Build Job Research Articles

A new approach of preheating and powder sintering in electron beam powder bed fusion

Preheating is an essential process step in electron beam powder bed fusion. It has the purpose of establishing a sintered powder bed and maintaining an elevated temperature. The sintered powder bed reduces the risk of smoke and in combination with the elevated temperature improves the processability. Today, the line-ordering preheating scheme is widely used. This scheme does not take the previously built layers into account and results in an inhomogeneous elevated temperature and consequently in a variety of sinter degrees, which is disadvantageous for the process. The main challenge is now to modify this scheme to establish a homogeneous temperature distribution. This study addresses this challenge and analyses as well as optimises this scheme. A GPU-parallelised thermal model reveals a heterogeneous temperature distribution during preheating because of varying thermal conditions within a build job. In addition, a work-of-sintering model predicts that the sinter degree of the current powder layer on top of previously consolidated material is smaller than on top of the surrounding powder bed. This work aims to invert this trend to improve powder re-usage and material consolidation. Consequently, this work proposes an extension of the current scheme, compensating for the specific energy loss with local adjustments to the energy input. This adaption results in a uniform temperature distribution and advantageous sintering. Applying the proposed numerical model proves to be an effective method to analyse the evolving process conditions and tailor the local energy input, thereby improving the efficiency of the preheating step.

Effect of a process interruption on the mechanical properties of AlSi10Mg components produced by laser powder bed fusion (PBF-LB/M)

Due to various causes, process interruptions during powder bed fusion of metal with laser beam (PBF-LB/M) can occur. This can be performed deliberately, e.g. as part of sensor integration or hybrid manufacturing. However, unplanned interruptions are also possible, for example, due to a power outage. In particular, long-term interruptions may result in significant quality losses, making it necessary to cancel the build job. Depending on the extent of the print job and the location of the interruption, this can mean a major economic loss. Most of the previous studies have found only minor reduction of the mechanical properties. However, these studies often dealt with the effect of planned interruptions and relatively short interruptions of about 1 h. Significantly longer interruption times are also realistic, especially when they occur overnight or during weekends. The aim of this study is to investigate the effects on the component quality of a process interruption when the manufacturing process is continued several hours after the interruption. For this purpose, the effects of different interruption durations (1, 4, 10 and 16 h) on the microstructure, hardness, tensile strength and fatigue properties of the PBF-LB/M-fabricated components made of AlSi10Mg were investigated. In addition, the layer shift resulting from the interruption is measured and described geometrically. The results show that for AlSi10Mg specimens in which the layer shift was removed and which are not highly loaded, an unplanned long-term process interruption up to 10 h does not result in a significant loss of quality under the considered conditions. Furthermore, it is apparent that the procedure for restarting the process is very important for the resulting component quality.

Reuse of Smoulder in Laser Powder-Bed Fusion of AlSi10Mg—Powder Characterization and Sample Analysis

Metal additive manufacturing technologies, such as Laser Powder-Bed Fusion, often rate as sustainable due to their high material efficiency. However, there are several drawbacks that reduce the overall sustainability and offer potential for improvement. One such drawback is waste emerging from the process. These smoulder particles form when the laser hits the powder-bed surface, are blown away from the part by the shielding gas stream and accumulate on the edge of the build chamber. Usually, smoulder does not contribute to the circular reuse of powder that was part of the powder-bed but was not integrated into a part. Instead, it marks an end-of-life state of powder. Significant amounts of smoulder accumulate depending on the irradiated area or the build volume in one job, respectively. This results in the waste of powder that was produced with low energy efficiency. This study investigates the question of whether smoulder can transform from waste to resource via common powder characterization methods and first build jobs using processed smoulder. The investigation of process-relevant powder properties like apparent density and flowability showed no significant difference between virgin powder and smoulder. Sample characterization indicated that neither porosity, surface quality nor mechanical properties deteriorate when samples contain about 50% smoulder. This allows for the reuse of smoulder in terms of powder characterization and part quality.

