This thesis aims to investigate the manufacturability of nickel-based superalloys, IN625 and IN718, using the laser powder bed fusion (LPBF) process. The study provides a better understanding of the process-structure-property of nickel-based superalloys, their fatigue life, and subsequent post-processing. First, the process-structure-property was investigated by selecting a wide range of process parameters to print coupons for IN625 and IN718. Next, a subset of process parameters was defined that would produce high relative density (>99%), low surface roughness (~2 μm), and a low tensile RS. Second, a multi-scale finite element model was constructed to predict the temperature gradients, cooling rates, and their effect on RS. At constant energy density, RS is affected by scan speed, laser power, and hatch spacing, respectively. Third, the optimum set of parameters was used to manufacture and test as-built and shot-peened samples to investigate the fatigue life without costly heat treatment processes. It was found that shot peening resulted in a fatigue life comparable to wrought heat-treated unnotched specimen. Additionally, IN625 had a better fatigue life compared to IN718 due to higher dislocations density as well as the absence of γ´ and γ´´ in IN718 due to the rapid cooling in LPBF. Finally, the effect of post-processing on dimensional accuracy and surface integrity was investigated. A new approach using low-frequency vibration-assisted drilling (VAD) proved feasible by enhancing the as-built hole accuracy while inducing compressive in-depth RS compared to laser peening, which only affects the RS. These favorable findings contributed to the scientific knowledge of LPBF of nickel-based superalloys by determining the process parameters optimum window and reducing the post-processes to obtain a high fatigue life, a better dimensional accuracy, and improved surface integrity. / Thesis / Doctor of Philosophy (PhD)

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This study develops a method to simulate a full-scale inertia friction weld with a sub-scale specimen and modifies a direct drive friction stir welder to perform the welding process. A torque meter is fabricated for the FSW machine to measure weld torque. Machine controls are modified to enable a force control during the IFW process. An equation is created to measure weld upset due to deflection of the FSW machine. Data obtained from a full-scale inertia friction weld are altered to account for the geometrical differences between the sub-scale and full-scale specimens. The IFW are simulated with the sub-scale specimen while controlling spindle RPM and matching weld power or weld RPM. The force used to perform friction welding is scaled to different values accounting for specimen size to determine the effects on output parameters including: HAZ, upset, RPM, torque, power and energy of the weld. Increasing force has positive effects to upset, torque, power and energy of the welds, while reducing the size of the HAZ.

The Ni-base superalloy, IN-718, has been coated with alumina sols. Coated surfaces, carrying alumina layers having thicknesses between 0.6 μm and 3.6 μm show a significantly reduced oxidation rate when compared with uncoated reference surfaces, even if heating temperature is increased up to 900 °C and heating time is extended to 800 h. Alumina layers were prepared via sol-gel processing using a modified Yoldas procedure to obtain alumina sols. No change in rheological sol behavior was observed for more than 1 year of aging under static conditions at room temperature. Depending on pH value, modified Yoldas sols contain a manifold of Al species, among them Al13 polycations. Thermal evolution of sol derived alumina powders depends on Al speciation of parent sols. Depending on sol composition, both gamma-Al2O3 and eta-Al2O3 occur as intermediate transition aluminas. Phase composition and gas phase velocity influence oxygen permeability of thin layers prepared with modified Yoldas sols.

Inconel 718 (IN 718) is widely used in applications, such as aircraft and power turbine components. Recently, fiber laser welding has become an attractive joining technique in industry for fabrication and repair of service-damaged components. However, a major limitation in the laser welding of IN 718 is that liquation cracking occurs. In the present work, autogenous fiber laser welding of IN 718 was used to study the effects of welding parameters and different pre-weld heat treatments on liquation cracking. Contrary to previous studies, a dual effect of heat input on cracking is observed. A rarely reported effect of heat input is attributed to process instability. Liquation cracking increases with pre-weld heat treatment temperatures that increase grain size and/or, possibly, intregranular boron segregation. The study shows that pre-weld heat treatment at 950oC can be used for repair welding of IN 718 without significant loss in cracking resistance.

