To address the concerns of simultaneously improving crash performance and fuel efficiency in the automotive industry, multi-material car bodies are becoming increasingly popular. Aluminium and steel are two materials whose properties complement each other well for this application. However, ultra-high strength steel (UHSS) and aluminium alloy is a hard-to-join material combination. In the last decades different solutions to this problem have been proposed, one of these being friction element welding (FEW). In this work, different UHSS have been joined to different aluminium alloys through the FEW equipment EJOWELD developed by EJOT. The joints have been evaluated though visual inspection, cross-section analysis, and mechanical tests. All materials could successfully be joined by the EJOWELD process.

Les pièces des structures aéronautiques telles que les arbres des turboréacteurs, les roues, les freins ou les trains d’atterrissage sont fabriquées avec des aciers à très haute résistance. Leur structure martensitique, renforcée par des précipités de taille inférieure au micromètre, confère à ces aciers une excellente résistance : leur limite d'élasticité peut dépasser les 1900 MPa et leur résistance mécanique atteindre les 2300 MPa. Ces matériaux sont choisis pour ces excellentes propriétés mécaniques sur la base de leur comportement en traction. Toutefois, leur déformation à striction (maximum de la charge) est de quelques pourcents seulement. Les méthodes de dimensionnement sous chargement critique actuelles considèrent qu’aucun point de la structure ne doit être soumis à une déformation supérieure à la déformation à striction. Ce type d’approche est ici très conservatrice puisque les aciers THR continuent de se déformer plastiquement, ceci jusqu’à plusieurs dizaines de pourcent après le début de la striction. L’objet de ces travaux est de définir un modèle d’amorçage de la rupture applicable au dimensionnement de ces structures pour un acier type : le ML340. Ce matériau est actuellement utilisé dans les arbres de turboréacteur LEAP.Le comportement élasto-plastique du matériau a été étudié grâce à des essais menés sur différents types d’éprouvettes : tractions lisses, axisymétriques entaillées, déformation plane, plates entaillées, traction-torsion. Un modèle simple de von Mises à écrouissage isotrope permet de reproduire l’ensemble de la base. Ce modèle est ajusté sur les essais de traction pour lesquels un suivi optique de la variation du diamètre minium a été mis en place. La loi d’ ́écrouissage est ensuite ajustée en prenant en compte à la fois l'élongation et la variation du diamètre. On montre en particulier que l'extrapolation du comportement, méthode consistant à prolonger les données obtenues avant l'apparition de la striction, peut conduire à une mauvaise prédiction du comportement des éprouvettes.La base expérimentale a également été employée pour étudier l’amorçage de la rupture. L’observation des faciès montre un mode de rupture ductile avec des cupules fines. Cependant, l'initiation est brutale pour tous les essais et le développement de l’endommagement en volume reste très limité, voire nul. Ces constatations conduisent à proposer l’emploi d’un critère d’amorçage découplé : ce modèle fait intervenir la triaxialité des contraintes et le paramètre de Lode. Cette double dépendance est nécessaire pour bien décrire la rupture sur toute la base expérimentale. L’identification des paramètres du modèle se base sur l'évaluation des champs mécaniques à partir des simulations élasto-plastiques par éléments finis représentant les essais. Le modèle est également capable de prédire les emplacements des points d’amorçages observés, ces informations peuvent être utilisées pour ajuster plus finement le modèle. / Aeronautical structures such as jet engines shafts, wheels, brakes or landing gears are made of Ultra High Strenth Steels (UHSS). Due to their hard martensitic matrix reinforced with second phase particles, such steels exhibit extreme mechanical resistance, their yield strength can overcome 1900 MPa and their ultimate tensile strength can reach 2300 MPa. Such materials are selected based on their tensile properties, however strain at necking (maximum load) is only few percent. Conventional certified design methods assume that failure occurs when a given point of the structure reaches this strain. Regarding UHSS this approach is very conservative; those materials are still able to bear large strains after necking start. The aim of this work is to define a failure initiation model able to predict ductile failure of such structures for extremes loadings. The ML340 steel, material of LEAP jet-engine shafts, have been selected for this study.Elasto-plastic behavior is investigated with various mechanical tests. Uniaxial tensile test were performed on round bars specimens, either smooth or notch, and flat specimens, either u notched or plane strain. Traction/torsion and compression/torsion biaxial tests were performed on tubes specimens. A simple isotropic von Mises plasticity model was found sufficient to describe mechanical behavior of this experimental database. This model was calibrated based on round smooth tensile tests, a longitudinal extensometer and a non contact method, measuring diameter reduction, were used in order to monitor strains. Hardening law was adjust with both sets of data using a reverse method, hence material striction is take into account during the identification. Identification method extrapolating plastic behavior based on tensile data measured before the striction begining is shown to overperdict plastic behavior.Failure initiation was also investigated through fracture tests. Every fractography display very fine dimples related to ductile fracture, however fracture apears to be very brutal and very few damage was observed underneath fracture surface. As a result an uncoupled fracture initiation model is proposed, damage indicator is driven by both stress triaxiality and a Lode parameter. This dual dependency is necessary in order to represent fracture for the whole database. Model parameters identification relies on the evaluation of local stress state for each test, this could be achieved with 3D elasto-plastic simulations. As a result fracture model was able to predict correct fracture initiation point positions observed on round tensile tests and flat u notch tests.