Laser powder bed fusion recoater selection guide—Comparison of resulting powder bed properties and part quality

Spreading devices used to create powder layers in the laser powder bed fusion of metals (PBF-LB/M) were found to have a significant impact on the additive manufacturing process. However, previous research primarily focused on theoretical investigations, including recoater concepts that are not available on the market, while no comprehensive comparison of commercially available spreading devices currently exists. The aim of this study is therefore to examine the powder bed properties and part qualities that can be achieved with the three most common types of recoater: carbon fiber brushes, polymer lips, and high speed steel (HSS) blades. Identical build jobs were produced using each of the spreading devices. Their capabilities were assessed by nine evaluation criteria, including dimensional, metallurgical, and mechanical properties and criticality of particles abraded from the spreading devices. Based on these quantitative findings, a spreading device selection guide was compiled for the benefit of PBF-LB/M practitioners. All recoaters yielded processes with high stability and part properties that were on a par with or even outperformed the nominal values from the literature. However, the HSS blade was found to provide higher accuracy and stability in steady-state processes. In turn, the brush and lip are better suited for parameter development and design studies. Additionally, the lip was found to have economic benefits over the brush, while the brush was deemed an effective all-rounder.

Towards data driven quality monitoring: alignment and correlation of photodiode-based co-axial melt pool monitoring signals to part quality in laser powder bed fusion

Online quality control of advanced manufacturing processes often utilize process monitoring as a qualification method to reduce or eliminate time consuming and costly destructive and non-destructive tests. The need for process monitoring based qualification becomes more apparent with the advent of laser based additive manufacturing for complex geometries and lot-size-one production. However, the usability of the process monitoring solution heavily relies on the accuracy of the established alignment and assignment of process signals to specific part and defects herein. This research proposes a data fusion strategy for data pre-processing to structure and align multi-source sensor data for improving correlation accuracy in process monitoring. A laser powder bed fusion system equipped with a photodiode-based co-axial melt pool monitoring is used for the study. First, the monitoring system collects photon emission data from the melt pool using photodiodes with high frequency (>100 kHz) which are aligned spatially and temporally using position data (xy-coordinates) of the galvanometer scanner and the laser ON/OFF signals. Next, a clustering approach is used to assign each photodiode signal with the individual parts on the build platform. Finally, correlation between melt pool monitoring signals and part level porosity is studied using micro-computed X-ray tomography (μCT) from a build job containing parts produced under varying process conditions.

Effects of powder recycling on laser-based powder bed fusion produced SS316L parts

Laser-based powder bed fusion (PBF-LB/M) is one of the extensively used additive manufacturing (AM) methods as the parts printed by PBF-LB/M have high resolution due to low layer thickness. Recycling and Reusing of the powder in this process has a significant impact on the surface roughness, mechanical properties such as hardness, elastic modulus and fracture strength etc. of the manufactured parts. The aim of the present study is to understand the effects of powder recycling on the properties of SS316L parts such as hardness and surface roughness. The developed method featured the use of SS316L powder five times over with weight percentage adjusted as required for the build job. The printed parts were checked for surface roughness across different surfaces with respect to the build direction along with Vickers hardness test. The surface roughness of the parts before polishing showed a steady increasing trend of about 40% with the recycling count while there was no significant effect on hardness as the values stayed in the range of 230+/-5 HV. Powder morphology studies with SEM displayed visible changes in terms of satellite formations, broken particles etc. with the increase in counts of recycling and particle size distribution showed a linear increase with the increase in recycling counts. This study was performed within the limited scope of a bigger study for which a detailed methodology of powder recycling will be developed.

Diagnosis of COVID-19 impact on the construction employment: Decline and recovery

The Coronavirus Disease 2019 (COVID-19) had an unprecedented impact on U.S. construction employment. The assessment of employment declines and recoveries across various construction sectors, workforce demographics, and geographic regions helps develop inclusive long-term plans to overcome the setbacks of natural hazards, such as the COVID-19 pandemic. Existing methods for quantifying the employment decline from sudden shocks do not consider trends and seasonality of construction employment under normal conditions. Hence, it is not clear whether the employment declines are due to the impact of a disaster or associated with trends and seasonal patterns of employment. The objective of this research is to develop an approach based on time series models, i.e., seasonal autoregressive integrated moving average (SARIMA) models, and cumulative sum (CUSUM) control charts to statistically quantify the impact of the COVID-19 pandemic on construction employment. Seasonal ARIMA models are developed using the pre-pandemic employment data from January 2011 to December 2019, and employment estimates under normal conditions are projected for the period from January 2020 to August 2021. CUSUM control charts are then used to detect employment declines and recovery timeframes for different construction employment. Results show declines in all construction sectors, faster recovery for women and Hispanic workers, and residential building jobs rebounding first. By August 2021, 14 states recovered construction employment, but 36 states lagged. It is anticipated that construction workforce decision-makers can benefit from this study by enhancing their understanding of the employment declines and the recovery status across industry sectors, gender, race, and geographical regions.

