Dislocation Density-Based Finite Element Method Modeling of Ultrasonic Consolidation

A dislocation density-based constitutive model has been developed and implemented into a crystal plasticity quasi-static finite element framework. This approach captures the statistical evolution of dislocation structures and grain fragmentation at the bonding interface when sufficient boundary conditions pertaining to the Ultrasonic Consolidation (UC) process are prescribed. The hardening is incorporated using statistically stored and geometrically necessary dislocation densities (SSDs and GNDs), which are dislocation analogs of isotropic and kinematic hardening, respectively. Since the macroscopic global boundary conditions during UC involves cyclic sinosuidal simple shear loading along with constant normal pressure, the cross slip mechanism has been included in the evolution equation for SSDs. The inclusion of cross slip promotes slip irreversibility, dislocation storage, and hence, cyclic hardening during the UC. The GND considers strain-gradient and thus renders the model size-dependent. The model is calibrated using experimental data from published refereed literature for simple shear deformation of single crystalline pure aluminum alloy and uniaxial tension of polycrystalline Aluminum 3003-H18 alloy. The model also incorporates various local and global effects such as (1) friction, (2) thermal softening, (3) acoustic softening, (4) surface texture of the sonotrode and initial mating surfaces, and (6) presence of oxide-scale at the mating surfaces, which further contribute significantly specifically to the grain substructure evolution at the interface and to the anisotropic bulk deformation away from the interface during UC in general. The model results have been predicted for Al-3003 alloy undergoing UC. A good agreement between the experimental and simulated results has been observed for the evolution of linear weld density and anisotropic global strengths macroscopically. Similarly, microscopic observations such as embrittlement due to grain substructure evolution at the UC interface have been also demonstrated by the simulation. In conclusion, the model was able to predict the effects of macroscopic global boundary conditions on bond quality. It has been found that the normal pressure enhances good bonding characteristics at the interface, though beyond a certain magnitude enhances dynamic failure. Similarly, lower oscillation amplitudes result in a lower rate of gap closure, whereas higher oscillation amplitude results in an enhanced rate of gap relaxation at the interface. Henceforth, good bonding characteristics between the constituent foils are found at an optimum oscillation amplitude. A similar analogy is also true for weld speed where the longitudinal locations behind the sonotrode rip open when higher weld speeds are implemented in the UC machine, leading to lower linear weld density and poor bonding characteristics

A New Finite Element Solver using Numerical Eigen Modes for Fast Simulation of Additive Manufacturing Processes

A new efficient numerical technique has been formulated for dimensional reduction and phenomenological multi-scale simulation of additive manufacturing processes using finite element analysis. This technique is demonstrated using prismatic build volumes to represent the Selective Laser Melting powder bed fusion additive manufacturing process. The Eigen modes determined as an outcome of implementation of this technique will help to reduce the time necessary for optimization of process parameters and closed loop control. In addition to thermal simulations of the Selective Laser Melting process, this technique is also applicable to the simulation of lattice structures, layered materials such as ultrasonically consolidated laminates, thin walled coatings and development of high fidelity beam and plate theories for parts made using additive manufacturing processes. A future integration of this method with analytical Eigen wavelets will provide infinite support compared to finite support provided by directional polynomial shape functions currently used for implementation of finite element strategies. The present Eigen modes will be also useful in analysis and optimization of mask projection based additive manufacturing processes.Mechanical Engineerin

Modeling and Experimental Validation of Nickel-based Super Alloy (Inconel 625) Made Using Selective Laser Melting

The formation of constituent phases in Selective Laser Melting of Inconel 625 is a function of local temperatures, hold times at those temperatures, local cooling rates and local compositions in the melt pool. These variables are directly correlated with input process parameters such as beam power, scan speed, hatch spacing, beam diameter and thermo-mechanical characteristics of the powder bed. The effect of these process parameters must be understood in order to properly control the machines and predict the properties of parts being fabricated. To understand the effects, IN625 coupons using eight different sets of processing parameters have been fabricated and microstructure and mechanical properties were compared. These properties will be then used to validate a dislocation density based crystal plasticity finite element model (DDCP-FEM).Mechanical Engineerin

An Energy Dissipative Constitutive Model for Multi-Surface Interfaces at Weld Defect Sites in Ultrasonic Consolidation

A new finite element based constitutive model has been developed for quantification of energy dissipation due to friction and plastic deformation at the mating interface of two surfaces during the Ultrasonic Consolidation process. This work will include bridging the mesoscopic response of a dislocation density based crystal plasticity finite element framework at inter and intra-granular scales and a point at the macroscopic scale. This response will be used to develop an energy dissipative constitutive model for multi-surface interfaces at the macroscopic scale. The constitutive model will be used for quantification of energy consumed at lack of fusion and trapped oxide defects present in the build and the amount of energy input required to compensate for it. This numerical procedure will help in real time optimization of process parameters and closed loop control.Mechanical Engineerin

