A numerical model of the temperature field during pulsed laser cutting of thin sheets (approximately 2.5 x l0⁻⁵ m) was developed. Cutting was simulated through removal of nodes from a finite difference scheme based on sensible heating to the phase change temperature and a single value of latent heat (melting or vaporization). The pulsed laser model predicts a heat-affected zone of less than 0.02 mm for pulsed laser cutting. For comparable cutting with a continuous power laser, a heat-affected zone between 0.05 and 0.10 mm is predicted. Thermal stress levels were predicted to be an order of magnitude lower for pulsed laser cutting than for continuous power cutting. The stress levels predicted by the model also increased with cut speed. Experimentally, pulsed laser cutting yielded better cut quality, based on less cracking, than continuous power cutting. In addition, the cut quality deteriorated as the cutting speed was increased for the continuous power laser. Presently, application of pulsed laser cutting is limited by its low cutting speed, which is restricted by the energy density of the laser. The model predicts that increasing energy density will decrease the size of the heat-affected zone and increase the maximum cutting speed. Therefore, pulsed laser cutting at high speeds should be attainable without deterioration in cut quality. / Master of Science

Os aços maraging pertencem a uma classe de aços de ultra-alta-resistência que diferem da maioria dos aços pelo mecanismo de endurecimento. Enquanto nos aços convencionais o endurecimento é obtido por transformação martensítica, nos aços maraging o aumento das propriedades mecânicas é obtido pela precipitação de compostos intermetálicos em uma matriz martensítica de baixo teor de carbono, em temperaturas da ordem de 480°C. Para o presente trabalho, empregou-se um aço maraging 300 na forma de barras cilíndricas com diâmetro de 8mm e comprimento de 120mm, cedido na condição solubilizada pelo Departamento de Ciência e Tecnologia Aeroespacial (DCTA). Os corpos-de-prova foram soldados em cheio empregando-se um laser a fibra de 2kW e um dispositivo especialmente desenvolvido para solda orbital. As condições de envelhecimento foram inicialmente estudadas e a melhor combinação de resistência mecânica e ductilidade foi obtidas na condição de para 460°C por 3 horas, que foi então adotada para a caracterização do comportamento em fadiga. Os ensaios de fadiga foram realizados em uma máquina de flexão rotativa (R = -1), em níveis de tensão correspondentes à faixa de vida de 104 a 107 ciclos. Os corpos-de-prova soldados a laser foram submetidos a três condições de tratamento pós-soldagem: a) envelhecimento, b) solubilização + envelhecimento e c) homogeneização + solubilização + envelhecimento. Os resultados permitiram avaliar e comparar o efeito da solda a laser, seguida por tratamentos térmicos pós-soldagem, no comportamento em fadiga do aço maraging 300. Os tratamentos pós-soldagem não resultaram em diferença significativa na vida em fadiga do aço maraging 300, o fator determinante para a duração da vida em fadiga foi a quantidade de poros presente na superfície de fratura. / The maraging steels belong to a class of ultra-high-resistance steels that differ from most of steel for the hardening mechanism. While the conventional steel is obtained by hardening for martensitic transformation in maraging steel the increase of mechanical properties is obtained by the precipitation of intermetallic phases in a martensitic matrix of low carbon, at temperatures about 480°C. For the present study, we used a maraging 300 steel in form of cylindrical bars with a diameter of 8mm and length of 120mm, received as solution treated by the Department of Aerospace Science and Technology (DCTA). The test specimens were welded using a 2 kW fibre laser coupled to a special lathe in order to obtain an orbital weld line. The ageing contitions of the steel were initially investigated using base material samples. The best combination of hardness, tensile strength and ductility were achieved for 460°C for 3 hours , which was then chosen for the fatigue characterization..The fatigue tests were conducted in a rotating bending machine (R = -1). The stress levels were found for fatigue lives in the range of 104 to 107 cycles. The laser welded test pieces underwent three distinct post-weld heat treatments: a) ageing , b) solution + ageing and b) homogenization + solution + ageing .The results allowed assessing and comparing the effect of laser beam welding, followed by post-weld heat treatments, on the fatigue behavior of the maraging 300 steel. The post-weld heat treatments no results in significant difference in fatigue life of maraging 300 steel , the determinant factor for fatigue life was the amount of pores present in the fracture surface.

