Importance of Characterizing the Variability for Batch Production using Laser Powder Bed Fusion

Additive Manufacturing (AM) of metals is passingthe research stage and finding application in the industrialenvironment as a manufacturing technology of choice. However,the quality of products fabricated using metal AM technologycould be considered inferior when used in batch manufacturing.Quality is generally defined as conformance to specification andis inversely proportional to process variation. Variation in aprocess signifies the number of possible defects per millionopportunities in a given production. This gives rise to the needto characterize process variation to improve quality. This paperexplores the characterization of variability in laser powder bedfusion (LPBF) AM of metals to improve the quality of partproduction with specific focus on batches. It summarizes thefactors that influence the variation and discusses the tools usedto improve part quality

Material characteristics of Ti-6AL-4V samples additively manufactured using laser-based direct energy deposition

Although additive manufacturing is fast gaining traction in the industrial world as a reputable manufacturing technique to complement traditional mechanical machining, it still has problems such as porosity and residual stresses in components that give rise to cracking, distortion, and delamination, which are important issues to resolve in structural load-bearing applications. This research project focused on the characterization of the evolution of residual stresses in Ti-6Al-4V extra-low interstitial (ELI) additive-manufactured test samples. Four square thin-walled tubular samples were deposited on the same baseplate, using the direct energy deposition laser printing process, to different build heights. The residual stresses were analysed in the as-printed condition by the neutron diffraction technique and correlated to qualitative predictions obtained using the ANSYS software suite. Good qualitative agreement between the stress measurements and predictions were observed. Both approaches revealed the existence of large tensile stresses along the laser track direction at the sections that were built last, i.e., centre of the top layers of the samples. This in addition leads to large tensile stresses at the outer edges (corners) which would have the effect of separating the samples from the baseplate should the stresses exceed the yield strength of the material. Such extreme conditions did not occur in this study, but the stresses did lead to significant distortion of the baseplate. In general, the microstructures and spatial elemental mapping revealed a strong correlation between the macro-segregation of elemental V and the distribution of the β-phase in the printed parts.http://www.saimm.co.za/journal-papersam2024Materials Science and Metallurgical EngineeringSDG-09: Industry, innovation and infrastructur

Performance assessment of a ti6al4v(eli) light aircraft nose wheel fork produced through laser powder bed fusion

Thesis (Master: Engineering: Mechanical Engineering)--Central University of TechnologyA scaled-down nose wheel fork for a light aircraft was redesigned by applying topology optimization for manufacturing in Ti6Al4V(ELI) through laser powder bed fusion (LPBF). This scaled-down nose wheel fork was built together with the test specimens for tensile and fatigue testing in this study. The test specimens were quality checked, tested, and analyzed through standard procedures to obtain the porosity levels, tensile and fatigue properties, and fracture characteristics. The effect of the inherent surface roughness on the high-cycle fatigue properties of LPBF Ti6Al4V(ELI) test specimens was investigated. These test specimens were built to the standard geometry without subsequent machining in three orthogonal directions. They were tested under constant load in a tension–tension fatigue testing machine in accordance with the ASTM E 466 standard. The data was collected and complied with the ASTM F3001 – 14 standard for additive manufacturing (AM) Ti6Al4V(ELI) with laser powder bed fusion. The fatigue performance of the Ti6Al4V(ELI) specimens built to the standard geometry without subsequent machining was compared to that of machined test specimens. It was found that the inherent surface roughness of the specimens built to the standard geometry reduced their fatigue life by about half that of the machined specimens. A customized jig was designed and manufactured to simulate the operational conditions applicable to the scaled-down nose wheel fork. This jig allowed three critical load cases to be tested. The experimental results of the fatigue test specimens and the performance testing of the scaled-down nose wheel fork under static loading were used to evaluate the feasibility of LPBF for production of structural aircraft components, particularly the nose wheel fork. Based on the outcome of the study, it was concluded that it would be justifiable to build a fullscale prototype of the nose wheel fork for testing under operational conditions

