PSI - Issue 42

Litton Bhandari et al. / Procedia Structural Integrity 42 (2022) 529–536 Bhandari et al. / Structural Integrity Procedia 00 (2019) 000–000

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Stress-relieving heat-treatment is generally done for the relaxation of these residual stresses. Process-induced defects are of di ff erent types and originates from di ff erent locations. These distinct types of defects are di ff erent in shapes and a ff ects the crack initiation and propagation behavior. Porosity is one of the such defects, that occurs due partial vaporization and over-melting of the metal powder during the fabrication process. Lack of fusion (LOF) is another such type process-induced defects that occurs due to under-melting during fabrication and gas porosity comes from the metal powder. All these defects cannot be eliminated completely but can be reduced by using optimized process parameters. These defects act as a stress raiser under the action of load and acts as a potential crack initiation site within the fabricated components (Gaur et al. (2015)). This eventually leads to premature failure without any prior indication. Hence, these defects become a much of our concern when it comes to the additively manufactured parts subjected to cyclic loading and thus, needs to be investigated in order to have an insight on the e ff ect of defects and inhomogeneity on the fatigue behavior of the alloy. An investigation has been made in this research to evaluate the e ff ect of process-induced defects on the fatigue behaviour of additively manufactured Ti6Al4V alloy and endurance limit has been estimated using the experimentally calibrated defect model based on the mechanical properties, the defects’ area and its location.

2. Materials and Method

2.1. Powder Characterization and sample fabrication

Ti6Al4V pre-alloyed powders was used as raw material for SLM process and are almost spherical with an average diameter of 26.67 µ m. These powder particles are manufactured by gas atomization techniques and the powder mor phology and powder distribution is shown in Figure 1. The Chemical composition of pre-alloyed Ti6Al4V alloy has been presented in Table 1.

Table 1. Chemical composition (% by wt.) of Ti6Al4V alloy powder Al V Fe Ni

Sn Mo Nb Si O N H Ti 5.77 4.15 0.34 0.14 0.08 0.07 0.04 0.02 0.14 0.02 0.01 Bal.

(a)

(b)

Fig. 1. Ti6Al4V alloy powder (a) Morphology (b) Powder size distribution

Ti6Al4V tensile, fatigue and cube samples were then fabricated using Selective Laser Melting (SLM) technique using optimized process parameters (Table 2) with a layer thickness of 60 µ m. The surface morphology of as-fabricated specimen was analysed for surface roughness using 3D profilometry. The density of the fabricated specimen was also evaluated using the Archimedes’ principle as per ASTM B311-13 in order to compute the relative density with respect to conventional counterparts. The as-fabricated samples were subjected to heat treatment (HT) at a temperature of 800°C for 2 hours in order to relieve the tensile residual stress induced due to high thermal gradient during the