A 316 austenitic stainless-steel alloy, with modified alloy composition, manufactured by laser powder bed fusion (L-PBF) has been investigated. The modification of the alloy composition included addition of niobium (Nb), tungsten (W) and copper (Cu), together with a reduction in the amount of molybdenum (Mo) and an increased amount of carbon (C). To find suitable process parameters, a parameter study by varying laser power, hatch distance and scan speed was performed, centered on typical parameters used for normal 316 L. As-built material from a selected parameter configuration was then subjected to different stress relief annealing heat treatments and ageing heat treatments. The effectiveness of the stress annealing was ranked using a deformation-based method. Microstructural characterization, hardness and room temperature tensile testing were done to evaluate the effect of stress relief and aging heat treatments.

It was found that a higher volumetric energy was needed to build dense material, about ∼50 % higher compared to the volumetric energy input for normal 316 L. A subsequent aging heat treatment at 725 °C for 3 h increased the strength and hardness of the material. A reinforcement of the cellular microstructure by precipitation of carbides in between the cells is believed to be the main reason for this. To completely alleviate the residual stresses it was necessary to carry out a stress relief annealing process at 950 °C, which resulted in a removal of the cellular structure and a lower strength material.

Metal based additive manufacturing techniques such as laser powder bed fusion (LPBF) can produce parts with complex designs as compared to traditional manufacturing. The quality is affected by defects such as porosity or lack of fusion that can be reduced by online control of manufacturing parameters. The conventional way of testing is time consuming and does not allow the process parameters to be linked to the mechanical properties. In this paper, ultrasound data along with supervised learning is used to estimate the manufacturing parameters of 316L steel cubes. Nine cubes with varying manufacturing parameters (speed, hatch distance and power) are examined with ultrasound using focused transducers. The volumetric energy density (VED) is calculated from the process parameters for each cube. The ultrasound scans are performed in a dense grid in the built and transverse direction. The ultrasound data is used in partial least square regression algorithm by labelling the data with speed, hatch distance and power and then by labelling the same data with the VED. These models are computed for both measurement directions and as the samples are anisotropic, we see different behaviours of estimation in each direction. The model is then validated with an unknown set from the same 9 cubes. The manufacturing parameters are estimated and validated with a good accuracy making way for online process control.

The alloy 21-6-9 is a nitrogen-strengthened austenitic stainless steel often used in aerospace applications due to its high strength, good fabrication properties, and toughness at cryogenic temperatures. However, minimal research has been conducted on alloy 21-6-9 using the additive manufacturing process laser powder-bed fusion (L-PBF). The L-PBF technique has been seen as a key to reducing production time and avoiding costly machining. Therefore, there is an interest in investigating L-PBF-processed 21-6-9 to determine the effects of L-PBF on properties at elevated and cryogenic temperatures. In this study, prior to tensile testing the alloy 21-6-9 underwent heat treatments that simulated aerospace applications and the alloy was analyzed and characterized to evaluate phase stability. The effects of elevated and cryogenic temperatures (77K) on the tensile behavior and microstructure were investigated using X-ray diffraction (XRD) and electron backscatter diffraction (EBSD). The tensile tests showed that the yield strength and ultimate tensile strength improved, while ductility varied depending on the conditions and test environment. The ultimate tensile strength was approximately 80% higher at 77K than at room temperature, although the elongation decreased by around 90%, possibly due to the formation of strain-induced martensite.

The laser powder bed fusion (L-PBF) process is an additive manufacturing (AM) process of building parts that uses the high power of the laser to melt the fine powder bed and form a structure, as shown in figure 1 (a). As a result, L-PBF is a promising technique that has likely demonstrated great interest in producing a complex part with near-net-shape design in the area of high-performance applications [1-4]. However, the defects formed during the manufacturing process affect the mechanical properties of a component, as seen in Figure 1 (b). Therefore, track remelting is required to avoid defects and thus low process efficiency [4], as shown in Figure 1 (c). In this study, five different hatch distances of 20 µm, 50 µm, 80 µm, 110 µm, and 140 µm of 316 L stainless steel were studied. To understand the effect of different hatch distances on microstructure, including crystallographic orientation and hardness, EBSD and nanoindentation hardness techniques are used. In addition, the porosity formed is calculated and distinguished (different defects, such as lack of fusion, gas pores, and keyhole defects) using image analysis software MIPAR.(a)(b)(c) Figure 1:(a) Arrangement of the building of tracks and layers during the L-PBF process,(b) illustration of cavity formation, and (c) various times of remelting while building a new build track [4].

