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

Surface functionalization under water using picosecond and femtosecond laser pulses - first observations and novel effects

Functionalization by changing the topography of workpiece surfaces is interesting for many applications. Lasers - especially ultrashort pulse lasers - are flexible tools for this kind of tasks. Besides deterministic structures they allow for processing surfaces in a way that stochastic structures appear. Ripple and spike structures can be fabricated in air. In liquids, however, there are only a few investigations and many open questions. The idea of this paper is to check how water in contact to the workpiece surface affects femtosecond laser surface processing and what kind of structures appear. Results obtained with a picosecond laser are presented for comparison

Optical coherence tomography for laser transmission joining processes in polymers and semiconductors

Laser transmission welding applications become wide spread technology in joining transparent and turbid materials. The requirement of higher throughputs and lesser rejects support the need of alternative methods to commonly used machine vision or temperature measurements. Optical coherence tomography delivers the opportunity to qualitatively assess transmission welding seems and welding spots even in three dimensions. Fourier Domain Mode Lock Lasers (FDMLs) used in OCT enable high throughput by high sweep rates. Preliminary results of OCT used for qualitatively assessment of laser transmission welded polymers and fiber reinforced plastics are shown as well as first approaches of using OCT in a silicon chip bonding process

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

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

Inhalation exposure to hazardous substances during powder-bed processes

This work resumes first results concerning the identification of workers' inhalation exposure by means of workplace measurements in the field of additive manufacturing. In particular for powder-bed processes, risks caused by mixed exposures due to released particles and relevant chemical constituents or possible volatile compounds have to be evaluated. The workplace measurements are focused on processes in the automotive as well as the aviation and tool construction industry. The aim is to gain exposure data concerning the application of metal-containing powders and alloys as well as polymer powders in powder-bed processes and to derive instructions for good working practice