TENSILE AND HIGH CYCLE FATIGUE PROPERTIES OF ANNEALED TI6AL4V (ELI) SPECIMENS PRODUCED BY DIRECT METAL LASER SINTERING

Lerata Botsane Malefane,M Maring,A Du Plessis,Wb Du Preez

doi:10.7166/29-3-2077

Lerata Botsane Malefane, M Maring + Show 2 more

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https://doi.org/10.7166/29-3-2077

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Abstract

The tensile and high cycle fatigue (HCF) properties of high temperature annealed (HTA) direct metal laser sintered (DMLS) Ti6Al4V extra low interstitial (ELI) machined and polished specimens were investigated. The HTA heat treatment of the specimens resulted in the nucleation and growth of the alpha and beta grains from the acicular ?′ martensite grains, improving their elongation to failure. The specimens were micro-CT scanned in an attempt to relate the pores in the specimens to their fatigue properties. The micro-CT pore information from suspected crack initiation pores on the surfaces of eventual fracture was used to calculate the stress intensity factors, which correlated well with the decreasing cycles to failure of the fatigue test specimens for all three build directions. Three representative specimens were analysed, and the ‘killer pore’ was identified in each micro-CT scan and fractograph, all of which were proximal to the surface of the specimen.

Highlights

Direct metal laser sintering (DMLS) uses the principle of slicing a solid 3D computer aided design (CAD) model into multiple layers, and uses a heat source to build up the part, layer on layer [1]
The tensile properties of high temperature annealed (HTA) DMLS Ti6Al4V (ELI) specimens showed a level of anisotropy with respect to the three mutually orthogonal DMLS build directions
The high cycle fatigue (HCF) crack initiation of HTA DMLS Ti6Al4V (ELI) specimens started from surface and subsurface DMLS process-related pores

Summary

Introduction

Direct metal laser sintering (DMLS) uses the principle of slicing a solid 3D computer aided design (CAD) model into multiple layers, and uses a heat source to build up the part, layer on layer [1]. The lack of fusion pores is bigger than the gas entrapment pores, which are oval shaped and located between two consecutive layers formed during the DMLS process [3, 4] These pores, inherent in the DMLS specimens, act as stress concentration sites, and are expected to lower the tensile and fatigue properties of the specimens and determine the locations of the fracture planes in the specimens [5, 6]. The projected areas of these pores in the fracture plane reduce the actual cross-sectional area of the specimen, increasing the stress experienced by the built material to a value exceeding the UTS at a force below the maximum applied force These pores result in a change in stress intensity factor (∆K) during dynamic loading, the magnitude of which depends on the applied change in stress (∆σ), the projected pore area normal to ∆σ, and its location, whether internal or at the surface. The initiation of cracks from these pores can be explained using the theory of linear elastic fracture mechanics (LEFM) applied to fatigue loading, where the initiation and propagation of cracks from small defects and non-metallic inclusions is described by Equation 1 [6, 7, 8]:

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