Understanding the Residual Stress in DMLS CoCrMo and SS316L using X-ray diffraction

Abstract

Laser additive manufacturing (LAM) is the next in line revolutionary technology for various turbine applications, as an enabler of new designs and parts and during repair and refurbishment of service returned parts. In LAM process, residual stresses play a key role during component build up as well as while removing the components from the build plate, to ensure distortion free parts, as a standalone or as a hybrid build on an existing part. The present study comprises an experimental determination of residual stresses of two direct metal laser sintered (DMLS) alloys, CoCrMo and SS316L, using the X-ray diffraction sin2 ψ technique. In both cases, the process parameters were optimized to result in dense (> 99.95%) samples. Surface residual stress measurements were carried out on the DMLS samples in the as printed condition and after heat treatment. While SS316L measured a surface tensile stress of 127 MPa, CoCrMo showed a tensile stress of 265 MPa, in the as printed condition. Upon grit blasting, both samples undergo a peening effect and record a compressive stress of 300-400 MPa. Heat treatment of SS316L at 650°C, tends to relieve these stresses and results in zero stress, whereas for CoCrMo there was a systematic variation in the response to heat treatment at different temperatures, correlating with the microstructural evolution. At 1050°C CoCrMo shows compressive stresses because of partial recrystallization and at 1150°C having a completely recrystallized microstructure comprising equiaxed grains of around 40 µm and a completely stress relieved condition.