D.G. McCartney

The effect of electromagnetic stirring during solidification on the structure of Al-Si alloys

A series of Al-Si alloy ingots were directionally solidified vertically downwards in a rectangular cavity with varying conditions of bulk liquid flow obtained with a controllable electromagnetic stirrer. Examination of their macrostructures showed that the columnar—equiaxed transition was promoted both by increasing bulk liquid velocity and increasing silicon content. Temperature measurements during solidification showed that the electromagnetic stirring resulted in the rapid removal of bulk liquid superheat and therefore the impact of a relatively cold liquid on the solidification front. Fluid flow modelling showed that bulk liquid velocities of up to 0.25 m s−1 were obtained. Fragmentation of the solidification front was the most probable mechanism for the promotion of the columnar—equiaxed transition with high velocity flows. However, with low velocity flows, such as were obtained with natural convection or a limited application of electromagnetic stirring, the columnar—equiaxed transition was probably significantly influenced by heterogeneous nucleation effects. Evidence for this came from comparing the behaviour of alloys prepared using aluminum and silicon of different levels of purity.

Microstructure formation and corrosion behaviour in HVOF-sprayed Inconel 625 coatings

The nickel-based alloy Inconel 625 was thermally sprayed by two different variants of the high velocity oxy-fuel process. In this study, coatings deposited by a liquid-fuelled gun were compared with those produced by a gas-fuelled system; in general, the former generates higher particle velocities but lower particle temperatures. Investigations into the microstructural evolution of the coatings, using scanning electron microscopy and X-ray diffraction, are presented along with results on their aqueous corrosion behaviour, obtained from salt spray and potentiodynamic tests. It is inferred from coating microstructures that, during spraying, powder particles generally comprised three separate zones as follows: fully melted regions; partially melted zones; and an unmelted core. However, the relative proportions formed in an individual powder particle depended on its size, trajectory through the gun, the gas dynamics (velocity/temperature) of the thermal spray gun and the type of gun employed. Cr2O3 was the principal oxide phase formed during spraying and the quantity appeared to be directly related to the degree to which particles were melted. The salt spray test provides a sensitive means of determining the presence of interconnected porosity in coatings and those produced with the liquid-fuelled gun exhibited reduced interconnected porosity and increased corrosion resistance compared with deposits obtained from the gas-fuelled system. In addition, potentiodynamic tests revealed that passive current densities are 10–20 times lower in liquid-fuel coatings than in those sprayed with the gas-fuelled gun.

Microstructural characterisation of a Ni-Cr-B-C based alloy coating produced by high velocity oxy-fuel thermal spraying

Abstract High velocity oxy-fuel (HVOF) thermal spraying was used to deposit coatings, approximately 200 μm thick, of a commercially available Ni-Cr based alloy containing boron and carbon [Ni-23.5Cr-3.8Cu-0.8Fe-5.9Mo-3.4W-2.8B-4.3C (at.%)]. Powder and coating microstructures were investigated by a combination of X-ray diffraction, scanning electron and transmission electron microscopy. The coatings had layered morphologies due to the deposition and solidification of successive molten or semi-molten splats. The splat microstructures consisted of a Ni-rich metallic matrix containing a small fraction of M 23 C 6 particles ∼50 nm in size. The Ni-rich matrix had a predominantly crystalline structure with only a small fraction of amorphous phase regions. Cr 2 O 3 and NiCr 2 O 4 oxide phases occurred in the form of either intersplat lamellae or globules, with Cr 2 O 3 being the predominant oxide. Coating microhardness values were found to be ∼6.0 GPa which is significantly higher than that of B- and C-free nickel-based alloys similarly deposited.

