High Cycle Fatigue Behavior of Recycled Additive Manufactured Electron Beam Melted Titanium Ti6Al4V

Metal Additive Manufacturing (AM) has become a popular method for producing complex and unique geometries, especially gaining traction in the aerospace and medical industries. With the increase in adoption of AM and the high cost of powder, it is critical to understand the effects of powder recycling on part performance to move towards material qualification and certification of affordable printed components. Due to the limitations of the Electron Beam Melting (EBM) process, current as-printed components are susceptible to failure at limits far below wrought metals and further understanding of the material properties and fatigue life is required. In this study, a high strength Titanium alloy, Ti-6Al-4V, is recycled over time and used to print fatigue specimens using the EBM process. Uniaxial High Cycle Fatigue tests have been performed on as-printed and polished cylindrical specimens and the locations of crack initiation and propagation have been determined through the use of a scanning electron microscope. This investigation has shown that the rough surface exterior is far more detrimental to performance life than the powder degradation occurring due to powder reuse. In addition, the effects of the rough surface exterior as a stress concentration is evaluated using the Arola-Ramulu. The following is a preliminary study of the effects powder recycling and surface treatments on EBM Ti-6Al4V fatigue life.

Force Controlled Electrical Pulse Assisted Forming

Electrically-assisted manufacturing refers to the direct application of electrical current to a workpiece during a manufacturing process. This assistance results in several benefits such as flow stress reduction, increased elongation, reduced springback, increased diffusion, and increased precipitation control. These effects are also associated with traditional thermal assistance. However, for over half a decade it has been argued whether or not these observed effects are due to electroplasticity, a term which describes effects that cannot be fully explained through resistive heating. Several theories have been proposed as to the mechanism responsible for these purported athermal effects. Conflicting results within literature have enabled this debate over electroplasticity since its discovery in the mid 20th century. While the effects of electrically-assisted manufacturing are clearly characterized throughout literature, there is a lack of research related to control systems which may be used to take advantage of its effects. Typically, control systems are developed using an empirical approach, requiring extensive testing in order to fully characterize the stress-strain behavior at all conditions. Additionally, current research has primarily focused on reducing flow stresses during electrically-assisted processes without regard for the strength of the material subsequent to forming. Therefore, there is a strong need for a control system which can quickly be deployed for new materials and does not significantly reduce the subsequent strength of the material. Herein, a novel control approach is developed in which electrical pulses are triggered by a predetermined stress level. This stress value would be set according to the manufacturer’s stamping die strength. Once the material reaches this stress value, current is deployed until a minimum stress level is reached. At that point, the electricity is turned off and the material allowed to cool; at that stage the stress begins to elevate and the cycle continues. This approach does not require extensive pre-testing and is robust to a range of strain rate. This type of implementation can also be adapted to different levels of capability. For example, since the current is controlled by force and not by time, a low-current power supply will stay on for each pulse longer than a power supply with higher capabilities; however, each will achieve a similar effect. This study investigates the effect of several different minimum stress levels and strain rates. The strain rates chosen are relatively similar to common stamping process. This system was experimentally tested using 1018 CR steel. This control approach was found to be a successful method of maintaining a desired stress level.

Machining Characteristics Estimation in WEDM Process While Machining Titanium Grade-2 Material Using ANN

Wire Electrical Discharge Machining (WEDM) provides an effective solution for machining hard materials with intricate shapes. WEDM is a specialized thermal machining process is capable to accurately machining parts of hard materials with complex shapes. However, selection of process parameters for obtaining higher machining efficiency or accuracy in wire EDM is still not fully solved, even with the most up-to-date CNC WED machine. The study presents the machining of Titanium grade 2 material using L’16 Orthogonal Array (OA). The process parameters considered for the present work are pulse on time, pulse off time, current, bed speed, voltage and flush rate. Among these process parameters voltage and flush rate were kept constant and the other four parameters were varied for the machining. Molybdenum wire of 0.18mm is used as the electrode material. Titanium is used in engine applications such as rotors, compressor blades, hydraulic system components and nacelles. Its application can also be found in critical jet engine rotating and airframes components in aircraft industries. Firstly optimization of the process parameters was done to know the effect of most influencing parameters on machining characteristics viz., Surface Roughness (SR) and Electrode Wear (EW). Then the simpler functional relationship plots were established between the parameters to know the possible information about the SR and EW. This simpler method of analysis does not provide the information on the status of the material and electrode. Hence more sophisticated method of analysis was used viz., Artificial Neural Network (ANN) for the estimation of the experimental values. SR and EW parameters prediction was carried out successfully for 50%, 60% and 70% of the training set for titanium material using ANN. Among the selected percentage data, at 70% training set showed remarkable similarities with the measured value then at 50% and 60%.

