As the manufacturing industry looks for better ways to create complex parts using sustainable and efficient methods, the additive manufacturing sector is taking a step forward. Additive manufacturing brings with it a large reduction in material used and the complex structures that can be created are not something to overlook. This study was focused on ways to improve additive manufacturing (AM), or more specifically Electron Beam Melting (EBM), by determining how powder characteristics were connected to build characteristics and how these factors could be improved. By using a sieve shaker and tapped density analyzer the two characteristics of particle size distribution (PSD) and tapped density were determined for three different powders. The build characteristics of surface roughness and porosity were measured for builds using the same powders. Parallels were drawn between powder and build characteristics and compared to the available and recent research. The results of the study showed that the average particle diameter was related to the powder coarseness and the PSD and that the tapped density of a powder could be improved upon by mixing coarse and fine powders. No clear connection between powder and build characteristics could be made. It was found that the powder characteristics could be improved by the mixing of the powders but how these correlates to build characteristics was not established. The data show that a coarse powder gives the densest part and that a fine powder the smoothest part. Further investigation into the process parameters and how they relate to build characteristics is needed. / I och med att tillverkningsindustrin letar efter bättre sätt att skapa komplexa delar med hållbara och effektiva metoder, tar additiv-tillverkningssektorn ett steg framåt. Additiv tillverkning för med sig en minskning av materialanvändning och de komplexa strukturer som kan skapas är inte något att förbise. Denna studie fokuserade på sätt att förbättra additiv tillverkning (AM), eller mer specifikt Electron Beam Melting (EBM), genom att fastställa hur pulveregenskaper kopplade till de slutgiltiga egenskaperna och hur dessa faktorer kunde förbättras. Genom att använda en siktskakare och tappdensitetsanalysator bestämdes två egenskaper, partikelstorleksfördelning (PSD) och tappdensitet för tre olika pulver. De slutgiltiga egenskaperna ytjämnhet och porositet mättes för kuber tillverkade av samma pulver. Paralleller drogs mellan pulveregenskaper och de slutgiltiga egenskaperna och jämfördes med ny och tillgänglig forskning. Resultaten av studien visade att den genomsnittliga partikeldiametern var relaterad till pulvrets grovhet och PSD och att tappdensiteten hos ett pulver kunde förbättras genom att blanda grova och fina pulver. Ingen tydlig koppling mellan pulveregenskaper och de slutgiltiga egenskaperna kunde göras. Det visade sig att pulvrets egenskaper kunde förbättras genom att blanda pulvren, men hur dessa korrelerar med de slutgiltiga egenskaperna fastställdes inte. Datan visar att ett grovt pulver ger den tätaste delen och att ett fint pulver den jämnaste delen. Ytterligare utredning av processparametrarna och hur de relaterar till de slutgiltiga egenskaperna behövs.

Utveckling av elmotorer under de senaste åren har lett till stora förändringar inom bilindustrin. Detta medför att industrin ständigt är i behov av utveckling för att kunna anpassa sig till nya metoder och material. Examensarbetet undersöker en del av detta. Lasersvetsning av kopparstänger, kallade hairpins, är en viktig del vid produktion av en elmotor. Vid lasersvetsning av koppar-hairpins uppstår defekter i materialet som resulterar i en dålig svetskvalitet som i sin tur leder till sämre strömflöde i elmotorn. I detta arbete utförs en litteraturundersökning tillsammans med experiment för att bestämma vilka ytbeläggningar samt hur ytgrovheten/ytfinheten påverkar lasersvetsningen av koppar-hairpins. 10 olika prover väljs fram, där förorenade, oxiderade och svartmålande ytor undersöks med slipade och polerade ytor. Resultatet visar att hairpins med en grov yta utan någon ytbeläggning har bäst svetskvalitet, medans hairpins i sin ursprungliga frästa form visar näst bäst svetskvalitet. Slutsatsen är att ytbeläggningar ökar porositeten vilket ger en lägre svetskvalitet. Fler tester behöver utföras för att säkerställa resultatet. / Development of electric motors in recent years has led to major changes in the automotive industry. This means that the industry is constantly in need of development in order to be able to adapt to new methods and materials. The thesis examines part of this. Laser welding of copper rods, called hairpins, is an important part of the production of an electric motor. During laser welding of copper hairpins, defects occur in the material which results in a poor welding quality which in turn leads to poorer current flow in the electric motor. In this work, a literature review is performed together with experiments to determine which coatings and how the roughness / surface fineness affects the laser welding of copper hairpins. 10 different samples are selected, where contaminated, oxidized and black-painted surfaces are examined with sanded and polished surfaces. The results show that hairpins with a rough surface without any coating have the best welding quality, while hairpins in their original milled shape show the second best welding quality. The conclusion is that coatings increase the porosity, which results in a lower welding quality. More tests need to be performed to ensure the result.

