Originally developed for prototyping, today it is mainly used for the production of geometrically complex components in small batch sizes between 1 and 1, Here, additive manufacturing is for the most part more economical than conventional methods. One of the most promising approaches for optimized production processes is the integration of laser beam sources in machine tools.
For example, Laserline's LDM diode lasers are implemented into a five-axis milling machine, where they render change between additive and subtractive processing possible: the laser realizes the powder coating while the milling head realizes the machining. The integrated diode laser applies the powder extensively and thus creates the basic structure of the component.
The downstream milling head post-processes in a chipping manner the generated part though only at necessary areas. The flexible change between laser and milling processing makes the post-processing of component segments possible, which would be out of reach at the finished part. Design and production, at which undercuts are not a problem anymore or the production of overhanging contours without support structure, are now possible.
Basic new application and geometry opportunities are available. The various application possibilities of Laserline's diode lasers in additive manufacturing also offer other options that go way beyond the interplay between additive powder coating and subtractive machining. For example, lasers can be integrated into a twelve-axis milling machine and can be used, besides powder cladding, for welding and hardening.
Besides the laser and powder nozzle, corresponding optics are implemented for this purpose, between which you can switch back and forth depending on the processing operation. As a result, complex production processes that are based on a single beam source can be realized by laser additive manufacturing. Additive Manufacturing Process Examples Product request. Your browser does not support the video tag. Laser additive manufacturing of metal.
Manufacturing applications — overview How does additive manufacturing work? A linear scaling of vapor depression depth with laser power is not necessarily expected for keyhole mode heat transport where strong vaporization and melt pool dynamics play important roles. In this keyhole regime, the melt is rapidly displaced away from the laser beam under the effect of recoil momentum and Marangoni shear flow. The absorbed laser energy therefore not only leads to melting, but also to melt motion. The linear dependence observed here appears to indicate that the absorbed laser energy is spent mainly to melt the solid even in the keyhole regime, which helps to explain the linear scaling of melt depth behavior with power.
Interestingly, for the case of the maximum vapor depression depth during the turn point, the depression relationship with power is also linear Fig. Inspection of the velocity of galvanometer-based X—Y scanning mirrors during the turn point indicates that at the turn point itself, the programmed steady-state scan speed has no influence on the measured scan speed see Supplementary Fig.
As the laser approaches the turn point, the mirrors decelerate and then accelerate back to full steady-state scan speed immediately after the turn. When the laser scan speed approaches zero at the turn point, the instantaneous energy density increases, leading to localized overheating and an increase in depression depth.
All laser turn conditions were performed at full laser power. Note the different Y -axis scale between the two panels. The maximum vapor depression depth occurring post-turn is caused by a build-up of heat in the turn point region from the long dwell time of the near-stationary laser. This increase in vapor depression depth highlights a transition between the onset of keyhole mode present during steady state and a deep keyhole mode regime at the turn region. For the case of keyhole mode welding the power density of the laser is sufficient to evaporate metal from the surface and initiate a plasma plume Evaporation of metal from the surface during keyhole mode allows the laser to drill into the material leading to the formation of a vapor depression.
This increase in laser absorption and therefore localized energy density is accompanied by formation of a high aspect ratio keyhole vapor depression as observed in Fig. The steady-state vapor depression depth during the post-turn scan is higher than the pre-turn scan due to thermal lag associated with preheating of the material this can be directly observed in Fig. Pores form at all laser powers under these processing conditions, with pores forming only post-turn during vapor depression collapse.
From these direct observations, we ascribe the mechanism of pore formation at laser turn points to rapid collapse of the vapor depression. After the depression depth increases during the laser turn, the mirrors accelerate away from the turn too quickly for the depression to smoothly return to the steady state, prompting a rapid collapse of the depression. This is exemplified in Fig. The collapse of the vapor depression and subsequent pore formation can be described by hydrodynamics, and the exertion of force by gaseous metal on the molten pool surrounding the depression.
Increased localized energy density in material at the turn point leads to an increase in the vapor pressure of gaseous metal above the base of the vapor depression. The increase in vapor pressure causes metal to rapidly be ejected from the vapor depression driving the depression deep into the substrate and the recoil pressure of gaseous material inside the depression overcomes the force of molten metal flow into the void. Once the scan speed returns to steady state and a decrease in temperature at the depression surface is realized the recoil pressure from evaporating metal is reduced exponentially.
As the surface temperature is reduced the surface tension of the melt pool increases overcoming the force from the recoil pressure and the depression collapses Argon filled pores are then trapped in place by the quickly freezing melt pool, leaving pores trapped in the solidified material. This collapse mechanism is distinct from the traditional view of pore formation during keyhole mode, in which instabilities at the liquid metal—vapor interface stochastically form pores even in steady state Under turn point conditions where laser scan acceleration is maximum, pores are formed due to the transition of the vapor depression into a deep keyhole regime and the associated collapse of the walls of vapor depression, which is too rapid for the system to smoothly accommodate without the formation of pores.
