Influence of the laser position in laser-assisted WAAM process on weld bead shape and surface properties

The lateral use of the laser in laser-assisted WAAM processes, resulting in a directional dependence, can influence the bead shape and the bead surface. The influence of the laser position on the weld bead is investigated. Beads with different laser positions are applied and the height and width as well as the waviness of the beads are evaluated. In addition, claddings are welded and the waviness is measured. The waviness along the beads ranges from 8.77 to 34.66 µm, and no significant correlation with the welding direction could be determined. For the bead shape, the differences in height range from 3.54 to 3.90 mm and in width from 8.20 to 8.89 mm. Based on the results, a dependence on the laser position for surface properties and weld bead shape becomes clear

Determination of optimum process parameters for different Ti-6Al-4V powders processed by Laser-based Powder Bed Fusion

The PBF-LB (laser-based powder bed fusion) process is subject to a large number of variables, including the characteristics of the processed powder. Since a powder with a given specification can be supplied by various powder manufacturers, the transferability of optimized parameter settings and statistical processing models is of major interest. This work therefore investigates the processing windows of two Ti-6Al-4V powders supplied by different manufacturers following the Design of Experiments (DoE) approach. The fitted regression models for porosity and roughness demonstrate a significant influence of the powder and its size distribution. Further, the powder type significantly interacts with laser power, scanning speed and hatch spacing. It is shown that an increase of the powder size distribution quantiles by less than 10 µm leads to a shift of optimum settings towards a higher volume energy density by 6.4 J/mm3 as well as to higher roughness on the top and side surfaces

Development of a Laser Double-wire Directed Energy Deposition Process for Functionally Graded Materials and In-situ alloying

Development of a Laser Double-wire Directed Energy Deposition process (LD-DED). Utilizing high material efficiency of the laser wire process for multi-material additive manufacturing. Enabling omnidirectional wire based build up of in-situ fabricated alloys and Functionally Graded Material

Weakening of thick steel plates by laser radiation for the removal of hazardous substances

Hazardous substances such as unexploded ordnance represent a serious threat. It is necessary to develop new methods and equipment for their elimination. For this reason, a laser ablation process is developed where the shell of the objects are weakened in a defined manner to reduce the threat to the environment. This article shows how 25 mm thick steel sheets can be weakened with ablation rates of more than 3000 mm3/min and achieved ablation depths of more than 15 mm by means of an adapted process strategy and an off-axis process gas control. Despite the high incoming process energy, it is ensured that there is no danger to affect the hazardous substances. A critical temperature of 300 °C is not exceeded at the rear surface of the sheet metal

Residual oxygen content and powder recycling: effects on microstructure and mechanical properties of additively manufactured Ti-6Al-4V parts

The laser-based powder bed fusion of metals (PBF-LB/M) offers a variety of advantages over conventional processing techniques and the possibility to recycle and reuse powder increases its sustainability. However, the process and resulting part properties are influenced by a variety of factors including powder recycling grade and residual oxygen content of the process atmosphere. Especially in terms of reactive materials like Ti-6Al-4V, oxidation during processing and recycling determines process stability and reproducibility. This work therefore focusses on the influence of the conventionally varied processing parameters as well as atmosphere residual oxygen content process and powder recycling on the microstructure and mechanical properties. For this purpose, the design of experiments approach is used and by evaluation of regression models, effect sizes and interactions are given. Additionally, two different etching techniques were employed to reveal different aspects of the microstructure. While no significant influence of powder recycling and residual oxygen on the microstructure could be observed, they both significantly influence the mechanical properties. A maximum hardness of 470 HV0.1, a maximum ultimate tensile strength of 1252.3 MPa, and a maximum elongation at break of 17.8 % were obtained. The results demonstrate the importance of the processing atmosphere’s residual oxygen content and of taking into account the changing powder characteristics during recycling as well as its effect on the part properties

Discoloration of AISI 420 stainless steel in dependence of inter layer time during Laser-based Powder Bed Fusion

