The optical systems for temperature monitoring of SLS/SLM process are developed and integrated with industrial
SLS/SLM machines. The system provides the possibility to spatial distribution of brightness temperature at two
wavelengths and selected temperature profiles, calculation of colour temperature and express analysis of possible
deviations of the maximum temperature from its optimal value. Optimal regimes of SLS process for the sintering of the
high porosity powder body was determined.
To up-grade selective laser melting (SLM) process for manufacturing real components, high mechanical properties of
final product must be achieved. The properties of a part produced by SLM technology depend strongly on the properties
of each single track and each single layer. In this study, effects of the processing parameters such as laser power,
scanning speed and powder layer thickness on the single tracks formation are analyzed. It is shown that, by choosing an
optimal technological window and appropriate strategy of SLM, it is possible to manufacture highly complex parts with
mechanical properties comparable to those of wrought material.
Optical monitoring of temperature evolution and temperature distribution in laser machining provides important
information to optimise and to control technological process under study.
The multi-wavelength pyrometer is used to measure brightness temperature under the pulsed action of Nd:YAG laser on
stainless steel substrates. Specially developed "notch" filters (10-6 transparency at 1.06 μm wavelength) are applied to
avoid the influence of laser radiation on temperature measurements. The true temperature is restored based on the
method of multi-colour pyrometry.
Temperature monitoring of the thin-walled gilded kovar boxes is applied to detect deviation of the welding seam from its
optimum position.
The pyrometers are used to control CO2-laser welding of steel and Ti plates: misalignment of the welded plates, variation
of the welding geometry, internal defects, deviation of the laser beam trajectory from the junction, etc. The temperature
profiles along and across the welding axis are measured by the 2D pyrometer.
When using multi-component powder blends in laser cladding, for example metal matrix composite with ceramic
reinforcement, one needs to control temperature of the melt to avoid thermal decomposition of certain compounds (as
WC) and to assure melting of the base metal (as Co).
Infra-red camera FLIR Phoenix RDAS provides detailed information on distribution of brightness temperature in laser
cladding zone. CCD-camera based diagnostic system is used to measure particles-in-flight velocity and size distribution.
The set of original pyrometers and the special diagnostic CCD-camera were applied for monitoring of Nd:YAG laser cladding (Pulsed-Periodic and Continuous Wave) with coaxial powder injection and on-line measurement of cladded layer temperature. The experiments were carried out in course of elaboration of wear resistant coatings using various powder blends (WC-Co, CuSn, Mo, Stellite grade 12, etc.) applying variation of different process parameters: laser power, cladding velocity, powder feeding rate, etc. Surface temperature distribution to the cladding seam and the overall temperature mapping were registered. The CCD-camera based diagnostic tool was applied for: (1) monitoring of flux of hot particles and its instability; (2) measurement of particle-in-flight size and velocity; (3) monitoring of particle collision with the clad in the interaction zone.
The possible manifestations of the thermodynamical instability (explosive vaporization) are discussed for different regimes of laser heating of the target. It is shown that repeated explosive vaporization regime can be realized during nanosecond laser pulses provided that nucleation time τ < 10-10s. This regime can be observed if the surface pressure is lower than the critical pressure Pc of the liquid-vapor phase transition. The laser vaporization front instabilities are investigated for the steady state vaporization regime of absorbing condensed matter. The results of the numerical analysis of the dispersion equation are presented for various values of the target absorption coefficient α and different Mach number M ≤ 1 in the vapor flow. In several cases of bulk absorption (α ≤ 104 cm-1) the instability increment maximum γmax is of the order of 109s-1 and the corresponding wavelength λmax ≥ 0.3 μm. The dependence of these results and vaporization dynamics on the sticking (condensation) coefficient in vaporization boundary conditions is also analyzed.
Relatively new method of producing 3D objects with Functionally Graded Material (FGM) structure is realized by coaxial powder injection with variable composition into the zone of laser beam action. The desired 3-dimensional material distribution is realized by repetitive deposition process. Theoretical analysis and experimental results show essential role of radiation mode and powder granularity as optimization parameters. Applied laser sources are continuous wave Nd:YAG(HAAS 2006D, 2kW), pulse-periodic Nd:YAG(HAAS HL304P, avg. power 300 W), quazi-cw CO2 (Rofin-Sinar, 300 W). Among applied materials are nanostructured WC/Co, CuSn, Stainless steel 316L, 430L, Co-base alloy, nanostructured FeCu, etc. The originality of obtained results is that different gradient types are produced "in situ" and combined within one sample: smooth, sharp or multilayered gradients. The number of samples is produced and examined with metallographical and SEM analysis. The minimal spatial gradient resolution (transition zone between two different materials) is starting from 10 microns and can be varied in a wide range; the surface roughness depends from powder granularity, best value of Ra is about 5 μm, microhardness of differet zones of samples is varied from 120 to 450 HV. The achieved geometry spatial resolution is 200 μm.
The overview presents the state-of-the-art in application of laser assisted Rapid Prototyping (RP) methods and their possibility to produce Functionally Graded Material (FGM) models. As Rapid Prototyping is a stepwise process constructing the object layer by layer, material composition may be changed from layer to layer and/or within the layer, this way a FGM object with 3D material gradient can be created. The free-form FGM model with engineered shape, composition, structure and even protective coating can be manufactured within one single process. Several techniques are analyzed and particular attention is paid to the lateral and coaxial powder injection into the laser beam. These technologies are shown as most effective tool for creation of FGM parts. The best results are obtained in laboratory conditions: spatial resolution of fabrication up to 200 μm with Ra up to 5 μm, transition zone between layers of different composition can be varied in a wide range starting from 10 μm.