Thermally sprayed Ni(Cr)–TiB2 coatings using powder produced by self-propagating high temperature synthesis: microstructure and abrasive wear behaviour

Abstract High velocity oxy-fuel (HVOF) thermal spraying was used to deposit Ni(Cr)–TiB2 cermet coatings onto steel substrates from a feedstock powder produced by a self-propagating high-temperature synthesis (SHS) reaction. The powder and coating microstructures were investigated by scanning electron microscopy and X-ray diffraction (XRD), whilst dry sand rubber wheel abrasive wear and microhardness tests were performed on the coatings. The SHS reacted powder was found to comprise principally 1–5 μm sized TiB2 in a crystalline, Ni-based solid solution matrix, with approximately 60 vol.% TiB2 and 40 vol.% metallic binder. XRD revealed the presence of only around 2% of the following phases: TiB, Ni2B, NiTi, TiO. The reacted compact was crushed, ground and classified into a powder of size range (+8 to −38) μm for thermal spraying. The HVOF sprayed deposits had layered, splat-like morphologies typical of thermally sprayed cermets. The coating microstructure consisted primarily of TiB2 in the Ni-rich binder phase but the boride fraction was reduced compared to that of the feedstock powder as a result of TiB2 dissolution in the molten alloy during spraying. XRD analysis of the coating indicated that the binder solidified to, in part, an amorphous/nanocrystalline structure with ∼2% of Ti-containing oxide phases also present. The TiB, Ni2B and NiTi phases were not detected in the coating. Under the wear conditions employed, angular alumina generated significantly higher wear rates than the rounded silica abradent, and the wear mechanism changed from microcutting with alumina to pull-out with silica. The wear coefficients obtained with the coating were significantly reduced compared with those of mild steel, in the case of alumina abrasive a factor of four reduction. Additionally, a significant improvement in wear resistance was also found when compared to HVOF-sprayed chromium carbide–nickel/chromium cermet coatings produced from commercially available powders.

Microstructure formation in high velocity oxy-fuel thermally sprayed Ni-Cr-Mo-B alloys

High velocity oxy-fuel (HVOF) thermal spraying was used to deposit Ni‐Cr‐Mo‐B alloy coatings, approximately 200 mm thick, onto mild steel substrates. Gas atomised powders of three different compositions were used in the experiments. Powder and coating microstructures were investigated by a combination of X-ray diffraction, scanning electron and transmission electron microscopy and differential thermal analysis (DTA). The coatings had layered morphologies due to the deposition and solidification of successive molten or semi-molten splats. The splat microstructures consisted of a Ni-rich metallic matrix containing a small fraction of M3B2 particles 10‐30 nm in size and with a tetragonal crystal structure. The Ni-rich matrix comprised both crystalline and amorphous regions. The former was predominantly nanocrystalline with a grain size of ca. 50 nm and DTA showed that the crystallization temperature of the amorphous phase varied from 800 to 860 K depending on the alloy constitution. Cr2O3 and NiCr2O4 oxide phases occurred in the form of either intersplat lamellae or globules. The thin lamellar oxides exhibited the a-Cr2O3 crystal structure whereas globular oxides, up to 1 mm in size, were found with both a-Cr2O3 and NiCr2O4 structures.

The corrosion behavior and microstructure of high-velocity oxy-fuel sprayed nickel-base amorphous/nanocrystalline coatings

The corrosion characteristics of two Ni-Cr-Mo-B alloy powders sprayed by the high-velocity oxy-fuel (HVOF) process have been studied using potentiodynamic and potentiostatic corrosion analysis in 0.5 M H2SO4. The deposits were also microstructurally characterized using x-ray diffraction (XRD), scanning electron microscopy (SEM) (utilizing both secondary electron and backscattered electron modes), and transmission electron microscopy (TEM). Results from the microstructural examination of the two alloys have revealed a predominantly amorphous/nanocrystalline face centered cubic (fcc) matrix containing submicron boride precipitates as well as regions of martensitically transformed laths.Apparent recrystallization of the amorphous matrix has also been observed in the form of cellular crystals with a fcc structure. The oxide stringers observed at splat boundaries were found to be columnar grained α-Cr2O3, though regions of the spinel oxide NiCr2O4 with a globular morphology were also observed. The coatings of the two alloys exhibited comparable resistance to corrosion in 0.5 M H2SO4, as revealed by potentiodynamic tests. They both had rest potentials approximately equal to −300 mV saturated calomel electrode (SCE) and passive region current densities of ∼1 mA/cm2. Microstructural examination of samples tested potentiostatically revealed the prevalence of degradation at splat boundaries, especially those where significant oxidation of the deposit occurred.