Direct metal laser sintering of titanium alloys for biomedical applications

Published ThesisOngoing scientific progress shifts conventional methods to the much celebrated Additive Manufacturing (AM) due to its freedom of design, flexibility in feedstock and material optimization. It has shown that Direct Metal Laser sintering (DMLS), one of the AM technologies, is an attractive manufacturing route for the biomedical applications. Ti6Al4V is the most widely used titanium alloy for the implants. However, there still remain issues of relative low ductility of DMLS Ti6Al4V and infections after implantation which have triggered the current research into producing implants of high ductility with antibacterial properties by DMLS, while establishing a body of knowledge about the relationship between the laser-matter interaction, microstructure, and mechanical properties. The type of material used in biomedical applications depends on specific implant applications and different types of implant need different mechanical properties. The current study is designed to investigate DMLS lattice structures from traditional Ti alloy such as T6Al4V ELI and the possibility of producing novel alloys by in-situ alloying for DMLS process. Learning from nature, it can be understood that cellular structures would be more preferable for biomedical implants than dense solid structures’ since the architecture of bone tissues in the human body are not completely dense and solid. Cellular structures of different nodes and strut sizes were produced and mechanically investigated to mimic the anisotropic porous nature of bones. A finite element analysis (FEA) was conducted to determine the applicability of graded/gradient implant based on each patient requirement. From the FEA it was hypothesized that implant design with cellular structures with relative low Elastic modulus would bridge the Elastic modulus gradient between dense solid metallic implants and the porous bones. An advanced lightweight mandible model was proposed whereby a damaged mandible could be replaced with a graded material based on the functional requirements of the damaged part. Mixing different elemental powders for in-situ alloying by DMLS would definitely increase the material pallet for AM. Understanding the effects of the parameters on DMLS process is paramount to gaining full control over density, microstructure and the mechanical properties of the DMLS parts. Only a careful combination of the process parameters would result in optimum process parameters for each type of powder. A wide range of process parameters were investigated to gain in-depth knowledge into the interaction between the laser beam and the powder bed by in-situ alloying powders with vastly different melting points and similar particle size distribution (45 μm). Due to difference in thermo-physical properties between the powders (Ti6Al4V, Cu, Mo, Ti), sintered materials were inhomogenous. Rescanning was employed but there was no significant change in the volume fraction of the unmelted Mo particles in the Ti15Mo alloy matrix. Due to the inherent high rate of heating and cooling simultaneously of the DMLS process, martensitic phase was found in the as-built Ti15Mo and Ti6Al4V–1at.%Cu samples. The martensitic properties reduce the ductility of the as-built samples significantly. Optimum process parameters were determined for both molybdenum-bearing titanium alloy (85% Ti and 15% Mo) and copper-bearing titanium alloy (Ti6Al4V and 1at.%Cu). Successful manufacturing of non-porous samples was done. In-situ alloying Ti6Al4V+1%Cu was successful and therefore there are promising ways to manufacture materials with embedded antibacterial properties. Incorporating copper into the bulk material by in-situ alloying would prevent the fall-off of antibacterial deposition coatings used in the past, since the material matrix (implant) would be antibacterial agent. The mechanical properties investigations with mini-samples presented ductility values below what was recommended for biomedical materials. It was concluded that finer Mo particles have to be chosen for in-situ alloying Ti15Mo for producing biomedical objects. Future work have to be done with elaboration of heat treatment procedures for higher ductility for structural bearing implants in a single step by the DMLS process. The results obtained developed new knowledge that is important for understanding the in situ alloying process during DMLS and new material production. The illustrated effects of process parameters on the properties of the synthesized material would be paramount for advanced implants with unique properties

Enhancing Fracture Toughness of Ultrahigh Strength Aerospace Components made by Additive Manufacturing

