Residual Stress Distribution and the Concept of Total Fatigue Stress in Laser and Mechanically Formed Commercially Pure Grade 2 Titanium Alloy Plates

Abstract

This paper discusses the investigation of residual stresses developed as a result of mechanical and laser forming processes in commercially pure grade 2 titanium alloy plates as well as the concept of total fatigue stress (TFS). The intention of the study was to bend the plates using the respective processes to a final radius of 120 mm using both processes. The hole drilling method was used to measure residual strains in all the plates. High stress gradients were witnessed in the current research and possible cases analyzed and investigated. The effects of processing speeds and powers used also played a significant role in the residual stress distribution in all the formed plates. A change in laser power resulted in changes to residual stress distribution in the plates evaluated. This study also dwells into how the loads that are not normally incorporated in fatigue testing influence fatigue life of commercially pure grade 2 titanium alloy plates. Also, the parent material was used to benchmark the performance of the two forming processes in terms of stresses developed. Residual stresses developed from the two forming processes and those obtained from the parent material were used. The residual stress values were then added to the mean stress and the alternating stress from the fatigue machine to develop the concept of TFS. This exercise indicated the effect of these stresses on the fatigue life of the parent material, laser and mechanically formed plate samples. A strong link between these stresses was obtained and formulae explaining the relationship were formulated. A comparison between theory and practical application shown by test results is found to be satisfactory in explaining concerns that may arise. The laser forming process is more influential in the development of residual stress, compared to the mechanical forming process. With each parameter change in laser forming, there is a change in residual stress arrangement. Under the influence of laser forming, the stress is more tensile in nature making the laser formed plate specimens more susceptible to early fatigue failure. The laser and mechanical forming processes involve bending of the plate samples and most of these samples experienced a two-dimensional defect, which is a dislocation. The dislocation is the defect responsible for the phenomenon of slip by which most metals deform plastically. Also, the high temperatures experienced in laser forming were one of the major driving factors in bending.

Microstructure and Mechanical Properties of Laser and Mechanically Formed Commercially Pure Grade 2 Titanium Plates

“…The graph obtained is totally different when compared to other plates evaluated in this study. With the other power levels in laser formed plates, there were changes in residual stress and strain with changes in drill depth [8]. This figure also shows an even distribution of residual strains on the material, and, unlike the laser formed plates, it seems possible that the temperature gradient on the parent plates during fabrication was not steep.…”

“…The laser formed plates showed higher values of stress than both mechanically formed and the parent materials. The effect of these stresses is therefore evident in fatigue testing and is documented in the results obtained [8]. The mechanical forming process resulted in minor changes to the relieved stress and strain, when compared to the parent material results.…”

“…Surface residual stress is of high importance to mechanical design engineers as they show areas of high residual stress. The high residual stress areas help contribute to fatigue failure of the material [8]. All values obtained in the analysis of residual stress and strain of CP grade 2 titanium plates are shown in Table 6, and results obtained allude to the performance of these plates during fatigue testing.…”

“…Readings were taken to fully understand the incident power hitting the titanium plate surface. As an example for a laser power setting of 3500 W, the pocket monitor reader would show 3250 W. This value indicates a 10% loss in power on the irradiated sample [8]. This assisted in understanding and acknowledging the presence of losses in laser irradiation in material processing.…”

“…Line energy is the most important variable in the laser forming process. It was also decided to determine what influence the variation in power levels would have on the microstructure and the mechanical properties of titanium [8]. There is a widespread belief that a line energy threshold must be exceeded in order to commence with permanent deformation by the temperature gradient method (TGM) [9].…”