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The combination of laser powder bed fusion (LPBF), known for its geometrical freedom and accuracy, and the nozzle-based laser metal deposition process (LMD), known for its high build-up rates, has great potential to reduce the additive manufacturing times for large metallic parts. For the industrial application of the LPBF-LMD hybrid process chain, it is necessary to investigate the infuence of the LMD process on the LPBF substrate. In addition, the build plate material also has a signifcant impact on the occurrence of distortion along the additive manufacturing process chain. In the literature, steel build plates are often used in laser-based additive manufacturing processes of Inconel 718, since a good metallurgical Bonding can be assured whilst reducing costs in the production and restoration of the build plates. This paper examines the distortion caused by LMD material deposition and the infuence of the build plate material along the hybrid additive manufacturing process chain. Twin cantilevers are manufactured by LPBF and an additional layer is subsequently deposited with LMD. The distortion is measured in the as-built condition as well as after heat treatment. The efect of diferent LMD hatch strategies on the distortion is determined. The experiments are conducted using the nickel-base alloy Inconel 718. The results show a signifcant infuence of LMD path strategies on distortion, with shorter tool paths leading to less distortion. The remaining distortion after heat treatment is considerably dependent on the material of the build plate.
This study investigates the room‐ and high‐temperature (650 °C) tensile and low‐cycle‐fatigue behavior of Inconel 718 produced by laser powder bed fusion (PBF‐LB/M) with a four‐step heat treatment and compares the results to the conventional wrought material. The microstructure after heat treatment is characterized on different length scales. Compared to the wrought variant, the elastic and yield properties are comparable at both test temperatures while tensile strength, ductility, and strain hardening capacity are lower. The fatigue life of the PBF‐LB/M variant at room temperature is slightly lower than that of the wrought material, while at 650 °C, it is vice versa. The cyclic stress response for both material variants is characterized by cyclic softening, which is more pronounced at the higher test temperature. High strain amplitudes (≥0.7%) at room temperature and especially a high testing temperature result in the formation of multiple secondary cracks at the transitions of regions comprising predominantly elongated grain morphology and columns of stacked grains with ripple patterns in the PBF‐LB/M material. This observation and pronounced crack branching and deflection indicate that the cracks are controlled by sharp micromechanical gradients and local crystallite clusters.
Laser welding is an appropriate technique for joining Laser Powder Bed Fusion (L-PBF) parts together and to conventional wrought ones. The potential consists of profiting from synergies between additive and conventional manufacturing methods and overcoming the existing limitations of both. On the one hand, L-PBF is a widely spread metal-based additive manufacturing technique suitable for generating complex parts which can present intrinsic designed cavities, conformal cooling channels, and filigree structures contributing to sustainable manufacturing and efficiency-oriented designs. On the other hand, chamber sizes for producing L-PBF parts are limited, and the process is time-consuming. Thus, its employment is not for every geometry justified. Additionally, they are in most cases individual elements of a larger assembly and need to be joined together to conventionally fabricated parts.
The present research suggests laser welding parameters to adequately bond pairs of wrought and L-PBF processed plates and two L-PBF plates of Inconel 718 in butt position. L-PBF samples are printed in three different build-up orientations. Additionally, the influence of as-built L-PBF roughness qualities and usual pre-weld preparations such as edge milling are examined. The effect of normed pre-weld heat treatments is also contemplated. Identified cracks are analyzed by means of EDS in order to confirm the present phases on the areas of interest. EBSD is also employed to obtain a clear depiction of the crystallographic texture and distribution of the hot cracks.
The quality of the weldment was examined according to existing standards. Substantial differences in seam geometry and microstructure across different edge’s surface qualities and build directions of stress relieved L-PBF parts have not been detected. Nevertheless, even if no other irregularities are present in the seam, variability in liquation cracking susceptibility has been confirmed. This defect is prone to happen when parts made of this nickel-based superalloy are welded together when not enough precautions are taken. It has been determined that grain size and ductility of the material before welding play a crucial role and mitigating or intensifying these imperfections. Moreover, recommendations are presented to avoid this potential welding defect.
The present research focuses on the weldability of PBF-LB/M lattice structure parts of Inconel 718 by means of laser beam welding. The integration of complex and lightweight AM structures into assemblies employing conventional joining methods is challenging. During the welding process, dissipation of the laser heat input through thin-walled structures of these lightweight structures is strongly limited, resulting in heat accumulation, atypical cooling rates, and affecting weld geometry, quality of the joint and microstructure. PBF-LB/M parts have been manufactured in three characteristic build directions. They were all stress relieved, then a group of them were submitted to an additional solution annealing and the second group to a two-step aging treatment. Afterwards, these parts were welded to identical PBF-LB/M parts and to wrought ones in butt position. To gain insight into the welding temperature field and to explain segregation mechanisms, grain growth and asymmetries, thermocouples have been attached on the surface’s regions and struts nearby. Joining a lattice structure with a full material wrought sample results in clear asymmetric welds due to dissimilar heat dissipation on both sides of the seam. Selected pre-weld heat treatments have a crucial influence on the quality of the seam on the AM part.
