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Additive manufacturing processes offer extensive advantages for the design freedom of structures through layer-by-layer production. This enables high weight savings as well as the integration of functions such as cooling channels. This technology thus offers great potential to contribute to a sustainable future. The pioneer among these manufacturing processes is the powder bed fusion of metals with laser beams (PBF-LB/M). This process is characterised by high laser scanning speeds and highly localised heat input, which have a strong effect on the microstructure and thus also on the mechanical properties. For example, the austenitic steel 316L exhibits a cellular structure at the subgrain level. This microstructure feature leads to higher yield strengths and comparable ductility to conventionally processed 316L. In addition to the traditional applications of 316L steel in the petrochemical and nuclear industries, this enables new applications such as medical stents or bipolar plates for fuel cells with proton exchange membranes. However, the layer-by-layer production with high scanning speeds and localised heat input induces cooling rates in the order of 106 K.s-1. The large temperature gradients and the shrinkage restraints of each weld bead and layer lead to the development of complex residual stress fields. These reduce the material performance and can even lead to premature failure. Thus, the fatigue properties are severely affected by rapid crack growth or prematurely developing cracks. Furthermore, specimens may warp during PBF-LB/M or immediately when the components are separated from the build plate. Therefore, residual stress is one of the main disadvantages of PBF-LB/M, making it difficult for this technology to be more widely accepted in the industry. Based on the current state of the literature, the procedure for determining residual stress employing diffraction methods, the influence of the component geometry, as well as the inter-layertime (ILT) on residual stress and, lastly, suitable heat treatment strategies for relaxing residual stress in PBF-LB/M/316L, were identified as insufficiently researched areas.
Determining residual stress is a major challenge. X-ray and neutron diffraction are particularly suitable for filigree structures, which can preferably be produced using PBF-LB/M. Here, the microscopic strain of the lattice planes is used to calculate the macroscopic residual stress. These methods are nondestructive and allow the spatial resolution of the bi-axial and tri-axial residual stress. In the present work, in-situ neutron diffraction tensile tests were performed to analyse the micromechanical behaviour of PBF-LB/M/316L. The suitability of the lattice planes for calculating the macroscopic residual stress was investigated. The (311) lattice plane was found to be the best option for determining the macroscopic residual stress in PBF-LB/M/316L. Furthermore, it was shown that the Kröner model can be used to calculate the X-ray diffraction constants despite the texture. Currently, both aspects are common practices in the determination of residual stress. The results presented here support the validity of this approach and increase the confidence in the experimentally determined residual stress, which has a positive effect on the assessment of quality concerning the safety of a component manufactured by PBF-LB/M.
The geometry of a structure manufactured by PBF-LB/M determines the component stiffness and influences the thermal gradients during manufacture and ultimately the residual stress. The effect of smaller or larger dimensions (larger than 10 mm) on the residual stress is rarely considered. To investigate this aspect, representative test specimens with different thicknesses and lengths were produced. Hence, the influence of the geometry i.e., component stiffness on the residual stress was evaluated. The residual stress was determined using X-ray and neutron diffraction. The analysis of the residual stress showed that an increase in thickness leads to overall higher residual stress. In addition, it was shown that increasing the sample dimension leads to smaller residual stress gradients. Above a threshold value of a few millimetres, no significant change in the residual stress was observed.
The ILT is inherent in every PBF-LB/M construction job and influences the thermal gradients during production and thus the residual stress. A change in wall thickness in a geometrically complex structure or a variation in the number of specimens in the construction process leads directly to a change in the ILT. To simulate this, specimens with different ILT were produced. The residual stress was determined by X-ray and neutron diffraction. The use of a short ILT resulted in higher surface residual stress, but lower volume residual stress. Here, the surface residual stress and the residual stress in the volume showed contrary behaviour. This was attributed to the complex heat conduction during the process, as shown by the thermographic measurements.