Os aços maraging pertencem a uma classe de aços de ultra-alta-resistência que diferem da maioria dos aços pelo mecanismo de endurecimento. Enquanto nos aços convencionais o endurecimento é obtido por transformação martensítica, nos aços maraging o aumento das propriedades mecânicas é obtido pela precipitação de compostos intermetálicos em uma matriz martensítica de baixo teor de carbono, em temperaturas da ordem de 480°C. Para o presente trabalho, empregou-se um aço maraging 300 na forma de barras cilíndricas com diâmetro de 8mm e comprimento de 120mm, cedido na condição solubilizada pelo Departamento de Ciência e Tecnologia Aeroespacial (DCTA). Os corpos-de-prova foram soldados em cheio empregando-se um laser a fibra de 2kW e um dispositivo especialmente desenvolvido para solda orbital. As condições de envelhecimento foram inicialmente estudadas e a melhor combinação de resistência mecânica e ductilidade foi obtidas na condição de para 460°C por 3 horas, que foi então adotada para a caracterização do comportamento em fadiga. Os ensaios de fadiga foram realizados em uma máquina de flexão rotativa (R = -1), em níveis de tensão correspondentes à faixa de vida de 104 a 107 ciclos. Os corpos-de-prova soldados a laser foram submetidos a três condições de tratamento pós-soldagem: a) envelhecimento, b) solubilização + envelhecimento e c) homogeneização + solubilização + envelhecimento. Os resultados permitiram avaliar e comparar o efeito da solda a laser, seguida por tratamentos térmicos pós-soldagem, no comportamento em fadiga do aço maraging 300. Os tratamentos pós-soldagem não resultaram em diferença significativa na vida em fadiga do aço maraging 300, o fator determinante para a duração da vida em fadiga foi a quantidade de poros presente na superfície de fratura. / The maraging steels belong to a class of ultra-high-resistance steels that differ from most of steel for the hardening mechanism. While the conventional steel is obtained by hardening for martensitic transformation in maraging steel the increase of mechanical properties is obtained by the precipitation of intermetallic phases in a martensitic matrix of low carbon, at temperatures about 480°C. For the present study, we used a maraging 300 steel in form of cylindrical bars with a diameter of 8mm and length of 120mm, received as solution treated by the Department of Aerospace Science and Technology (DCTA). The test specimens were welded using a 2 kW fibre laser coupled to a special lathe in order to obtain an orbital weld line. The ageing contitions of the steel were initially investigated using base material samples. The best combination of hardness, tensile strength and ductility were achieved for 460°C for 3 hours , which was then chosen for the fatigue characterization..The fatigue tests were conducted in a rotating bending machine (R = -1). The stress levels were found for fatigue lives in the range of 104 to 107 cycles. The laser welded test pieces underwent three distinct post-weld heat treatments: a) ageing , b) solution + ageing and b) homogenization + solution + ageing .The results allowed assessing and comparing the effect of laser beam welding, followed by post-weld heat treatments, on the fatigue behavior of the maraging 300 steel. The post-weld heat treatments no results in significant difference in fatigue life of maraging 300 steel , the determinant factor for fatigue life was the amount of pores present in the fracture surface.

