Multimaterial assemblies made by resistance spot welding (RSW) process offer a cost-efficient lightweight solution for automotive body-in-white production. However, joining different materials, such as steel (Fe) and aluminum (Al), brings issues due to their different physical properties and low weldability. The present work investigated the evolution of the weld between a thin galvanized low carbon steel sheet and an AA6056-T4 aluminum alloy from macroscopic observations to the evolution of the intermetallic compound layer (IMC) during RSW. Comparisons between homogeneous Al-Al RSW, Fe-Al RSW and Fe-Al adhesive weld bonding for different mechanical solicitations were also realized. Observations showed that the interface for short welding periods is composed of a thin and discontinuous IMC layer. The interface also presents defects that facilitate crack initiation and propagation, leading to an interfacial fracture mode. The different mechanical tests carried out demonstrated that the Al-Al and Fe-Al welds reached a similar maximum tensile load under the shear–tensile test. However, Fe-Al assemblies under cross-tensile test showed poor mechanical properties. The use of an adhesive layer with an Fe-Al spot weld seems to be a potential solution, as it is easily implementable on production lines and improves the mechanical properties of Fe-Al assemblies.

The automotive industry is undergoing significant changes driven by factors such as reducing carbon dioxide emissions, advancing technology, evolving regulations, and the emergence of new energy sources. Lightweight materials, particularly aluminum alloys, are being extensively researched and integrated into vehicles to reduce weight and improve performance. However, the heating process during vehicle production can cause thermal buckling in thin aluminum alloy structures, affecting their appearance and quality. While thermal buckling has been studied in other industries, research in the automotive sector, particularly for non-structural parts like car roofs, is limited. This study uses numerical simulation to predict thermal buckling and post-buckling behavior of a EN AW 6016-T4 alloy car roof assembled in a predominantly steel body-in-white. The research findings indicate that roof buckling occurs at a relatively low temperature difference of approximately 60 °C, which is lower than the maximum temperatures experienced during the painting phases in the automotive industry. Consequently, undulations in the roof's shape become apparent, underscoring the importance of design modifications to ensure visual conformity. Validation through physical testing confirms the model's accuracy, providing valuable insights for designing lightweight structures with improved performance and aesthetics.

Aluminum‑silicon (AlSi) coating is used to protect Press Hardened Steel sheets against oxidation during the austenitization inside the furnace before the stamping stage. Heat treatment conditions, i.e. temperature and dwell times, influence the diffusion of elements between the AlSi coating and the substrate. During resistance spot welding, the occurrence of molten metal splashing at sheet-sheet interface increases with more severe heat transfer conditions, i.e. over-heated press hardened steel sheets or too long dwell times. The welding current range is dramatically reduced in this case. Measurements of electrical contact resistance at sheet-sheet interface reveal higher values, which could involve interfacial overheating. The AlSi coating, which has lower melting point compared with the substrate, is extruded early during welding stage at the faying interface and forms an accumulation at the notch root close to the nugget. The potential role of this accumulation to prevent the splashing phenomena is not clear. The stiffness of this accumulation is investigated through the analysis of the microstructure and the chemical composition of the coating. The results have suggested that the aluminum content in the interdiffusion layer can change with the heat treatment conditions and may modify its capacity to confine and seal the molten zone.

This work presents a methodology to estimate the in-plane thermal diffusivity through infrared thermography. Experimentally, a vertical thin-thickness sample is briefly heated on its center by Joule effect and one face is filmed by an infrared camera. The experimental temperature field is compared with an analytical axisymmetric semi-infinite thermal model. The in-plane thermal diffusivity and a modified Biot number are estimated through a least square method based on the minimization of the quadratic error between the experimental and the theoretical temperatures. The methodology is applied to four samples with a broad range of thermal conductivity: balsa, PVC, mild steel, and aluminum alloy. The analysis of the residuals and of the parameter correlation coefficients show the feasibility of the method for isotropic materials for which a rather good agreement with literature data is found. When applied to anisotropic materials like balsa, the method shows the possibility with one experiment to characterize in every material directions.

Spectrometric analysis is one of the most widely used approaches to characterize the chemical nature of microplastics. Despite recent developments, this key step remains time consuming. The aim of this paper is to propose a new method for the pre-detection of microplastics based on mid-infrared imaging. Plastic particles were mixed with sand particles and placed on a glass filter. Infrared observation with a thermal camera shows a stronger thermal contrast measured between the filter and the plastics than between the filter and the sand, which reveals the plastic particles in a few tens of seconds. An image processing tool is then used to amplify this contrast. Furthermore, this pre-detection method makes it possible to propose hypotheses on the most probable chemical nature of the particles identified. Consequently, pre-detection using active thermography constitutes a promising way of significantly accelerating microplastic study.

