Search

In this paper, high-throughput ultrashort pulse laser machining is investigated on various industrial grade metals (Aluminium, Copper, Stainless steel) and Al2O3 ceramic at unprecedented processing speeds. This is achieved by using a high pulse repetition frequency picosecond laser with maximum average output power of 270 W in conjunction with a unique, in-house developed two-axis polygon scanner. Initially, different concepts of polygon scanners are engineered and tested to find out the optimal architecture for ultrafast and precision laser beam scanning. Remarkable 1,000 m/s scan speed is achieved on the substrate, and thanks to the resulting low pulse overlap, thermal accumulation and plasma absorption effects are avoided at up to 20 MHz pulse repetition frequencies. In order to identify optimum processing conditions for efficient high-average power laser machining, the depths of cavities produced under varied parameter settings are analyzed and, from the results obtained, the characteristic removal values are specified. The maximum removal rate is achieved as high as 27.8 mm3/min for Aluminium, 21.4 mm3/min for Copper, 15.3 mm3/min for Stainless steel and 129.1 mm3/min for Al2O3 when full available laser power is irradiated at optimum pulse repetition frequency.

Laser micro processing using a high power single-mode continuous wave fibre laser in combina-tion with a fast galvanometer scanner as well as an ultra fast polygon scan systems was investigated. As a key technology in high rate laser ablation a maximum laser power up to 3 kW and scan speeds up to 18,000 m/minwere applied. With the ultra fast laser beam deflection and laser a smallfocal spot diameter of 21 µm laser dwell times less than 100 nanoseconds were achieved. As a result laser intensities comparable to the q-switched lasers in the range of

Laser micro sintering (LMS) was developed by the research group at University of Applied Sciences Mittweida and the associated Laserinstitut Mittelsachsen e.V. as the result of research started in 2001 with a project on the possibility of generating parts by selective laser sintering (SLS) with improved resolution. For the successful generation of solid bodies from various metal powders the technology uses essentially sub-micrometer powders, a cylindrical coating blade and a q-switched solid state laser. The resolution and the surface roughness are by more than one order of magnitude better than those achieved by previous selective laser sinter technologies. Presently the technology shows advancements in selective laser sintering of highly resolved specimens of densely sintered Al2O3 and SiC ceramics too. This paper reports the process mechanism of LMS and its principal differences compared to other SLS methods. A variety of laser micro sintered parts from different metals and the newest results in laser ...

Since the discovery of selective laser sintering/melting, numerous modifications have been made to upgrade or customize this technology for industrial purposes. Laser micro sintering (LMS) is one of those modifications: Powders with particles in the range of a few micrometers are used to obtain products with highly resolved structures. Pulses of a q-switched laser had been considered necessary in order to generate sinter layers from the micrometer scaled metal powders. LMS has been applied with powders from metals as well as from ceramic and cermet feedstock’s to generate micro parts. Recent technological progress and the application of high brilliant continuous laser radiation have now allowed an efficient laser sintering/melting of micrometer scaled metal powders. Thereby it is remarkable that thin sinter layers are generated using high continuous laser power. The principles of the process, the state of the art in LMS concerning its advantages and limitations and furthermore the latest results of the recent development of this technology will be presented. Laser Micro Sintering / Laser Micro Melting (LMM) offer a vision for a new dimension of additive fabrication of miniature and precise parts also with application potential in all engineering fields.

Additive manufacturing (AM) offers a quick and efficient method for prototyping and small batch production with a short delivery time or even individual parts. Components of high geometrical complexity can be directly shaped from metals, metal-alloys, metal-matrix composites and from ceramic materials. By a suitable choice of the process parameters AM produced components can meet the demanding requirements of the aerospace, automotive and biomedical industries. Laser micro sintering (LMS) as a further development of selective laser sintering uses fine grained powders below 10 µm to produce high accuracy 3D structures (

Since the discovery of selective laser sintering/melting, numerous modifications have been made to upgrade or customize this technology for industrial purposes. Laser micro sintering (LMS) is one of those modifications: Powders with particles in the range of a few micrometers are used to obtain products with highly resolved structures. Pulses of a q-switched laser had been considered necessary in order to generate sinter layers from these µm-scaled metal powders. However, despite the high resolution, the process repeatability of LMS and the material property of the products have never been completely satisfactory. Recent technological and theoretical progress and the application of brilliant continuous laser radiation have now allowed for efficient laser melting of µm-scaled metal powders. Thereby, it is remarkable that thin sinter layers are generated with a very high laser power. The resulting product resolution is comparable to the one achieved by the LMS regime with q-switched pulses. From the experimental results the performance and potential of this high resolution laser melting regime is demonstrated and the limits of the applicable parameters are deduced.

