Fatigue behavior of Ti-6Al-4V foams processed by magnesium space holder technique

Porous Ti-6Al-4V alloys are widely used in the biomedical applications for hard tissue implantation due to their elastic moduli being close to that of bone. In this study, porous Ti-6Al-4V alloys were produced with a powder metallurgical process, space holder technique, where magnesium powders were utilized to generate porosity in the range of 51-65 vol%. The production of porous Ti-6Al-4V alloys was composed of three steps. Firstly, spherical Ti-6Al-4V powders with an average size of 55 μm were mixed with spherical magnesium powders, which have been sieved to an average size of 375 μm. Secondly, the mixtures were compacted with a hydraulic press under 500 MPa pressure by using a double-ended steel die. Finally, the green compacts were heated to 1200 °C, during which magnesium powder evaporates, and sintered for 2 h under high purity argon gas atmosphere. Processed foams were investigated under scanning electron microscope and yielded that the foams were composed of spherical, interconnected macropores and irregular shaped micropores. Monotonic compression tests were conducted under quasi-static test conditions to the processed foams. Yield strengths of the foams were found to vary between 69 and 167 MPa and elastic moduli were between 4 and 12 GPa. Processed foams were also dynamically tested under compression-compression fatigue with a stress ratio of 0.1. The foams exhibited similar fatigue response when maximum applied stress was normalized with the average yield strength of the corresponding porosity content. It was found that foams were fatigue immune with a practical limit of 1 million cycles under a maximum applied stress of 0.75σ (normalized).