This thesis studies dwell fatigue crack growth in an advanced nickel disc alloy RR1000 at elevated temperature. RR1000 with different grain sizes and γ’ precipitate distributions are tested at both 650 and 700°C to investigate the effect of microstructure and testing temperature on dwell fatigue crack growth. Dwell fatigue tests with different dwell times at the peak load are conducted to study the effect of dwell time on crack growth rate and crack propagation mechanism. A transition from cycle-dependent transgranular fatigue crack growth to time-dependent oxide induced intergranular crack growth is found when the dwell time exceeds a critical value. Linear elastic stress intensity factor (K) threshold values for intergranular crack growth under sustained load are also measured to evaluate conditions when intergranular crack growth occurs progressively during the dwell period. Impacts of prior dwell fatigue loading on subsequent fatigue crack growth are also studied.

Martensitic 9-12%Cr-based creep-resistant steels are currently employed in a number of power generation applications as a cheaper substitute for the more expensive Ni-based superalloys. A tempered martensite microstructure with a uniform, fine prior austenite grain size is desired to give the required strength and creep-resistant properties at service entry. Refinement of austenite grain size can be achieved via recrystallisation. During hot opendie forging of high-volume rotor discs utilised in power plants non-uniformities of parameters determining austenite grain size, such as temperature, initial grain size, strain and strain rate across the component occur. The main focus of the research was to gain a comprehensive understanding of the influence of the aforementioned parameters on the development of grain size and kinetics of recrystallisation. Experimental efforts were channelled towards the investigation of static recrystallisation as the process governing the grain size development between the forging sequences. Experimental material for this study was provided by the industrial partner, FOMAS, in a form of a 5-tonne ESR ingot of FB2 steel, stabilised at 760 °C for 24 hours, heat treated at 1150 °C for 10 hours and tempered. Cylindrical axisymmetric test specimens were deformed on the Gleeble 3500TM Thermomechanical Simulator in uniaxial compression mode in the range of temperatures 900-1200 °C, strains 0.15-0.45, strain rates 0.1-5 s-1 and two different initial grain sizes, utilising stress relaxation and double hit methods. Stress relaxation softening behaviour over the range of experimental conditions has been analysed with the final recrystallised fraction being confirmed by optical microscopy. Aspect ratio development, along with the grain size, have been employed as a recrystallisation progress criterion. Considerable scatter and unusual shapes have been shown by the softening curves during the stress relaxation. Deviations in the amount of recrystallisation, when compared with predictions based on the Dutta-Sellars equations have been identified and discussed, based on a modification of the Avrami exponent used. Grain size measurements allowed the necessary parameters for the modelling of recrystallised grain size to be obtained.

The first phase of this thesis characterised the currently commercially available L169 and A201 aluminium alloys in terms of their response to testing at the operating parameters predicted for next generation aero-engine components. The L169 and A201 alloys were initially subjected to ageing trials at 205°C, specimens of both alloys were then fatigue tested at ambient temperature and at 205°C following 1000 hours exposure at 205°C. Detailed micrographic characterisation was undertaken to assess the impact of prolonged elevated temperature exposure on the alloy microstructure. Fractography was undertaken on the failed fatigue specimens to assess the impact of ageing temperature and temperature exposure on fatigue behaviour. The L169 alloy exhibited a significant reduction in properties following 1000 hours exposure at 205°C due to extensive precipitate coarsening. The A201 exhibited comparably better elevated temperature performance due to the increased stability of the Ω- phase precipitate however the extensive shrinkage porosity observed in the alloy had a negative impact on fatigue performance and will limit its use in a pressure tight environment. In addition to the investigation into currently commercially available alloys a detailed investigation was taken into a novel dilute aluminium-copper based castable metal matrix composite with the potential for use at elevated temperatures. This alloy exhibits unique solidification mechanisms which result in an increased resistance to conventional aluminium copper alloy casting defects such as shrinkage porosity, segregation and hot tearing. A detailed investigation was undertaken to assess the impact of chemical composition on the alloys unique solidification behaviour and to assess whether there was any possibility for further optimisation. Following on from this investigation the alloy was characterised in similar terms to the L169 and A201 alloys in terms of its fatigue behaviour at both ambient and elevated temperatures to provide an assessment of the alloys potential to meet the predicted next generation aero-engine component operating conditions.

