Effect of Multi-Pass Ultrasonic Surface Rolling on the Mechanical and Fatigue Properties of HIP Ti-6Al-4V Alloy

Abstract

The main purpose of this paper was to investigate the effect of a surface plastic deformation layer introduced by multi-pass ultrasonic surface rolling (MUSR) on the mechanical and fatigue properties of HIP Ti-6Al-4V alloys. Some microscopic analysis methods (SEM, TEM and XRD) were used to characterize the modified microstructure in the material surface layer. The results indicated that the material surface layer experienced a certain extent plastic deformation, accompanied by some dense dislocations and twin generation. Moreover, surface microhardness, residual stress and roughness values of samples treated by MUSR were also greatly improved compared with that of untreated samples. Surface microhardness and compressive residual stress were increased to 435 HV and −1173 MPa, respectively. The minimum surface roughness was reduced to 0.13 μm. The maximum depth of the surface hardening layer was about 55 μm. However, the practical influence depth was about 450 μm judging from the tensile and fatigue fracture surfaces. The ultimate tensile strength of the MUSR-treated sample increased to 990 MPa from the initial 963 MPa. The fatigue strength of the MUSR-treated sample was increased by about 25% on the base of 107 cycles, and the lifetime was prolonged from two times to two orders of magnitude at the applied stress amplitudes of 650–560 MPa. The improved mechanical and fatigue properties of MUSR-treated samples should be attributed to the combined effects of the increased microhardness and compressive residual stress, low surface roughness, grain refinement and micro-pore healing in the material surface-modified layer.

The influence of ultrasonic surface rolling on the fatigue and wear properties of 23-8N engine valve steel

“…and Suh et al that the crack initiation position was shifted to the subsurface region after the surface enhancement with ultrasonic vibration technology [16,23]. As presented in Fig.…”

“…Dense dislocation can provide the strain energy with high density. The new boundaries of grains can be formed by the mode of annihilating, accumulating and rearranging of dislocation [16]. With the dislocation accumulating at the grain boundaries, the dislocation density was increased.…”

“…With the dislocation accumulating at the grain boundaries, the dislocation density was increased. titanium material was only about 20 m to 60 m [15][16][17]. According to the Hall-Petch relationship [26], the microhardness of the material surface and severe plastic deformation layer could be increased by good grain refinement, and the work hardening effect will be produced by plastic compressive deformation, which resulted from the high frequency ultrasonic surface impacts.…”

“…Based on the methodology presented in the previous investigations of Qu et al on titanium alloy with hardness of 325 HV [15][16][17], the residual stress and hardened depth of modified layer was increased when the static force was increased from 600 N to 900 N. Hence, optimized processing parameters of USR were chosen and they are listed in Table 3. The applied static force was chosen at 900 N and the amplitude of vibration was set at 10 m. The frequency of ultrasonic vibration was respectively chosen at 20 kHz and 30 kHz, and an air compressor was used to support the static force at a certain air pressure.…”

“…These strikes can be considered as having a micro-cold-forging process [14]. Similarly, ultrasonic surface rolling (USR) also has been applied to in a titanium alloy in previous work of Qu et al [15][16][17].…”