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Review Article

J. Manuf. Sci. Eng. 2016;139(5):050801-050801-21. doi:10.1115/1.4034439.

This article presents a review of current methods for production of metallic open-cell porous materials through space holders. The methods are divided into two major groups: on the basis of sintering and using liquid phase processing. Details about technologies are given, and their relations to structure parameters of obtained materials are discussed. Methods with 11 different space holders are described. The space holders could be metallic or nonmetallic (organic and inorganic) materials which could be leached or burned depending on removal technique. It is concluded that the flexible application of different space holders offers opportunities for obtaining large variety of metallic porous structures. A new line of development should be elaboration of complex techniques for production of porous structure with graded pore size and/or porosity which will meet various engineering requirements and will open new possibilities for applications as functional and structural elements. The next part of this work is devoted to the structure, the properties, and application of the open-cells porous materials obtained through space holders.

Commentary by Dr. Valentin Fuster
J. Manuf. Sci. Eng. 2016;139(5):050802-050802-31. doi:10.1115/1.4034440.

This work presents an overview of structural characteristics and basic mechanical properties of the open-cell metallic foams obtained by different space-holder methods, which are discussed in Part I of the same review. The presentation is arranged with respect to foam material, and the structure and properties are compared for different space holders and production techniques. In order to have more clear information for the structures obtained and their relation with production techniques, many images are provided and discussed. Compressive behavior of the foams is shown, and stress–strain curves are analyzed with respect to the energy absorption characteristics. The analysis are made on the basis of different porosities and processing parameters. Some applications of the open-cell metallic foams are discussed in the end of the article.

Commentary by Dr. Valentin Fuster

Research Papers

J. Manuf. Sci. Eng. 2016;139(5):051001-051001-16. doi:10.1115/1.4034715.

Selective laser melting (SLM) has been attracting a growing interest in different industrial sectors as it is one of the key technologies for metal additive manufacturing (AM). Despite the relevant improvements made by the SLM technology in the recent years, process capability is still a major issue for its industrial breakthrough. As a matter of fact, different kinds of defect may originate during the layerwise process. In some cases, they propagate from one layer to the following ones leading to a job failure. In other cases, they are hardly visible and detectable by inspecting the final part, as they can affect the internal structure or structural features that are difficult to measure. This implies the need for in-process monitoring methods able to rapidly detect and locate defect onsets during the process itself. Different authors have been investigating machine sensorization architectures, but the development of statistical monitoring techniques is still in a very preliminary phase. This paper proposes a method for the detection and spatial identification of defects during the layerwise process by using a machine vision system in the visible range. A statistical descriptor based on principal component analysis (PCA) applied to image data is presented, which is suitable to identify defective areas of a layer. The use of image k-means clustering analysis is then proposed for automated defect detection. A real case study in SLM including both simple and complicated geometries is discussed to demonstrate the performances of the method.

Commentary by Dr. Valentin Fuster
J. Manuf. Sci. Eng. 2016;139(5):051002-051002-14. doi:10.1115/1.4035035.

Geometrical accuracy of microfeatures in micromilling is strictly related with the choice of cutting parameters. Their correct selection is a challenging task in particular when the target feature geometry is a high aspect ratio feature with tight tolerance requirements. Metallic micromilled pins are adopted in many different industrial applications as in the micromold technology field, in the microelectromechanical systems, and in the biomedical devices and their geometrical accuracy represents a fundamental property for their functionality. This work outlines the connection between the achieved geometrical accuracy and the micromilling parameters and cutting strategies on pins with diameter = 100 μm and height = 2 mm (i.e., aspect ratio = 20). Pin geometrical error features are extracted from three-dimensional optical measurements and then correlated with cutting parameters to support machining process setup. A proper fitting based on Chebyshev functions is applied and a statistical analysis assesses the importance of each deviation component in relation to the imposed cutting conditions. The proposed methodology fills the specific lack in the literature domain about micropin machining and can easily extend to different types of geometrical microfeatures. Finally, correlation between part geometrical errors and machining forces is analyzed. Cutting force analysis is adopted in conventional machining for implementing online geometrical errors assessment or compensation methods. However, this study confirms that the applicability of this approach in high aspect ratio pin micromilling is prevented from the predominant scale-effects and the large part bending that generates a low direct correlation between forces and part geometrical errors.

