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Improving metal additive manufacturing part design and final part precision using feedback from X-ray computed tomography
Abstract
Additive metal manufacturing processes, such as laser powder bed fusion, still show difficulties when producing overhang features or internal structures such as channels or bores. Channels are often mutilated by sag defects and dross formation at their upper part, when the channel-axis is close to parallel to the base plate and in the particular case when support structures cannot be used as it would be impossible to remove them after the build. The problem is still not completely solved, although various design guidelines have been developed for various processes and materials in use. So far, a general approach is to tweak the processing parameters or to orient the design on the build plate to reduce downfacing regions at the most critical features of the parts. This work proposes to use feedback from X-ray computed tomography measurements and a new evaluation approach for the additive manufacturing process-chain to obtain improved geometrical accuracy of internal channels. Preliminary results on the evaluation are presented, with the future scope of reducing sag and dross defects by adapting the channels and bores during the design stage.
... An increasing number of researchers are trying to use the gained knowledge to devise methods providing feedback for design and process. For instance, with the aid of high-resolution CT scans, it was possible to identify a high anisotropy in the porosity distribution of manufactured channel artefacts, where the majority of the pores was located in between the bulk and contour region, indicating misalignments between the hatch strategies of each region [23]. More recently, Baier et al. [24] proposed a method to evaluate sag and dross defects and general form deviations of internal channels, use the obtained information to define identifiers separating sag and dross and calculate the onset of the defects in a quantitative manner. ...
In recent years, the non-destructive characterisation via X-ray computed tomography of internal features, often produced with novel manufacturing methods, has gained increasing attention. Reducing the rate of defects during the production of internal features is of particular interest since post-processing steps are highly cumbersome, or removal of defects is, in many cases, even impossible for complex channels or bores. However, it is not sufficient to characterise the defects solely, but their root causes need to be understood in depth to allow for a-priori compensation approaches fostering first-time right part production. To achieve this, it is imperative to fully understand the connections and influences between the different factors, to enable the creation of versatile and scalable correction and compensation approaches. Within this scope, this work presents a characterisation of additively manufactured internal channels, focussing on dross defects encountered in overhang regions of the channel and their dependency as a function of a broad range of channel diameters and inclination angles. The extraction of the required information from the channels is based on cross-sectional images along the channel's axis of the reconstructed 3D volume, obtained with high-resolution X-ray computed tomography. With the data obtained through an analysis algorithm presented in an earlier publication, it is possible to extract characteristic identifiers that can be used to calculate potential compensations counteracting the defects emerging during the printing. With the aid of these identifiers, it is possible to chart out general trends of the channel defects as a function of channel diameter and inclination angle. Therefore, the results allow to identify parameters where certain defects remain almost constant for a variation of the channel diameter.
Additive manufacturing (AM) is a fast-growing sector with the ability to evoke a revolution in manufacturing due to its almost unlimited design freedom and its capability to produce personalised parts locally and with efficient material use. AM companies, however, still face technological challenges such as limited precision due to shrinkage, built-in stresses, and limited process stability and robustness. Moreover, often post-processing is needed due to the high roughness and remaining porosity. Qualified, trained personnel are also in short supply.
In recent years, there have been dramatic improvements in AM design methods, process control, post-processing, material properties and material range. However, if AM is going to gain a significant market share it must be developed into a true precision manufacturing method. The production of precision parts relies on three principles:
1. Production is robust (i.e. that all sensitive parameters can be controlled).
2. Production is predictable (for example, the shrinkage that occurs is acceptable because it can be predicted and compensated in the design).
3. Parts are measurable (as without metrology, accuracy, repeatability and quality assurance cannot be known).
AM of metals is inherently a high-energy process, with many of sensitive and inter-related process parameters, making it susceptible to thermal distortions, defects and process drift. The complete modelling of these processes is beyond current computational power and novel methods are needed to practicably predict performance and inform design. In addition, metal AM produces highly textured surfaces and complex surface features that stretch the limits of contemporary metrology. With so many factors to consider, there is a significant shortage of background material on how to inject precision into AM processes. Shortage in such material is an important barrier for a wider uptake of advanced manufacturing technologies and a comprehensive book is thus needed.
This book aims to inform the reader how to improve the precision of metal AM processes by tackling the three principles of robustness, predictability and metrology, and by developing computer-aided engineering methods that empower rather than limit AM design.
X-ray computed tomography (CT) is a non-destructive approach to verify internal features of various industrial components built by additive manufacturing (AM) or other processing methods. However, the measurement results was highly impacted by numerous factors. In this study, DoE (Design of Experiments) was conducted to statistically study impacts of error source of X-ray CT metrology; optimal settings were recommended for different internal geometrical features. Measurement comparison between X-ray CT and CMM (Coordinate Measuring Machine) is also provided in this paper to analyze the principle difference of these two measurement technology.
3D printing or additive manufacturing is a novel method of manufacturing parts directly from digital model using layer-by-layer material build-up approach. This tool-less manufacturing method can produce fully dense metallic parts in short time, with high precision. Features of additive manufacturing like freedom of part design, part complexity, light weighting, part consolidation, and design for function are garnering particular interests in metal additive manufacturing for aerospace, oil and gas, marine, and automobile applications. Powder bed fusion, in which each powder bed layer is selectively fused using energy source like laser, is the most promising additive manufacturing technology that can be used for manufacturing small, low-volume, complex metallic parts. This review presents overview of 3D Printing technologies, materials, applications, advantages, disadvantages, challenges, economics, and applications of 3D metal printing technology, the DMLS process in detail, and also 3D metal printing perspectives in developing countries.
