA, while alloy NS was produced at commercial level at Pilot Tools Pty Ltd (South Africa) using un-recycled powders. The alloy properties were evaluated following standard procedures for hardmetals. When the zinc recycled material was mechanically disintegrated, about 70 % of the recycled material was recovered as fine powder, while 30% was recovered as coarse oversize particles. The oversize particles were quite tough due to a high Co content, and it was difficult to disintegrate them through milling or repeated zinc recycling. The recycled powders took twice as much time to mill to the desired size as the new un-recycled powders, and had predominantly angular particles, while the new powders had smaller more rounded particles. A cubic free layer (CFL) was formed in all the alloys during sintering, although the recycled alloys R and RA had a narrower CFL compared to the new alloys NS and N. The recycled alloys R and RA had carbide grain size, carbide contiguity, binder mean free path, hardness, fracture toughness and wear rate which were generally within the same ranges as the new un-recycled alloys. / MT2018

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1979. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING / Vita. / Includes bibliographical references. / by Bruce M. Kramer. / Ph.D.

Laser cutting is one of the most important applications of laser in manufacturing industry; it is mainly used for sheet metal cutting. In laser cutting, performing real-time evaluation of laser cut quality is very important to the advancement of this process in industry. However, due to the dynamic nature of the laser cutting process specially when cutting ferrous alloys using oxygen as an assist gas, laser cut quality cannot be easily predicted; therefore, the quality inspection of the laser cut is performed by off line inspections of the edges of the metal by skilled operators. This methodology is carried out after the process and thus cannot maintain a good quality if the process performance is out of control. Therefore, the objective of the research project is to qualify and develop a sensor system that ensure fault recognition online and can automatically control the laser metal cutting process to achieve good quality cut. For the realization of this objective the following has been done: - study the relationship between process parameters and cut quality characteristics; - identify the best sensors that can be used to monitor the process; - design and develop an experimental setup to test the proposed sensors; - collect and analyze data from the proposed sensors and correlate them to specific cut quality characteristics (process state variables); - develop direct relationships between the process signals and cut quality; - develop appropriate strategy for process control; - design and develop an integrated monitoring and control system; - test and evaluate the proposed system using simulation. In this study, a new technique for the determination of cut quality of sheet steels under the CO2 laser cutting process has been established. It is based on on-line detection and post-processing analysis of light radiation and acoustic emissions from the cut kerf. Determination of machining quality during cutting is best done through the measurement of surface roughness and kerf widths, as these are the two parameters that vary in successful through cuts. These two quality parameters can further be correlated to the two dominant process parameters of laser power and cutting speed. This study presents an analysis of acoustic emissions and reflected light for CO2 laser cutting of steel plates, and discusses their use for the estimation of cut quality parameters of kerf width and striation frequency for mild steel plates of 3mm, 5mm, 8mm, and 10mm thicknesses. Airborne acoustic and light signals are acquired with a microphone and a photodiode respectively, and recorded with a PC based data acquisition system in real time. The signals are then analyzed to establish a correlation between the signals obtained and the cut quality achieved. Experimental evidence shows that the energy levels of acoustic emission signals (RMS analysis) can be used to maintain the cutting process under steady state condition. On the other hand, the light intensity signal fluctuates with a frequency that corresponds to the frequency of striations formed on the cut surface; therefore it can be used to regulate cutting speed and laser power to obtain an optimum cutting condition and best cut quality. The validity of the proposed control strategy was tested experimentally by simulating the variations of cutting speed and examining their effect on the signals. So far, the prototype used for experimentation has been successful in providing correct information about cut quality in terms of striation frequency, and also about the state of the process where the microphone signal was successful in determining system failure or improper cutting conditions. A microprocessor based control system utilizing the PID control algorithm is recommended for the implementation of the control strategy. The implementation requirements of the proposed system for industrial use are then discussed. A new setup for the coaxial monitoring of CO2 laser cutting using a photodiode is proposed to enhance the quality of the signal and also to protect the photodiode from the harsh cutting environment. It is also proposed that an open control architecture platform is needed to enhance the integration of the proposed process control functions. Conclusions and future research directions towards the achievement of Autonomous Production Cell (APC) for the laser cutting process are then given.

Improvement of manufacturing productivity is dependent on the successful automation of manufacturing processes, the success of which is based in turn upon the availability of information which describes the state of manufacturing operations. Acoustic Emission (AE) signals related to the cutting process and tool wear have been recently applied to monitor manufacturing processes, and various AE parameters can be used to provide process information. For example, when cutting tools become worn, AE energy generated at the interface of tool flank and work piece increases. This study is thus an experimental investigation of the AE spectrums representing AE signals energy distribution to determine the possibility of extracting useful parameters to provide on-line information about drilled-hole quality and drill-bit wear. An experiment conducted using a radial-arm drilling machine was employed to collect on-line AE drilling process spectrums, yielding eight indicator parameters. Drill wear states were measured using a machine vision system. Assessment of the drilled hole quality was based on tolerances established in Geometric Dimensioning and Tolerancing (GD&T). Correlations among drill wear, drilled-hole quality measurements, and the AE spectrum indicator parameters were examined by regression analysis. A forward-stepwise variable selection procedure was used to select the best-fit regression model for each drilled hole quality measurement associated with the set of one AE parameter raised to different powers. According to quality measurements, drilled holes were categorized as either "acceptable" or "unacceptable" holes, using cluster analysis with a group-averaging method. The usage of AE parameters to decide to which group a drilled hole belonged was also examined. From the experimental evidence, it was observed that there are strong relationships between AE parameters and drill-wear state and the quality measurements of drilled holes. AE parameters could be useful predictor variables to provide information to controller/operators to evaluate current drilling processes. Based on the status information of drill wear and the quality measurements, drilling processes can be adjusted accordingly. / Graduation date: 1997

The present thesis focuses on characterisation of microstructure and the resulting mechanical and tribological properties of CVD and PVD coatings used in metal cutting applications. These thin and hard coatings are designed to improve the tribological performance of cutting tools which in metal cutting operations may result in improved cutting performance, lower energy consumption, lower production costs and lower impact on the environment.  In order to increase the understanding of the tribological behaviour of the coating systems a number of friction and wear tests have been performed and evaluated by post-test microscopy and surface analysis. Much of the work has focused on coating cohesive and adhesive strength, surface fatigue resistance, abrasive wear resistance and friction and wear behaviour under sliding contact and metal cutting conditions. The results show that the CVD deposition of accurate crystallographic phases, e.g. α-Al2O3 rather than κ-Al2O3, textures and multilayer structures can increase the wear resistance of Al2O3. However, the characteristics of the interfaces, e.g. topography as well as interfacial porosity, have a strong impact on coating adhesion and consequently on the resulting properties.  Through the deposition of well designed bonding and template layer structures the above problems may be eliminated. Also, the presence of macro-particles in PVD coatings may have a significant impact on the interfacial adhesive strength, increasing the tendency to coating spalling and lowering the surface fatigue resistance, as well as increasing the friction in sliding contacts. Finally, the CVD-Al2O3 coating topography influences the contact conditions in sliding as well as in metal cutting. In summary, the work illuminates the importance of understanding the relationships between deposition process parameters, composition and microstructure, resulting properties and tribological performance of CVD and PVD coatings and how this knowledge can be used to develop the coating materials of tomorrow.

published_or_final_version / Industrial Engineering / Doctoral / Doctor of Philosophy

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