Mechanical Behavior of Ti-6Al-4V Manufactured by Electron Beam Additive Fabrication

Additive Layer Fabrication, in particular Electron Beam Additive Fabrication (EBAF), has recently drawn much attention for its special usability to fabricate intricately designed parts as a whole. It not only increases the production rate which reduces the production lead time but also reduces the cost by minimizing the amount of waste material to a great extent. Ti6Al4V is the most common type of material that is currently being fabricated using EBAF technique. This material has been used in aerospace industry for several reasons such as excellent mechanical properties, low density, great resistance to corrosion, and non-magnetism. The effects of build direction of layers (namely, addition of layers along one of the x, y & z directions with respect to the build table) and the anisotropy effect caused by it has not been explored vigorously. This anisotropy effect has been investigated in this work. Different mechanical properties such as Yield Strength (YS), Ultimate Tensile Strength (UTS), and Modulus of Elasticity (E) of these three types of Ti6Al4V are determined using tensile tests and are compared with literature. The tensile test results show that YS and UTS for flat-build samples have distinguishably higher values than those of the side-build and top-build samples.

Characterization of Flow Stress for Commercially Pure Titanium Subjected to Electrically-Assisted Deformation

Uniaxial tension tests were conducted on thin commercially pure titanium sheets subjected to electrically-assisted deformation using a new experimental setup to decouple thermal-mechanical and possible electroplastic behavior. The observed absence of stress reductions for specimens air-cooled to near room temperature motivated the need to reevaluate the role of temperature on modeling the plastic behavior of metals subjected to electrically-assisted deformation, an item that is often overlooked when invoking electroplasticity theory. As a result, two empirical constitutive models, a modified-Hollomon and the Johnson-Cook models of plastic flow stress, were used to predict the magnitude of stress reductions caused by the application of constant DC current and the associated Joule heating temperature increase during electrically-assisted tension experiments. Results show that the thermal-mechanical coupled models can effectively predict the mechanical behavior of commercially pure titanium in electrically-assisted tension and compression experiments.

A New Approach for a Simulation-Based Prediction of Torsional Chatter in Deep Hole Drilling With Extra-Long Twist Drills

Numerous investigations work on torsional chatter vibrations in drilling. Particularly in terms of productivity, torsional chatter is detrimental because of a reduction of tool life and an undesirably high level of noise emissions due to the increased process dynamics. To achieve a deeper understanding of the process dynamics, a new numerical simulation model was developed to predict torsional chatter for extra-long twist drills. It is used to determine the influence of numerous factors such as cutting parameters, drill torsional stiffness, rotary moment of inertia and torsional-axial coupling. In this paper, the general structure of the model and the tool model is presented.

Statistical Cutting Force Model for Orthogonal Cutting of Polytetrafluoroethylene (PTFE) Composites

This paper presents an experimental investigation of the cutting forces response during the orthogonal cutting of polytetrafluoroethylene (PTFE) and PTFE-based composites using the Taguchi method. Cutting experiments were conducted using the L27 orthogonal array and the effects of the cutting parameters (feed rate, cutting speed and rake angle) on the cutting force were analyzed using the S/N ratio response and the analysis of variance (ANOVA). Statistical models that correlate the cutting force with process variables were developed using ANOVA and polynomial regression. The variation of the apparent friction coefficient was analyzed with respect to tool geometry and the cutting process. The results indicated that cutting and thrust forces increase with increasing feed rate, and decrease with increasing rake angles from negative to positive values and increasing cutting speed. A power law relationship between the apparent friction coefficient and the normal force exerted by the chip on the tool-rake face was identified, the former decreasing with an increasing normal force.

Experimental Investigation of Chip Formation and Surface Topology in Ultrasonic-Assisted Milling of X20Cr13 Stainless Steel

In the present paper, by using longitudinal one dimensional ultrasonic vibrations, characteristics of side milling of X20Cr13 martensitic stainless steel has been investigated. In order to experimentally investigate the chip formation and machined surface topology of workpiece, conventional milling (CM) and ultrasonic-assisted milling (UAM) processes have been applied and compared in certain cutting conditions. Imaging by digital microscope shows that applying ultrasonic vibrations on milling process leads to thinner and smaller formed chips and it also makes machined surface of workpece flatter. In both CM and UAM processes, as feed rate increases, chips become thicker and machine surface loses its flatness.

