Axial Fatigue of PBF-LB Ti6Al4V: Process and Surface Effects

Overview

This investigation examines how layer thickness and surface condition influence the microstructure and mechanical performance of Ti6Al4V components manufactured through Laser Powder Bed Fusion (PBF-LB). Specimens were fabricated using two layer thicknesses—40 micrometers and 80 micrometers—while maintaining identical process parameters across both conditions. The research addresses the relationship between process-induced microstructural variations and fatigue behavior, with particular emphasis on the relative contributions of surface integrity versus microstructural morphology to fatigue performance. Both processing conditions yielded fully martensitic alpha-prime microstructures, though the 40 micrometer builds exhibited finer lamellae and smaller prior-beta grains attributable to higher cooling rates during solidification. The study situates its findings within the broader context of additive manufacturing productivity trade-offs, where enhanced mechanical performance through finer layers and surface finishing must be balanced against reduced build rates.

Methods and approach

The experimental program involved tensile and axial fatigue testing of Ti6Al4V specimens produced via PBF-LB with two layer thickness conditions. Microstructural characterization employed field-emission scanning electron microscopy (FE-SEM) and electron backscatter diffraction (EBSD) techniques to assess phase composition, grain morphology, and lamellar structure. The fatigue testing protocol examined multiple variables including layer thickness, surface condition (as-built versus electropolished), and build orientation (diagonal versus other orientations). Process parameters excluding layer thickness were held constant to isolate the effects of this single variable on resultant microstructure and mechanical properties. The characterization methodology enabled direct correlation between cooling rate-dependent microstructural features and measured mechanical response under both monotonic and cyclic loading conditions.

Key Findings

Both layer thickness conditions produced fully martensitic alpha-prime microstructures, with the 40 micrometer builds demonstrating finer lamellae and smaller prior-beta grain sizes due to elevated cooling rates during fabrication. Tensile testing revealed that 40 micrometer specimens exhibited higher ductility while maintaining strength levels comparable to the 80 micrometer condition. Axial fatigue testing demonstrated superior performance for specimens built with 40 micrometer layers, electropolished surfaces, and diagonal build orientation. The data establish that fatigue performance in PBF-LB Ti6Al4V is primarily controlled by surface integrity and defect population rather than by variations in microstructural morphology between the two layer thickness conditions. Finer layer thickness and electropolishing both contribute to enhanced endurance strength, though these improvements are accompanied by reduced build productivity due to increased processing time.

Implications

The findings confirm that surface-related factors dominate fatigue behavior in PBF-LB Ti6Al4V, with microstructural refinement playing a secondary role despite measurable differences in lamellar spacing and grain size between layer thickness conditions. This hierarchy of influence has practical implications for process optimization, suggesting that post-process surface treatments such as electropolishing may deliver fatigue improvements more efficiently than microstructural refinement through reduced layer thickness alone. The documented trade-off between mechanical performance and build productivity represents a critical design consideration for industrial implementation, as the enhanced endurance strength achieved through 40 micrometer layers and surface finishing must be justified against extended manufacturing time and increased production costs. The primacy of surface integrity over microstructural morphology in governing fatigue life indicates that defect mitigation strategies and surface finishing protocols warrant prioritization in process development for fatigue-critical applications of additively manufactured Ti6Al4V components.

Scope and limitations

This summary is based on the study abstract and available metadata. It does not include a full analysis of the complete paper, supplementary materials, or underlying datasets unless explicitly stated. Findings should be interpreted in the context of the original publication.