Increasing the use of Ultra High Strength Steels (UHSS) in vehicle bodystructures is important for reducing weight and cutting CO2 emissions. This thesis investigates challenges in resistance welding that can be a barrier to implementing UHSS as a replacement for low strength steels in vehicle structures. Empirical research has been performed to offer new approaches for improved joint strength and to increase knowledge on cracking mechanisms in resistance projection welding and resistance spot welding of UHSS. By optimising the current build-up phase and peak current during the first milliseconds of weld time, it was shown that the strength could be improved by up to two-fold for projection welded joints. An approach to improve the ductility and strength of resistance spotwelds in UHSS using reduced cooling time was unsuccessful. The reduced cooling rate after weld metal solidification did not fully create the desired softened microstructure. The study on the surface cracking mechanism in resistance spot welded dual-phase UHSS showed that cracking is linked to the galvanization method. It is proposed that formation of aluminium oxide layers on the electrode tips increases the surface temperature and thereby increases the probability for liquid metal embrittlement and surface cracking. / <p>QC 20150526</p>

Friction stir spot welding (FSSW) is quickly becoming a method of interest for welding of high strength steel (HSS) and ultra high strength steel (UHSS). FSSW has been shown to produce high quality welds in these materials, without the drawbacks associated with fusion welding. Tool grade for polycrystalline cubic boron nitride (PCBN) tools has a significant impact on wear resistance, weld quality, and tool failure in FSSW of DP 980 steel sheet. More specifically, for a nominal composition of 90% CBN, the grain size has a significant impact on the wear resistance of the tool. A-type tools performed the best, of the three grades that were tested in this work, because the grain size of this grade was the finest, measuring from 3-6 microns. The effect of fine grain size was less adhesion of DP 980 on the tool surface over time, less abrasive wear, and better lap shear fracture loads of the welds that were produced, compared to the other grades. This is explained by less exposure of the binder phase to wear by both adhesion and abrasion during welding of DP 980. A-type tools were the most consistent in both the number of welds per tool, and the number of welds that reached acceptable lap shear fracture loads. B-type tools, with a bimodal grain size distribution (grain size of 4 – 40 microns) did a little bit better than C-type tools (grain size of 12-15 microns) in terms of wear, but neither of them were able to achieve consistent acceptable lap shear fracture load values after the first 200 welds. In fact only one out of five C-type tools was able to produce acceptable lap shear fracture loads after the first 100 welds.

