Characterization of machining-induced residual stresses in titanium-based alloys

Machining-induced residual stresses (RS) have a major impact on the fatigue life of critical aero-engine parts subjected to dynamic loads in harsh environments.Their state and magnitude can be controlled by careful selection of cutting conditions.Tensile RS are extremely harmful as they accelerate crack nucleation and propagation, diminishing the resistance to fatigue failures.It is crucial to identify cutting parameters that promote desirable compressive RS in critical parts without compromising other aspects of surface integrity.Limited information is available in the open literature on machining-induced RS in Ti-alloys.In this research, an extensive experimental investigation is performed of the effect of cutting parameters on RS in two aerospace grade Ti-alloys, Ti-64 and Ti-6246, used for aeroengine fan and compressor parts.Cutting is performed at conditions relevant to industry.This is coupled with a comprehensive evaluation of surface integrity including RS, surface roughness, the near-surface microstructure, and hardness distribution.Based on x-ray diffraction measurements, empirical models are developed that offer fast and accurate predictions of surface RS.For the investigated finish turning regime, RS are compressive in nature.Due to a conflict between RS and surface finish, guidelines are established for the optimal selection of cutting parameters.Empirical models are non-generic, cannot be extrapolated, and cannot offer a physical interpretation of the phenomena that govern the cutting process.Available FE models for Tialloys mostly focus on chip formation and force prediction, and rarely extend to RS.In this work, a 2D FE model is constructed using DEFORM TM for the prediction of machininginduced RS in Ti-alloys, and is optimized for accuracy and computational efficiency.This is preceded by a numerical study on the relative contribution of thermal loads, mechanical loads, and phase transformations to the resultant stress state in commercially available materials.The FE model is firstly validated against experimental machining forces, cutting temperatures, and RS.It is then used as a virtual machining medium to gain insight into the effect of cutting parameters, tool edge preparation, flank wear, and chip segmentation on residual stress formation.FE predictions for Ti-6246 show that RS are highly sensitive to flank wear, which can cause a severe shift from the compressive to the tensile state.For the investigated cutting regime, residual stress prediction errors for Ti-64 are limited to 10% provided that chip segmentation is modeled at relatively high cutting speeds.ii RSUMLes contraintes rsiduelles (CR) induites par l'usinage ont un impact majeur sur la rsistance la fatigue des composantes soumises aux charges dynamiques dans les environnements svres.Leur tat peut tre contrl par la slection soigneuse des conditions de coupe.Les CR en tension rduisent la rsistance la dfaillance par fatigue.Par consquent, l'identification des paramtres de coupe qui promeuvent des CR compressives, en vitant l'endommagement de l'intgrit de la surface, est extrmement importante.Les tudes sur les CR produites par l'usinage des alliages de titane son trs limite.Dans ce travail, une enqute approfondie est effectue sur les effets des paramtres de coupe sur les CR gnres dans deux alliages de titane, Ti-64 et Ti-6246, utilises dans les moteurs d'avions.L'usinage et ralis des conditions pertinentes pour l'industrie.Cela et suivi par une valuation de l'intgrit de la surface incluant les CR, la rugosit, la microstructure, et les profils de duret.Bas sur des mesures des CR par diffraction des rayons X, des modles empiriques rapides et prcis sont dvelopps.Pour les conditions de tournages de finition enqutes, les CR sont compressives.Du un conflit entres les CR et la rugosit, des lignes directrices sont dveloppes pour la slection des paramtres de coupe.Les modles empirique qui sont non-gnrique ne peuvent pas tre extrapols et ne peuvent pas offrir une interprtation des phnomne physiques.Les modles d'lment finis disponibles pour les alliages de titane abordent rarement la formation des CR.Dans cette recherche, un modle 2D d'lment finis est dvelopp travers DEFORM TM pour la prdiction des CR gnres par l'usinage.Le modle est optimiser pour l'efficacit et la prcision.Cela est prcd par une tude numrique sur la contribution relatif des charges thermiques, charges mcaniques, et transformations de phases a l'tat final des CR dans des nombreux matriaux.Le modle est valid contre des mesurent de forces, temprature, et CR produites par l'usinage.Par la suite, le modle est utilis comme un milieu d'usinage virtuel pour l'analyse des effets des paramtres de coupe, l'usure, et la segmentation des copeaux sur l'volution des CR.Les tudes numrique lies au Ti-6246 dmontres que les CR sont trs sensibles l'usure qui peut promouvoir un changement d'tat des CR de compression en tension.Pour les conditions de tournages de finition enqutes, la marge d'erreur de prdiction des CR gnres dans le Ti-64 est de 10% en condition que la segmentation des copeaux est modlis a des vitesses de coupe relativement leves.iii CLAIMS OF ORIGINALITYThis research addresses important gaps related to machining-induced residual stresses (RS) in Ti-alloys used in the manufacture of critical aero-engine parts subjected to dynamic loads in harsh environments.The knowledge gained through this work is highly significant to increasing the reliability, safety, and service life of aero-engines:1. Prior numerical studies on the machining of Ti-alloys focused on chip formation and force prediction.Very limited attention was given to machining-induced residual stresses (RS) although they critically affect the fatigue resistance of machined components.Using the FE model developed in this work, an in-depth analysis of the effect of cutting parameters, tool edge radius, flank wear, and material properties on RS was performed that highlights important aspects that should be considered in order to generate compressive RS in Tialloys.Such an analysis for Ti-alloys is practically not available in the open literature and yields crucial information for the optimization of the machining process.2. Analytical studies of the origins of RS are mainly focused on the grinding process.This research provided fundamental understanding of the significance and relative contributions of mechanical, thermal and metallurgical sources of RS generated during cutting.It allowed isolating the effect of these sources and gaining knowledge of how they are affected by material properties and process conditions.Without this information, emerging technologies such as laser-assisted machining (LAM) and cryogenic machining cannot be optimized to generate favorable residual stress distributions.3. The effect of chip segmentation on machining-induced RS is not fully explored and in the case of Ti-alloys was only addressed for high speed machining.In this work, through FE analysis of state variable distributions, this effect was determined for conventional cutting speeds, and its impact on the accuracy of residual stress predictions was analyzed.4. There is a lack of information on machining-induced RS in Ti-alloys, especially for the finish cutting regime.In this work, extensive experimental investigation was carried out to fill this gap for two aerospace grade / Ti-alloys, Ti-64 and Ti-6246, used in the manufacture of aero-engine disks and blades.Guidelines were established for the optimal selection of cutting parameters that attain desired levels of compressive RS and surface quality.

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