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IN THIS ISSUE

### Review Article

J. Manuf. Sci. Eng. 2017;139(11):110801-110801-15. doi:10.1115/1.4036716.

Increasingly strict fuel efficiency standards have driven the aerospace and automotive industries to improve the fuel economy of their fleets. A key method for feasibly improving the fuel economy is by decreasing the weight, which requires the introduction of materials with high strength to weight ratios into airplane and vehicle designs. Many of these materials are not as formable or machinable as conventional low carbon steels, making production difficult when using traditional forming and machining strategies and capital. Electrical augmentation offers a potential solution to this dilemma through enhancing process capabilities and allowing for continued use of existing equipment. The use of electricity to aid in deformation of metallic materials is termed as electrically assisted manufacturing (EAM). The direct effect of electricity on the deformation of metallic materials is termed as electroplastic effect. This paper presents a summary of the current state-of-the-art in using electric current to augment existing manufacturing processes for processing of higher-strength materials. Advantages of this process include flow stress and forming force reduction, increased formability, decreased elastic recovery, fracture mode transformation from brittle to ductile, decreased overall process energy, and decreased cutting forces in machining. There is currently a lack of agreement as to the underlying mechanisms of the electroplastic effect. Therefore, this paper presents the four main existing theories and the experimental understanding of these theories, along with modeling approaches for understanding and predicting the electroplastic effect.

Commentary by Dr. Valentin Fuster

### Research Papers

J. Manuf. Sci. Eng. 2017;139(11):111001-111001-10. doi:10.1115/1.4037320.

Ultrasonic welding is a well-known technique for joining thermoplastics and has recently been introduced to joining carbon fiber-reinforced composites (CFRC). However, suitable models for predicting joint performance have not yet been established. At present, most failure models for bonded composites are built based on uniform adhesive joints, which assume constant joint properties. Nevertheless, the joint properties of ultrasonic spot welds for CFRC are variable, which depend on the input welding parameters. In this paper, the effect of welding energy, which is the most important welding parameter, on the joint properties is investigated. Then, a surface-based cohesive performance model based on mode-II (in-plane) shear loading is developed to predict the joint performance, wherein the critical fracture parameters in the model are described via the functions of welding energy. After comparing the simulated results with experiments, the model is proven feasible in predicting the joint properties of the ultrasonic spot welds under shear loading condition, and hence, a mix-mode cohesive-zone model is practical to predict the joint performance under any loading conditions with the predicted fracture parameters.

Commentary by Dr. Valentin Fuster
J. Manuf. Sci. Eng. 2017;139(11):111002-111002-14. doi:10.1115/1.4037322.

Textured functional surfaces are finding applications in the fields of bioengineering, surface energy, hydrodynamics, lubrication, and optics. Electrical discharge machining (EDM), which is normally used to generate smoother surface finish on various automotive components and toolings, can also generate surfaces of rough finish, a desirable characteristic for texturing purposes. There is a lack of modeling efforts to predict the surface textures obtained under various EDM operating conditions. The aim of the current work is to capture the physics of the electrical discharge texturing (EDT) on a surface assuming random generation of multiple sparks with respect to (i) space, (ii) time, and (iii) energy. A uniform heat disk assumption is taken for each individual spark. The three-dimensional (3D) texture generated is utilized to evaluate a 3D roughness parameter namely arithmetic mean height, $Sa$. Surface textures obtained from the model are validated against experimentally obtained ones by comparison of distribution of $Ra$ values taken along parallel sections along the surface. It was found that the distribution of simulated $Ra$ values agrees with that of experimental $Ra$ values.

Commentary by Dr. Valentin Fuster
J. Manuf. Sci. Eng. 2017;139(11):111003-111003-11. doi:10.1115/1.4037554.