High-strength low-alloyed (HSLA) steels with yield strength ≥ 690 MPa have increasing importance in steel construction and civil engineering. However, weld processing of those steels is a major challenge. The susceptibility for degradation of mechanical properties of weld joints significantly increases in presence of hydrogen and can result in hydrogen assisted cracking (HAC). Generally, risk for HAC increases with increasing yield strength of HSLA steels. To minimize the incidence of HAC, it is essential to gain knowledge about both the (1) absorbed hydrogen amount and its distribution in the weld seam and (2) options to lower
the amount of introduced hydrogen. Existing standards recommend heat treatment procedures (interpass temperature or post weld heat treatment) to reduce the diffusible hydrogen concentration in weldments. In this context, different weld seam geometries should be considered. For HSLA steel fabrication weld processing with seam opening angles of 45° to 60° is typical. Modern weld technologies allow welding with seam opening angles of 30° - reduced welding time and costs. In the present study, the hydrogen distribution in multi-layer welds of a 960 MPa HSLA steel was analysed. Influence of different seam opening angles as well as heat input, interpass temperature and post weld heat treatments were investigated. The welded samples were quenched in ice water immediately after welding and subsequently stored in liquid nitrogen. After defined warming up, small specimens were machined from the weld seam by water jet cutting. The diffusible hydrogen concentration was measured by carrier gas hot extraction with coupled mass spectrometer. The results showed, that low heat input and post weld heat treatment procedures can lower hydrogen concentrations in welds. Furthermore, a gradient of the hydrogen concentration was
identified with increasing weld pool depth. By varying the seam opening angles different hydrogen concentrations were
measured.
High-strength low-alloyed (HSLA) steels with yield strength ≥ 690 MPa are gaining popularity in civil engineering and construction of heavy vehicles. With increasing yield strength, the susceptibility for degradation of the mechanical properties in the presence of diffusible hydrogen, i.e., hydrogen-assisted cracking (HAC), generally increases. HAC is a result of the critical interaction between local microstructure, mechanical load, and hydrogen concentration. In existing standards for welding of HSLA-steels, recommendations including working temperatures and dehydrogenation heat treatment (DHT) are given to Limit the amount of introduced hydrogen during welding. These recommendations are based on investigations into conventional arc welding processes. In the past decade, modern weld technologies were developed to enable welding of narrower weld seams with V-grooves of 30°, e.g., the modified spray arc process. In that connection, a reduced number of weld runs and weld volume are important technical and, economic benefits. In the present study, the hydrogen distribution in S960QL multi-layer welds with thickness of 20 mm was analyzed. The influence of different weld seam opening angles, heat input, working temperature and DHT were investigated. The results show that weldments with narrow grooves contained an increased amount of diffusible hydrogen. Hydrogen concentration has been reduced by decreasing both the heat input and working temperature. Hydrogen-free weldments were only achieved via subsequent DHT after welding. Furthermore, hydrogen distribution was experimentally determined across the weld seam thickness in HSLA gas metal arc welded multi-layer welds for the first time.
High-strength low-alloyed (HSLA) steels with yield strength ≥ 690 MPa are gaining popularity in civil engineering and construction of heavy vehicles. With increasing yield strength, the susceptibility for degradation of the mechanical properties in presence of diffusible hydrogen, i.e. hydrogen-assisted cracking (HAC) generally increases. HAC is a result of the critical interaction of local microstructure, mechanical load and hydrogen concentration. In existing standards for welding of HSLA steels, recommendations like working temperatures and dehydrogenation heat treatment (DHT) are given to limit the amount of introduced hydrogen during welding. The recommendations are based on investigations with conventional arc welding processes. In the past decade, modern weld technologies were developed to enable welding of narrower weld seams with V-grooves of 30°, e.g. the modified spray arc process. In that connection, a reduced number of weld runs and weld volume are important technical and, hence, economic benefits. In the present study, the hydrogen distribution in S960QL multi-layer welds with thickness of 20 mm was analyzed. The influence of different weld seam opening angles, heat input, working temperature and DHT was investigated. The results show that weldments with narrow groove contained increased diffusible hydrogen amount. Hydrogen concentration has been reduced by decreasing both the heat input and working temperature. Hydrogen free weldments were only achieved via subsequent DHT after welding. Furthermore, hydrogen distribution was experimentally determined across the weld seam thickness in HSLA GMA welded multi-layer welds for the first time.
