Laser-induced spallation of minerals common on asteroids

The ability to deflect dangerous small bodies in the Solar System or redirect profitable ones is a necessary and worthwhile challenge. One well-studied method to accomplish this is laser ablation, where solid surface material sublimates, and the escaping gas creates a momentum exchange. Alternatively, laser-induced spallation and sputtering could be a more efficient means of deflection, yet little research has studied these processes in detail. We used a 15-kW Ytterbium fiber laser on samples of olivine, pyroxene, and serpentine (minerals commonly found on asteroids) to induce spallation. We observed the process with a high-speed camera and illumination laser, and used X-ray micro-tomography to measure the size of the holes produced by the laser to determine material removal efficiency. We found that pyroxene will spallate at power densities between 1.5 and 6.0 kW cm(-2), serpentine will also spallate at 13.7 kW cm(-2), but olivine does not spallate at 1.5 kW cm(-2) and higher power densities melt the sample. Laser-induced spallation of pyroxene and serpentine can be two- to three-times more energy efficient (volume removed per unit of absorbed energy) than laser-induced spattering, and over 40x more efficient than laser ablation.Peer reviewe

Laser processing of minerals common on asteroids

Asteroid mining and redirection are two trends that both can utilize lasers, one to drill and cut, the other to ablate and move. Yet little is known about what happens when a laser is used to process the types of materials we typically expect to find on most asteroids. To shed light on laser processing of asteroid material, we used a 300-W, pulsed Ytterbium fiber laser on samples of olivine, pyroxene, and serpentine, and studied the process with a high-speed camera and illumination laser at 10 000 frames per second. We also measure the sizes of the resulting holes using X-ray micro-tomography to find the pulse parameters which remove the largest amount of material using the least amount of energy. We find that at these power densities, all three minerals will melt and chaotically throw off spatter. Short, low-power pulses can efficiently produce thin, deep holes, and long, high-power pulses are more energy efficient at removing the most amount of material.Peer reviewe

Morfologi för laserhybridsvetsar

This thesis is about formation of surface imperfections formed in welding when using the manufacturing methods of laser welding and laser arc hybrid welding. In hybrid welding a traditional arc welding source and a laser share the same melt pool, making the process even more complex. Laser welding is often considered as a non-traditional but highly advanced manufacturing technique in industry. As more is getting known about these advanced welding techniques, coupled with reduced prices of laser sources, the interest in industry gradually increases. In welding, control over the quality is essential, particularly to suppress imperfections and unfavourable surface geometries, like undercuts. The mechanical behaviour of a product in service, in particular fatigue life, can suffer from these small, sometimes hardly visible weld imperfections. The final weld quality results from a very complex process, involving non-linear multi-physics. The documentation of parameters, process conditions and the resulting quality is also complex, difficult and so far unsatisfactory. Therefore, the survey manuscript Paper i address the mechanisms and challenges for the documentation of knowledge in laser welding. In addition to the survey manuscript, six journal publications utilize the study of macrographs, surface scanning and High Speed Imaging as methods for capturing and identifying why weld surface imperfection formation can take place. Paper I studies the surface geometry of welds resulting from fibre laser welding with various parameters, also applying the new documentation method called Matrix Flow Chart, MFC. In Paper II, III, IV and VI the formation and shape of undercuts under various circumstances are compared, mapped, analysed and explained. When causes for undercuts are known, counter measures are suggested. Paper V is about accepting undercut formation and instead “repairing” the welded surface by re-melting it with a defocussed laser, effectively eliminating previous surface imperfections. Finally, Paper VI also provides a survey on undercuts in welding and describes the different parameter causes and physical mechanisms.Godkänd; 2014; 20140401 (jankar); Nedanstående person kommer att disputera för avläggande av teknologie doktorsexamen. Namn: Jan Frostevarg Ämne: Produktionsutveckling/Manufacturing Systems Engineering Avhandling: The Morphology of Laser Arc Hybrid Welds Opponent: Professor Veli Kujanpää, VTT Technical Research Centre of Finland, Lappeenranta, Finland Ordförande: Professor Alexander Kaplan, Avd för produkt- och produktionsutveckling, Institutionen för teknikvetenskap och matematik, Luleå tekniska universitet Tid: Torsdag den 5 juni 2014, kl 09.00 Plats: E632, Luleå tekniska universite

Differences between arc models in laser hybrid arc welding upon weld bead stability and undercut formation

In this study, three different arc modes are studied in laser hybrid arc welding with a gasmetal arc, i.e. Standard, Pulsed and Cold Metal Transfer mode. Originally developed forbeing able to weld thin materials, the pulsed mode is the favoured arc mode in both ordinaryarc welding and hybrid welding. The pulsed mode is a more controlled gas metal arc weldingprocess that uses less heat and is able to weld thinner materials than the spray mode processwith globular drop transfer. The cold metal transfer mode utilizes surface tension droptransfer, compared to the free flying drops governing the other modes and is thus even morecontrolled than the pulsed mode. The cold metal transfer mode is much colder than the otherarc modes and is considered to generate less undercuts and spatter than the other modes, byboth developers and users alike.This study compares welds made by the three arc modes for both low and highdeposition rates. The welds are studied by macrographs, scanning and high speed imaging.This study shows that the differences between drop transfer modes are partially eliminateddue to the presence of a laser keyhole. The main arguments to use either arc mode arediscussed.Godkänd; 2013; 20131210 (andbra

Laboratory experiments with a laser-based attachment mechanism for spacecraft at small bodies

