Heidi Piili

Monitoring of temperature profiles and surface morphologies during laser sintering of alumina ceramics

Abstract Additive manufacturing of alumina by laser is a delicate process and small changes of processing parameters might cause less controlled and understood consequences. The real-time monitoring of temperature profiles, spectrum profiles and surface morphologies were evaluated in off-axial set-up for controlling the laser sintering of alumina ceramics. The real-time spectrometer and pyrometer were used for rapid monitoring of the thermal stability during the laser sintering process. An active illumination imaging system successfully recorded the high temperature melt pool and surrounding area simultaneously. The captured images also showed how the defects form and progress during the laser sintering process. All of these real-time monitoring methods have shown a great potential for on-line quality control during laser sintering of ceramics.

Correlation between pyrometer monitoring and active illuminaton imaging of laser assisted additive manufacturing of stainless steel

Laser assisted additive manufacturing which can e.g. be used for manufacturing of 3-D parts from stainless steel powder layer-by-layer by assist of laser beam. Stainless steel is molten selectively with laser beam and solidified layers of material, attached to each other, form desired 3-D structure.Nowadays range of materials that can be used for laser assisted additive manufacturing is widened and high automation level of equipments has made this method more interesting process for manufacturing. This is why also methods to control the process have started to gain an essential role in laser assisted additive manufacturing processes. Control of process parameters is also essential to get good appearance, good quality and good strength properties of work piece still maintaining the optimum processing speed. Control of temperature in of process is most important of all numerous process factors.The objective of this study was to monitor selective laser melting process simultaneously with pyrometer and video camera system with active illumination. Aim of this study was also to find out correlation between phenomena captured with imaging system and pyrometer measurement data.All tests in this study were carried out at Lappeenranta University of Technology with trial set-up of IPG 200 W SM CW fiber laser and a layer of metal powder. An inert atmosphere was simulated by nitrogen gas flow during process. Material used in this study was EOS StainlessSteel PH1.Results indicate that several phenomena observed from video imaging system had correlation to pyrometer measurements. This makes pyrometer monitoring suitable method for controlling laser sintering process.Laser assisted additive manufacturing which can e.g. be used for manufacturing of 3-D parts from stainless steel powder layer-by-layer by assist of laser beam. Stainless steel is molten selectively with laser beam and solidified layers of material, attached to each other, form desired 3-D structure.Nowadays range of materials that can be used for laser assisted additive manufacturing is widened and high automation level of equipments has made this method more interesting process for manufacturing. This is why also methods to control the process have started to gain an essential role in laser assisted additive manufacturing processes. Control of process parameters is also essential to get good appearance, good quality and good strength properties of work piece still maintaining the optimum processing speed. Control of temperature in of process is most important of all numerous process factors.The objective of this study was to monitor selective laser melting process simultaneously with pyrometer and video ...

Color change in laser cutting of paper material

Laser technology has been developed for cutting of paper materials since the middle of 1970’s. Due to high cost of equipment and maintenance the laser technology has been applied in paper industry only in 1990’s. During its development in the field of processing with different wood-based materials laser technology was confronted with problem of color change in the area of interaction between laser beam and paper material. The main problems in laser cut paper are yellowing and carbonization of the cutting edge. It is commonly known that lignin is the main cause of color change. Due to its optical characteristics lignin is a good absorber of light of various wavelengths compared to other wood compounds like cellulose and hemicelluloses. But structure of lignin is still unresolved nowadays and only few studies concentrate on the phenomena of interaction between laser beam and paper material.The aim of this study was to clarify the nature of color change of the samples which were exposed to influence of light with different wavelength. CO2 laser was employed to carry out cutting of paper material samples. This choice was caused by its high suitability for cutting of wood based materials. CO2 laser operated with wavelength of 10600 nm which is easily absorbed by paper material. The experimental part covers measurement of optical characteristics of the samples after laser cutting as well as during storage time of 21 days. The effect of laser parameters, such as laser power and cutting speed, was considered.Laser technology has been developed for cutting of paper materials since the middle of 1970’s. Due to high cost of equipment and maintenance the laser technology has been applied in paper industry only in 1990’s. During its development in the field of processing with different wood-based materials laser technology was confronted with problem of color change in the area of interaction between laser beam and paper material. The main problems in laser cut paper are yellowing and carbonization of the cutting edge. It is commonly known that lignin is the main cause of color change. Due to its optical characteristics lignin is a good absorber of light of various wavelengths compared to other wood compounds like cellulose and hemicelluloses. But structure of lignin is still unresolved nowadays and only few studies concentrate on the phenomena of interaction between laser beam and paper material.The aim of this study was to clarify the nature of color change of the samples which were exposed to influence of light...