Inconel 718 (IN 718) is widely used in applications, such as aircraft and power turbine components. Recently, fiber laser welding has become an attractive joining technique in industry for fabrication and repair of service-damaged components. However, a major limitation in the laser welding of IN 718 is that liquation cracking occurs. In the present work, autogenous fiber laser welding of IN 718 was used to study the effects of welding parameters and different pre-weld heat treatments on liquation cracking. Contrary to previous studies, a dual effect of heat input on cracking is observed. A rarely reported effect of heat input is attributed to process instability. Liquation cracking increases with pre-weld heat treatment temperatures that increase grain size and/or, possibly, intregranular boron segregation. The study shows that pre-weld heat treatment at 950oC can be used for repair welding of IN 718 without significant loss in cracking resistance.

A reduction in voluntary feed intake is a major factor in the lost productivity of grazing lambs infected by gastrointestinal parasites yet the mechanisms involved are poorly understood. Potential pathways involved in parasite-induced feed intake depression were investigated in lambs with minimal previous exposure to parasites and artificially infected by the small intestinal parasite Trichostrongylus colubriformis. Six in vivo experiments were conducted on lambs housed in individual pens or metabolism crates with similar feeding and experimental procedures. In Experiment 1 (Chapter 4) the effect of T. colubriformis infection on short term feed intake in lambs and of some pharmacological agents on feed intake depression were investigated. Prior to and for the duration of infection, lambs were fed once per day and feed intake recorded at regular intervals over the day (8 h). Following the onset of feed intake depression in the infected group (9 weeks after commencing dosing), all animals were treated with an analgesic (codeine phosphate per os), an anti-inflammatory agent (indomethacin per os), a CCK antagonist (L364-718 by subcutaneous injection) or saline (control) in a replicated Latin square design (n = 8). Although the pattern of feed consumption was similar in infected and non-infected lambs, average daily intake was reduced 32 % and short term intake (recorded at 10 minute intervals for the first hour of feeding, 15 minute intervals for the second hour and hourly for the next 6 hours of feeding) reduced 40 % by infection. This identified the key component by which intake was depressed and enabled the use of a short term intake model and short duration of action compounds to identify the pathways involved in intake depression in this sequence of experiments. None of the pharmacological treatments increased intake in the infected group. These results suggest a reduction in the rate of consumption due to reduced hunger signals, rather than change of meal eating patterns, is the major cause of feed intake depression. Specific conclusions about the pathways investigated using the pharmacological agents could not be obtained. Experiment 2 (Chapter 5) was designed to investigate the roles of pain and osmolality on feed intake depression. Digesta samples collected prior to and during parasite infection and before and after feeding had similar osmolalities (240-260 mosmol/l) which indicated that feeding or infection had no effect on osmolality of digesta. Following the onset of feed intake depression in infected animals, all animals were treated in a Latin square design (n = 4) with no treatment, saline, local anaesthetic (xylocaine) or analgesic (codeine phosphate) solution 15 minutes before feeding, by slow injection into the duodenum. There was no effect of these treatments on food intake. In the second part of the experiment, hyperosmotic solutions (mannitol and NaCI) markedly depressed short term intake in non-infected animals, suggesting a role for osmoreceptors in intake regulation. However these effects were not blocked by local anaesthetic so the depressed intake may have resulted from generalised malaise rather than from specific osmoreceptor effects. In Experiment 3 (Chapter 6) the role of peripheral CCK on intake depression was examined by a dose-response study utilising the CCK antagonist, loxiglumide. Intravenous injection of 5, 10 or 20 mg/kg LW of loxiglumide to infected lambs 10-15 minutes before feeding (n = 6) had no effect on feed intake at any of the dose levels. In experiment 4 (Chapter 7) loxiglumide was infused intravenously for 10 minutes (30 mg/kg/h) before feeding and for the first 2 h (10 mg/kg/h) after feed was offered to minimise any effect of the rate of clearance of loxiglumide on the lack of feed intake response. As well, the rate of marker disappearance from the abomasum was recorded in both infected and non-infected animals. Continuous infusion of loxiglumide did not attenuate parasite induced intake depression nor did it have any effect on abomasal emptying. Abomasal volume was reduced by infection (66.3 vs 162 ml) as was the fractional outflow rate (2.2 vs 2.8 ml/min) but these differences were accounted for by the lower level of feed intake in the infected animals. In Experiment 5 (Chapter 8) brotizolam, a benzodiazepine appetite stimulant, thought to act on the hypothalamus, was administered in a dose-response study to infected and non-infected animals (n = 4) immediately prior