EM - Escola de Minas

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Heat treatment effects on the hardness and wear behavior of laser-welded AISI40 martensitic steel plates.
(2021) Gonçalves, Karina Aparecida Martins Barcelos; Faria, Geraldo Lúcio de; Siqueira, Rafael Humberto Mota de; Oliveira, Tarcísio Reis de; Lima, Milton Sérgio Fernandes de
The use of laser beams for joining materials has grown along with the need to weld more challenging materials such as martensitic steels. In this work, an AISI 420 martensitic steel plate was autogeneously welded using a fiber laser. The process parameters were 2000 W power and weld speed of 16 mm/s with the focus on the plate surface. Before welding, the plates were prepared in two conditions, tempered, or annealed, and tempering was performed at temperatures of 300°C, 400°C, 500°C, and 700°C after welding. The finite element analysis and dilatometric tests allowed to determine the phase transformations of this steel under development by the company Aperam. The observed microstructure varies according to the initial state of the plate and subsequent heat treatments. The hardness of the fusion and heat-affected zones varied according to the samples. The welded sample in the tempered state softened moderately due to the reheating of the laser beam. In the case of the annealed sample, the hardness of the molten zone was the highest obtained. This high hardness was reflected by a high resistance to wear by reciprocal sliding in the annealed case, compared to the other conditions. The fusion zone generally proved to be much more resistant to wear than the base material.
A comparative study of the heat input during laser welding of aeronautical aluminum alloy AA6013-T4.
(2018) Coelho, Bruno Nazário; Lima, Milton Sérgio Fernandes de; Carvalho, Sheila Medeiros de; Costa, Adilson Rodrigues da
The heat input is the amount of energy supplied per unit length of the welded workpiece. In this study, the effect of two different heat inputs in laser beam welding of a high strength aluminum alloy AA6013-T4 was evaluated from macrostructural and microstructural points of view. The experiments were performed using a continuous wave 2 kW Yb-fiber laser with 100 μm spot size on the upper surface of the workpiece. Keeping the heat input at a given level, 13 or 30 J/mm, the laser power was changed from 650 W to 2 kW and the welding speed from 33 to 150 mm/s. In the condition of higher heat input 30 J/mm it was possible to obtain both cutting and welding processes. For 13 J/mm, welding processes were obtained in conduction and keyhole modes. The equiaxed grain fraction changed with changing speed for the same heat input. The laser processing induced a decrease in the hardness of the weld bead of about 25% due to the solubilization of the precipitates. The estimated absorptivities of the laser beam in the liquid aluminum changed largely with experimental conditions, from 4.6% to 10.5%, being the most significant source of error in measuring the real amount of energy absorbed in the process. For the same heat input the macrostructure of the welded surfaces, i.e., humps and dropouts, changed as well. All these facts indicate that the heat input is not a convenient method to parameterize the laser beam welding parameters aiming the same weld features.

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