Effect of high-pressure cooling on the residual stress in Ti-alloys during machining

Sammanfattning: Titanium alloys are widely used in aerospace industry but also in other industry sectors. Details for compressors used for generation and petrochemical plants and medical devices can be mentioned as a few examples. Plastic deformation during forming of the metals introduces residual stresses. Fatigue, creep and corrosion are typical failure mechanisms that are stopped or accelerated in the presence of tensile residual stress. Metal cutting as a manufacturing method generates residual stresses in the surface layer. The stress state in the near-surface zone of components is of special interest as the surface often suffers loads and consequently the cracks initiate and begin to grow at that location. The scope of this thesis is to investigate the properties of the machined surface with regards to measurement of residual stresses. The results achieved by X-ray diffractometry were compared with the results using a simulating method with the same cutting data for checking the accuracy of these two methods. This review highlighted the residual stress character and how they can be measured by X-ray diffractometry in two different directions, the transversal, the cutting direction, and longitudinal, the feed direction. For measurement of residual stress under the surface, the material was removed by an electro-chemical polishing method for not introducing more stress. The same procedure was checked using Finite Element simulations. The results obtained from the investigation clarified that: The residual stress measurement by X-rays was quite accurate in comparison with finite element simulation on the surface generated by turning on titanium alloy. There were compressive residual stresses in the cutting and feed directions of the cut. Residual stress is beneficial for delay of crack propagation. High-pressure cooling produced and increased compressional residual stresses.