Tailor welded blanks (TWBs) are used in the automotive industries as a method to meet economic, environmental and governmental demands. Conventionally, TWBs incorporated mild and low strength steels such as interstitial free and draw quality steels because of their excellent formability traits. However, due to their low strength they are unsuitable for energy absorption applications; thus, the interest of incorporating advanced high strength steels (AHSS) into the TWBs. Dual phase (DP) steel is a type of AHSS that is of interest because of its combination of high strength and good formability that is comparable to high strength low alloy (HSLA) steels. However, welding DP steel causes softening in the heat affected zone (HAZ), which leads to premature failure and reduces formability. The aim of this thesis was to study the effect of weld design on the formability of TWBs with DP steels and with HSLA steel. This thesis is divided into three parts; the first part examines TWBs with different weld line positions, weld line orientations and strain paths. The second part investigates bead-on plate curvilinear blanks and its effect on formability of the blanks. The last part examines the effects of multiple welds on the formability of TWBs.

Tailor welded blanks (TWBs) are used in the automotive industries as a method to meet economic, environmental and governmental demands. Conventionally, TWBs incorporated mild and low strength steels such as interstitial free and draw quality steels because of their excellent formability traits. However, due to their low strength they are unsuitable for energy absorption applications; thus, the interest of incorporating advanced high strength steels (AHSS) into the TWBs. Dual phase (DP) steel is a type of AHSS that is of interest because of its combination of high strength and good formability that is comparable to high strength low alloy (HSLA) steels. However, welding DP steel causes softening in the heat affected zone (HAZ), which leads to premature failure and reduces formability. The aim of this thesis was to study the effect of weld design on the formability of TWBs with DP steels and with HSLA steel. This thesis is divided into three parts; the first part examines TWBs with different weld line positions, weld line orientations and strain paths. The second part investigates bead-on plate curvilinear blanks and its effect on formability of the blanks. The last part examines the effects of multiple welds on the formability of TWBs.

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The use of high-strength steel is more and more frequent in the car industry. I focused on USIBOR® 1500 steel welded by Nd-YAG laser. For my experiment, there were used various welding parameters and shielding gases. To compare the weld samples, I used the tensile, bending, macrostructure, microstructure and Vickers hardness test with different combinations of thickness as well as welded material.

This research investigation explores the feasibility of implementing a laser welded sandwich steel panel bridge deck system as a viable alternative to standardized reinforced concrete bridge decks. Generally used in naval ship building applications, steel sandwich panels possess attractive characteristics towards the integration with bridge infrastructure such as service life in excess of 100 plus years, dead load reduction, rapid construction, decreased closure time, and automated mass production. The lack of fatigue data for the laser "stake" welds used to create the enclosed sandwich panel geometry raised concerns with respect to fatigue life. The primary focus of this study was to determine whether or not infinite fatigue life was possible. Two different laser welding technologies were investigated, Laser Beam Welding (LBW) and Hybrid-Laser Arc Welding (HLAW). Test specimens were fabricated and tested in order to examine fatigue resistance based on a localized load effect between adjacent core stiffeners. Finite element models were used to obtain the stress range for each individual test due to complex geometry and partially restrained boundary conditions. In order to assess the fatigue performance of the overall deck system, additional finite element models were created to study the local and global behavior of different sandwich panel configurations. As a whole the investigation yielded promising results. Infinite fatigue life is achievable due to outstanding fatigue performance. The HLAW stake welds demonstrated superior fatigue resistance in comparison to the LBW process. Localized load effects can be minimized through the modification of different panel parameters. Pushing forward, full scale testing is essential to the future employment of this innovative bridge deck system. / Master of Science

L’allégement des véhicules est un objectif incontournable dans l’industrie automobile. Parmi les voies exploitées par les constructeurs, le déploiement d’alliages d’aluminium pour le pavillon permet un allégement de plusieurs kilogrammes. C’est pourquoi, le procédé de soudo-brasage laser occupe une place de choix pour l’assemblage hétérogène pavillon aluminium/caisse acier. Cependant, en plus des problèmes liés à l’hétérogénéité du joint soudé (formation de composés intermétalliques par exemple), des difficultés inhérentes à la production en grande série émergent : un environnement avec de fortes contraintes industrielles, un niveau de conformité élevé et robuste ainsi qu’une cadence de production imposant des grandes vitesses de soudo-brasage comprises entre 4 et 6 m/min et adaptée à des configurations exploitables industriellement. L’objectif de ce travail de thèse est de conduire une analyse physique et technologique du process de soudo-brasage laser pour un assemblage hétérogène allégé pavillon/caisse. Dans un premier temps, un ensemble d’hypothèses sur l’influence de la physique du procédé, des variables process et de l’environnement d’étude sur la conformité et la reproductibilité de ces assemblages furent établies. Leur validation ensuite a été réalisée par des essais à l’échelle 1 sur une installation laser de production industrielle, prolongée par des analyses de la métallurgie et de la microstructure des assemblages ainsi que d’une simulation numérique du procédé. La physique du procédé en lien avec les paramètres du process de soudo-brasage laser acier/aluminium à haute vitesse a été identifiée. Les variables énergétiques ont montrées un fort impact sur le niveau de conformité atteint ainsi que sur la dispersion des résultats. Celles-ci tendent à influencer fortement les phénomènes physiques mises en jeu et notamment la thermique à l’interface acier/aluminium, impactant directement la formation des composés intermétalliques et leurs microstructures. Différentes tailles de grains ont été observés en fonction des paramètres process, contrôlant ainsi la tenue mécanique des assemblages soudo-brasés. La modulation in fine du cycle thermique de soudo-brasage, piloté par la physique activée du process, permet de discriminer les configurations les plus optimales pour une application industrielle du procédé. / Weight reduction of vehicles is a key objective in the automotive industry. Particularly, the use of aluminum alloys for the roof is one of the most promising path studied by manufacturers to save several kilograms. Therefore, the laser brazing process has a place of choice for assembling heterogeneous aluminum roof / steel body-side. However, in addition to issues related to the heterogeneity of the welded joint (formation of intermetallic compounds, for example), issues related to the mass production emerge: an environment with strong industrial constraints, high and robust levels of compliance, a production rate involving high brazing speeds between 4 and 6 m / min and fitting with industrial configurations. The objective of this thesis is to conduct a physical and technological analysis of laser brazing process in the case of a heterogeneous lightweight roof / body-side assembly. Firstly, a set of assumptions about the influence of the physical process, the process variables and the industrial environment on the compliance and the reproducibility of these assemblies are made. Then, validation was conducted by tests at scale 1 on a laser industrial production set-up, extended by analysis of metallurgy and microstructure of assemblies and a numerical simulation of the process. The link between the physical phenomena of the process and the high speed steel-aluminium brazing process parameters has been identified. Energy variables showed a strong impact on the level of compliance achieved and on the dispersion of results. These variables tend to strongly influence the physical phenomena of the process and especially thermal behaviors at the steel / aluminum interface, directly impacting the formation of intermetallic compounds and their microstructures. Different grain sizes were observed depending on the process parameters, influencing hence the mechanical strength of assemblies. Finally, the modulation of the brazing thermal cycle, driven by the activated physical phenomenon of the process, allows bringing out the most optimal configuration for an industrial application of the process.