Burr formation and poor hole quality can be detrimental to fatigue life as well as hinder the assembly and functionality of drilled components, particularly those made from metallic-composite stacks. Following a detailed literature review, four phases of experimental work were carried out to evaluate the effects of varying cutting parameters, tool geometries/coatings, workpiece configuration and machining strategies on hole quality/integrity, burr formation and subsequent workpiece fatigue performance, following drilling of several 2-layer stack configurations (CFRP/AA2024, AA2024/AA7010 and CFRP/Ti-6A1-4V). This was complemented by the development of numerical models to predict burr formation when drilling metallic alloys. Key contributions of the research were: (i) improved understanding with regard to the influence of cutting speed and feed rate on burr formation and hole quality when drilling individual Ti-6A1-4V, AA2024 and AA7010 alloys together with various stack assemblies; (ii) identification of appropriate tool type and cutting conditions in addition to possible alternative burr suppression methods; (iii) understanding of the significance of burr formation on fatigue life of individual metallic materials and; (iv) derivation of an analytical model for entrance and interlayer burrs as well as formulation of an FE model for enhanced burr formation (entrance and exit) predictions when drilling individual metallic materials.

Understanding of the mechanism of the vibration needed to fill thin section or one with sharp edges in profile shapes and clarifying the dominant control parameters of the vibration in thin wall investment casting is key to producing sound casting (one free of misrun defects). It's also a central issue for study in this thesis. The filling capability in thin wall investment casting method was assessed in relation to metal head. It was found that the effect of the vibration on the metal head is markedly dependent on acceleration. Generally, it was observed that the metal head required to force the metal in thin sections in the casting vibrated at (1g) acceleration is approximately half that used in castings made without vibration. Two potential mechanisms were observed from the experimental result during the filling process in thin wall casting i] discontinuous propagation flow in vibration conditions; and ii] continuous propagation flow without vibration. These mechanisms may be acting to modify the contact angles between liquid metal and a wall of the mould. Experiments also showed that two features of the transition can be observed from the front of the morphology; i] a coherent liquid metal front - this occurs in thin wall investment casting when the acceleration due to vibration is less than (1g); and ii] jetting at the free surface - this occurs in thin wall investment casting when the acceleration due to vibration exceeds 1g. This is present in terms of a unifying concept, using a frequency and amplitude ( f - a ) map. The time of the vibration operation has a moderate effect on the relative filling area when the acceleration is less than 1g. However, it is more effective when the acceleration of the vibration is greater than 1g. The mathematical models comprised one-dimensional heat transfer with phase change and had an established flow field for molten A356 alloys flow in the thin section ceramic channel mould. The work was concerned with the fluidity of A356 alloys in thin wall investment casting with and without vibration in two type of filling (flowability and fillability filling types), combining heat and metal flow in addition to the simultaneous solidification stage. The results of the mathematical model, produced agreement with the experimental test carried out in the foundry and also agreed with other published data. The results on fluidity indicated that the fluidity of the molten metal was affected by mould temperature, pouring temperature, the velocity of the molten metal flow relative to the surface tension and the channel thickness. The data used in the mathematical model of the fluidity in thin section under vibration condition were deduced experimentally; namely, velocity of the molten metal and the heat transfer coefficient between the liquid metal and the chilled surface of the mould. This model was used to estimate the fluidity characteristics in thin wall investment casting with and without vibration. Real-time X-ray observation and computer modelling of the metal head-driven mould filling sequences reveal that no surface turbulence occurred when the liquid metal flowed into the thin section and the advance metal front continued to flow under surface tension control. X-ray was also used to measure the flow time and the velocity of the metal inside the thin channel and confirm the modification on Bernoullis Equation (kinetic energy+ potential energy = constant) to estimate the velocity relative to surface tension in the fluidity mathematical model. Flow-3D software was used to calculate the velocity of the liquid metal in the flowability filling type and the fluidity characteristics. Weibull analysis identifies the acceleration vibration as practical criterion to judge the reliability of casting. A vibration mould with vertical direction in the thin wall investment casting after filling can make the liquid metal flow into the thin section under surface tension control. This technique is used to achieve mould filling free from misrun defects and surface turbulence and this makes vibration casting a promising technique for producing high quality castings. On the basis of these findings, an operation window for the production of reliable castings has for the first time been developed in this research.