Thermal expansion of the workpiece during cylinder boring process is one of the sources causing the bore cylindricity error. To study thermal expansion induced bore distortion, detailed workpiece temperature distribution in cylinder boring is required. Four finite element models, namely, the advection model, surface heat model, heat carrier model, and ring heat model, were developed to predict the workpiece temperature in cylinder boring. Cylinder boring experiments were conducted utilizing the tool–foil and embedded thermocouple experimental approaches to measure the workpiece temperature, predict the temperature distribution using the inverse heat transfer method, and evaluate the capability of the four models in terms of accuracy and efficiency. Results showed an accurate global temperature prediction for all models and a good correlation with the embedded thermocouple experimental measurements. Good correlation was also obtained between the tool–foil thermocouple measurement of machined surface temperature and model predictions. Advantages and disadvantages as well as applicable scenarios of each model were discussed. For studying detailed cylinder boring workpiece temperature, it is suggested to use the ring heat model to estimate the moving heat flux and the heat carrier model for local workpiece temperature calculation.

Commentary by Dr. Valentin Fuster
J. Manuf. Sci. Eng. 2017;139(11):111004-111004-12. doi:10.1115/1.4037238.

Conductive viscoelastic polymer composites (CVPCs) consisting of conductive fillers in viscoelastic polymer matrices find numerous applications in emerging technologies such as flexible electronics, energy storage, and biochemical sensing. Additive manufacturing methods at micro- and mesoscales provide exciting opportunities toward realizing the unique capabilities of such material systems. In this paper, we study the direct-ink-writing (DIW) process of CVPCs consisting of electrically conductive additives in a poly(ethylene oxide) (PEO) matrix. We particularly focus on the deposition mechanisms of the DIW process and the influence of these mechanisms on the printed structure geometry, morphology, and functional properties. To this end, we utilized a novel practical approach of modeling the ink extrusion through the nozzles considering the non-Newtonian viscous effects while capturing the viscoelastic extensional flow (drawing) effects through the variation of the nozzle exit pressure. We concluded that inks containing higher amounts of high molecular weight (HMW) PEO exhibit drawing type deposition at high printing speeds and low inlet pressures enabling thinner, higher aspect ratio structures with ideal three-dimensional stacking. Under this deposition mechanism, the electrical conductivity of the anodic structures decreased with increasing printing speed, indicating the effect of the drawing mechanism on the printed structure morphology.

Commentary by Dr. Valentin Fuster
J. Manuf. Sci. Eng. 2017;139(11):111005-111005-12. doi:10.1115/1.4037428.

Bending complex curved steel plates for constructing ship hull has long been a challenge in shipbuilding industry. This paper presents a novel incremental bending process to obtain complicated curved steel plates by a series of sequential and layered punches. Taking advantage of this process, the blank plate that is fixed and held by a flexible supporting system can incrementally be bent into the target shape by a press tool along a planned tool path step by step and layer by layer. Acting as a “lower die,” the flexible supporting system can provide flexible and multifunctional supports for the work piece during the forming process, whose four general motion modes are demonstrated in this paper. Meanwhile, the procedures of tool path planning and forming layering are also explained in detail. In addition, aiming at different motion modes of the flexible supporting system, two springback compensation methods are given. Furthermore, according to the forming principle presented in this paper, an original incremental prototype equipment was designed and manufactured, which is mainly composed of a three-axis computer numerical control (CNC) machine, a flexible supporting system, and a three-dimensional (3D) scanning feedback system. A series of forming experiments focusing on a gradual curvature shape were carried out using this prototype to investigate the feasibility and validity of this forming process.

Commentary by Dr. Valentin Fuster
J. Manuf. Sci. Eng. 2017;139(11):111006-111006-8. doi:10.1115/1.4037429.

Four-dimensional (4D) printing is a new category of printing that expands the fabrication process to include time as the fourth dimension, and its simulation and planning need to take time into consideration as well. The common tool for estimating the behavior of a deformable object is the finite element method (FEM). However, there are various sources of deformation in 4D printing, e.g., hardware and material settings. To model the behavior by FEM, a complete understanding of the process is needed and a mathematical model should be established for the structure–property–process relationship. However, the relationship is usually complicated, which requires different kinds of testing to formulate such models due to the process complexity. With the insight that the characteristic of shape change is the primary focus in 4D printing, this paper introduces geometry-driven finite element (GDFE) to simplify the modeling process by inducing deformation behavior from a few physical experiments. The principle of GDFE is based on the relationship between material structure and shape transformation. Accordingly, a deformation simulation can be developed for 4D printing by applying the principles to the GDFEs. The GDFE framework provides an intuitive and effective way to enable simulation and planning for 4D printing even when a complete mathematical model of new material is not available yet. The use of the GDFE framework for some applications is also presented in this paper.

