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1. The Effect of Bulk Residual Stress on Milling-Induced Residual Stress and Distortion

   https://doi.org/10.1007/s11340-022-00843-9  

   Chighizola, C. R. ; D’Elia, C. R. ; Jonsson, J. E. ; Weber, D. ; Kirsch, B. ; Aurich, J. C. ; Linke, B. S. ; Hill, M. R. ( April 2022 , Experimental Mechanics)

   Distortion arises during machining of metallic parts from two main mechanisms: 1) release of bulk residual stress (BRS) in the pre-form, and 2) permanent deformation induced by cut tools. Interaction between these mechanisms is unexplored.

   Assess this interaction using aluminum samples that have a flat surface with variations of BRS, where that surface is subsequently milled, and we observe milling-induced residual stress (MIRS) and distortion.

   Plate samples are cut from two kinds of large blocks, one kind stress-relieved by stretching and a second kind solution heat treated, quenched and aged. The BRS field in the plates is known from a recent series of measurements, being small in the stress relieved plates (within ±20 MPa) and large (±100 MPa) in the quenched plates, varying from tension to compression over the surface that is milled. MIRS is measured following milling using hole-drilling. Distortions of thin wafers cut at the milled surfaces are used to elucidate BRS/MIRS interactions. A finite element (FE) model and a strength of materials model are each used to assess consistency between wafer distortion and measured MIRS.

   Milling in samples with high BRS magnitude changes the directions of MIRS and distortion relative to the milling direction, with the direction of maximum curvature rotating toward or away from the milling direction depending on the sign and direction of BRS. High magnitude BRS was also found to increase the wafer peak arc height, nearly doubling the amount found in low BRS samples.

   Measured residual stress and observed wafer distortion both show interactions between MIRS and BRS. Stress analysis models show that the differences in measured MIRS are consistent with the differences in observed distortion.

    

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2. Physics-Informed Uncertainty Quantification in Modeling of Machining-Induced Residual Stress

   https://doi.org/10.1016/j.procir.2023.03.025  

   Hasan, Md Mehedi ; Schoop, Julius ( January 2023 , Procedia CIRP)

   Machining processes involve various sources of uncertainty which lead to inaccurate interpretation of results in the surface integrity of machined products. This work presents a physics-informed, data-driven modeling framework for achieving comprehensive uncertainty quantification (UQ) of the impact of process and material variability on machining-induced residual stress (RS). Uncertainty due to the variation in bulk material properties and model input parameters in machining are considered. Preliminary results showed that variations in calibration parameters have a substantial effect on modeling RS, while the variation in material properties has a smaller effect. Further research directions for UQ in machining are also outlined. 

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3. Increased ductility of Ti-6Al-4V by interlayer milling during directed energy deposition

   https://doi.org/10.1016/j.addma.2023.103818  

   Karunakaran, Rakeshkumar ; Sotelo, Luz D ; Maharaja, Hitarth ; Nez, Calsey ; Ramoni, Monsuru ; Halliday, Scott ; Mishra, Sushil ; Karunakaran, KP ; Turner, Joseph A ; Sealy, Michael P ( September 2023 , Additive Manufacturing)

   Additive manufacturing (AM) often results in high strength but poor ductility in titanium alloys. Hybrid AM is a solution capable of improving both ductility and strength. In this study, hybrid AM of Ti-6Al-4 V was achieved by coupling directed energy deposition with interlayer machining. The microstructure, residual stress, and microhardness were examined to explain how interlayer machining caused a 63% improvement in ductility while retaining an equivalent strength to as-printed samples. Interlayer machining introduced recurrent interruptions in printing that allowed for slow cooling-induced coarsening of acicular α laths at the machined interfaces. The coarse α laths on the selectively machined layers increased dislocation motion under tensile loads and improved bulk ductility. The results highlighted in this publication demonstrate the feasibility of hybrid AM to enhance the toughness of titanium alloys. 

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4. Intermethod comparison and evaluation of near surface residual stress in aluminum parts subject to various milling parameters

   https://doi.org/10.1007/978-3-030-30098-2  

   Chighizola, Christopher R. ; D’Elia, Christopher R. ; Hill, Michael R. ( June 2019 , Residual Stress, Thermomechanics & Infrared Imaging and Inverse Problems, Volume 6; Proceedings of the 2019 Annual Conference on Experimental and Applied Mechanics)

   Near surface residual stress (NSRS) induced by machining (e.g., milling) is known to drive distortion in machined aluminum, particularly in thin complex geometries with tight tolerance requirements where large distortion is undesirable. The understanding and characterization of NSRS in milled aluminum parts is important and should be included in the design and manufacturing process. There exist a variety of experimental tests characterizing these stresses. The objective of this paper is to assess the quality of three experimental methods for evaluating NSRS in prismatic aluminum parts subject to various milling parameters. The three methods are: hole-drilling, slotting, and x-ray diffraction, all of which include incremental material removal. The aluminum parts are cut from stress-relieved plate, AA7050-T7451. A combination of milling table and tool speeds are used to machine a flat surface in the parts. Measurements are made at specified locations and depths on each part. NSRS data from the hole drilling and slotting measurements were comparable; NSRS data from x-ray diffraction differed and was less repeatable. NSRS data for different milling parameters shows that the depth of NSRS increases with feed per tooth but is unaffected by different cutting speeds. 

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5. Influence of clamping technique on the resulting surface roughness in diamond machining of CaF2

   Adam, B. ; Riemer, O. ; Rickens, K. ; Lucca, D.A. ; Tunesi, M. ( June 2022 , Proceedings of the 20th euspen International Conference and Exhibition)

   Single crystal calcium fluoride (CaF2) is widely used for transmissive optics in the ultraviolet and vacuum ultraviolet (UV and VUV) spectral regions because of its high optical transmission. Optical components made of CaF2 are usually manufactured by precision machining to generate high quality surfaces with low surface roughness. However, the influence of the clamping technique on the resulting surface roughness of diamond machined CaF2 has not been reported. In this research, two clamping techniques, vacuum clamping and gluing with wax, are used in off-axis diamond turning experiments with zero degree and negative rake angle diamond tools. Surface characterization by white light interferometry and atomic force microscopy show surfaces with low surface roughness. Furthermore, a significant influence of the clamping technique on the generated surface topography is observed. 

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