K. Chattopadhyay

Surface nanostructuring of Ti-6Al-4V alloy through ultrasonic shot peening

Solution treated samples of titanium alloy Ti-6Al-4V were subjected to ultrasonic shot peening (USSP) for different durations of 5, 15, and 30 minutes with hard steel balls of 3 mm diameter and examined for modification of microstructure, surface roughness and hardness profile. Based on microhardness profile a nanostructured layer of approximately 100 µm thickness resulted from the USSP. The shot peened surfaces were examined also by XRD for phase transformation, if any, resulting from USSP. Grain refinement was observed to nano scale due to USSP without any phase transformation. However, surface cracking was observed in the sample subjected to longer duration of 30 minutes and surface roughness was increased with duration of USSP. Microhardness of the 30 minutes shot peened surface was increased by ∼34%, however, the increased hardness level was reduced by ∼14%, following stress relieving treatment (400°C−1 h). The results are discussed in terms of the processes of grain refinement through USSP.

Effect of Ultrasonic Shot Peening on Surface Microstructure and Fatigue Behavior of Structural Alloys

Fatigue resistance of structural materials is considered as one of the most important factors in design of structural components for integrity, efficiency and safe functioning under repeated mechanical/thermal loading. Fatigue resistance of structural alloys is characterized in terms of appropriate parameters based on high cycle/low cycle fatigue loading. Fatigue life comprises of two components, number of cycles for crack initiation and for crack propagation. While there is dominant role of the process of crack initiation in high cycle fatigue, low cycle fatigue is controlled by crack propagation. Invariably, fatigue cracks initiate from the surface and surface modification has been used for enhancement of fatigue life, delaying the process of crack initiation. Inducement of compressive residual stress in surface region through shot peening has been one of the most widely used conventional process of enhancing high cycle fatigue life. This paper presents the effect of surface grain refinement, using the novel technique of ultrasonic shot peening, on enhancement of fatigue life of structural alloys at low stress/strain.

Optimization of USSP duration for enhanced corrosion resistance of AA7075

HighlightsUSSP treatment resulted in grain refinement near the surface region.Sample treated for 15 s exhibited superior corrosion properties.The passive layer mainly constituted Al(OH)3 with Al2O3.Pitting was mainly associated with the cathodic second phase precipitates. ABSTRACT This investigation was carried out following our earlier work on the effect of ultrasonic shot peening (USSP) on corrosion resistance of the 7075 aluminium alloy in 3.5 wt% NaCl solution to optimize the duration of USSP. The samples not treated with USSP and different samples treated with USSP were subjected to potentiodynamic polarization and electrochemical impedance spectroscopy. Among the specimens USSP treated from 5 to 30 s, the one USSP treated for 15 s (USSP 15) was found to exhibit highest corrosion potential (Ecorr) and lowest corrosion current density (icorr). Corrosion products were characterized by Scanning Electron Microscopy (SEM) and X‐ray Photoelectron Spectroscopy (XPS). Scanning Kelvin Probe Force Microscopy (SKPFM) was used to measure the surface free potential. The enhanced corrosion resistance of the USSP 15 sample was found to be due to combined effect of surface nanostructure of the matrix, homogeneity and refinement of second phase precipitates. There was enhancement in formation of adherent passive layer in the USSP15 specimen.