The present work is aimed at determining the conditions for which synthesis of coatings composed of solid solution TiBN is possible using cathodic arc evaporation, a member of the physical vapor deposition family of coating techniques. Such coatings are interesting from a machining point of view because they have previously been shown to exhibit excellent tribological properties at elevated temperatures. A combinatorial approach was employed, in which several coatings with differing B-content were grown at once. Several such coating experiments were performed with varying process parameters, e.g. gas mixture composition and temperature. Analysis of the deposited coatings was carried out using X-ray powder diffraction, scanning electron microscopy and wavelength dispersive X-ray spectroscopy to determine the crystal structure, morphology and chemical composition. The results of these measurements are discussed at length.

AbstractThe continuous advancement of medical implants technology presents exciting possibilities.Laser Powder Bed Fusion (L-PBF) additive manufactured Ti-6Al-4V alloy implants showtremendous promise as they offer the potential for highly personalized implants, improved implantfunctionality and enhanced long-term outcomes. However, the microstructure and microhardnessproperty of components produced through L-PBF are inferior when compared to theircounterparts manufactured using traditional methods. Recent investigation presents aninnovative method that has potential to address these microstructure and microhardnesschallenges. This method creates a spatially modulated Ti alloy by adding 316L powder into Ti-6Al-4V powder before printing with L-PBF. The resulting alloy shows a reduced formation ofcoarse β columnar grains into more preferred equiaxed grains. However, the effect of heattreatment operation on this spatially modulated alloy has not yet been explored.Thus, this present study reports on the effect of heat treatment operation on the microstructureand hardness property of Laser Powder Bed Fusion (L-PBF)- fabricated Ti-6Al-4V and Ti-6Al-4V-4.5wt.%316L alloys respectively. The heat treatment processes include both super- and sub- betatransus temperature (Tβ). After the heat treatment processes, the samples were either water-quenched or air-cooled. X-ray Diffraction (XRD), Light Optical Microscopy (LOM), and ScanningElectron Microscopy (SEM) were the characterization techniques performed on these alloys andthese qualitative data gathered were correlated with their microhardness measurements. The as-fabricated Ti-6Al-4V alloy exhibited a martensite α՛ microstructure. On the other hand, the Ti-6Al-4V-4.5wt.%316L alloy possessed a β grain structure. Ti-6Al-4V alloy, subjecting it to the 1020˚Csuper-transus treatment followed by water quenching resulted in the formation of new α՛martensite microstructure and fine primary alpha. When the Ti-6Al-4V specimen was subjectedto 1020˚C and then air cooled, a bimodal microstructure comprising coarse primary alpha andα+β lamellar was formed. The 920˚C sub-transus heat treatment also produced α+β lamellarstructure. For the 1020˚C water-quenched Ti-6Al-4V sample, the microhardness value increasedby 7.3% and 11% in the transverse and cross-sections to the build direction axis, respectivelywhen compared to the as-built parts. Conversely, Ti-6Al-4V-4.5wt.%316L alloy exhibited evenlydistributed primary alpha plates in beta matrix for both the water-quenched and air-cooledsamples at 1020˚C and 920˚C. On the other hand, heat treatment at 800 ˚C retains the as-builtbeta grain morphology while alpha plates precipitate along the grain boundaries. Themicrohardness results suggest that performing only transus heat treatment on the Ti-6Al-4V-4.5wt.%316L alloy was insufficient to significantly alter its hardness properties.Key words: Additive manufacturing, Ti-6Al-4V, Laser powder bed fusion, microstructure control.

