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Additive manufacturing (AM) processes enable the efficient production of advanced constructions. New developments in topology optimization are leading to weight-optimized designs of increasing complexity. Direct energy deposition processes (DED) such as wire and arc-based additive manufacturing are an important method of additive manufacturing. The wire filler metals enable a wide range of materials, while the arc process provides a high deposition rate compared to laser and powder-based processes. Combined with the use of high-strength steels, the thickness of walls or components can be significantly reduced in the context of lightweight construction, which results in significant savings in energy, costs, time and resources. Suitable high-strength steel filler metals are commercially available for DED-arc AM processes. However, guidelines and quantitative knowledge about welding stresses and cold cracking issues during component production and service are lacking. This limits the industrial application considerably. In a joint project of BAM and Chemnitz University of Technology, the main influences and complex interactions of material, production process, design and processing steps on the residual stress level are investigated. The aim is to develop processing recommendations and a cold cracking test for economical processing and stress-related design of high-strength steels with DED-arc. This study focuses on residual stress analysis by neutron diffraction (ND) and X-ray diffraction (XRD) on defined test specimens. The ND analysis were performed at the Paul Scherrer Institute- Villigen, Switzerland (PSI) and the XRD analysis at BAM. The study shows a quantitative and qualitative comparison of the residual stress magnitudes and distribution between the component bulk (ND) and surface (XRD) analyses. The ND analysis reveals that in DED-arc AM walls the residual stresses dominate in the direction of welding and are negligibly small in each case transverse to the direction of welding. The topology of the analyzed residual stresses shows almost identical residual stress maps compared to XRD. In addition, the residual stresses are significantly influenced by the solid phase transformation of the material due to low cooling times and less post heat treatment cycles of following AM layers in the area of the top layer.
Additive manufacturing (AM) processes enable the efficient production of advanced constructions. New developments in topology optimization are leading to weight-optimized designs of increasing complexity. Direct energy deposition processes (DED) such as wire and arc-based additive manufacturing are an important method of additive manufacturing. The wire filler metals enable a wide range of materials, while the arc process provides a high deposition rate compared to laser and powder-based processes. Combined with the use of high-strength steels, the thickness of walls or components can be significantly reduced in the context of lightweight construction, which results in significant savings in energy, costs, time and resources. Suitable high-strength steel filler metals are commercially available for DED-arc AM processes. However, guidelines and quantitative knowledge about welding stresses and cold cracking issues during component production and service are lacking. This limits the industrial application considerably. In a joint project of BAM and Chemnitz University of Technology, the main influences and complex interactions of material, production process, design and processing steps on the residual stress level are investigated. The aim is to develop processing recommendations and a cold cracking test for economical processing and stress-related design of high-strength steels with DED-arc. This study focuses on residual stress analysis by neutron diffraction (ND) and X-ray diffraction (XRD) on defined test specimens. The ND analysis were performed at the Paul Scherrer Institute- Villigen, Switzerland (PSI) and the XRD analysis at BAM. The study shows a quantitative and qualitative comparison of the residual stress magnitudes and distribution between the component bulk (ND) and surface (XRD) analyses. The ND analysis reveals that in DED-arc AM walls the residual stresses dominate in the direction of welding and are negligibly small in each case transverse to the direction of welding. The topology of the analyzed residual stresses shows almost identical residual stress maps compared to XRD. In addition, the residual stresses are significantly influenced by the solid phase transformation of the material due to low cooling times and less post heat treatment cycles of following AM layers in the area of the top layer.
The damage tolerance approach is widely used in the design and estimation of inspection intervals of safety-relevant metallic components subject to fatigue loading. The approach relies on the knowledge of the fatigue crack propagation characteristics, wherein a relevant role is played by the fatigue crack propagation threshold. Nevertheless, the use of material data determined by testing on conventional specimens is not straightforward in the case of thin-walled components such as turbine blades or additively manufactured parts, in which the local variation of material properties in highly stressed regions must be considered. In these cases, the possibility of investigating the fatigue crack propagation properties on a limited portion of material is crucial. For this purpose, a new test procedure has been developed for small-scale specimens which allows the determination of the intrinsic fatigue crack propagation threshold and the near-threshold regime. The validity and limitations of the method are demonstrated on the high strength steel S960QL, along with a comparison with data determined by testing on conventional geometries.
Three high strength Nb-, Ti- and Ti+V- bearing S690QL steels were welded to investigate and compare the effects of microalloy addition on HAZ toughness. Charpy V notch impact tests from three microalloyed welds under different cooling rates have been performed. Fractographic examination shows that several factors including large-sized grain, upper bainite or hard second phase, interact to determine the brittle fracture mode and impaired toughness of Nb bearing weld at high heat input. In contrast to this reduced toughness, Ti bearing welds exhibits satisfied toughness regardless of at a fast or slow cooling rate as a result of limited austenite grain and refined favourable intragranular acicular ferrite structure. Moreover, in the case of such refined structure as matrix, TiN particles are found to be irrelevant to the fracture process and crystallographic misorientation results also confirms that high angle boundaries between fine ferrites plates provide much effective barrier for crack propagation and contribute to improved toughness.
Effect of cooling rate on microstructure and properties of microalloyed HSLA steel weld metals
(2015)
Two high strength Nb/Ti microalloyed S690QL steels were welded with identical filler material, varying welding parameters to obtain three cooling rates: slow, medium and fast cooling. As cooling rate increased, the predominantly acicular ferrite in Nb weld metal (WM) is substituted by bainite, with a consequence of obvious hardness increase, but in Ti WM, no great variation of acicular ferrite at all cooling rates contributed to little increment of hardness. The transition between bainite and acicular ferrite has been analysed from the point view of inclusions characteristics, chemical composition and cooling rate. Excellent Charpy toughness at 233 K was obtained with acicular ferrite as predominantly microstructure. Even with bainite weld of high hardness, the toughness was nearly enough to fulfill the minimal requirements. WM for Ti steel showed to be markedly less sensitive to the variations of cooling rate than that for Nb steel.