The plasma-controlled evaporation of the Al target induced by the laser pulse with intensity of 8 x 108 W/cm2 and wavelength of 1.06 μm is analyzed with account for the two-dimensional effects. The self consistent model is applied, consisting of the heat transfer equation in condensed medium, the system of radiation gas dynamics in evaporated substance, and the Knudsen layer model at the two media boundary. It is established that the phase transition of the target surface is controlled by the two factors: the surface temperature that depends on the transmitted radiation intensity and the plasma pressure, governed by the expansion regime. The process comes through three characteristics stages -- the sonic evaporation at the beginning, the condensation during the period of plasma formation and initial expansion and, finally, the recommence of evaporation in subsonic regime after the partial brightening of the plasma. During the subsonic evaporation stage the vapor flow and the mass removal rate is much higher near the beam boundaries than in the center due to smaller plasma counter-pressure. The vapor plasma pattern is characterized by the dense hot zone near the surface where the deposition of laser energy occurs, and rapid decrease of density outside the zone due to three-dimensional expansion. The application of the laser beam of smaller radius at the same intensity leads to the formation of more rarefied and more transparent plasma, that allows to improve the mass removal efficiency.
A thorough experimental study was made of the processes determining formation of metastable surface alloys in a film-substrate system (Au-Ni, Sn-Cr) irradiated by laser pulses of nanosecond duration. The irradiation was performed in conventional conditions (on air) and, following the new method, through transparent overlay. A physical model of laser induced processes has been developed. The model predicts an increases of the depth of the melt, elevation of the liquid-phase temperature, rise of the diffusion speed of atoms in the liquid phase, and decrease of the cooling rate of the alloy during irradiation through the transparent overlay.
Coaxial powder injection into a laser beam was applied for the laser-assisted direct manufacturing of 3D functionally graded (FG) objects. The powders of Stainless Steel 316L and Stellite grade 12 were applied. The following laser sources were used: (1) quasi-cw CO2 Rofin Sinar laser with 120 μm focal spot diameter and (2) pulsed-periodic Nd:YAG (HAAS HL 304P) with 200 μm focal spot diameter. The objects were fabricated layer-by-layer in the form of "walls", having the thickness of about 200 μm for CO2 laser and 300 μm for Nd:YAG laser. SEM analysis was applied for the FG objects fabricated by CO2 laser, yielding wall elements distribution in vertical direction. It was found that microhardness distribution is fully correlated with the components distribution. The compositional gradient can be smooth or sharp. Periodic multi-layered structures can be obtained as well. Minimal thickness of a layer with the fixed composition (for cw CO2 laser) is about 50 μm. Minimal thickness of a graded material zone, i.e. zone with composition variation from pure stainless steel to pure stellite is about 30 μm.
A bi-dimensional monochromatic and a 1-spot multiwavelengths pyrometers were applied for surface temperature monitoring in Nd:YAG continuous laser welding. The experiments were performed for stainless steel and titanium alloy applying variation of laser parameters: power, welding velocity, focalisation distance, shielding gas flow rate, presence of surface pollution, with fixed or variable gap and misalignment between the dissimilar plates. Surface temperature evolution along and perpendicular to the welding seam and full temperature image versus laser action parameters were measured with the help of the bi-dimensional pyrometer. The dynamics of temperature gradients, transient periods and steady-state temperature distributions are analysed. The same approach is applied for Nd:YAG laser cladding of stellite on steel substrate. A new type of CCD based optical diagnostic tool has been applied for monitoring the particle in flight size and velocity.
A 2D gas-dynamic model of laser ablation an ambient gas atmosphere is proposed. To obtain the boundary conditions at the evaporated target surface, a nonlinear heat transfer problem in the target including the dynamics of the melt and evaporation fronts is considered. Back condensation of the vapor at the target is taken into account. At later stages, compete absorption of the vapor and back condensation thereof with a local sound speed are assumed. The gas- dynamic problem in divided into the initial 1D and final 2D stages. The 1D stage describes the ablation plume formation under the action of laser pulse. The 2D stage is responsible for the formation of the energy and angular distributions of the ablated material. A considerable compression of the ambient gas around the expanding plume of the laser- evaporated material and a shock front propagating through the undisturbed ambient one. Once the laser pulse is over, the vapor pressure eventually drops down to the value comparable to the compressed ambient gas pressure. From this time on, the gas considerably suppresses the vapor expansion. There is a noticeable difference between the vapor distribution in vacuum and the one in the ambient atmosphere: the vapor fills the entire plume volume in vacuum while in the presence of ambient atmosphere it is accumulated near the plume boundary and tends to form a thin shell. The angular and energy distributions of the ablated material are especially sensitive to the nature and pressure of the ambient gas. Both the kinetic energy of the ablated atoms and the width of their angular distribution decrease with the ambient pressure.
Special techniques of non-contact optical diagnostics under actual industrial conditions is required for accurate temperature monitoring and control in a wide range of laser applications. The set of pyrometers was developed and applied for surface temperature monitoring in pulsed periodic Nd:YAG laser welding and surface treatment, deep penetration welding by CO2 and Nd:YAG lasers, and electron beam; laser assisted machining; laser cladding, etc.
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