On the Influence of Volumetric Energy Density and Inter-Layer Time on the Material Properties of Case-Hardening Steels

Case-hardening steels are gaining increasing interest in the field of laser powder bed fusion (PBF-LB/M) due to their excellent weldability. In combination with post-process carburization heat treatment, the surface properties can be improved to generate high-strength products. When manufacturing larger products by means of PBF-LB/M, the in situ heat accumulation and the altered cooling rates affect the resulting material properties. Therefore, the fabrication of larger products requires an understanding on the influencing factors that affect the material properties. This work investigates the effect of different volumetric energy densities (VED) on the resulting microstructural and mechanical properties. It is found that the hardness decreases continuously along the build direction. The gradient depends on the applied energy and is stronger for higher energy inputs due to heat accumulation and lowered cooling rates. Furthermore, countering strategies are investigated to avoid process-specific hardness reduction along the build direction. This includes a reduced number of parts within the build job as well as a modified inter-layer time (ILT) between consecutive layers of the specimen. Applying a moderate inter-layer time helps to counter process-specific overheating, which is indicated by an almost homogeneous material hardness and melt pool size along the build direction.

Recoater crashes during powder bed fusion of metal with laser beam: simulative prediction of interference and experimental evaluation of resulting part quality

In powder bed fusion of metal with laser beam (PBF-LB/M), repetitive melting and solidification of newly added layers lead to thermal stresses and distortions during part build-up. Particularly at critical component features such as unsupported overhangs, super-elevated edges pose a risk in terms of crashes with the recoating system during powder spreading. Damaged recoater lips lead to irregularities in the form of stripes in the powder bed. These local inhomogeneities cause lack-of-fusion porosity and geometric defects on the part surface. However, quantitative information on important quality aspects, such as tensile properties, dimensional accuracy, roughness, and hardness of parts printed under irregular powder bed conditions is scarce. Here, we show that samples from build jobs with recoater crashes maintain their elastic tensile properties and hardness, but lose elongation at break. Finite-element simulations of in-process distortions are used to design an artifact that intentionally damages the silicone rubber lip of the recoater but does not cause machine breakdown. The lowest mean yield strength of the damage-affected samples is 243 MPa, which is still within the material data sheet limits for AlSi10Mg. Therefore, recoater crashes do not necessarily result in rejects, but users must consider the likely presence of porosity.

Comparative Study of Material Management in Public and Private Building Construction Project Inside Kathmandu Valley

Material management is crucial to timely and cost-effective project delivery. Material management was key to every building job. Material management tasks slowed project delivery. This study compares material management in public and private building projects. The study aimed to improve material management in Kathmandu Valley development projects.The study used 34 projects. DUDBC and NRA executed 16 of them. 18 were by private developers. Primary data was collected by questionnaire and key informant interview. Secondary data was collected using desk study. Relative Importance Index ranked Likert questionnaire responses (RII).The study indicated that the public and private sectors have comparable views on material management in building construction projects. Both sectors agreed on the presence of a material schedule in their respective project, but are unsatisfied with their material management method. They also agreed on the delay in their respective project owing to late materials. Both sectors demanded a separate material schedule for successful material management and favoured the ABC classification approach for inventory planning and material classification. Both sectors insisted on a joint visit to find the material source.The study finds discrepancies between public and private sectors. The public sector highlighted "The source must be able to offer materials that fulfil project specifications in the quantities" as the best guideline for locating material purchasing sources, while the private sector emphasized "The supplier must always be honest." Public sector material management focuses on inventory control, while the private sector prioritizes planning and programming. Public projects use pricing performance to grade vendors, while private builders emphasize quality. The public project emphasizes timetable adherence over quality schedule as the key to delivery performance.The analysis showed severe weaknesses in material management in both industries. In the public sector, material procurement delays, wastage, and quality and specification problems are important material management deficiencies. For the private sector, late supplier answers, transportation, delivery, and stock management challenges.Possible solutions include preparing a material schedule in line with the construction schedule for material requirement planning, verifying materials and locating sources, purchasing required materials in advance before work begins, classifying materials and managing inventory based on usage value, and eliminating and reducing wastages during overall material management. How to cite this article: Shahi S, Shreshta HM, Bhattarai SK et al. Comparative Study of Material Management in Public and Private Building Construction Project Inside Kathmandu Valley. J Adv Res Prod Ind Engg 2022; 9(3&4): 1-11. DOI: https://doi.org/ 10.24321/2456.429X.202202