To avoid distortion of the specimens or real components upon separation from the build plate or during post-processing steps, stress relief annealing is usually performed after the PBF-LB/M process. Based on standards for heat treatment of welded austenitic steels, heat treatments were performed at low (450 °C for four hours) and high (800 °C and 900 °C for one hour) temperatures. The results show that the heat treatment at 450 °C relaxed the residual stress by only 5 %. This low relaxation is due to the stability of the cell structures. The high-temperature heat treatment showed that 900 °C is required to dissolve the cell structure and achieve a relaxation of about 85 %. This result is in good agreement with the standards for stress relief annealing of welded austenitic steels.
The interest to additively manufacture Nickel-based superalloys has substantially grown within the past decade both academically and industrially. More specifically, additive manufacturing processes such as laser powder bed fusion (LPBF) offer the ability to produce dense parts within a single manufacturing step. In fact, the exceptional freedom in design associated with the layer-based nature of the processes is of particular interest for the complex shapes typically required in turbine applications. In certain cases, the overall part performance can be achieved by tailoring the microstructure and the crystallographic texture to the specific application. However, these advantages must be paid at a price: the large local temperature gradients associated with the rapid melting and solidification produce parts that inherently contain large residual stress in the as-manufactured state. In addition, the presence of pores in the final part may further affect the in-service part failure. As among Nickel-based alloys Inconel 718 exhibits excellent weldability, this alloy has been widely studied in open research in the domain of LPBF. However, significant microsegregation of the heavier alloying elements such as Niobium and Molybdenum accompanied by dislocation entanglements may preclude the application of conventional heat treatment schedules. Therefore, different post processing heat treatments are required for laser powder bed fused Inconel 718 as compared to conventional variants of the same alloy.
In this study, we investigated two different heat treatment routes for LPBF Inconel 718. In a first routine, the samples were stress relieved and subsequently subjected to hot isostatic pressing (HIP) followed by a solution heat treatment and a two-step age (referred to as FHT). In a second routine, the samples were subjected to a single-step direct age post stress relieving heat treatment (referred to DA). We investigated the consequences of such heat treatment schedules on the microstructure, texture, and mechanical behavior. We show that by applying a DA heat treatment the typical columnar microstructure possessing a crystallographic texture is retained, while an equiaxed untextured microstructure prevails in case of an FHT heat treatment. We further evaluate how these heat treatments affect the mechanical behaviour on the macroscopic and microscopic scale.
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.
Directed energy deposition additive manufactured parts have steep stress gradients and an anisotropic microstructure caused by the rapid thermo-cycles and the layer-upon-layer manufacturing, hence heat treatment can be used to reduce the residual stresses and to restore the microstructure. The numerical simulation is a suitable tool to determine the parameters of the heat treatment process and to reduce the necessary application efforts. The heat treatment simulation calculates the distortion and residual stresses during the process. Validation experiments are necessary to verify the simulation results. This paper presents a 3D coupled thermo-mechanical model of the heat treatment of additive components. A distortion-based validation is conducted to verify the simulation results, using a C-ring shaped specimen geometry. Therefore, the C-ring samples were 3D scanned using a structured light 3D scanner to compare the distortion of the samples with different post-processing histories.
Properties of pipe steels for CCS (carbon capture and storage) technology require resistance against the corrosive environment of a potential CCS-site (heat, pressure, salinity of the aquifer, CO2-partial pressure). The influence of austenitzing in heat treatment routines of two different injection pipe Steels (1.4034, X46Cr13 and 1.4021, X20Cr13) was evaluated. Steel coupons were austenitized at different temperatures (900- 1050 °C) for different lengths of time (30-90 min) before quenching and annealing prior to long term corrosion experiments (60°C, 100 bar, artificial brine close to a CCS-site in the Northern German Basin, Germany). In general, fewer pits are found on X46Cr13. Comparing steels with 13% chromium each the higher carbon content of X46Cr13 (0.46% C) results in a lower number of pits compared to X20Cr13 (0.20% C). It is found that neither the carbon content of the steels nor austenitizing temperature has much influence, but local corrosion behaviour is most susceptible towards austenitzing time.