Lasers in Manufacturing Conference 2015 Flow diagnostics produced by selective laser melting of cutting nozzles

S.Ulricha, S.Lorenza S. Jahna, S.Sändiga, B.Fleckb

aGünter-Köhler-Institut für Fügetechnik und Werkstoffprüfung GmbH bErnst-Abbe-Hochschule Jena

Abstract

The increasing spread of laser technology in materials processing leads inter alia increasingly individual solution strategies in order to cope with the growing demands on the process control. The focus of this work is the fluidic analysis of the cutting nozzles, which were usually produced either by selective laser melting or conventional methods. The Schlieren measurement was utilized in order to visualize flows. Through the adjustment of optical components, the Schlieren-Aufnahmegerät 80 was coupled with a high speed camera. Based on these measurement results, the influence of manufacturing technology has been evaluated on the flow behaviour. With the help of cutting tests a direct proof of the achievable quality of the cutting edge has been evaluated. The results from both research methods provide a statement on the quality of the gas stream and the achievable cutting quality of manufacturing technology.

Keywords: laser cutting, selektiv laser melting, nozzle, flow visualization

1. Introduction

Nowadays, the decisive factors for financial success are on the one hand innovative products, and on the other hand the acquisition of knowledge through research and development. In materials processing, the application of lasers in technological fields like cutting and welding, enables shorter lead times. The understanding of the process plays a crucial role for the quality of the component. Regarding cutting, the quality of the cut edge and the dimensional accuracy is affected by many parameters. Among others, the quality of the gas jet has been identified as a crucial factor determining the efficiency of the melt ejection [VDI93]. It can be assumed, that there is a complex interaction between gas and melt flow, so the quality of the formative processes in the kerf is closely related to the gas jet. With increasing complexity of the

* Stefan Ulrich. Tel.: +49-3641-204-116; fax: +49-3641-204-110. E-mail address: [email protected].

components, the selective supply of gas to the knitting point is more difficult. Nevertheless, for the production of high-quality components, the use of alternative designs is necessary. Currently, these nozzles are manufactured elaborately and subject to production technical restrictions. In order to manufacture complex internal geometries selective laser melting is an alternative technique to traditional methods. Using this method, complex components can be built monolithically in a single production step. The current development of cutting nozzles is often dominated by experimentally trials, therefore, the developer relies on experience and conventional nozzle concepts. This heuristic method often leads to solutions, but which have not exploited the full potential of an arbitrary 3D geometry to influence the cutting gas flow. An alternative to this “trial and error” method is to measure or calculate the flow field. The use of simulation programs requires extensive knowledge in the field of fluidic mechanics. Under transient conditions, e.g., pressure shock, and simultaneously complex structures one encounters with the limits of numerical simulation, making it difficult to accurately predict the influence of a structure or even more the actual flow processes. As an alternative to simulation, a variety of diagnostic methods can be applied to gain knowledge on the outflow behavior. In the field of non-contact methods, the Schlieren measurement has been established in order to visualize flows. This method bases on the physical phenomenon, that a beam of light is reflected differently depending on the refractive index. As the index of refraction of the gas is determined by the type of gas and the applied pressure one can gather knowledge of local pressure and in a continuous measurement on the gas flow. The resulting image is a very good source for interpretation one. In previous investigations on the influence of the inner contour, the main points of study were the flow and the formulation of recommendations for manufacturing [KOV09, SHA06]. In the framework of this work, the impact of the selective laser melting technology on the flow behavior will be investigated and discussed. Due to the selective laser melting process, the components have a higher surface roughness and a lower contour accuracy of conventional method.

2. Operating principle and experimental setup of flow diagnostics

When fluids or gases are subject to a temporal or local change density, pressure and temperature variations arise. As a result thereof, inhomogeneity occurs for deflecting the propagating direction of light in an optically transparent medium, i.e. a change in the optical refractive index n in the flow field. That implies the change in density ρ takes place primarily by the expansion of the gas stream from the nozzle. The Schlieren measurement technology allows for visualizing the local change of the refractive index density and refractive index are linearly interdependent [SET01].

(1)

Developed by A. Toepler, an arrangement (see Fig. 1) is nowadays used in industrial monitoring and research applications most commonly [HER93]. The rays of the light source can be projected via a condenser in the plane of the aperture, which is located in the focal plane of the first schlieren lens. Using this lens a parallel beam path in the measuring area between the two lenses –the object field- is generated. In the back focal plane of the schlieren head are simultaneously the real image of the gap and the knife edge (Foucault-level). The "radiation beam" from the measurement area is mapped by the second schlieren lens in the Foucault- level and then falls on the table. If there is a change in refractive index, arise in the focal plane of the objective two slit images (an image of the original radiation and an image of the deflected radiation). Hence, the insertion of the knife edge causes a change in the intensity. [SET01].