Building on the Competency Pivot: Helping Students Build Job Portfolios for Employment

The purpose of this article is to share teaching strategies within business and professional communication that assisted students in developing a job portfolio, potentially enhancing students’ ability to secure an internship or job after course completion. This article details the application of these strategies over the course of three semesters in 2018 and 2019 at Monmouth University. Student comments suggested these strategies enhanced their chances of employment success. Using course curriculum to put students in position to secure employment should be a goal of the course and curriculum should be adjusted periodically to meet this goal.

A Novel Methodology for the Thermographic Cooling Rate Measurement during Powder Bed Fusion of Metals Using a Laser Beam

Powder bed fusion of metals using a laser beam (PBF-LB/M) is a process that enables the fabrication of geometrically complex parts. In this process, a laser beam melts a metallic powder locally to build the desired geometry. The melt pool solidifies rapidly, which results in high cooling rates. These rates vary during the process in line with the geometric characteristics of the part, which leads to a non-uniform microstructure along with anisotropic mechanical properties. The unknown part characteristics prevent the process from being used in safety-critical applications. Thermographic in situ process monitoring provides information about the thermal field, enabling predictions of the resulting material properties. This study presents a novel methodology for the thermographic measurement of cooling rates during the PBF-LB/M process using a high-speed thermographic camera. The cooling rates occurring during the manufacturing of 316L tower-like specimens were measured. The cooling rate decreased with increasing build height, due to the heat accumulation in the parts. The microhardness profile of the parts was tested perpendicularly and parallel to the build direction. A significant decrease in hardness values was observed along the build height. The measured cooling rate was correlated to the microhardness profile of the specimens using a Hall–Petch type relationship. The results show a high level of reproducibility of the cooling rates between different specimens in the same build job as well as between subsequent build jobs. The presented methodology allows studying the effects of the geometry on the cooling rates and the resulting mechanical properties of 316L specimens.

Online monitoring for error detection in vat photopolymerization

ABSTRACT Sensor implementation together with data processing in manufacturing provides continuous measurements enabling process monitoring, optimization, and automation. Additive manufacturing (AM) technologies are shifting from one-off and prototyping production to batch, mass, and continuous production. Current AM technologies are mostly focused on small production devices, without monitoring systems. This study aims to bring AM closer to automation by proposing an online monitor system for bottom-up photopolymerization AM (VPP) systems. The sensor-generated data is used to capture the detachment error of built parts from the build platform that otherwise cannot be observed physically by the machine operator. The detachment itself will not stop the build job, which results in lost material and operating time. The online monitoring procedure consists of two phases, an offline and an online one, respectively. The offline phase is used for training a prediction model to be used in connection with a control chart for online monitoring. The monitoring control chart is advantageous as only the detachment predictions need to be recorded. The research carried out in this article brings novelty in both the vat photopolymerization set-up, as well as in an online monitoring procedure that can be easily extended to similar technologies.

Material properties of AISI H10 (32CrMoV12-28) hot work tool steel processed by Laser Powder Bed Fusion with 200°C substrate preheating temperature

In this study the processability of hot work tool steel AISI H10 (32CrMoV12-28) using the Laser Powder Bed Fusion (L-PBF) process was investigated. The tool steel was processed in two build jobs with different parameter settings. The produced samples were examined for defects such as cracks, pores and lack of fusion. Due to the high cooling rates in the L-PBF process and the resulting residual stresses, cracks were found despite substrate preheating. In a second build job the process parameters were adapted without changing the preheating temperature of 200°C. Changing the process parameters led to crack-free AISI H10 hot work tool steel samples. Process parameters with the highest density and lowest number of defects were used to produce tensile and notch impact samples which were used to determine the mechanical properties.