Commentary by Dr. Valentin Fuster
J. Manuf. Sci. Eng. 2017;139(11):111007-111007-11. doi:10.1115/1.4037430.

Nerve conduits with topographical guidance have been recognized as the efficient repair of damaged peripheral nerves. In this study, polymeric hollow fiber membranes (HFMs) with grooved inner surface have been fabricated from a microstructured spinneret using a dry-jet wet spinning process for nerve regeneration studies. The effectiveness of HFM inner grooves has been demonstrated during an in vitro study of chick forebrain neuron outgrowth. It is of great importance that the groove geometry can be controllable to meet various needs in promoting nerve regeneration performance. While the overall groove geometry is determined by the spinneret design, fabrication conditions are also indispensable in fine-tuning the final groove geometry such as the groove height and width on the order of 10 μm or less. It is found that the bore fluid flow rate can be utilized to effectively adjust the resulting groove height by at most 52% and groove width by at most 61%, respectively, without modifying the spinneret geometry. This enables a new approach to fabricate different grooved HFMs using the same spinneret. By comparing to the influences of bore fluid flow rate, the dope fluid flow rate is less effective in regulating the groove height and width when using the same microstructured spinneret. Both bore and dope fluid flow rates should be carefully selected for fine groove width tuning.

Commentary by Dr. Valentin Fuster
J. Manuf. Sci. Eng. 2017;139(11):111008-111008-6. doi:10.1115/1.4037436.

Electrohydrodynamic jet (e-jet) printing is a microscale additive manufacturing technique used to print microscale constructs, including next-generation biological and optical sensors. Despite the many advantages to e-jet over competing microscale additive manufacturing techniques, there do not exist validated models of build material drop formation in e-jet, relegating process design and control to be heuristic and ad hoc. This work provides a model to map deposited drop volume to final spread topography and validates this model over the drop volume range of 0.68–13.4 pL. The model couples a spherical cap volume conservation law to a molecular kinetic relationship for contact line velocity and assumes an initial contact angle of 180 deg to predict the drop shape dynamics of dynamic contact angle and dynamic base radius. For validation, the spreading of e-jet-printed drops of a viscous adhesive is captured by high-speed microscopy. Our model is validated to have a relative error less than 3% in dynamic contact angle and 1% in dynamic base radius.

Commentary by Dr. Valentin Fuster
J. Manuf. Sci. Eng. 2017;139(11):111009-111009-10. doi:10.1115/1.4037240.

Setting optimum process parameters is very critical in achieving a sound friction stir weld joint. Understanding the formation of defects and developing techniques to minimize them can help in improving the overall weld strength. The most common defects in friction stir welding (FSW) are tunnel defects, cavities, and excess flash formation, which are caused due to incorrect tool rotational or advancing speed. In this paper, the formation of these defects is explained with the help of an experimentally verified 3D finite element (FE) model. It was observed that the asymmetricity in temperature distribution varies for different types of defects formed during FSW. The location of the defect also changes based on the shoulder induced flow and pin induced flow during FSW. Besides formation of defects like excess flash, cavity defects, tunnel/wormhole defects, two types of groove like defects are also discussed in this paper. By studying the different types of defects formed, a methodology is proposed to recognize these defects and counter them by modifying the process parameters to achieve a sound joint for a displacement-based FSW process.

Commentary by Dr. Valentin Fuster
J. Manuf. Sci. Eng. 2017;139(11):111010-111010-11. doi:10.1115/1.4037437.