(2)

schlieren slit filament aperature ds=60 µm

hemiplanar adjustable mirror

mapping lens

matt screen schlieren schlieren objective objective objective for projektion condensor

camera on light source dn/ds table tripod

Fig. 1. Schlieren-Aufnahmegerät 80 coupled with high-speed camera (Toepler´s dual-field-lens schlieren arrangement) Depending on the cutting position, the schliere appears dark on light background or light on dark background, the second variant due to the light sensitivity of the eye is the more appropriate (see Fig. 2). The quality of the Schlieren image and the sensitivity of the test object are determined by:  the type of knife edge  the knife edge and aperture orientation  the light source  a uniform illumination of the measuring field  the focal length of the schlieren lenses  exposure time distributed slit image slit image

knife-edge

schlieren objective knife-edge

Fig. 2. Knife-edge arrangement in a double lens structure (left: lateral view, right: front view) To the visual evaluation of the real gas flow, a modified system was used (Schlieren-Aufnahmegerät 80 of the Carl Zeiss AG). To meet the requirements of the measuring task, the apertures were initially modified and the construction was coupled with a high-speed camera (Fastcam SA4 of Photron). The sensitivity of the measuring set-up could be increased by the reduction of the gap width of the aperture to 60 µm and substitute the cutting edge by a filament aperture. In addition, the reduction of the gap width led to the reduction of the resolution of the picture. By the increased sensitivity of the construction the smallest pressure differences can be visualized therefore. A crucial factor for a high-quality image is the setting of the exposure time. A longer

exposure time yields a brighter schlieren photograph (up to the overexposure). By changing the gap and filament orientation the different gradient directions can be visualised, i.e., representation of the ray border or the compression front.

3. Flow characterization

For the flow characterization, mainly the results from the optical analysis are used. Fig. 3 depicts that the flow gas is divided into three areas:  the conical core with constant density (supersonic inviscid core) [MAR77, GAU70]  the transition zone with variable density (supersonic mixing region)  and the similarity area in which a free jet is completely formed and it is for mixing with the secondary fluid (subsonic mixing zone) [SHA06]. Especially during laser cutting, the core area should be within the kerf to guarantee a controlled melt ejection. At sufficient high pressure, expansion and compression waves are visible in the schlieren image, (see Fig. 3). At the nozzle outlet opening in the formation of expansion waves, which intersect at the beam axis and are reflected at the free jet boundary.. The reflected waves continues as compression waves (compression front), intersect in the gas jet axis and are reflected back as an expansion wave. This process continues periodically until the energy is consumed by friction between the edge beam and environment [CAR56]. intersection point

supersonic compression inviscid core shock

length boundary of the free jet expansion wave

supersonic supersonic supersonic mixing region

secondary fluid compression wave

subsonic mixing zone

Fig. 3. Free jet of a non-conformist exit opening (left: schematic view, right: compression and expansion behaviour) For the study, the copper nozzle (HG 10.281 by Precitec) was used because it has a wide industrial relevance. To ensure comparability for the investigation, the same nozzle was produced with the selective laser melting process. At 푝 = 1.5 푏푎푟 and 푝 = 6.0 푏푎푟 recordings of the free jet have been made. The gas jet exits the nozzle with a shortened supersonic inviscid core, which leads to a more rapid expansion of the supersonic mixing region. The reason for that are production-related influences, which amongst others yield a varying the layer thickness or an increased surface roughness of the component. The elevated surface roughness leads to a decrease of the kinetic energy of the gas jet already inside the nozzle. Based on the length of the core beam, which is 3 times greater for the copper nozzle, and the number of shocks, this influence is demonstrated. A shortening of the core jet range, for the cutting process means a reduction in the distance between nozzle and workpiece. With increasing pressure to pressure shocks built on. Their number depends

mainly on the production method. Is the shape of the pressure shocks less, this means less turbulence and thus a more rapid homogenization of the gas flow. This effect was studied in detail by simulating a workpiece in the form of a baffle plate. The pictures reveal very clearly what is expected with the standard nozzle up to a distance of two millimetres a safer melt ejection. However, if the selective laser melting nozzle (SLMN) is considered, the outlet gas stream is homogenous. Compared to the free jet, the pressure distribution is constant. It is assumed that the pressure balance is brought to the surroundings and to the interior of the nozzle and hence there a homogeneous velocity distribution is achieved outside the nozzle. If the distance between plate and nozzle is increased, the gas jet is tapered, which can be explained by higher energy consumption. For larger material thicknesses, this could lead to problems when extruding the melt.