Effect of Scan Pattern on the Microstructural Evolution of Inconel 625 during Selective Laser Melting

Selective laser melting (SLM) involves highly localized heat input and directional solidification, which enables novel microstructure control through the development of scanning strategies and related process variables. A careful study of scan pattern is important to understand microstructural evolution during SLM. In this study, various types of scanning strategies were used to build samples of Inconel 625. Microstructure differences due to different scan patterns in as-built Inconel 625 samples were then studied in detail. Microstructure samples showed grains with cellular substructure with enriched regions of Nb and Mo in the inter arm spacing. The grains were observed to grow preferentially in the build direction, but there were also clear effects of grain orientation differences due to scan direction effects.Mechanical Engineerin

Melt Pool Characterization for Selective Laser Melting of Ti-6Al-4V Pre-alloyed Powder

Parameter optimization for metal powders in Selective Laser Melting (SLM) is usually carried out by experimental investigations of the influence of significant parameters (such as laser power, scan speed, hatch spacing, layer thickness, scan pattern, etc.) on microstructure and/or mechanical properties. This type of experimental optimization is extremely time- and cost-consuming. In order to accelerate the optimization process, a study was undertaken to develop a method for rapid optimization of parameters based on melt pool characterizations. These characterizations began with investigations of SLM single bead experiments. Pre-alloyed Ti-6Al-4V powder was used for single bead fabrication with multiple laser power and scan speed combinations. Surface morphology and dimensions of single beads were characterized. Geometrical features of melt pools were measured after polishing and etching of the cross section of each single bead. It was found that melt pool characteristics provide significant information that is helpful for process parameters selection. These experiments are being extended to characterize test pads with multiple layers.Mechanical Engineerin

A Review of Thermal Analysis Methods in Laser Sintering and Selective Laser Melting

Thermal analysis of laser processes can be used to predict thermal stresses and microstructures during processing and in a completed part. Thermal analysis is also the basis for feedback control of laser processing parameters in manufacturing. A comprehensive literature review of thermal analysis methods utilized in Laser Sintering (LS) has been undertaken. In many studies, experimental methods were commonly used to detect and validate thermal behavior during processing. Coupling of thermal experiments and FEM analyses were utilized in many of the latter studies. Analytical solutions were often derived from the Rosenthal solution and other established theories. In recent years, some temperature measuring systems have been implemented to validate the simulation results. The main characteristics of LS temperature distribution and effects of process parameters to temperature are also summarized and shown by a case study.Mechanical Engineerin

Establishing Flow Stress and Elongation Relationships as a Function of Microstructural Features of Ti6Al4V Alloy Processed using SLM

Selective laser melting (SLM) is an attractive technology for fabricating complex metal parts with reduced number of processing steps compared to traditional manufacturing technologies. The main challenge in its adoption is the variability in mechanical property produced through this process. Control and understanding of microstructural features affected by the SLM process is the key for achieving desirable mechanical properties. Numerous studies have been published related to microstructure and mechanical properties of SLM printed parts; however, few of those reported end-to-end process–structure–property relationship. Therefore, the current study aims to comprehensively present the widespread microstructure information available on SLM processed Ti6Al4V alloy. Furthermore, its effects on the magnitude and anisotropy of the resultant mechanical properties, such as the yield strength and elongation, has been established. A Hall–Petch relationship is established between α lath size and yield strength magnitude for the as-built, heat-treated, transverse, and longitudinal built samples. The anisotropy in flow stress is established using the α lath size and prior β grain orientation. Percentage elongation was identified to be affected by both α lath size and powder layer thickness, due to its correlation with the prior β columnar grain size. A linear relationship was established between percentage elongation and combined size of α lath and powder layer thickness using the rule of mixtures

Prediction of Mechanical Properties of Electron Beam Melted Ti6Al4V Parts Using Dislocation Density Based Crystal Plasticity Framework

Parts produced using Electron Beam Melting (EBM) with Ti6Al4V powders are generally tested for two important mechanical properties, namely tensile strength and fatigue life. The optimization of the process input parameters, such as part orientation, initial powder size and hatch pattern, for the abovementioned mechanical properties has been attempted using two numerical finite element methods. First, the dislocation density based crystal plasticity framework (DDCP-FEM) has been used to evaluate the localized stress-strain evolution, dislocation density evolutions and non-local deformations as a function of loading, sample geometry, microstructural phase, grain size and shape. This analysis has been compared against simulations based on continuum plasticity based finite element techniques. Though the localized evolutions as a function of microstructural attributes are missing in the continuum analysis, the low computational costs involved makes this technique an ideal candidate for spatial homogenization of the DDCP-FEM framework. The simulations conducted in the current work only validate the mechanical properties for tensile and fatigue specimens fabricated with known process parameters. These simulations will form the basis for future modeling efforts to optimize these parameters for required mechanical properties in service.Mechanical Engineerin