One of the most popular additive manufacturing processes is laser-based direct metal laser sintering (DMLS) process, which enables us to make complex three-dimensional (3D) parts directly from computer-aided design models. Due to layer-by-layer formation, parts built in this process tend to be anisotropic in nature. Suitable heat treatment can reduce this anisotropic behavior by changing the microstructure. Depending upon the applications, a wide range of mechanical properties can be achieved between 482 °C and 621 °C temperature for precipitation-hardened stainless steels. In the present study, effect of different heat treatment processes, namely solution annealing, aging, and overaging, on tensile strength, hardness, and wear properties has been studied in detail. Suitable metallurgical and mechanical characterization techniques have been applied wherever required, to support the experimental observations. Results show H900 condition gives highest yield strength and lowest tensile strain at break, whereas solution annealing gives lowest yield strength and as-built condition gives highest tensile strain at break. Scanning electron microscope (SEM) images show that H900 and H1150 condition produces brittle and ductile morphology, respectively, which in turn gives highest and lowest hardness value, respectively. X-ray diffraction (XRD) analysis shows presence of austenite phases, which can increase ductility at the cost of hardness. Average wear loss for H900 condition is highest, whereas it is lowest for solution annealed condition. Further optical and SEM images have been taken to understand the basic wear mechanism involved.

Commentary by Dr. Valentin Fuster
J. Manuf. Sci. Eng. 2017;139(11):111011-111011-9. doi:10.1115/1.4037570.

This paper proposes an integrated approach to determine optimal build orientation for powder bed fusion by laser (PBF-L), by simultaneously optimizing mechanical properties, surface roughness, the amount of support structure (SUPP), and build time and cost. Experimental data analysis has been used to establish the objective functions for different mechanical properties and surface roughness. Geometry analysis of the part has been used to estimate the needed SUPP and thus evaluate the build time and cost. Normalized weights are assigned to different objectives depending on their relative importance allowing solving the multi-objective optimization problem using a genetic optimization algorithm. A study case is presented to demonstrate the capabilities of the developed system. The major achievements of this work are the consideration of multiple objectives and the establishment of objective function considering different load direction and heat treatments. A user-friendly graphical user interface was developed allowing to control different optimization process factors and providing different visualization and evaluation tools.

Commentary by Dr. Valentin Fuster
J. Manuf. Sci. Eng. 2017;139(11):111012-111012-9. doi:10.1115/1.4037432.

In this article, we describe an experimental method for investigating the autoclave co-cure of honeycomb core composite sandwich structures. The design and capabilities of a custom-built, lab-scale “in situ co-cure fixture” are presented, including procedures and representative results for three types of experiments. The first type of experiment involves measuring changes in gas pressure on either side of a prepreg laminate to determine the prepreg air permeability. The second type involves co-curing composite samples using regulated, constant pressures, to study material behaviors in controlled conditions. For the final type, “realistic” co-cure, samples are processed in conditions mimicking autoclave cure, where the gas pressure in the honeycomb core evolves naturally due to the competing effects of air evacuation and moisture desorption from the core cell walls. The in situ co-cure fixture contains temperature and pressure sensors, and derives its name from a glass window that enables direct visual observation of the skin/core bond-line during processing, shedding light on physical phenomena that are not observable in a traditional manufacturing setting. The experiments presented here are a first step within a larger research effort, whose long-term goal is to develop a physics-based process model for autoclave co-cure.

Commentary by Dr. Valentin Fuster
J. Manuf. Sci. Eng. 2017;139(11):111013-111013-9. doi:10.1115/1.4037569.

Multi-axis electrical discharging machining (EDM) is the main manufacture method for shrouded blisks, which are key components of aero and rocket engines. Involving both linear and rotational axes, a feeding path for machining a narrow and twisted channel consists of a large number of G-code lines. Accelerations and decelerations at junctions, which connect two neighboring G-code lines, can significantly reduce the machining efficiency. In this paper, a new simplification of feeding paths in roughing EDM for shrouded blisks is proposed in order to reduce the number of junctions on a feeding path. However, deviating from the original feeding path, a simplified feeding path can bring over contour errors which can cause geometrical errors of workpieces. Contour error can thus serve as a criterion for simplifying the original path. Eight vertices of a hexahedron, which contains the electrode, are used to represent all points inside and on an electrode. Forward kinematics of a six-axis EDM machine is used to calculate the contour errors of the eight vertices when the electrode feeds along a simplified path. A simplified feeding path can be found provided that the contour error constraint is respected. Machining tests show that the use of a simplified feeding path in roughing EDM machining can reduce the average total machining time by 26.5% without significant impact on surface roughness and white layer thickness.

