Hybrid laser welding of stainless and high strength steels

In manufactory industry, welding is a joining technology with noticeable properties. Among fusion welding technologies, laser welding employs high power laser beam with high density. Laser beam welding debuted several decades later, after the invention of the laser in 1960, and has demonstrated to be especially useful in high-speed, automated welding. The laser beam produces a reduced melting zone, which has always been seen positive for the industrial products. Low thermal distortion, high speed and little heat input per unit of volume can be reached by fiber laser with some limitations like crack susceptibility, porosity, strict tolerances for edges preparation, and high reflectivity. Hybrid laser arc welding can overcome some of those limitations since its tendency to flexibility, low crack formation and porosity, high precision, narrow melting zones, high cooling and heating speeds, deep penetration. Mixing and diffusion phenomena can be controlled and that reduces the formation of intermetallic compounds in the case of dissimilar joints. In fact, dissimilar metal welding implicates joining two different metals or alloys by melting, which the addition of filler metal as an option. Among dissimilar metal weld, aluminum/steel (Al/Fe), advanced high strength steel (AHSSs) /austenitic stainless steel have increased the attention from aeronautical and automotive industry because of the properties such as reduced weight, good formability, high mechanical strength, and more. Dissimilar joining of hybrid Al-Fe structures is often challenging because of the inevitable formation of brittle intermetallic compounds (IMCs), which can compromise the mechanical properties of the weld due to their poor ductility. Therefore, it is necessary to limit their formation to obtain good quality welds. The major goal of this thesis is to analyze the microstructural and mechanical aspects of dissimilar joints used in the automotive industry, which were welded by different laser techniques. Particularly, great importance was given to the microstructure of the fuse (FZ) and heat affect zones (HAZ), being those zones mainly associated to the mechanical behavior of the weld. The mechanical and microstructural aspect of the joint were related with the process parameters to study the defects and to obtain the better mechanical properties. After the state-of-art on fusion welding processes, the research activity concerned the study of the fiber laser/TIG weldability of AISI 304 and AISI 410 stainless steel (SS), weldability of Twinning-induced plasticity steels (TWIP) and Dual Phase (DP) steel dissimilar joint by laser arc hybrid welding with austenitic filler and finally the assessment of weldability of TWIP and DP steels with AISI 316 stainless steel. Since the automotive industry has been reducing the vehicle weight and fuel consumption, advanced high strength steels and stainless steel can be help to this achievement. Research activity concerned also the study of the laser welding of the annealed alloy AA5754 with austenitic stainless steel AISI 316. Particularly, the laser off-set welding (LOW) was used to limit the formation of brittle intermetallic compounds during the welding process. The laser beam was directed onto the stainless steel side of the butt weld at a small distance from the edge of the aluminum sheet. The keyhole formed and the full penetration was reached in the SS side of the weld. Thin layer of intermetallic compounds of different stoichiometry was found at the interfaces between SS and Al fusion zone (FZ). Finite Element (FE) analysis was employed to evaluate both the temperature distribution, residual stress and distortions in the joints. Using Simufact commercial software, the numerical model of the joint was created and validated through the available experimental data. The target of this part of the research was to build a methodology to build a numerical model for laser and hybrid welding process, with a particular attention to the set-up of the heat source model.