Experimental Investigation on Forming Limit Curve at Elevated Temperature Through Dome and Biaxial Test

The characteristics of metal and materials are very important to design any component so that it should not fail in the life of the service. The properties of the materials are also an important consideration while setting the manufacturing parameters which deforms the raw material to give the design shape without providing any defect or fracture. For centuries the commonly used method to characterize the material is the traditional uniaxial tension test. The standard has been created for this test by American Standard for Testing Materials (ASTM) – E8. This specimen is traditionally been used to test the materials and extract the properties needed for designing and manufacturing. It should be noted that the uniaxial tension test uses one axis to test the material i.e., the material is pulled in one direction to extract the properties. The data acquired from this test found enough for manufacturing operations of simple forming where one axis stretching is dominant. Recently a sudden increase in the usage of automotive vehicles results in sudden increases in fuel consumption which results in an increase in air pollution. To cope up with this challenge federal government is implying the stricter environmental regulation to decrease air pollution. To save from the environmental regulation penalty vehicle industry is researching innovation which would reduce vehicle weight and decrease fuel consumption. Thus, the innovation related to light-weighting is not only an option anymore but became a mandatory necessity to decrease fuel consumption. To achieve this target, the industry has been looking at fabricating components from high strength to ultra-high strength steels or lightweight materials. This need is driven by the requirement of 54 miles per gallon by 2025. In addition, the complexity in design increased where multiple individual parts are eliminated. This integrated complex part needs the complex manufacturing forming operation as well as the process like warm or hot forming for maximum formability. The complex forming process will induce the multi-axial stress states in the part, which is found difficult to predict using conventional tools like tension test material characterization. In many pieces of literature limiting dome height and bulge tests were suggested analyzing these multi-axial stress states. However, these tests limit the possibilities of applying multi-axial loading and resulting stress patterns due to contact surfaces. Thus, a test machine called biaxial test is devised which would provide the capability to test the specimen in multi-axial stress states with varying load. In this paper, two processes, limiting dome test and biaxial test were experimented to plot the forming limit curve. The forming limit curve serves the tool for the design of die for manufacturing operation. For experiments, the cruciform test specimens were used in both limiting dome test and biaxial test and tested at elevated temperatures. The forming limit curve from both tests was plotted and compared. In addition, the strain path, forming, and formability was investigated and the difference between the tests was provided.

Investigation of Grinding Force and Surface Integrity of IC10 in Creep Feed Grinding

The IC10 superalloy material is one of the most important materials for aero-engine turbine blade due to its excellent performances. However, it is difficult to be machined because of its special properties such as terrible tool wear and low machined efficiency. The creep feed grinding is widely used in machining IC10 superalloy due to the advance in reducing tool wear, improving material removal rate and surface quality. The creep feed grinding is a promising machining process with the advantages of high material removal rate due to large cutting depth, long cutting arc and very slow workpiece, and its predominant features might have significant influence on the grinding force and surface quality for the workpiece. Hence, it is of great importance to study the grinding force and surface integrity in creep feed grinding IC10 superalloy. In this paper, a series of orthogonal experiments have been carried out and the effects of grinding parameters on the grinding force and the surface roughness are analyzed. The topographies and defects of the machined surface were observed and analyzed using SEM. The results of the experiments show that the tangential force is decreased with the workpiece speed increasing. However, there is no significant change in tangential force with the increasing of grinding depth and wheel speed. The normal force is decreased with the workpiece speed increasing when the workpiece speed is less than 150 mm/min, but when the workpiece speed is more than 150 mm/min the normal force is increased tardily. Moreover, the normal force is increased sharply with the increase of grinding depth and is increased slowly with the increase of wheel speed. In general, the surface roughness is increased with workpiece speed and grinding depth increasing, while the trend of increase corresponding that of workpiece speed is more evident. The value of the surface roughness is decreased with wheel speed increasing. And it is found out that the main defect is burning of the IC10 superalloy material in creep feed grinding by energy spectrum analysis of some typical topography in this study.

A Study of the Effect of Extended Hounsfield Unit Range and Voxel Size on Defect Detection in Friction Stir Welds

Friction stir welding (FSW) is a novel welding method that is garnering attention, in part, due to its ability to join dissimilar materials. One of the challenges in producing dissimilar friction welded joints is ensuring the welds are defect-free. Nondestructive testing (NDT) methods such as ultrasonic waves, gamma rays, X-rays, and X-ray CT, are gaining popularity as a method to detect internal defects in FSW joints. In this study, dissimilar AA1050-AA6061-T6 FSW lap welds are Manufactured and then examined using an NDT X-ray CT technique. The effects of two critical X-ray CT scanning parameters (voxel size and Hounsfield unit (HU)) on the detection of internal defects are investigated. The samples are scanned via X-ray CT at two different voxel sizes (2.457 E−02 and 1.420 E−03 mm3) and two HU ranges (12-bit and 16-bit depth). The generated Digital Imaging and Communications in Medicine (DICOM) images are segmented based on a proper HU threshold found via the Otsu thresholding method. The findings show that Small voxel size (higher resolution) improves the ability of detecting internal defects and improves the effectiveness of the thresholding process. Higher HU range results in a wider separation between detected material peaks, thus enhancing the effectiveness of the thresholding process as well.

