M. Qian

Topological design and additive manufacturing of porous metals for bone scaffolds and orthopaedic implants: A review

One of the critical issues in orthopaedic regenerative medicine is the design of bone scaffolds and implants that replicate the biomechanical properties of the host bones. Porous metals have found themselves to be suitable candidates for repairing or replacing the damaged bones since their stiffness and porosity can be adjusted on demands. Another advantage of porous metals lies in their open space for the in-growth of bone tissue, hence accelerating the osseointegration process. The fabrication of porous metals has been extensively explored over decades, however only limited controls over the internal architecture can be achieved by the conventional processes. Recent advances in additive manufacturing have provided unprecedented opportunities for producing complex structures to meet the increasing demands for implants with customized mechanical performance. At the same time, topology optimization techniques have been developed to enable the internal architecture of porous metals to be designed to achieve specified mechanical properties at will. Thus implants designed via the topology optimization approach and produced by additive manufacturing are of great interest. This paper reviews the state-of-the-art of topological design and manufacturing processes of various types of porous metals, in particular for titanium alloys, biodegradable metals and shape memory alloys. This review also identifies the limitations of current techniques and addresses the directions for future investigations.

Characteristic zirconium-rich coring structures in Mg-Zr alloys

The most characteristic feature of the microstructure of a magnesium alloy that contains more than a few tenths per cent soluble zirconium is the zirconium-rich cores that exist in most grains. The morphology, distribution and composition of cores observed in a Mg-0.56%Zr alloy and the small particles present in them were investigated

Grain Refinement of Magnesium Alloys: A Review of Recent Research, Theoretical Developments, and Their Application

This paper builds on the “Grain Refinement of Mg Alloys” published in 2005 and reviews the grain refinement research on Mg alloys that has been undertaken since then with an emphasis on the theoretical and analytical methods that have been developed. Consideration of recent research results and current theoretical knowledge has highlighted two important factors that affect an alloy’s as-cast grain size. The first factor applies to commercial Mg-Al alloys where it is concluded that impurity and minor elements such as Fe and Mn have a substantially negative impact on grain size because, in combination with Al, intermetallic phases can be formed that tend to poison the more potent native or deliberately added nucleant particles present in the melt. This factor appears to explain the contradictory experimental outcomes reported in the literature and suggests that the search for a more potent and reliable grain refining technology may need to take a different approach. The second factor applies to all alloys and is related to the role of constitutional supercooling which, on the one hand, promotes grain nucleation and, on the other hand, forms a nucleation-free zone preventing further nucleation within this zone, consequently limiting the grain refinement achievable, particularly in low solute-containing alloys. Strategies to reduce the negative impact of these two factors are discussed. Further, the Interdependence model has been shown to apply to a broad range of casting methods from slow cooling gravity die casting to fast cooling high pressure die casting and dynamic methods such as ultrasonic treatment.

Settling of undissolved zirconium particles in pure magnesium melts

Abstract Zirconium is a remarkable grain refiner for magnesium alloys and is currently introduced into magnesium alloys primarily in the form of a Mg–Zr master alloy (Zirmax ® a Mg–33.3Zr alloy supplied by Magnesium Elektron). Owing to the difficulties of attaining complete dissolution, undissolved zirconium particles are often observed in Zirmax ® alloyed magnesium microstructures in the form of both isolated individual particles varying from submicrometre to greater than 10 μm, and various sizes of clusters that contain a number of zirconium particles. Aimed at improving the alloying efficiency with Zirmax ® and eliminating these undesirable large zirconium inclusions prior to pouring, this paper provides a detailed theoretical and experimental study of the settling behaviour of undissolved zirconium particles in pure magnesium melts. Various characteristics of the settling behaviour of zirconium particles are clarified based on the good agreement achieved between the experimental observations and theoretical predictions. In addition, it is also shown that wet chemical analysis of the total zirconium content in samples taken after different settling times at temperature can be an effective approach for evaluating the settling behaviour of undissolved zirconium particles in pure magnesium melts.

Microstructure and mechanical behavior of metal injection molded Ti–Nb binary alloys as biomedical material

The application of titanium (Ti) based biomedical materials which are widely used at present, such as commercially pure titanium (CP-Ti) and Ti-6Al-4V, are limited by the mismatch of Youngs modulus between the implant and the bones, the high costs of products, and the difficulty of producing complex shapes of materials by conventional methods. Niobium (Nb) is a non-toxic element with strong β stabilizing effect in Ti alloys, which makes Ti-Nb based alloys attractive for implant application. Metal injection molding (MIM) is a cost-efficient near-net shape process. Thus, it attracts growing interest for the processing of Ti and Ti alloys as biomaterial. In this investigation, metal injection molding was applied to the fabrication of a series of Ti-Nb binary alloys with niobium content ranging from 10wt% to 22wt%, and CP-Ti for comparison. Specimens were characterized by melt extraction, optical microscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and transmission electron microscopy (TEM). Titanium carbide formation was observed in all the as-sintered Ti-Nb binary alloys but not in the as-sintered CP-Ti. Selected area electron diffraction (SAED) patterns revealed that the carbides are Ti2C. It was found that with increasing niobium content from 0% to 22%, the porosity increased from about 1.6% to 5.8%, and the carbide area fraction increased from 0% to about 1.8% in the as-sintered samples. The effects of niobium content, porosity and titanium carbides on mechanical properties have been discussed. The as-sintered Ti-Nb specimens exhibited an excellent combination of high tensile strength and low Youngs modulus, but relatively low ductility.