Interaction between laser beam and paper materials

It is commonly known that laser beam is suitable for cutting of paper materials. Even tough laser technology has been applied to paper material cutting since 1970’s; there exists quite a few published research results about issue.Purpose of this literature review is to give comprehensive point of view about interaction between laser beam and paper materials by examining the issue from different aspects: • Optical phenomena of interaction • Absorption phenomena of interaction • Thermo-chemical phenomena of interaction When realized that paper materials is a miniature 3D net structure consisting fibres, pores and small pigment particles, it can be foreseen that there are many optical boundaries and surfaces inside material that seems to have even smooth surface for outside observer. Actually paper is optically much more complicated than a metal surface. That is why interaction of laser light can be transmitting, reflecting, scattering, refracting, diffracting, absorbing etc., when it interacts with paper material and the phenomena happens both on surface and inside the structure.There exist only a few articles about the absorption of laser beam to paper materials. The problem of these articles is that they concentrate in visible wavelength range and usually the used power is low. Based on literature, it was found that paper materials have absorption maximum in wavelength ranges of around 3 µm and 8-10 µm.It is also important to understand what happens to paper material at high temperatures. Chemical mechanism of thermal decomposition of cellulose (basic component of paper materials) in laser processing is called pyrolysis; solid cellulose decomposes at high temperatures and in absence of oxygen into small volatile compounds, like COx, water and furanes.It is commonly known that laser beam is suitable for cutting of paper materials. Even tough laser technology has been applied to paper material cutting since 1970’s; there exists quite a few published research results about issue.Purpose of this literature review is to give comprehensive point of view about interaction between laser beam and paper materials by examining the issue from different aspects: • Optical phenomena of interaction • Absorption phenomena of interaction • Thermo-chemical phenomena of interaction When realized that paper materials is a miniature 3D net structure consisting fibres, pores and small pigment particles, it can be foreseen that there are many optical boundaries and surfaces inside material that seems to have even smooth surface for outside observer. Actually paper is optically much more complicated than a metal surface. That is why interaction of laser light can be transmitting, reflecting, scattering, refracting, diffracting, absorbing etc., when it interacts with paper m...

Design of a microdistillation column

Distillation is the predominantly used separation method in chemical engineering. The device presented in this paper is called a microdistillation column, because the distilled volumes are measured in milliliters and dimensions in millimeters or less. There are two main reasons for a small distillation unit; firstly, it could theoretically be used in production, maybe in series with many such units. It could be used for example for hazardous or expensive materials, or in any case for applications which do not require large volumes to be distilled. More importantly, however, it could be used to study the distillation process in very small scale for the industrial scale device. Process development could then move from laboratory scale straight to industrial scale without need for expensive pilot plants.This study focuses on the design and manufacturing process of such a microdistillation column. To the knowledge of the authors it is the smallest continuous distillation column in the world at the moment. As such, the small size of the device posed several challenges in manufacturing. This case serves as a single and relatively simple example on how to design for advanced manufacturing methods; in this case laser processing. Another purpose of this paper is to give a clearer view of the advantages as well as disadvantages and limits of laser processing in small scale manufacturing. It is important to understand where and when the use of laser processing is really justified and when more conventional methods are more suitable.Distillation is the predominantly used separation method in chemical engineering. The device presented in this paper is called a microdistillation column, because the distilled volumes are measured in milliliters and dimensions in millimeters or less. There are two main reasons for a small distillation unit; firstly, it could theoretically be used in production, maybe in series with many such units. It could be used for example for hazardous or expensive materials, or in any case for applications which do not require large volumes to be distilled. More importantly, however, it could be used to study the distillation process in very small scale for the industrial scale device. Process development could then move from laboratory scale straight to industrial scale without need for expensive pilot plants.This study focuses on the design and manufacturing process of such a microdistillation column. To the knowledge of the authors it is the smallest continuous distillation column in the world at the moment. As ...

