The effect of surface preparation on the precipitation of sigma during high temperature exposure of S32205 duplex stainless steel

Although the formation of sigma phase in duplex stainless steels is reasonably well documented, the effect of surface finish on its formation rate in surface regions has not been previously noted. The growth of the sigma phase precipitated in the subsurface region (to a maximum depth of 120 μm) has been quantified after heat treatment of S32205 duplex stainless steel at 1073 K (800˚C) and 1173 K (900˚C) after preparation to two surface finishes. Here, results are presented that show that there is a change in the rate of sigma phase formation in the surface region of the material, with a coarser surface finish leading to a greater depth of precipitation at a given time and temperature of heat treatment. The growth rate and morphology of the precipitated sigma has been examined and explored in conjunction with thermodynamic equilibrium phase calculations

The use of holographic optical elements (HOE's) to investigate the use of a flat irradiance profile in the control of heat absorption in wire-fed laser cladding

This work investigates the use of holographic optical elements (HOE's) to control the applied heat profile and thermal absorption of a wire used for laser cladding. The two thermal distributions compared were a circular beam with a Gaussian heat profile (Gaussian beam) and a square beam with a flat profile (pedestal beam). Heat absorption calculations between these were carried out to show the potential differences in absorption, with empirical results created to show how the differences in absorption affect the cladding properties. Micrographs of the clad cross-sections were created using optical microscopy and were analysed with respect to wetting angle and clad dilution. These results were compared to an alternative method of applying an even beam profile; enlarging the Gaussian beam relative to the wire diameter. The results showed that the use of a HOE to create a more even beam profile gave superior wetting behaviour and less dilution

Interface study by dual-beam FIB-TEM in a pressureless infiltrated Al(Mg)–Al2O3 interpenetrating composite

This paper considers the microstructures of an Al(Mg)/Al2O3 interpenetrating composite produced by a pressureless infiltration technique. It is well known that the governing principle in pressureless infiltration in Al/Al2O3 system is the wettability between the molten metal and the ceramic phase; however, the infiltration mechanism is still not well understood. The objective of this research was to observe the metal / ceramic interface to understand the infiltration mechanism better. The composite was produced using an Al-8wt.% Mg alloy and 15% dense alumina foams at 915°C in a flowing N2 atmosphere. After infiltration, the composite was characterized by a series of techniques. Thin film samples, specifically produced across the Al(Mg)-Al2O3 interface, were prepared using a Dual Beam Focused Ion Beam (FIB) and subsequently observed using Transmission Electron Microscopy (TEM). XRD scan analysis shows that Mg3N2 formed in the foam at the molten alloy-ceramic infiltration front whilst TEM analysis revealed that fine AlN grains formed at the metal / ceramic interface and MgAl2O4 and MgSi2 grains formed at specific points. It is concluded that it is the reactions between the Al, Mg and N2 atmosphere that improve the wettability between molten Al and Al2O3 and induce spontaneous infiltration

Processing of ceramic-metal interpenetrating composites

Interpenetrating composites consist of 3-dimensionally interpenetrating matrices of two different phases; in the present work they were alumina and aluminium-magnesium alloys and were produced by infiltrating ceramic foams with molten alloys. The foams were made by mechanically agitating ceramic suspensions to entrain gases and then setting the structure via the in-situ polymerisation of organic monomers, a process known as gel casting. This resulted in the foams having a very open and interconnected structure that could be easily infiltrated by the molten metals. Previous composites have been produced at Loughborough University using squeeze casting; however, whilst infiltration was usually accomplished in a matter of seconds, the resulting size and shape of the composite was limited. Hence the present work has focused on investigating the potential for using gravity-fed infiltration. Whilst this was much slower, often taking several minutes, when optimised it is believed it will offer the potential for the production of large and complex-shaped pieces. The composites were produced at atmospheric pressure by infiltrating 2-10 wt.% magnesium content Al-Mg alloys into 20% dense Al2O3 foams with highly interconnected porosity. The processing parameters of temperature, ≥ 900°C, and atmosphere, flowing N2- Ar, were investigated to determine the processing window and infiltration kinetics. In-situ observation of the process shows that infiltration is faster at higher temperatures, Mg contents and N2 partial pressures. Both optical and scanning electron microscopy (SEM) have been used to characterize the composites

Using wire shaping techniques and holographic optics to optimise deposition characteristics in wire-based laser cladding

In laser cladding, the potential benefits of wire feeding are considerable. Typical problems with the use of powder, such as gas entrapment, sub-100% material density and low deposition rate are all avoided with the use of wire. However, the use of a powder-based source material is the industry standard, with wire-based deposition generally regarded as an academic curiosity. This is because, although wire-based methods have been shown to be capable of superior quality results, the wire-based process is more difficult to control. In this work, the potential for wire shaping techniques, combined with existing holographic optical element knowledge, is investigated in order to further improve the processing characteristics. Experiments with pre-placed wire showed the ability of shaped wire to provide uniformity of wire melting compared with standard round wire, giving reduced power density requirements and superior control of clad track dilution. When feeding with flat wire, the resulting clad tracks showed a greater level of quality consistency and became less sensitive to alterations in processing conditions. In addition, a 22% increase in deposition rate was achieved. Stacking of multiple layers demonstrated the ability to create fully dense, three-dimensional structures, with directional metallurgical grain growth and uniform chemical structure

