Dilution is an unavoidable consequence of multi-material fusion processing, i.e. welding, cladding etc. In this paper we propose a novel method for controlled dilution experiments, analyzing microstructural trends of steel filler wire diluted with steel base metal. The highlight of this method is the control of processing conditions used to melt a pre-determined dilution of two high strength steels. The materials involved are S960QL base metal machining chips and a chopped under-matched wire consumable, which is used to increase the toughness of welded joints. These materials were combined in specific mass ratios in a prepared cavity and then melted by a pulsed laser beam. A high-speed RGB camera evaluated the relative spatial temperature of the melt surface. The molten mass then solidified into a uniform nugget, confirmed by energy dispersive x-ray spectrometry (EDS) to have a homogenous chemical composition (a ‘Snapshot’ nugget). Hardness values obtained for different dilution levels were compared to a narrow gap multi-layer laser weld (NGMLW), with a decreased dilution rate yielding a decreased hardness. The Snapshot microstructures created are similar to the different regions of the NGMLW, in the weld cap and in the body of the weld. Snapshot nuggets were also evaluated for non-metallic inclusion (NMI) size distributions relating to the dilution levels (NMIs are important indicators for acicular ferrite, which has been shown to increase impact toughness).

A novel, near-vertical approach to the usually horizontal laser Narrow Gap Multi-Layer Welding process is introduced. The process is applied to join X100 pipeline steel and studied through High Speed Imaging. The produced welded joints are compared to their horizontally welded counterparts using 3D scanning, longitudinal & perpendicular cross sections and Computed Tomography analysis. The near-vertical approach is found to be robust and produce welded joints with a uniform appearance. The top surface exhibits certain reoccurring morphological features, and variations in internal track melting boundaries are observed. Any observed cavities appear similar to those produced using the horizontal process, with the difference of their orientation. A combination of the horizontal and the near-vertical process could be beneficial; the near-vertical approach offers potential for shorter inter-layer time and the horizontal method for better surface finish than that of its counterpart. Potential benefits of, and improvements to, the near-vertical process are discussed.

Narrow Gap Multi-Layer Welding (NGMLW) using a laser as the main heat source and metal wire for material addition has been a growing topic of interest in the last decade. This is in part due to its potential for joining much thicker sheets of steel than what is usually considered possible when using autogenous laser welding. The process has shown great potential but improvements can still be made, e.g. through increased process control to decrease welding imperfections. Using closed-loop control, where the process is continuously monitored and regulated automatically, can help to account for variations during manufacturing. However, achieving functional closed loop control can be challenging due to limitations in data gathering and processing speeds. Important initial steps include identifying what data can be useful and how frequently this data has to be recorded. Too much data takes too long to process while too little causes risks of missing important details. In this study, 20 mm thick X80 pipeline steel sheets are joined together using this multi-layer approach; the samples are examined using 3D scanning and Computed Tomography (CT) analysis and the process is observed using High-Speed Imaging (HSI). The quality of the welded joint and welding imperfections are discussed and potential points of formation are identified. Suggestions on how to mitigate imperfections to improve the quality of the welded joint are presented, including the potential to use camera imaging for closed-loop process control and additional industrial uses of the HSI footage.

In welding, wire-feeding enables alteration of the resulting microstructure and, in turn, the mechanical behavior of the welded joint. For pipeline steel grades, very few commercial wires are matching at high strength and simultaneously ensure sufficient toughness. New wire chemistries need to be investigated. Promising consumable chemistries can be studied through metal cored wires. One promising concept is alloys that promote acicular ferrite instead of bainite. Interlocking instead of parallel laths can lead to higher toughness. In the gouge range of 15–19 mm, laser-arc hybrid welding has been studied for pipeline steel grades X80 and X100. For efficient mapping of various weld metal conditions, a simplifying “snapshot” method was developed. A pulse shaped laser beam melts wire pieces in a controlled manner, reproducing thermal cycles in welding. The weld metal tends to form bainite, but under certain conditions, complex microstructures with interlocking laths can be generated. Slow thermal cycles can lead to coalescence of the laths to coarser structures, while fast cycles favored finer structures and occasionally lath interlocking. The formation of acicular ferrite was difficult to achieve. Advanced wire chemistries lowered the hardness of the weld metal, as did preheating.