The impact of geometric complexity on manufacturing process efficiency of Selective Laser Sintering

One of the major advantages of additive manufacturing (AM) is often cited as freedom of design. In comparison to traditional manufacturing processes, this is valid because an increase in geometric complexity is not equally followed by an increase in costs. However, there are numerous challenges in this regard, since the proposed freedom of design is often presented in an undifferentiated manner, neglecting the various processes and principles of this technology. A comparison of different AM-processes (e.g., powder bed fusion and material extrusion of polymers) shows that the geometry to be generated sometimes has significant effects on the production effort in terms of building time and costs. However, these effects are insufficiently explored at the present stage.This paper addresses the aforementioned issues in order to evaluate the impact of geometric complexity on the manufacturing process. The methodology is demonstrated using the process of selective laser sintering (SLS) for polymers. The study is conducted with a thermoplastic polyurethane (TPU) powder. The main focus lies on the analysis of geometrical influencing factors, parameters of the manufacturing process as well as resulting production efforts. Building on these theoretical findings, the relationships are proven experimentally. For this purpose, a variety of complex geometric shapes are produced with two different SLS systems in order to analyse generation time. While maintaining typical component criteria such as height and volume, it is shown that the generation time can be changed by more than 30 % by varying the complexity. Compared to existing work, new insights into the productivity of the manufacturing process are gained regarding component design, component orientation as well as the layout of build jobs.

In situ detection of cracks during laser powder bed fusion using acoustic emission monitoring

• Crack-related AE activity can be registered by a simple threshold approach • AE signals from cracks differ from the noise by waveform and statistical parameters • There is a clear relation between cracks directly observed by µ-CT and AE signals • The time between a layer was built and a crack emergence can take up to an hour • AE method enables to detect cracks regardless of their spatial or temporal location Despite rapid development of laser powder bed fusion (L-PBF) and its monitoring techniques, there is still a lack of in situ crack detection methods, among which acoustic emission (AE) is one of the most sensitive. To elaborate on this topic, in situ AE monitoring was applied to L-PBF manufacturing of a high-strength Al 92 Mn 6 Ce 2 (at. %) alloy and combined with subsequent X-ray computed tomography. By using a structure borne high-frequency sensor, even a simple threshold-based monitoring was able to detect AE activity associated with cracking, which occurred not only during L-PBF itself, but also after the build job was completed, i.e. in the cooling phase. AE data analysis revealed that crack-related signals can easily be separated from the background noise (e.g. inert gas circulation pump) through their specific shape of a waveform, as well as their energy, skewness and kurtosis. Thus, AE was verified to be a promising method for L-PBF monitoring, enabling to detect formation of cracks regardless of their spatial and temporal occurrence.

Parametric design optimisation of tree-like support structure for the laser-based powder bed fusion of metals

Support structure design for Laser Powder Bed Fusion has received little attention in Design for Additive Manufacturing although its importance for a successful and efficient build job. The resulting knowledge insufficiency on support design can lead to in-process failures of the build. Also, parts can crack due to high, unaccounted residual stresses. This, and overestimated support designs, lead to an increased manufacturing time and cost. The presented research contributes to a better understanding of support structure design by developing a workflow for the design of tree-like support structure and its subsequent meta-model based parameter optimisation. The design parameters of resulting support structures for two generic geometries in a cantilever shape with a planar and an arched down-facing surface are compared with each other, showing consistent values for the minimum distance between branches and the branch length. These and other design parameters govern the geometry of the tree-like structure within the design space. An additionally performed sensitivity study revealed high correlations of the stem diameter with the part’s displacements. Based on 500 individual support designs and the meta-model evaluation of 9000 parameter sets, a Pareto frontier emerges for the trade-off between the minimum support structure volume and the part’s displacements.

