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Additively manufactured metallic materials have already started to find application in safety-relevant components. However, this has only happened for certain materials and specific applications and loading conditions, since there is still an extensive lack of knowledge as well as of historical data regarding their mechanical behaviour. This contribution aims to address this lack of understanding and historical data concerning the creep behaviour of the austenitic stainless steel 316L manufactured by Laser-Powder-Bed-Fusion (L-PBF) and the low-cycle-fatigue behaviour of the titanium alloy Ti-6Al-4V manufactured by Laser-Metal-Deposition (LMD). Furthermore, it aims to assess their mechanical behaviour against their conventional counterparts. With that in mind, specimens from conventional and additive materials are tested and their mechanical behaviour analysed based on characteristic curves. To understand the damage behaviours the materials are characterized by destructive and non-destructive techniques before and after failure.
Creep and fracture behavior of conventionally and additively manufactured stainless steel 316L
(2020)
A critical task within the frame of establishing process-structure-property-performance relationships in additive manufacturing (AM) of metals is producing reliable and well-documented material behavior’s data and knowledge regarding the structure-property correlation, including the role of defects. After all, it represents the basis for developing more targeted process optimizations and more reliable predictions of performance in the future. Within this context, this contribution aims to close the actual gap of limited historical data and knowledge concerning the creep behavior of the widely used austenitic stainless steel 316L, manufactured by Laser-Powder-Bed-Fusion (L-PBF). To address this objective, specimens from conventional hot-rolled and AM material were tested under application-relevant conditions according to existing standards for conventional material, and microstructurally characterized before and after failure. The test specimens were machined from single blocks from the AM material. The blocks were manufactured using a standard scan and build-up strategy and were subsequently heat-treated. The creep behavior is described and comparatively assessed based on the creep lifetime and selected creep curves and characteristic values. The effect of defects and microstructure on the material’s behavior is analyzed based on destructive and non-destructive evaluations on selected specimens. The AM material shows shorter creep lives, reaches the secondary creep stage much faster and at a lower strain, and features lower creep ductility compared to its conventional counterpart. The creep damage behavior of the AM material is more microstructure than defect controlled and is characterized by the formation and accumulation of single intergranular damage along the whole volume. Critical features identified are the grain morphology and the grain-boundary as well as the dislocation’s density. Micro-computed tomography (µCT) proves to be an alternative to metallography to analyze the creep damage.
Several Additive Manufacturing (AM) processes are based on the deposition of a powder to form a powder-bed layer-by-layer which typically has a low packing density (35-50%) and consequently hinders the ability of sintering ceramic parts to full density.
The layerwise slurry deposition (LSD) is an innovative process for the deposition of layers in AM. In the LSD, a slurry with no or small organic content is repetitively spread as thin layers on each other by means of a doctor blade. During the deposition, the ceramic particles settle to form thin layers with a high packing density (55-60%). When coupled with a printing head or with a laser, the LSD enables novel AM technologies inspired to the 3D printing or selective laser sintering, but taking advantage of having a highly dense powder-bed. This approach has been successfully applied to silicate ceramics, but implementing technical ceramic slurries is more challenging, because the water is drained too quickly from the suspension into the pores of the previous porous layer forming a cast.
In this presentation, the variables involved in the LSD will be analyzed and the latest improvements in the deposition setup will be described. The application of the LSD to technical ceramics has the potential of generating additive manufacturing parts which in the green state are comparable to those produced by slip casting.
The layerwise slurry deposition (LSD) is an innovative process for the deposition of layers in AM. In the LSD, a slurry with no or low organic content is repetitively spread as thin layers on each other by means of a doctor blade. During the deposition, the ceramic particles settle to form thin layers with a high packing density (55-60%). The LSD therefore shares aspects both of tape casting and slip casting. The LSD differentiates from the classical powder-based AM layer deposition, which typically are able to achieve only a low packing density (35-50%), consequently hindering the ability of sintering ceramic parts to full density. The LSD is coupled with the principles of selective laser sintering (SLS) or binder printing, to generate novel processes which take advantage of the possibility of achieving a highly dense powder-bed.