This licentiate thesis focuses on the fatigue cracking behaviour of laser and hybrid laser-MAG welded structures. Beside the welding process and the resulting weld, several topics related to fatigue of welded structures are treated such as; macro and micro surface geometry, weld defects and their influence on fatigue performance of welded structures, fatigue analysis by the nominal and effective notch stress method, fatigue life prediction using LEFM (Linear Elastic Fracture Mechanics), fatigue testing, metallurgical analysis, elastic and elastic-plastic finite element analysis. The main objective is to gain understanding of the impact of weld defects and weld shape details on the fatigue behaviour of laser and hybrid laser welded joints. The first paper is a literature survey which compiled useful information regarding fracture and fatigue analysis of various welded joints. In the second paper fatigue testing by bending of laser hybrid welded eccentric fillet joints was carried out. The weld surface geometry was measured and studied in order to understand the crack initiation mechanisms. The crack initiation location and the crack propagation path were studied and compared to Finite Element stress analysis, taking into account the surface macro- and micro-geometry. Based on the nominal stress approach, SN-curves were designed for laser hybrid welded eccentric fillet joints. The competing criteria of throat depth and stress raising by the weld toe radii and by the surface ripples are explained, showing that surface ripples can be critical.The third paper is the continuation of the second paper, but studying the fatigue crack propagation of laser hybrid welded eccentric fillet joints. Microscopic analysis was carried out to identify internal weld defects. Nominal and effective notch stress analysis was carried out to compare standardized values. LEFM analysis was conducted for this joint geometry for four point bending load in order to study the effect of LOF on fatigue life. In good agreement between simulation and metallurgy, cracking starts and propagates from the lower toe, but for certain geometries alternatively from the weld bead or upper toe, even in case of Lack of Fusion, as was well be explained. Improved understanding of the crack propagation for these geometrical conditions was obtained and in turn illustrated. Lack of fusion surprisingly was not critical and only slightly lowered the fatigue life. Two dimensional linear elastic finite element analyses is carried out in the fourth paper on laser welding of a beamer in order to study the impact of geometrical aspects of the joint design and of the weld root on the fatigue performance. Critical geometrical aspects were classified and then studied by FE-analysis with respect to their impact on the fatigue behaviour. Stress comparison of full 15 mm and partial 6 mm weld penetration of the beam was done by varying the toe and root geometry to identify the critical details. Generalization of the knowledge by new methods was an important aspect, particularly to apply the findings for other joints. Together the papers provide better understanding of fatigue behaviour for complex geometries and are therefore suitable guidelines for improved weld design.

