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Abstract
Laser cladding is a well-established technique, with the majority of prior numerical modelling work focused on delivery and melt pool behaviour of powder-based processes. This research presents new investigations into optimised laser beam shaping for the unique characteristics of wire-based processes, where direct substrate heating, as well as heat transfer between the wire and substrate, is important. The value of this subject is the improved deposition rates and dense metallic structures that can be achieved by wire-based deposition processes compared to powder-based material delivery. The within-wire temperature distribution (AISI 316 stainless steel), the heat transfer and direct heating of the substrate (mild steel) are modelled via heat transfer simulations, with three laser beam irradiance distributions. This analysis identified the removal of localised high-temperature regions typically associated to standard Gaussian distributions, and the improved substrate heating that a uniform square beam profile can provide. Experiments using pre-placed wire and a 1.2 kW CO2 laser were analysed using cross-sectional optical microscopy to provide model validation and evidence of improved wire-substrate wetting, while maintaining favourable austenitic metallurgy in the clad material. A key finding of this work is a reduction, from 480 to 190 W/mm2, in the required irradiance for effective melt pool formation when changing from a Gaussian distribution to a uniform square distribution. This also provided a 50% reduction in total energy. The potential improvements to energy efficiency, cost reductions and sustainability improvements are recognised and discussed.
Highlights
A widely utilised technique for depositing a metallic clad material onto a metallic substrate is Laser Directed Energy Deposition (DED), known as Laser Cladding or Laser Deposition, for such purposes as surface protection [1], surface repair [2] and additive manufacturing [3].While laser cladding processes date back to the 1970s, there have been many recent developments in the simulation1.1 Laser thermal modellingResearch into the simulation of laser surface treatment began with welding, with Rosenthal’s models in the 1930s [7]
Other welding simulations were created by Limmaneevichitr and Kou [9], Int J Adv Manuf Technol (2021) 114:3021–3036 who made a study of Marangoni convection with a defocussed CO2 laser beam, showing this as responsible for the vast majority of melt pool thermal transport
The literature review of DED modelling has identified two areas of opportunity for the development of wire-based deposition, the control of heat input, and the unique heat transfer situation in terms of geometry and heat flux before wire melting. This paper investigates these phenomena through the following research question: To what extent does the control of heat input geometry and substrate heating affect the temperature distribution in the solid phase of wire-fed DED?
Summary
Research into the simulation of laser surface treatment began with welding, with Rosenthal’s models in the 1930s [7] This model was computationally relatively simple, and did not account for metal flow or phase changes [8]. Kim and Peng [11] carried out additional studies of melt pool shape and dilution This analysis showed that it was necessary for laser parameters to adapt to account for increasing temperature throughout the process. Cho et al [14] used ABAQUS to create a 3D heat transfer model, investigating latent heat (heat absorbed during a solid-liquid phase change) which usually neglected for simplicity This inclusion both lowered the peak predicted temperature and shrank the melt pool, less strongly for a stationary beam than for a moving one. This was focused on melt pool dynamics and Marangoni flow, not on pre-melt heating
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