Direct Metal Laser Sintering (DMLS) and Selective Laser Melting (SLM) are an Additive Manufacturing (AM) technique that produces complex three- dimensional parts by adding layer upon layer of powder materials from bottom to top. Recently, AM has received a significant amount of press and is set to have an enormous impact such as decreasing the cost of production, fast and flexible, design freedom and increase the innovation opportunities. The powder base nature allows these techniques to process a variant of materials as well as produce complex composite parts and develop new materials system for Aerospace industries. The biggest problems in the process are limited surface quality and residual porosity in SLM and DMLS parts that are undesirable for some applications where fatigue resistance and high strength are essential. This research aims to improve the fracture toughness, ductility and fatigue of the metallic components, which is essential to be able to exploit the potential of the SLM and DMLS of these alloys for aerospace applications. In an additional development of the AM technology is not only limited to new machines but also processes, new materials, and methods, as it offers high mechanical properties and performance. This research focuses on DMLS and SLM of titanium and stainless steel alloys to investigate the effect of processes parameter and different build direction on toughness and fatigue crack growth property to change the physical and mechanical properties. Also, manipulate the process parameters and their effect on strength, fracture toughness and quality for both bulk and cellular lattice structure parts. The novelty in this study lies in using additive manufacturing process to evaluate the local failure mechanism of 316L bulk and cellular lattice structures made by SLM under uniaxial tension and three-point bending load. The effect of different build directions of the 316L lattice structure on the fracture toughness properties is compared to the Ashby and Gibson models. The findings demonstrate that the build direction does have an effect on the microstructure of parts, which subsequently has an effect upon mechanical properties and the surface quality of manufactured parts. Results found in this study will enable the designer to understand the important factors which affect the SLM and DMLS process and quality of final parts at different build direction. The comparison between micromechanics model and experimental results will help the designer to predict fracture toughness of AM cellular structures without the need of experimental tests. Finally, the results of mechanical properties of these bulk and lightweight parts will give a confidence to the designer to use and tailor their properties to specific applications

Development of a qualification procedure, and quality assurance and quality control concepts and procedures for repairing and reproducing parts with additive manufacturing in MRO processes

This MSc by Research is focused mainly on Quality Assurance (QA) and Qualification Procedures for metal parts manufactured using new Additive Manufacturing (AM) techniques in the aerospace industry. The main aim is to understand the state of the art of these technologies and the strong regulatory framework of this industry in order to develop correct QA/QC procedures in accordance with the certification process for the technology and spare parts. These include all the testing and validation necessary to implement them in the field, as well as to maintain their capability throughout their lifecycle, specific procedures to manufacture or repair parts, workflows and records amongst others. At the end of this MSc by Research, an entire Qualification Procedure for Electron Beam Melting (EBM) and Selective Laser Melting (SLM) for reproduction of an aerospace part will be developed and defined. Also, General Procedures, Operational Instructions, and Control Procedures with its respective registers, activities, and performance indicators for both technologies will be developed. These will be part of the future Quality Assurance and Quality Management systems of those aerospace companies that implement EBM or SLM in their supply chain

Investigation of the high strain rate behaviour and impact toughness of Ti6Al4V (ELI) parts built by the EOS M280 DMLS System with standard process parameters : as-built and stress relieved