Commentary by Dr. Valentin Fuster
J. Manuf. Sci. Eng. 2017;139(11):111014-111014-9. doi:10.1115/1.4037421.

Hybrid welding/joining of lightweight metals to carbon fiber reinforced polymers (CFRPs) typically relies on the adhesive bond created when the molten polymer matrix hardens in contact with the metallic surface. It is hypothesized that these bonds can be improved upon by fully displacing the polymer and infiltrating the carbon fibers with the metallic constituent to create load-bearing fibers that bridge the two materials. Friction stir welding (FSW) holds potential to melt and displace the polymer matrix, plasticize the metal constituent, and force the plasticized metal to flow around the fibers. Preliminary investigations were performed by FSW in AA 6061-T6 plates sandwiched against dry carbon fiber bundles. The FSW process plasticizes the aluminum while applying pressure, forcing the material to flow around the fibers. Cross-sectional images of the samples were used to measure the distance of infiltration of the aluminum into the carbon fiber bed. A fiber infiltration model previously developed to describe the infiltration of carbon fibers with epoxy resins during resin transfer molding was applied to this solid-state infiltration situation, thus modeling the plasticized aluminum as a fluid with an effective viscosity. Promising agreement was seen between the measured distances of infiltration and the predicted distances of infiltration when using effective viscosity values predicted by computational fluid dynamics (CFD) simulations of FSW found in literature. This work indicates that the well-established epoxy infiltration model can form the basis of a model to describe solid-state infiltration of carbon fibers with a plasticized metal.

Commentary by Dr. Valentin Fuster
J. Manuf. Sci. Eng. 2017;139(11):111015-111015-10. doi:10.1115/1.4037423.

Blades are essential parts used in thermal and nuclear power generation. Its machining precision is a vital factor that influences the efficiency and life of those industries. Blades are thin-walled parts, which could easily deform under cutting forces, and hence deteriorate the machining precision. In our previous work, a milling process with twin tool for blade is proposed, in which two tools are assigned to machine the basin and dorsal surfaces simultaneously. It is expected that the cutting forces acted on the basin and dorsal surfaces can be counteracted to reduce the deformation of the blade. In this study, a method of twin-tool paths generation is developed. The tool center points and tool axis vectors are generated with consideration of the cutting forces balance, the machine tool kinematics, the surface geometric precision, and the same number of tool paths on basin and dorsal surfaces. Virtual machining, finite element analysis, and trial cutting are carried out and verified that the method which is used for generating the twin-tool paths is successful. The basin and dorsal surfaces have the same number of tool paths and tool contact point coordinates, which guarantees that the two surfaces can be completely machined and can be machined and finished simultaneously. Furthermore, the cutting forces acted on the basin and dorsal surfaces can achieve the balance along the twin-tool paths. Therefore, the deformation of a blade caused by cutting force is obviously reduced compared with a conventional machining process with a single tool.

Commentary by Dr. Valentin Fuster
J. Manuf. Sci. Eng. 2017;139(11):111016-111016-8. doi:10.1115/1.4037425.

Selective laser foaming is a novel process that combines solid-state foaming and laser ablation to fabricate an array of microliter tissue engineering scaffolds on a polymeric chip for biomedical applications. In this study, a finite element analysis (FEA) model is developed to investigate the effect of laser processing parameters. Experimental results with biodegradable polylactic acid (PLA) were used for validation. It is found that foaming always occurs before ablation, and once it occurs, the temperature increases dramatically due to an enhanced laser absorption effect of the porous structure. The geometry of the fabricated scaffolds can be controlled by laser parameters. While the depth of scaffolds can be controlled by laser power and lasing time, the diameter is more effectively controlled by the laser power. The model developed in this study can be used to optimize and control the selective foaming process.