High-throughput ultrashort pulse laser machining is investigated on various industrial grade metals (aluminum, copper, and stainless steel) and Al2O3 ceramic at unprecedented processing speeds. This is achieved by using a high-average power picosecond laser in conjunction with a unique, in-house developed polygon mirror-based biaxial scanning system. Therefore, different concepts of polygon scanners are engineered and tested to find the best architecture for high-speed and precision laser beam scanning. In order to identify the optimum conditions for efficient processing when using high-average laser powers, the depths of cavities made in the samples by varying the processing parameter settings are analyzed and, from the results obtained, the characteristic removal values are specified. For overlapping pulses of optimum fluence, the removal rate is as high as 27.8 mm3/min for aluminum, 21.4 mm3/min for copper, 15.3 mm3/min for stainless steel, and 129.1 mm3/min for Al2O3, when a laser beam of 187 W average laser powers irradiates. On stainless steel, it is demonstrated that the removal rate increases to 23.3 mm3/min when the laser beam is very fast moving. This is thanks to the low pulse overlap as achieved with 800 m/s beam deflection speed; thus, laser beam shielding can be avoided even when irradiating high-repetitive 20-MHz pulses.

Laser microsintering of tungsten powder is investigated as a function of laser output power, pulse interval and vacuum level. The intensities are calculated for the evaporation thresholds of tungsten powder particles of various sizes. In addition, the powder layer generation and the resulting layer thicknesses are calculated. The powder abrasion occurring during the process was taken into consideration. Polished sections and REM images were prepared in order to analyse the experimental outcomes. The dependence of sinter density on the parameters is discussed.

Laser micro sintering, a modification of selective laser sintering for the freeform fabrication of micro-parts, has been developed continuously since its first application. The main feature of the original regime was the application of non-overlapping q-switched pulses which was required because of the poor density of the powder layers and the concomitant need for a compacting effect during the laser sinter process. Next to some unanticipated positive side effects of this regime, poor compactness of the products had to be taken into account frequently as a detrimental consequence. Recently an upgraded coating routine has been developed that contains a compaction-step of the powder coating prior to the sintering of each layer. After appropriate adaption of the laser regime micro-parts with considerably higher densities can be achieved now. Process observations and the properties of the sintered solids give evidence that a different sinter mechanism takes place with its own negative and positive side effects. The conclusion can be made that, with sufficiently compacted powder layers and sufficient control of the coating, even laser micro sintering with continuous radiation might be feasible. Direct generation of highly miniaturized machines with encased freely movable entities by the upgraded technique has been proved. The new technology is named HD-LMS (“High Density-Laser Micro Sintering”).Laser micro sintering, a modification of selective laser sintering for the freeform fabrication of micro-parts, has been developed continuously since its first application. The main feature of the original regime was the application of non-overlapping q-switched pulses which was required because of the poor density of the powder layers and the concomitant need for a compacting effect during the laser sinter process. Next to some unanticipated positive side effects of this regime, poor compactness of the products had to be taken into account frequently as a detrimental consequence. Recently an upgraded coating routine has been developed that contains a compaction-step of the powder coating prior to the sintering of each layer. After appropriate adaption of the laser regime micro-parts with considerably higher densities can be achieved now. Process observations and the properties of the sintered solids give evidence that a different sinter mechanism takes place with its own negative and positive side effect...

Many materials have already been investigated for laser micro sintering. Nearly all technical metals can be processed with this rapid prototyping technology. A new field of investigation is the sintering of ceramics. For industrial and also for medical, especially dental, application silicon dioxide is a multiply applicable material. One of its interesting features is that the properties of the resulting material can be varied between ceramic on the one and vitreous on the other side, depending on the extent of crystalline or amorphous character of the nano-scale structure. A special problem with laser micro sintering of ceramics is the poor absorption of Nd:YAG laser radiation by most of the materials. Besides that, laser micro sintering of ceramics, in contrary to the process with metals, is not merely a series of aggregational transitions. A solution for the micro part generation of SiO2 is reported. Typical laser sintering results from this material are presented. Material specific behaviors during laser micro sintering are discussed.© (2008) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The selective laser micro sintering (SLS), which was developed by the Laserinstitut Mittelsachsen e.V. (Ebert 1999) is used for generation of micro parts. Tungsten and tungsten alloys (especially tungstenaluminium) were employed as the first usable metal powders for laser micro sintering (Regenfuss 2003), (Exner 2003). Meanwhile many new materials were investigated (Regenfuss 2005).