Commentary by Dr. Valentin Fuster
J. Manuf. Sci. Eng. 2016;139(5):051003-051003-16. doi:10.1115/1.4034846.

Energy consumption in five-axis machining of freeform surfaces can be considerably large for large-size parts. This paper presents a study on how to setup the workpiece in order to minimize the energy consumption without modifying the toolpath itself. For an arbitrary freeform workpiece, the way how it is setup on the working table highly affects the machine's kinematic behavior, which dominates the overall processing time and energy consumption. Taking into account the speed and acceleration limit of each axis of the machine, we first establish the energy consumption model as a function of the workpiece setup. However, this original model involves certain critical physically pertinent coefficients (such as the moment of inertial of a rotary table) which are usually unavailable in practice. Instead, by exploring insightful geometric characteristics of the five-axis machine, an alternative energy consumption model is established which is independent of those hard-to-obtain coefficients. A simple algorithm is then designed to optimize this model. Both computer simulations and physical cutting experiments demonstrate that, when compared with an arbitrary setup, the optimized workpiece setup is able to achieve a significant saving (as much as 50%) in both energy consumption and total machining time, both using a same tool path.

Commentary by Dr. Valentin Fuster
J. Manuf. Sci. Eng. 2016;139(5):051004-051004-12. doi:10.1115/1.4034895.

A coupled thermal–mechanical model based on the Eulerian formulation is developed for the steady-state dissimilar friction stir welding (FSW) process. Multiple phase flow theories are adopted in deriving analytical formulations, which are further implemented into the fluent software for computational fluid dynamics analysis. A shear stress boundary at the tool/workpiece interface yields a much more reasonable material distribution compared with a velocity boundary condition when the involved two materials have quite different physical and mechanical properties. The model can capture the feature of embedded steel strip in aluminum side, as observed in weld cross sections from experiments. For further evaluation, the calculated flow and thermal response are compared with experimental results in three welding conditions, which generally show good agreements.

Commentary by Dr. Valentin Fuster
J. Manuf. Sci. Eng. 2016;139(5):051005-051005-13. doi:10.1115/1.4034897.

Miniature components with complex shape can be created by micromilling with excellent form and finish. However, for difficult-to-machine materials, such as Ti-alloys, failure of low-flexural stiffness microtools is a big limitation. High spindle speeds (20,000–100,000 rpm) can be used to reduce the undeformed chip thickness and the cutting forces to reduce the catastrophic failure of the tool. This reduced uncut chip thicknesses, in some cases lower than the cutting edge radius, can result in intermittent chip formation which can lead to dynamic variation in cutting forces. In addition, the run-out and the misalignment effects are amplified at higher rotational speeds which can induce dynamic force variation. These dynamic force variations coupled with low-flexural rigidity of micro end mill can render the process unstable. Consequently, accurate prediction of forces and stability is essential in high-speed micromilling. Most of the previous studies reported in the literature use constant cutting coefficients in the mechanistic cutting force model which does not yield accurate results. Recent work has shown significant improvement in the prediction of cutting forces with velocity–chip load dependent coefficients but a single-function velocity–chip model fails to predict the forces accurately at very high speeds (>80,000 rpm). This inaccurate force prediction affects the predicted stability boundary at those speeds. Hence, this paper presents a segmented approach, wherein a function is fit for a given range of speeds to determine the chip load dependent cutting coefficients. The segmented velocity–chip load dependent cutting coefficient improves the cutting force prediction at high speeds, which yields much accurate stability boundary. This paper employs two degrees-of-freedom (2DOF) model with forcing functions based on segmented velocity–chip load dependent cutting coefficients. Stability lobe diagram based on 2DOF model has been created for different speed ranges using Nyquist stability criterion. Chatter onset has been identified experimentally via accelerometer data and the power spectral density (PSD) analysis of the machined surface topography. Critical spatial chatter frequencies and magnitudes of PSD corresponding to onset of instability have also been determined for different conditions.

Commentary by Dr. Valentin Fuster
J. Manuf. Sci. Eng. 2016;139(5):051006-051006-17. doi:10.1115/1.4035037.

