Generation and detection of defects in metallic parts fabricated by selective laser melting and electron beam melting and their effects on mechanical properties.

Application of Additive Manufacturing (AM) technology to fabricate complex three-dimensional components is one promising direction within the manufacturing industry. This approach is rapidly changing the way designers and engineers create objects with desired shape and structures. Selective Laser Melting (SLM) and Electron Beam Melting (EBM) are two common powder bed fusion processes within AM for fabricating metallic parts. In order to give designers and engineers more insights into employing AM, the quality and long-term behavior of SLM- and EBM-produced parts need to be carefully investigated. Thus, this research project aims to understand how processing parameters affect defect generation and distribution during SLM and EBM processes, to study the morphological features of defects, to identify effective non-destructive method(s) to detect these defects, and to characterize the effect of defects on mechanical properties of SLM- and EBM-produced parts. The study began by generating stochastic defects via adjustment to process parameters from optimal parameters to marginal parameters, in order to correlate the porosity to the marginal parameters. Archimedes method was employed to estimate porosity of SLM- and EBM-produced specimens. After this, by using destructive characterization techniques, the defective specimens were sectioned. The morphology of stochastic defects was investigated based on their contour features on the cross sections. Micro CT was primarily used to evaluate the stochastic defects in the SLM and EBM parts and demonstrate their morphological characteristics and porosity in the single slices and reconstructed models. Finally, tensile and fatigue tests were carried out on Ti-6Al-4V parts with identified porosity. The fracture mechanism was analyzed. This study established a fundamental understanding of defects in parts made by SLM and EBM processes. Porosity was quantitatively correlated to the marginal parameters of SLM and EBM processes. Defects were differentiated based on their morphological properties and features. Micro CT was confirmed to be an effective non-destructive method for evaluating stochastic defects in SLM- and EBM-produced parts. The effects of stochastic defects on Ti-6Al-4V parts were determined based on tensile and fatigue tests. It was found that both microstructure and porosity have an impact on the mechanical properties of SLM- and EBM-produced parts

Numerical simulation of flow and heat transfer of continous cast steel slab under traveling magnetic field

A unified numerical model for simulating solidification transport phenomena (STP) of steel slab in electromagnetic continuous casting (EMCC) process was developed. In order to solve the multi-physics fields coupled problem conveniently, the complicated bidirectional coupled process between EM and STP was simplified as a unidirectional one, and a FEM/FVM-combined numerical simulation technique was adopted. The traveling magnetic fields (TMFs) applied to the EMCC process were calculated using the ANSYS11.0 software, and then the EM-data output by ANSYS were converted to FVM-format using a data-format conversion program developed previously. Thereafter, the governing equations were solved using a pressure-based Direct-SIMPLE algorithm. The simulation results of the STP in CC-process show that, due to the influences of Lorentz force and Joule heat, the two strong circulating flows and the temperature field can be obviously damped and changed once TMF with one pair of poles (1-POPs) or 2-POPs is applied, which would accordingly improve the quality of casting. It was found in the present research that the integrated actions of 2-POPs TMF are superior to 1-POPs. All the computations indicate that the present numerical model of EM-STP as well as the FEM/FVM-combined technique is successful

Cardiovascular calcification in chronic kidney disease: Risk factors and effect of α-keto acid tablets

Purpose: To investigate the effect of α-keto acid tablets, and risk factors for cardiovascular calcification in patients with chronic kidney disease (CKD).Methods: A total of 128 CKD patients were enrolled in this study. They were randomly assigned to study and control groups, each with 64 patients. Control patients received symptomatic treatment, while the study group patients received α-keto acid tablets plus. Indices of cardiovascular calcification, blood lipids and mineral metabolism were determined in the 2 groups of patients and compared. Risk factors for cardiovascular calcification were also analyzed.Results: After treatment, the two groups had decreased CACS scores and reduced serum FGF-23levels, with lower values in patients in the study group. Levels of Klotho and fetuin-A were significantly elevated after treatment, with higher values observed in study group patients. The degree of cardiovascular calcification was markedly lower in study group than that in controls. There was no significant difference in blood Ca level between the control and study groups before and after treatment. Logistic multivariate analysis demonstrated that hyperlipidemia, hyperphosphatemia, hypercalcemia, hypertension and diabetes put patients at risk for cardiovascular calcification.Conclusion: Compound α-keto acid tablets delay cardiovascular calcification in patients with CKD, and alleviate symptoms of related risk factors for cardiovascular calcification

The Effects of Processing Parameters on Defect Regularity in Ti-6Al-4V Parts Fabricated By Selective Laser Melting and Electron Beam Melting

Processing parameter has an important effect on Selective Laser Melting (SLM) and Electron Beam Melting (EBM) processes. Defects are easily formed by deviating from optimized processing parameters. This study purposely fabricated Ti-6Al-4V specimens with defects by varying process parameters from the factory default settings in both SLM and EBM equipment. Specimen’s density was measured based on the Archimedes method for estimating porosity. Microscopy of specimen’s top surface were observed to compare melt pool and overlap. “Marginal Parameters” is identified to describe the processing parameters which are capable of fabricating specimens with certain porosity. As a result, a correlation between defect regularity and marginal parameters has been established. The effect of marginal parameters on the melt pool is discussed to explain defect formation.Mechanical Engineerin

An Investigation of Thin Feature Generation in Direct Metal Laser Sintering Systems

The fabrication of metal structures with thin features (<1mm) with additive manufacturing processes is of interest for various lightweight applications. This paper investigates the geometrical and microstructural characteristics of thin features fabricated using an EOS direct metal laser sintering (DMLS) system. Both 1D and 2D thin features were fabricated using various process themes, and subsequently analyzed for microstructure geometrical characteristics. It was found that the selection of process parameters has significant influence on both the geometrical accuracy and the microstructure of the thin features. The results can be used for the further development of process guidelines for lightweight structures such as cellular structures and support structures.Mechanical Engineerin

Influence of Small Particles Inclusion on Selective Laser Melting of Ti-6al-4v Powder

The particle size distribution and powder morphology of metallic powders have an important effect on powder bed fusion based additive manufacturing processes, such as selective laser melting (SLM). The process development and parameter optimization require a fundamental understanding of the influence of powder on SLM. This study introduces a pre-alloyed titanium alloy Ti-6Al-4V powder, which has a certain amount of small particles, for SLM. The influence of small particle inclusion is investigated through microscopy of surface topography, elemental and microstructural analysis, and mechanical testing, compared to the Ti-6Al-4V powder provided by SLM machine vendor. It is found that the small particles inclusion in Ti-6Al-4V powder has a noticeable effect on extra laser energy absorption, which may develop imperfections and deteriorate the SLM fatigue performance

Rheological Properties of Two Stainless Steel 316L Powders for Additive Manufacturing

This study measures the rheological properties of two stainless steel 316L powders which are used for the powder-bed-fusion based additive manufacturing process. The purpose is to evaluate the newly acquired powder in comparison with the used and recycled powder, so that both powders can be mixed with each other to supplement the powder usage. The powder rheology properties, such as dynamic property, bulk property, and shear property, are tested and compared. The results and analysis confirm the compatibility of powder mixing

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