The purpose of this report is to evaluate the hardness and microstructure in bent and welded samples of Alloy 718. The results will be used by GKN aerospace to evaluate the simulated values of the production process of vines in a jet engine. In total, eleven samples from three different production chains are evaluated. All samples are bent and go through different stages within the production including bending, solution treatment and welding. The samples are cut and mounted in bakelite then polished and etched in order for the microstructure to be seen and evaluated. Hardness tests were made on the mounts to evaluate how bending, solution treatment and welding affects the hardness. The bent samples without the solution treatment became harder depending on the amount of cold deformation. The amount of cold deformation controls how fast the material recrystallizes during subsequent solution treatment. During the solution treatment, the δ-phase (Ni3Nb) is precipitated in the grain boundaries which prevents a coarser grain size and therefore promotes a smaller grain size.  The laser weld creates a small HAZ (heat affected zone) that becomes softer because the heat dissolves the δ-phase which therefore triggers the grains to grow. The weld consists of eutectic γ-dendrites with interdendritic pools of alloying elements. After the solution treatment, many needle shaped δ-phases arise from the pools and HAZ. The hardness measurements were tested on a manual machine which makes potential human error important to consider when the measurements are evaluated. When the grain size measurements are done on the solution treated samples, the grain boundaries can be difficult to determine because the δ-phases and twins create wide and incorrect boundaries. The results will be used by GKN Aerospace in order to verify their simulations. The heterogenic material after solution treatment will probably be studied further. / Syftet med projektet är att undersöka hårdhet och mikrostruktur i bockade och svetsade prover tillverkade av Alloy 718. Totalt undersöks elva stycken prover som är tillverkade på olika sätt enligt tre tillverkningskedjor. Alla provbitar är bockade och har sedan genomgått olika många steg i tillverkningen som består utav bockning, upplösningsbehandling och svets. Genom att kapa upp provbitarna i mindre bitar, baka in de i bakelit och etsa de studerades mikrostrukturen och hur den påverkas av bockning, upplösningsbehandling och svets. Vidare gjordes hårdhetsmätningar över olika delar av proven för att undersöka hur mycket hårdare materialet blir vid kalldeformation och hur upplösningsbehandling och svets påverkar hårdheten i Alloy 718. Endast bockade prov blir hårdare beroende på hur mycket kalldeformation provbiten har utsatts för. Hur mycket provet har deformerats styr hur snabbt rekristallisationen sker vid en efterföljande upplösningsbehandling. Under upplösningsbehandlingen utskiljs även δ-fas (Ni3Nb) i korngränser vilket främjar en mindre kornstorlek. Lasersvetsen ger ett litet värmepåverkat område men värmen från svetsen bidrar till att δ-fasen löses upp och större korn bildas. Svetsen består av långa eutektiska γ-dendriter med interdendritiska poler av mycket legeringsämnen. Vid upplösningsbehandling efter utförd svets bildas det δ-fas i de värmepåverkade och i svetsen. Hårdhetsmätningarna har utförts på en manuell hårdhetsmaskin vilket innebär att felmarginalen blir större då den mänskliga felfaktorn spelar en stor roll. Vid beräkning av kornstorlek har det varit svårt att utskilja vad som är en korngräns i upplösningsbehandlade prov då mängden δ-fas efter upplösningsbehandling ger tjocka korngränser.  Resultaten kommer användas av GKN Aerospace för att verifiera sina simulationer. Det heterogena materialet efter upplösningsbehandlingen kommer troligtvis studeras vidare.

This thesis is focused on the analysis of current state of production technology and quality control of high-pressure reservoir. It deals mainly on the macro-metallographic test of the quality and position of the weld, the posibility of it´s implementation, organization of workplace. Task of this thesis is to find alternative methods of testing. Part of thesis is design and implementation of a new workplace´s layout based on experiment with using of alternative method of testing and assessment of implementation to achal process. The main objective is to improve the working condition and reduction of time for testing.