The turn midpoint was determined by analysis of the in situ X-ray images. Black circles denote pore formation events. Error bars represent uncertainty in the distance between the base of the vapor depression and the surface caused by surface roughness. To further our understanding of pore formation, a series of simulations were performed to ascertain the dynamics of the collapsing vapor depression.
Simulations were performed using the ALE3D multi-physics software tool 39 and parameters for the validated, stainless steel SSL simulation environment. Simulated laser rays strike the surface from the source in a direct line of sight and the energy deposited into the sample is determined by an effective absorption coefficient 0.
The bulk of the incident laser energy is deposited over the front inclined wall of the vapor depression which consists of a flat liquid surface. The energy deposited into the front wall location dominates the melt pool response and melt pool depth via the recoil pressure. This is due to the exponential temperature dependence of the recoil physics and because the highest surface temperature is realized immediately below the laser at the point of incidence.
Previously, the model was shown to predict melt pool dimensions, as well as explain the formation mechanism of other defect modes such as end-of-track pore defects Furthermore, the simulation enables probing of the generality of the vapor depression dynamics observed in the materials. The linear dependence of vapor depression depth with respect to the laser power was also reproduced in the SSL simulation see Supplementary Fig.
A single frame of the 3D simulation is shown in Fig. This is a direct result of the increase in laser dwell time as the scan speed decreases for execution of the turn, and more energy is absorbed in the local volume causing the depth to increase. As the laser finishes the turn, a more pronounced depth increase occurs because the accumulated residual heat is higher in this pre-heated location compared to entry into the turn point. Immediately following this melt pool depth maximum, the vapor depression depth rapidly decreases as the laser accelerates away from the turn point Fig.
A significant increase of the vapor depression depth due to this overheating followed by rapid collapse Fig. The rapidly moving 0. The simulations show that at the turn point the process enters a deep keyhole regime and large pore defects are generated as the process returns to steady state, agreeing with the experimental observations in Ti—6Al—4V.
These complementary observations strongly suggest that this behavior is universal and occurs regardless of material in turn points during LPBF. The black contour line represents the melt pool boundary. From our experimental and modeling results, the increase in depth followed by a rapid collapse of the vapor depression is the mechanism that gives rise to the formation of pores during the laser turn point. This behavior is caused by a transient increase in energy density deposited by the laser and can therefore be mitigated by adjusting process parameters to keep energy density approximately constant.
Turning the laser off at the turn point a so-called sky writing method is not a viable solution for pore mitigation as it has previously been shown to result in pore formation Incidentally, the formation of pores at the end of a written track when using the sky writing method can also be explained by the observations here describing the collapse of the vapor depression and the subsequent formation of pores. The vapor depression collapse when the laser is turned off likely exhibits behavior comparable to the conditions described due to the removal of laser power within the 6.
In the laser off condition the transition from keyhole mode to zero laser input power would be even more abrupt than at the turn point. A scan strategy for mitigation of pore formation has the following mechanistic and physical requirements: i the vapor depression must not transition into the keyhole regime at the turn region, ii laser power must be controlled with no rapid oscillations, iii the power must be sufficient to maintain the melt pool during scanning of the hatch spacing, and iv the laser power must not increase rapidly when accelerating out of the turn point into the pre-heated region.
Most importantly, pores were not detected in the processed track using this scan strategy. When combined with contour and border hatch scan strategies which have been shown to reduce porosity 41 this power modulation scan strategy could further improve final component quality. This is particularly important in island scan sequences where the number of turn points per volume slice is increased significantly Also shown is the commanded laser power as a function of time used in the scan strategy red line.
Depression depth values were measured for the case of a bare plate experiment because depression depth measurements in bare plate were less uncertain than the powder case, but the same trend is observed in both cases. An analytical approach, utilizing normalized enthalpy 18 , 34 , 43 , was used to investigate the outcomes of the mitigation strategy.
Normalized enthalpy is a term commonly used in the welding literature 43 , 44 and recently has expanded to characterize LPBF conditions 18 , Previous studies have shown a linear dependence on the depth of molten material with normalized enthalpy under varied laser conditions.
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The normalized enthalpy approach was applied to the vapor depression depth as a function power data in Fig. This reveals that the vapor depression depth is linear with normalized enthalpy and the relationship is identical for different laser scan speeds under these material processing conditions. Normalized enthalpy as a function of laser position for the constant power and mitigation scan strategy are shown in Fig. During the mitigated scan strategy, the normalized enthalpy is near constant, peaking at a value of This short increase however did not result in a transition to a deep keyhole vapor depression likely due to thermal lag required to induce changes in overall melt pool behavior under these conditions.
