Measurement and Simulation of Low Carbon Steel Alloy Deposit Temperature in plasma Arc Welding Additive Manufacturing

Additive manufacturing has the potential to produce near-net shape parts directly from weld metal. Prior work has proved that it is possible to directly manufacture components with complex geometric features and with good productivity. However, under high productivity conditions, deposit temperature increases to a level that it is no longer possible to develop appropriate deposit microstructure and therefore mechanical properties. In this study, Plasma Arc welding was used to produce experimental deposits of 1018 low carbon steel under various conditions. An analytical heat flow model was developed to study the influence of interlayer wait time on deposit temperature and therefore grain size and hardness. The results of the model indicated that as wall height increased, the rate of deposit heat removal by conduction to the substrate decreased leading to a higher preheat temperature after a fixed interlayer wait time causing grain size to increase as wall height increased. However, the model results also show that as wall height increased, the deposit surface area from which heat energy is lost via convection and radiation increased. The model also demonstrated that the use of a means of forced convection to rapidly remove heat from the deposit could be an effective way to boost productivity and maintain smaller grain size and therefore higher hardness and strength in the deposit

Microstructural and mechanical properties control during additive manufacturing

The microstructure of Inconel superalloy thin walls has been characterised. The thin walls were built using direct laser deposition (DLD) with variable parameters: a wide range of continuous laser powers with different pulsing types, frequencies, powder feed rates, scanning speeds and strategies. The walls were analysed to assess the role of the DLD parameters on the microstructure, elemental segregation, mechanical properties, discontinuities, cracks and build geometry, in order to identify the sensitivity of the grain and precipitate morphology to the various process parameters. The builds were examined using optical microscopy, scanning electron microscopy, energy dispersive X-ray analysis and electron backscattered diffraction. An analytical thermal camera and thermocouples were used to study the influence of deposit temperature on grain size and hardness. The power mode was used to control the heat input and cooling rates, which resulted in significant modifications in the microstructure and properties of the deposits. In this work, DLD was performed on IN718 and CM247LC Ni- superalloy using continuous wave laser power modes, and different pulsed wave modes to understand the impact on the product using advanced material characterisation techniques. The study showed that increasing the laser power gave rise to a columnar grain structure, and the grains became coarser as the laser power increased. Furthermore, the deposition path appeared to affect the orientation of the dendrites; this change can be attributed to the variation in the heat flux within the melt pool. Power pulsing increased the cooling rate and reduced the average heat input, and most importantly this broke the dendrites and disturbed the segregation process, thereby reducing the formation of Laves phase. The microstructure could be tailored to a specific size using pulsing, which could also reduce Nb segregation and so reduce the time needed for heat treatment after deposition. Furthermore, the pulse duration played a significant role in reducing segregation and eliminating cracking. A significant variation was observed in the grain size distribution and morphology while the porosity volume fraction was limited, and this varied marginally with the process parameters

Measurement and Simulation of Low Carbon Steel Alloy Deposit Temperature in plasma Arc Welding Additive Manufacturing

Abstract Additive manufacturing has the potential to produce near-net shape parts directly from weld metal. Prior work has proved that it is possible to directly manufacture components with complex geometric features and with good productivity. However, under high productivity conditions, deposit temperature increases to a level that it is no longer possible to develop appropriate deposit microstructure and therefore mechanical properties. In this study, Plasma Arc welding was used to produce experimental deposits of 1018 low carbon steel under various conditions. An analytical heat flow model was developed to study the influence of interlayer wait time on deposit temperature and therefore grain size and hardness. The results of the model indicated that as wall height increased, the rate of deposit heat removal by conduction to the substrate decreased leading to a higher preheat temperature after a fixed interlayer wait time causing grain size to increase as wall height increased. However, the model results also show that as wall height increased, the deposit surface area from which heat energy is lost via convection and radiation increased. The model also demonstrated that the use of a means of forced convection to rapidly remove heat from the deposit could be an effective way to boost productivity and maintain smaller grain size and therefore higher hardness and strength in the deposit

Direct laser deposition of crack-free CM247LC thin walls:mechanical properties and microstructural effects of heat treatment

CM247LC is classified as a non-weldable Ni alloy due to the high Ti + Al content, which makes it susceptible to cracking. It is particularly prone to microcracking when processed by direct laser deposition (DLD). In this work, multiple single walls were manufactured by DLD in CM247LC using two different laser modes: continuous wave and pulse wave (PW). The manufactured walls were studied during processing and post-processing, and characterised by scanning electron microscopy, electron backscattered diffraction, thermal analysis and X-ray diffraction. The results indicate that crack-free conditions during the deposition process and the subsequent thermal processing can be achieved using the PW laser mode and that this can also lead to outstanding mechanical properties