Mechanism of Fatigue Performance Enhancement in a Superhard Nanoparticles Integrated Nanocomposites by a Hybrid Manufacturing Technique

A hybrid manufacturing process, which contains Laser Sintering (LS) and Laser shock peening (LSP), is introduced to generate iron-TiN nanoparticle nanocomposites. It is a two-step process including LS followed with LSP. Before LS, TiN nanoparticles mixed with iron powders are coated on samples surface. After LS, TiN nanoparticles are embedded into iron matrix to strengthen materials. Then LSP is performed to introduce work hardening and compressive residual stress. The existed nanoparticles increase the dislocation density and also help to pin the dislocation movement. Better residual stress stability under thermal annealing can be obtained by better dislocation movement stabilization, which is beneficial for fatigue performance.

Microcellular Injection Molding of Thermoplastic Polyurethane (TPU) Scaffolds Using Carbon Dioxide and Water as Co-Blowing Agents

In this study, a novel microcellular injection foaming method employing supercritical CO2 (scCO2) and water as co-blowing agents was developed to produce thermoplastic polyurethane (TPU) tissue engineering scaffolds with a uniform porous structure and no solid skin layer. Various characterization techniques were applied to investigate the cell morphology, crystallization behavior, and static and dynamic mechanical properties of solid molded samples, foamed samples using CO2 or water as a single blowing agent, and foamed samples using both CO2 and water as co-blowing agents. Compared with CO2 foamed scaffolds, scaffolds produced by the co-blowing method exhibit much more uniform cell morphologies without a noticeable reduction in mechanical properties. Moreover, these TPU scaffolds have almost no skin layer, which permits free transport of nutrients and waste throughout the samples, which is highly desirable in tissue engineering. The effect of these blowing agents on the shear viscosity of various samples is also reported.

Empirical Modeling of Direct Electric Current Effect on Machining Cutting Force

Metallic materials can be made more ductile and be formed at lower forces through the application of electrical current during deformation, termed Electrically-Assisted Forming (EAF). The current provides a degree of resistive heating, but also facilitates deformation by direct electrical mechanisms (termed the electroplastic effect). It is envisioned that this approach, currently applied to bulk/sheet deformation, could also be used to reduce the flow stress in the deformation zone of the machining shear plane. The objective of this work is to study and model the effect of electric current on forces in machining in order to relate the force reduction to the current level and machining process parameters. To perform this, skiving tests and orthogonal machining tests are performed with varying electrical conditions. It is shown that application of electric current does reduce machining force by up to 60% under certain conditions.

Characterization of Sintered Ti-6Al-4V Powders in Electron Beam Additive Manufacturing

In the powder-based electron beam additive manufacturing (EBAM) process, preheating is applied, prior to the melting stage, to aggregate precursor powders and to reduce the residual stresses in the build parts. Preheating results in sintering of the powders, which serve as the initial work material for the subsequent melting stage. In this study, sintered Ti-6Al-4V alloy powders from preheating were obtained and studied. The specimens of sintered powders, also processed to prepare metallographic samples, were observed and characterized by optical microscopy (OM) and scanning electron microscopy (SEM). The results show that after preheating, some powders are partially “melted” and necks between adjacent particles are formed with metallurgical bonds. The sintering evidence, necking, can be noted on both the build plane and the side surface (normal to the build plane). The Baktetwave α-β structure is identified in the powders, while the martensitic structure is formed in the solid EBAM part.

A Novel Hybrid Process for Drawing Operations

A novel hybrid process for drawing operations is proposed. This process combines the conventional drawing and hydroforming features. The hybrid drawing die assembly is designed to incorporate multiple die segments engraved with high pressure fluid channels. Preliminary results on drawn Al 6061 specimens under two fluid pressure levels showed that the drawing load can decrease significantly. The hybrid drawing process has also shown that varying the fluid pressure can alter the surface asperities at the tool-workpiece interface in real-time, promoting micro-pool lubrication. This was evidenced by distinct surface topographies observed via scanning electron micrographs and optical micrographs.