Parts built by laser-based powder bed fusion (PBF-LB) experience intervals of heating and cooling during the powder deposition and the selective melting of successive layers. Short time intervals of cooling can lead to heat accumulation resulting in discoloration of AISI 420 (X20Cr13) stainless steel parts. Discoloration occurs due to the formation of oxide layers, which negatively affect the corrosion resistance. This process is determined by the time-dependent influence of temperature and oxygen. Therefore, this study investigates effects of varied inter layer time on mechanical PBF-LB part properties and surface characteristics to prevent discoloration. EDX is used to analyze the chemical composition with regard to the chromium content as an indicator of reduced corrosion resistance. The results emphasize the need for implementing an minimum inter layer time greater than 12 s to prevent discoloration during the PBF-LB process with a layer thickness of 20 µm and a volume energy density of 113.3 J/mm

Influence of ultrasound on pore and crack formation in laser beam welding of nickel-base alloy round bars

Welding by laser beam is a method for creating deep and narrow welds with low influence on the surrounding material. Nevertheless, the microstructure and mechanical properties change, and highly alloyed materials are prone to segregation. A new promising approach for minimizing segregation and its effects like hot cracks is introducing ultrasonic excitation into the specimen. The following investigations are about the effects of different ultrasonic amplitudes (2/4/6 µm) and different positions of the weld pool in the resonant vibration distribution (antinode, centered, and node position) for bead on plate welds on 2.4856 nickel alloy round bars (30 mm diameter) with a laser beam power of 6 kW. The weld is evaluated by visual inspection and metallographic cross sections. The experiments reveal specific mechanisms of interaction between melt and different positions regarding to the vibration shape, which influence weld shape, microstructure, segregation, cracks and pores. Welding with ultrasonic excitation in antinode position improves the welding results

Design of additively manufacturable injection molds with conformal cooling

Additive manufacturing enables the production of intricate geometries including internal structures. This design freedom can be used advantageously to enhance heat transfer in injection molds by means of conformal cooling. The main goal is to reduce cycle times and to improve part quality through uniform cooling of the plastic products. This paper presents cooling design concepts for mold inserts. Their underlying approaches differ with respect to the shape and the cross-sectional geometries of cooling channels. Distinct inserts are additively manufactured by laser-based powder bed fusion (PBF-LB) of AISI 420 stainless steel. Experiments are carried out on a custom thermal test bench. Infrared thermography is used to examine the surface temperature, showing a reduction in cooling time by up to 41 % compared to conventional concepts. Additionally, the coolant flow is measured. The evaluation of the cooling characteristics reveal a critical trade-off between cycle time and uniformity of the surface temperature

Deep Learning-Based Weld Contour and Defect Detection from Micrographs of Laser Beam Welded Semi-Finished Products

Laser beam welding is used in many areas of industry and research. There are many strategies and approaches to further improve the weld seam properties in laser beam welding. Metallography is often needed to evaluate welded seams. Typically, the images are examined and evaluated by experts. The evaluation process qualitatively provides the properties of the welds. Particularly in times when artificial intelligence is being used more and more in processes, the quantization of properties that could previously only be determined qualitatively is gaining importance. In this contribution, we propose to use deep learning to perform semantic segmentation of micrographs of complex weld areas to achieve the automatic detection and quantization of weld seam properties. A semantic segmentation dataset is created containing 282 labeled images. The training process is performed with DeepLabv3+. The trained model achieves a value of around 95% for weld contour detection and 76.88% of mean intersection over union (mIoU). © 2022 by the authors. Licensee MDPI, Basel, Switzerland

Advances in powder bed based Additive Manufacturing of metal-glass-hybrid-components

As powder bed based additive manufacturing (AM) is gaining more traction in the research and industry, significant efforts are made to expand the capability of AM in combining different materials. Conventionally, multiple materials, sealants and processes are necessary in order to join metal and glasses in assemblies. The development of a single-step process for vacuum-tight and strong bonding between glass and metal could drastically reduce the part count and assembly cost. This paper investigates the feasibility of joining the iron-nickel-cobalt alloy Kovar with borosilicate glass. The powder bed fusion by laser beam is used to melt Kovar structures directly onto glass substrates. Suitable process parameter for an iron-silicon-oxide material-bonding layer of 3 µm thickness were developed using design of experiments. With an energy input of 333 J/mm3 for the initial layer and 60 J/mm3 for the bulk structure, tensile testing of the metal-glass-connection revealed bonding forces up to 0.5 MPa