O conjunto de tubos denominado sistema pneumático, tem a função de conduzir o ar quente pressurizado desde o motor até os pontos de utilização pelos sistemas ambientais da aeronave. A ocorrência de falhas em serviço de peças desse sistema soldadas pelo processo TIG (Tungsten Inert Gas), motivou o estudo da substituição desse processo pelo de soldagem a laser. Foi utilizado um laser a fibra de 2 kW de potência e tubos de titânio comercialmente puro grau 2, de 0,5 mm de espessura e 50 mm de diâmetro. Para efeito de comparação foram realizadas soldas pelo processo TIG, semi-automático, cujos parâmetros foram obtidos junto a uma empresa fabricante de aviões. Os tubos de titânio foram soldados em cheio, sem adição de arame, e o argônio foi utilizado como gás de proteção. Os parâmetros de soldagem a laser selecionados foram 200 W e 2000 mm/min e, 250 W e 300 mm/min. Os tubos soldados a laser apresentam cordões com aproximadamente 1 mm de largura e grãos de Ti-CP ?, enquanto que as soldas TIG resultam em cordões de 5 mm e grãos grosseiros serrilhado de Ti-CP ?. O refinamento microestrutural observado nas amostras soldadas a laser, em relação à TIG, deve-se a alta taxa de resfriamento, que no laser foi cerca de 70 vezes maior do que no TIG. Os tubos soldados a laser e TIG foram ciclados na bancada pneumática, 44.000 vezes, a 350 oC, sem fraturarem ou iniciarem uma trinca que comprometesse os testes de pressurização cíclica. Os resultados de fadiga evidenciaram uma maior resistência mecânica da solda a laser na condição de 200 W e 2000 mm/min, quando comparado com as demais condições de soldagem. As avaliações microestruturais e mecânicas dos cordões de solda evidenciaram as vantagens do processo de soldagem a laser para substituir o TIG, em junções do sistema pneumático de aeronaves. / The engine bleed air system, also known as pneumatic system, conducts the pressurized hot air from the engine to the environmental systems of aircrafts. In-service failures of the arc welded pneumatic parts have driven further developments of laser beam welding as an alternative method. Here, a fiber laser with 2 kW power had been employed to weld commercial purity titanium alloy tubes with 0.5 wall thickness and 50 mm diameter. For comparison purposes, semiautomatic TIG welding was realized, based on the previous experience of an aircraft company. All the welds were autogenous, bead-on-plate, and under protective argon atmosphere. The chosen parameters speed and laser powers for laser welding were 200 W - 2000 mm/min and 250 W - 300 mm/min. The laser welded tubes presented 1-mm wide weld beads composed by grained ?-Ti. The TIG welded tubes showed 5-mm wide beads composed by acicular ?-Ti grains. These observed differences had been associated to the cooling rate, which is seventy times higher in the laser case. Both laser and TIG welded tubes were cycled 44.000 times in a pneumatic bench at 350 oC without failures or cracks that could break the internal pressure. The condition 200W-2000mm/min presented superior fatigue resistance values compared to other welding conditions, and being similar to the tubes in the unwelded condition. On the other hand, the condition 250W-3000mm/min presented the highest dispersion for the fatigue lives. The microstructural and mechanical results shown that the current laser technology could replace with advantages the arc welding for the joining of the titanium tubes in the aircrafts pneumatic system.

The current thesis was designed to examine the welding characteristics of laser and electron beam welding of the superplastic metal matrix composites (MMCs) reinforced with 1~20% SiC and to differentiate the difference between the 6061 aluminum alloy and 6061/SiC composites. The 6061/20%SiCw MMC was found to exhibit poor welding characteristics under electron beam welding. This was because that the SiC whiskers would induce poor fluidity of molten Al matrix and the electron beam continuously bombared the MMC resulting in material loss through sputtering, and this effect induced an "V" groove formed at the center of the fusion zone. The laser beam welding of the 20% SiCw MMCs caused the decomposition of the SiCw into Al4C3 platelets at the center region of the fusion zone, as well as cavities along the outer region due to thermal expansion differences. The post-weld tensile test results showed that the brittle weld zone lead to the degradation of strength, and the 6061/20%SiCw MMC after welding would lose superplastic properties. There were some differences between the 6061 alloy and MMC upon subjected to laser beam welding. The absorption of laser energy by the MMC was better than that by the alloy; the absorption of laser energy increased with increasing SiC content. The shape of the reinforced material could also influence the quantity of Al4C3 formed. The total surface area of SiC particles was smaller than that of the SiC whiskers under equal volume fraction, therefore more SiC whiskers were decomposed. In the wetting experiment, the wettability and fluidity of molten material was observed to decrease with increasing SiC volume fraction at the same temperature. The wettability could be improved at higher temperatures. For the 20%SiCw MMC, the wettability and fluidity could not be sufficiently improved even at a high temperature of 1300¢J, which appeared to be the cause for the lack of feeding in the central fusion zone.