This presentation will describe the most recent developments in the application of the LSD to technical ceramics, in particular Al2O3 and SiC, discussing potential and issues connected to the layer deposition and the selective laser sintering of such layers.
Functional materials for energy conversion are important technology drivers needed for the implementation of low carbon energy. Therefore, researchers commonly focus on improving the intrinsic properties of a functional material. However, for applications, the extrinsic properties are at least as important as the intrinsic ones. Consequently, it is important to investigate and understand the external and internal structure of semi-finished products and especially defect dependent properties. The extrinsic properties may change during application and the life cycle of the material as well as through processing and molding steps.
Our studies show how X-ray tomographic (XCT) investigations can contribute to structure investigations in composites and massive samples using the example of magnetic materials for energy conversion. The components are tested non-destructively in 3D in order to localize and characterize cracks, pores, inclusions as well as other defects and their influence on the functional properties and also “in-time” during the life cycle of the material. Exsitu and in-situ experiments performed with non-destructive XCT are predestinated to follow damaging mechanisms of materials under certain load conditions, atmospheres or liquids, e.g. went through several working cycles of a functional material. By combining microtomography with other methods of magnetic and classical material characterization, unique statements about the structure and the functional properties can be made.
From the applications point of view, sometimes complex, three-dimensional geometries are needed to fully exploit the functional properties of the materials, e.g. to ensure a high surface area for heat exchange. Since many functional materials are brittle and difficult to form, shaping is often a big challenge. In principle, additive manufacturing processes offer the possibility to produce complex, porous components from poorly formable alloys.
If all stages of additive manufacturing are accompanied by X-ray tomographic imaging, the process of finding the optimal parameters for material processing can be significantly accelerated.
Based on the quality control of the initial powder material used and also investigations of the shape and arrangement of defects within the molten structure and their relationship with the melting path scanning strategy, Xray tomography has proven to be an ideal tool for additive manufacturing, even for functional materials. Overall, tomographic methods are important tools for the development of functional materials to application maturity.
High-strength steels offer great potential in weight-optimised modern steel structures. Additive manufacturing processes, such as Wire Arc Additive Manufacturing (WAAM), enable near-net-shape manufacturing of complex structures and more efficient manufacturing, offering significant savings in costs, time, and resources. Suitable filler materials for WAAM are already commercially available. However, the lack of knowledge or technical guidelines regarding welding residual stresses during manufacturing and operation in connection with cold cracking risk limit their industrial application significantly. In a project of BAM and TU Chemnitz, the influences and complex interactions of material, manufacturing process, design and processing steps on residual stress evolution are investigated. By developing process recommendations and a special cold cracking test, economic manufacturing, and stress-appropriate design of high-strength steel WAAM components are main objectives.
The present study focuses on determining the influence of heat control (interpass temperature, heat input, cooling time) and the design aspects of the components on the hardness and residual stresses, which are analysed by X-ray diffraction. Defined reference specimens, i.e., hollow cuboids, are automatically welded with a special WAAM solid wire. The influences of wall length, wall thickness and wall height on the residual stresses are analysed. Geometric properties can be selectively adjusted by wire feed and welding speed but cannot be varied arbitrarily. This was addressed by adapted build-up strategies. The results indicate a significant influence of the heat control and the wall height on the residual stresses. The interpass temperature, wall thickness and wall length are not significant. These analyses allow recommendations for standards and manufacturing guidelines, enabling a safe and economic manufacturing of high-strength steel components.
Rapid cooling rates and steep temperature gradients are characteristic of additively manufactured (AM) parts and important factors for residual stress formation which have implications on structural integrity. This study examined the influence of heat input on the distribution of residual stresses in two prisms produced by laser powder bed fusion (L-PBF) of austenitic stainless steel 316L.
The layers of the prisms were exposed using two distinct helix scanning strategies: one scanned from the centre to the perimeter and the other from the perimeter to the centre. Residual stresses were characterised at one plane perpendicular to the building direction at half of its build height using neutron diffraction. In addition, the defect distribution was analysed via micro X-ray computed tomography (µCT) in a twin specimen.