Accelerating Large-Format Metal Additive Manufacturing: How Controls R&D Is Driving Speed, Scale, and Efficiency

This article highlights work at Oak Ridge National Laboratory’s Manufacturing Demonstration Facility to develop closed-loop, feedback control for laser-wire based Directed Energy Deposition, a form of metal Big Area Additive Manufacturing (m-BAAM), a process being developed in partnership with GKN Aerospace specifically for the production of Ti-6Al-4V pre-forms for aerospace components. A large-scale structural demonstrator component is presented as a case-study in which not just control, but the entire 3D printing workflow for m-BAAM is discussed in detail, including design principles for large-format metal AM, toolpath generation, parameter development, process control, and system operation, as well as post-print net-shape geometric analysis and finish machining. In terms of control, a multi-sensor approach has been utilized to measure both layer height and melt pool size, and multiple modes of closed-loop control have been developed to manipulate process parameters (laser power, print speed, deposition rate) to control these variables. Layer height control and melt pool size control have yielded excellent local (intralayer) and global (component-level) geometry control, and the impact of melt pool size control in particular on thermal gradients and material properties is the subject of continuing research. Further, these modes of control have allowed the process to advance to higher deposition rates (exceeding 7.5 lb/hr), larger parts (1-meter scale), shorter build times, and higher overall efficiency. The control modes are examined individually, highlighting their development, demonstration, and lessons learned, and it is shown how they operate concurrently to enable the printing of a large-scale, near net shape Ti-6Al-4V component.

Investigation of Contact Conditions During Stamping of a Thin Plate

Stamping is one of the most used manufacturing processes, where real-time monitoring is quite difficult due to high speed of the mechanical press, which leads to deterioration of the accuracy of the products In the present work, a method is developed to model elastic waves propagation in solids to measure contact conditions between die and workpiece during stamping. A two-dimensional model is developed that reduces the wave propagation equations to two-dimensional equations. To simulate the wave propagation inside the die-workpiece model, the finite difference time domain (FDTD) method and modified Yee algorithm has been employed. The numerical stability of the wave propagation model is achieved through courant stability condition, i.e., Courant-Friedrichs-Lewy (CFL) number. Two cases, i.e., flat die-workpiece interface and inclined die-workpiece interface, are investigated in the present work. The elastic wave propagation is simulated with a two-dimension (2D) model of the die and workpiece using reflecting boundary conditions for different material properties. The experimental and simulation-based results of reflected and transmitted wave characteristics are compared for different materials in terms of reflected and transmitted wave height ratio and material properties such as acoustic impedance. It is found that the numerical simulation results are in good agreement with the experimental results.

Investigation of Spatial Variability on Strut Diameters of Additively Manufactured Lattice Structures

In this study, the influence of the spatial variability of geometric uncertainties on the strut members of the lattice structures fabricated by additive manufacturing is investigated. Individual struts are fabricated with various printing angles and diameters using a material extrusion process and PLA material. The diameter values of the fabricated samples are measured along the printing and radial directions at each layer under an optical microscope. Spatial correlations are characterized based on the measurements using the experimental autocorrelation function. Candidate autocorrelation functions are fitted to the measured data to identify the best fitted one for each diameter parameter and the corresponding correlation lengths are evaluated for random field. The applicability of the Karhunen-Loeve expansion (KLE) is investigated to reduce the dimensionality of the random field discretization. The results show that the diameters of the strut members at each layer are spatially dependent and the KLE method was found to give a good representation of the random field.

Minimizing Residual Stress in Brazed Joints by Optimizing the Brazing Thermal Profile

Ceramic to metal brazing is a common bonding process used in many advanced systems such as automotive engines, aircraft engines, and electronics. In this study, we use optimization techniques and finite element analysis utilizing viscoplastic and thermo-elastic material models to find an optimum thermal profile for a Kovar® washer bonded to an alumina button that is typical of a tension pull test. Several active braze filler materials are included in this work. Cooling rates, annealing times, aging, and thermal profile shapes are related to specific material behaviors. Viscoplastic material models are used to represent the creep and plasticity behavior in the Kovar® and braze materials while a thermo-elastic material model is used on the alumina. The Kovar® is particularly interesting because it has a Curie point at 435°C that creates a nonlinearity in its thermal strain and stiffness profiles. This complex behavior incentivizes the optimizer to maximize the stress above the Curie point with a fast cooling rate and then favors slow cooling rates below the Curie point to anneal the material. It is assumed that if failure occurs in these joints, it will occur in the ceramic material. Consequently, the maximum principle stress of the ceramic is minimized in the objective function. Specific details of the stress state are considered and discussed.