Chapter 12 Damage Evolution in 304L Stainless Steel Partial Penetration Laser Welds Sharlotte Kramer, Amanda Jones, John Emery, and Kyle Karlson Abstract Partial penetration laser welds join metal surfaces without additional filler material, providing hermetic seals for a variety of components. The crack-like geometry of a partial penetration weld is a local stress riser that may lead to failure of the component in the weld. Computational modeling of laser welds has shown that the model should include damage evolution to predict the large deformation and failure. We have performed interrupted tensile experiments both to characterize the damage evolution and failure in laser welds and to aid computational modeling of these welds. Several EDM-notched and laser-welded 304L stainless steel tensile coupons were pulled in tension, each one to a different load level, and then sectioned and imaged to show the evolution of damage in the laser weld and in the EDM-notched parent 304L material (having a similar geometry to the partial penetration laser-welded material). SEM imaging of these specimens revealed considerable cracking at the root of the laser welds and some visible micro-cracking in the root of the EDM notch even before peak load was achieved in these specimens. The images also showed deformation-induced damage in the root of the notch and laser weld prior to the appearance of the main crack, though the laser-welded specimens tended to have more extensive damage than the notched material. These experiments show that the local geometry alone is not the cause of the damage, but also microstructure of the laser weld, which requires additional investigation. Keywords Damage evolution • Ductile fracture • Laser weld • Computational modeling • Void nucleation 12.1 Introduction Laser welds are a type of joint formed by the melting of two metals at a seam with a high-powered laser. Oftentimes these welds do not fully penetrate the thickness of the material to prevent damage to nearby heat-sensitive material [1]. These partial penetration laser welds have a crack-like geometry with a local stress riser, leading to failure in the weld of a component under loading. In this study, we consider 304L stainless steel (SS) partial penetration laser welds; Fig.12.1a is a cross-section of a typical butt-type laser weld in this study with approximately 0.75-mm depth in a 1.55-mm thick sheet. 304L SS is a highly ductile metal, leading to questions of whether the deformation damage mechanisms at the root of the laser weld are dominated by plastic deformation, ductile rupture, void nucleation and growth, or growth of pre-existing pores from the welding process. Previous research has considered deformation of laser welds with the unwelded ligament removed to exclude the stress riser in an initially partial penetration laser weld in 304L SS [2, 3] and quantified the variations in weld root porosity [4, 5], but this is the first experimental study to systematically characterize damage evolution in these laser welds with the full partial penetration geometry. A computational study of laser weld deformation, based on limited experimental tensile data, combined the material and geometric effects into a surrogate model to represent the laser weld deformation for a component-level analysis with some success [6]. Due to the pragmatic approach of this research, the details of different damage mechanisms were neglected. A different computational study considered the effect of porosity on deformation in laser welds with the unwelded ligament removed [3]. Recent computational efforts in conjunction with this study, with higher-fidelity modeling of full partial penetration laser weld geometry, have highlighted many of the unknowns of the damage evolution. Computational modeling of tensile loading of the local geometry of the laser weld, based on material model calibration of the tensile tests of the base material, cannot predict the early load-drop in the load-displacement behavior of the laser weld geometry. This implies that damage mechanisms based purely on the ductile rupture of the base 304L are S. Kramer ( ) • A. Jones • K. Karlson Sandia National Laboratories, 1515 Eubank Blvd. SE, Albuquerque, NM, 87123, USA e-mail: slkrame@sandia.gov J. Emery Component Science and Mechanics, Sandia National Laboratories, Albuquerque, NM, 87185, USA © The Society for Experimental Mechanics, Inc. 2018 J. Carroll et al. (eds.), Fracture, Fatigue, Failure and Damage Evolution, Volume 7, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-62831-8_12 85
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