to feeding or following termination of the first meal (45 minutes after feeding) and the feed intake response recorded. Brotizolam elevated both the short term (0-0.75 h), daily (22 h) intake and all time intervals in the first 5 h after feeding in infected and non-infected animals when administered after the first meal but when administered prior to feeding elevated intake only over the first 6 h of feeding. In both cases the magnitude of the response was greater in infected animals than in non-infected animals. Brotizolam appeared to increase the rate of eating without having a major impact on meal eating patterns when administered before feeding. Where administration was after the first meal, the effect was due to an "extra" meal being consumed. These findings showed that infected animals can respond to central stimulators of intake although the mechanism of the response is not known. Opioids were implicated in intake depression as the rate of intake rather than meal patterns appeared to be the major parameter depressed under parasitism. This was examined in experiment 6 (Chapter 9) where animals (n = 6) were fasted for 26 h or not fasted, then treated with saline (control), brotizolam (intake stimulant) or naloxone (opioid antagonist) immediately prior to feeding. Fasting stimulated feed intake in the short term (100 % increase in 75 min) and over the day (12 % increase) in both infected and non-infected animals. Following fasting, infected animals ate a similar amount of feed to the non-infected, fasted animals and more than the non-infected, non-fasted animals. The signals resulting from a one day fast were sufficient in the short term to override parasite induced mechanisms causing feed intake depression. Naloxone suppressed the intake stimulatory effects of a 26 h fast in both infected and non-infected animals, which supports a role for endogenous opioids as hunger signals. Where animals were not fasted, naloxone reduced intake only in the non-infected animals which suggested endogenous opioid levels may be lower in infected animals than in non-infected animals. In the final experiment (Experiment 7, Chapter 10) the role of central hunger and satiety mechanism were investigated. Infected and non-infected animals (n = 6) were treated with naloxone or saline by intravenous injection, or saline and met-enkephalinamide (an opioid analogue) by intracerebral infusion, or naloxone and the opioid analogue simultaneously to investigate the role of central opioids in feed intake depression. To determine the role of CCK induced satiety signals on feed intake at a central level, loxiglumide and CCK were infused separately and in combination for 30 minutes prior to feeding and for the first 60 minutes of feed on offer, into a lateral cerebral ventricle of the brain of infected and control animals (n = 6). The opioid analogue tended to increase intake in infected animals but the effect was not significant probably because the dose used was too low to elicit a response in sheep. Naloxone depressed intake only in the infected animals, which conflicted with the results of Experiment 4. As a consequence these results were inconclusive because of the single low dose of opioid analogue used and the conflicting naloxone responses. CCK alone depressed intake by 39-52 % only in infected animals and this effect of the 90 minute infusion was evident over the 8 h short term recording period. Loxiglumide attenuated the feed intake depressive effects of CCK in the infected animals to the extent that intake was elevated above control levels. Loxiglumide alone was an intake stimulant in both infected and non-infected animals. Intake was increased over the entire 8 h but mostly in the second hour when intake was increased by 188 % in infected animals and by 16 % in the non-infected animals and resulted in almost continuous eating. These results showed loxiglumide will temporarily block the effect of parasite infection on feed intake in sheep when administered centrally and the fact that it blocked the effects of exogenous CCK on intake indicated that the effect is mediated via CCK receptors. In conclusion GIT parasite infection reduced both short term and daily feed intake apparently by a change in rate of intake rather than any alteration in meal patterns. It was further suggested that anyone of a number of potential peripheral pathways, including changes to osmolality, gut emptying, pain and inflammation of the gut, alone is not involved in anorexia in sofar as the compounds used could block these factors and the results support the idea that intake depression is mediated via a central mechanism. Intake in infected animals responded to a much greater extent when fasting, i.c.v. loxiglumide or brotizolam were employed. Feed intake thus appears to be regulated through the same mechanisms in infected and non-infected animals. The results from compounds affecting the central mechanism suggest central CCK receptors are important in parasite induced anorexia, possibly by changing the onset of satiety or by interacting with endogenous opioids to reduce the rate of feed intake. Secondly reduced endogenous opioids may be causing the reduction in the rate of feed consumption alone or as a result of other interactions. It was concluded that intake in parasitised animals could be increased to that of control animals by employing procedures and compounds thought to act on the hypothalamus.

