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The definition of process parameters depending on the geometry of the workpiece is one of the main challenges for Selective Laser Melting (SLM). The possibility to use different emission modes is an essential feature of the contemporary fiber lasers, which still requires further attention for controlling the melt pool size. Most of the commercially available SLM systems operates with continuous wave (CW) emission lasers. As a consequence, a substantial effort has been directed towards obtaining full densification by considering the variation of process parameters in CW modality. Pulsed wave (PW) emission achieved by power modulation of a fiber laser is preferred by a smaller fraction of the industrial SLM systems. The differences between the two emission regimes, advantages and disadvantages in their use have not been fully understood.Accordingly, this work proposes a comparative study between the two emission regimes in SLM, namely PW and CW. For this purpose, a single mode fiber laser is coupled to a prototype SLM system composed of an automated powder-bed and a scanner head. The laser source is extensively characterized for pulsed wave emission characteristics in a power modulated regime. Conditions providing the same energy content over the single track were determined and their effect on single track densification is studied. High speed imaging (HSI) is used to observe the differences in the melt pool formation in situ. The overall results confirm that CW emission provides a larger and more stable molten pool during the process, resulting in higher deposition rates. On the other hand, under stable conditions, PW emission provides relatively narrow tracks, which might be problematic for porosity formation and at the same time useful for the production of fine geometries.

The demand for additive manufacturing (AM) of Cu and its alloys shows an increased trend from the energy and heat transfer related applications. Selective laser melting (SLM) is amongst the key AM processes for metals, providing high geometrical accuracy and design flexibility. The technology is most commonly employed using high brilliance fiber lasers operating at 1 m. However, the elevated reflectivity of Cu at this wavelength, combined with its high thermal conductivity is the cause for a highly unstable process, whereby pore-free products are difficult to obtain. Accordingly, the present work explores the limitations in processing pure Cu powders with a 1 kW single mode fiber laser providing solutions and different strategies for improving part quality. In particular, the power level requirements, as well as build plate material, is assessed through an analytical model. Later on, the process parameters were studied for single and multi-pass melting strategies.The results demonstrate that a correct sequence of multi-pass strategy can improve the part density up to 99.10.2% with an industrially acceptable build rate of 12.6 cm3/h.

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In the industrial panorama, laser powder bed fusion (LPBF) systems enable the near net shaping of metal powders into complex geometries with unique design features. This makes the technology appealing for many industrial applications, which require high performance materials combined with lightweight design, lattice structures and organic forms. However, many of the alloys that would be ideal for the realisation of these functional components are classified as difficult to weld due to their cracking sensitivity. γ-TiAl alloys are currently processed via electron beam melting (EBM) to produce components for energy generation applications. The EBM process provides crack-free processing thanks to the preheating stages between layers, but lacks geometrical precision. The use of LPBF could provide the means for higher precision, and therefore an easier post-processing stage. However, industrial LPBF systems employ resistive heating elements underneath the base plate which do not commonly reach the high temperatures required for the processing of γ-TiAl alloys. Thus, elevated temperature preheating of the build part and control over the cooling rate after the deposition process is concluded are amongst the features which require further investigations. In this work, the design and implementation of a novel inductive high temperature LPBF system to process Ti-48Al-2Cr-2Nb is presented. Specimens were built with preheating at 800°C and the cooling rate at the end of the build was controlled at 5°C min −1 . Crack formation was suppressed and apparent density in excess of 99% was achieved.

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Melt pool monitoring in selective laser melting (SLM) is a key challenge to enhance the understanding of the basic process physics as well as the in-situ identification of drifts. The main issues are related to its fast dynamics and small spatial extent, which require high performance monitoring systems in terms of temporal and spatial resolution, often resulting in unmanageable data rates. Furthermore, the broadband thermal emission from the process can be viewed in different spectral ranges depending on the sensor and optical system employed. Accordingly, a wide range of sensing configurations is possible. The choice of monitoring parameters is crucial to achieve a solution capable of describing process dynamics with industrial applicability in terms of data transport and analysis. This work discusses the design of a coaxial monitoring module to assess melt pool dynamics in SLM using temporal and spatial cues related to the process physics. Restrictions regarding data management were considered to ensure a future inline process control. The system was tested with an external illuminator to reveal the actual melt pool geometry employing an acquisition rate of 1200 fps, 4.3 mm x 4.3 mm field of view and 14 µm/pixel spatial resolution. The process emission in visible (640 nm) and nearinfrared regions (850-1000 nm) was also acquired and the band choice discussed. The proposed solution captured successfully the melting conditions from both a spatial and temporal viewpoint. The monitoring system depicted variations of the melt pool shape when processing different geometries using modulated and continuous wave laser emission. Index Terms-Selective laser melting, process monitoring, thermal emission, infrared imaging, melt pool geometry.