Both scanning strategies reveal residual stress distributions typical for AM: compressive stresses in the bulk and tensile stresses at the surface. However, temperature gradients and maximum stress levels differ due to the different heat input. Regarding the X-ray µCT results, they show an accumulation of defects at the corners where the laser direction turned through 90°.
The results demonstrate that neutron diffraction and X-ray µCT can be successfully used as non-destructive methods to analyse through-thickness residual stress and defect distribution in AM parts, and in the presented case, illustrate the influence of scanning strategies. This approach contributes to deeper assessment of structural integrity of AM materials and components.
Nickel-iron-alloy modification to enhance additively welded microstructure for subsequent milling
(2021)
The aerospace industry uses nickel-iron-alloys to create moulding tools for composite materials because of the low coefficient of thermal expansion. These tools have a large-sized and complex structure, making them cost-intensive and difficult to manufacture. Therefore, the focus is set on additive manufacturing, which can additionally enable the repair of components in order to eliminate local defects. However, the process usually results in a heterogeneous microstructure and anisotropic mechanical properties. As there is a high demand for a precise and exact fit of the precision moulds and thus the surface quality, the welded components must be subsequently machined. Nickel-iron alloys are difficult to machine and an inhomogeneous microstructure also leads to unstable cutting forces.
Consequently, a refinement and homogenisation of the microstructure morphology is achieved through specific alloy modifications in order to stabilise and improve the subsequent machining process. Studies on the refinement of FeNi 36 based on vacuum arc melting furnaces are used as a starting point. Therefore, titanium and niobium are chosen as modification elements with a maximum 1 % weight percent and are added to nickel-iron base alloy. The elements are alloyed and build-up welded by using plasma-transferred-arc welding. The resulting microstructure morphology of the welded wall structure and the machining properties are then determined. Furthermore, the influence on the coefficient of thermal expansion is investigated in connection with the modification and the welding process itself. It can be shown that even small amounts of niobium have a significant influence on the structural morphology of the welded layers during plasma-transferred-arc welding.
Having been introduced almost two decades ago, Additive Manufacturing (AM) of metals has become industrially viable for a large variety of applications, including aerospace, automotive and medicine. Powder bed techniques such as Selective Laser Melting (SLM) based on layer-by-layer deposition and laser melt enable numerous degrees of freedom for the geometrical design. Developing during the manufacturing process, residual stresses may limit the application of SLM parts by reducing the load bearing capacity as well as induce unwanted distortion depending on the boundary conditions specified in manufacturing.
The residual stress distribution in IN718 elongated prisms produced by SLM was studied non-destructively by means of neutron (bulk) and laboratory X-ray (surface) diffraction. The samples with different scanning strategies, i.e. hatching length, were measured in as-build condition (on a build plate) and after removal from the build plate.
The absolute values of all stress components decreased after removal from the build plate. Together with surface scan utilizing a coordinate-measuring machine (CMM), it is possible to link the stress release to the sample distortion. Obtained results indicated different residual stress states for each of the transversal, longitudinal and normal component depending on the thermal gradient in the respective direction.
Having been introduced almost two decades ago, Additive Manufacturing (AM) of metals has become industrially viable for a large variety of applications, including aerospace, automotive and medicine. Powder bed techniques such as Selective Laser Melting (SLM) based on layer-by-layer deposition and laser melt enable numerous degrees of freedom for the geometrical design. Developing during the manufacturing process, residual stresses may limit the application of SLM parts by reducing the load bearing capacity as well as induce unwanted distortion depending on the boundary conditions specified in manufacturing.
The residual stress distribution in IN718 elongated prisms produced by SLM was studied non-destructively by means of neutron (bulk) and laboratory X-ray (surface) diffraction. The samples with different scanning strategies, i.e. hatching length, were measured in as-build condition (on a build plate) and after removal from the build plate.
The absolute values of all stress components decreased after removal from the build plate. Together with surface scan utilizing a coordinate-measuring machine (CMM), it is possible to link the stress release to the sample distortion. Obtained results indicated different residual stress states for each of the transversal, longitudinal and normal component depending on the thermal gradient in the respective direction.