B.G. Mellor

Multiple material additive manufacturing – Part 1:a review

Interest in multifunctional structures made automatically from multiple materials poses a challenge for todays additive manufacturing (AM) technologies; however the ability to process multiple materials is a fundamental advantage to some AM technologies. The capability to fabricate multiple material parts can improve AM technologies by either optimising the mechanical properties of the parts or providing additional functions to the final parts. The objective of this paper is to give an overview on the current state of the art of multiple material AM technologies and their practical applications. In this paper, multiple material AM processes have been classified and the principles of the key processes have been reviewed comprehensively. The advantages and disadvantages of each process, recent progress, challenging technological obstacles, the possible strategies to overcome these barriers, and future trends are also discussed.

Fracture toughness and crack morphologies in eroded WC–Co–Cr thermally sprayed coatings

Abstract The fracture toughness of two thermally sprayed tungsten carbide/cobalt–chromium coatings of nominally identically composition, 86WC–10Co–4Cr, one produced by the Detonation Gun (D-Gun) process and one by the High Velocity Oxy-fuel (HVOF) process, has been determined by the indentation method. Indentation testing revealed that both coatings exhibited anisotropic crack propagation and hence, fracture toughness, crack propagation being much easier parallel to the coating/substrate interface than transverse to it. The sample produced by the D-Gun process had a lower fracture toughness, and greater range of fracture toughness values, both parallel and transverse to the coating/substrate interface, than the HVOF coating reflecting the greater microstructural inhomogeneity of this coating. The subsurface cracks produced by indentation have a similar morphology to those produced on erosion testing and hence, the fracture toughness values obtained by this method are appropriate for use in erosion rate prediction equations. However, the range in fracture toughness values found should be taken into account when modelling erosion as regions of low fracture toughness, where subsurface crack growth will be easier, will tend to control the rate of erosion. The cracks produced by indentation testing and erosion tend to propagate along the boundary between material that has been completely molten during thermal spraying and the inner core of the projected particle.

Sand erosion performance of detonation-gun applied tungsten carbide/cobalt-chromium coatings

Abstract Sand erosion studies of thermally sprayed WCCoCr (Denotation-Gun LW45) have been undertaken using a sand/water jet impingement rig. Results are presented which show that the erosion rate of sprayed compared to sintered tungsten carbide-cobalt-chrome is similar for low energy impacts but the sintered material outperforms by 4 times the sprayed material for high energy impacts. This reflects the anisotropic microstructure of the thermally sprayed coating with a preferred crack propagation parallel to the coating surface followed by crack interlinking and spalling. This is the dominant erosion mechanism present. A minor erosion mechanism consists of micro-cutting and ploughing at low angles of particle impact. The coatings have a relatively high density of defects including thermal stress induced transverse cracks, voids, oxides, and grit blasting remnants. Such defects are shown to accelerate the erosion process considerably because they aid crack initiation and growth leading to partial, mono or multi-splat spalling of loose material. The influence of slurry jet angle was found to be more pronounced under low energy conditions where maximum erosion occurred at 90° and the minimum at 30° in contrast to the high energy erosion rates which were independent of jet angle. This is a result of the lower levels of fluctuating stresses imparted to the coating during low energy impacts leading to the impact angle having a greater effect on sub critical growth rate than for the high energy conditions.

Surface coatings for protection against wear

Understanding surface wear in engineering materials Mechanic testing of coatings The range of surface coating methods Chemical vapour deposition methods for coating against wear Physical vapour deposition methods for protection against wear Electroless plating for protection against wear Electroplating for protection against wear Thermal spraying methods for protection against wear Welding surface treatment methods for protection against wear Laser surface treatment methods for protection against wear Future trends in surface coatings for protection against wear.

Wavelet analysis of electrochemical noise measurements during corrosion of austenitic and superduplex stainless steels in chloride media

A wavelet variance analysis procedure has been used for the characterisation of electrochemical noise measurements (ENM) made during corrosion of three grades of austenitic and superduplex stainless steels, with varying sulphur contents, when exposed to chloride media. The resulting electrochemical noise has been interpreted in terms of coupled current and free corrosion potential measurements. The wavelet variance exponent has been used to characterise the behaviour of the signals. It is shown that the wavelet analysis technique is able to discriminate between various corrosion processes covering a wide range of ENM.

Acoustic emission study on WC–Co thermal sprayed coatings

Thermally sprayed coatings contain residual stresses that are produced in the spraying process. These may reduce the coating lifetime. In order to determine the optimum spraying conditions with respect to the residual stress level present, the acoustic emission (AE) during four-point bend tests on tungsten carbide–cobalt coatings sprayed onto mild steel substrates was investigated. Samples tested at different levels of deformation were studied by means of scanning electron microscopy and AE in order to understand the cracking mechanisms. Relationships between the number and amplitude of AE events detected and the type of cracking processes occurring were established. It has been possible to compare the residual stresses caused by the effect of different spraying parameters, such as coating thickness, spraying distance and high velocity oxy-fuel gun.