In automotive industry, to simultaneously impact safety and light weighting for reducing energy consumption, new families of high strength steel have been introduced in the design of body in white. Some combinations of dissimilar sheets, including a very thin sheet, cause weldability problems and difficulties to the optimization of the process parameters setting. Thanks to the progress achieved in numerical and computer engineering fields, modelling and numerical simulation is a relevant approach, to understand the difficulties encountered during resistance spot welding of these assemblies, and to search solutions to improve the weldability. This work aims at the improvement of the weldability of a dissymmetric combination of three dissimilar sheets: a very thin (0.57 mm) zinc coated low carbon steel sheet, a thick (1.47 mm) zinc coated advanced high strength steel sheet, and a thick (1.2 mm) aluminumsilicon coated press hardened sheet. A numerical axisymmetric 2D Electro-Thermo-Mechanical model developed with the software FORGE® is used to improve the knowledge about the mechanisms which influence the nugget formation and growth, and its penetration inside the cover thin sheet. Experimental evolutions of thermal and electrical contact resistances evolutions, at electrode/sheet and sheet/sheet interfaces, strongly dependent of coatings properties, are embedded in the model and considered dependent on contact temperature and normal stress. The contact radius evolutions, involving the normal contact stresses and the current density distributions in the assembly, are calculated during squeezing, welding, and forging stages. The model is consistent with several experimental observations (nugget size, contact radii, dynamic resistance) issued from welding tests. Thanks to this model, the important effect of the interfacial mechanisms on the formation and the growth of the molten pool have been highlighted. Furthermore, the influence of the process parameters (current, force) and of the curvature radius of the rounded tip electrodes on the penetration of the welding pool into the thin sheet have been investigated.

In automotive industry, to simultaneously impact safety and light weighting for reducing energy consumption, new families of high strength steel have been introduced in the design of body in white. Some combinations of dissimilar sheets, including a very thin sheet, cause weldability problems and difficulties to the optimization of the process parameters setting. This work aims at the improvement of the weldability of a dissymmetric combination of three dissimilar sheets: a very thin (0.57mm) zinc coated low carbon steel sheet, a thick (1.47mm) zinc coated advanced high strength steel sheet, and a thick (1.2mm) aluminium-silicium coated press hardened sheet. A numerical axisymmetric 2D Electro-Thermo-Mechanical model developed with the software FORGE® is used to improve the knowledge about the mechanisms which influence the nugget formation and growth, and its penetration inside the cover thin sheet. The model is consistent with several experimental observations (nugget size, contact radii, dynamic resistance) issued from welding tests. The numerical results show that the initial heating is done at the interfaces with the aluminized sheet, opposite the very thin sheet. The lack of penetration of the nugget into the thin sheet is a consequence of the overheating at the interfaces with the aluminized sheet and the uncontrolled growth of the nugget.

During a resistance spot weld, a current of several kiloamperes passes through a stack of sheets clamped between two electrodes. We are interested here in the very first moments of the welding, when the combined effects of electrical and thermal contact resistances at sheet/sheet (S/S) and electrode/sheet (E/S) interfaces are at the origin of intense overheating. In order to estimate the contact temperatures, non-contact infrared thermography measurements coupled with an instrumentation by micro-thermocouples welded on the sheets are performed. The evolution of the emissivity is measured and the temperatures at the interfaces are evaluated.

An experimental apparatus was designed to reproduce a resistance spot welding operation and allow observation via infrared camera. A specific ex situ device was used to measure the variations of electrical contact resistances, versus pressure and temperature, for the electrode-sheet and sheet–sheet interfaces. The high values of the electrical contact resistances associated to the Al-Si coated Press Hardened Steel sheet explains the spark effect observed at these interfaces by infrared thermography. The initiation and the growth of the nugget from the Al-Si coated sheet to the galvanized low carbon thin sheet are also due to these high contact resistances. The different observation methods and measurements concur to explain the nugget formation.

Dans l’industrie automobile, les exigences en matiere d’emissions polluantes conduisent a alleger les vehicules, notamment en reduisant l’epaisseur des toles. Ce travail en partenariat avec ArcelorMittal porte sur le soudage par resistance par point de toles fines d’acier. L’objectif est d’identifier les phenomenes qui induisent les difficultes de soudabilite operatoire rencontrees avec une combinaison dissymetrique de trois toles revetues, incluant une tole tres mince galvanisee de moins de 0,6 mm, une tole de DP600 et une tole en Usibor® emboutie a chaud. Des observations par camera infrarouge montrent que les echauffements initiaux se produisent principalement au niveau des interfaces avec la tole d’Usibor®1500, et que le noyau se forme du cote de cette tole, loin de la tole mince. Les valeurs tres elevees des resistances de contact electrique et thermique, mesurees aux interfaces avec la tole d’Usibor®, sont imputables au revetement Alusi® et sont a l’origine des forts echauffements initiaux observes a ces interfaces. Un modele numerique, limite aux aspects electrothermiques et developpe sur COMSOL Multiphysics®, a permis de montrer que la zone fondue s’initie tres rapidement dans la tole d’Usibor® 1500 sous l’effet des fortes resistances de contact adjacentes, et que son developpement en epaisseur et diametre est pilote par les evolutions des rayons de contact electrode-tole. Les resistances de contact entre electrode et tole mince, le profil du courant de soudage et les rayons de courbure des faces actives des electrodes sont les parametres preponderants a optimiser pour ameliorer la penetration du noyau dans la tole mince.