Laser assisted manufacturing of microscale chemical device of TT reactor

The latest development trend even in chemical industry is to use smaller components even in mill and micro meter scale. Microscale devices and systems have a number of advantages for process development and reaction studies in chemical engineering. Microreactors are small-scale reactors or other processing units for chemical industry with typical channel or chamber diameters in the range of 10-500 micrometers. One of the main features of microreactors is their high surface area to volume ratio, which leads to high heat and mass transfer rates. The advantages of microreactors compared to the conventional reactors are e.g. more optimal reaction conditions, efficient temperature control and smaller volume of raw materials. In process development, number-up by microreactor units would guarantee identical process conditions and eliminate costly redesign and pilot experiments that are usually routines in conventional scale-up and thereby the development time from laboratory to production is shortened.The laser micro processing is one of the fastest spreading and developing areas of laser processing. The processes of laser micromachining are numerous: micro cutting, welding, marking, drilling etc. This wide field of different processes makes laser a novel tool for micro processing and gives lots of new ideas, solutions and applications for designing of microscale process devices for chemical industry. With the use of lasers engineers are given an opportunity to design reactors based on process rather than manufacturing technologies.Aim and purpose of this study was to manufacture a microstructured TT-reactor with laser processing. Material used for this study was HDPE (high density polyethylene) and laser equipment used were IPG 200 W fibre laser and Laserline 200 W diode laser. The lasers were selected in order to test the productivity and reached quality of low power CW lasers.It was concluded that laser processing provides a unique tool for industrial manufacturing of these devices, even in planning and testing stage of these devices in laboratory cases. When a certain structure is proven to be most efficient in a laboratory, number-up means only production of larger number of similar structures.The latest development trend even in chemical industry is to use smaller components even in mill and micro meter scale. Microscale devices and systems have a number of advantages for process development and reaction studies in chemical engineering. Microreactors are small-scale reactors or other processing units for chemical industry with typical channel or chamber diameters in the range of 10-500 micrometers. One of the main features of microreactors is their high surface area to volume ratio, which leads to high heat and mass transfer rates. The advantages of microreactors compared to the conventional reactors are e.g. more optimal reaction conditions, efficient temperature control and smaller volume of raw materials. In process development, number-up by microreactor units would guarantee identical process conditions and eliminate costly redesign and pilot experiments that are usually routines in conventional scale-up and thereby the development time from laboratory to production is shortened.The laser ...

Preliminary characterization of phenomena occurring during single track fabrication in laser additive manufacturing of stainless steel

Laser additive manufacturing (LAM) is a layer wise fabrication technology which enables the production of complex shaped, individually designed parts with mechanical properties comparable to conventionally manufactured parts. However, the part manufacturing is relatively slow and via this whole production feasibility is not yet totally studied for real series production, as findings from literature shows. It is obvious that many of those studies are carried out in companies “behind locked doors” and because of this whole era of research is suffering of this lack of information.Even though the throughput time from idea to real metal product is short, the throughput time of the actual LAM phase could still be improved to gain more feasible fabrication method. Due to this, it is necessary to increase the build rate in order to improve the process efficiency and also improve whole production feasibility of LAM. It was observed that there are only few public studies about process efficiency of laser additive m...