Additive manufacturing (AM) can be used for the fabrication of large metal parts, e.g., aerospace/space applications. Wire arc additivemanufacturing (WAAM) can be a suitable process for this due to its high deposition rates and relatively low equipment and operationcosts. In WAAM, an electrical arc is used as a heat source and the material is supplied in the form of a metal wire. A known disadvantageof the process is the comparably low dimensional accuracy. This is usually compensated by generating larger structures than desired andmachining away excess materials. So far, using combinations of arc in atmospheric conditions with high precision laser heat sources forAM has not yet been widely researched. Properties of the comparable cheap arc-based process, such as melt pool stability and dimensionalaccuracy, can be improved with the addition of a laser source. Within this paper, impacts of adding a laser beam to the WAAMprocess are presented. Differences between having the beam in a leading or a trailing position, relative to the wire and arc, are alsorevealed. Structures generated using the arc-laser-hybrid processes are compared to ones made using only an arc as the heat source. Bothgeometrical and material aspects are studied to determine the influences of laser hybridization, applied techniques including x ray,energy-dispersive X-ray spectroscopy, and high precision 3D scanning. A trailing laser beam is found to best improve topological capabilitiesof WAAM. Having a leading laser beam, on the other hand, is shown to affect cold metal transfer synergy behavior, promotinghigher deposition rates but decreasing topological accuracy.

Purpose

The steadily growing popularity of additive manufacturing (AM) increases the demand for understanding fundamental behaviors of these processes. High-speed imaging (HSI) can be a useful tool to observe these behaviors, but many studies only present qualitative analysis. The purpose of this paper is to propose an algorithm-assisted method as an intermediate to rapidly quantify data from HSI. Here, the method is used to study melt pool surface profile movement in a cold metal transfer-based (CMT-based) AM process, and how it changes when the process is augmented with a laser beam.

Design/methodology/approach

Single-track wide walls are generated in multiple layers using only CMT, CMT with leading and with trailing laser beam while observing the processes using HSI. The studied features are manually traced in multiple HSI frames. Algorithms are then used for sorting measurement points and generating feature curves for easier comparison.

Findings

Using this method, it is found that the fluctuation of the melt surface in the chosen CMT AM process can be reduced by more than 35 per cent with the addition of a laser beam trailing behind the arc. This indicates that arc and laser can be a viable combination for AM.

Originality/value

The suggested quantification method was used successfully for the laser-arc hybrid process and can also be applied for studies of many other AM processes where HSI is implemented. This can help fortify and expand the understanding of many phenomena in AM that were previously too difficult to measure.

Several laser materials processing technologies using metal wire addition have been researched during the last decades. Especially in the field of joining, as well as in the field of Additive Manufacturing (AM), multiple major benefits have been reached, e.g. higher welding speeds and lower heat input. With laser and arc hybrid welding techniques, additional prospects become accessible. These can combine and improve both deep penetration of autogenous laser welding and gap bridging capabilities of traditional arc welding. In the field of AM, wire feed has been a much-appreciated way of supplying additional material. Reasons include clean and easy handling, high utilisation and availability. A high intensity heat source, e.g. a laser beam or an electrical arc, continuously melts a metal wire; the melt being deposited onto a substrate in one or multiple layers to generate a new surface or three dimensional structure. An alternative joining process is Narrow Gap Multi-Layer Welding (NGMLW). This technique utilises the former mentioned AM processes to fill a gap to join sheets together, instead of depositing on an open surface. NGMLW is a capable competitor to the above-mentioned joining processes due to its prospects of being able to join essentially any thickness of sheets, as long as the beam and wire can accurately reach the gap floor and a sufficient number of layers are used.

In this thesis, multiple types of NGMLW, Papers A – D, and hybrid material deposition, Papers E and F, using laser and hybrid heat sources with metal wire addition have been studied. Techniques such as High-Speed Imaging (HSI), 3D and Computed Tomography (CT) scanning have been used to gain greater insight into the workings of these modern manufacturing processes. The multi-layered way of material deposition within a gap to form a welded joint and onto a surface for AM have many similarities, e.g. wire melting behaviour and melt flow.

Paper A introduces the workings of NGMLW, highlighting possible welding imperfections and welded joint morphology. HSI of the process is analysed both qualitatively and quantitatively: qualitative analysis identifying possible causes for said imperfections; quantitative analysis highlighting the potential for using similar and lower frame rate camera footage for closed loop control to suppress the formation of such imperfections.

In Paper B, an alternative near-vertical building strategy for NGMLW is presented and compared to its more common horizontal counterpart. This upright strategy is found to be fully capable of producing sound welded joints, sporting less than 0.3% cavities. The near-vertical welded joints also have potential for unique material properties due to their much different thermal history.

Papers C and D return to the topic of horizontal NGMLW, but with resistance heating of the metal wire for easier processing, also referred to as Laser Hot-Wire Welding (LHWW). Process behaviour and the resulting morphology of welded joints are the main topics of Paper C. Theoretical reasoning for the formation of occasional centre-line cracks, relating to the shape of the melt pool during solidification, are presented. Arcing is observed in some of the experiments, although prior theory indicates that the applied wire voltage was too low for arcing to occur. This arcing phenomenon is further covered in Paper D, where HSI observations are used to correlate process parameters to arcing probability and a theoretical explanation of why arcing can occur is suggested.