Table 1: Comparison of the nozzle (left: free jet, right: baffle plate)

p HG 10.281 SLMN H HG 10.281 SLMN

5 bar 5

0 mm 0

1.

1.

bar

0 0

0 mm 0

6. 6.

The cutting quality is a direct indicator of the quality of the gas flow because all other process parameters are kept constant during the nozzle change. Thus, the results can be compared with each other. The rectilinear grooves, as shown in Fig. 4, demonstrate that the gas jet (from HG 10.281) owns a high kinetic energy and thus the melt can be driven out well. In this figure, the measured roughness of the cut edge is illustrated. With a thickness of b=1.0 mm, the values are with ≈40% above the maximum allowable roughness. The horizontal line can be assigned to a local compression shock within the kerf. It can be concluded, that there is a very good transfer of kinetic energy on the melt. As evidence, the shape and the intensity of the shower of sparks is used in practice. This is used as a guide for the achievable quality during the process. The shower of sparks during the cutting experiment was evaluated as very good. Confirmation was the following estimation by visual assessment of the cut edge. This showed no abnormalities, which meant that a further optimization of the cutting parameters was not performed. A similar behavior shows the sintered nozzle. The conclusions drawn from the schlieren measurement can be confirmed by the visual assessment. Based on this, it is visible, that the edge quality is slightly direction dependent. Through the determined values this statement was confirmed. The values for the tolerances depend on the cutting direction fluctuations. Already during the cutting process first signs through a low intensity of the radio-flight (lower melt ejection) could be detected. Thus, the formation of the process of typical vertical grooves is not uniform. At the lower edge of the workpiece one observes a formation of a “string of solidified droplets”, which means good removability. If the nozzle distance is reduced, an improvement in quality (no string of solidified droplets) was achieved and thus the shower of sparks are improved.

Fig. 4. Influence of the nozzle wall on the cutting edge quality (cut faces topography) When cutting of acrylic glass, only a visual evaluation of the sample is performed. The best edge quality was achieved only with the HG 10.281. The increase of the rounded cutting edge at the beam entrance side correlated with the decrease in the quality of the nozzle outlet opening. This means, i.a., a reduced flow rate, thus more material is melted during the process, but not completely removed from the kerf. At the cut edges a blistering and brownish coloration occurred, which means the transition temperature TG was exceeded. If TG is exceeded the chemical bonds are broken and the thermal decomposition starts. In the reference nozzle, the material, which has passed TG, was completely removed. Thus, no resolidified material was deposited on the cutting edge. By adapting the gas pressure, the cutting material was completely removed, and thus the same edge quality can be achieved as in the HG 10.281.

4. Adjustment of cutting gas nozzle

In the processing of silica glass the nozzle is exposed to considerable thermal stress. This results in long cutting distances to damage of the nozzle outlet opening at the sintered and the copper nozzle. In order to remove the process heat from the nozzle more quickly, we improved the internal geometry by including two cooling channels between the two walls (see Table 2). With the cooling, the lifetime was significantly increased and the deformation of the outlet opening can be avoided. In a further step, an increase in quality was achieved by adapting the internal geometry and the change in the orientation of the part in construction space. Due to these purposeful changes support structures are omitted in the manufacturing process. Thus, the surface roughness is reduced. Another quality improvement has been achieved by optimizing the process parameters of the selective laser melting. After the selective laser melting manufacturing a final machining to the exit surface was carried out, inter alia, the risk for adhesions sublimate to minimize. The additive manufacturing process offers a high design freedom. Using the advantage and optimizing the selective laser melting process led to significant increase in the surface quality and dimensional accuracy. Thus, we optimized the flow behaviour of the gas stream and achieved an improvement in the quality of cut, see Table 2.