Microstructure and texture development during solid consolidation recycling of Ti-6Al-4V

Ti-6Al-4V machining chips were recycled using equal channel angular pressing (ECAP). The microstructural and texture evolution of the recycled Ti-6Al-4V have been investigated using scanning electron microscopy, electron backscattered diffraction and transmission electron microscopy. For samples consolidated at 500 °C with a back-pressure of 100 MPa, the as-pressed density reached up to 99.9% after 8 passes. Pronounced grain refinement was also observed with increasing number of passes. The morphology of the grains has been changed from elongated and coarse to equiaxed and ultrafine as the number of passes increases. Strong textures were also introduced during multiple passes via Bc route. Texture has been developed with basal planes parallel to the inclination direction which is at 21° of the extrusion direction. After 4 and 8 passes, basal planes were rotated towards the transverse direction. No oxide can be detected at the chip-chip boundaries when the Ti-Al-4 V machining chips was consolidated at 500 °C. When the sample was processed at 550 °C, significant grain growth and clear oxide layers at the chip-chip interface were observed. In addition, the c-axis were rotated towards the longitudinal direction due to the non-basal slip activity. TEM observation revealed the dislocations presence in the 550 °C ECAP-processed sample

Weld pool shaping and microstructural control using novel computer generated holographic optic laser welding of steel and stainless steel

This work considers the use of Holographic Optical Elements (HOEs) to shape the weld beam and control the microstructure of the weld bead. The beam profiles investigated are a standard Gaussian and an Offset Rugby Post produced by a HOE. Autogenous welds have been produced on plain carbon steel with the introduction of a nickel alloy filler powder, using different energy densities. Cross sections of the welds have been analysed in terms of the weld profile, weld pool shape, HAZ and the extent of the deposit/substrate mixing. Electron BackScatter Diffraction (EBSD) coupled with Energy Dispersive X-ray Spectroscopy (EDS) has been used to study the microstructures developed. The results have shown that by utilising HOE’s the weld pool shape can be modified so that a squarer profile can be obtained. The grain structure within the weld pool can be controlled such that a finer more equiaxed grain structure can be developed when compared with the coarse columnar grains seen with a Gaussian beam with a marked difference in the microstructures in the HAZ

The use of holographic optics for heat flow control in wire-based laser cladding

Laser cladding with wire utilises a focussing lens to melt the surface of the substrate, into which the wire is fed to build up a clad track on the surface. Process reliablity issues in practice include; clad tracks with high levels of dilution, surface cracking and other defects. Key to this is wire reflectivity calculations. Here using Fresnel equations that relate angle of incidence to heat absorption, we are able to show a direct correlation between the applied heat profile of the laser beam and the absorption profile of the wire surface; this has been modelled using COMSOL multiphysics conduction simulations which showed that the heat profile of the applied laser beam has a direct effect on the size and shape of the resulting melt pool. Using computer generated Holographic Optical Elements (HOE), a novel form of optic that alters the heat profile of the laser beam to a user-specified 3d profile, a conventional 1.25 mm diameter Gaussian beam shape and a 1.25 mm square uniform ‘pedestal’ HOE-derived beam shape were tested and compared, using a 1 mm diameter AISI 316 stainless steel wire on a 0.8mm mild steel substrate. These results were also compared to an enlarged 3.5 mm diameter Gaussian beam, in order to evaluate different methods of altering the heat distribution applied to the wire. The HOE generated beam gave superior results, due to its shorter thermal cycle, which reduced the amount of heat going into the clad track and resulted in lower dilution

An investigation on oxidation/carburisation of 9Cr-1Mo steel heat exchanger tube in an AGR environment

9Cr-1Mo steels have been used extensively in the power generation industry. In this study, a wide range of experimental samples exposed at different times and temperatures in a CO2 environment were analysed to look at the development of the metal and oxides over time. The main objective of this work was to obtain a better understanding of the carburisation and oxidation behaviour of 9Cr 1Mo steels as a function of temperature/time, with special attention paid to the transition from protective to breakaway oxidation. In addition, experiments were also carried out to investigate any links between oxidation transition and carburisation behaviour of these materials

Grain size control in the weld pool and heat affected zone using holograms

This paper considers the use of Holographic Optical Elements (HOEs) to shape the weld beam and hence control the grain size of the weld bead and the grain growth and phase transformations in the HAZ. Welds have been produced on carbon steel with the introduction of a nickel based filler powder, using different energy densities produced by the HOEs. Cross sections of the welds have been analysed in terms of the weld profile, weld pool shape and grain size in the deposit and the HAZ. Electron BackScatter Diffraction (EBSD) coupled with Energy Dispersive X-ray Spectroscopy (EDS) has been used to study the microstructures developed. The results have shown that by utilising HOEs the grain size within the weld pool can be controlled such that a more equiaxed grain structure is developed when compared with the coarse columnar grains seen with a Gaussian beam