A Fractographic Analysis of Additively Manufactured Ti6Al4V by Electron Beam Melting: Effects of Powder Reuse

Abstract

Metal additive manufacturing (AM) is being rapidly adopted in the aerospace and biomedical industries. Powder bed fusion AM processes are leading this trend. To maximize process economy, excess ''unmelted'' powder retrieved from the build chamber is used in subsequent build cycles. The metal properties and component reliability could undergo degradation with powder reuse. This study investigates the effects of powder reuse on fracture surface characteristics of Ti6Al4V specimens fabricated by electron beam melting AM over 30 sequential build cycles. Optical microscopy and scanning electron microscopy were used to evaluate the changes in fracture surface features of tensile failures with powder reuse. Macroscopically, slant fractures were most common in early builds, which transitioned to orthogonal fracture surfaces with poorly defined shear lips with increasing reuse. Regardless of the build number, the fracture origins were consistently from the as-built surfaces. Microscopically, ductile features such as micro-void coalescence were evident throughout the 30 build cycles. However, increasing flute content with reuse suggests that rising oxygen levels causes solution strengthening and limits the participation of active slip systems. These results highlight the importance of surface roughness and powder oxidation to metal performance in AM, and the evolution of fractographic features with powder reuse.

Oxidation in Reused Powder Bed Fusion Additive Manufacturing Ti-6Al-4V Feedstock: A Brief Review

“…72 With increasing reuse and powder oxidation, the as-built parts in both horizontal and vertical build directions saw increases in yield and ultimate tensile strengths while showing a decrease in elongation. 71 Fractography of the as-built parts from the Ghods et al study revealed that failure occurred at the surface of all as-built specimens as well as microvoid coalescence in all of the fracture surfaces regardless of reuse times. 71 Schur et al highlight that failure of the horizontal specimens occurred at the last printed surface, which they attribute to the likelihood of a finer surface microstructure of the surface.…”

“…71 Fractography of the as-built parts from the Ghods et al study revealed that failure occurred at the surface of all as-built specimens as well as microvoid coalescence in all of the fracture surfaces regardless of reuse times. 71 Schur et al highlight that failure of the horizontal specimens occurred at the last printed surface, which they attribute to the likelihood of a finer surface microstructure of the surface. The appearance of fracture flutes were observed in all builds after the 12th build, which the authors attribute to a reduction in slip of the a phases due to the increased oxygen content with powder reuse.…”

“…The authors noted a decrease in flowability, attributed to a reduction in spherical particles and the introduction of defects and irregularities from the reuse process. 69 Three articles stemming from the same 30 builds of reused Ti-6Al-4V powder were recently published that detail the oxygen content and effects on mechanical properties of the built parts, 70 fractographic features of the parts built with the reused powder, 71 and the general powder morphology and part microstructure of EB-PBF Ti-6Al-4V built from the reused powder over the 30 builds. 72 The 30 builds consisted of Grade 5 Ti-6Al-4V used in an Arcam A2X EBM machine.…”

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

“…A summary of different papers that examine such conditions and characteristics in EBM-manufactured samples, as well as their mechanical characterization by means of tensile testing, is presented below. In [ 158 ], the reuse of Ti–6Al–4V alloy powder in EBM was analyzed in samples subjected to tensile strength testing. This alloy powder was reused up to 30 times in horizontally- and vertically-built parts.…”

“…This, in turn, suggests a greater yield strength, ultimate tensile strength, and percentage of elongation in horizontally-built samples although their modulus of elasticity is slightly lower than that of vertically-built samples. With regard to the reuse of Ti–6Al–4V alloy powder, the authors of [ 158 ] found that the higher the number of times the powder is reused, the higher the modulus of elasticity, yield strength, and ultimate tensile stress, but the lower the percentage of elongation. In addition, they reported that, after reusing the powder in 25 sequential cycles, the modulus of elasticity decreased.…”