Effect of process parameters to monitoring of laser assisted additive manufacturing of alumina ceramics

Nowadays the widening range of materials suitable for laser assisted additive manufacturing (laser sintering and laser direct melting) and high automation level of equipments has made this method more interesting process for rapid manufacturing. Also use of alumina (Al2O3) as a raw material in these cases has raising interest among different industries, since has very favorable properties like high hardness and high melting point. Alumina is used industrially for example as abrasive, filler, isolator, catalyst and catalyst support.Laser assisted additive manufacturing of alumina has been very difficult according to literature. There exist a lot of methods to build-up 3-D structure of work piece with the assist of laser beam. In those cases, typically a binder is mixed to alumina and laser melts/evaporates this binder which is joining the particles together thus giving the shape to work piece, such that in final construction alumina particles are very close to each others. This is followed with post-heating during which the closely packed alumina particles are actually sintered/melted together. There are only a few articles of direct laser beam sintering of alumina. In this case laser beam directly melts material and a 3-D structure is formed from alumina powder layer-by-layer as solidified layers of material build on top of each others.The aim of this study was to examine effect of heat profiles by changing laser power and scanning speed to monitoring of additive manufacturing with direct laser melting /sintering of alumina ceramics. The monitoring was done by using spectrometer, pyrometer and video camera system with active illumination. All tests in this study were carried out with a commercial laser sintering facility EOS M270 installed at Stockholm University (Sweden) consisting of IPG 200W fiber laser and inert atmosphere. The pure alumina powder was used as precursor material.Process was examined with fixed monitoring devices previously mentioned. The obtained results were collected to be for afterwards analyzed. The microstructure of laser sintered alumina ceramics was characterized both by optical microscope and scanning electron microscope. Results indicate that as laser assisted additive manufacturing is a sensitive process; also change of the process parameters has strong effect to monitoring results. This could also be noticed from micrographs taken from sintered parts of alumina.Nowadays the widening range of materials suitable for laser assisted additive manufacturing (laser sintering and laser direct melting) and high automation level of equipments has made this method more interesting process for rapid manufacturing. Also use of alumina (Al2O3) as a raw material in these cases has raising interest among different industries, since has very favorable properties like high hardness and high melting point. Alumina is used industrially for example as abrasive, filler, isolator, catalyst and catalyst support.Laser assisted additive manufacturing of alumina has been very difficult according to literature. There exist a lot of methods to build-up 3-D structure of work piece with the assist of laser beam. In those cases, typically a binder is mixed to alumina and laser melts/evaporates this binder which is joining the particles together thus giving the shape to work piece, such that in final construction alumina particles are very close to each others. This is followed with post-heatin...