The flow stress in the high-speed machining of titanium alloys depends strongly on the microstructural state of the material which is defined by the composition of the material, its starting microstructure, and the thermomechanical loads imposed during the machining process. In the past, researchers have determined the flow stress empirically as a function of mechanical state parameters, such as strain, strain rate, and temperature while ignoring the changes in the microstructural state such as phase transformations. This paper presents a microstructure-sensitive flow stress model based on the self-consistent method (SCM) that includes the effects of chemical composition, α phase and β phase, as well mechanical state imposed. This flow stress is developed to model the flow behavior of titanium alloys in machining at speed of higher than 5 m/s, characterized by extremely high strains (2–10 or higher), high strain rates (104–106 s−1 or higher), and high temperatures (600–1300 °C). The flow stress sensitivity to mechanical and material parameters is analyzed. A new SCM-based Johnson–Cook (JC) flow stress model is proposed whose constants and ranges are determined using experimental data from literature and the physical basis for SCM approach. This new flow stress is successfully implemented in the finite-element (FE) framework to simulate machining. The predicted results confirm that the new model is much more effective and reliable than the original JC model in predicting chip segmentation in the high-speed machining of titanium Ti–6Al–4V alloy.

Commentary by Dr. Valentin Fuster
J. Manuf. Sci. Eng. 2016;139(5):051007-051007-7. doi:10.1115/1.4034935.

Optimizing the energy consumption of robot movements has been one of the main focuses for most of today's robotic simulation software. This optimization is based on minimizing a robot's joint movements. In many cases, it does not take into consideration the dynamic features. Therefore, reducing energy consumption is still a challenging task and it involves studying the robot's kinematic and dynamic models together with application requirements. This research aims to minimize the robot energy consumption during assembly. Given a trajectory and based on the inverse kinematics and dynamics of a robot, a set of attainable configurations for the robot can be determined, perused by calculating the suitable forces and torques on the joints and links of the robot. The energy consumption is then calculated for each configuration and based on the assigned trajectory. The ones with the lowest energy consumption are selected. Given that the energy-efficient robot configurations lead to reduced overall energy consumption, this approach becomes instrumental and can be embedded in energy-efficient robotic assembly.

Commentary by Dr. Valentin Fuster
J. Manuf. Sci. Eng. 2016;139(5):051008-051008-7. doi:10.1115/1.4034625.

The manufacturing industry is evolving and starting to use three-dimensional (3D) models as the central knowledge artifact for product data and product definition, or what is known as model-based definition (MBD). The model-based enterprise (MBE) uses MBD as a way to transition away from using traditional paper-based drawings and documentation. As MBD grows in popularity, it is imperative to understand what information is needed in the transition from drawings to models so that the models represent all the relevant information needed for processes to continue efficiently. Finding this information can help to define what data are common amongst different models in different stages of the lifecycle, which could help to establish a common information model. The common information model is a source that contains common information from domain specific elements amongst different aspects of the lifecycle. To help establish this common information model, information about how models are used in the industry within different workflows needs to be understood. To retrieve this information, a survey mechanism was administered to industry professionals from various sectors. Based on the results of the survey a common information model could not be established. However, the results gave great insight that will help in further investigation of the common information model.

Commentary by Dr. Valentin Fuster
J. Manuf. Sci. Eng. 2016;139(5):051009-051009-7. doi:10.1115/1.4034850.

The manufacturing industry contributes over 19% to the world's greenhouse gas emissions (U.S. Energy Information Administration, 2008, “Rep: Annual Energy Review 2008,” Report No. DOE/EIA-0384; Diaz et al., 2010, “Environmental Analysis of Milling Machine Tool Use in Various Manufacturing Environments,” 2010 IEEE International Symposium on Sustainable Systems and Technology.) and 31% of the total energy consumed annually in the U.S. (Herzog, T., 2005, “World Greenhouse Gas Emissions in 2005,” World Resources Institute, Washington, DC 2; Diaz et al., 2010, “Environmental Analysis of Milling Machine Tool Use in Various Manufacturing Environments,” 2010 IEEE International Symposium on Sustainable Systems and Technology.). There is therefore an increasing demand for sustainable solutions for the production technology industry. At the Technische Universitat (TU) Berlin, Germany, a collaborative research center (CRC) is focusing on new solutions for the sustainable machining of high performance alloys, with developments from machine tool frames to cutting tool technology being undertaken. An innovative machine tool concept with a modular frame, which allows a high level of flexibility, has been developed. Furthermore, add-on upgrading systems for older machine tools, which are particularly relevant for developing countries, have been developed. These systems allow the accuracy of outdated machine tools to be increased, thus making the machine tools comparable to modern systems. Finally the cutting process also requires solutions for dry machining, as the use of cooling lubricant is environmentally damaging and a significant cost contributor in machining processes. Two solutions are being developed at the TU Berlin: an internally cooled cutting tool and a heating concept for ceramic tools to allow dry machining of high temperature alloys, for example, for the aerospace industry.