High-strength low-alloyed (HSLA) steels with yield strength ≥ 690 MPa have increasing im-portance in steel construction and civil engineering. However, weld processing of those steels is a major challenge. The susceptibility for degradation of mechanical properties of weld joints sig-nificantly increases in presence of hydrogen and can result in hydrogen assisted cracking (HAC). Generally, risk for HAC increases with increasing yield strength of HSLA steels. To min-imize the incidence of HAC, it is essential to gain knowledge about both the (1) absorbed hydro-gen amount and its distribution in the weld seam and (2) options to lower the amount of intro-duced hydrogen. Existing standards recommend heat treatment procedures (interpass tempera-ture or post weld heat treatment) to reduce the diffusible hydrogen concentration in weldments. In this context, different weld seam geometries should be considered. For HSLA steel fabrication weld processing with seam opening angles of 45° to 60° is typical. Modern weld technologies allow welding with seam opening angles of 30° - reduced welding time and costs. In the present study, the hydrogen distribution in multi-layer welds of a 960 MPa HSLA steel was analysed. Influence of different seam opening angles as well as heat input, interpass temperature and post weld heat treatments were investigated. The welded samples were quenched in ice water imme-diately after welding and subsequently stored in liquid nitrogen. After defined warming up, small specimens were machined from the weld seam by water jet cutting. The diffusible hydrogen concentration was measured by carrier gas hot extraction with coupled mass spectrometer. The results showed, that low heat input and post weld heat treatment procedures can lower hydrogen concentrations in welds. Furthermore, a gradient of the hydrogen concentration was identified with increasing weld pool depth. By varying the seam opening angles different hydrogen concen-trations were measured.
Al-Si alloys produced by Laser Powder Bed Fusion (L-PBF) techniques allow the fabrication of lightweight free-shape components that find space in aerospace, automotive, biomedical and military applications. Due to the high cooling rates occurring during the building process, L-PBF AlSi10Mg alloys exhibit an ultra-fine microstructure that leads to superior mechanical properties in the as-built condition compared to conventional cast Al-Si materials. Nevertheless, L-PBF processing induces high thermal gradients, leading to deleterious residual stress levels that must be considered to avoid part distortion and unpredicted failures. In order to relax detrimental residual stress and to increase the ductility, post-processing stress relief treatments are generally performed. In as-built condition the hypoeutectic AlSi10Mg microstructure consist of fine α-Al cells containing uniformly dispersed silicon nanoparticles, which are, in addition, surrounded by a eutectic Si network. Above 260°C the silicon interconnectivity starts to breakdown into spheroidized particles and to coarsen. At the same time, the heating residual stresses are relieved.
The objective of the contribution is to investigate, under different heat treatment conditions, the evolution of microstructure and residual stresses in view of optimizing the fatigue performance of the alloy. To this purpose various heat treatments in a range of temperatures between 265°C and 300°C for a duration between 15 minutes and 2 hours are performed. The microstructure modifications are analysed using a scanning electron microscope and the residual stress state is measured by laboratory X-ray diffraction.
The microstructure has a great influence on short fatigue crack growth in metallic materials. Laser-based Powder Bed Fusion AlSi10Mg alloys exhibit in the as-built condition a fine fibrous Si structure and a supersaturated solid solution of Si in the α-Al matrix, which is significantly modified by heat treatments starting already at temperatures under 260 °C. This study focuses on the influence of post-process heat treatments on the microstructural evolution and the resulting fatigue crack growth resistance. As compared to the as-built condition, two heat treatments at 265 °C/1 h and at 300 °C/2 h are found to be beneficial to the fatigue crack growth resistance of the investigated material.
Al-Si alloys produced by Laser Powder Bed Fusion (L-PBF) techniques allow the fabrication of lightweight free-shape components. Due to the high cooling rates occurring during the building process, L-PBF AlSi10Mg alloys exhibit an ultra-fine microstructure that leads to superior mechanical properties in the as-built condition compared to conventional cast Al-Si materials. Nevertheless, L-PBF processing induces high thermal gradients, leading to deleterious residual stress. In order to relax detrimental residual stress and to increase the ductility, post-processing stress relief treatments are performed. The objective of the contribution is to investigate, under different heat treatment condition, the evolution of microstructure and residual stresses in view of optimizing the fatigue performance of the alloy. To this purpose various heat treatments in a range of temperatures between 265°C and 300°C for a duration between 15 minutes and 2 hours are performed.
Directed energy deposition is a metal additive manufacturing process that builds parts by joining material in a layer-by-layer fashion on a substrate. Those parts are exposed to rapid thermo-cycles which cause steep stress gradients and the layer-upon-layer manufacturing fosters an anisotropic microstructure, therefore stress relief heat treatment is necessary. The numerical simulation can be used to find suitable parameters for the heat treatment and to reduce the necessary efforts to perform an effective stress relieving. Suitable validation Experiments are necessary to verify the results of the numerical simulation.
In this paper, a 3D coupled thermo-mechanical model is used to simulate the heat treatment of an additive manufactured component to investigate the application of a C-ring geometry for the distortion-based validation of the numerical simulation.
Therefore, the C-ring samples were 3D scanned using a structured light 3D scanner to quantify the distortion after each process step.