Šviesos netiesinio fokusavimosi reiškinys tyrinėjamas jau nuo pat lazerio sukūrimo. Ypatingai aktyviai nagrinėjamas šviesos gijų susidarymas bei jų dinamika skirtingos fazinės būsenos medžiagose. Šviesos gijos įdomios tuo, kad šviesa jose sklinda beveik nepatirdama difrakcijos ir dispersijos. Mokslininkai pasiūlė nemažai šviesos gijos taikymų, kaip pvz. atmosferos sluoksnių spektrinė analizė, atosekundinių impulsų generacija inertinėse dujose ar bangolaidžių užrašymas kietose skaidriose terpėse. Šviesos saviveikos metu pradinė bangų paketų forma erdvėje bei laike gali stipriai pakisti. Standartiniai impulsų charakterizavimo metodai, kurie remiasi erdvėje integruotų autokoreliacinių ir kryžminių koreliacinių funkcijų matavimu, įgalina gauti tik dalinę informaciją apie vykstančias šviesos impulso bei terpės savybių transformacijas ir jų fizikinę prigimtį. Todėl, norint gerai įvaldyti minėtus taikymus bei suprasti vykstančius fizikinius reiškinius, visiškam šviesos impulsų sąveikos su medžiaga ir šviesos gijų formavimo proceso charakterizavimui reikalingi nauji matavimo metodai, kuriuos pasitelkus būtų galima išmatuoti bangų paketo intensyvumo pasiskirstymą trijų matmenų erdvėje bet kuriuo laiko momentu bei registruoti momentinius terpės struktūros pokyčius. Antra vertus, šviesos ir medžiagos sąveikos procesai yra labai spartūs, tad jų tyrimui reikalinga labai didelė tiek erdvinė (μm eilės), tiek laikinė (fs eilės) skyra. Šiame darbe pasiūlėme didelės laikinės (20 fs) ir... [toliau žr. visą tekstą] / In the disertation, the self-focusing phenomenon of intense femtosecond light pulses in media with cubic nonlinearity is investigated and the origin of formation of light filaments is revealed. In this work, new measurement techniques are introduced and applied experimentally, which allowed high resolution temporal, spatial and spectral mapping of light wave-packet dynamics during the nonlinear propagation in transparent media, and enabled to observe and make accurate quantitative evaluation of the ultrafast change of medium properties. By means of high temporal (20 fs) and spatial (1 μm) resolution laser 3D mapping technique, it was shown that the initial Gaussian wave packet during self-action in Kerr media redistributes its energy in a way that in spatio-temporal domain the wave packet takes a complex X-type intensity distribution. This transformation is universal and is determined by temporal and spatial spectral broadening (as a consequence of self-focusing and self phase modulation), conical emission (as a consequence of four-wave mixing) and nonlinear losses caused by multiphoton absorption. By means of imaging spectrometer technique it was shown that in the medium with normal group velocity dispersion the far-field angular spectrum of the wave packet takes a characteristic X shape, whereas in anomalous group velocity dispersion regime – a characteristic O shape. In both cases the localization of the wave packet is observed, however the quantitative differences of the... [to full text]

Haynes alloy 282 is a new gamma prime (γ’) precipitation strengthened nickel-base superalloy developed for high temperature applications in land-based and aero turbine engines. Joining is a crucial process both during the manufacturing of new components and repair of service-damaged turbine parts. Unfortunately, the new superalloy cracks during laser beam welding (LBW), which is an attractive technique for joining superalloys components due to its low heat input characteristic that preclude the geometrical distortion of welded components. This research is therefore initiated with the goal of studying and developing an effective approach for preventing or minimizing cracking during LBW of the new superalloy Haynes 282. Careful and detailed electron microscopy and spectroscopy study reveal, for the first time, the formation of sub-micron grain boundary M5B3 particles, in the material. Microstructural study of welded specimens coupled with Gleeble thermo-mechanical physical simulations shows that the primary cause of weld heat affected zone (HAZ) cracking in the alloy is the sub-solidus liquation reaction of intergranular M5B3 borides in the material. Further weldability study showed that the HAZ liquation cracking problem worsens with reduction in welding heat input, which is normally necessary to produce the desired weld geometry with minimum distortion. In order to minimize the HAZ cracking during low heat input laser welding, microstructural modification of the alloy by heat treatment at 1080 - 1100oC has been developed. The pre-weld heat treatment minimizes cracking in the alloy by reducing the volume fraction of the newly identified M5B3 borides, while also minimizing non-equilibrium grain boundary segregation of boron liberated during dissociation of the boride particles. Further improvement in resistance to cracking was produced by subjecting the material to thermo-mechanically induced grain refinement coupled with a pre-weld heat treatment at 1080oC. This approach produces, for the first time, crack-free welds in this superalloy, and the benefit of this procedure in preventing weld cracking in the new material is preserved after post-weld heat treatment (PWHT), as additional cracking was not observed in welded specimens subjected to PWHT.