This paper discusses results obtained in high-rate laser microprocessing by using a high average power high-pulse repetition frequency ultrashort pulse laser source in combination with an in-house developed polygon scanner system. With the recent development of ultrashort pulse laser systems supplying high average power of hundreds watts and megahertz pulse repetition rates, a significant increase of the productivity can potentially be achieved in micromachining. This permits upscaling of the ablation rates and large-area processing, gaining increased interest of the ultrashort pulse laser technology for a large variety of industrial processes. However, effective implementation of high average power lasers in microprocessing requires fast deflection of the laser beam. For this, high-rate laser processing by using polygon scanner systems provide a sustainable technological solution. In this study, a picosecond laser system with a maximum average power of 100 W and a repetition rate up to 20 MHz was used. In raster scanning using the polygon scanner, the laser beam with a focus spot diameter of 44 μm was deflected with scan speeds of several hundred meters per second. The two-dimensional scanning capability of the polygon scanner supplied a scan field of 325 × 325 mm. The investigations were focused on high-rate large-area laser ablation of technical grade stainless steel as well as selective thin film ablation from bulk substrates. By variation of the processing parameters laser fluence, as well as temporal and spatial pulse-to-pulse distance, their impact onto the ablation process was evaluated with respect to the ablation rate, processing rate, surface quality, and ablation efficiency.

Under consideration of already published approaches, energy absorption and conversion of a laser beam penetrating into a powder layer is described as a function of grain size, grain density, laser beam intensity, and material properties. The simulations are based on a ray tracing algorithm and show typical results of the early dissipation phase regarding the energy form and the spatial distribution in the irradiated powder. An approach for the estimation of the powder layer thickness is proposed; optimum thicknesses are derived. The findings are applied to interpret observations that have been made during laser micro sintering of molybdenum powder.

PurposeThe purpose of the paper is the elucidation of certain mechanisms of laser material processing in general and laser micro sintering in particular. One major intention is to emphasize the synergism of the various effects of q‐switched laser pulses upon metal and ceramic powder material and to point out the non‐equilibrium character of reaction steps.Design/methodology/approachRecent results and observations, obtained in development of “laser micro sintering,” are surveyed and analyzed. By breaking down the overall process into relevant steps and considering their possible kinetics, an approach is made towards interpreting specific phenomena of laser micro sintering. Thermodynamics upon heating of the material as well as its photo‐electronic response to the incident radiation are considered.FindingsThe findings corroborate a model whereby short pulses of high intensity provide non‐equilibrium pressure conditions at the location of incidence, that allow for the melting of metal powder with an almost immediate expansion of a plasma and/or vapor bulb. Thereby the molten material is condensed and propelled towards the substrate. A final boiling eruption after each pulse is the reason for the morphology of the laser micro‐sintered surfaces and can prevent oxidation when the process is conducted under normal atmosphere. In sintering of ceramics, the short pulsed and intensive radiation increases the chance to excite the material even with photon energies below the bandgap value and it lowers the risk of running into a destructive avalanche.Research limitations/implicationsOwing to the stochastic character of the respective sintering event, that is initiated by each individual pulse, the gathered data are not suitable yet for the formulation of an exact quantitative function between sintering behavior and laser parameters.Practical implicationsThe qualitative findings yield a good rule of thumb for the choice of parameters in laser sintering on a micrometer scale and the model is conducive for advanced interpretation of other phenomena in laser material processing besides sintering.Originality/valueThe kinetics and thermodynamics of laser sintering with q‐switched pulses are approached by a qualitative explanation. The heterogeneous and non‐equilibrium character of the processes is taken into account; this character is often neglected by researchers in the area.

Laser micro sintering, a modification of selective laser sintering for freeform fabrication of micro-parts, was continuously upgraded since its first application. Poor density of the powder layers has been a persisting problem that had to be dealt with from the beginning. One solution was the application of high intensity q-switched laser pulses. Compaction of the material and improvement of the sinter resolution was achieved. But with these pulse-regimes only limited density of the sintered body has been achievable. Recently special efforts have been made to get rid of or at least reduce these drawbacks by markedly higher compaction of the respective powder layers. There is clear evidence that with sufficiently compacted powder layers even laser micro sintering with continuous radiation should be feasible. Till recently laser sintering of metal had been applied mainly to produce monolithic components. With the upgraded technique direct generation of micro devices with freely movable subassemblies can be possible.

Silicon carbide – a solid with covalent bonds - is conventionally synthesized via the Acheson process. Usually solid bodies of silicon carbide with definite shapes are generated from the grained material via hot isostatic pressing or liquid phase sintering. Both processes are conducted under well-controlled temperature regimes. Applying the freeform fabrication technique “Laser Micro Sintering” poses a big challenge to experimental skill due to the nonequilibrium conditions that are characteristic features of laser material processing. Successive layers SiC layers with a thickness of 1μm were processed with coherent radiation of 1064 nm. The specific behavior of two different silicon carbide powders - one of them blended with additives - are reported along with interpretational approaches.