Tensile and Creep Properties Improvement of Ti-6Al-4V Alloy Specimens Produced by Electron Beam Powder Bed Fusion Additive Manufacturing

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.

Mechanical Properties of Powder Metallugry (Ti-6Al-4V) with Hot Isostatic Pressing

“…While Al tends to enter the α phase, the V tends to enter the β phase [24]. In the images taken for the 8 specimens at the same magnification, the dark regions show the α phase, while the light regions present the β phase [25]. The presence of delamination wear, abrasion and debris in the SEM pictures of Ti-6Al-4V is in accordance with the test results of other titanium alloy testing [26].…”

Additive manufacturing of Ti6Al4V alloy via electron beam melting for the development of implants for the biomedical industry

“…Other studies have assessed the machinability of EBM-fabricated samples in assisted surface finishing treatments using different cooling techniques [ 133 ], [ 134 ]. The internal porosity defect can be reduced by subjecting samples to subsequent treatments such as hot isostatic pressing and other heat processes [ 135 ]. Another important aspect are the scanning strategies employed during the EBM process [ 136 ], as they could play a role in minimizing internal porosity, a characteristic that reduces fatigue strength.…”

“…Their mechanical performance, for instance, has been assessed by means of Creep tests. Aliprandi et al [ 135 ], [ 160 ] investigated the mechanical behavior of EBM-fabricated parts at temperatures between 400 and 600 °C. They reported that a better tensile performance at these temperatures is driven by a subsequent surface smoothing and observed in samples whose microstructure (basket weave) is aligned to the loading direction and has elongated grains along the tensile direction.…”

Surface Roughness Considerations in Design for Additive Manufacturing - A Literature Review

“…Furthermore, defects can be caused by lack of fusion, unmelted or partially melted powder and gas entrapment occurring during the LPBF build process (Eliaz et al, 2020). These issues can impact the surface roughness of an LPBF part which in turn can influence properties (Aliprandi et al, 2019).…”