Commentary by Dr. Valentin Fuster
J. Manuf. Sci. Eng. 2017;139(11):111017-111017-7. doi:10.1115/1.4037798.

Electrically assisted (EA) wire drawing process is a hybrid manufacturing process characterized by enhancement of the formability, ductility, and elongation of the wire drawn specimen. A thermomechanical model to describe the change of the mechanical response due to the thermal contribution is proposed in this work. Additionally, a numerical simulation was conducted to study the potential and limitations of this hybrid process by using two different hardening laws: a phenomenological and a dislocation-based hardening laws. The results show how the flow stress, the effective plastic strain, and residual stresses behave under the electroplusing effect. In addition, electron backscattered diffraction was used to study the electropulsing treatments on the microstructure during cold drawing. It is observed a decrease of the high- and low-angle grain boundaries (LAGB) for samples deformed with electropulsing. This detwinning process has a strong influence on the strain hardening by improving the material formability. It was shown that the two proposed hardening laws adequately describe the EA wire drawing process showing a similar mechanical behavior. Nevertheless, the dislocation-based hardening law has the potential to be generalized to many other material and process configurations without extensive number of material tests as the phenomenological hardening law would require.

Commentary by Dr. Valentin Fuster
J. Manuf. Sci. Eng. 2017;139(11):111018-111018-10. doi:10.1115/1.4037609.

In this study, the forming limit of aluminum alloy sheet materials is predicted by developing a ductile failure criterion (DFC). In the DFC, the damage growth is defined by Mclintock formula, stretching failure is defined at localized necking (LN) or fracture without LN, while the critical damage is defined by a so-called effect function, which reflects the effect of strain path and initial sheet thickness. In the first part of this study, the DFC is used to predict forming limit curves (FLCs) of six different aluminum sheet materials at room temperature. Then, the DFC is further developed for elevated temperature conditions by introducing an improved Zener–Hollomon parameter ($Z′$), which is proposed to provide enhanced representation of the strain rate and temperature effect on limit strain. In warm forming condition, the improved DFC is used to predict the FLCs of Al5083-O and failure in a rectangular cup warm draw process on Al5182 + Mn. Comparison shows that all the predictions match quite well with the experimental measurements. Thanks to the proposal of effect function, the DFC needs calibration only in uniaxial tension, and thus, provides a promising potential to predict forming limit with reduced effort.

Commentary by Dr. Valentin Fuster

### Technical Brief

J. Manuf. Sci. Eng. 2017;139(11):114501-114501-8. doi:10.1115/1.4037422.

The paper presents a computational study of steady, laminar, two-dimensional (2D) mixed convection heat transfer from a continuously moving isothermal vertical plate to alumina–water nanofluid as in hot extrusion. The simulation is based on a heterogeneous flow model which takes into account Brownian diffusion and thermophoresis of nanoparticles. The finite difference method is used to discretize the governing equations. The SIMPLE algorithm has been applied to obtain flow, thermal, and nanoparticle concentration fields. The numerical results have been validated satisfactorily with the published results for pure fluids. A detailed parametric study reveals that in the mixed convection regime, the enhancement factor (EF) (defined as the ratio of average heat transfer coefficient in nanofluid to that in base fluid) increases with nanoparticle concentration. The enhancement is more at lower Richardson number (Gr/Re2), that is, closer to forced convection regime. In the regime close to free convection, the EF is found to be very small. Larger plate velocity (that is, higher Reynolds number) has a positive effect on heat transfer enhancement but higher plate-fluid temperature difference results in lower EF. An enhancement in heat transfer coefficient as high as 22% is realized at the plate velocity of 0.4 m/s. The effectiveness (defined as the ratio of average heat transfer coefficient in nanofluid to the power required to pull the plate), in general, falls with higher volume fraction of nanoparticles and plate velocity and escalates with a rise in Richardson number and plate-fluid temperature difference.

Commentary by Dr. Valentin Fuster