Fatigue Fracture Characteristics of Ti6Al4V Subjected to Ultrasonic Nanocrystal Surface Modification

Abstract

Abstract:The influence of ultrasonic nanocrystal surface modification (UNSM) on the fatigue fracture characteristics of Ti6Al4V was investigated. Two groups of specimens were separated due to different heat treatment conditions. Group one was stress-relief annealed at 650 • C, and group two was then treated with solid solution-aging. UNSM with the conditions of a static load of 25 N, vibration amplitude of 30 µm, and 36,000 strikes per unit produced about 40 µm surface severe plastic deformation (SPD) layers on both groups of specimens. UNSM improved the microhardness and the compressive residual stress. UNSM also helped achieve a neat surface, almost without changing the surface roughness. The fatigue strengths of these two groups were improved by 7% and 11.7%, respectively. After UNSM, fatigue cracks mainly initiated from the surface of the specimen before the fatigue life of 10 6 cycles, while they appeared at the internal compress deformed α-phase at the zone between the SPD layer and the core after the fatigue life of 106 cycles. The cracks usually extended along the deformation overflow bands and the process traces on the surface. Through the change of micro-dimples in the fatigue final rupture region, nanocrystals were achieved in the SPD layer. The crystal slip and the surface remodeling together influenced the energy field of crack evolution.

Effect of Ultrasonic Surface Impact on the Fatigue Properties of Ti3Zr2Sn3Mo25Nb

“…It effects the position and the shape of inner crack initiation. It is reported that the high compressive residual stress of TC4 which is treated by UNSM rapidly decreases from the depth of 50 μm to 200 μm [5]. Thus, cracks are initiated easily in this zone without the nanograins and the compressive residual stress.…”

“…Noticeably, the inner crack cores are at this zone [35]. In addition, the compressed residual stress decreases quickly, the hardness is closed to the core and plasticity and tenacity are weakened in this area [5]. Therefore, indented slip traces of crystals are obvious in the crack initiation.…”

“…Nanograins prevent the initiation of surface crack. Microcracks which are generated by the plastic overflow after ultrasonic surface strike are observed to be surface defects [5]. Because of the surface maximum bending normal stress and the surface micro defects, some specimens have surface crack initiation with a fatigue life of more than 10 6 cycles.…”

“…Mechanical properties of some titanium alloys for biomedical use. Pure Ti [4] 170-485 240-550 15-24 25-30 ≈103 Ti6Al4V(annealed) [5] 820-870 900-930 6-10 20-25 110-114 Ti6Al7Nb [6] 880-950 900-1050 8-15 25-45 114 Ti13Nb13Zr(aged) [7] 830-910 970-1040 10-16 27-53 79-84 Ti15Mo(annealed) [8] 511 874 21 82 78 Ti12Mo6Zr2Fe(annealed) [9] 1000 Titanium and its alloys have been qualified in many fields, including space engineering, ocean engineering and biomedical engineering. However, high friction coefficient, poor wear resistance and low hardness limit its application [11].…”

Improving the Reliability of Mechanical Components That Have Failed in the Field Due to Repetitive Stress

“…Cracks in the helix dispenser require the manufacturer to redesign the product to keep it functioning for its expected lifetime. By utilizing the above failure analysis (and tests), fractures that started in voids inside mechanical parts such as gear have been shown to propagate to their ends [23][24][25][26][27][28][29][30]. In particular, intergranular fractures of austenitic stainless steels have been identified and discussed in a similar way to the failure of the helix dispenser [31].…”