Abstract In order to meet the increasing demand for high‐speed joining processes, laser beam micro welding is used to reproducibly weld metallic joining partners such as steel. Long weld seams or high cycle speeds, however, remain a challenge to achieve a constant welding depth throughout the entire welding process without major fluctuations. This paper shows the development and evaluation of a weld penetration depth control system based on interferometric measurement of the depth of the vapor capillary. High path speeds and small weld depths are challenging for real‐time depth control. Therefore, the offline data of the interferometric measurements are used to implement an iterative learning control of the weld penetration depth. The quality of the control is verified by welding with and without spatial power modulation on 1.4301 steel while following a linear and sinusoidal trajectory.
To design future laser manufacturing processes for welding of copper materials, more and more high-end analysis methods are required. A fundamental process understanding by analyzing cause-and-effect relations of process dynamics using inline in situ diagnostics allows for an improved description of laser material interaction. Strategies for a reliable and robust welding process are derived from the findings. In this study, a four-step advanced methodical approach is presented and discussed. In the first step, a fundamental process description of the geometry of the vapor capillary and the formation of weld defects is developed. Therefore, welds on electrolytic tough pitch copper (Cu-ETP) and CuSn6 are carried out to analyze the temporal and spatial vapor capillary dynamics depending on laser power, welding speed, and focal diameter. This fundamental process understanding is transferred to the welding of copper pins in the form of I-pins. For this purpose, impurities and imperfections were applied to the pin surface to investigate the effects on the process result. As a third step, strategies by means of laser intensity distributions were adapted to compensate for imperfections in the welding process. Finally, a sensor vision system is adapted for ideal welding results. Investigations are based on in situ synchrotron analysis at Petra III, DESY in Hamburg. For the experiments, a TRUMPF TruDisk laser (100/400 μm fiber diameter), a TRUMPF TruFiber 6000P (50/100 μm fiber diameter), and a single-mode fiber laser (14 μm fiber diameter) were used. The focal diameter was adjusted with the optical system depending on the investigation.
Laser beam shaping is a novel and relatively little explored method for controlling the melt pool conditions during metal additive manufacturing (MAM) processes, however it sets the scene for achieving site-specific tailored properties. In this work, a comprehensive numerical and experimental campaign is carried out to explore this subject within metal laser powder bed fusion (LPBF). More specifically, a multiphysics numerical simulation is developed for modelling the heat and fluid flow conditions during LPBF of Ti6Al4V using arbitrary circular beam shapes with various power distributions spanning from a pure Gaussian beam (6:0), to a pure ring beam profile (0:6). The model is subsequently coupled with a cellular automata procedure to describe the beam shape effects on the microstructure evolution during LPBF of Ti6Al4V alloy. Model validation is carried out in a two-fold manner. First we compare the predicted melt pool cross-section with the one from ex-situ single track experiments and we find a deviation of less than 9% in melt pool dimensions. Secondly, advanced in-situ X-ray monitoring is carried out to unravel the melt pool dynamics during LPBF using the ring beam profile. We find that the predicted melt pool morphology closely matches the in-situ X-ray results and it is shown that at lower laser powers, a bulge of liquid metal forms at the center of the melt pool when employing ring profiles and this is ascribed to the absent recoil pressure at the center of the ring beam. Furthermore, increasing the laser power seems to destabilize the melt pool regime, as the central bulge transforms into a liquid metal jet that periodically collapses and breaks up into hot spatters. Based on the results, we believe that our multiphysics modelling methodology, in which we simulate accurately the melt pool morphology as well as link process and microstructure, opens up new pathways for predicting how laser beam shaping influences porosity, surface roughness as well as microstructure formation in LPBF processes.