The thesis is about laser welding, in particular about geometrical defects, but it also addresses how to improve the documentation of the findings. Laser welding is an important and highly advanced manufacturing technique in industry. An emerging option is laser hybrid arc welding. In welding, control over the quality is essential, particularly to suppress defects and unfavourable surface geometries. The mechanical behaviour of a product in service can even suffer from small, hardly visible welding defects like undercuts. The process causing the quality involves complex multi-physics and is nonlinear. The documentation of parameters, process conditions and of the resulting quality is difficult and unsatisfactory so far. Therefore Paper 1 addresses the mechanisms and challenges for the documentation of laser welding knowledge. According to the knowledge lifecycle theory, these difficulties not only address how welding knowledge is generated, identified, captured and stored, but also how it is accessed, shared and eventually used. In Paper 2 the geometry resulting from fibre laser welding of a corner joint, including defects like undercuts, is studied for varying parameters. The results are categorized and mapped. Then they are documented by a new method, the Matrix Flow Chart, MFC, as an attempt to improve knowledge formalizing, storing and access, which is also discussed in Paper 1. In Paper 3 and 4 two different kinds of undercuts, caused by different metal surface conditions during the laser hybrid arc welding process, are investigated by visual analysis, SEM, chemical analysis and high speed imaging. In Paper 3 the observed findings are documented in common letter journal format as an extremely brief type of documentation. In Paper 4 the physical mechanisms of the welding process causing the two types of undercuts are explained in regular manuscript format. Here a sequential flow chart turned out to be suitable for documentation of the findings. / Avhandlingen behandlar lasersvetsning, speciellt dess geometriska defekter och hur dokumentering av genererad kunskap kan förbättras. Lasersvetsning är en viktig och avancerad tillverkningsmetod inom industrin. En variant av lasersvetsning som börjat användas är laserhybridsvetsning med MAG. Det är väsentligt inom svetsning att kunna kontrollera svetsprocessen för att få önskad kvalitet, speciellt med avseende på undertryckande av defekter och ogynnsamma ytgeometrier. De mekaniska egenskaperna för en produkt i drift kan lida av små och knappt synliga svetsdefekter, exempelvis svetsdiken. Processen som avgör den resulterande kvaliteten är multifysikalisk och ickelinjär. Dokumentering av parametrarna, processvillkor och kvalitetskriterier är svårt och i dagsläget otillfredsställande. Därför behandlar artikel 1 mekanismer och utmaningar för effektiv dokumentering av kunskap inom lasersvetsning. Enligt livscykelteorin beror inte dessa problem enbart på när kunskapen genereras, identifieras, fångas och lagrad, utan även hur den hämtas, delas och används. I artikel 2 studeras resulterande geometrier för en hörnfog med varierande parametrar. Bl.a. svetsdiken undersöks. Resultaten kategoriseras, kartläggs och dokumenteras. I ett försök att bättra kunskaps- formalisering, förvaring och åtkomst som diskuteras i artikel 1, används en ny metod kallad Matrix Flow Chart, MFC. Svetsdiken orsakas av olika villkor hos de metalliska ytorna i laserhybridsvetsning, vilket i artikel 3 och 4 två olika sorters svetsdiken undersöks med hjälp av optisk analys, SEM, kemisk analys och höghastighetsfilmning. I artikel 3 dokumenteras de observerade fynden i ett vanligt letter-format, som är en extremt kortfattad dokumenteringsmetod. I artikel 4 förklaras de fysiska mekanismerna för svetsprocessen som skapar de två typerna av svetsdiken i ett vanligt manuskriptformat. Ett sekventiellt flödesschema visas passa dokumentering av fynden.

After the laser was invented in 1960, it was not long until someone started using this powerful source of light to weld parts together. Laser welding became an industrial application during the 1970's and the field has developed ever since. In 2008 a new 15kW fibre laser was installed at Luleå University of Technology, and at the same time a considerable investment was made in new digital high speed cameras. This combination of equipment enabled research on a new level, and a first step towards a more general understanding of laser welding.This thesis presents the results acquired by analyzing high speed videos of laser welding.Qualitative and quantitative results from these high speed videos have revealed a considerable amount of information about the physics which underlies the laser welding process, including direct measurements of fluid flow within the melt pool and the interpretation of electromagnetic signals which emanate from the welding process.The thesis comprises three papers which are thematically linked by their concentration on the analysis of high speed imaging of the laser welding process.Paper A concerns the quantitative evaluation of high speed imaging of the time-dependent metal vapour jet that streams out of the laser welding vapour capillary. This work has revealed an important correlation. The output of commercially available process monitoring photodiodes (used for detecting infrared radiation) correlates with the fluctuating vapour jet above the weld, instead of, as was previously assumed, with the radiation from the molten and solid surface.In paper B, for the first time, ultra-high speed images of the surface of the laser welding vapour capillary have been obtained, (at a rate of 180 000 frames per second) with good spatial resolution and contrast. In addition, a streak technique was developed that measures and the time-dependent melt flow velocity along a selected line. Wave-like patterns that flow down the capillary have been directly observed. These phenomena are of essential importance for a basic understanding of the laser welding process and are a very powerful support for future research, e.g. for modelling and simulation.Using the above method, the velocity of the flowing vapour capillary waves was quantitatively evaluated in Paper C for varying process parameters, like laser power, focus position or welding speed, revealing clear, important trends of the laser welding process.

