Experimental study on interlayer temperatures in HSLA steel wire arc additive manufacturing aiming for self-tempering

Abstract

Directed Energy Additive Manufacturing (GMA-DED) of hardened, high-strength, low-alloy (HSLA) steel components can produce hard, potentially brittle deposits that require subsequent heat treatment to achieve the desired properties. In this thesis, a thermal management strategy for HSLA thin walls manufactured by GMA-DED was developed and validated, in which progressively increasing interlayer (IT) temperatures were actively controlled to keep the entire thin wall at the same temperature and intensify an in situ heat treatment (self-tempering), preserving the wall geometry and avoiding productivity losses due to excessive waiting times between layers. The basic criterion of analysis was to maintain always the same wall width, keeping the wire feed speed constant and adjusting the deposition speed, even at the expense of variations in the heat source's energy per unit length of the deposited layers. Using the hardened ER90S-B3 consumable, the shielding gas for thin-wall deposition under short-circuit transfer was initially selected by combining operational stability metrics with geometry-based criteria, resulting in the choice of the Ar + 8% CO₂ mixture. This gas provided the selected parameters with the highest operational stability and the most consistent wall quality, and was adopted as a reference condition for investigating the influence of IT on the microstructural and geometric evolution of the wall. The Near-Immersed Active Radiation Heating (NIARH) approach was developed, which allowed control of the IT up to 550 °C during thin-walled deposition while maintaining stable weld pool dynamics and short-circuit transfer. An original pyrometry-based system was designed and evaluated to monitor the IT and the cooling rates of the layers. A graphical methodology was developed to estimate the dilution between layers and to predict the architecture of heat treatments applied sequentially to the deposited layers during multiple thermal cycles. The results showed that the thermal history was mainly governed by the IT, counterbalancing the effects of preheating with those of lower deposition energy per unit of layer length (the cooling rates were independent of the IT, although the permanence time of each layer in the tempering temperature range increased). Porosity was measured using Archimedes' principle and showed no significant effect of the TI. However, the harshness showed clear dependence on IT: it remained relatively high and stable at low ITs and decreased as IT increased. The microstructural observations of the walls indicated the predominant presence of bainite, except in the condition with the highest TI, which showed a tendency toward increased carbide formation. Overall, the results demonstrated the feasibility of promoting in situ self-tempering through high, controlled interlayer temperatures in thin walls produced by GMA-DED, offering a practical way to adjust properties while maintaining the process's geometric integrity and stability.