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.
This paper illustrates the effects of the laser and mechanical forming on the hardness and microstructural distribution in commercially pure grade 2 Titanium alloy plates. The two processes were used to bend commercially pure grade 2 Titanium alloy plates to a similar radius also investigate if the laser forming process could replace the mechanical forming process in the future. The results from both processes are discussed in relation to the mechanical properties of the material. Observations from hardness testing indicate that the laser forming process results in increased hardness in all the samples evaluated, and on the other hand, the mechanical forming process did not influence hardness on the samples evaluated. There was no change in microstructure as a result of the mechanical forming process while the laser forming process had a major influence on the overall microstructure in samples evaluated. The size of the grains became larger with increases in thermal gradient and heat flux, causing changes to the overall mechanical properties of the material. The thermal heat generated has a profound influence on the grain structure and the hardness of Titanium. It is evident that the higher the thermal energy the higher is the hardness, but this only applies up to a power of 2.5kW. Afterwards, there is a reduction in hardness and an increase in grain size. The cooling rate of the plates has been proved to play a significant role in the resulting microstructure of Titanium alloys. The scanning speed plays a role in maintaining the surface temperatures of laser formed Titanium plates resulting in changes to both hardness and the microstructure. An increase in heat results in grain growth affecting the hardness of Titanium.
From previous research, Friction Stir Processing (FSP) was identified as a unique processing method that could be applied to improve the microstructure of welded non-ferrous alloys. FSP is similar to Friction Stir Welding (FSW) in that the material is processed below the melting point of the material, but the emphasis is not to produce a joint as in the case of FSW, but to improve mechanical performance. The difference between the two processes is that FSP is used to improve an existing weld's mechanical properties through microstructure refinement. This investigation was conducted on an aluminium alloy due to the relative difficulty with which defect free welds can be produced using conventional fusion welding processes. The aluminium alloy sections were joined using a MIG-laser hybrid process with varying gap widths between the adjoining plates. Similarly, a number of plates were also prepared using a conventional FSW process for comparative purposes. After fabrication, samples were sectioned from the welded plates for mechanical testing in order to evaluate the integrity of the welds. Mechanical properties were evaluated by tensile, fatigue and microhardness testing. Based on comparative results between the FSP MIG-laser hybrid (FSPMLH), the MIG-laser hybrid (MLH) and FSW for the three different gap widths evaluated, several general trends could be observed: weld efficiency (based on tensile strength) was the highest for the 0mm gap MIG-laser hybrid welded plates; FSP had a negligible effect on the tensile strength of the MIG-laser hybrid welded plates, but the percentage elongation values increased significantly. The effect of FSP on fatigue life became more noticeable as the width of the gap between adjoining plates increased. Microstructure evaluation revealed that FSP resulted in a refinement of the as-welded dendritic structure as well as eliminating porosity in the weld.
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