The thesis is about the analysis of experimental results on the laser arc hybrid welding (LAHW) process as well as about the methodology and tools behind. Due to the high potential on weld penetration and welding speed, the industrial importance of LAHW techniques has been increasing in the recent years. However the massive use of them is constrained because of high investment costs, complexity and novelty. These obstacles provoke a lack of experienced operators and they desire a base of knowledge related to optimal parameters to obtain good quality welds. In this scenario, understanding of the physical LAHW phenomena has been proven useful for better control of the process,particularly to predict and avoid groups of parameters that can originate defects. For this reason, it is valuable to carry out experiments and to systematize the analysis methods.The presented work is focused on the impact of geometrical joint fit-up properties on the weld surface quality. The papers included here are organized as a comprehensive study of the effects and impact of various geometrical aspects of the laser-arc-workpiece arrangement on the surface quality of the welds, i.e. tracking from joint fit-up tolerances like gap width to critical weld shape aspects like undercuts via observation of fluid flow at the weld pool surface. Three frequent and critical geometrical aspects were identified from industrial edge preparation, namely gap width variations, vertical edgemismatch and vertical plate position to the laser-arc tool. These aspects can cause defects when the surface is bended or when the LAHW tool is automatically moved. Although in production they arise all simultaneously, to understand the respective contribution, systematic experiments were designed. These experiments were carried out to measure and surpass the stability threshold related to each aspect, to observe the flow behaviour and to evaluate the physical phenomena related to weld bead formation.The three Papers I, II, III describe a systematic methodology based on High Speed Imaging, HSI, on quantitative weld surface measurement obtained from a laser triangulation scanner and on statistical analysis of different experimental results. This methodology is based on the observation, measurement, automatic location and calculation of fundamental bead variables (top and root undercuts, reinforcement, melt pool length, pool width and flow speed, or mass balance) related to the melt flow.In Paper I, using a pulsed arc mode, the effects of increasing the standoff between the LAHW tool and the workpiece are described and a mechanism is identified to explain the flow behaviour and its consequences on the resulting weld pool shape. In Paper II, again using pulsed arc mode in LAHW, the effects of the vertical edge mismatch and of the gap width on the weld pool shape are described, again followed by a theoretical description. Paper III compares the pulsed arc mode with the CMT mode in LAHW,again for the effect of the gap width on the weld surface shape, in particular the top bridging phenomenon that was identified. Also these results are theoretically described, explaining the mechanisms via HSI.Altogether, the three papers comprise a theoretical description of the LAHW weld shape behaviour and weld quality depending on the fit-up tolerances, based on experimental evidence and analysis. From this chain of evidence and associated understanding, conclusions were drawn and practical guidelines were derived.

