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

Online Monitoring Of Laser-Based Powder Bed Fusion By Acoustic Emission

ThesisMetal additive manufacturing (AM) has seen great advances in capabilities and the technology has matured to the point where industries, such as aerospace, are readily implementing it for production. The main concern remains qualification and quality control. Laser powder bed fusion (LPBF) is one of the more popular AM technologies which, as the name suggests, uses a laser to melt and solidify a powder in such a way as to create a three-dimensional part. The part is built in a layer-wise fashion, stacking each layer on top of the previous layer. The quality of the part being built is dependent on the quality of the previous layer as it forms the foundation. The advantage of the layer-wise process is that it also makes online monitoring of the building process a viable option. Monitoring the process can allow for very tight control and thus improve the quality or notify the operator that the component has defects and, therefore, is not fit for service. Current commercial online monitoring systems are mostly in the form of some sort of imaging or temperature monitoring system. These have the ability to monitor any defects in the powder delivery and laser scanning (melt pool). The size and shape of single tracks ultimately determine the quality of the parts, as it is the building block of the LPBF process. All the different process parameters interplay with each other and operate within a process window. The powder layer should be carefully controlled because the input energy from the laser is set and any change in material volume/powder thickness will change the resulting track’s shape. This study investigates whether gas-borne acoustic emission (AE) signal can be used for online monitoring during LPBF. The amazing amount of information that can be interpreted through listening has been proved for manufacturing processes such as laser welding and monitoring of components in service, such as electrical generators. The experiments were carried out on a commercial machine, EOS M 280, with Ti6Al4V ELI alloy. The influence of the machine noise, microphone and scanning position is investigated, and the signal filtered accordingly. Defects due to changes in process parameters are shown, and more specifically, laser power, scanning speed and powder layer thickness. The AE is correlated to the resulting single track shape. Each combination of sets of process parameters produces a specific sound. The sound pressure level and frequency of AE signal are clearly correlated to defects in single tracks that are supported by physical cross-sectioning. The information about the characteristics of LPBF and its AE is used to develop two possible methods which can detect a defective layer thickness. The algorithm compares the test signal to signals from optimal parameters and parameters which produce defects. The signals are run through a series of processing steps and the results are then correlated to each other. It is shown that the proposed algorithm can detect a defective layer with high accuracy

Effect of design and tensile testing specimen geometry on final tensile properties of powder bed fusion plastic

In Laser Powder Bed Fusion there are certain considerations that need to be accounted for when designing for thin-walled or complex shapes. Much is known of how parameters such as build orientation and scanning strategy can affect the resultant tensile properties. The tensile property results are also influenced by factors such as the shape of the specimen. The specimens’ cross-sectional geometry and length ratio are carefully considered to obtain accurate and reliable tensile properties. Eighteen different tensile geometries were manufactured using an EOS P110 and PA12 powder. These different geometries were chosen to evaluate different influencing factors such as width, gauge length, specimen geometry and scanning strategy. This knowledge is used in conventional standards when determining specimen geometries. This work aims to combine conventional tensile specimen shape and L-PBF factors to best represent the actual tensile properties of different polymer geometries

Acoustic Diagnostic of Laser Powder Bed Fusion Processes

Online monitoring of Laser Powder Bed Fusion is critical to advance the technology and its applications. Many studies have shown that the acoustic signal from the laser powder bed fusion process contains a large amount of information about the process condition. In this research, we used an acoustic system for the in-situ characterization of a wide variety of different single-track geometries. The internal acoustic system includes a microphone and accelerometer. The melting mode, cross-sectional shape and dimensions of Ti6Al4V single tracks at different process parameters are presented. We have established a correlation between track geometry, internal defects and acoustic signals. The parameters are varied and tested against the acoustic frequency measurements to determine the sensitivity. We determined the patterns of signal behaviour in the event of anomalies (spatter, balling, pores, undercut). The characteristic features of the process are traced to a commercial machine. Well described dataset with correlated monitoring data and signal tracks properties obtained and can be used for building classification model and quality prediction. All this is aimed at creating a database of experimental data that will be a key for LPBF digitalization and control, allowing real-time control of the process to optimize part quality and, more importantly, help with decision-making algorithms.

Developing a Ti6Al4V specimen to induce residual stress deformations and cracks for use in metal additive manufacturing online monitoring

In laser powder bed fusion factors such as residual stresses within the part, lead to deformations and cracks which impact the quality of the final product. Although residual stress and deformations have been thoroughly studied research and development of in-situ online monitoring requires a specimen that cracks in a predictable manner. This paper aims to show which sample geometry can be used to replicate cracks. The Ti6Al4V sample was designed based on known residual stress phenomena from literature of rapid heating and cooling cycles inducing compressive and tensile stresses during L-PBF. The sample was developed with the aid of computer aided design and simulation software using the inherent strain method. For the purpose of consistency, two identical samples were built simultaneously, and for the purpose of repeatability, two different builds were conducted. It was shown that the sample failed as predicted by the simulations due to the effective plastic strain and equivalent stress exceeding that of the mechanical properties. The sample developed can be used to test if cracks that form during the L-PBF build process can be predicted and detected

Numerical and Experimental Study on the Effect of Artificial Porosity in a Lattice Structure Manufactured by Laser Based Powder Bed Fusion

Additively manufactured lattice structures are used in various applications due to their unique properties, especially low weight with relatively good strength and stiffness. While lattices have been investigated widely, the effect of manufacturing flaws on the lattice performance was not yet analyzed in detail. One important type of manufacturing flaw which can be relatively easily analyzed numerically and experimentally is unwanted voids or porosity. In this work, using a simple cubic lattice structure as a test case, pores with varying sizes were induced in a single strut and compressive loading simulated. Ti6Al4V ELI (extra low interstitial) lattices produced by laser powder bed fusion, with and without induced pores, were subjected to mechanical compression tests. MicroCT images validated the presence and size of the induced voids in produced samples. The mechanical compression results show that even relatively large pores in individual loadbearing struts do not affect the ultimate compressive strength of these lattices, for these particular lattice shapes studied and for individual large pores.Mechanical Engineerin

Mechanical Properties and In Situ Deformation Imaging of Microlattices Manufactured by Laser Based Powder Bed Fusion

This paper reports on the production and mechanical properties of Ti6Al4V microlattice structures with strut thickness nearing the single-track width of the laser-based powder bed fusion (LPBF) system used. Besides providing new information on the mechanical properties and manufacturability of such thin-strut lattices, this paper also reports on the in situ deformation imaging of microlattice structures with six unit cells in every direction. LPBF lattices are of interest for medical implants due to the possibility of creating structures with an elastic modulus close to that of the bones and small pore sizes that allow effective osseointegration. In this work, four different cubes were produced using laser powder bed fusion and subsequently analyzed using microCT, compression testing, and one selected lattice was subjected to in situ microCT imaging during compression. The in situ imaging was performed at four steps during yielding. The results indicate that mechanical performance (elastic modulus and strength) correlate well with actual density and that this performance is remarkably good despite the high roughness and irregularity of the struts at this scale. In situ yielding is visually illustrated