Focus shift analysis, to manufacture dense and crack-free SLM-processed CM247LC samples

Nickel-base alloy CM247LC is particularly sensitive for weld-cracking during powder-bed fusion, caused by an inappropriate laser spot diameter due to focus shifting. In this paper, the influence of a 4 mm focus shift on the melt pool geometry, intensity, relative Archimedean density, crack density, microstructure and texture is investigated, by using two different calibration procedures for the laser system. The hot calibration method is a procedure to overcome the thermally induced focus shift of 4 mm by setting the focus plane 4 mm below the build plane. After heating up to the operating temperature of the laser system by scanning two times an area of 10 x 10 mm2, a stable focus diameter in the build plane is guaranteed. This procedure enables fully dense and crack-free CM247LC samples. Due to deeper melt pools, realized by higher intensity and a reduced laser beam diameter in the focus plane, the cracks in the layers below are re-molten and crack healing occurs. Between the cold and hot calibration procedure, the crack-density has been reduced relatively more than 98-times. Therefore, the laser beam diameter has been identified as an essential SLM parameter for healing hot cracks. Samples, which are manufactured with the hot calibrated system, indicate a stronger texture parallel to the build direction, compared to the cold calibrated one. Furthermore, Archimedean density, the lack of bonding and the porosity have been analyzed with varying laser scan speeds for the thermal steady state system. The increased Archimedean density correlates well with the decreased lack of bonding and porosity.