Joachim Gussone

Inducing Stable α + β Microstructures during Selective Laser Melting of Ti-6Al-4V Using Intensified Intrinsic Heat Treatments

Selective laser melting is a promising powder-bed-based additive manufacturing technique for titanium alloys: near net-shaped metallic components can be produced with high resource-efficiency and cost savings. For the most commercialized titanium alloy, namely Ti-6Al-4V, the complicated thermal profile of selective laser melting manufacturing (sharp cycles of steep heating and cooling rates) usually hinders manufacturing of components in a one-step process owing to the formation of brittle martensitic microstructures unsuitable for structural applications. In this work, an intensified intrinsic heat treatment is applied during selective laser melting of Ti-6Al-4V powder using a scanning strategy that combines porosity-optimized processing with a very tight hatch distance. Extensive martensite decomposition providing a uniform, fine lamellar α + β microstructure is obtained along the building direction. Moreover, structural evidence of the formation of the intermetallic α2-Ti3Al phase is provided. Variations in the lattice parameter of β serve as an indicator of the microstructural degree of stabilization. Interconnected 3D networks of β are generated in regions highly affected by the intensified intrinsic heat treatment applied. The results obtained reflect a contribution towards simultaneous selective laser melting-manufacturing and heat treatment for fabrication of Ti-6Al-4V parts.

Peritectic titanium alloys for 3D printing

Metal-based additive manufacturing (AM) permits layer-by-layer fabrication of near net-shaped metallic components with complex geometries not achievable using the design constraints of traditional manufacturing. Production savings of titanium-based components by AM are estimated up to 50% owing to the current exorbitant loss of material during machining. Nowadays, most of the titanium alloys for AM are based on conventional compositions still tailored to conventional manufacturing not considering the directional thermal gradient that provokes epitaxial growth during AM. This results in severely textured microstructures associated with anisotropic structural properties usually remaining upon post-AM processing. The present investigations reveal a promising solidification and cooling path for α formation not yet exploited, in which α does not inherit the usual crystallographic orientation relationship with the parent β phase. The associated decrease in anisotropy, accompanied by the formation of equiaxed microstructures represents a step forward toward a next generation of titanium alloys for AM.3D printing of titanium alloys today is based on known alloy compositions that result in anisotropic structural properties. Here, the authors add lanthanum to commercially pure titanium and exploit a solidification path that reduces texture and anisotropy.

Anodic dissolution of vanadium in molten LiCl–KCl–TiCl2

The mechanism of anodic dissolution of pure vanadium in a titanium-enriched alkali chloride molten salt was investigated to determine whether it can be used as an ion source for a continuous Ti–V alloy deposition process. This study represents the first step towards the preparation of ternary Ti–Al–V alloys. Cyclic voltammetry as well electrochemical impedance spectroscopy (EIS) was performed and potentials for dissolution experiments were determined. Additionally, the influence of anode morphologies on the dissolution process, as a consequence of pre-treatment, was investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Results indicate that anodic vanadium dissolution is possible, but hindered by the electroless formation of a thin titanium layer. Additionally, a secondary reaction, namely the oxidation of Ti2+ ions, takes place, lowering the current efficiency of the process. Morphology investigations revealed the risk of grain detachment (material loss) from the vanadium electrode, which is critical in direct dissolution, whereas under indirect dissolution conditions, passivation impedes the controlled process. Thus, electrolysis is best carried out with coarse-grained vanadium electrodes in the direct dissolution range.Graphical Abstract

Effect of vanadium ion valence state on the deposition behaviour in molten salt electrolysis

The electrodeposition process of vanadium from LiCl–KCl base electrolytes was investigated by means of cyclic voltammetry, galvanostatic electrolyses and micro analytical analysis of the deposits. It is demonstrated that the valence state of the vanadium ions has a critical influence on the feasibility of performing a reproducible and stable coating process aiming to obtain compact vanadium films. When the electrolyte contained predominantly trivalent vanadium ions, the process was unstable and the deposit consisted of dendrites. In contrast, making use of a comproportionation reaction of metallic vanadium and VCl3 to divalent vanadium ions led to a stable deposition behaviour and allowed to obtain thick deposits with high current efficiencies. The disadvantageous behaviour of melts with mostly trivalent ions is explained by the fact that deposition is interfered by the reduction of trivalent to divalent ions under limiting current conditions.Graphical Abstract