Review of effect of oxygen on room temperature ductility of titanium and titanium alloys

Abstract Room temperature tensile ductility is an important property of titanium (Ti) and titanium alloys for structural applications. This article reviews the dependency of tensile ductility on oxygen for α-Ti, (α+β)-Ti and β-Ti alloys fabricated via traditional ingot metallurgy (IM), powder metallurgy (PM) and additive manufacturing (AM) or three-dimensional printing methods and recent advances in understanding the effect of oxygen on ductility. Seven mechanisms have been discussed based on case studies of individual titanium materials reported in literature. The dependency of ductility on oxygen is determined by both the composition and microstructure of the titanium alloy. For Ti–6Al–4V (wt-%), as sintered Ti–6Al–4V shows a critical oxygen level of about 0·33 wt-% while additively manufactured Ti–6Al–4V exhibits different critical levels ranging from about 0·22% to well above 0·4% depending on microstructure. Rare earth (RE) elements are effective scavengers of oxygen in titanium materials even just with a small addition (e.g. 0·1 wt-%), irrespective of the manufacturing method (IM, PM and AM). High cycle fatigue experiments revealed no initiation of fatigue cracks from the resulting RE oxide particles over the size range from submicrometres to a few micrometres. A small addition of RE elements offers a practical and affordable approach to mitigating the detrimental effect of oxygen on ductility.

Alloying of pure magnesium with Mg 33.3 wt-%Zr master alloy

Abstract To capitalise on the strengthening potential of zirconium as a potent grain refiner for magnesium alloys, the mechanisms of adding zirconium to magnesium and its subsequent grain refining action need to be understood. Using a Mg- 33.3Zr master alloy (Zirmax supplied by Magnesium Elektron Ltd) as a zirconium alloying additive, the influence of different alloying conditions on the dissolution of zirconium in magnesium was investigated. It was found that owing to the highly alloyable microstructure of Zirmax, the dissolution of zirconium was generally complete within a few minutes in the temperature range 730 to 780°C. Prolonging and/or intensifying stirring were found to have no conspicuous influence on further enhancing the dissolution of zirconium. In all cases studied, the average grain size increased with increasing holding time at temperature while the total zirconium content decreased. The finest grain structure and highest total zirconium content corresponded to sampling immediately after stirring. Pick up of iron by molten magnesium from the mild steel crucibles used for melting and holding, was significantly delayed or avoided in the temperature range 730 to 780°C by coating the crucibles with boron nitride. It is therefore feasible to conduct zirconium alloying at 730°C without the need of a considerable excess of Zirmax addition using a properly coated or lined steel crucible.

The Contribution of Constitutional Supercooling to Nucleation and Grain Formation

The concept of constitutional supercooling (CS) including the term itself was first described and discussed qualitatively by Rutter and Chalmers in order to understand the formation of cellular structures during the solidification of tin, and then quantified by Tiller et al. On that basis, Winegard and Chalmers further considered ‘supercooling and dendritic freezing of alloys’ where they described how CS promotes the heterogeneous nucleation of new crystals and the formation of an equiaxed zone. Since then the importance of CS in promoting the formation of equiaxed microstructures in both grain refined and unrefined alloys has been clearly revealed and quantified. This paper describes our current understanding of the role of CS in promoting nucleation and grain formation. It also highlights that CS, on the one hand, develops a nucleation-free zone surrounding each nucleated and growing grain and, on the other hand, protects this grain from readily remelting when temperature fluctuations occur due to convection. Further, due to the importance of the diffusion field that generates CS, recent analytical models are evaluated and compared with a numerical model. A comprehensive description of the mechanisms affecting nucleation and grain formation and the prediction of grain size is presented with reference to the influence of the casting conditions applied during the practical casting of an alloy.

An approach to assessing ultrasonic attenuation in molten magnesium alloys

Few experimental data are available on ultrasonic attenuation in molten light alloys due to lack of means of characterization. An approach has been proposed and demonstrated to assessing ultrasonic attenuation in molten magnesium alloys based on the finding that the grain density in ultrasonicated magnesium alloy samples depends linearly on ultrasonic amplitude along the propagation direction. Hence, the attenuation of the ultrasonic amplitude with propagation distance can be effectively assessed according to the variations in the grain density with propagation distance. Metallographic analyses revealed that the dependence of grain density on propagation distance is best described exponentially with respect to different amplitudes. Consequently, the attenuation behavior of the ultrasonic amplitude with propagation distance can be described by the same exponential law. The characteristic ultrasonic attenuation coefficients in three benchmark molten magnesium alloys investigated were determined accordingly....

Grain nucleation and formation in Mg-Zr alloys

Abstract Zirconium is the most potent nucleant discovered to date for magnesium. This work reports the grain nucleation and formation characteristics in zirconium-inoculated magnesium melts. It was found that nucleation of magnesium grains occurred on two types of zirconium nucleants, where the first type were small and round, suggesting that they were formed by precipitation, and the other type were larger and irregularly shaped, similar to undissolved particles. Subsequent growth of the magnesium halo structures after nucleation could occur either spherically or dendritically under similar cooling conditions but under well-inoculated conditions they grew spherically in general. Hence, achieving a high level of dissolved Zr content is not only essential to the nucleation process but also important for control of the subsequent growth morphologies of the halo structures. The nucleation process can be understood by an adsorption mechanism.