Abstract
Thanks to their excellent mechanical strength in combination with low density, high melting point, and good resistance to corrosion, titanium alloys are very useful in many industrial and biomedical fields. The new additive manufacturing methods, such as Electron Beam Powder Bed Fusion based on the deposition of metal powders layers progressively molten by electron beam scanning, can overcome many of the machining problems concerning the production of peculiar shapes made of Ti alloys. However, the processing route is strictly determinant for mechanical performance of products, especially in the case of Ti alloys. In the present work flat specimens made of Ti-6Al-4V alloy produced by Electron Beam Powder Bed Fusion (or Electron Beam Melting) have been built and post-processed with the purpose of obtaining good tensile and creep performance. Preliminarily, the process parameters were set according to literature evidence and machine producer recommendations, validated by the results of a thermal analysis, aimed at satisfying the best processing conditions to reduce defects, as unmelted regions, microstructure coarsening or porosity, that are detrimental to mechanical behavior. Subsequently, Hot Isostatic Pressing and surface smoothing were considered, respectively, in order to reduce any internal porosity and lower roughness. Microstructure of the investigated specimens was characterized by optical and scanning electron microscopy observations and by X-ray diffraction measurements. Results show enhanced tensile behavior after the hot pressing treatment that allows to relieve stresses and reduce defects detrimental to mechanical properties. The best ductility was obtained by the combined effects of machining and densification. Creep test results verify the beneficial effects of surface smoothing.
Highlights
Titanium alloys are widely used in many fields, such as in the chemical, automotive, and aerospace industries, thanks to their excellent mechanical strength, together with the ability to maintain it within a wide range of temperatures which make these alloys suitable for engine parts
Machinability problems can be overcome by additive manufacturing, that includes a variety of technologies, applied for more than 20 years for porous structures and prototypes, some of which promise to become commonly used to produce near-net-shape components of complex geometry reducing production time and cost [5]
Temperature distribution around the coordinate system fixed to the movable point source has been calculated by Equation (3), fixing the powder bed temperature To and the scanning speed v, and varying the spatial coordinates
Summary
Titanium alloys are widely used in many fields, such as in the chemical, automotive, and aerospace industries, thanks to their excellent mechanical strength (especially considering their low density and high melting point), together with the ability to maintain it within a wide range of temperatures which make these alloys suitable for engine parts. The experience shows that titanium alloys have high reliability in seawater corrosion resistance: pipes and fittings made of titanium can reduce weight and extend significantly their life. Titanium applications are more reasonable if its high cost is compensated by long lifetime, especially when repair and maintenance service are difficult to perform [1]. Besides these properties, the non-toxicity of titanium makes it useful for biomedical applications (implant devices, pacemakers, artificial hearts) [2]. Machinability problems can be overcome by additive manufacturing, that includes a variety of technologies, applied for more than 20 years for porous structures and prototypes, some of which promise to become commonly used to produce near-net-shape components of complex geometry reducing production time and cost [5]
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