The validation of laser welding of metallic materials is challenging due to its highly dynamic processes and limited accessibility to the weld. The measurement of process emissions and the processing laser beam are one way to record highly dynamic process phenomena. However, these recordings always take place via the surface of the weld. Phenomena on the inside are only implicitly recognizable in the data and require further processing. To increase the validity of the diagnostic process, the multispectral emission data are synchronized with synchrotron data consisting of in situ high-speed images based on phase contrast videography. The welding process is transilluminated by synchrotron radiation and recorded during execution, providing clear contrasts between solid, liquid, and gaseous material phases. Thus, dynamics of the vapor capillary and the formation of defects such as pores can be recorded with high spatial and temporal resolution of <5 μm and >5 kHz. In this paper, laser welding of copper Cu-ETP and CuSn6 is investigated at the Deutsches Elektronen-Synchrotron (DESY). The synchronization is achieved by leveraging a three-stage deep learning approach. A preprocessing Mask-R-CNN, dimensionality reduction PCA/Autoencoders, and a final LSTM/Transformer stage provide end-to-end defect detection capabilities. Integrated gradients allow for the extraction of correlations between defects and emission data. The novel approach of correlating image and sensor data increases the informative value of the sensor data. It aims to characterize welds based on the sensor data not only according to IO/NIO but also to provide a quantitative description of the defects in the weld.
Abstract The X-ray phase contrast imaging is a powerful method to understand the fundamental behavior of the melt and keyhole during the laser beam welding process. In this paper, the keyhole-induced vapor capillary formation in the melt pool is investigated by using an adjustable laser beam source. For this purpose, the aluminum A1050 specimen with a thickness of 0.5 mm is molten only with the heat conduction welding regime by using the ring-mode laser beam. Once the specimen is molten through, the core multi-mode laser beam is then applied to vaporize the melt and a transition to keyhole welding regime occurs. Therefore, the core multi-mode laser beam with an intensity value of 33.3 MW/cm 2 is investigated. The correlation between the keyhole-induced vapor capillary and the melt behavior is further investigated in this paper which was recorded with a high sampling rate of 19 kHz. In addition, a theoretical calculation about the keyhole depth is discussed in this paper.
Soil fertility is at stake at a global scale, putting pressure on food security, poverty alleviation and environmental protection, under scenarios of climate change that in most cases aggravate the threat. In sub-Saharan Africa, a combination of depleted soils and population growth adds particular pressure to smallholder farmers and society. Their capacity to innovate in a social, economic, political and cultural context is seen as decisive to reverse the trend of declining soil fertility. However, many technologies with a potential to protect, maintain and build up soil fertility are hardly used by small-scale farmers, triggering the urgent question on their reasoning not to do so. Exploring and understanding the constraints and complexity of the social systems interacting with the implied institutional dynamics are essential steps in designing appropriate agricultural innovations that are scalable and adoptable. The focus of the inter- and transdisciplinary approach applied in the project ORM4Soil (Organic Resource Management for Soil Fertility; www.orm4soil.net) lies at the heart of this project. We are combining qualitative and quantitative methods from agronomy, sociology and communication sciences in order to bring soil-fertility-enhancing-technologies and their adoption to the center of the decision-making process of farmers’ as well as local and regional institutions. At local and regional innovation platforms, stakeholders from business, government, academia and farmer organizations are discussing the outcomes of agronomic trials and sociological research. We are expecting to create bridges between the needs and concerns of farmers, relevant segments of society and policymaking, with the new common goal to enhance soil fertility.
Soil fertility decline is a significant challenge to the agroecosystems in sub-Saharan Africa (SSA). Accurate and demand-driven soil fertility information is vital for improving agricultural production. We achieved two objectives, i) identifying soil fertility information needs and access and ii) assessing socioeconomic determinants of soil fertility information needs and access among smallholder farmers in the central highlands of Kenya. We sampled 397 smallholder farming households in Murang'a and Tharaka-Nithi counties. We found high soil fertility information need indices among farmers in the two regions. The main soil fertility information needs for farmers in Murang'a County were knowing the correct method of manure application, knowing sources of information, and how to determine soil fertility levels. In Tharaka-Nithi County, farmers' information priorities were how to apply conservation agriculture, knowing soil erosion control methods, and how to apply animal manure. We found mixed results on soil fertility information access ranging from low to high across different information items and study sites. The binary logistic regression results highlight the influence of farmer perceptions and other factors on soil fertility information need and access. To promote soil fertility information access, agricultural policies should consider site-specific information priorities and socioeconomic contexts.
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