Laser welding of micro-VLE-measurement device and its practical application

The knowledge of phase equilibrium is critical for the modeling and operation of reactors and separation units. The use of incorrect vapor-liquid equilibrium (VLE) data for distillation leads to tower malfunction with varying end results. The only reliable method for obtaining valid VLE data for a non-ideal system is to measure it. When studying components that are either very expensive or hazardous the amount of chemicals used is preferably minimized. Typical volume of chemicals used in a VLE-measurement is 100 cm3 or above. In a VLE-measurement the temperature, pressure and composition of both phases are determined for a range of concentrations.The laser micro/fine processing is one of the fastest spreading and developing areas of all laser processes in the world. The wide field of applications makes laser a novel tool for micro processing and gives lots of new ideas, solutions, opportunities and applications for designing these milli and micro scale process devices for chemical industry.In this study an exceptionally small VLE-measurement device was designed and manufactured by utilization of laser processing. Even though the application itself is in micro scale the laser processing used in fine processing scale gave opportunity to reach the minimum volume. Laser welding has unique possibilities for this kind of welding when heat input can be controlled and only small heat affected zone and thereby minor distortions are caused. Laser welding also enables welding of demanding structures, like this micro-VLE-device.The volume of the measurement cell of micro-VLE-device was approximately 2.5 cm3, which was made possible by using the pressure transducer cavity as the equilibrium cell. The chemical consumption is therefore reduced by up to a factor of 50. The valves were also welded to the structure. The welding would not have been possible with conventional methods due to overheating of the transducer electronics. The cell was initially tested by measuring pure component vapor pressures of alkanes.The knowledge of phase equilibrium is critical for the modeling and operation of reactors and separation units. The use of incorrect vapor-liquid equilibrium (VLE) data for distillation leads to tower malfunction with varying end results. The only reliable method for obtaining valid VLE data for a non-ideal system is to measure it. When studying components that are either very expensive or hazardous the amount of chemicals used is preferably minimized. Typical volume of chemicals used in a VLE-measurement is 100 cm3 or above. In a VLE-measurement the temperature, pressure and composition of both phases are determined for a range of concentrations.The laser micro/fine processing is one of the fastest spreading and developing areas of all laser processes in the world. The wide field of applications makes laser a novel tool for micro processing and gives lots of new ideas, solutions, opportunities and applications for designing these milli and micro scale process devices for chemical industry.In this study a...

Laser scribing of stainless steel with and without work media

The advantages such as tight restrictions for heat and mass transfer make micro-/milli scale devices of mixing and droplet formation viable to become widely used in specialty chemical industry. Small dimensions and simple geometry ensure laminar flow and mixing through diffusion, ensuring well-defined behavior of mixing and short reaction times. Combining dry etching by laser beam with wet chemical etching is expected to reduce the production costs of these novel devices.In this study the manufacturing of grooves in stainless steel SS 316L by means of laser micro-/milli processing was investigated. Deep and narrow channels with depth to width ratio of 1:1 at least are preferable, width is allowed to vary from 10-500 micrometer. Lasers used for this study were: 5 kW IPG YLR-5000 S, 200 W IPG YLS-200-SM-WC, 1 kW IPG YLR-1000-SM fiber lasers, 400 W Powerlase diode pumped Nd:YAG laser and 14 W diode pumped Nd:YVO4 laser. Chemical assisted laser scribing is also included in this study.Preliminary results show possibility to obtain channels with desired parameters in pulsed mode laser machining. However, investment cost for CW (continuous wave) lasers per kilowatt are in order of magnitude smaller than for pulsed lasers, that’s why the study was essentially focusing on lasers operating in CW mode. Optimal scribing parameters were defined by adjusting laser power, number of repetitions and speed. Preliminary experiments done without any media resulted in low quality grooves with moderate depth and burned edges. It was concluded in this study that finding a suitable chemical to improve to scribing process is a key moment of getting channels with acceptable quality.The advantages such as tight restrictions for heat and mass transfer make micro-/milli scale devices of mixing and droplet formation viable to become widely used in specialty chemical industry. Small dimensions and simple geometry ensure laminar flow and mixing through diffusion, ensuring well-defined behavior of mixing and short reaction times. Combining dry etching by laser beam with wet chemical etching is expected to reduce the production costs of these novel devices.In this study the manufacturing of grooves in stainless steel SS 316L by means of laser micro-/milli processing was investigated. Deep and narrow channels with depth to width ratio of 1:1 at least are preferable, width is allowed to vary from 10-500 micrometer. Lasers used for this study were: 5 kW IPG YLR-5000 S, 200 W IPG YLS-200-SM-WC, 1 kW IPG YLR-1000-SM fiber lasers, 400 W Powerlase diode pumped Nd:YAG laser and 14 W diode pumped Nd:YVO4 laser. Chemical assisted laser scribing is also included in this study.Preliminary results show ...