Commentary by Dr. Valentin Fuster
J. Manuf. Sci. Eng. 2016;139(5):051010-051010-11. doi:10.1115/1.4035184.

In milling of flexible workpieces, like axial-flow compressor impellers with thin-wall blades and deep channels, interference occurrence between workpiece and tool shaft is a great adverse issue. Even though interference avoidance plays a mandatory role in tool path generation stage, the generated tool path remains just a nominally interference-free one. This challenge is attributed to the fact that workpiece flexibility and dynamic response cannot be considered in tool path generation stage. This paper presents a strategy in process parameters planning stage, aiming to avoid the interference between tool shaft and flexible workpiece with dynamic response in milling process. The interference problem is formulated as that to evaluate the approaching extent of two surfaces, i.e., the vibrating workpiece and the swept envelope surface generated by the tool undergoing spatial motions. A metric is defined to evaluate quantitatively the approaching extent. Then, a minimax optimization model is developed, in which the optimization objective is to maximize the metric, so as the interference-free can be guaranteed while constraints require the milling process to be stable and process parameters to fall into preferred intervals in which material removal rate is satisfactory. Finish milling of impeller using a conical cutter governed by a nominally interference-free tool path is numerically simulated to illustrate the dynamics responses of the spatially distributed nodal points on the thin-wall blade and approaching extent of the time-varying vibrating blades to the tool swept envelope surface. Furthermore, the present model results suggest to use an optimal process parameters set in finish milling, as a result improving machining efficiency in addition to ensuring the interference-free requirement. The model results are verified against milling experiments.

Commentary by Dr. Valentin Fuster
J. Manuf. Sci. Eng. 2017;139(5):051011-051011-8. doi:10.1115/1.4035720.

High-speed machine tools typically provide high spindle speeds and feedrates to achieve an effective material removal rate (MRR). However, it is not possible to realize the full extent of their high-speed capabilities due to the sharp corners of toolpaths which are introduced by conventional machining strategies, such as contour- and direction-parallel toolpaths. To address this limitation, spiral toolpaths that can reduce the magnitude of sudden direction changes have been developed in previous researches. Nevertheless, for some pockets, the average radial cutting width is significantly decreased while the total length of the toolpath is significantly increased as compared to contour- and direction-parallel toolpath. In this situation, spiral toolpath may take more machining time. To overcome these drawbacks, an aggressive spiral toolpath generation method based on the medial axis (MA) transformation is proposed in machining pocket without islands inside, which refers to no additional material inside the counter. The salient feature of this work is that it integrates the advantages of both conventional contour-parallel machining strategy and the existing spiral toolpath machining strategy. The cutting width at each MA point is determined based on the diameter of the locally inscribed circle (LIC) of the MA point and the topological structure of MA. A distance-constrained contour determination algorithm is utilized to calculate the toolpath for each pass. Finally, a circular arc transition strategy is used to transform all the isolated passes into a spiral toolpath. Experiments are conducted to show the effectiveness of the proposed method.

Commentary by Dr. Valentin Fuster
J. Manuf. Sci. Eng. 2017;139(5):051012-051012-10. doi:10.1115/1.4035469.

