Residual stress reduction of LPBF-processed CM247LC samples via multi laser beam strategies

Abstract

Based on SLM parameters from previous works, which guarantee fully dense and crack free CM247LC samples, multi laser beam strategies have been pursued to reduce residual stresses or rather distortion during LPBF processing. By using a second post heating and non-melting laser source with a defocused laser beam and lateral offset, cantilever distortion is reduced more than 7.5%, compared to the reference. Based on pre-tests with 9 different offset parameters, the optimum offset has been identified. Also, an upper limit for the laser power of 65 W is identified for the second heat laser beam with a spot diameter of 380 μm, to avoid re-melting and creating new defects. A theoretical “two bar model,” to explain the residual stress behavior and reduction with multi laser beam offset strategy during the LPBF process, is presented. Furthermore, re-melting cracks, defects, and microstructure are analyzed in conjunction with the second defocused offset laser, in case of a 200 W laser power, an increased scan speed of 1300 mms/s, and a reduced hatch distance. Secondary electron signal (SE) images of re-melting cracks are analyzed and compared to SE-image of hot cracks (solidification cracks). Based on electron backscatter diffraction (EBSD), the results of the microstructure from the last mentioned multi laser beam approach, which creates re-melting cracks, are presented and analyzed.

High power selective laser melting of crack sensitive nickel-base alloy CM247LC, including dimensional analysis and modelling

“…To proof this theory, the influence of further increased laser beam diameters with the last-mentioned laser power and scan velocity have to be investigated. The used layer thickness and hatch distance can be taken from our previous studies, [22] and [23].…”

“…Based on dense and crack-free reference samples in conjunction with the corresponding process parameters at 200 W with a beam diameter of 90 µm, as explained in the research of [22] and summarized in the study of [23], the bi-directional scan strategy is applied, not only to reduce the process time further, but also for the comparison with the high power parameter samples, manufactured with 500 and 600 W. The same peak intensity and the same track energy density, as known from the crack-free reference samples, are used. By simply changing the laser beam diameter in the build plane, higher laser powers and scan velocities should be achieved with nearly equal results regarding density and crack density.…”

“…Based on reference parameters with 200 W, which results in dense and crack free CM247LC samples, high power and high scan velocity parameters are re-scaled and optimized to decrease production time further with similar results regarding Archimedean density and crack density, as obtained from the reference samples. The reference process and laser parameters for dense and crack-free CM247LC are described in detail in the research of [22] and summarized in the study of [23]. To increase the productivity further, all samples are manufactured with a bidirectional scan strategy.…”

On the hot isostatic pressing of Inconel 625 structures built using laser powder bed fusion at higher layer thickness

“…Laser powder bed fusion (LPBF) is one of the most commonly used additive manufacturing processes in industries to build complex-shaped engineering components due to the shape design freedom offered by the technology 14,15 . It starts with a 3D model sliced into several layers (2D sections) and material addition is carried out in a layer-by-layer sequence by selective fusion of the preplaced material using high power lasers 15,16 . LPBF provides the additional advantages of reduced tooling, geometric freedom, and mass customization over conventional manufacturing processes (i.e., machining or forming) 17,18 .…”

A digital twin of synchronized circular laser array for powder bed fusion additive manufacturing

“…The numerical simulation of four laser arrays showed the dependency of the anisotropy of residual stress on the scanning strategy [16]. Moreover, the part distortion was reduced by 7.5% using a dual laser array with a lateral offset; the second beam is dedicated to the post-heating [17].…”