This thesis concerns experimental investigations of laser cutting with theoretical and practical discussions of the results. The thesis is made up of three papers which are linked in such a way that each of them studies a different aspect of laser cutting: In paper I the two major laser types in cutting, namely CO2 and fiber lasers, are compared to each other by a self-defined cut efficiency. Next in paper II the laser cutting process is observed with a high speed imaging, HSI, camera to give information about the melt flow in the cut zone. In paper III the initiation of the laser cutting process, called piercing, is studied. Paper I is about investigating the effect of material type, material thickness, laser wavelength, and laser power on the efficiency of the cutting process for industrial state-of-the-art cutting machines. Here the cutting efficiency is defined in its most fundamental terms: as the area of cut edge created per Joule of laser energy. This paper presents phenomenological explanations for the relative cutting efficiencies of fiber lasers and CO2 lasers and the mechanisms affecting these efficiencies for stainless steels and mild steel over a range of thicknesses. The paper also involves a discussion of both theoretical and practical engineering issues.In Paper II a new experimental technique has been developed which enables High Speed Imaging of laser cut fronts produced using standard, commercial parameters. The results presented here suggest that the cut front produced when cutting 10 mm thick medium section stainless steel with a fibre laser and a nitrogen assist gas is covered in humps which themselves are covered in a thin layer of liquid. A combination of HSI results and theoretical analysis has revealed that these humps move down the cut front at an average speed which is a factor three less than the liquid flow speed. Paper III addresses a specific topic: Before any cut is started the laser needs to pierce the material. The two most important aspects of the piercing process are: a) How long does it take to pierce the material? And b) How wide is the pierced hole? If the hole is no wider than the cut line, the material can be pierced on the line to be cut. In this paper the laser piercing process is investigated using a wide range of laser pulse parameters, for stainless steel using a fibre laser, to discover their influence on pierce time and pierced hole diameter. A high speed imaging camera is used to time the penetration event and to study the laser-material interactions involved in drilling the pierced holes. Optimum parameters have been identified for both pierce time and pierce hole width.

Since the invention of the laser in 1960, its use has been growing steadily. New laser sources with high beam power and high beam quality provide potential for further growth. High quality beams can be shaped by optical tools, such as scanners or Diffractive Optical Elements, DOE, to almost any beam shape, enabling innovative laser process solutions. For welding in particular, a tailored beam can be used to control the melt pool and to optimise the temperature field and cycle. For example, joining of electrical components like battery cells becomes more common due to the shift to electrical vehicles. This is a field of applications where laser welding with a tailored beam has high potential due to the need of tightly controlled design tolerances or processing temperatures and in turn electrical and mechanical properties. The research presented in the thesis encompasses the heat flow generated from tailored laser beams, the thermal effects on the weld shape and on other quality criteria, the generated residual stress and its influence on fatigue crack propagation. For the sake of simplicity, melt flow was not considered in the calculations, which was discussed, too. The first three papers apply predictive mathematical modelling for the temperature field while the fourth paper experimentally derives the thermally induced residual stress distribution back from measured fatigue crack propagation.Paper I contains a FEM-based numerical heat flow study of a conduction mode laser welding case where a C-shaped overlap joint is desired. The quality criteria demand the welding process to be tightly controlled in terms of laser power and pulse time. Contrary to expectations, the joint geometry can significantly deviate from the laser beam C shape. As a continuation, in Paper II various quantitative indicators were derived and studied as part of the numerical simulation, in order to identify a suitable beam shape and in turn a DOE-design.Paper III presents a semi-analytical mathematical model that was developed for the heat flow in pulsed conduction mode welding for spatially and temporally shaped laser beams. As an alternative to FEM, the model is fast due to its analytical nature, which enables iterative beam shape optimization and DOE-design. By studying different beam shapes and the induced temperature fields, the potential and limits of the model were demonstrated and discussed. Paper IV is a study on residual stress that is thermally induced during the heating and cooling cycle of laser keyhole welding. Acceleration measurement of the crack propagating across the weld during fatigue testing turned out to be a suitable method to derive the residual stress distribution along the crack, including its alteration during the cracking. Comparisons with FEM-based stress analysis provide a link back to the temperature field induced by the laser, which enables optimization, e.g. by beam shaping.