While coupled three-dimensional (3D) nonisothermal finite-element (FE) models can be used to predict distortion in weldments, computational costs remain high, and the development of alternate FE-based engineering approaches remains an important topic. In the present study, a plane stress model is proposed for analyzing angular distortion in butt-welded plates having appreciable levels of weld reinforcement. The approach is based on an analysis of contractile shrinkage forces and only requires knowledge of the plastic zone geometry to develop the input data needed for an isothermal linear elastic FE model. Results show that the proposed method significantly reduces the computational time and provides acceptable accuracy when plane stress conditions are satisfied. The effect of weld reinforcement was also analyzed using the method. The results indicate that the contraction force from the bead is dominant, and that the primary effect of the crown is to increase eccentricity of the in-plane contraction force. A steel liner from a nuclear plant cooling tower was also analyzed to demonstrate the method. The results showed that the model was able to predict the distortion pattern and demonstrated fair accuracy.

Commentary by Dr. Valentin Fuster
J. Manuf. Sci. Eng. 2017;139(5):051013-051013-7. doi:10.1115/1.4035722.

Experimental investigations of process emissions from atomic layer deposition (ALD) of Al2O3 are accomplished under various temperatures and purge times to understand its environmental sustainability performance. About 93% of Trimethylaluminum (TMA) is found flowing through ALD system without deposition. 2–9 × 104 of ultrafine nanoparticles containing 51.9 ± 4.6% of C, 16.6 ± 0.9% of Al, 31.4 ± 4.1% of O are generated during each cycle of reactions. 0.34–0.38 cm3 of CH4 (25 °C, 1 atm), which takes up 45–51% of C contained in TMA is produced simultaneously. The concentration of nanoparticles drops with the increase of purge time. CH4 also has a trend of decreasing but acts more complex with the largest emission at a short purge time. Compared with temperature, which has limited effects on reactants, purge time changes the time of reaction as well as the degree of gas phase mixing, and therefore greatly influences ALD emissions.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Manuf. Sci. Eng. 2017;139(5):054501-054501-5. doi:10.1115/1.4035492.

Bipolar forceps are a type of electrosurgical device (ESD) widely used for tissue welding in modern surgeries. ESDs have many advantages over traditional surgical tools including reduced blood loss, improved efficiency, and lower surgeon fatigue. However, these devices suffer from tissue sticking and damage due to overheating which leads to poor tissue joint quality. The problem is potentially caused by uneven power distribution due to nonuniform compression applied by the bipolar forceps. In this study, the effect of compression force uniformity was investigated with an experimental setup to achieve a uniform and consistent compression force at the jaws of bipolar forceps. Comparative tissue welding experiments were conducted under both uniform and nonuniform compression force conditions with tissue mimicking material. In situ welding process parameters including compression force, electrical voltage, and current were collected and analyzed to understand the effect of compression force uniformity. The results indicate that tissue impedance is lower due to increased tool–tissue contact area; the electrical power is initially higher during the first few milliseconds of welding. The experimental device developed in this study provides an important platform to understand the difference of tissue welding process under uniform and nonuniform compression force conditions.

Commentary by Dr. Valentin Fuster
J. Manuf. Sci. Eng. 2017;139(5):054502-054502-9. doi:10.1115/1.4035531.

In this paper, an innovative approach for the description of the functional properties of a grinding wheel surface is discussed. First, the state of the art in the description of grinding wheel topographies is summarized. Furthermore, the fundamentals for a new approach for the quantitative description of grinding wheel topographies are provided. In order to analyze the functional properties of a grinding wheel's topography depending on its specification, grinding experiments were carried out. For the experimental investigations vitrified, synthetic resin bonded and electroplated grinding wheels with varied compositions were analyzed. During the experiments, the topographies of the investigated grinding wheels have been analyzed by means of the topotool in detail. The developed software tool allows a detailed description of the kinematic cutting edges depending on the grinding process parameters and the grinding wheel specification. In addition to the calculation of the number of kinematic cutting edges and the area per cutting edge, a differentiation of the cutting edge areas in normal and tangential areas of the grinding wheel's circumferential direction is implemented. Furthermore, the topotool enables to analyze the kinematic cutting edges shape by calculating the angles of the grain in different directions. This enables a detailed analysis and a quantitative comparison of grinding wheel topographies related to different grinding wheel specifications. In addition, the influence of the dressing process and wear conditions to the grinding wheel topography can be evaluated. The new approach allows a better characterization of the contact conditions between grinding wheel and workpiece. Hence, the impact of a specific topography on the grinding process behavior, the generated grinding energy distribution, and the grinding result can be revealed.

Commentary by Dr. Valentin Fuster

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