Purpose
For geometries exhibiting overhanging surfaces, support structures are needed to dissipate process heat and to minimize geometrical distortions attributed to internal stresses. The use of support structures is often time- and cost-consuming. For this reason, this study aims to propose an approach which minimizes the use of such structures.
Design/methodology/approach
For minimizing the use of support structures, process parameters in combination with a contour-like exposure strategy are developed to realize support-less overhanging structures of less than 35°. These parameters are implemented in a shell-core strategy, which follows the idea of applying different processing strategies to the critical (overhanging) shell and the uncritical core of the part. Thereby, the core is processed with standard parameters, aiming a dense material. On the critical shell, optimized processing parameters are applied, reaching good results in terms of surface quality, especially at extreme overhang situations.
Findings
The results show that the selective laser melting (SLM) technology is able to realize support-less overhanging surfaces by choosing suitable scan strategies and process parameters. Particularly good results are always obtained when the exposure direction of the shell is parallel to the contour of the sample.
Originality/value
The validity of the results is demonstrated through the successful reproduction of the build strategy on two commercial SLM machines, reaching support-free builds of surfaces with an angle to the horizontal of less than or equal to 30°.
This study focuses on the comparison of porosity testing methods for the quality assessment of selective laser melted parts. Porosity is regarded as important quality indicator in metal additive manufacturing. Various destructive and non-destructive testing methods are compared, ranging from global to local observation techniques and from quick low-cost to expensive time-consuming analyses. Forty test specimens were produced using five varying control factors. The experimental results show that Archimedes and CT methods compare well, Archimedes can be deployed to inspect parts in small series and CT pre- and post-cut analysis show that post-cut porosity results are systematically higher.
Today, laser additive manufacturing (LAM) is used in more and more industrial applications. Due
to a new freedom in design it offers a high potential for weight saving in lightweight applications,
e.g., in the aerospace industry. However, most design engineers are used to design parts for
conventional manufacturing methods, such as milling and casting, and often only have limited
experience in designing products for additive manufacturing. The absence of comprehensive
design guidelines is therefore limiting the further usage and distribution of LAM. In this paper,
experimental investigations on the influence of part position and orientation on the dimension
accuracy and surface quality are presented. Typical basic shapes used in lightweight design have
been identified and built in LAM. Thin walls, bars, and bore holes with varying diameters were
built in different orientations to determine the process limits. From the results of the experiments,
comprehensive design guidelines for lightweight structures were derived in a catalog according to
DIN 2222 and are presented in detail. For each structure a favorable and an unfavorable example is
shown, the underlying process restrictions are mentioned and further recommendations are given.
Selective laser melting (SLM) is an additive manufacturing technique in which metal products are manufactured in a layer-by-layer manner. One of the main advantages of SLM is the large geometrical design freedom. Because of the layered build, parts with inner cavities can be produced. However, complex structures, such as downfacing areas, influence the process behavior significantly. The downfacing areas can be either horizontal or inclined structures. The first part of this work describes the process parameter optimization for noncomplex, upfacing structures to obtain relative densities above 99%. In the second part of this research, parameters are optimized for downfacing areas, both horizontal and inclined. The experimental results are compared to simulations of a thermal model, which calculates the melt pool dimensions based on the material properties (such as thermal conductivity) and process parameters (such as laser power and scan speed). The simulations show a great similarity between the thermal model and the actual process.
X-ray computed tomography (CT)has recently started to be used for evaluating the surface topography of metal parts produced by additive manufacturing (AM). In particular, CT can overcome the main limitations of contact and optical measuring techniques, as CT enables non-destructive measurements of both internal and difficult-to-access surfaces, including micro-scale re-entrant surface features. This work aims at improving the understanding of CT-based surface topography characterisation, including the use of new generalised surface texture parameters suited for AM surfaces. Experimental investigations are performed on Ti6Al4V reference samples fabricated by laser powder bed fusion to determine the uncertainty of CT surface topography measurements.
With the advance of additive manufacturing (AM) processes, complex designs can be created with engineering metals. One specific advantage of this greater design space is the ability to create small internal channels and passageways for cooling high heat flux or temperature applications such as electronics and gas turbine airfoils. These applications can have complex shapes, which when coupled with the required small channel sizes, make traditional finishing processes a challenge for additively manufactured parts. Therefore, it is desirable for designers to be able to use AM parts with small internal channels that are as-built. To achieve this goal, however, designers must know how the AM process affects internal channel tolerances and roughness levels, since both impact the amount of cooling that can be achieved in actual applications. In this study, the direct metal laser sintering (DMLS) process, more generically referred to as selective laser melting (SLM), was used to additively manufacture test coupons. The AM build direction was varied to study its effect on small microsized, circular channels. Specifically, X-ray computed tomography (CT-scan) was used to nondestructively inspect the interior of the test coupons. Using the data from the CT-scans, internal surface roughness, geometric tolerances, and deviations from the computer-aided design (CAD) model were calculated. In comparing the data, significant differences were seen between the three different build directions.
The paper gives a survey of the upcoming use of X-ray computed tomography (CT) for dimensional quality control purposes: i.e. for traceable measurement of dimensions of technical (mechanical) components and for tolerance verification of such components. It describes the basic principles of CT metrology, putting emphasis on issues as accuracy, traceability to the unit of length (the meter) and measurement uncertainty. It provides a state of the art (anno 2011) and application examples, showing the aptitude of CT metrology to: (i) check internal dimensions that cannot be measured using traditional coordinate measuring machines and (ii) combine dimensional quality control with material quality control in one single quality inspection run.