Unlike conventional materials removal methods, additive manufacturing (AM) is based on a novel materials incremental manufacturing philosophy. Additive manufacturing implies layer by layer shaping and consolidation of powder feedstock to arbitrary configurations, normally using a computer controlled laser. The current development focus of AM is to produce complex shaped functional metallic components, including metals, alloys and metal matrix composites (MMCs), to meet demanding requirements from aerospace, defence, automotive and biomedical industries. Laser sintering (LS), laser melting (LM) and laser metal deposition (LMD) are presently regarded as the three most versatile AM processes. Laser based AM processes generally have a complex non-equilibrium physical and chemical metallurgical nature, which is material and process dependent. The influence of material characteristics and processing conditions on metallurgical mechanisms and resultant microstructural and mechanical properties of AM proc...

Laser additive manufacturing of metallic components: materials, processes and mechanisms

A significant requirement for a bone implant is to replicate the functional gradient across the bone to mimic the localization change in stiffness. In this work, continuous functionally graded porous scaffolds (FGPSs) based on the Schwartz diamond unit cell with a wide range of graded volume fraction were manufactured by selective laser melting (SLM). The micro-topology, strut dimension characterization and effect of graded volume fraction on the mechanical properties of SLM-processed FGPSs were systematically investigated. The micro-topology observations indicate that diamond FGPSs with a wide range of graded volume fraction from 7.97% to 19.99% were fabricated without any defects, showing a good geometric reproduction of the original designs. The dimensional characterization demonstrates the capability of SLM in manufacturing titanium diamond FGPSs with the strut size of 483-905µm. The elastic modulus and yield strength of the titanium diamond FGPSs can be tailored in the range of 0.28-0.59GPa and 3.79-17.75MPa respectively by adjusting the graded volume fraction, which are comparable to those of the cancellous bone. The mathematical relationship between the graded porosity and compression properties of a FGPS was revealed. Furthermore, two equations based on the Gibson and Ashby model have been established to predict the modulus and yield strength of SLM-processed diamond FGPSs. Compared to homogeneous diamond porous scaffolds, FGPSs provide a wide range of mutative pore size and porosity, which are potential to be tailored to optimize the pore space for bone tissue growth. The findings provide a basis of new methodologies to design and manufacture superior graded scaffolds for bone implant applications.

Continuous functionally graded porous titanium scaffolds manufactured by selective laser melting for bone implants.

This paper looks at the tribology of thermal sprayed WC–Co based coatings and covers the high energy air–sand erosion resistance and slurry jet impingement erosion performance, dry and wet sliding tribology of thermal spray WC–Co based coatings as well as the abrasion and abrasion–corrosion of these coatings. The tribological and tribo-corrosion performance of the coatings will be related to their mechanical and corrosion properties as well as deposition parameters, microstructure and actual composition. For example, the anisotropic microstructure of thermally sprayed WC–Co–Cr coatings, in particular the low fracture toughness in a direction parallel to the substrate, has been observed to affect the nature of crack formation under 200 μJ air–solid particle erosion conditions. Voids and occasionally other microstructural features (i.e., cobalt lakes, splat boundaries, interfacial inclusions) in the coating act as crack initiation sites. The erosion rate was dominated by cracks within 5 μm of the surface and was relatively insensitive to total length of cracks, showing a near-surface damage front controls the erosion rate and this region is coincident with the region of maximum shear stress induced by erodent impacts. Optimisation of the deposition parameters of HVOF 86WC–10Co–4Cr coatings show an improvement in erosion resistance of more than 50% over the conventional D-gun applied coating of identical nominal composition. The variation in the slurry erosion performance of the thermally sprayed coatings is also linked to directional fracture toughness and crack propagation paths which are influenced by the presence of pores, inhomogeneous carbide distributions and substrate grit blast remnants. The influence of slurry jet angle is more pronounced under 0.4 μJ energy conditions where maximum erosion occurred at 90° and the minimum at 30° in contrast to 7 μJ slurry erosion rates which were independent of jet angle. This reflects the lower levels of fluctuating stresses imparted to the coating during low energy slurry impacts leading to the impact angle having a greater effect on sub critical crack growth rate than for higher energy conditions. The abrasion resistance of these coatings was found comparable to sintered cermets of the same composition. The synergistic effects between micro and macro abrasion and corrosion for detonation gun (D-gun) sprayed WC–10Co–4Cr coatings are shown to be significant and depend on the environment. The size effect of the abradant relative to the microstructure and splat size is important as well as the propensity for the various phases to passivate to control corrosion levels. Comparisons between exposed and freshly polished coating surfaces in strong NaOH solutions (pH 11) show that significantly lower wear rates were seen for the exposed surface due to a negative wear–corrosion synergy due to selective phase removal and the effects of localised passivation. Dry and wet sliding wear resistance of these coatings is shown to be high (wear rates of 10 −16 –10 −18  m 3 /Nm) with modest coefficient of friction levels between 0.2 and 0.5. The presence of oxides on the binder phases appears to influence the friction and wear levels. Wear appears to be by carbide ejection and/or by tribo-chemical processes.

