Microstructure and mechanical properties of Ti6Al4V (ELI) parts produced by DMLS

Abstract

Direct Metal Laser Sintering (DMLS) is an Additive Manufacturing (AM) technology where powder is melted by the laser beam track-by-track, layer-by layer to produce complex components. The advantage of DMLS is manufacturing functional parts, tools and also medical implants with complex shape and about 100% density. Microstructure and mechanical properties of as-built DMLS objects depend on not only material properties, but also process-parameters such as laser power density, scanning speed, powder layer thickness, scanning strategy, preheating and building strategies, etc. Titanium (Ti) alloys have superior qualities compared to other biomaterials: they are strong, lightweight, high corrosion resistant, non-toxic, biocompatible, long-lasting, and non-ferromagnetic, it has osseointegration capabilities. Ti6Al4V is one of the most widely used - Ti alloy containing 6 wt% Aluminium and 4 wt% Vanadium. Ti6Al4V (ELI) alloy is a higher-purity ("Extra-Low Interstitial") version of Ti6Al4V, with lower specified limits of iron and the interstitial elements C and O. Ti6Al4V (ELI) has an excellent combination of strength and toughness along with excellent corrosion resistance. High cooling rates at DMLS result in a formation of the ´ martensitic phase in Ti6Al4V alloy. As built Ti6Al4V DMLS parts have higher yield strength and lower ductility in comparison with cast material used for implants. Literature review revealed high variations in mechanical properties of DMLS Ti6Al4V under different process-parameters and building strategies. The main aim of the study was to analyse the microstructure and mechanical properties of DMLS Ti6Al4V (ELI) samples and comparison with the appropriate quality standard for the conventional alloy for biomedical applications. EOSINT M280 machine was used to produce samples for defect analysis, microstructural and mechanical studies. In this work DMLS Ti6Al4V (ELI) specimens with different building strategy, surface qualities and shapes were investigated. The low level of porosity in DMLS samples was observed by CT scans and the material had >99.9 % density. XRD measurements of residual stresses in as-built DMLS specimens attached to the substrate were performed and high tensile stresses in wide range of 220–800 MPa were measured. The direction of the maximal principal stress coincided with the direction of the laser scanning. It was found that second principal stress which is perpendicular to the scanning direction, correlates with surface roughness. Defect analysis by CT scans in pre-strained samples was used to detect the crack formation mechanism during tensile loading of as-built and heat-treated samples. The material in all the experiments showed a moderate ductile behaviour through necking formation, pores coalescence and cup and cone fracture. Vertical and horizontal samples showed similar mechanical properties. The results of this study show that if the DMLS process parameters are properly selected, the properties of DMLS Ti6Al4V (ELI) fully meet the requirements of the standards, and the only post-processing that is required is stress-relieving. After the stress-relief of Ti6Al4V (ELI) at 650°C for 3 hours in Ar argon atmosphere, no beta phase was detected by TEM, EBSD, and XRD, very small (500 nm) globular grains of alpha phase were found. The stress-relieved DMLS samples had good tensile properties that correspond to ASTM standard (F1108-14) for biomedical applications. It also was shown that mini-samples are effective for the determination of the basic mechanical properties of DMLS material.