Table 2: Comparison of the nozzle for SiO2 cutting

p = 1.5 bar cut edge in the cutting insert draft

HG 10.281HG

p = 1.5 bar cut edge in the cutting insert CT-scan

)

optimized

(

N SLM

For current standard applications, as the laser material processing of flat metal sheets, a variety of standardized gas nozzles are available [REG11], with which very good results can be achieved. Once, however, deviated from the standardized parameters or geometries, for example, commercially available gas nozzles often reach their limits due to an increasing complexity of components launched. Another example of process adapted nozzles is the laser drilling in deep cavities. This nozzle has the primary task to convey the melted part of the process, with the kinetic energy of the gas flow exiting the hole. The unfavourable space conditions in injection moulds between the cavity and the machining head (including standard nozzle) is a challenge. With the processing system (machining head and gas nozzle) of the manufacturer it is not possible to achieve the ingrained plane of the cavity. The solution represents an increase of the working distance. For this purpose, the focusing of the machining head was replaced with a longer focal length. The laser-drilling nozzle was adapted in the design process of the new beam geometry, thereby the standard nozzle serves as construction base (see Figure 5). In the following, the nozzle was produced or generated by the selective laser beam melting. Due to the process-related high surface roughness of the component, the nozzle in the interface area to the processing head has been provided with an allowance, which was removed by machining in a further stage. With the optimized nozzle it is possible to drill micro-holes with an outlet diameter of 60 µm reliably produced up to 80 mm deep cavities [LOR14].

Fig. 5. Geometric adaptation a standard nozzle (left: boundary conditions, mid; generated nozzle compared to standard nozzle, right: SEM picture of a 60 hole)

5. Summary

In the article the influence of the manufacturing process of laser cutting nozzles was introduced. The conventional method (micro-electrical discharge machining) is compared to the novel and more flexible selective laser melting technique. We used the Schlieren measurement technology for the flow diagnostics and the effects of the production technology are investigated by means of cutting tests and respective cutting result. The comparison of the schlieren recordings shows that the selection of the manufacturing technology has a significant impact on the flow behaviour. The greatest impact on the flow behaviour is caused by the increased roughness of the functional surfaces. This increased roughness results in a shortening of the core jet region and in an associated reduction of the flow velocity, which finally results in limitations in laser cutting. Based on the cutting tests the effectiveness of melting ejection investigated. The formation of a “string of solidified droplets” (which is good removability) are due to the lower melt ejection. A substantial increase in flow quality and hence the achievable cutting quality has been achieved by the geometrical adjustment of the nozzle, by an optimization of the manufacture process and by adapting the cutting parameters. Are production-specific requirements into account, the laser melting of metals is an alternative to the conventional methods. Taking advantage of the many degrees of freedom in the structural design of the nozzle, it could be proved that can be a positive influence on the flow behaviour and thus the cutting quality has been improved. In addition to the presented cutting nozzles have been developed shielding gas nozzle for welding processes at the Günter Köhler Institute for Joining Technology and Materials Testing GmbH (IFW). Further, the fabricated gas nozzles have been successfully used in numerous laser material processing projects.

6. References

[CAR56] Carafoli, E.: High-Speed Aerodynamics (Compressible Flow). In: EDITURATEHNICĂ, Bucharest 1956. [GAU70] Gauntner, J.W; Livingood, J.N.B.; Hrycak, P.: Survey of literature on flow characteristics of a single turbulent jet impinging on a flat plate. In: NASA TN D-5652 N70-18963, Washington D.C. 1970.

[HER93] Herzinger, G.; Loosen, P.(Hrsg.): Werkstoffbearbeitung mit Laserstrahlung, Grundlagen-Systeme- Verfahren. In: Carl Hanser Verlag, München 1993. [KOV09] Kovalev, O.B.; Yudin, P.V.; Zaitsev, A.V.: Modeling of flow separation of assist gas as applied to laser cutting. In: Applied Mathematical Modelling, Novosibirsk 2009. [LOR14] Lorenz, S.; Ulrich, S.; Jahn, S.; Sändig, S..: DVS Congress 2014-Große Schweißtechnische Tagung-DVS Studentenkongress. In: DVS Media GmbH, Band 306, S.4-9 ,Düsseldorf 2014. [MAR77] Martin, H.: Heat and mass transfer between impinging gas jets and solid surfaces. In: Advances in Heat Transfer Volume 13, S. 1-60, London 1977. [REG11] Reg, Y.; Leitz, K.-H. Schmidt, M. (2011): Influence of Processing Gas on the Ablation Quality at ns- Laser Beam Ablation. In: Physics Procedia, Volume 12, Part B, S. 182-187, Erlangen 2011. [SET01] Settles, G.S.: Schlieren and shadowgraph techniques: visualizing phenomena in transparent media. In: Springer-Verlag New York 2001. [SHA06] Shapario, A.H.: The dynamics and thermodynamics of compressible fluid flow. In: John Wiley & Sons published online, New York 2006. [VDI93] VDI-Technologiezentrum Physikalische Technologien (Hrsg.) : Schneiden mit CO2-Lasern, Handbuchreihe Laser in der Materialbearbeitung, Band 1. In: VDI-Verlag GmbH, Düsseldorf 1993