Determination of the fatigue and fracture properties and improvement of the dynamic behavior of additively manufactured titanium alloys used in the aviation industry at different temperatures

This study aims to analyze the fatigue behavior and fracture mechanics properties of the Ti6Al4V alloy produced by the Selective Laser Melting (SLM) method under different service conditions. SLM is one Additive Manufacturing (AM) technique where metal powders are melted layer-by-layer using a computer-controlled high-energy laser to produce complex geometries. Due to their high strength-to-weight ratio, titanium alloys are widely used in automotive, defense, and aerospace applications. Turbine blades produced by SLM are subjected to repetitive loads under various service conditions, which can trigger crack formation leading to failure. In the aerospace and defense industries, turbines are expected to operate efficiently when exposed to high temperatures or extreme cold environments. Additionally, materials can be subjected to sudden overloads under variable loads. Therefore, it is crucial to investigate the fatigue behavior of SLM-produced materials under constant and variable amplitude loads and to enhance their fatigue life. Since fatigue damage is known to be directly related to surface properties, improving these properties can enhance fatigue performance. In this context, the Ti6Al4V fatigue and fracture mechanics samples produced by SLM were evaluated in three groups: as-built, heat-treated at 850ºC for 2 hours followed by electrochemical polishing (secondary treated) and coated with DLC using the Closed Area Non-Uniform Magnetic Spraying method after heat treatment. The fatigue and crack propagation behaviors of the samples at different ambient temperatures (25 ºC, -50 ºC, 50 ºC, 250 ºC, and cyclic -50/+50 ºC) were examined, and their structural, morphological, and mechanical properties were analyzed using XRD, SEM, XPS, optical microscopy, 3D optical profilometry, and microhardness testing. Changes in crack sizes during fatigue crack propagation were detected using digital cameras and the Digital Image Correlation (DIC) method. The results showed that the best fatigue resistance was achieved at 25 ºC for all sample groups; however, the formation of subsurface oxide layers at 250 ºC, embrittlement due to low temperatures at -50 ºC, and thermal expansion differences caused by cyclic -50/+50 ºC conditions negatively affected the material's fatigue life. Heat treatment and surface processes increased the material's hardness, creating residual compressive stresses at the surface, thereby improving fatigue life under all test temperature conditions.