High Cycle Fatigue Properties of as-Built Ti6al4v (Eli) Produced by Direct Metal Laser Sintering

Abstract

The high cycle fatigue (HCF) properties of direct metal laser sintering (DMLS) of Ti6Al4V (ELI) in three mutually orthogonal build directions were investigated by cycling specimens under load control, in a tension-tension fatigue testing machine. Semi-log graphs of maximum stress (S) against life (N) of the specimens produced along the respective three build directions were plotted, and the displayed endurance limits compared. Optical and scanning electron microscopy were used to compare and analyse the crack initiation and propagation characteristics of the specimens. The influence of the build direction on the fatigue properties of the tested specimens is discussed here. OPSOMMINGDie hoësiklus vermoeidheidseienskappe van direkte metaallaser sintering van Ti6Al4V in drie onderlinge ortogonale bourigtings is ondersoek deur monsters te onderwerp aan beheerde sikliese ladings met behulp van ʼn trek-trek vermoeidheidstoetsmasjien. Semi-log grafieke van maksimum spanning teen leeftyd (S-N grafieke) is vir die monsters saamgestel, saam met die onderskeie bou rigtings. Sodoende kon die vermoeidheidslewe vergelyk word. Optiese-en skandeer elektronmikroskopie is gebruik om die kraakvorming en -voortplanting te vergelyk en te analiseer. Die invloed van die bourigting op die vermoeidheidseienskappe van die monsters is bespreek. INTRODUCTIONWork done by the Centre for Rapid Prototyping and Manufacturing (CRPM) of the Central University of Technology Free State on the manufacture of parts using direct metal laser sintering (DMLS) of Ti6Al4V (ELI) powder has thus far been based on the static mechanical properties of such DMLS parts that have been stress-relieved. These static mechanical properties were found to conform to the properties of standard wrought Ti6Al4V (ELI) [1,2]. However, for DMLS Ti6Al4V (ELI) parts that are expected to experience cyclic loading, there is uncertainty that they will last their full operational life. This is due to limited available fatigue life data for the DMLS-produced alloy.This study presents baseline data and analyses on as-built DMLS Ti6Al4V (ELI), as a precursor to studies of DMLS Ti6Al4V (ELI) that is heat treated in order to tailor its microstructure to give the best high cycle fatigue (HCF) properties possible.

High Cycle Fatigue Performance of Ti6al4v(eli) Specimens Produced With Inherent Laser Powder Bed Fusion Surface Roughness

“…The fatigue data of these specimens are presented in Figure 2. These fatigue results showed characteristics similar to the curve profile for Ti6Al4V(ELI) observed by Malefane et al [14] on machined test specimens. A comparison between the X-and Y-direction specimens showed no significant effect of the layer orientation on the fatigue performance.…”

“…In the study of Malefane et al [14], the vertically built specimens showed a shorter fatigue life than the horizontal specimens owing to the effect of internal defects (entrapped sub-surface gas pores) that were inherent in the LPBF process. In the current study, specimens X8 and Y7 could reach an endurance limit of 5 million cycles at 190 MPa, while the Z8 specimen reached the limit at the maximum stress of 225 MPa.…”

Tensile and High Cycle Fatigue Properties of Annealed Ti6al4v (Eli) Specimens Produced by Direct Metal Laser Sintering

“…Extrapolation of the linearised curve for the fractured specimens and the horizontal curve for the fatigue endurance limit at = 450 MPa resulted in an estimate of the knee point at 55,690,000 load cycles, as indicated in Figure 2. This knee point estimate is higher than the estimated value for the as-built X-build direction DMLS Ti6Al4V (ELI) specimens in reference [16] and assumed here during testing. This implies an increase in the number of load cycles to failure of DMLS Ti6Al4V (ELI) specimens after the application of the HTA process [16].…”

“…This knee point estimate is higher than the estimated value for the as-built X-build direction DMLS Ti6Al4V (ELI) specimens in reference [16] and assumed here during testing. This implies an increase in the number of load cycles to failure of DMLS Ti6Al4V (ELI) specimens after the application of the HTA process [16]. Table 5 shows the values of the maximum tension-tension fatigue stresses and the corresponding number of load cycles to failure or run-out of the test specimens.…”

“…These are indicative of transverse propagation of cracks in the central flat areas of the fracture surface (circled in Figure 5(a)) and shear propagation of cracks inclined to the longitudinal direction of the specimens in the cup-and-cone lip areas on the periphery of the specimens [17]. On increasing the magnification of the areas of transverse propagation of cracks, the fibrous nature of the fracture surfaces became more pronounced, indicating a ductile fracture at a microscale [16]. A further increase in magnification in these areas of transverse propagation of cracks revealed the presence of dimples, as seen in Figure 5(b) (magnifications from the circled areas in Figure 5(a)).…”