Ti-6Al-4V alloys have gained popularity in chemical, aerospace, and biomedical applications owing to their high strength to weight ratio along with excellent corrosion resistance. In addition, high biocompatibility as well as elastic moduli identical to that of bone makes this alloy interesting for load bearing implants such as the hip, dental crews, and elbow joints. Unfortunately, the typical production procedures like machining, deformation, powder metallurgy, and casting cannot produce near net shape implants that are required for each patient’s anatomical structure.
These traditional production procedures cannot be used to produce functionally graded parts and complex geometries. They are both time and cost inefficient considering that they entail multiple production stages and might entail metal removal. For this reason, there is a high demand for new manufacturing procedures that could allow the manufacture of products with high geometrical complexity as well as dimensional accuracy at a single step.
Selective laser melting has been used for decades now for the manufacture of complex and near net shaped models. Even though selective laser melting can be adopted to fabricate custom designed components in short durations, it produces microstructures identical to those of cast parts reference to the local solidification and melting, and residual stresses due to rapid cooling of the melt formed during the laser interaction.
In addition, as the melt cools, undesirable non-equilibrium phases and segregation could occur between each layer of the deposited powder bed. The development of these non-equilibrium phases is common in Ti-6Al-4V alloys. For this reason, to enhance the ductility of the selective laser melting produced Ti-6Al-4V alloys and to make them appropriate for load bearing applications in the fields of aerospace and biomedicine, subsequent treatment is needed.
Güney Mert Bilgin and Arcan F. Dericioglu at Middle East Technical University in collaboration with Ziya Esen and Şeniz Kuşhan Akın at Çankaya University in Turkey applied a 2-step Thermo-Hydrogen Process incorporating hydrogenation and dehydrogenation stages to Ti-6Al-4V alloy fabricated by selective laser melting in a bid to refine the microstructure and improve the ductility of the alloy without losing much strength. Their research work is published in journal, Materials Science & Engineering A.
Thermo-Hydrogen Processing relieved the residual stresses appearing during selective laser melting reference to treatment at higher temperatures and refined the microstructure by alloying and de-alloying of hydrogen simultaneously. The 2-step Thermo-Hydrogen processing was used to remove the α-phase formation at the grain boundary, which occured owing to betatizing and eutectoid decomposition.
The research team observed that the as-fabricated alloy surface was made of oxides of titanium and aluminum. Hydrogen treatment at 650 °C for 1 h transformed the starting non-equilibrium α ¢-martensitic phase to β- and δ phases. Dehydrogenation at 700 °C for 18 h resulted into a very fine β discontinuous phase being formed along with an α phase.
The authors recorded approximately 110 % and 240 % increments in % elongation and % reduction in area values, respectively. This was as a result of 2-step Thermo-hydrogen processing, although there was a 10% decrease in strength. Improvement in ductility was confirmed by the change of the flat and shiny facture observed in selective laser melting fabricated samples to a fracture surface with equiaxed dimples after Thermo-Hydrogen treatment. The decrease in hardness after hydrogenation and dehydrogenation treatments could be referenced to a relief of residual stress as well as the formation of equilibrium α and β-phases.
Güney Mert Bilgin, Ziya Esen, Şeniz Kuşhan Akın, Arcan F. Dericioglu. Optimization of the mechanical properties of Ti-6Al-4V alloy fabricated by selective laser melting using thermohydrogen processes. Materials Science & Engineering A, volume 700 (2017), pages 574–582.
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