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Research Papers

J. Manuf. Sci. Eng. 2018;140(6):061002-061002-9. doi:10.1115/1.4039109.

Ultrasonic-assisted machining, which is the application of ultrasonic vibrations to standard or “conventional” machine tools for processes such as drilling, milling, and turning, is a rapidly developing technology aimed at increasing the productivity of machining processes. While a solid foundation is being established through laboratory-based research studies, typically these processes have not yet progressed to fulfill the demanding requirements of the factory floor. The objective of the current work is to transition the ultrasonic-assisted drilling (UAD) process from the laboratory to a production system compatible with automated machining systems. This work details the design and development of an ultrasonic drilling module that has sufficient strength, stiffness, and accuracy for production demands, while maintaining powerful levels of ultrasonic vibrations that result in lowered drilling forces and faster feed rates. In addition, this work will review prior work in UAD, including the development of a module based on a vibration-isolating case using a standard tool holder. Performance of the system is shown to provide thrust force reductions, while maintaining or improving surface finish and drilling accuracy. The results from drilling several materials are presented.

Commentary by Dr. Valentin Fuster
J. Manuf. Sci. Eng. 2018;140(6):061003-061003-11. doi:10.1115/1.4038570.

This paper formulates the generalized dynamics and stability of thread turning operations with custom multipoint inserts. The closed-loop chip regeneration mechanism is modeled by evaluating the effect of the current vibrations and the vibration marks left from the previous tooth. Using the developed chip discretization method, the dynamic cutting and process damping forces are obtained at each point along the cutting edge by projecting the three-dimensional (3D) vibrations of the tool and workpiece in the direction of local chip thickness. The equation of motion is derived in both physical and modal spaces, and stability is analyzed in frequency domain using Nyquist criterion. An iterative process optimization algorithm has been developed to maximize productivity while respecting machine tool's torque and power limits. Extension of the model to thin-walled workpieces along with the validating experiments on real-scale oil pipes is presented in Part II of this paper.

Commentary by Dr. Valentin Fuster
J. Manuf. Sci. Eng. 2018;140(6):061004-061004-11. doi:10.1115/1.4039063.

One of the key barriers to widespread adoption of additive manufacturing (AM) for metal parts is the build-up of residual stresses. In the laser-based powder bed fusion process, a laser selectively fuses metal powder layer by layer, generating significant temperature gradients that cause residual stress within the part. This can lead to parts exceeding tolerances and experiencing severe deformations. In order to develop strategies to reduce the adverse effects of these stresses, the stresses first need to be quantified. Cylindrical Nickel Alloy 625 samples were designed with varied outer diameters, inner diameters, and heights. Neutron diffraction was used to characterize the three-dimensional (3D) stress state throughout the parts. The stress state of the parts was generally comprised of tensile exteriors and compressive interiors. Regardless of part height, only the topmost scan height of each part experienced large reductions in axial and hoop stress. Improved understanding of the residual stress trends will aid in model development and validation leading to techniques to reduce negative effects of the residual stress.

Topics: Stress
Commentary by Dr. Valentin Fuster
J. Manuf. Sci. Eng. 2018;140(6):061005-061005-10. doi:10.1115/1.4037438.

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.

Commentary by Dr. Valentin Fuster

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