Laser powder bed fusion (L-PBF) is an innovative manufacturing technique that produces complex and high accuracy components by melting of powder layer-by-layer [1]. However, defects formed during the manufacturing process affect the mechanical properties of a component [2]. To avoid the defects, remelting of tracks is required and thus low process efficiency [3]. In Figure 1 (a) case of cavity formation in which Ac is, the area of the cavity and Av is remelting vertically area of cross-section, and AT is newly generated area of cross-section. (b) The cross-section of melt track is calculated from modeling in which different shades of yellow show the number of remelting during the building of the new track. In this study five samples of 316L stainless steel with hatch distance 20 μm, 50 μm, 80 μm, 110 μm, and 140 μm are studied. EBSD and nanoindentation hardness are used to understand the correlation between hatch distance and microstructure including crystallographic orientation and hardness. MIPAR image analysis software is used to calculate porosity and distinguish different types of defects such as lack of fusion, gas pores, and keyhole defects.

Laser powder bed fusion (L-PBF) has attracted great interest in the aerospace and medical sectors because it can produce complex and lightweight parts with high accuracy. Austenitic stainless steel alloy 316 L is widely used in many applications due to its good mechanical properties and high corrosion resistance over a wide temperature range. In this study, L-PBF-processed 316 L was investigated for its suitability in aerospace applications at cryogenic service temperatures and the behavior at cryogenic temperature was compared with room temperature to understand the properties and microstructural changes within this temperature range. Tensile tests were performed at room temperature and at −196 °C to study the mechanical performance and phase changes. The microstructure and fracture surfaces were characterized using scanning electron microscopy, and the phases were analyzed by X-ray diffraction. The results showed a significant increase in the strength of 316 L at −196 °C, while its ductility remained at an acceptable level. The results indicated the formation of ε and α martensite during cryogenic testing, which explained the increase in strength. Nanoindentation revealed different hardness values, indicating the different mechanical properties of austenite (γ), strained austenite, body-centered cubic martensite (α), and hexagonal close-packed martensite (ε) formed during the tensile tests due to mechanical deformation.

Selective Laser Melting technique, SLM, requires remelting of adjacent tracks to avoid cavities and other imperfections. Usually, very conservative process parameters are chosen to avoid imperfections, resulting in a low building rate. The process efficiency relates the energy required for the generation of a new track to the laser beam power. For SLM this efficiency is determined by the process parameters, specifically hatch distance, layer depth and scanning speed, independent of the resulting process mechanisms. For SLM the process efficiency often very low, typically 2‑20%. Apart from beam reflection losses of normally 50-60%, significant energy losses result from the remelting of surrounding layers. Some areas can even experience multiple remelting cycles. Further losses originate inevitably from substrate heating. A simplified mathematical model of the track cross section and the corresponding layer overlap geometry has been developed, to analyze the different loss contributions from remelting with respect to the process parameters. The model explains why increasing the hatch distance or the layer depth proportionally increases the process efficiency. However, these increases are limited by cavity formation. The cross section of the overlapping tracks generated by SLM can be regarded as an experimental fingerprint linked to the process conditions. The track cross section geometries can significantly fluctuate, in terms of area and coordinate position. The fluctuations require additional reduction of the hatch distance or layer depth, to ensure robust, cavity-free processing. Examples are presented for stainless steel where a 180 W laser beam has led to a process efficiency of 5-11%, proportional to a hatch distance that was increased from 50 to 110 µm, for 40 µm powder layer depth, at a speed of 50 m/min.