The effect of electromagnetic stirring on macrostructure and macrosegregation in the aluminium alloy 7150

Ingots of aluminium alloy 7150 (Al-Zn-Mg-Cu) were unidirectionally solidified vertically downwards under conditions of natural convection and electromagnetic stirring of the bulk liquid. Increased fluid flow in the bulk liquid was found to promote the formation of equiaxed grains and hence the columnar-equiaxed transition (CET). Increased fluid flow was also associated with an increase in macrosegregation of the alloying elements. The extent of segregation was greatest for Cu and least for Zn. The experimental observations suggest that macrosegregation in the ingots was caused by the displacement of solute-enriched liquid from the mushy zone as a result of the electromagnetically stirred bulk liquid being driven into the solidification front.

Two-dimensional tungsten oxide nanowire networks

The authors report the synthesis and characterization of two-dimensional (2D) single crystalline nanonetworks consisting of tungsten oxide nanowires with diameters of ca. 20nm. The 2D networks are believed to result from the nanowire growth along the four crystallographic equivalent directions of ⟨110⟩ in the tetragonal WO2.9 structure. These 2D tungsten oxide networks may be potential precursors for creating 2D networks comprising WS2 nanotubes.

Observations on the microstructure and performance of an AlTiC grain-refining master alloy

Abstract AlTiC grain-refining master alloys are of increasing importance in aluminium casting because they are believed to introduce a smaller volume fraction of insoluble particles into the melt than conventional AlTiB master alloys are. The main microstructural features of an Al-6wt.%Ti-0.02wt.%C master alloy have been examined using X-ray diffraction metallic phases were identified, namely titanium aluminide and titanium carbide. The titanium aluminides were found to contain small amounts of dissolved vanadium and to possess a range of morphologies from well faceted to rough and irregular. A combination of microanalysis and electron diffraction was used to show that clusters of submicron particles, rich in Ti but with virtually no Al, were TiC crystals. Individual particles were found to be single crystals with an octahedral morphology. A simple thermodynamic analysis of the stability of TiC in the Al-rich corner of the AlTiC phase diagram is presented which confirms the stability of TiC during master alloys synthesis. Using standard test procedures, the AlTiC alloy proved to be an effective grain refiner when added to commercial purity aluminium, and it is proposed that undissolved TiC particles were most likely to have been the heterogeneous nuclei.

Bonding between aluminium and copper in cold spraying: story of asymmetry

Abstract The bonding mechanism in cold spraying is still a matter of some debate, which requires further investigation. In the present work, aluminium powder was cold sprayed onto a copper substrate and copper powder was cold sprayed onto an aluminium substrate using the same process gas and spray parameters. Separate experiments were performed to produce thick (∼400 μm) coatings and isolated particle impacts. Deposits were characterised using scanning electron microscopy and image analysis. The coating–substrate interfacial bonding was assessed via a method in which, following a short heat treatment at 400°C, intermetallics grow at the interface where metal to metal contact has been established. In addition, the bond strength values of deposits were determined using a standard pull-off test. It was found that the copper particles deposited onto an aluminium substrate resulted in significant substrate deformation, whereas aluminium particles caused minimal deformation of the copper substrate. Furthermore, the former displayed a higher degree of metallurgical bonding at the coating/substrate interface in comparison with the latter. These results suggest that the removal of oxide films from the surfaces was greater when copper was the material being sprayed rather than aluminium. The impact behaviour of the two materials and the removal of oxide due to deformation at high strain rate are discussed with the aid of the Johnson–Cook plasticity model.