The analytical normalized enthalpy analysis reveals that for successful implementation of a defect mitigation scan strategy the normalized enthalpy should be kept near constant and below the transition point into the keyhole regime. A pore mitigation strategy is not viable if the resulting track is of low quality e.
The track produced using constant power Fig. For the case of the track produced using the mitigation strategy, Fig. There is no longer a bulge at the end of the track caused by overheating. This reveals that not only is the formation of pores mitigated using this scan strategy Fig. The mitigation strategy not only improves quality of material fabricated by LPBF-AM, but also is simple to implement, because it only requires linear ramping of the laser power over a few hundred microseconds.
This sort of power adjustment can be implemented with the hardware available in most commercial LPBF machines by constructing power maps with the 3D slicer software that converts the component geometry defined by a CAD file into machine instructions Pores formed during LPBF are highlighted in b.
No pores were detected in d. In summary, we have uncovered the mechanism of pore formation during laser turn points, a critical defect mode in serpentine scan-based LPBF. The pore formation process is observed experimentally in Ti—6Al—4V and via multi-physics modeling of SSL, revealing the general nature of the mechanism. Pores form at laser turn points due to the emergence and subsequent collapse of a deep keyhole depression caused by the deceleration and acceleration of the galvanometer-based scanning mirrors during the turn which results in dramatic variations in the local normalized enthalpy at the material surface.
As the laser accelerates away from the turn point, the keyhole depression collapses and molten metal fills the void, trapping gaseous argon which ultimately forms a pore as the material solidifies.
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This understanding based on in situ X-ray imaging and multi-physics modeling was harnessed to devise a pore mitigation strategy based on laser power modulation and implemented under typical Ti—6Al—4V build conditions. The mitigation strategy effectively prevents pore formation at the laser turn point by removing the rapid variation in depression depth inherent in the unmitigated case and improves the geometric tolerance of fabricated tracks by avoiding overheating.
The successful mitigation strategy presented here illustrates the potential of in situ X-ray measurements coupled with high fidelity modeling for driving process improvements and paves the way to increasing the quality of LPBF-built components. LPBF was performed using a laboratory-scale test bed described and characterized in detail elsewhere The Ti—6Al—4V substrate was sandwiched between two 1-mm-thick glassy carbon sheets which provided a trench to contain Ti—6Al—4V powder on the substrate surface.
Laser turn-around scanning conditions were programmed using the Waverunner scan control software and Pipeline-2 scan controller Nutfield Technology which compiled the required instruction routine for the galvanometer scanning mirrors and laser power interface. Two parallel, 2. The laser was programmed to irradiate this geometry based on internal triggering from the scanning mirror position.
The galvanometer scanning mirrors were controlled by the Pipeline-2 controller for all cases, with mitigation achieved by disabling the Pipeline-2 controller laser power interface and initializing the FPGA module to control the laser power via an analog voltage signal. The FPGA module controlled the laser power as a function of time using a lookup table.
Aims and Scope
The white-beam X-ray spectrum generated by the 1. The beam was aligned coincident with the Ti—6Al—4V substrate surface in the center of the vacuum chamber and the laser aligned to scan through the X-ray imaging system field of view during processing. Transmission X-ray images of the LPBF process were captured using a scintillator-based optical system. Synchronization of the laser and imaging system was realized using a custom FPGA-based timing circuit. X-ray images were analyzed using ImageJ 47 and Mathematica Version This routine provided an image where darker regions reveal a decrease in X-ray absorption or material and lighter regions reveal an increase in X-ray absorption or material.
A custom script in Mathematica utilized the built-in Binarize contrast threshold method to identify and characterize pores in the processed Ti—6Al—4V substrate. Simulations were performed using the ALE3D multi-physics software tool which utilizes arbitrary-Lagrangian-Eulerian techniques 39 , To simulate the thermal response and melt flow in the material, the heat conduction equation was coupled with the Navier—Stokes equations via operator splitting Material parameters for stainless steel L were used for the simulation Powder was not included as it significantly increases the physical complexity of the simulation and experimental results showed no significant change in the trend in pore formation between bare plate and powder.
A series of simulations was performed as a function of laser power during the turn point. The maximum melt depth as a function of time was determined as a function of turn point laser power. The laser followed a scan geometry and turn point scan speed informed by measurements of the galvanometer scan mirror response, which comprised of a straight line stretch ending in a turn point. The data that support the findings of this study are available from the corresponding author on reasonable request. All data generated using this code are available from the corresponding author on reasonable request.
Journal peer review information: Nature Communications thanks Marco Simonelli and the other anonymous reviewer for their contribution to the peer review of this work. Peer reviewer reports are available. Wohlers, T. Wohlers Report DebRoy, T.
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