Konventionelles Laserstrahlschweißen von Halbzeugen aus höherfesten Stahlfeinblechen führt zum drastischen Verlust an Umformbarkeit im Schweißnahtbereich. Durch integrierte induktive Erwärmung können der Temperaturverlauf beim Schweißen modifiziert, die Verbindungseigenschaften beeinflusst und die Umformbarkeit verbessert werden. Die Zusammenhänge zwischen Prozessparametern und mechanischen Verbindungseigenschaften werden beschrieben und Anwendungspotenziale aufgezeigt.

Konventionelles Laserstrahlschweißen von Halbzeugen aus höherfesten Stahlfeinblechen führt zum drastischen Verlust an Umformbarkeit im Schweißnahtbereich. Durch integrierte induktive Erwärmung können der Temperaturverlauf beim Schweißen modifiziert, die Verbindungseigenschaften beeinflusst und die Umformbarkeit verbessert werden. Die Zusammenhänge zwischen Prozessparametern und mechanischen Verbindungseigenschaften werden beschrieben und Anwendungspotenziale aufgezeigt.

Spot friction welding (SFW) was performed on ultra high strength steel (UHSS) steel sheet commonly used in automobile manufacturing. Alloys studied included DP780, DP780EG, DP980, and DF140T sheet steel of varying thickness from 1.2 mm to 1.4 mm. Welding was accomplished using a PCBN standard tool. Weld strengths were then compared to a proposed AWS standard. Initial hardness readings were taken in cross sectioned samples. Grain structure in a SFW is presented. Resistance spot welds were created in three steels. This study focuses on the strength of SFW joints as compared to traditional resistance spot welding (RSW) in welding like materials to one another. Cycle times of SFW were also evaluated and compared to production rate cycle times of RSW.

In an effort to increase the fuel efficiency of cars more widespread use of higher strength steels is seen for their high strength-to-weight ratio. Thesesteels are more limited in their formability and tendency to harden than conventional steels, complicating manufacturing. This thesis summarizes the available research on resistance projection nut welding to higher strength steels and investigates the accuracy of the simulation program SORPAS when simulating projection nut welds to AlSi-coated Boron steel. It was found that the greatest difficulties in welding coated ultra high strength steels were the metallurgical effects of both the high alloying content of the steel and the coatings interacting with the weld when melting. Although SORPAS was an intuitive program to use for resistance welding and had a wide library of materials available, it was not found to be able to predict the resistance characteristics or results of projection nut welds to coated Boron steel without significant changes to default material parameters. The biggest issue was the delaying effect the coating layer had on the peak resistance, something not observed experimentally. Better results are suggested to be possible after experimentally ensuring the properties of the materials used and importing those values into SORPAS. / I ett försök att öka bilars bränsleeffektivitet används i allt större utsträckning stål med högre hållfasthet på grund av deras goda förhållande mellan styrka och vikt. Dessa stål är mer begränsade i sin formbarhet och tendens att härda än konventionella stål, vilket försvårar tillverkning. Denna uppsats sammanfattar den tillgängliga forskningen om motståndssvetsning med projektionsmutter i höghållfasta stål och undersöker noggrannheten hos simuleringsprogrammet SORPAS vid simulering av projektionsmuttersvetsar i AlSi-belagt borstål. Det konstaterades att de största svårigheterna vid svetsning av belagda ultrahöghållfasta stål var de metallurgiska effekterna av både stålets höga legeringsinnehåll och beläggningarnas interaktion med svetsen vid smältning. Trots att SORPAS var ett intuitivt program att använda för motståndssvetsning och hade ett brett bibliotek av tillgängliga material kunde det inte förutsäga motståndsegenskaperna eller resultaten av projektionsmuttersvetsar mot belagt borstål utan betydande förändringar av standardmaterialparametrarna. Det största problemet var den fördröjande effekt som beläggningsskiktet hade på maximala resistansen, något som inte observerades experimentellt. Bättre överenstämmelse föreslås vara möjlig efter att experimentellt säkerställa egenskaperna hos de material som används och importera dessa värden till SORPAS.