Papers E and F take the step out of the gap, studying the impact of laser beam augmentation in different orientations on Wire-Arc Additive Manufacturing (WAAM). Paper E focuses on a method of quantifying melt pool movement. Fluctuations of the melt pool surface decreased by more than 35% with the introduction of a laser beam to the process. Paper F analyses the generated structures, evaluating the usable portion of the “as deposited” shapes and material composition. Surface irregularities decreased by more than 50% on application of a trailing laser beam. Additional aspects relating to the resulting morphology are also presented, including observations and reasoning for surface irregularities and sloping.

The knowledge gained and methods used in the presented work intertwine to form a strong insight into both laser and laser-hybrid materials processing with wire addition. They also introduce approaches for processing and quantifying HSI footage for process evaluation and improvement.

In order to lead to a competitive advantage, there is the need to carefully consider the pros and cons of state-of-the-art manufacturing techniques. This is frequently carried out in a competitive manner, but can also be done in a complementary way. This complementary approach is often used for the processing of difficult-to-machine materials with particular regard to high-tech parts or components. Hybrid machining processes or, more general, advanced machining processes can be brought to the point that the results would not be possible with the individual constituent processes in isolation [Hybrid Machining Processes Perspectives on Machining and Finishing (Springer International Publishing AG, 2016)]. Hence, the controlled interaction of process mechanisms and/or energy sources is frequently applied for a significant increase of the process performance [Advanced Machining Processes of Metallic Materials: Theory, Modelling, and Applications, 2nd ed. (2016)] and will be addressed within the present paper. A via electron beam melting manufactured gamma titanium aluminide nozzle is extended and adapted. This is done via hybrid laser metal deposition. The presented approach considers critical impacts like processing temperatures, temperature gradients, and solidification conditions with particular regard to crucial material properties like the phenomena of lamellar interface cracking [Laser-Based Manufacturing of Components using Materials with High Cracking Susceptibility (Laser Institute of America–LIA), pp. 586–592; Ti-2015: The 13th World Conference on Titanium, Symposium 5]. Furthermore, selected destructive and non-destructive testing is performed in order to prove the material properties. Finally, the results will be evaluated. This will also be done in the perspective of other applications.

In order to lead to a competitive advantage, there is the need to carefully consider the pros and cons of state-of-the-art manufacturing techniques. This is frequently carried out in a competitive manner, but can also be done in a complementary way. This complementary approach is often used for the processing of difficult-to-machine materials with particular regard to high-tech parts or components. Hybrid machining processes or, more general, advanced machining processes can be brought to the point that the results would not be possible with the individual constituent processes in isolation [Hybrid Machining Processes Perspectives on Machining and Finishing (Springer International Publishing AG, 2016)]. Hence, the controlled interaction of process mechanisms and/or energy sources is frequently applied for a significant increase of the process performance [Advanced Machining Processes of Metallic Materials: Theory, Modelling, and Applications, 2nd ed. (2016)] and will be addressed within the present paper. A via electron beam melting manufactured gamma titanium aluminide nozzle is extended and adapted. This is done via hybrid laser metal deposition. The presented approach considers critical impacts like processing temperatures, temperature gradients, and solidification conditions with particular regard to crucial material properties like the phenomena of lamellar interface cracking [Laser-Based Manufacturing of Components using Materials with High Cracking Susceptibility (Laser Institute of America–LIA), pp. 586–592; Ti-2015: The 13th World Conference on Titanium, Symposium 5]. Furthermore, selected destructive and non-destructive testing is performed in order to prove the material properties. Finally, the results will be evaluated. This will also be done in the perspective of other applications.

Many heavy industrial applications, e.g. shipbuilding and offshore, rely on thick-section, high-quality welds. Unfortunately, traditional arc-based techniques are often found wanting due to a limited penetration depth and excessive heat-affected zone. The former is typically solved by having a wide groove filled by multiple weld passes, which is both costly and time consuming. Other processes such as autonomous laser or electron beams can join thick materials, but have disadvantages such as increased hardness and solidification cracks inside the welds. A promising in-between technique to join thick sheets is narrow gap multi layer laser welding (NGMLW), using less filler material while also offering more control of weld properties. This technique is often used with laser scanning optics and cold wire, or a defocused laser and electrically heated wire. This paper investigates the limitations of the latter during NGMLW, mainly using high-speed imaging to directly observe and explain process behavior. Increased deposition rates are wanted, but heating also consequently needs to be increased for proper bead fusion. Arc occurrences are found to be the cause of instabilities. They are observed occasionally even at low voltages, but more frequently at higher outputs, and then are also more disruptive to the process.