There is a strong industrial driving force to find alternative production technologies in order to make the production of aero engine components of superalloys even more efficient than it is today. Introducing new and nonconventional machining technologies allows taking a giant leap to increase the material removal rate and thereby drastically increase the productivity. However, the end result is to meet the requirements set for today's machined surfaces.The present work has been dedicated to improving the knowledge of how the non-conventional machining methods Abrasive Water Jet Machining, AWJM, Laser Beam Machining, LBM, and Electrical Discharge Machining, EDM, affect the surface integrity. The aim has been to understand how the surface integrity could be altered to an acceptable level. The results of this work have shown that both EDM and AWJM are two possible candidates but EDM is the better alternative; mainly due to the method's ability to machine complex geometries. It has further been shown that both methods require post processing in order to clean the surface and to improve the topography and for the case of EDM ageneration of compressive residual stresses are also needed.Three cold working post processes have been evaluated in order to attain this: shot peening, grit blasting and high pressure water jet cleaning, HPWJC. There sults showed that a combination of two post processes is required in order to reach the specified level of surface integrity in terms of cleaning and generating compressive residual stresses and low surface roughness. The method of high pressure water jet cleaning was the most effective method for removing the EDM wire residuals, and shot peening generated the highest compressive residual stresses as well as improved the surface topography.To summarise: the most promising production flow alternative using nonconventional machining would be EDM followed by post processing using HPWJC and shot peening.

The Ni-base superalloy, IN-718, has been coated with alumina sols. Coated surfaces, carrying alumina layers having thicknesses between 0.6 μm and 3.6 μm show a significantly reduced oxidation rate when compared with uncoated reference surfaces, even if heating temperature is increased up to 900 °C and heating time is extended to 800 h. Alumina layers were prepared via sol-gel processing using a modified Yoldas procedure to obtain alumina sols. No change in rheological sol behavior was observed for more than 1 year of aging under static conditions at room temperature. Depending on pH value, modified Yoldas sols contain a manifold of Al species, among them Al13 polycations. Thermal evolution of sol derived alumina powders depends on Al speciation of parent sols. Depending on sol composition, both gamma-Al2O3 and eta-Al2O3 occur as intermediate transition aluminas. Phase composition and gas phase velocity influence oxygen permeability of thin layers prepared with modified Yoldas sols.

Das Ziel der Arbeit war die Weiterführung der Substrukturanalyse von plastisch verformten metallischen Werkstoffen mit kubischer Struktur auf der Basis des Disklinationskonzeptes unter Berücksichtigung unterschiedlicher homologer Temperaturen und Stapelfehlerenergien. Die Untersuchungen wurden an Stauchproben von Wolfram, Aluminium und der Legierung Inconel 718 mit Hilfe der TEM, der REM (EBSD) und der röntgenografischen Profilanalyse durchgeführt. Bei allen drei Werkstoffen konnten im Temperatur- und Umformbereich T/Ts<=0,3, Deformation>=0,3 auf der Basis von lokalen TEM-Desorientierungsmessungen Partialdisklinationen identifiziert und ihre Frankvektoren bzw. Defektstärken bestimmt werden. Die Ergebnisse der EBSD-Untersuchungen und der Röntgendiffraktometrie sind mit den TEM-Beobachtungen kompatibel und zeigen, dass die Kombination der drei Methoden für die systematische Substrukturanalyse an stark verformten Werkstoffen gut geeignet ist.