Thesis (Master of Engineering in Mechanical Engineering) -- Central University of Technology, Free State, 2018The high strain rate behaviour and the impact toughness of Direct Metal Laser Sintering (DMLS) produced Ti6Al4V (ELI) parts were investigated. The as-built (AB) specimens were also taken through stress-relieving heat treatment and are hereinafter referred to as SR samples or specimens. The high strain rate deformation of the two forms of the alloy (AB and SR) was studied using the Split Hopkinson Pressure Bar (SHPB) test, in compression and tension. Two high strain rates of 400 s-1 and 700 s-1 in compression and 250 s-1 and 360 s-1 in tension were used in these tests. The test results were used to investigate the relative strain rate dependence of flow stresses and fracture strain. Microstructural analysis was carried out to study the dominant fracture mechanisms on the deformed surfaces of the compression and tensile test samples using optical and scanning electron microscopy. Measurements of Vickers microhardness for the test samples were taken before and after the compression and tensile tests. The results of the high rate tensile and compression tests showed significant strain rate sensitivity in both compression and tension. Moreover, the rate sensitivity was seen to be relatively higher in the SR samples than in the AB samples. Microstructural analysis of the tested specimens showed that adiabatic shear bands dominated the deformed surfaces in the compression test samples. Multiple cracks that were more pronounced on the surfaces of the tested specimens were seen to initiate randomly on the deformed surfaces of the tensile test samples. The Vickers microhardness of the tested samples were observed to be higher for loaded specimens than for the unloaded specimens. The impact properties of the AB and SR DMLS Ti6Al4V (ELI) specimens were studied using an instrumented Charpy impact tester. The transition curves for the absorbed energy and lateral expansion were obtained by performing the experiments in the temperature range 130 ºC to 250 ºC. The effect of the orientation of the v-notch on the standard test specimen with relation to the base plate of the DMLS machine was investigated. Furthermore, the effects of stress-relieving heat treatment on the notch toughness of DMLS Ti6Al4V (ELI) specimens was also studied. The analysis of the fracture surfaces resulting from the tests at various temperatures was done using a scanning electron microscope (SEM). The values of absorbed energy and the lateral expansion of the specimens that were determined from this series of tests indicated that specimens built with the v-notch facing the base plate of the DMLS machine had better impact toughness and notch ductility in comparison to those built with the v-notch facing away from the base plate over most of the temperature range of testing. Besides improving toughness, stress-relieving heat treatment gave rise to a shift of the ductile-to-brittle transition temperatures (DBTT) to lower values. The study further established that the DMLS Ti6Al4V (ELI) retains appreciable notch toughness even at sub-zero temperatures

Non-destructive testing of the parts manufactured by Direct Metal Laser Sintering

Published ThesisInterest in Additive Manufacturing (AM) has grown considerably in the past decades and industry has gained great benefits from this type of technology. The main advantages are: geometrical freedom that allows the design of parts with complex shape, which are difficult or impossible to produce by conventional technology; shortened design-to-product time; customization and the possibility to use several materials in one process. Direct Metal Laser Sintering (DMLS) is one of the most promising AM technologies that utilizes metal powders. Due to the layer-by-layer nature of powder delivery used in DMLS, the drawbacks are: surface quality and accuracy, high residual stress in as-built parts and porosity – all of which depend on the powder material, process-parameters, scanning and building strategies. This can result in a substantial deterioration of the mechanical properties of the products and their performance characteristics. For this reason, it is very important to identify defective parts before enrolling into service. Non-destructive testing (NDT) is effective for detection of internal defects without causing damage. NDT also covers a wide group of methods of analysis used to evaluate the properties of a material. NDT techniques like visual, acoustic, ultrasonic, thermal, X-ray and 3Dcomputed tomography (CT) inspections are now widely used for various industrial applications. For the analysis of material properties and the detection of defects, each of these methods uses different physical principles that have their advantages and disadvantages. In this study, some of the NDT techniques are evaluated in terms of their applicability to the inspection of parts manufactured by DMLS technology: Visual, Ultrasonic, Computed Tomography and Acoustic Emission inspection. Artificial defects were used to determine the feasibility of each NDT method. DMLS samples were produced containing a range of artificial defects. These samples were than subjected to each method and the results compared. A comparison between the amount of defect information obtained is made. It was shown that the nature of the sample; shape, size, material and the type of defects present plays a vital role in the selection of testing methods. Ultrasonic-Total Focus Method indicated that some defects are present upon testing relatively big samples with simple geometry. X-ray Computed Tomography showed some limitations with regard to the possibilities and the amount of defect detail, the only drawback being the cost and time involved. Acoustic Emission showed to be a promising method for production parts although it requires an initial time investment; thereafter it is a simple and easy way of detecting defective samples