Tribology of thermal sprayed WC–Co coatings

Additive manufacturing (AM), nowadays commonly known as 3D printing, is a revolutionary materials processing technology, particularly suitable for the production of low-volume parts with high shape complexities and often with multiple functions. As such, it holds great promise for the fabrication of patient-specific implants. In recent years, remarkable progress has been made in implementing AM in the bio-fabrication field. This paper presents an overview on the state-of-the-art AM technology for bone tissue engineering (BTE) scaffolds, with a particular focus on the AM scaffolds made of metallic biomaterials. It starts with a brief description of architecture design strategies to meet the biological and mechanical property requirements of scaffolds. Then, it summarizes the working principles, advantages and limitations of each of AM methods suitable for creating porous structures and manufacturing scaffolds from powdered materials. It elaborates on the finite-element (FE) analysis applied to predict the mechanical behavior of AM scaffolds, as well as the effect of the architectural design of porous structure on its mechanical properties. The review ends up with the authors’ view on the current challenges and further research directions.

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Additively Manufactured Scaffolds for Bone Tissue Engineering and the Prediction of their Mechanical Behavior: A Review

his paper provides a comprehensive assessment of the sliding and abrasive wear behaviour of WC–10Co4Cr hardmetal coatings, representative of the existing state-of-the-art. A commercial feedstock po ...

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Tribology of HVOF- and HVAF-sprayed WC-10Co4Cr hardmetal coatings : A comparative assessment

Tungsten carbide–cobalt (WC–Co) has been extensively employed as an abrasion/wear protective material. However, carbon loss (decarburization) of WC–Co powders during processing reduces the efficiency of the material against abrasive wear. This paper examines the efficiency of three consolidation processing techniques to obtain coatings and bulk samples from WC–Co powder. In this work, high velocity oxy-fuel (HVOF) spray forming and laser engineered net shaping (LENS®) were applied to make WC–Co coatings; spark plasma sintering (SPS) was used to produce bulk WC–Co samples for comparison. The microstructures of two types of coatings and one bulk sample were analyzed by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) to recognize material modification. In addition, porosity and tribological properties of the WC–Co samples were tested. The ultra-microindentation technique was applied to measure the universal hardness and the elasto-plastic properties of the samples. Experiments using a tribometer (ball on disc configuration) under dry conditions have been performed in order to evaluate the friction and wear properties of the different systems.

Microstructure and wear resistance of WC–Co by three consolidation processing techniques

High Velocity Oxy-Fuel (HVOF) spray techniques can produce high performance alloy and cermet coatings for applications that require wear resistant surfaces. In HVOF process, the particle velocity and temperature determine the resultant coating properties and in many cases enables a better understanding of the process. The aim of this study is to investigate influences of different oxygen/fuel ratios on velocity and temperature of flying particles as well as properties of the HVOF thermal sprayed WC-CoCr coatings. Particle parameters were recorded just prior to impact on the substrate using in-flight particle diagnostic tool Accuraspray-g3®. Detailed correlation of particle parameters and the coating properties are evaluated in order to deduce particle parameter ranges providing coatings with optimum properties.

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Effect of oxygen/fuel ratio on the in-flight particle parameters and properties of HVOF WC–CoCr coatings

Powder metallurgy with space holder for porous titanium implants: A review

High-velocity oxy-fuel (HVOF) spray techniques can produce cermet coatings for applications that require corrosion resistant surfaces. The aim of this study was to investigate the influence of different oxygen/fuel ratios on velocity and temperature of flying particles and establish the effects on the coatings quality in relation to the corrosion behavior of HVOF WC–CoCr coating in a 0.1 N HCl. The results show that in-flight particle characteristics have a significant effect on the porosity, hardness and corrosion resistance. The highest corrosion resistance is obtained with those parameters that allow a sufficient melting of the agglomerates while minimizing the decarburization of the WC particles.

Influence of HVOF spraying parameters on the corrosion resistance of WC–CoCr coatings in strong acidic environment

Hard chrome plating has been used in several different applications in industries that require abrasive sliding wear resistance, such as hydraulic pistons, shafts or bearings. However, the increasing environmental and worker safety pressures on electrolytic hard chrome are leading companies to adopt alternatives. The improvements of the high-velocity-oxy-fuel (HVOF) thermal spray process allow the chromium coating replacement with a comparable or superior surface treatment and are more environmentally friendly. This HVOF process, as a flexible dry-coating technology, avoids high-volume waste streams and enables a flexible choice of coating material for each application. The cobalt–chromium-cemented tungsten carbides are some of the easiest materials to spray and the WC-10Co-4Cr coatings have demonstrated superior performance over hard chrome with regard to mechanical and tribological properties. In this work, this coating has been deposited with a Sulzer Metco WokaJet-400 kerosene fuel spray gun, and the spray conditions have been optimized in order to ensure the best properties of the coatings. The mechanical and tribological properties have been evaluated in coatings sprayed with four deposition conditions that involve different gas flow rates. The most wear-resistant coating is obtained with those HVOF parameters that prevent decarburization of WC particles and, at the same time, allow an adequate agglomerate melting giving a good intersplat adhesion. The results indicate that HVOF-sprayed WC-CoCr coatings are a reliable alternative to electrolytic hard chrome (EHC) in the aeronautical industry to coat landing gear components. In particular, in the dry wear tests, the WC-CoCr coatings outperform hard chrome coatings in wear resistance. Copyright © 2010 John Wiley & Sons, Ltd.

Miquel Punset

Papers

Microstructure and wear resistance of WC–Co by three consolidation processing techniques

Effect of oxygen/fuel ratio on the in-flight particle parameters and properties of HVOF WC–CoCr coatings

Powder metallurgy with space holder for porous titanium implants: A review

Influence of HVOF spraying parameters on the corrosion resistance of WC–CoCr coatings in strong acidic environment

Tribological evaluation of HVOF thermal‐spray coatings as a hard chrome replacement