We present the results of two sets of experiments that investigate laser-based metal-to-rock attachment techniques. Asteroids and comets have low surface gravity which pose a challenge to landers with moving parts. Such parts can generate torques and forces which may tip the lander over or launch it into deep space. Thus, if a lander on a small body is to have moving parts, the spacecraft must be equipped with an anchoring mechanism. To this end, we sought to use a laser to melt and bind a piece of metal mimicking a part of a spacecraft to a rock mimicking the surface of a typical asteroid. In the first set of experiments, extra material was not fed in during the processing. The second set were performed using a standard wire feeder used in laser welding, which added metal to the experiment during processing. During the first experiments, we discovered that a traditional weld, where two melt pools mix and solidify to form a strong bond, was not possible-the melt pools would not mix, and when they did, the resulting weld was extremely brittle. The second set of experiments resulted in a physico-mechanical bond, where a hole was drilled with a laser, and a wire was melted and fed into the hole. These latter experiments were successful in forming bonds as strong as 115 N. Such an attachment mechanism can also be used to maneuver small boulders on asteroid surfaces, to redirect small, monolithic asteroids, or in space-debris removal.Peer reviewe

Dropout formation in thick steel plates during laser welding

Laser welding is a promising technique for welding thick metal sheets, which is usually used for achieving full penetration in a single weld pass. However, among the imperfections that can occur, dropout formation becomes an increasingly larger problem when the sheets to be welded get thicker. When the sheets are 15 mm or thicker it is a challenge to suppress and countermeasures to suppress dropouts get less impact. If full penetration is not achieved, the dropout formation cannot be formed and the melt flows to form better weld caps. Therefore a typical method when welding 15 mm and thicker is to use partial penetration from both sides of the joint and thereby achieving full penetration, at a cost of increasing plate handling complexities and time losses.If the mechanisms behind the dropout formation can be understood, countermeasures may be developed and applied in order to be able to laser weld thicker plates with full penetration single pass welding. In order to understand the mechanics of the formation of dropout during full penetration welding in 15 mm thick plates, experiments have been conducted using laser hybrid arc welding (LAHW) using the laser in CW mode and also applying power modulation. During the experiments, the root has been observed with High Speed Imaging (HSI) to observe the mechanisms behind the formation. It is determined that the downward flow from the keyhole along with the surface tensional forces of the molten steel and its cooling rate play the most significant roles.Godkänd; 2015; 20151127 (jankar

Hybrid Welding of 45 mm High Strength Steel Sections

Thick section welding has significant importance for oil and gas industry in low temperature regions. Arc welding is usually employed providing suitable quality joints with acceptable toughness at low temperatures with very limited productivity compared to modern high power laser systems. Laser-arc hybrid welding (LAHW) can enhance the productivity by several times due to higher penetration depth from laser beam and combined advantages of both heat sources. LAHW was applied to join 45 mm high strength steel with double-sided technique and application of metal cored wire. The process was captured by high speed camera, allowing process observation in order to identify the relation of the process stability on weld imperfections and efficiency. Among the results, it was found that both arc power and presence of a gap increased penetration depth, and that higher welding speeds cause unstable processing and limits penetration depth. Over a wide range of heat inputs, the welds where found to consist of large amounts of fine-grained acicular ferrite in the upper 60-75% part of welds. At the root filler wire mixing was less and cooling faster, and thus found to have bainitic transformation. Toughness of deposited welds provided acceptable toughness at -50 °C with some scattering

Additive Manufacturing by laser-assisted drop deposition from a metal wire

The subject of Additive Manufacturing includes numerous techniques, some of which have reached very high levels of development and are now used industrially. Other techniques such as Micro Droplet Deposition Manufacture are under development and present different manufacturing possibilities, but are employed only for low melting temperature metals. In this paper, the possibility of using a laser-based drop deposition technique for stainless-steel wire is investigated. This technique is expected to be a more flexible alternative to Laser Metal Wire Deposition. Laser Droplet Generation experiments were carried out in an attempt to accurately detach steel drops towards a desired position. High-speed imaging was used to observe drop generation and measure the direction of detachment of the drops. Two drop detachment techniques were investigated and the physical phenomena leading to the drop detachment are explained, wherein the drop weight, the surface tension and the recoil pressure play a major role. Optimised parameters for accurate single drop detachment were identified and then used to build multi-drop tracks. Tracks with an even geometry were produced, where the microstructure was influenced by the numerous drop depositions. The tracks showed a considerably higher hardness than the base wire, exhibiting a relatively homogeneous macro-hardness with a localised softening effect at the interfaces between drops.Validerad;2021;Nivå 2;2021-07-26 (beamah);Ytterligare forskningsfinansiär: EIT Raw Materials (no. 18079)LAM-4DLAM-4D stage 2SAMO

Filler metal distribution and processing stability in laser-arc hybrid welding of thick HSLA steel

Welds made by high power laser beam have deep and narrow geometry. Addition offiller wire by the arc source,forming the laser-arc hybrid welding (LAHW) process, is very important to obtain required mechanical prop-erties. Distribution of molten wire throughout the entire weld depth is of concern since it tends to have lowtransportation ability to the root. Accurate identification offiller metal distribution is very challenging. Metal-cored wires can provide high density of non-metallic inclusions (NMIs) which are important for acicular ferritenucleation. Accuratefiller distribution can be recognized based on statistical characterization of NMIs in theweld. In the present study, it was found that the amount offiller metal decreased linearly towards the root. Thefiller metal tends to accumulate in the upper part of the weld and has a steep decrease at 45–55 % depth whichalso has wavy pattern based on longitudinal cuts. Substantial hardness variation in longitudinal direction wasobserved, where in the root values can reach > 300 HV. Excessive porosity was generated at 75 % depth due tounstable and turbulent meltflow based on morphology of prior austenite grains. The delicate balance of processparameters is important factor for both process stability andfiller metal distribution.publishedVersio