Sand erosion performance of CVD boron carbide coated tungsten carbide

The erosion performance and the interaction between the micro-mechanisms of erosion and the microstructure of a chemical vapour deposited boron carbide coating are presented. Samples were tested using both water–sand slurry and air–sand jet impingements at 90° incidence. Tests used angular quartz sand with a mean diameter between 135 and 235 ?m and jet impingement velocities between 16 and 268 m s?1. The chemical vapour deposition (CVD) boron carbide coatings were 15–20 ?m thick and deposited on a range of substrates of sintered tungsten carbide with 6 to 15 wt.% metal binder. The results, relative to the erosion resistance of the uncoated substrates, show the coatings to have higher resistance (10 times) under lower energy impacts but similar resistance at higher energy impacts. The sintered boron carbide had a similar erosion resistance to that of sintered tungsten carbide except at high energy impacts where it outperforms tungsten carbide and CVD boron carbide by a factor of 2. The performance of these coatings against erodent mass and impact energy are discussed and related to the nano and micro brittle fracture mechanisms identified by detailed microscopy and predicted by Hertzian cone crack theory. Partial concentric spalling of the coating was also evident in regions where circular Hertzian surface cracks are present. These erosion mechanisms, primarily nano-chipping and crack propagation, are also related to the microstructure and composition identified by XRD analysis and Raman spectroscopy. These results, in conjunction with fracture toughness and micro-hardness measurements, suggest that the coating composition is not pure B13C2 but has less erosion resistant forms of boron carbide present such as B50C2

The effect of fillers on the wear resistance of thermoplastic polymeric coatings

Abstract Thermoplastic polymer matrix composites are used as coating materials for the bore of downhole tubulars used as water injectors in the oil industry. These coatings are primarily employed for corrosion resistance but must also resist mechanical damage from the inspection tools lowered at speed down the tubing. This mechanical damage is produced by the wearing action of the supporting wire against the coating (wireline wear) and by direct impact of the tool against the coating. Filler materials are added to these polymeric coatings and these additions are known to affect the wear resistance of the coating. In this study, three types of thermoplastic polymeric coatings were subjected to wear tests. Both abrasive wear tests — using silicon carbide papers as the abrasive, and wireline wear tests, utilising a true tribocouple consisting of the coating and a length of “slickline” wire on a modified pin on disc apparatus — were carried out to study the wear resistance of these three coatings. Detailed scanning electron microscopy was performed on the wear tracks produced to elucidate the wear mechanism and in particular the role of fillers. In abrasive wear a polymer with a brittle filler has a higher wear rate than an unfilled polymer due to the fact that the brittle fillers can be easily fractured and detached from the polymer matrix. In general, the weak bond between the filler and a thermoplastic polymer matrix leads to the filler particles detaching from the matrix causing enhanced wear. In wireline wear the presence of voids and unmelted particles is particularly deleterious.

The mechanisms of long fatigue crack growth behaviour in Al–Si casting alloys at room and elevated temperature

Abstract Pistons are commonly made from multicomponent Al–Si casting alloys, which have complex, interconnecting three-dimensional (3D) networks of secondary phase particles. They are non-serviceable parts and so must be able to withstand high cycle fatigue while operating at temperatures between 30 and 80% of T m. Long fatigue crack growth tests were performed at room temperature (RT) and 350°C to assess the micromechanisms of fatigue. The fracture profiles at low and high da/dN were analysed; at low crack growth rates at both temperatures there is no crack path preferentiality with respect to the microstructure. At high da/dN in the RT sample the crack growth occurs preferentially via hard particles, while at 350°C there is a change in mechanism and the crack appears to avoid hard particles. X-ray tomography has been used to image the crack tips and gain a detailed insight into the mechanisms of fatigue in these complex 3D microstructures.

Mechanical-property relationships of Co/WC and CoNiFe/WC hard metal alloys

Abstract The microstructure and the mechanical property relationships of the different hard metal alloys based on Co/WC and CoNiFe/WC have been studied. The fractography of the different alloys has been also studied. The hard metals were studied metallographically by standard optical and scanning electron microscopy techniques. Using X-ray powder diffraction techniques, hard metals with pure cobalt as a metallic phase was found to be present in its two allotropic forms, face centered cubic and hexagonal close packed. Hardmetals with cobalt-nickel-iron as metallic phase was found to be present as a face centered cubic structure. The hardness, compressive strength and transverse rupture strength of hard metals studied with a cobalt-nickel-iron metallic phase are similar but slightly lower than those with a cobalt metallic phase for the same tungsten carbide level. The fracture toughness values of hard metals with 70–75% tungsten carbide and cobalt-nickel-iron metallic phase is higher than those with a cobalt metallic phase.