Influence of part geometry on spatter formation in laser powder bed fusion of Inconel 718 alloy revealed by optical tomography

The metal powder used during the Laser Powder Bed Fusion (L-PBF) process is usually cycled for reuse in subsequent build jobs for cost-effectiveness and sustainability. Qualification guidelines are being established based on testing results of powder properties in terms of flowability, chemistry and rheological behaviors, etc. for making decisions on whether a batch of reused powder is suitable for producing parts that meet certain requirements. The current paper aims to develop experimental strategies for tracking powder history using novel design of specimens and on-line monitoring. Powder-capturing containers designed with internal lattice structures of varied beam lengths and diameters were manufactured by the L-PBF process using an Inconel 718 powder to investigate the influence of part geometry on the degradation of reused powder. The L-PBF experiment was monitored by a commercial Optical Tomography (OT) system which records the thermal emissions from the build area. Data were extracted from the OT images to evaluate the emissions of spatter particles introduced to the powder bed, which is influenced by the local layer profiles of the lattices and the overall geometries of the container. The collected powder samples were tested by combustion analysis for oxygen content and characterized by Scanning Electron Microscopy (SEM). Surface chemistry analyses of the powders were performed by X-ray Photoelectron Spectroscopy (XPS). Depending on the lattice structure geometry, the oxygen uptake in the powder collected from the containers was increasing by 10 ppm in case of empty container and up to as high as 118 ppm in case of container with larger areas of overhangs and higher surface-to-volume ratio. XPS results revealed the presences of Al-rich and Cr-rich oxides on the surface of powder samples collected from the container filled with lattices of high surface-to-volume ratio and the container filled with lattices of large overhangs, which agrees with the analyses of OT data.

Effect of selected process parameters on dimensional accuracy in Arburg Plastic Freeforming

PurposeThe body of the literature on the Arburg Plastic Freeforming process is still very limited despite the increasing industrial importance of this technology. This paper aims to contribute to a better understanding of this technology by investigating relations between characteristic process parameters and part features. Particularly, the effects of nominal dimension, drop aspect ratio, build chamber temperature and part position on accuracy are investigated. The density of manufactured parts is also measured to understand its relation with dimensional error.Design/methodology/approachA benchmark part was designed and manufactured in Polycarbonate on an Arburg Plastic Freeformer 2K-3A. The process was repeated with two levels of drop aspect ratio (1.2125 and 1.2150) and two build chamber temperatures (90°C and 120°C). Each build job included five parts in different positions of the chamber. The dimensional accuracy of benchmarks was measured by using a digital caliper, while Archimede’s principle was used for density measurements. All the acquired results were processed through an analysis of variance to investigate the role of experimental factors.FindingsResults demonstrate that the linear shrinkage occurring at the end of the 3D printing process is the main source of inaccuracy. The higher the building chamber temperature, the most the part accuracy is influenced by the nominal dimension. The drop aspect ratio affects the dimensional error in the XY plane by increasing the overlap of adjacent droplets. On the other hand, this parameter does not influence the accuracy along the Z direction. The position of the parts inside the building chamber exhibited an influence on results, arguably due to the hot airflows.Research limitations/implicationsThis research did not allow for a complete understanding of the role of part positioning on part accuracy. Further study is needed to understand the detail of this phenomenon.Practical implicationsThe results of this study can aid the users of Arburg Plastic Freeforming technology by uncovering the role of the main process parameters.Originality/valueThis paper expands the body of knowledge on the Arburg Plastic Freeforming process by providing new information on the role of the main process parameters on dimensional accuracy and density. Particularly, the results answer a research question on the role of the drop aspect ratio, demonstrating that its main effect is to vary the droplets overlap, which, in turn, affects the thermal shrinkage.

On the Influence of AlSi10Mg Powder Recycling Behavior in the LPBF Process and Consequences for Mechanical Properties

By using additive manufacturing techniques like the laser powder bed fusion (LPBF) process, parts can be manufactured with high material efficiency because unfused powder material can be reconditioned and reused in consecutive manufacturing jobs. Nevertheless, process by-products like spatters may influence the powder quality and hence alter the mechanical properties/performance of parts. In order to investigate these dependencies, a methodology and a standard build job for the recycling behavior of the lightweight aluminum alloy AlSi10Mg was developed and built with ageing powder in 10 consecutive jobs with no refreshing between the cycles. The powder properties and mechanical performance of parts at static load for two build directions (horizontally and vertically to substrate plate) was evaluated. The influence of build height effects on mechanical performance was investigated as well. The findings may indicate that the coarsening of the powder material during recycling could lead to improved mechanical properties for the AlSi10Mg alloy.