Haynes alloy 282 is a new gamma prime (γ’) precipitation strengthened nickel-base superalloy developed for high temperature applications in land-based and aero turbine engines. Joining is a crucial process both during the manufacturing of new components and repair of service-damaged turbine parts. Unfortunately, the new superalloy cracks during laser beam welding (LBW), which is an attractive technique for joining superalloys components due to its low heat input characteristic that preclude the geometrical distortion of welded components. This research is therefore initiated with the goal of studying and developing an effective approach for preventing or minimizing cracking during LBW of the new superalloy Haynes 282. Careful and detailed electron microscopy and spectroscopy study reveal, for the first time, the formation of sub-micron grain boundary M5B3 particles, in the material. Microstructural study of welded specimens coupled with Gleeble thermo-mechanical physical simulations shows that the primary cause of weld heat affected zone (HAZ) cracking in the alloy is the sub-solidus liquation reaction of intergranular M5B3 borides in the material. Further weldability study showed that the HAZ liquation cracking problem worsens with reduction in welding heat input, which is normally necessary to produce the desired weld geometry with minimum distortion. In order to minimize the HAZ cracking during low heat input laser welding, microstructural modification of the alloy by heat treatment at 1080 - 1100oC has been developed. The pre-weld heat treatment minimizes cracking in the alloy by reducing the volume fraction of the newly identified M5B3 borides, while also minimizing non-equilibrium grain boundary segregation of boron liberated during dissociation of the boride particles. Further improvement in resistance to cracking was produced by subjecting the material to thermo-mechanically induced grain refinement coupled with a pre-weld heat treatment at 1080oC. This approach produces, for the first time, crack-free welds in this superalloy, and the benefit of this procedure in preventing weld cracking in the new material is preserved after post-weld heat treatment (PWHT), as additional cracking was not observed in welded specimens subjected to PWHT.

Cette thèse a pour but d’augmenter la durée de vie des moules pour l’injection plastique. Les principaux phénomènes à refréner sont l’usure par abrasion et l’usure par corrosion. Pour ce faire, deux familles de matériaux ont été présélectionnées. Il s’agit des aciers à outils inoxydables et des verres métalliques massifs. Ces travaux détaillent donc la sélection, le développement et la caractérisation d’une nuance pour chacune de ces familles. In fine, cette thèse délivre un nouvel acier adapté à la fabrication additive et aux moules d’injection plastique, allant de l’élaboration de la poudre à l’optimisation des paramètres LBM et de ceux du traitement thermique. Cet acier permet d’offrir un bon compromis en termes de propriétés thermiques, mécaniques et de résistance à la corrosion. Quant au verre métallique massif, ces travaux de thèse démontrent que la nuance choisie permet de conserver suffisamment de matériau amorphe pour induire des propriétés exceptionnelles. / The aim of this thesis is to increase molds lifetime. Mains phenomena to limit are abrasion wear and corrosion. For this purpose, two material families have been preselected. They have been identified among stainless steels and bulk metallic glasses. This work details their selection, development in additive manufacturing and characterization for both materials. In the end, this thesis delivers a new steel grade adapted to the additive manufacturing and plastic injection molds, which affords good corrosion resistance, high hardness and a comparatively good thermal conduction. Concerning the bulk metallic glass, this work shows that the chosen composition allowed to preserve enough amorphous material to induce exceptional properties and give strong hopes to continue in this vein.

Made available in DSpace on 2014-10-09T12:53:36Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:08:47Z (GMT). No. of bitstreams: 0 / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energéticas e Nucleares - IPEN-CNEN/SP