No description available.

A high γ' volume fraction Ni-base superalloy (RR1000) has been studied and its microstructural and mechanical response to the inertia welding process assessed. The bond line microstructure has been characterised in terms of process parameters and associated modelled temperature distributions. The high temperature mechanical behaviour has been interrogated via sustained load crack growth testing in air and vacuum. The weld microstructure is characterised by a uni-modal distribution of ultrafine γ' and a meta-dynamically recrystallised grain structure. The recrystallised grain size is determined by the width of the shear zone and the associated deformation behaviour, which varies with process parameter selection. Of particular importance is the welding pressure, which controls the upset rate, thereby limiting the shear zone width. A restricted shear zone can be related to increases in the peak bond line temperature and cooling rate. The high temperature crack growth behaviour is controlled by grain boundary oxide formation and crack tip stress state. In inertia welded RR1000 this stress state is governed by the reprecipitated γ'. The steady state crack growth rate increases with temperature, which is due to an increased rate of oxide formation. Near threshold growth behaviour is also dependent on localised microstructural features.

The adoption of lithium-ion and/or super-capacitor battery technologies is a current hot topic in the automotive industry. For both battery types, the terminals and busbars are manufactured from copper (Cu) and/or aluminium-based (Al-based) alloys, as a result of their high electrical and thermal conductivities. Laser welding is considered an attractive process to industry due to its easy auto-motability, high processing speed and highly repeatable cost-effective processing. However, laser welding of monometallic and dissimilar Cu and A1 presents several difficulties due to the high surface reflectivity at infrared (IR.) wavelengths. Three main areas were investigated in this research work: • Laser beam lap-welding of monometallic Cu sheets, with the aim of validating the developed welding procedures against target specifications addressed by the automotive industry, in line with production environment setups. • The suitability of using tailored energy distributions, produced using a 1070nm laser source delivered through a laser beam scanner, for welding multiple overlapping Al or Cu foils. • The effects of different laser processing parameters on the formation of deleterious brittle intermetallic phases when welding Alto Cu with a continuous-wave Yb-fibre laser.

Linear friction welding (LFW) is a solid-state welding process (i.e. the melting temperature of the material is not reached) that is used for the fabrication of titanium alloy bladed discs (Blisks) in the fan and compressor stage of modern aero engines. Blisk technology enables a 20-30 % weight saving through removing the need for the dovetail attachment thus enabling slimmer, lighter and more streamlined disc architecture. This significant weight saving helps the aero engine meet environmental targets (ACARE, 2000) by reducing fuel burn, noise and emissions as well as eliminating fretting fatigue around the dovetail attachment and extending component life. LFW is considered to be a self-cleaning process where contaminants trapped within the plasticised layer are expelled into the flash thus producing a high integrity weld that is stronger than the parent material. This high integrity is critical for Blisk application as contaminants in the final weld joint may result in reduced component life or sudden unexpected failure. Despite the importance of weld integrity for Blisk applications, little is known about the underlying process physics of the cleaning regime therefore the aim of this thesis is to provide a thorough mechanistic understanding of the weld evolution of Ti- 6Al-4V LFW by empirically evaluating weld efficiency, material flow behaviour and weld cleaning behaviour over a range of appropriate key process variables.

An evaluation of inertia friction welds in RR1000 has been conducted through microtensile and sustained load crack growth tests on pre-cracked testpieces. As-welded and two post weld heat treatment conditions have been considered. The fatigue crack growth behaviour of γ TiAl has also been considered, with a focus on establishing fatigue crack growth thresholds.